阅读说明：本技术 用于储存热量的方法和设备 (Method and apparatus for storing heat ) 是由 亨里克·普拉诺夫 于 2019-08-29 设计创作，主要内容包括：一种储热容器包括：将气态传热流体或过热液态传热流体引入储热容器的至少一个输入入口和从储热容器回收液态传热流体的至少一个液体回收系统；和/或从储热容器回收气态传热流体的至少一个气体出口和将液态传热流体引入储热容器的至少一个输出入口；并且还包括固体粒状材料的容积体,在传热流体与固体粒状材料接触时通过传热流体从气态到液态的相变向容积体传递热量和/或在传热流体与固体粒状材料接触时通过传热流体从液态到气态的相变从容积体送走热量,该容积体与至少一个输入入口和至少一个液体回收系统和/或至少一个气体出口和至少一个输出入口流体连接,并且减压系统与固体粒状材料的容积体流体连接,其特征在于减压系统被设置为减小仅由不可凝物质产生的气体压力贡献。(A heat storage container comprising: at least one liquid recovery system for introducing gaseous or superheated liquid heat transfer fluid into the at least one input inlet of the thermal storage vessel and recovering liquid heat transfer fluid from the thermal storage vessel; and/or at least one gas outlet for recovering gaseous heat transfer fluid from the thermal storage vessel and at least one input port for introducing liquid heat transfer fluid into the thermal storage vessel; and further comprising a volume of solid particulate material to which heat is transferred by the phase change of the heat transfer fluid from gaseous to liquid state when the heat transfer fluid is in contact with the solid particulate material and/or from which heat is transferred by the phase change of the heat transfer fluid from liquid to gaseous state when the heat transfer fluid is in contact with the solid particulate material, the volume being in fluid connection with at least one input inlet and at least one liquid recovery system and/or at least one gas outlet and at least one input/output port, and a pressure reduction system in fluid connection with the volume of solid particulate material, characterized in that the pressure reduction system is arranged to reduce the gas pressure contribution generated solely by the non-condensable matter.)

1. A thermal storage container, comprising:

at least one input inlet for introducing gaseous or superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering liquid heat transfer fluid from the thermal storage vessel; and/or

At least one gas outlet for recovering gaseous heat transfer fluid from the heat storage container and at least one output port for introducing liquid heat transfer fluid into the heat storage container,

and further comprising a volume of solid particulate material to which heat is transferred by means of a phase change of the heat transfer fluid from gaseous to liquid state upon contact of the heat transfer fluid with the solid particulate material and/or from which heat is transferred by means of a phase change of the heat transfer fluid from liquid to gaseous state upon contact of the heat transfer fluid with the solid particulate material,

the volume is in fluid connection with the at least one input inlet and the at least one liquid recovery system and/or with the at least one gas outlet and the at least one output port, and

a pressure reduction system is in fluid connection with said volume of said solid particulate material,

characterised in that the pressure reduction system is arranged to reduce the gas pressure contribution generated by non-condensable matter only.

2. The thermal storage container of claim 1 wherein the pressure relief system comprises a condenser operative to condense the gaseous heat transfer fluid and prevent its removal from the thermal storage container by the pressure relief system, and preferably wherein the condenser is maintained at a suitable temperature to condense the gaseous heat transfer fluid by exposure to the ambient environment.

3. The thermal storage container of claim 2 wherein the condenser is arranged such that the condensed heat transfer fluid returns to the thermal storage container by the action of gravity.

4. Heat storage vessel according to any of the preceding claims wherein the solid particulate material has a non-porous surface, preferably wherein the solid particulate material is capable of containing a volume of heat transfer fluid within the pores of the particles of less than 1% of the volume of the particles.

5. Heat storage vessel according to any of the preceding claims wherein the solid particulate material has a convex particle shape, preferably wherein the particle shape of the solid particulate material is such that a volume of heat transfer fluid of less than 1% of the volume of the particles can occupy recessed areas on the surface of the particles.

6. Heat storage container according to any of the preceding claims, wherein the particle diameter of the solid particulate material is 10mm to 500mm, preferably 10mm to 300 mm.

7. Heat storage container according to any of the preceding claims, wherein the filling rate of the solid particulate material in the volume of the solid particulate material is in the range of 0.5 to 0.9.

8. Heat storage vessel according to any of the preceding claims wherein the solid particulate material is selected from rock particles, mineral particles and mixtures thereof, and more preferably from granite, basalt, marble, diamond, quartz, flint, pebbles obtained from beaches, sea or river beds or mixtures thereof.

9. Heat storage container according to any of the preceding claims, wherein the heat storage container is designed to be pressurized to an overpressure of at most 1 bar.

10. A thermal storage system, comprising: an input system and/or an output system; and a heat storage container, wherein the heat storage container includes:

at least one input inlet for introducing gaseous or superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering liquid heat transfer fluid, wherein the at least one input inlet and the at least one liquid recovery system are included in the input system of the thermal storage system; and/or

At least one gas outlet for recovering gaseous heat transfer fluid and at least one output port for introducing liquid heat transfer fluid into the heat storage container, wherein the at least one output port and the at least one gas outlet are comprised in the output system of the heat storage system, and

the heat storage container further comprising a volume of solid particulate material to which heat is transferred by means of a phase change of the heat transfer fluid from a gaseous state to a liquid state upon contact of the heat transfer fluid with the solid particulate material and/or from which heat is transferred by means of a phase change of the heat transfer fluid from a liquid state to a gaseous state upon contact of the heat transfer fluid with the solid particulate material,

the volume is in fluid connection with the at least one inlet and the at least one liquid recovery system and/or with the at least one output port and the at least one gas outlet, and

wherein the input system further comprises an input heat source fluidly connected to the at least one input inlet and the at least one recovery system, the input heat source being arranged to evaporate liquid heat transfer fluid received by the at least one liquid recovery system prior to introducing gaseous heat transfer fluid into the heat storage vessel through the at least one input inlet, or to produce superheated liquid heat transfer fluid that is introduced into the heat storage vessel through the at least one input outlet and at least partially evaporates to form gaseous heat transfer fluid; and/or

Wherein the output system further comprises an output heat sink in fluid connection with the at least one output port and the at least one gas outlet, the output heat sink being arranged to condense gaseous heat transfer fluid received through the at least one gas outlet prior to introducing liquid heat transfer fluid through the at least one output port into the heat storage container,

characterized in that the heat storage container further comprises a pressure reduction system in fluid connection with the volume of solid particulate material.

11. The thermal storage system of claim 10 wherein the pressure relief system is configured to reduce a gas pressure contribution generated solely by non-condensable matter.

12. The heat storage system of claim 11 wherein the heat storage container is a heat storage container of any one of claims 1 to 9.

13. Heat storage system according to any one of claims 10 to 12 wherein the movement of gaseous or superheated heat transfer fluid from the input heat source to the heat storage vessel and/or the movement of gaseous heat transfer fluid from the heat storage vessel to the output heat sink is driven solely by the phase change of the heat transfer fluid upon contact with the solid particulate material.

14. Heat storage system according to any one of claims 10 to 13 wherein the movement of liquid heat transfer fluid from the heat storage container to the input heat source and/or the movement of liquid heat transfer fluid from the output heat sink to the heat storage container is driven solely by gravity.

15. A method of charging and/or discharging heat to/from a heat storage container, the heat storage container comprising:

at least one input inlet for introducing gaseous or superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering liquid heat transfer fluid; and/or

At least one gas outlet for recovering gaseous heat transfer fluid and at least one input port for introducing liquid heat transfer fluid into the heat storage container,

the heat storage container further comprises a volume of solid particulate material to which heat is transferred by means of a phase change of the heat transfer fluid from gaseous to liquid state upon contact with the solid particulate material and/or from which heat is transferred by means of a phase change of the heat transfer fluid from liquid to gaseous state upon contact with the solid particulate material, the volume being in fluid connection with the at least one input inlet and the at least one liquid recovery system and/or in fluid connection with the at least one gas outlet and the at least one input outlet, and

a pressure reduction system is in fluid connection with said volume of said solid particulate material,

the method comprises the following steps:

reducing the pressure within the volume of solid particulate material before and/or during introduction of a heat transfer fluid into the heat storage container through the at least one input inlet and/or the at least one output port to contact the volume of solid particulate material.

16. A method of charging and/or discharging heat to/from a heat storage system, the heat storage system comprising: an input system and/or an output system; and a heat storage container, wherein the heat storage container includes:

at least one input inlet for introducing a gaseous heat transfer fluid or a superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering liquid heat transfer fluid, wherein the at least one input inlet and the at least one fluid recovery system are included in the input system; and/or

At least one gas outlet for recovering gaseous heat transfer fluid and at least one output port for introducing liquid heat transfer fluid into the heat storage container, wherein the at least one output port and the at least one gas outlet are comprised in the output system,

wherein the heat storage container further comprises a volume of solid particulate material in fluid connection with the at least one input inlet and the at least one liquid recovery system and/or in fluid connection with the at least one output inlet and the at least one gas outlet, and

wherein the thermal storage vessel further comprises a pressure reduction system in fluid connection with the volume of solid particulate material,

wherein the input system further comprises an input heat source fluidly connected to the at least one input inlet and the at least one liquid recovery system, and/or wherein the output system further comprises an output heat sink fluidly connected to the at least one output port and the at least one gas outlet, the method comprising:

A. heating a liquid heat transfer fluid in the input system by transferring heat from the input heat source to evaporate or superheat the heat transfer fluid to charge the thermal storage vessel; introducing a vaporized or superheated heat transfer fluid into the volume of the solid particulate material through the input inlet such that the vaporized heat transfer fluid condenses upon contact with the solid particulate material, or the superheated heat transfer fluid vaporizes upon or after entering the volume of the solid particulate material and then condenses upon contact with the solid particulate material, thereby transferring heat from the heat transfer fluid to the solid particulate material; and collecting condensed liquid heat transfer fluid by the liquid recovery system; and/or

B. Discharging heat from the heat storage container by introducing a liquid heat transfer fluid into the volume of solid particulate material through the input port to evaporate the liquid heat transfer fluid upon contact with the solid particulate material, thereby transferring heat from the solid particulate material to the liquid heat transfer fluid; and allowing the vaporized heat transfer fluid to pass through the gas outlet to the output heat sink and transfer heat to the output heat sink to condense the vaporized heat transfer fluid,

wherein the pressure within the volume of solid particulate material is reduced prior to and/or during the introduction of a heat transfer fluid into the volume of solid particulate material through the at least one input inlet and/or the at least one output port.

17. A method according to claim 15 or 16, wherein reducing the pressure is reducing the gas pressure contribution produced only by non-condensable matter.

18. A method according to claim 17, wherein the gas pressure contribution due to non-condensable substances only is 200 mbar or less, more preferably 175 mbar or less or 150 mbar or less, such as 125 mbar or less, or 100 mbar or less, such as 75 mbar or less, or 50 mbar or less, such as 25 mbar or less, or 10 mbar or less, such as 5 mbar or less.

19. The method of claim 17 or 18 wherein the heat storage container is a heat storage container of any one of claims 1 to 9.

20. The method of any one of claims 15 to 19 wherein the heat storage system is a heat storage system of any one of claims 10 to 14.

21. The method of any of claims 15 to 20, wherein the degree of pressure reduction is varied to vary the boiling point of the heat transfer fluid such that the boiling point of the heat transfer fluid increases during charging of the heat storage container and/or decreases during discharging of heat from the heat storage container.

22. A method according to any one of claims 15 to 21, wherein the percentage of heat transfer to and/or from the heat storage vessel by phase change of the heat transfer fluid is preferably at least 50%, more preferably 60%, more preferably 70%, more preferably 80%, more preferably 90%, most preferably more than 95%.

23. The method of any one of claims 17 to 22 wherein the heat transfer fluid is stable relative to the operating temperature of the thermal storage vessel due to a reduction in the gas pressure contribution generated by the non-condensable matter.

Technical Field

The present invention relates to a heat reservoir for storing energy for later use, a heat storage container and a method for storing heat.

