AU2010201979A1

AU2010201979A1 - A solar energy collection and storage system

Info

Publication number: AU2010201979A1
Application number: AU2010201979A
Authority: AU
Inventors: Wayne Bliesner
Original assignee: SOLAR FUSION POWER Pty Ltd
Current assignee: SOLAR FUSION POWER Pty Ltd
Priority date: 2009-06-16
Filing date: 2010-05-17
Publication date: 2011-01-06
Also published as: NZ585425A; WO2010147674A2; WO2010147674A3

Description

P/00/0o11 Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: A solar energy collection and storage system The following statement is a full description of this invention, including the best method of performing it known to us: 2 A SOLAR ENERGY COLLECTION AND STORAGE SYSTEM Field of the invention This invention relates to a solar energy collection and storage system and more particularly to a solar energy collection and storage system which converts solar energy 5 into thermal energy. Background of the invention Solar energy provides a thermal power source which can supply a significant fraction of the world energy needs. A complete system which can absorb the sunlight through the day and produce continuous electricity over a 24 hour period maximises the benefit of 0 solar energy collection to the society. The power tower concept where a central receiver absorbs sunlight from a field of mirrors or heliostats focused on the tower has been demonstrated in facilities such as the Barstow facility, Solar 1 and Solar 2. In the case of Solar 1 and 2 in order to extend the operation of the facility beyond daylight hours, a thermal storage medium of potassium nitrate/sodium nitrate salts was used to provide 5 several hours of extended runtime beyond the solar sunlight hours. These systems used the heat of fusion within the salt, ie when a material transitions from a liquid to a solid as the storage medium. The energy density of a salt mixture requires significant quantities of salt to provide a few extra hours of operation due to the low energy density of the storage media. 20 Ideally it is desirable for the invention to convert solar energy into thermal energy which can continuously be converted to other forms of usable energy such as electricity to significantly increase the benefits and usefulness of solar as an energy source. Summary of the invention According to one aspect of the invention, there is provided a solar energy collection and 25 storage system including 3 a device for focusing solar energy onto a reaction chamber for the conversion of metal hydride to liquid metal and hydrogen, a metal/metal hydride chamber containing a metal/metal hydride mixture, a liquid transport conduit extending from the metal/metal hydride chamber to the 5 reaction chamber to transport liquid metal hydride to the reaction chamber, a return conduit for transporting liquid metal to the inner pressure chamber, and a hydrogen storage system. The invention utilises the heat of formation between the metal and hydrogen to form the metal hydride as the energy storage medium. The preferred metal hydride is calcium 0 hydride which provides up to 20 times the energy density of salt systems previously used. While calcium hydride is the preferred metal hydride, other metals which form metal hydrides may be used such as magnesium, strontium, barium, lithium, sodium, potassium, titanium or zirconium. Alternatively metal boro-hydrides may be used such as lithium, sodium or potassium boro-hydride. 5 The primary metal hydride is chosen based on the ability to disassociate the hydrogen from the liquid metal using a thermal heat source such as the sun. The process is endothermic absorbing the sun's energy during the day and storing it chemically. The process is reversed at night with the metal and hydrogen recombining to form the metal hydride in an exothermic reaction. The direction of the reaction is controlled by the 20 temperature and pressure in the reaction chamber. Calcium hydride is one of the more stable hydrides and will operate reversibly in the 1200OF to 2200OF (650 0 C to 12040C) temperature range with the direction of the hydriding reaction dependent on the pressure. At these temperatures, a stirling engine may be used integrated with a heat pipe to produce electricity from the thermal energy stored in the metal/metal hydride 25 pressure vessel In the preferred form of the invention, the