Background

Many energy generation technologies, especially renewable resources such as wind and solar, deliver energy in ways that are inconsistent with local energy consumption. Therefore, storing energy for later use is an important aspect of energy infrastructure. Today, there are indeed many such technologies, such as chemical batteries and heat storage solutions. However, most solutions are expensive compared to the amount of stored energy, or have a limited number of operating cycles (charge-discharge), substantially increasing the cost of the stored energy compared to the energy used directly. Therefore, it would be advantageous to be extendable to a solution that stores large amounts of energy at low cost and has a large number of operating cycles.

FR2981736 describes a thermal storage system for storing solar thermal energy, in particular for heating buildings, to allow the use of solar thermal energy collected in the summer in the winter. This heat-storage system includes: a sealed housing having an inlet and a heat transfer fluid outlet; an energy storage medium, preferably spherical particles of glass, ceramic or vitreous asbestos, preferably having a diameter of between 1mm and 1cm, located in a sealed enclosure; and a vacuum pump in fluid communication with the housing. It is proposed that once heat is transferred to the energy storage medium, heat loss from the sealed enclosure can be reduced by withdrawing the heat transfer fluid from the sealed enclosure and maintaining the pressure in the sealed enclosure below atmospheric during the storage phase. It is claimed to reduce heat loss by convective and conductive exchange between the energy storage medium and the heat transfer fluid.

GB2485836 describes the use of a pebble bed (pebble bed) to cool gas in the event that hot gas needs to be removed from the turbine, for example in the event of an emergency shutdown of a thermal power plant. The pebble bed is interposed between the hot gas stream and the cooler normally provided for such a case, and allegedly reduces the thermal stress on the cooler by absorbing some of the heat of the gas and thus reducing the temperature to which the cooler is initially subjected during the removal of the hot gas from the turbine. The ball pieces in the ball bed are made of a solid super refractory material such as silicon carbide, mullite or alumina and may range in diameter from 12mm to 30 mm. The pebble bed may be cooled by passing a fluid coolant, such as water, liquid nitrogen, steam or nitrogen therethrough, and a two stage cooling process is envisaged in which a gaseous coolant, such as steam, is delivered to the bed, followed by a liquid coolant, such as water. The ball bed may be provided with a condensate drain to remove any condensate that may be generated during heating or cooling of the bed. Cooling of the bed may be performed by recirculating a fluid coolant through the ball bed and a heat exchanger for cooling the coolant after it has passed through the ball bed.

GB2509894 describes a thermal energy storage system and a method of storing and discharging thermal energy in which a heated heat transfer liquid is passed through a bed of granular material (preferably rock or mineral material) to transfer heat from the heat transfer liquid to the granular material for storage, and in which a gas is passed through the bed of granular material in order to discharge stored heat from the granular material.

CH703413 describes a heat storage system for use with an air source heat pump, wherein the heat storage system is in the form of a heat box containing rock particles or gravel through which air is circulated to heat or cool the rock particles or gravel as required.

US2012/241122 relates to a thermal storage system in which the heat transfer fluid is a phase change fluid and transfers heat to the storage medium primarily through a phase change of the heat transfer fluid. The storage medium may also be a phase transfer material.

The storage of thermal energy can be done in a variety of ways. The most common way is to heat a large heat conducting mass (e.g. bulk concrete) using a heat transfer fluid such as air, diathermic oil or pressurized water, which flows through pipes embedded in the concrete. When the stored energy is to be used, cold fluid flows through the embedded pipes, thereby being heated by the concrete. The heated fluid may then be used to drive a thermal Carnot cycle (Carnot) process or other process that utilizes the stored heat. Instead of using a solid reservoir, a liquid reservoir may also be used, such as a reservoir of a larger diathermic oil or molten salt, wherein the heat extraction process is typically performed by flowing a fluid through a heat exchanger to heat a secondary fluid, which is used in a carnot cycle or other process. A third way of storing heat energy is to use phase change materials, e.g. materials that melt or boil at a certain temperature, wherein a relatively large amount of heat is used to promote the phase change. Once the phase change process is reversed, heat is released again at the boiling or melting point of the phase change material.

Disclosure of Invention

The present inventors have realised that it is important that the storage system selected can be charged with thermal energy in an efficient manner allowing maximum heat transfer and minimum heat loss between the heat source and the thermal storage system, and similarly that the storage system can be discharged with stored thermal energy in an efficient manner. In addition, when using larger solid heat conducting substances, it is desirable to avoid the need to provide embedded piping or similar heat distribution mechanisms that are complicated to manufacture, while also ensuring uniform heat distribution within the storage system, thereby avoiding the formation of hot zones that are in close contact with the heat source or cold zones that are not in sufficiently close contact with the heat source to be effectively heated by the heat source. Furthermore, it is preferred to use naturally available materials and/or low cost materials and/or locally available materials and materials that are reusable where possible, especially if the storage system is to be incorporated into a renewable energy generation system such as a solar power generation system, a wind power generation system, a wave power generation system or a heat pump system (e.g. a ground source heat pump or an air source heat pump).

It is an object of the present invention to provide an improved method for storing thermal energy.

It is an object of the present invention to provide an improved method for storing thermal energy and increasing internal heat transfer in a packed bed system.

It is another object of the invention to reduce the cost of thermal energy storage.

It is another object of the present invention to provide a thermal solution that uses a large proportion of natural materials with a low carbon footprint.

Another object of the invention is to simplify the construction of the thermal energy storage and to increase the flexibility in dimensioning the storage for the power of the input and output system and the size of the heat storage containers, respectively.

Another object of the invention is to enhance durability, simplify maintenance and reduce the obstacles to replacement of thermal energy storage.

It is another object of the present invention to provide an improved method of charging and/or discharging heat from a heat storage container.

It is another object of the invention to allow the use of heat transfer fluids that are generally unstable to the operating temperatures required for the thermal storage vessels.

It is another object of the present invention to provide an efficient heat transfer method.

It is a further object of the present invention to provide an alternative to the prior art.

Described herein are a heat storage container, a heat storage system, and methods of charging and/or discharging heat by such a heat storage container or heat storage system, all of which are intended to achieve at least one of the desired aspects described above.

Accordingly, in a first aspect, the present invention provides a thermal storage container comprising:

at least one input inlet for introducing gaseous or superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering liquid heat transfer fluid from the thermal storage vessel; and/or

At least one gas outlet for recovering gaseous heat transfer fluid from the heat storage vessel and at least one input port for introducing liquid heat transfer fluid into the heat storage vessel,

and further comprising a volume of solid particulate material to which heat is transferred by means of a phase change of the heat transfer fluid from gaseous to liquid state upon contact of the heat transfer fluid with the solid particulate material and/or from which heat is transferred by means of a phase change of the heat transfer fluid from liquid to gaseous state upon contact of the heat transfer fluid with the solid particulate material,

the volume being in fluid connection with at least one inlet and at least one liquid recovery system and/or with at least one gas outlet and at least one inlet and outlet, and

a pressure reduction system is in fluid connection with the volume of solid particulate material, characterized in that the pressure reduction system is arranged to reduce the gas pressure contribution generated solely by the non-condensable matter.

Preferably, the depressurization system includes a condenser operative to condense the gaseous heat transfer fluid and thereby prevent its removal from the heat storage container by the depressurization system. Preferably, the condenser is maintained at a suitable temperature to condense the gaseous heat transfer fluid by exposure to the ambient environment. Preferably, the condenser is arranged such that the condensed heat transfer fluid returns to the heat storage container by the action of gravity.

Preferably, the heat storage container further comprises insulation surrounding the volume of solid granular material to reduce heat loss from the heat storage container. Suitably, the insulation may be selected from mineral wool, glass wool, rock wool, ceramic wool, natural wool and mixtures thereof.

Preferably, the solid particulate material has a non-porous surface, and more preferably the volume of heat transfer fluid capable of being contained within the pores of the particles is less than 1% of the volume of the particles. Preferably, the solid particulate material has a convex particle shape, and more preferably a particle shape such that a volume of heat transfer fluid of less than 1% of the volume of the particle is able to occupy recessed regions on the surface of the particle. Preferably, the particles of solid particulate materialThe pellet diameter is 10mm to 500mm, for example 10mm to 300 mm. Preferably, the particle diameter of the solid particulate material is greater than 10 mm. Preferably, the maximum range of diameters of the particles of the volumetric body of solid particulate material is ± 50% of the diameter of the selected particle. Preferably, the filling rate of the solid particulate material within the volume of solid particulate material is in the range of 0.5 to 0.9. Preferably, the average width of the interstices between the particles is from 10mm to 30 mm. Preferably, the surface roughness R of the particles_aLess than 25 nm. Preferably, the receding contact angle of the surface of the particulate material with respect to the heat transfer fluid is at least 45 °. Suitably, the solid particulate material may be surface treated to obtain the desired receding contact angle. Suitably, the solid particulate material is surface treated with perfluorododecyl trichlorosilane, silicone or siloxane.

Preferably, the solid particulate material is selected from rock particles, mineral particles and mixtures thereof. Preferably, the solid particulate material is selected from granite, basalt, marble, diamond, quartz, flint, pebbles obtained through beaches, sea or river beds, or mixtures thereof. Suitably, the rock particles or mineral particles may be man-made particles. Preferably, the rock particles or mineral particles are obtained from naturally occurring rocks or minerals, more preferably naturally occurring rocks or minerals that have been divided into particles of appropriate size.

Alternatively, the solid particulate material may be an encapsulated phase change material. Suitably, the phase change material may be selected from eutectic salts or organic materials having melting points in a suitable range. Suitable eutectic salts include potassium nitrate and sodium nitrate. Suitable organic materials include waxes. Suitable encapsulating materials include metals, polymers or concrete. Preferably, the particles of encapsulated phase change material are spherical.

Preferably, the liquid recovery system comprises an inclined lower surface located below or forming a lower portion of the volume of solid particulate material.

Preferably, the outlet port is selected from one or more nozzles and one or more rotary distributors. Preferably, the input/output port is placed in an upper portion of the heat storage container, for example above a volume of solid granular material.

Preferably, the heat storage vessel is designed to be pressurized to an overpressure of at most 1 bar. That is, the heat storage vessel is preferably designed to withstand a maximum internal gauge pressure of 1 bar or an absolute pressure of 2 bar.

In a second aspect, the invention provides a heat storage system comprising an input system and/or an output system and a heat storage container, wherein the heat storage container comprises:

at least one input inlet for introducing a gaseous heat transfer fluid or a superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering the liquid heat transfer fluid, wherein the at least one input inlet and the at least one liquid recovery system are comprised in the input system of the thermal storage system; and/or

At least one gas outlet for recovering the gaseous heat transfer fluid and at least one outlet for introducing the liquid heat transfer fluid into the heat storage container, wherein the at least one outlet and the at least one gas outlet are comprised in an outlet system of the heat storage system, and

the heat storage container further comprises a volume of solid particulate material to which heat is transferred by means of a phase change of the heat transfer fluid from a gaseous state to a liquid state upon contact of the heat transfer fluid with the solid particulate material and/or from which heat is transferred by means of a phase change of the heat transfer fluid from a liquid state to a gaseous state upon contact of the heat transfer fluid with the solid particulate material,

the containment body being in fluid connection with at least one inlet and at least one liquid recovery system and/or with at least one output port and at least one gas outlet, and wherein the input system further comprises an input heat source in fluid connection with at least one input inlet and at least one recovery system, the input heat source being arranged to evaporate liquid heat transfer fluid received through the at least one liquid recovery system prior to introducing gaseous heat transfer fluid into the heat storage container through the at least one input inlet, or to produce superheated liquid heat transfer fluid which is introduced into the heat storage container through the at least one input outlet and at least partially evaporates to form gaseous heat transfer fluid; and/or wherein the output system further comprises an output heat sink in fluid connection with the at least one output port and the at least one gas outlet, the output heat sink being arranged to condense gaseous heat transfer fluid received through the at least one gas outlet prior to introducing liquid heat transfer fluid through the at least one output port into the heat storage container,

characterized in that the heat storage container further comprises a pressure reduction system in fluid connection with the volume of solid particulate material.

Preferably, the pressure reduction system is arranged to reduce the gas pressure contribution generated by the non-condensable matter only. Preferably, the depressurization system includes a condenser operative to condense the gaseous heat transfer fluid and thereby prevent its removal from the heat storage container by the depressurization system.

Preferably, the heat storage container is a heat storage container according to the first aspect of the invention.

Preferably, the input heat source comprises a system for generating a saturated vapour of a heat transfer fluid. Alternatively, the input heat source comprises a system for producing a superheated liquid heat transfer fluid. Suitably, the input heat source comprises a primary fluid circuit and a heat exchanger for transferring heat from the primary fluid to the heat transfer fluid. Preferably, the input heat source comprises a solar heat source, more preferably a solar concentrator. Preferably, a solar thermal source is used to heat the primary fluid in the primary fluid loop. Where the input heat source comprises a system for generating a saturated vapour of a heat transfer fluid, preferably the system further comprises an evaporator.

Preferably, the output radiator comprises a turbine. Preferably, the output heat sink comprises a heat exchanger for transferring heat from the heat transfer fluid.

Preferably, the heat transfer system does not comprise a pump arranged to move gaseous or superheated heat transfer fluid from the input heat source to the thermal storage vessel and/or to move liquid heat transfer fluid from the thermal storage vessel to the input heat source and/or to move gaseous heat transfer fluid from the thermal storage vessel to the output heat sink and/or to move liquid heat transfer fluid from the output heat sink to the thermal storage vessel. Preferably, the movement of the heat transfer fluid through the inlet system and/or the outlet system is driven solely by the phase change of the heat transfer fluid when in contact with the solid particulate material for gaseous or superheated heat transfer fluids and/or solely by the action of gravity for liquid heat transfer fluids.

Preferably, the heat storage system is designed to be pressurized to an overpressure of at most 1 bar. That is, the heat storage vessel is preferably designed to withstand a maximum internal gauge pressure of 1 bar or an absolute pressure of 2 bar.

The one or more heat transfer fluids used in the input and output systems may be the same or different and are selected based on the heat source of the input system and the heat sink of the output system, which may optimally transfer heat in the same temperature range or in different temperature ranges.

In case more than one heat transfer fluid needs to be used, the heat storage system may suitably be provided with a first input system for a first heat transfer fluid, a second input system for a second heat transfer fluid, and so on for the third, fourth, fifth and further heat transfer fluids. Similarly, the thermal storage system may suitably be provided with a first output system for a first heat transfer fluid, a second output system for a second heat transfer fluid, and so on for third, fourth, fifth and further heat transfer fluids. Furthermore, the combination of heat transfer fluids used in the input and output systems may be the same or different depending on the heat source in the input system and the heat sink in the output system, particularly the temperature to which the heat transfer fluid can be heated by the heat source and the temperature to which the output heat sink needs to be brought. Alternatively, the use of more than one heat transfer fluid may be achieved by using a single input system and/or output system, wherein separate input inlets and liquid recovery systems and/or input and gas outlets are provided, respectively, each in fluid connection with an input heat source and an output heat sink, respectively. Alternatively, the same inlet and outlet are used for each of a series of heat transfer fluids, which may be preferred if the heat transfer fluids are easily separable or inexpensive or if the first heat transfer fluid can be substantially completely removed from the thermal storage vessel prior to introduction of a subsequent heat transfer fluid. Where this last arrangement is used, separate heat transfer fluids may be provided with separate fluid paths between the liquid recovery system of the input system, the heat source and the input inlet and/or between the gas outlet, the heat sink and the input/output port of the output system, or they may use a common fluid path.

The thermal storage system may also suitably comprise one or more containers for storing one or more heat transfer fluids to be used, each container being in fluid contact with the input and/or output system in a suitable manner to receive heat transfer fluid from the system when not in use and to supply heat transfer fluid to the system when in use.

The heat storage system may suitably also comprise a system for separating the heat transfer fluids from each other, wherein the fluids have been mixed during use. For example, the heat storage system may further include: phase separation devices for density-based separation of immiscible heat transfer fluids, such as separatory funnels or centrifuges; or a distillation apparatus for separating heat transfer liquid based on boiling point difference.

The heat storage system may also suitably comprise a system for removing all heat transfer fluid from the heat storage vessel and/or the input system and/or the output system. This is useful in cases where the heat storage system is taken out of service for cleaning or maintenance or at the end of its useful life.

In a third aspect, the present invention provides a method of charging and/or discharging heat to/from a heat storage container, the heat storage container comprising:

at least one input inlet for introducing gaseous or superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering the liquid heat transfer fluid; and/or

At least one gas outlet for recovering the gaseous heat transfer fluid and at least one input port for introducing the liquid heat transfer fluid into the thermal storage container,

the heat storage container further comprises a volume of solid particulate material to which heat is transferred by means of a phase change of the heat transfer fluid from gaseous to liquid state upon contact of the heat transfer fluid with the solid particulate material and/or from which heat is removed by means of a phase change of the heat transfer fluid from liquid to gaseous state upon contact of the heat transfer fluid with the solid particulate material, the volume being in fluid connection with at least one input inlet and at least one liquid recovery system and/or with at least one gas outlet and at least one input outlet, and

a pressure reduction system is fluidly connected to a volume of solid particulate material, the method comprising:

the pressure within the volume of solid particulate material is reduced before and/or during the introduction of the heat transfer fluid into the heat storage container through the at least one input inlet and/or the at least one output port to contact the volume of solid particulate material.

Preferably, the pressure reduction system is arranged to reduce the gas pressure contribution generated by the non-condensable matter only. Preferably, the depressurization system includes a condenser operative to condense the gaseous heat transfer fluid and thereby prevent its removal from the heat storage container by the depressurization system.

Preferably, the contribution of gas pressure resulting solely from non-condensable substances is below 200 mbar, more preferably below 175 mbar, or below 150 mbar, such as below 125 mbar, or below 100 mbar, such as below 75 mbar, or below 50 mbar, such as below 25 mbar, or below 10 mbar, such as below 5 mbar.

Preferably, the heat storage container is a heat storage container according to the first aspect of the invention.

Preferably, the percentage of heat transfer to and/or from the heat storage container by phase change of the heat transfer fluid is preferably at least 50%, more preferably 60%, more preferably 70%, more preferably 80%, more preferably 90%, most preferably more than 95%.

In a fourth aspect, the present invention provides a method of charging and/or discharging heat to and/or from a heat storage system comprising an input system and/or an output system and a heat storage container, wherein the heat storage container comprises:

at least one input inlet for introducing a gaseous heat transfer fluid or a superheated liquid heat transfer fluid into the thermal storage vessel and at least one liquid recovery system for recovering the liquid heat transfer fluid, wherein the at least one input inlet and the at least one fluid recovery system are comprised in the input system; and/or

At least one gas outlet for recovering the gaseous heat transfer fluid and at least one output port for introducing the liquid heat transfer fluid into the heat storage container, wherein the at least one output port and the at least one gas outlet are comprised in an output system,

wherein the heat storage vessel further comprises a volume of solid particulate material in fluid connection with at least one inlet and at least one liquid recovery system and/or with at least one inlet and at least one outlet for gas, and

wherein the thermal storage vessel further comprises a pressure reduction system in fluid connection with the volume of solid particulate material,

wherein the input system further comprises an input heat source fluidly connected to the at least one input inlet and the at least one liquid recovery system, and/or wherein the output system further comprises an output heat sink fluidly connected to the at least one output port and the at least one gas outlet, the method comprising:

A. heating a liquid heat transfer fluid in an input system by transferring heat from an input heat source to vaporize or superheat the heat transfer fluid, thereby charging a thermal storage vessel; introducing a vaporized or superheated heat transfer fluid into the volume of solid particulate material through the input inlet such that the vaporized heat transfer fluid condenses upon contact with the solid particulate material, or the superheated heat transfer fluid at least partially vaporizes upon or after entering the volume of solid particulate material and then condenses upon contact with the solid particulate material, thereby transferring heat from the heat transfer fluid to the solid particulate material; and collecting the condensed liquid heat transfer fluid by a liquid recovery system; and/or

B. Releasing heat from the heat storage container by introducing a liquid heat transfer fluid into the volume of solid particulate material through the input/output port to cause the liquid heat transfer fluid to evaporate upon contact with the solid particulate material, thereby transferring heat from the solid particulate material to the liquid heat transfer fluid; and allowing the vaporized heat transfer fluid to pass through the gas outlet to the output heat sink and transfer heat to the output heat sink to condense the vaporized heat transfer fluid,

wherein the pressure within the volume of solid particulate material is reduced prior to and/or during the introduction of the heat transfer fluid into the volume of solid particulate material through the at least one inlet and/or at least one outlet.

Preferably, the pressure reduction system is arranged to reduce the gas pressure contribution generated by the non-condensable matter only. Preferably, the depressurization system includes a condenser operative to condense the gaseous heat transfer fluid and thereby prevent its removal from the heat storage container by the depressurization system.

Preferably, the contribution of gas pressure resulting solely from non-condensable substances is below 200 mbar, more preferably below 175 mbar, or below 150 mbar, such as below 125 mbar, or below 100 mbar, such as below 75 mbar, or below 50 mbar, such as below 25 mbar, or below 10 mbar, such as below 5 mbar.

Preferably, the heat storage container is a heat storage container according to the first aspect of the invention. Preferably, the heat storage system is a heat storage system according to the second aspect of the invention.

Preferably, in the third and fourth aspects of the invention, the degree of pressure reduction is varied to vary the boiling point of the heat transfer fluid to maximise the efficiency of heat transfer between the heat transfer fluid and the solid particulate material. Suitably, the extent of the pressure reduction during charging of the thermal storage container is reduced during charging from a maximum value (i.e. minimum pressure) at the start of charging with any one of the heat transfer fluids to a minimum value (i.e. maximum pressure) at the completion of charging. Thus, the actual boiling point of the heat transfer fluid changes from a minimum value at the start of charging to a maximum value at the completion of charging. Similarly, the degree of pressure reduction upon heat release from the heat storage container is suitably increased during heat release from a minimum value (i.e., a maximum pressure) at the start of heat release with any one of the heat transfer fluids to a maximum value (i.e., a minimum pressure) at the completion of heat release. Therefore, the actual boiling point of the heat transfer fluid changes from the maximum value at the start of heat release to the minimum value at the completion of heat release. This allows the percentage of heat transfer by phase change of the heat transfer fluid to be maximized.

Preferably, the percentage of heat transfer to and from the heat storage vessel by phase change of the heat transfer fluid is preferably at least 50%, more preferably 60%, more preferably 70%, more preferably 80%, more preferably 90%, most preferably more than 95%.

Preferably, in the third and fourth aspects of the invention, the heat transfer fluid is stable with respect to the operating temperature of the heat transfer and storage vessels due to the reduced gas pressure contribution from the non-condensable matter.

All features described may be used in combination where they are not mutually exclusive.

Drawings

Fig. 1 shows an embodiment of the heat storage system and the heat storage container of the invention.

FIG. 2 is a flow chart depicting the thermal storage system of the present invention and its use in the method of the present invention.

Detailed Description

The present invention utilizes a heat storage system comprising a heat storage container containing a volume of solid particulate material, and in particular to an apparatus and method for charging and/or discharging a heat storage container by using a process that prevents the formation of hotter or colder regions in the storage container and allows for efficient heat transfer between the solid particulate material and a heat transfer fluid, in an efficient manner without the need for embedded piping to transfer energy to and/or extract energy from such a storage container.

The heat storage system according to the invention comprises an input system, a heat storage container and an output system. Furthermore, the invention may include systems for recovering different percentages of the heat transfer fluid used, as well as systems for removing all of the heat transfer fluid from the thermal storage vessel, which is preferred for maintenance or end-of-life deconstruction.

The input system includes a system for generating saturated vapor of heat transfer liquid(s) at pressures near or below ambient pressure in the thermal storage vessel. A typical implementation is to have a primary fluid circuit (heat source) and a heat transfer fluid to be evaporated to pass through a heat exchanger that transfers heat from the heat source to the heat transfer fluid, thereby evaporating the heat transfer fluid. The vaporized heat transfer fluid then enters the thermal storage container as a vapor. Alternatively, the heat transfer fluid may be superheated by an input heat source such that it remains in a liquid state until it is introduced into the thermal storage container 102 and at least partially evaporates while or shortly after it is introduced into the thermal storage container. This latter arrangement enables a simpler apparatus than including an evaporation device in the input system.