reaction chamber is located in a power tower at the focus of a heliostat or field of mirrors. The reaction vessel is preferably located 4 above the metal/metal hydride pressure vessel and the liquid transport conduit extends from the metal hydride pressure vessel to the reaction chamber. Preferably, the liquid transport conduit is substantially vertical. The metal/metal hydride pressure vessel is preferably located within an outer pressure 5 vessel and an annular space exists between the metal hydride pressure vessel and the inner wall of the outer pressure vessel. In one embodiment of the invention, hydrogen separated during the daytime may be stored in a separate low temperature hydrogen storage vessel which allows a low cost bulk storage technique for the hydrogen. Sodium aluminium hydride provides a low cost 0 storage solution for the hydrogen although other metal or metal alloy hydrides may also be used. The hydrogen storage system includes a primary gas stream extending from the reaction chamber and a secondary conduit preferably from the primary conduit in fluid communication with the annular space of the outer pressure vessel. 5 The preferred hydrogen storage system comprises a hydrogen storage vessel containing a hydrogen storage material, the hydrogen storage material comprising a metal or metal alloy capable of reacting with or absorbing hydrogen. The hydride storage material may be hydrides of a number of metal or metal alloys such as magnesium nickel hydride, lithium aluminium hydride, magnesium iron hydride, 20 lanthanum nickel aluminium hydride, calcium nickel hydride, titanium iron hydride, or magnesium hydride. Alternatively, the hydrogen may also be stored as a compressed gas or liquid and used as the supply of hydrogen for the hydriding reaction in the metal/ metal hydride pressure vessel. 25 Heat exchangers with thermal storage may be used to store the heat in the primary hydrogen gas stream resulting from the disassociation of the metal hydride into metal and hydrogen gas in the reaction chamber. The hydrogen gas stream is cooled from temperatures of approximately 2000*F to 100'F. Thus in order to maximise the heat 5 extracted from the hydrogen gas stream, it is further preferred that the hydrogen storage system comprise a heat exchange and heat storage system between the primary hydrogen storage vessel and the reaction chamber to extract and store heat from the primary hydrogen gas stream. 5 The heat exchange and storage system has a heat supply arrangement to supply heat to the hydrogen storage vessel to release hydrogen from the hydrogen storage material when demand requires. The heat exchange and storage system comprises a first high temperature heat exchanger utilising at least one high temperature liquid heat exchange material. The 0 high temperature heat exchange material (first heat exchange material) undergoes heat exchange with the hydrogen gas stream. The first heat exchange material is stored in a first heat storage vessel and the material circulates between the first heat exchanger and the first heat storage vessel and is capable of absorbing heat by heat exchange with the hydrogen leaving the reaction chamber. It is preferable that the first heat 5 exchange materials includes a boron oxide liquid or lead, tin, or lead tin alloys in the counter current heat exchanger with the hydrogen gas stream. The heat exchange and storage system may further include a second low temperature heat exchanger utilising at least one second low temperature heat exchange material. The second low temperature heat exchange material undergoes heat exchange with the 20 hydrogen gas stream. The second heat exchange material is stored in a first heat storage vessel and the material circulates between the second heat exchanger and the second heat storage vessel and is capable of absorbing heat by heat exchange with the hydrogen leaving the first heat exchanger. It is preferable that the second heat exchange material is a nitrate salt mixture in counter current heat exchange with the 25 hydrogen gas stream, the nitrate salt being selected from the group of alkaline metal and earth metal nitrates such as potassium, sodium, calcium and lithium nitrates. In both cases the heat exchange with the hydrogen gas stream occurs without mixing between the hydrogen gas and the heat exchange material.

6 In another aspect of the invention there is provided a method of solar energy collection and storage comprising the steps of supplying a metal hydride from a metal/metal hydride vessel to a reaction vessel focusing solar energy onto the reaction vessel containing metal hydride to 5 disassociate the hydrogen from the metal hydride to produce hydrogen gas and liquid metal, returning the liquid metal to the metal/metal hydride vessel, and storing the hydrogen in a hydrogen storage system. The preferred metal hydride is calcium hydride but other metals which form metal 0 hydrides such as magnesium, strontium, barium, lithium, sodium, potassium, titanium or zirconium. Alternatively metal boro-hydrides may be used such as lithium, sodium or potassium boro-hydride. In a preferred form of this aspect of the invention, storing the hydrogen in a hydrogen storage system includes the step of passing the hydrogen to a hydrogen storage vessel 5 containing a hydrogen storage material which reversibly absorbs hydrogen. The hydride storage material may be selected from hydrides of a number of metal or metal alloy systems such as magnesium nickel hydride, lithium aluminium hydride, magnesium iron hydride, lanthanum nickel aluminium hydride, calcium nickel hydride, titanium iron hydride, or magnesium hydride. ?O Alternatively, the hydrogen could also be stored as a compressed gas or liquid and used as the supply of hydrogen for the hydriding reaction in the metal/ metal hydride pressure vessel. This aspect of the invention may further include the steps of passing the hydrogen gas through one or more heat exchange systems where the hydrogen gas stream ?5 exchanges heat with one or more secondary heat exchange fluids without mixing the 7 hydrogen and heat exchange fluids. The heat exchange fluids are then used to store heat which may later be used to desorb hydrogen from the hydrogen storage material when a supply of hydrogen is required to convert liquid metal to metal hydride in the metal/metal/hydride vessel. 5 The advantage of the invention is that it is a completely reversible closed cycle. The intermittent sunlight can be chemically stored and released at a controlled rate for electric power production. The system uses materials which are low cost and provides a competitive electrical production facility for very large scale application. In a further aspect of the invention there is provided a hydrogen storage system 0 comprising a hydrogen storage vessel; a heat exchange and heat storage system to recover heat from the primary hydrogen gas stream to the hydrogen storage vessel, the heat exchange and heat storage system having a heat supply arrangement to supply heat to the hydrogen storage vessel to 5 release hydrogen from the hydrogen storage material. the heat exchange and storage system comprising a first high temperature heat exchanger utilising at least one high temperature heat exchange material, the high temperature heat exchange material undergoing heat exchange with the hydrogen gas stream, the first heat exchange material being stored 20 in a first heat storage vessel and the material circulating between the first high temperature heat exchanger and the first heat storage vessel; and a second low temperature heat exchanger utilising at least one second low temperature heat exchange material, the second low temperature heat exchange material undergoing heat exchange with the hydrogen gas stream subsequent to heat exchange 25 with the first high temperature heat exchange material and prior to storage in the hydrogen storage vessel, the second heat exchange material being stored in a second heat storage vessel and the second heat exchange material circulating between the second heat exchanger and the second heat storage vessel.

8 Preferably the hydrogen storage vessel contains a hydrogen storage material, the hydrogen storage material comprising a metal or metal alloy capable of reversibly reacting with or absorbing hydrogen to release heat and releasing hydrogen when heated. 5 Other features and advantages of the invention will be come apparent from the following description of the preferred embodiment and accompanying drawings. Brief description of the figures Figure 1 shows an embodiment of the solar energy collection system, the heat exchange and heat storage system and the hydrogen storage system; 0 Figure 2 gives a detail view of a multi wall vessel (1) containing the metal/metal hydride vessel; Figure 3 is a flowchart of the operation of the hydrogen storage and heat exchange and heat storage systems during discharge of the thermal cell; Figure 4 is a flowchart of the operation of the hydrogen storage and heat exchange and 5 heat storage systems during charging of the thermal cell; Figure 5 provides an alternative arrangement of the metal/metal hydride storage cell and reaction chamber, with ancillary processes similar to those described and shown in Figure, Figure 3 and Figure 4. Detailed description of the embodiments 20 A preferred embodiment of the invention will now be described. In a preferred embodiment of the invention, the solar energy collection and storage system comprises a solar energy collection system, a heat recovery and storage system and a hydrogen storage system. The solar energy collection system comprises a metal/metal hydride storage