The heat storage container comprises a volume of a granular material, wherein the particles of said material are preferably non-porous. The granular nature of the material will ensure that voids are formed between the particles such that the voids form an interconnected network through which heat transfer fluid from the input system can flow. If the particles are not porous and the temperature of the particles is below the boiling point of the heat transfer fluid, the vaporized heat transfer fluid will condense on the surface of the particles, thereby releasing the heat of vaporization of the fluid, which is absorbed by the particles, thereby storing heat. After condensation, the now liquid (and thus denser) heat transfer fluid will be collected at the bottom of the vessel (by gravity) and removed by mechanical means (e.g., by a pump). The higher the percentage of heat transfer fluid removed in the liquid phase rather than the vapor phase, the higher the thermodynamic efficiency of the system.

When the particles that pass this heat absorption reach a temperature close to the boiling point of the heat transfer fluid, the process will no longer be able to move energy from the evaporating heat transfer medium to the heat storage container. However, by using multiple heat transfer liquids with different boiling points used in series, heat can be transferred to the reservoir until the reservoir reaches the boiling temperature of the heat transfer fluid with the highest boiling point. The reason for not using a single fluid with a high boiling point in the input system is that typical heat sources (e.g., concentrated solar power plants) transfer heat to the heat transfer fluid more efficiently the colder the heat transfer fluid is. The temperature will be set by the boiling point of the heat transfer liquid used, since the heat source liquid will not cool below the boiling point of the heat transfer fluid in the heat exchanger. The control and selection of which heat transfer fluid to introduce is typically done by temperature monitoring of the thermal reservoir. By using condensation of the gas phase fluid to transfer heat to the vessel, three main advantages over the use of piping are obtained. Firstly, no piping is required in the heat reservoir, thereby significantly reducing the cost of the vessel. Second, the granularity of the reservoir can be adjusted to obtain different input/output powers of the system (by controlling the surface to volume ratio of the system). A final major advantage is that the temperature distribution of such a system is self-leveling with respect to the heat reservoir. This effect is due to the volume change as the vaporized heat transfer fluid condenses. The condensation rate in the volume will be higher in view of the colder volume of the heat reservoir, and therefore the mass flow to this volume will increase, increasing the heating rate of this particular colder volume until the temperature is the same as the rest of the volume. This feature is particularly important in view of the exchange of different heat transfer fluids depending on the reservoir temperature. If a high proportion of the vaporized heat transfer liquid supplied does not condense (or is re-vaporized by a higher vaporization point fluid), the heat transfer efficiency of the system will be reduced. Therefore, good volume control of the temperature is an important feature of the system, which is here achieved by using a heat transfer process (evaporation/condensation), which also causes changes in volume and density.

Furthermore, it has been found that reducing the residual air pressure in the thermal storage container (i.e., the gas pressure contribution from the non-condensable species) during heat transfer greatly increases the internal heat transfer rate. This is due to the increased evaporation and condensation rate of the heat transfer fluid, and the increased diffusion rate of the evaporated liquid between the cooler and hotter regions in the reservoir.

In addition, a system for maintaining a low residual air pressure in the system has been developed. Most systems operating at sub-atmospheric pressures will have a small leak, resulting in air entering the system, slowly increasing the residual air pressure, which will reduce the internal heat transfer over time. By using a standard vacuum pump connected directly to the system, residual air and heat transfer fluid will be evacuated (to some extent), which is not preferred. However, by inserting an elongated conduit maintained at ambient temperature between the reservoir and the vacuum pump, in case the vacuum pump is placed at a higher level than the reservoir, the active fluid will condense in the conduit due to gravity and return to the reservoir as a liquid, which can be recovered using the conventional recovery system of the heat reservoir, while the remaining air will not condense at ambient temperature and is thus evacuated. To further minimize the extracted working fluid, the pipe or a portion thereof may be actively cooled to a temperature at which the vapor pressure of the working fluid is lower.

Another feature of the system is that the heat reservoir particles should preferably not be porous, as condensation may occur in the pores of the material, which largely prevents condensed liquid from flowing down to the mechanical liquid collection system. If downward flow is prevented, the liquid will evaporate again once the next heat transfer fluid (at a higher temperature) is used, resulting in less thermodynamic efficiency. In addition, it may also be desirable to use a larger volume of (often expensive) heat transfer fluid in the system, thereby rendering the system more expensive. One way to further reduce the need for heat transfer fluid and one way to improve the charging/discharging characteristics of the system is to surface treat the particles so that the liquid heat transfer fluid forms droplets on the surface and thus flows away more quickly.

The output system works in the opposite way to the input system; a spray means of liquid heat transfer fluid is provided at the top of the vessel. Once the liquid heat transfer liquid comes into contact with the hot particles of the heat storage container, the liquid heat transfer medium will evaporate, absorbing energy and increasing in volume. The increase in volume will cause the evaporated heat transfer liquid to escape from the heat storage vessel (which is designed to withstand high overpressure, e.g. greater than 1 bar gauge pressure/2 bar absolute pressure, but sealed from the gas) to the output heat exchanger system, where the hot and evaporated heat transfer fluid will condense, transferring the heat of evaporation for another process (e.g. water/steam in a steam turbine or pressure fluid in an Organic Rankine Cycle (ORC) system), or to water/steam in a steam generator. After condensation in the heat exchanger, the liquid fluid may re-enter the vessel during the cycle. Once the temperature of the heat storage container reaches the boiling point of the fluid, a lower boiling fluid must be used. The reason for not starting to use the lowest boiling point liquid is that the temperature at which the thermal energy is extracted (which is equal to the boiling point of the fluid used) should generally be as high as possible, for example, to ensure higher power generation efficiency during the carnot cycle (e.g., a steam turbine/ORC generator).

Since the system can use multiple heat transfer fluids in both the input and output systems, it is advantageous to include a mechanism for separating and separately storing the different heat transfer liquids so they can be used multiple times in an optimal thermodynamic manner in both systems.

Another feature of the system is that the movement of heat transfer liquid from the inlet system to the reservoir and from the reservoir to the outlet system, respectively, does not require the use of mechanical pumps. The phase change from liquid to gas in the output system and the resulting expansion drive the heat transfer fluid through the gas outlet. The phase change from a gaseous state to a liquid state in the input system results in a volume reduction which further draws gaseous heat transfer liquid into the vessel. Furthermore, by arranging the inlet and outlet of the container accordingly, gravity can be used to collect condensed liquid from the container or the output system, respectively.

A typical implementation of the heat storage container is to use stones or rocks with a relatively narrow size range. Typical dimensions (depending on how fast energy extraction is required and how large the volume of the container is) will be in the range of 10-500 mm. A typical size range is +/-50% of the diameter to form the desired network of voids around the particles, as having a very broad size distribution will generally result in a tightly packed structure. Furthermore, this will also depend on the local material source. Another implementation may be to use a metal container with the phase change material inside. This increases costs but allows more energy to be stored at the phase transition temperature of the phase change material. This may be a preferred solution in case the volume of the container is limited.

The disclosed heat reservoir has the advantage of combining granular non-porous materials with evaporation/condensation processes using input and output of thermal energy of multiple heat transfer liquids with different boiling points, which solves the challenge of controlling the heat distribution in the granular material by forced flow (without any volume change) and the problem of having limited thermodynamic efficiency by using only a single liquid. Furthermore, the use of multiple heat transfer fluids increases the possible operating temperature range of the heat storage system without the need to design the heat storage system to operate at significant excess pressure, since once the boiling point of a given heat transfer fluid at ambient pressure is exceeded, additional heat transfer fluids having a higher boiling point at ambient pressure may be used to increase the maximum storage temperature that the heat storage system may reach. Thus, the cost and complexity of the system is reduced.

The invention also preferably relates to a heat reservoir, wherein said phase change drives the required mass transfer due to the volume change associated with said phase change of the heat transfer fluid in said heat storage vessel and/or the input and/or output system, respectively, thereby avoiding the use of mechanical pumps to move evaporated heat transfer liquid between the non-porous granular material and the input and/or output system, respectively.

The invention also relates to a heat reservoir wherein the receding contact angle of the particles is at least 45 degrees, more preferably more than 50 degrees, more preferably more than 55 degrees, more preferably more than 60 degrees, more preferably more than 65 degrees, more preferably more than 70 degrees, more preferably more than 75 degrees, more preferably more than 80 degrees, more preferably more than 85 degrees, most preferably more than 90 degrees, and preferably wherein the contact angle is formed by a surface treatment process of the particulate material.

The invention also relates to a heat reservoir, wherein the heat storage vessel is preferably designed to be maximally pressurized at an overpressure of less than 1 bar, more preferably at an overpressure of less than 0.5 bar, more preferably at an overpressure of less than 0.25 bar, more preferably at an overpressure of less than 0.1 bar, most preferably not at a pressure above ambient pressure. Similarly, the method of the invention is preferably carried out at a maximum pressure of 1 bar overpressure (i.e. 1 bar gauge pressure, 2 bar absolute) in the heat transfer and storage vessel, more preferably at an overpressure of less than 0.5 bar, more preferably at an overpressure of less than 0.25 bar, more preferably at an overpressure of less than 0.1 bar, most preferably without an increase in pressure of the heat storage vessel compared to ambient pressure.

Furthermore, the invention preferably relates to a heat reservoir, wherein the operating temperature is in the range of ambient temperature to 500 ℃. Suitably, the maximum temperature to which the volume of solid particulate material may be heated in the present invention is 250 ℃ or 300 ℃ or 350 ℃ or 400 ℃ or 450 ℃, for example 475 ℃ or 500 ℃.

Furthermore, the invention relates to a heat reservoir or a method of charging and/or discharging a heat reservoir, wherein a plurality of heat transfer fluids are used having different boiling points and are used sequentially during charging and discharging of the heat reservoir.

The invention also relates to a heat reservoir or a method of charging and/or discharging a heat reservoir, wherein a heat transfer fluid is used having a boiling point based on pressure, and the pressure is variable to set the boiling point of the heat transfer fluid in dependence on the temperature state of the heat reservoir.

Furthermore, the invention relates to a heat reservoir without any gas phase mechanical pump for pumping heat transfer fluid between the input system, the heat storage vessel and the output system.

The heat of vaporization refers to the enthalpy of vaporization.

By effective fluid is meant a heat transfer fluid for transferring heat from a heat source to a reservoir by evaporation in an input system and condensation in a reservoir, or a fluid for transferring heat from a reservoir to an output system by evaporation in a reservoir and condensation in an output system.

The gas pressure contribution from the non-condensable matter refers to the pressure of the inert gas in the system, e.g. the pressure of air, nitrogen or other gas of the atmosphere that is not evaporated or condensed in the operating temperature window of the heat reservoir. This is also referred to as "residual air pressure".

Convex particles refer to the shape of a particle in which no significant amount of liquid will collect in the recessed areas on the surface of the particle, and therefore a significant amount of liquid will flow away due to gravitational pull in the liquid. For the purposes of this application, a particle is defined as convex if it can collect in the concave surface region of the particle with a liquid volume that is less than 1% of the volume of the particle.

Granular refers to a material consisting of individual cohesive portions capable of forming mechanically stable aggregates with voids (or air) between the individual particles.

The diameter of a given object refers to the equivalent diameter of a spherical object having the same mass and density. Thus, the requirement for a size range of the granular material defined by the diameter does not imply that the granular material needs to be composed of spherical objects.

The size distribution refers to the relative expansion of the size of the object. The distribution may follow a normal distribution or other distribution, and the expansion is defined to be equal to two standard deviations of 95% of the objects within the expansion.

Pressurization refers to a configuration designed to be mechanically stable under significant internal overpressure. In this context, this significance is defined as an overpressure exceeding 1 bar.

Phase change materials refer to materials that change between a solid phase and a liquid phase at a particular temperature.

By heat transfer fluid is meant a fluid capable of being in a liquid and a gaseous state, wherein the two states are separated by a phase change at the associated enthalpy of vaporization.

Thermodynamic efficiency refers to the loss of energy quality (or entropy gain) from the input to the output system. For example, in the case where the temperature gradient between the heat transfer fluid and the solid material particles is very large, the temperature loss in heat transfer between the fluid and the particles is large, and thus the thermodynamic efficiency of the transfer is low. On the other hand, where the temperature gradient is small but sufficient to cause the heat transfer fluid to undergo a phase change upon contact with the particles to an extent sufficient to give an acceptable rate of transfer, the thermodynamic efficiency of the transfer is much higher and the temperature losses associated with the transfer are much less.

Boiling point refers to the boiling point at atmospheric pressure.

"X is fluidly connected to Y" means that a fluid flow path is provided between element X and element Y.

The method and apparatus according to the invention will now be described in more detail with reference to the accompanying drawings. The drawings illustrate one way of implementing the invention and should not be construed as limited to other possible embodiments falling within the scope of the appended claims.

Fig. 1 depicts an embodiment of a thermal storage system 100 according to the present invention comprising a thermal storage container 102 according to the present invention. When the various inlets and outlets provided are closed, the thermal storage vessel 102 is substantially airtight. The heat storage container may be made of any material suitable for withstanding the conditions of use in terms of temperature and pressure, and suitable materials include concrete or metal. For use in the context of the present invention, it is not necessary to design the heat storage container to withstand a large positive overpressure, since it is not envisaged to pressurize the container to a pressure significantly above atmospheric pressure, for example a gauge pressure exceeding 1 bar.

The heat storage vessel also includes a barrier 104 to prevent heat loss from the vessel, where suitable barriers are materials such as mineral wool, glass wool or rock wool, and other ceramic wool or rock wool, or other suitable materials well known to those skilled in the art.

Within the heat storage container 102 is a volume 106 of solid particulate material 108 which stores heat by receiving heat from an incoming heat transfer fluid causing a phase change of the heat transfer fluid from a gaseous state to a liquid state and which causes a phase change of the heat transfer fluid from a liquid state to a gaseous state by transferring heat to an outgoing heat transfer fluid when the stored heat is required.

In order that the solid particulate material may perform this function, it is preferred that the solid particulate material is a non-porous material at least on its surface and preferably throughout each particle so that the heat transfer fluid passing over the surface of the solid particulate material is not trapped to any significant extent within the pores of the material. Furthermore, it is preferred that the solid particulate material has a generally convex shape to again prevent heat transfer fluid from collecting in recessed areas or grooves on the surface of the material particles. For these reasons, it is preferred that the solid particulate material be capable of containing a volume of heat transfer fluid within the pores of the particles of material into and out of which the heat transfer fluid may enter and exit, of less than 1% of the volume of the particles. Similarly, the solid particulate material preferably has a shape that allows a volume of heat transfer fluid of less than 1% of the volume of the particles to occupy the valley or groove regions of the particle surface. For the purposes of these definitions, it is believed that the diameter of the pores is less than 10mm, and the depressed or recessed regions on the particle surface are believed to have a diameter of 10mm or greater, since the former dimension is believed to allow capillary forces exerted through the pores to retain liquid on the particle surface, which is not true for the larger depressed or recessed regions.

It is also preferred that the solid particulate material be non-porous to enhance the heat storage capacity of each particle.

It is therefore contemplated that heat transfer occurs through contact between the heat transfer fluid and the surface of the solid particulate material, and that the heat transfer fluid is able to pass freely over the surface of the solid material particles within the thermal storage container 102. Thus, the size and shape of the solid particulate material must be selected to allow the heat transfer fluid to flow freely around and between the solid particles of the material. The inventors have found that a diameter of the material particles of between 10 and 500mm is suitable, for example greater than 10mm or more to 500mm, for example 10 to 300mm, or 10mm or more to 300 mm. For example, an average diameter of 150mm may be selected. Furthermore, it is preferred that the diameter of the particles of the solid material is relatively uniform and that the maximum range of particle diameters will be ± 50% of the selected particle diameter. For example, if an average particle diameter of 150mm is selected, an expansion of 75mm in diameter may be allowed, and a variation of 50mm in particle diameter is preferred. In order to provide a suitable proportion of voids between the particles of solid material in the heat storage container for the passage of the heat transfer fluid therethrough, the fill rate of the solid particulate material in the heat storage container within the total volume of solid particulate material provided in the heat storage container is suitably in the range 0.5 to 0.9, with a preferred value of 0.75. The average width of the interstices between the particles may suitably be in the range of 10 to 30 mm.

Preferably, the surface of the particulate material is a relatively smooth surface which promotes the rolling off of droplets of the heat transfer fluid from the surface to maximise the circulation of the heat transfer fluid within the volume of solid particulate material. Suitably, the surface roughness R of each particle_a(arithmetic mean deviation of surface profile) may be less than 25 nm. The receding contact angle between the surface of the particulate material and the heat transfer fluid may suitably be at least 45 °, such as at least 50 °, such as at least 55 °, such as at least 60 °,for example at least 65 deg., for example at least 70 deg., for example at least 75 deg., for example at least 80 deg., for example at least 85 deg., for example at least 90 deg.. While the roll-off angle (i.e., the angle at which a drop of fluid rolls off a surface) can be measured for a given combination of surface, fluid, and drop size, the receding contact angle related to the roll-off angle is better defined and the measurement is more reliable. Receding contact angle is the contact angle between a liquid and a solid that has been wetted by the liquid and is in the process of being dewetted. The receding contact angle may be suitably determined by reducing the volume of an already positioned sessile drop in a drop shape analysis, where the volume of the drop is reduced manually or with an electric piston. At the same time, the images were recorded and evaluated. Suitable instruments for making such measurements include DSA100 available from Kruss Scientific (www.kruss-Scientific. com).

Many materials inherently have large receding contact angles on their surface. However, in the case where the surface of the selected solid particulate material does not naturally have a suitably large receding contact angle, the particles may be surface treated to increase the contact angle to a desired value. Suitable surface treatments may include coating the solid material particles with, for example, FDTS (perfluorododecyl trichlorosilane), silicone or siloxane. Furthermore, in addition to the benefits described above, it is preferred to use non-porous materials for the solid particulate material to use less surface treatment material. However, where the selected material is more porous than desired, the properties of the material may be improved to some extent by surface treatment with a coating as described above, as these treatments may reduce the capillary forces holding the liquid in the pores, thereby allowing the liquid to flow more freely across the particle surface than would be the case without the surface treatment.

The requirement for the surface of the solid material particles to be smooth, non-porous and convex is particularly important in the case where the heat transfer fluid or fluids to be used (for example diathermic oil) are expensive, since it is desirable to minimize losses during fluid circulation due to oil remaining on the surface and/or in the pores of the particles. However, where less expensive heat transfer fluids can be used, higher porosity, concavity and/or roughness of the particle surface can be tolerated and thus higher losses of fluid during cycling can be tolerated.

Suitably, the solid particulate material may be selected from rock particles or mineral particles. The material should be selected to be suitable for withstanding multiple heating and cooling cycles to the desired maximum storage temperature without breaking into significantly smaller pieces or decomposing. It is expected that the use of temperatures up to 500 ℃ for most rock particles or mineral particles will not cause such problems. Suitable natural rock or natural mineral materials for the solid particulate material may be granite, basalt, marble, diamond, quartz, flint or other similar rocks or minerals or mixtures thereof. Furthermore, it is envisaged that rocks which have been crushed, shaped into particles and then sintered to reduce their porosity compared to the natural state, or other artificial or non-natural rock or mineral products, such as synthetic diamond or artificial granite, may be used if desired. However, especially in the case of providing a thermal reservoir for a renewable energy generating system, it is preferred to use natural materials, more preferably locally available natural materials. In view of the above considerations regarding the size and shape of the granular material, particularly preferred materials may be pebbles obtained from the sea, beach or river bed, as they have been ground into a more smooth and uniform convex shape than is typically obtained by rock harvesting on land. Of course, however, the harvested rock may be suitably smoothed, polished, sized and/or shaped to achieve the appropriate particle size and shape for use in the present invention. Many naturally occurring rocks or minerals have a large receding contact angle on the surface; however, if this is not the case, the rock or mineral particles may be surface treated as described above to improve the receding contact angle. This may allow the use of native rock or mineral deposits that do not inherently have the desired surface characteristics.

Alternatively, and in particular where space for a container for storing heat is limited, encapsulated phase change material may be used as the solid particulate material to which heat is transferred for storage. The phase change material is in a solid state after releasing heat from the heat storage container and in a liquid state while transferring heat thereto from the heat transfer fluid. Generally, for the same heat storage capacity, the phase change material allows the volume of the heat storage container to be about half that of a heat storage container containing rock or mineral particles, since the phase change allows the phase change material to have a much larger heat capacity at the temperature at which the phase change occurs compared to a material which does not undergo a phase change at that temperature. In addition, if the phase change occurs at high temperatures, this property may allow a higher proportion of stored heat to be obtained at high temperatures compared to solid particulate materials in which no phase change occurs, since a large portion of the stored heat is released as the phase changes from a liquid to a solid state, rather than simply due to the temperature drop. However, the cost is significantly higher. Suitable phase change materials may include eutectic salts (e.g., potassium nitrate or sodium nitrate), or organic materials (e.g., waxes) having melting points within a suitable range for the heat transfer to be performed. It is contemplated that the phase change material may be encapsulated in a suitable material, such as metal, polymer, or concrete, to form solid material particles. The choice of the packaging material will depend on the intended use of the thermal storage container, particularly the rate at which heat transfer must occur. For example, if seasonal heat storage is required, heat transfer may occur slowly, and the material used to encapsulate the phase change material need not be a highly thermally conductive material, so concrete may be a suitable choice. However, where heat transfer must be faster (e.g., over a period of minutes or hours), materials that conduct heat faster must be selected to encapsulate the phase change material, polymers or metals are more suitable because they can form thin capsules and/or conduct heat faster than concrete. The properties of the particles of the encapsulated phase change material are preferably as given above for the solid particulate material as a whole. When manufacturing particles of encapsulated phase change material, the size, size distribution, shape and surface properties of the particles can be fully controlled. Preferably, the particles of encapsulated phase change material are spherical.

The heat storage system 100 also includes an input system 126 for introducing heat into the heat storage container 102 (i.e., for charging the heat storage container 102). The input system 126 includes an input inlet 128 for introducing a gaseous heat transfer fluid into the thermal storage vessel 102, a liquid recovery system 116 for collecting liquid heat transfer liquid from the thermal storage vessel 102, and an input heat source 130 for evaporating the liquid heat transfer fluid. The liquid recovery system 116, the input heat source 130, and the input inlet 128 are fluidly connected to one another to recirculate the heat transfer fluid through the input system 126.

The input inlet 128 may have any suitable form for introducing gaseous heat transfer fluid into the thermal storage vessel 102 such that the gaseous heat transfer fluid contacts the solid particulate material 108; for example, although fig. 1 shows the input inlet 128 positioned toward an upper portion of the volume 106 of solid particulate material 108 and extending across a portion of the width of the volume 106, these locations should not be construed as limiting. Furthermore, separate inlets may be provided at locations where different heat transfer fluids are to be used.

Below the lower portion 114 of the volume of solid particulate material 106 is a liquid recovery system 116 for collecting liquid discharged downwardly through the volume of solid particulate material. As shown in fig. 1, the lower portion 114 of the volume 106 of solid particulate material may be provided in a form that facilitates the downward drainage of liquid under the influence of gravity, such as an inclined surface located at the lower portion of the volume 106. Suitably, the liquid collected at the liquid recovery system 116 may be pumped away, for example using a mechanical centrifugal pump (not shown), and/or further discharge of the fluid from the surface of the solid particulate material 108 may be facilitated by means such as applying vibration or sound waves to the particulate material 108. However, it is understood that other arrangements may be used that allow liquid to collect at the lower portion of the volume 106. Further, separate liquid recovery systems may be provided at locations where different heat transfer fluids are to be used.

Input heat source 130 may include any device or method of adding heat to a heat transfer fluid, such as direct electrical heating, a heat exchanger with a hot fluid on the opposite side of the heat transfer fluid, concentrated sunlight, a solar thermal source (e.g., a solar concentrator for heating a fluid), or any other means of heating a heat transfer fluid in an input system. Input heat source 130 may include or may also include a heat exchanger for transferring heat to a heat transfer fluid to cause evaporation thereof. Where different heat transfer fluids are to be used, it may be preferable to provide separate heat exchangers for each fluid, or separate flow paths through a single heat exchanger for each fluid.