cell (1) and reaction chamber (9). The hydrogen storage system using a low 25 temperature hydride storage material in a hydrogen storage vessel (2). The heat 9 recovery and storage system comprises one or more heat exchangers for recovering heat from a gas stream and storing the energy/heat storage material in a vessel. The heat recovery system provide thermal recovery and storage by extracting the energy from the hydrogen gas stream from the reaction chamber prior to the hydrogen 5 being stored in the low temperature hydrogen storage system (2). Multiple vessels may be used to store the thermal energy from the hydrogen gas as it cools from 2000*F to room temperature. The heat recovery system comprises a series of temperature storage tanks (17 and 22) with associated counter-current heat exchangers (18A and 18B) to cool the hydrogen from 2000'F to 400 0 F. The temperature storage tanks (17 0 and 22) include two internal chambers, which contain the heat exchange fluid in a closed loop. To cool the hydrogen stream, heat exchange material is introduced at a lower temperature from one chamber, extracts heat from the hydrogen stream, and then is stored in the second chamber with the extracted heat energy. In this embodiment of the invention, there is a high temperature storage tank (17) with associated counter 5 current heat exchanger (18A) that cools the hydrogen gas stream from 2000*F to 1000*F, similarly the heat exchange material is heated from 1000*F to 2000'F. This is followed by a low temperature storage tank (22) with associated counter-current heat exchanger (18B) to cool the hydrogen gas stream from 1000*F to 400'F via a similar mechanism. The thermal energy stored in the high temperature and low temperature 20 storage vessels may be used to heat the hydrogen gas stream, during the 'discharge' mode of operation, prior to it entering the metal/metal hydride storage cell (1) to support hydriding of the metal. Figure 1 shows an embodiment of the solar energy collection system, the heat exchange and heat storage system and the hydrogen storage system of the invention. 25 The metal/metal hydride storage cell (1) comprises an outer tank (3A), a mid tank (3B), and metal/metal hydride inner tank (4) with an insulation layer (6) between the three tanks. The inner tank (4) is maintained at suitable temperatures dependent on the metal/metal hydride used, while the outer tank (3A) is slightly above room temperature. The insulation layer (6) extends completely around both the inner tank (4) and mid tank 30 (3b) including the top and bottom. Insulation (6) maintains a minimal heat loss between the three tanks. The inner tank (4) contains the metal and metal hydride in which the 10 metal floats above the metal hydride. The metal is selected from the group of calcium, magnesium, strontium, barium, lithium, sodium, potassium, titanium or zirconium with calcium the preferred metal. In the case of a calcium/calcium hydride system, the temperature in the metal/metal hydride vessel is maintained at a temperature of 1500*F 5 to 2000 0 F. Above 1800*F both materials are liquid. The reaction between metal, particularly calcium and hydrogen to produce metal hydride i.e. calcium hydride is exothermic. Thus the conversion of metal to metal hydride provides the heat source to drive a thermal engine such as a Stirling engine or steam turbine. Furthermore the direction of the reaction may be controlled by the 0 temperature and pressure conditions. For example, at a given temperature, the reaction can be reversed by simply varying the pressure, and at a given pressure the reaction can be reversed by varying the temperature. To illustrate this, calcium, calcium hydride, and hydrogen gas are in equilibrium with the hydrogen pressure as a function of the reaction temperature. At approximately 1800 0 F, this equilibrium is 1 atmosphere 5 pressure whereas at 2000 0 F, the equilibrium is at approximately 3 atmospheres pressure. Thus the system pressure may be used to drive the reaction in either the exothermic or endothermic reaction direction. A Stirling engine (20) is located near the base of the multi walled metal/metal hydride storage cell (1). A heat pipe (21) connects from the Stirling engine (20) into the inner 20 tank (4). Multiple heat engines or a single engine could be used to extract the heat from the inner tank (4). The heat engines could include any variation of brayton, stirling, or rankine cycles. A secondary steam cycle could also be used to supply peaking power utilising a steam turbine. In a separate embodiment of the invention, the reaction tower can be removed as 25 shown in Figure 5. The mode of operation of this system is the same as that described for the previous embodiment, except that this arrangement removes the need for vertical transportation of the metal/metal hydride. The advantages of this system include lower pump energy usage, reduced heat loss due to a lower exposed surface area, and potentially lower capital costs. This system will require the sunlight to be collected and 30 focussed using a different methodology than that for the scheme shown in Figure 1. The 11 operation of the invention will now be described by reference to calcium as the hydriding metal but it would be appreciated by those skilled in the art that other hydridable metals could be used