The thermal storage system 100 also includes an output system 132 for extracting heat from the thermal storage vessel 102 (i.e., discharging the thermal storage vessel 102). The output system 132 includes an output port 110 for introducing liquid heat transfer fluid into the thermal storage container 102, a gas outlet 118 for allowing vaporized heat transfer fluid to exit the thermal storage container 102, and an output heat sink 134 for condensing the vaporized heat transfer fluid. The gas outlet 118, the output radiator 134, and the input/output port 110 are fluidly connected to one another to recirculate the heat transfer fluid through the output system 132.

The outlet port 110 may have any suitable form for introducing a liquid heat transfer fluid into the volume 106 of solid particulate material. Suitably, the outlet 110 may be in the form of one or more distributors, such as one or more spray nozzles or one or more rotary distributors, in the upper portion of the thermal storage container 102. Fig. 1 depicts a single rotating distributor arm 112, but those skilled in the art will appreciate that alternative distributors may be provided, and that an appropriate number and type of such distributors may be provided to ensure uniform distribution of the heat transfer fluid across the top of the volume of solid particulate material in heat exchange relation with the heat transfer fluid. Furthermore, separate inlets may be provided at locations where different heat transfer fluids are to be used. Preferably, the input port 110 is positioned above the volume of solid particulate material so that the liquid heat transfer fluid can be dispensed through the volume of solid particulate material by gravity alone.

Gas outlet 118 may have any suitable form that allows gaseous heat transfer fluid to escape from heat storage container 102 and travel to output heat sink 134; for example, although fig. 1 depicts the gas outlet 118 positioned toward an upper portion of the volume 106 of solid particulate material 108 and extending across a portion of the width of the volume 106, these locations should not be construed as limiting. Furthermore, separate gas outlets may be provided at locations where different heat transfer fluids are to be used.

The output radiator 134 may suitably comprise any device requiring a heat source, such as a turbine. Output radiator 134 may also include a heat exchanger for transferring heat away from the gaseous heat transfer fluid to cause it to condense. Where different heat transfer fluids are to be used, it may be preferable to provide separate heat exchangers for each fluid, or separate flow paths through a single heat exchanger for each fluid.

Although the figures depict a single input system 126 and a single output system 132, it should be understood that more than one of each system may be provided, for example, to allow separate cycles of different heat transfer fluids to be used during different phases of the heat transfer process, during charging and/or discharging of the heat storage system, as desired.

The heat storage vessel suitably further comprises a temperature monitor arranged to monitor the temperature of the volume of solid particulate material.

The thermal storage vessel 102 also includes a residual pressure reduction system 120 fluidly connected to the thermal storage vessel 102 to maintain a low residual air (or gas) pressure within the volume of solid granular material 106, where residual air (or gas) pressure refers to the pressure created by air or gas that is not and does not include a vaporized heat transfer fluid or other gas that undergoes a phase change within the operating temperature range of the thermal storage vessel. The residual pressure reduction system may suitably comprise an outlet to a vacuum pump. Preferably, the depressurization system further includes a condenser 122 maintained at a temperature sufficiently below the boiling point of the heat transfer fluid such that contact between the condenser and the gaseous heat transfer fluid causes the heat transfer fluid to condense, thereby reducing the expulsion of the heat transfer fluid from the thermal storage vessel through the residual pressure reduction system. Suitably, the condenser 122 may take the form of a conduit included in the fluid connection between the vacuum pump or other pressure reducing device and the volume of solid particulate material 106 and sloping downwardly from the vacuum pump or other pressure reducing device towards the volume of solid particulate material 106 to assist in returning the condensed heat transfer liquid to the heat storage container 102. A suitable form of condenser 122 is the coil shown in figure 1. Suitably, the condenser 122 is maintained at a suitable temperature to condense the vaporized heat transfer fluid simply by exposure to the ambient environment outside the heat storage container 102; it will be appreciated, however, that other forms of temperature control are possible, including for example a flowing water jacket around the flow path connecting the vacuum pump or other pressure reducing device and the volume 106 of solid particulate material. Alternatively, the condenser 122 may take the form of a cold finger or cold trap placed in fluid connection between a vacuum pump or other pressure reduction device and the volume 106 of solid particulate material. A pressure monitor may be provided that monitors the gas pressure contribution of the non-condensable matter, e.g. interposed between the pump and the condenser.

The heat storage system may also suitably comprise a system for removing all heat transfer fluid from the heat storage vessel and/or the input system and/or the output system. This is useful in cases where the heat storage system is taken out of service for cleaning or maintenance or at the end of its useful life. Suitably, the removal of the heat transfer fluid may include heating the heat transfer fluid present in the heat storage system to cause it to evaporate so that they pass through the gas outlet 118, condense through the output heat sink 134 and then drain through a suitable valve provided in the output system 132, for example, into a reservoir. Alternatively, the removal of the heat transfer fluid may be accomplished by heating the heat transfer fluid present in the thermal storage system to vaporize it and pumping the vaporized heat transfer fluid out of the thermal storage vessel using a pressure reduction system. In such a case, the condenser 122 may need to be maintained at a suitable temperature to prevent condensation and return the heat transfer fluid to the heat storage container, or be provided with a suitable outlet to allow the condensed heat transfer fluid to be discharged, for example, into a reservoir without being returned to the heat storage container. Further alternatives for removing the heat transfer fluid from the thermal storage reservoir may also be provided as will be apparent to those skilled in the art.

In use, the liquid heat transfer fluid in the input system 126 is heated, either directly or indirectly, by the input heat source 130 to vaporize it, and it is introduced in a gaseous state through the input inlet 128 to contact the solid particulate material 108 in the volume 106 within the thermal storage container 102. At this stage, the particles of solid material 108 are at a temperature below the boiling point of the heat transfer fluid. Thus, when the vaporized heat transfer fluid comes into contact with the surface of the solid particulate material 108, the heat transfer fluid transfers heat to the solid particulate material 108 and condenses on the surface of the particles. Due to the smooth concave shape of the granular material, its large contact angle with the heat transfer fluid, its low surface porosity, and the void space provided between the particles of the granular material, the condensed heat transfer fluid flows freely downward in the heat storage container to be collected in the lower portion of the container 114 and discharged through the liquid recovery system 116. The discharged condensed heat transfer fluid is recirculated through the input heat source 130 to be reheated and evaporated and reintroduced into the thermal storage vessel through the input inlet 128 to further transfer heat to the solid particulate material 108.

When the solid particulate material 108 reaches a temperature very close to the boiling point of the heat transfer fluid, the process will no longer be able to transfer energy efficiently from the evaporated heat transfer medium to the heat storage container, since a sufficient degree of condensation of the heat transfer fluid cannot reliably occur upon contact with the solid particulate material 108 (this is an equilibrium process). If it is desired to further transfer heat into the heat storage container, a second heat transfer liquid with a higher boiling point may be used instead of the first heat transfer liquid. The use of two or more heat transfer liquids with progressively higher boiling points allows heat to be transferred into the solid particulate material until the temperature of the solid particulate material reaches or approaches the boiling temperature of the heat transfer fluid with the highest boiling point.

In an alternative embodiment of the invention, the thermal storage system is based on the description above with respect to fig. 1, but the input system 126 comprises an input inlet 128 for introducing a superheated, rather than gaseous, heat transfer fluid into the thermal storage vessel 102.

The input inlet 128 may have any suitable form that introduces a superheated heat transfer fluid into the thermal storage container 102 to contact the gaseous heat transfer fluid with the solid particulate material 108. For example, the input inlet 128 may include a pressure relief valve at any selected location between the input heat source 130 and the heat storage container 102, but is preferably disposed proximate the heat storage container 102. The pressure relief valve allows the pressure upstream of the valve to be maintained at a level such that the superheated heat transfer fluid does not evaporate to any significant extent, but is predominantly in the liquid state. However, downstream of the pressure relief valve, the pressure is maintained at a level such that the heat transfer fluid may at least partially vaporize and enter the thermal storage container 102 in a gaseous state. Alternatively, there is no need to provide a pressure relief valve: with the pressure in the thermal storage vessel 102 reduced by the residual pressure reduction system 120, the pressure in the thermal storage vessel 102 can be maintained sufficiently lower than the pressure in the input inlet 128 so that the superheated heat transfer fluid remains in a liquid state in the input inlet 128, but evaporates upon entering the thermal storage vessel 102. Preferably, in this case, the input inlet 128 enters the thermal storage vessel 102 at the upper end of the vessel, and the input heat source 130 is positioned lower than the input inlet 128 enters the thermal storage vessel 102, so that the superheated liquid will not evaporate in the input heat source, but will only evaporate at the highest point of the input system, as this is the lowest pressure point.

In use, the liquid heat transfer fluid introduced into the system 126 is heated, directly or indirectly, by the input heat source 130 so that it is superheated above its normal boiling point at atmospheric pressure but does not evaporate, and is introduced through the input inlet 128 in a volatile liquid state and evaporates upon or after entering the volume 106. The gaseous heat transfer fluid then contacts the solid particulate material 108 in the volume 106 within the thermal storage container 102. At this stage, the particles of solid material 108 are at a temperature below the boiling point of the heat transfer fluid. Thus, when the vaporized heat transfer fluid comes into contact with the surface of the solid particulate material 108, the heat transfer fluid transfers heat to the solid particulate material 108 and condenses on the surface of the particles. Some heat transfer may also occur through contact between the heated liquid heat transfer fluid and the solid particulate material 108 in the event that the heat transfer fluid does not completely vaporize upon or after entering the volume 106. Although the temperature of the solid particulate material 108 is below the boiling point of the heat transfer fluid at the pressure maintained in the volume 106, but the temperature of the superheated heat transfer fluid is above the boiling point of the heat transfer fluid at the pressure maintained in the volume 106, the liquid heat transfer fluid introduced into the volume 106 will evaporate and any liquid heat transfer fluid that does not evaporate will flow to the bottom of the chamber and heat the condensed heat transfer fluid collected at the bottom of the chamber, causing it to partially evaporate with sufficient mixing of the heat transfer fluid at a temperature above the boiling point. The vaporized heat transfer fluid will then contact the solid particulate material in a gaseous form to transfer heat to the solid particulate material by condensing onto the surface of the solid particulate material. Due to the smooth concave shape of the granular material, its large contact angle with the heat transfer fluid, its low surface porosity, and the void space provided between the particles of the granular material, the condensed heat transfer fluid flows freely downward in the heat storage container to be collected in the lower portion of the container 114 and discharged through the liquid recovery system 116. The discharged condensed heat transfer fluid is recirculated through the input heat source 130 to be reheated and evaporated and reintroduced into the heat storage vessel through the input inlet 128 to further transfer heat to the solid particulate material 108.

When the solid particulate material 108 reaches a temperature very close to the boiling point of the heat transfer fluid, the process will no longer be able to transfer energy efficiently from the evaporated heat transfer medium to the heat storage container, since a sufficient degree of condensation of the heat transfer fluid cannot reliably occur upon contact with the solid particulate material 108 (this is an equilibrium process). If it is desired to further transfer heat into the heat storage container, a second heat transfer liquid with a higher boiling point may be used instead of the first heat transfer liquid. The use of two or more heat transfer liquids with progressively higher boiling points allows heat to be transferred into the solid particulate material until the temperature of the solid particulate material reaches or approaches the boiling temperature of the heat transfer fluid with the highest boiling point.

In any of the above embodiments, the selection of the amount and type of heat transfer fluid depends on the temperature of the heat source and the intended use. The choice will affect thermodynamic efficiency, as the boiling point of each heat transfer liquid will define the possible input and output temperatures. By having a small amount of fluid (immiscible or constant boiling), a relatively large boiling point difference will be achieved, and by having more constant boiling fluid, the system will have better thermodynamic performance, but at increased cost and complexity. Typical differences in boiling points of the different liquids will be in the range 10 ℃ to 80 ℃. Smaller boiling point differences by using more constant boiling fluid will increase thermodynamic performance to the maximum level, but may also require more advanced systems to control the mixture and collect and store the fluid.

The present inventors have determined that using a series of heat transfer fluids with progressively higher boiling points is more efficient than using a single high boiling point heat transfer fluid because the typical input heat source 130 will transfer heat to the heat transfer liquid more efficiently when the heat transfer liquid is cooler because of the larger temperature gradient between the source and the heat transfer liquid. The use of a high boiling point heat transfer fluid will first result in the temperature of the returned heat transfer fluid exiting through the liquid recovery system 116 being only slightly below its boiling point. Thus, the temperature gradient between the input heat source 130 and the heat transfer liquid to be evaporated is less than if a lower boiling point fluid were used.

However, the inventors have realised that in the present invention it is advantageous to select a smaller amount of heat transfer fluid for use, for example one fluid, two fluids with different boiling points, three fluids or four fluids, because the pressure relief system used in the present invention may allow for pressure regulation during charging and/or discharging of the thermal storage vessel, which in turn may regulate the boiling temperature of a given heat transfer fluid to a desired value, thereby achieving the same effect as using a series of heat transfer fluids with different boiling points. Thus, a given heat transfer fluid may be used in a pressure range from the lowest pressure selected during charging or discharging of the thermal storage vessel to ambient pressure, and thus may be used in the effective boiling point range expressed by the pressure or phase change of the material in the volume of solid particulate material.

It will be appreciated that the heat transfer fluid used in the present invention must be one which is capable of undergoing a phase change from gaseous to liquid (during charging) or from liquid to gaseous (during discharging) under the temperature and pressure conditions prevailing in the volume of solid particulate material or in the volume of phase change material when in use. The heat transfer fluid may be one that is solid at standard ambient temperatures and pressures, provided it is in the correct state to undergo the required phase change under the conditions of use. The heat transfer fluid should be one that is stable to repeated heating and cooling over the operating temperature range of the thermal storage system and is non-flammable at any operating temperature of the thermal storage system. Preferably, the heat transfer fluid used in the present invention has a low melting point and a high standard boiling point. The first heat transfer fluid used to charge the heat storage container is preferably liquid at or slightly above ambient temperature. Preferably, the heat transfer fluid used in the present invention reaches a vapour pressure of 1 bar at as high a temperature as possible. Suitable heat transfer fluids are oils (biological or mineral), silicones or other substances whose molecular weight can be adjusted to obtain feasible properties or sulphur in the case of higher temperature use.

In many cases, the upper working temperature limit of the heat transfer fluid is determined by the decomposition temperature of the fluid or the decomposition temperature of the particular fluid molecular bonds of the organic components or silicones that decompose in the range of 350-400 ℃ particularly in the presence of oxygen. The inventors have contemplated that such heat transfer fluids may be used in the present invention at temperatures exceeding this range due to the low residual air pressure used and thus the very low oxygen content in the atmosphere in the thermal storage container.

From an economic and environmental perspective, the heat transfer fluid should preferably be inexpensive, non-toxic, and non-corrosive.

It will be appreciated that the heat transfer between the heat transfer fluid and the volume of solid particulate material or the volume of phase change material will be due in part to the heat of phase change that occurs when the heat transfer fluid condenses (upon charging) or evaporates (during discharging) on contact with the solid particulate material or the phase change material and in part to the change in temperature of the heat transfer fluid that brings the heat transfer fluid to a temperature at which its phase change occurs. Preferably, the percentage of heat transfer into and out of the thermal storage container by the phase change of the heat transfer fluid is at least 50%, more preferably 60%, more preferably 70%, more preferably 80%, more preferably 90%, most preferably greater than 95%. Those skilled in the art will appreciate that the proportion of heat transferred due to the phase change can be calculated or modeled in a known manner. The higher the proportion of heat transfer caused by the phase change, the smaller the temperature gradient between the heat transfer fluid and the solid particulate material or phase change material on which the process relies; thus, heat transfer can be performed under almost isothermal conditions, so that a large degree of heat transfer can be performed without providing a large temperature gradient. The magnitude of the required temperature gradient depends on factors such as the characteristics of the heat transfer fluid, its surface tension, the pressure in the volume of the solid particulate material, the volume of the heat transfer material, and the purpose of storage (e.g. seasonal heat storage uses a large amount of solid particulate material and is expected to charge and discharge slowly, e.g. over weeks or months, so that a smaller temperature gradient is feasible). Suitably, there may be a maximum temperature difference of 50 ℃, for example 45 ℃ or 40 ℃, for example 35 ℃ or 30 ℃, for example 20 ℃ or 15 ℃ between the heat transfer fluid and the volume of solid particulate material or volume of phase change material at the time of contact. In order for the phase change of the heat transfer fluid to occur upon contact with the solid particulate material or the phase change material, there may be a minimum temperature difference of 2 ℃, for example 5 ℃ or 10 ℃, for example 15 ℃ or 20 ℃, for example 25 ℃ or 30 ℃ between the heat transfer fluid and the volume of solid particulate material or volume of phase change material upon contact.

The present inventors have also found that there are significant advantages to using condensation of a gas phase stream to transfer heat to a solid particulate material, as compared to using a system in which pipes are provided in a solid material such as a concrete block. First, no piping is required in the thermal storage vessel, thereby reducing the cost and complexity of manufacturing the vessel. Second, the size of the solid particulate material 108 used in the thermal storage vessel 102 may be adjusted to adjust the input/output power of the system by controlling the surface to volume ratio of the solid particulate material 108. Third, the thermal storage system 100 of the present invention inherently equalizes the temperature distribution within the thermal storage container due to the volumetric change of the vaporized heat transfer fluid as it condenses upon contact with the solid particulate material. Any cooler region of the volume of solid particulate material will result in a higher rate of condensation of the vaporized heat transfer fluid, resulting in an increased mass flow to the cooler region and an increased heating rate of the cooler region until the temperature is the same as the remainder of the volume. When it is desired to charge the thermal storage container 102 with a series of heat transfer fluids having an elevated boiling point, it is particularly important to be able to achieve self-equalization of the temperature as if the heat transfer efficiency of the system would be reduced without condensing or re-evaporating the higher rate of supply of vaporized heat transfer liquid by introducing the higher boiling point heat transfer fluid. Fourth, the condensation of the heat transfer fluid upon contact with the solid particulate material 108 allows a significant amount of heat to be transferred from the heat transfer fluid to the solid particulate material 108 almost independently of the temperature difference between the fluid and the particulate material 108. As long as the solid particulate material 108 is sufficiently below the boiling point of the heat transfer fluid to cause it to condense, the amount of heat transferred will be equal to (in the case where the temperature difference between the fluid and the particulate material is the minimum required for the heat transfer fluid to condense) or greater than (in the case where the temperature difference between the fluid and the particulate material is greater than the minimum required for the heat transfer fluid to condense) the enthalpy of the gas-liquid phase change of the heat transfer fluid.

The present inventors have also recognized that reducing the residual air pressure within the thermal storage container 102 significantly increases the internal heat transfer rate. Without wishing to be bound by theory, the inventors believe that this is due to the increased rate of evaporation (during heat release) and/or condensation (during heat charge) of the heat transfer fluid and the increased rate of diffusion of the evaporated heat transfer liquid between the colder and hotter regions within the thermal storage container.

It has been described in FR2981736 that draining the heat storage chamber after charging thereof is advantageous in reducing heat losses of the heat storage chamber during storage. However, the inventors' findings relate in particular to reducing the residual pressure in the heat storage container during charging and/or discharging of the heat storage container, rather than during storage. Although the choice in the present invention is to reduce the residual pressure in the thermal storage container during charging and to maintain a lower pressure during storage to minimize the work required to reduce the residual pressure during storage to discharge heat, the purpose of the residual pressure reduction of the thermal storage container is to increase the internal heat transfer rate between the heat transfer fluid and the solid particulate material in the thermal storage container, and to adjust the boiling point of the heat transfer fluid to maximize the heat transfer efficiency. The use of a residual pressure reduction during charging and/or discharging of the thermal storage system is not taught in FR2981736 and does not in fact lead to any improvement of the system, which is designed to use air as heat transfer fluid and therefore does not exploit the phase change of the heat transfer fluid as required by the present invention. Furthermore, only the residual air pressure that is to be reduced during the charging and discharging of the heat storage container. The total pressure of the system will include contributions from both the pressure of the residual air (or other uncondensed gas) and the vapor pressure of the heat transfer fluid. Removing only non-condensable gases from the thermal storage vessel may improve heat transfer between the heat transfer fluid and the solid particulate material.

It has been found that reducing the residual air pressure to 200 mbar or less allows the advantages of the invention in heat transfer to be achieved, with lower residual air pressures being preferred. For example, a maximum residual air pressure of 175 mbar or 150 mbar, such as 125 mbar, or 100 mbar, such as 75 mbar, or 50 mbar, such as 25 mbar, or 10 mbar, such as 5 mbar, may be used. Depending on the type of residual pressure reduction system used, a minimum residual air pressure of 25 mbar, 20 mbar, 15 mbar, 10 mbar, 5 mbar, 2 mbar, 1 mbar, or less than 1 mbar may be achieved.

Thus, the residual air pressure of the thermal storage vessel is reduced by the residual pressure reduction system 120, suitably by operation of a vacuum pump attached to an outlet in the thermal storage vessel, before and/or during introduction of the vaporized or superheated heat transfer fluid through the input inlet 128. The residual pressure reduction system preferably operates during the entire cycle of introducing heat into the heat storage container to prevent any small leaks in the heat storage container that would cause air or gas to enter the system and thus increase the residual air pressure and reduce the internal heat transfer rate over time. However, as an alternative, it is conceivable to reduce the residual pressure by operation of the residual pressure reduction system before the evaporated or superheated heat transfer fluid is led to the solid granular material, and to stop the operation of the residual pressure reduction system before the evaporated or superheated heat transfer fluid is introduced into the heat storage container.

The present inventors have noted that simply using a vacuum pump or similar device attached to the outlet of the thermal storage container may result in the removal of residual air and heat transfer fluid, particularly gaseous heat transfer fluid, within the thermal storage container when the depressurization system is to be operated during introduction of vaporized or superheated heat transfer fluid into the thermal storage container, which is not preferred when attempting to increase the internal heat transfer rate and/or to recirculate the heat transfer fluid for reuse. The present inventors therefore conceived to overcome this difficulty by using a condenser 122 between the solid particulate material and the vacuum pump, which condenser 122 is maintained at a temperature lower than the boiling point of the heat transfer liquid, but higher than the temperature at which components of air, especially oxygen, can condense, to condense any evaporated heat transfer fluid in contact with the condenser 122 and return it to the heat storage container, thus preventing its removal by the vacuum pump, and is suitably maintained at ambient temperature. Suitably, the condenser 122 may take the form of an elongate tube maintained at a temperature below the boiling temperature of the heat transfer liquid and placed between the reservoir and a vacuum pump, wherein the vacuum pump is placed at a higher level than the reservoir, so that any evaporated heat transfer fluid condensing on the cooling tube flows along the tube to be returned to the heat storage container. Suitably, the condenser 122 is maintained at ambient temperature by being placed outside the heat storage container 102. The residual air will not condense at the temperature of the device 122 and will therefore be expelled through the outlet 120. Another option may be to incorporate a cold trap or cold finger as known in the art, wherein active cooling is applied to a portion of the tubing to prevent the discharge of the heat transfer fluid.

Once heat transfer to the thermal storage vessel is complete, the outlet and inlet(s) may be closed in any suitable manner to seal the thermal storage vessel for the time needed to store heat. During this time, the residual air pressure may be maintained at its reduced pressure, or may be returned to ambient pressure. The former is preferred to reduce the energy consumption when the residual pressure is reduced.

When it is desired to release heat from the thermal storage container 102, a liquid heat transfer fluid is supplied through the output port 110 to the top of the thermal storage container 102 so that it contacts the hot solid particulate material 108. Once the liquid heat transfer fluid contacts the hot particles 108 of the vessel 102, the liquid heat transfer fluid will evaporate, absorb energy from the hot solid particulate material 108 and cause a significant increase in volume. This increase in volume will cause the vaporized heat transfer fluid to escape from the thermal storage vessel through gas outlet 118 where it can travel to output heat sink 134. For example, the vaporized heat transfer fluid may be moved to a heat exchanger system to transfer its vaporized heat to another process, such as water or steam in a steam turbine or steam generator or a pressurized fluid in an organic rankine cycle system. Once heat has been extracted from the vaporized heat transfer liquid and it has returned to a liquid state, the liquid heat transfer fluid may be recirculated to the thermal storage vessel 102 again through the outlet port 110 to further deliver heat from the heated solid particulate material 108. Once the temperature of the solid particulate material 108 in the heat storage vessel reaches the boiling point of the heat transfer fluid, a lower boiling point heat transfer fluid must be employed for further heat extraction in the same manner and for the same reasons as a series of heat transfer fluids with increasing boiling points are used during charging of the heat storage vessel.

Again, as explained above for charging the thermal storage container, it was found that a reduction in residual air pressure during heat release from the thermal storage container 102 increases the heat transfer rate. Similar to the case of charging the thermal storage container, it is preferred to operate the residual pressure reduction system during the entire cycle of introducing the liquid heat transfer fluid into the thermal storage container to prevent any small leaks in the thermal storage container that would cause air or gas to enter the system and thus increase the residual air pressure and reduce the internal heat transfer rate over time. However, as an alternative, it is conceivable to reduce the residual pressure by operating the residual pressure reduction system before the liquid heat transfer fluid is introduced into the solid particulate material, and the residual pressure reduction system is stopped from operating before the liquid heat transfer fluid is introduced into the heat storage container. If the reduced residual air pressure is maintained in the thermal storage vessel during storage, the residual air pressure will be low enough to obtain the benefits of the invention in heat transfer and not run the residual pressure reduction system further before releasing heat; however, this will depend on the degree of gas tightness of the heat storage container and the duration of the storage period. It is possible that an additional residual pressure reduction step may be required before and/or during the heat release of the heat storage container.

FIG. 2 shows a flow diagram of one embodiment of the heat transfer system of the present invention. The heat source 1 provides a hot fluid stream 2 which enters a heat exchanger 3 where a portion of its thermal energy is transferred to return to the heat source in the form of a cold return stream 4. Thermal energy is delivered to the liquid heat transfer fluid stream 5, which upon receiving the thermal energy evaporates to form a gaseous heat transfer fluid 6 or is superheated to form a superheated liquid heat transfer fluid 6. In the former case, a gaseous heat transfer fluid is introduced into the heat storage container 7, and in the latter case, a superheated liquid heat transfer fluid is introduced into the heat storage container 7, where it evaporates to form a gaseous heat transfer fluid. The gaseous heat transfer fluid then contacts the solid particulate material, where it condenses, thereby transferring thermal energy to the vessel. After condensation, the now liquid heat transfer fluid collects at the bottom of the container 7, preferably by means of gravity, and moves through the heat exchanger 3 again. Any uncondensed heat transfer fluid will be collected in the condenser 9 and the condensate will be stored in the reservoir 10.

When the energy in the heat storage container 7 is to be used, a liquid heat transfer fluid 11 is distributed into the heat storage container, evaporated therein to form a gaseous heat transfer fluid 12, which is transferred to a heat exchanger 13, condensed therein to release thermal energy. The released energy may be used to vaporize the condensed working fluid 14 to form a vaporized working fluid 15 that may drive a turbine 16.

Examples

Example 1

A concentrated solar power plant delivering heat transfer oil at 350 c was used as the heat source. The heat transfer oil flows through a counter-current heat exchanger, heats and evaporates a heat transfer fluid having a normal boiling point of 300 ℃. The internal pressure in the vessel and the connected input/output system is kept below 0.01 bar by using a vacuum pump with a connecting tube kept at ambient temperature. The vapour pressure of a thermal oil at ambient temperature is typically about 10^-5Bar, resulting in very little drainage of heat transfer liquid from the system, while maintaining a low residual air pressure. When the temperature of the thermal storage container is low, the total pressure in the system is low, equal to the vapor pressure of the heat transfer liquid plus the residual air pressure. Under this pressure, heat transfer liquidWill evaporate at a reduced temperature in the input system, thereby enabling a relatively low return temperature to the solar farm to be maintained. As the storage temperature increases, the return temperature will also gradually increase. As the temperature increases towards the normal boiling point of the heat transfer liquid, this liquid can eventually be exchanged for a liquid with a higher normal boiling point, thereby enabling the temperature of the reservoir to be further increased.

The heat storage container is composed of stone contained in a gas-tight metal container having a size of 12m (length) x2.35m (width) x2.6m (height) and externally isolated with ceramic rock wool. The average diameter of the stone was 150mm and the size distribution (spread) was 50 mm. The stones are circular in shape, so that an interconnected air network is formed between them, having an average width, measured at the maximum distance from a point in the interspace to the nearest stone, of 10-30mm, so that the flow of the heat transfer fluid is relatively unimpeded. The bottom of the container is slightly inclined so that a smaller area defines the lowest point of the container where a mechanical extraction mechanism in the form of a pump is placed. At the top of the vessel, the spray nozzles were placed in an arrangement of 11x2 and at a distance of 1m, each nozzle being capable of delivering a liquid stream of 0.3 kg/s. For the heat transfer fluid, the average heat of vaporization is 300kJ/kg, which corresponds to a maximum extraction rate of 2 MW. The fill rate of rock material in the vessel was 75%, resulting in a total specific heat capacity of 44.5kWh/K (the specific heat of the rock material used was 0.84 kJ/(kg. multidot.K) and the density of the rock material was 2600kg/m³). For a full vessel (300 ℃), this corresponds to an available energy content of about 10MWh (when the exotherm goes to rock temperature of 50 ℃). An output system collects the hot vaporized heat transfer fluid through a conduit into a container. The vaporized heat transfer fluid passes through a heat exchanger where heat is transferred to the working gas in the ORC generator, thereby generating electricity. The condensed heat transfer fluid is then re-injected into the container. The fluid used for energy extraction may be the same fluid used to heat the reservoir. As the temperature in the reservoir decreases during the exotherm, the heat transfer rate will gradually decrease and the temperature that can be reached in the output system will also decrease. If the heat transfer rate is reduced below the effective threshold, a second heat transfer liquid with a lower normal boiling point may be injected, thereby increasing the rate of heat transferHigh heat transfer rates, but without increasing the maximum output temperature (which is limited by the storage temperature regardless of the liquid used).

Example 2

A concentrated solar power plant delivering heat transfer oil at 350 c was used as the heat source. The heat transfer oil flows through a counter-current heat exchanger, heats and evaporates linseed oil with a normal boiling point of 287 ℃. The internal pressure in the vessel and the connected input/output system is reduced to about 0.01 bar before the vessel is charged by using a vacuum pump with a connecting tube maintained at ambient temperature. Linseed oil typically has a vapor pressure of about 10 at ambient temperature^-5Bar, resulting in very little drainage of heat transfer liquid from the system, while maintaining a low residual air pressure. When the temperature of the thermal storage container is low, the total pressure in the system is low, equal to the vapor pressure of the heat transfer liquid plus the residual air pressure. At this pressure, the heat transfer liquid will evaporate at a reduced temperature in the input system, thereby enabling a relatively low return temperature to the solar farm to be maintained. As the storage temperature increases, the return temperature will also gradually increase and the pressure in the container will increase to near ambient pressure. When the temperature rises to 270 ℃, the charging of the container is stopped and the remaining linseed oil in the container is removed (first by gravity and time, followed by a cooler volume in which the remaining oil can condense). As the remaining linseed oil is removed from the system, its vapour pressure contribution to the total pressure is also eliminated and the system pressure is again very low, e.g. 0.01 bar, due to the reduction of the residual air pressure. Thereafter, sulfur (melting point 115 ℃ C., boiling point 444 ℃ C.) was used as the second heat transfer fluid for heating the heat storage container to a temperature of 420 ℃. As with linseed oil, the contribution of the vapor pressure to the total pressure in the vessel increases with increasing temperature in the heat storage vessel, so the boiling point of sulfur will increase from 270 ℃ at about 0.04 bar to 444 ℃ at its normal boiling point at 1 bar. Once the heat transfer to the heat storage container is completed, the residual sulfur in the heat storage container is removed by gravity and time, and the liquid sulfur is discharged to the container by vibration of solid particlesThe lower part of the vessel then condenses in a cooler place than the reservoir.

When heat is required to be released from the container, the residual air pressure in the system is reduced, liquid sulfur is injected into the heat storage container through the input port, and heat is transferred thereto to vaporize it. The vaporized sulfur passes through the gas outlet and transfers its heat to the output heat sink, causing it to condense. Once the temperature of the heat storage vessel is reduced to about 150 to 200 ℃, sulfur is removed from the heat storage system and replaced with linseed oil to continue to release heat from the heat storage vessel to a lower temperature.

Example 3

A concentrated solar power plant delivering heat transfer oil at 350 c was used as the heat source. The thermal oil flows through a counter-current heat exchanger, heating a heat transfer fluid with a normal boiling point of 300 ℃, which is maintained at a pressure of 5 bar in said heat exchanger. In the heat exchanger, the heat transfer fluid was heated to 345 ℃ at 5 bar, after which it was injected into the vessel through a pressure relief valve. The internal pressure in the vessel and the connected input/output system is kept below 0.01 bar by using a vacuum pump with a connecting tube kept at ambient temperature. Upon injection of the superheated heat transfer fluid, it partially evaporates at the inlet of the vessel and subsequently condenses on the coldest part of the vessel. To prevent ambient air from accumulating in the container (e.g., entering through a small crack or leaky joint in the enclosure), the vacuum pump will continuously draw on the chamber. The vacuum pump will be connected to a long tube that is kept at ambient temperature. The vapour pressure of a heat transfer oil at ambient temperature is generally about 10^-5Bar, resulting in very little discharge of heat transfer liquid from the system (because it condenses in the long tube) while maintaining a low residual air pressure (because air does not condense in the long tube at ambient temperature). When the temperature of the thermal storage container is low, the total pressure in the system is low, equal to the vapor pressure of the heat transfer liquid plus the residual air pressure. At this pressure, the heat transfer liquid will evaporate at a reduced temperature in the input system, thereby enabling a relatively low return temperature to the solar farm to be maintained. As the storage temperature increases, the return temperature will also gradually increase. When the temperature is towards the standard of the heat transfer liquidAs the boiling point increases, the liquid may eventually be exchanged for a liquid having a higher normal boiling point, thereby enabling further increase in the temperature of the reservoir.

The heat storage container is composed of stone contained in a gas-tight metal container having a size of 12m (length) x2.35m (width) x2.6m (height) and externally isolated with ceramic rock wool. The average diameter of the stone was 150mm and the size distribution (spread) was 50 mm. The stones are circular in shape, so that an interconnected air network is formed between them, having an average width, measured at the maximum distance from a point in the interspace to the nearest stone, of 10-30mm, so that the flow of the heat transfer fluid is relatively unimpeded. The bottom of the container is slightly inclined so that a smaller area defines the lowest point of the container where a mechanical extraction mechanism in the form of a pump is placed. At the top of the vessel, the spray nozzles were placed in an 11x2 arrangement and a distance of 1m, each nozzle being capable of delivering a liquid stream of 0.3 kg/s. For the heat transfer fluid, the average heat of vaporization is 300kJ/kg, which corresponds to a maximum extraction rate of 2 MW. The fill rate of rock material in the vessel was 75%, resulting in a total specific heat capacity of 44.5kWh/K (the specific heat of the rock material used was 0.84 kJ/(kg. multidot.K) and the density of the rock material was 2600kg/m³). For a full vessel (300 ℃), this corresponds to an available energy content of about 10MWh (when the exotherm goes to rock temperature of 50 ℃). An output system collects the hot vaporized heat transfer fluid through a conduit into a container. The vaporized heat transfer fluid passes through a heat exchanger where heat is transferred to the working gas in the ORC generator, thereby generating electricity. The condensed heat transfer fluid is then re-injected into the container. The fluid used for energy extraction may be the same fluid used to heat the reservoir. As the temperature in the reservoir decreases during the exotherm, the heat transfer rate will gradually decrease and the temperature that can be reached in the output system will also decrease. If the heat transfer rate is reduced below the effective threshold, a second heat transfer liquid with a lower normal boiling point can be injected to increase the heat transfer rate, but not the maximum output temperature (which is limited by the storage temperature regardless of the liquid used).

While the invention has been described in connection with specific embodiments, it should not be construed as being limited to the examples set forth herein in any way. The scope of the invention is set forth in the appended claims. In the context of the claims, the term "comprising" does not exclude other possible elements or steps. Also, references to items such as "a" or "an" should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall not be construed as limiting the scope of the invention either. Furthermore, individual features mentioned in different claims may be advantageously combined, and the mentioning of these features in different claims does not imply that a combination of features is not feasible and advantageous.

All patent and non-patent references cited in this application are also incorporated herein by reference in their entirety.