with suitable adjustment of the temperature and pressure conditions. Solar energy collection system 5 The solar energy collection system includes a device for focusing solar energy onto a reaction chamber (9) for the conversion of metal hydride to liquid metal and hydrogen. This device is preferably a reaction tower at the focus of a heliostat with a reaction chamber in the tower. The power tower has at its base the metal/metal hydride cell including an inner tank (4) containing a metal/metal hydride mixture, a hydride transport 0 line (8) extending from the inner tank (4) to the reaction chamber (9) to transport liquid metal hydride to the reaction chamber and a return conduit for transporting liquid metal to the inner tank (4). The level of metal and metal hydride, within the inner tank (4), varies depending on the state of charge of the storage cell. A fully charged cell is all metal with hydrogen stored 5 separately in the hydrogen storage tank (2). A fully discharged cell is all metal hydride. The metal/metal hydride pressure vessel is preferably located within an outer pressure vessel and an annular space exists between the metal hydride pressure vessel and the inner wall of the outer pressure vessel. An inert gas (7) is used to pressurise the outer pressure vessel (3A). The inert gas (7) may be argon or nitrogen. 0 A liquid hydride transport line (8) is located above the bottom of the inner tank (4) and extends to the reaction chamber (9) located above the metal/metal storage cell (1). Liquid metal hydride transports through hydride transport line (8) from the inner tank (4) to the reaction chamber (9). An insulation zone (10) thermally isolates the reaction chamber (9) from the metal hydride located in the lower section of the hydride transport 25 line (8). This may also be accomplished by the hydride transport line (8) having a reduced size relative to the reaction chamber (9). The reaction chamber (9) consists of a chamber for thermally decomposing the metal hydride using a thermal heat source such as sunlight. Two exit lines are located near 12 the top of the reaction chamber (9). A liquid metal outlet line (12) is located near the top of the reaction chamber (9). A hydrogen outlet (11 a) is located above the metal outlet line (12). A liquid metal pump (13) is located between the reaction chamber (9) and the inner tank (4). In figure 1, the pump (13) is shown resting on mid tank (3B). The liquid 5 metal exits the pump and flows into the inner tank (4). As mentioned above the reaction tower (14) is located above the metal/metal hydride storage cell (1) and attaches to the top of outer pressure vessel (3A) and surrounds the reaction chamber (9). A bolt flange (14A), at the top of tanks (3A), and (3B) join together and provide the fitting for the reaction tower (14). A quartz window (15) is 0 located near the top side of the reaction tower (14) so that sunlight can project onto the reaction chamber (9) from a series of mirrors located on the ground around the solar thermal reaction tower. The region between the reaction tower (14), the hydride transport line (8), and part of the reaction chamber (9) is filled with insulation (6). The area between the quartz window (15) and the reaction chamber (9) does not have 5 insulation. Argon or nitrogen gas (7) fills the reaction tower (14). Hydrogen gas can be used in tower (14) fed through a filter similar to (24) between the tower (14) and the lower shells of metal/metal hydride storage cell (1). The inert gas within the outer tank(3A) could be nitrogen or other inert gas which does not react with the liquid metal. The gas in the outer tank (3A) could also be hydrogen through filter (24). 20 In the embodiment shown in figure 5, the same numbers are used in the drawings to represent similar pieces of equipment. In figure 5, the solar energy from the heliostat is focused through a window (15) preferably made of quartz in the storage cell (41) so that sunlight can project into reaction chamber (9) located in the storage cell (41). As with the embodiment of figure 1, calcium hydride draw from the inner tank (4) is converted to 25 calcium and hydrogen. Figure 2 gives a detail view of a metal/metal hydride storage cell (1). The figure shows an outer vessel (3A) which operates near room temperature and maintains the system pressure and supports the weight of the inner tank metal and metal hydride through the bolt flange (14A). The mid vessel (3B) is shown suspended from the bolt flange on the 30 outer tank (3A). Attachment at the top allows for thermal expansion to occur between 13 the three vessels without significant induced stresses. The inner tank (4) holds the liquid metal and metal hydride. The region below and on the sides of the inner tank (4), on the inside of mid vessel (3b), uses a structural silicon dioxide glass ceramic to support the inner tank (4). The mid tank (3B) shell serves as a containment shell for the 5 liquids during operation. The temperature drop through the insulation is sufficient so that the mid vessel (3B) operates below the melting point of the metal. If a leak occurs the liquid metal freezes prior to reaching the mid tank (3B). The use of silicon dioxide insulation is beneficial due to the reaction which occurs with the liquid metal. Free silicon will form which solidifies stopping the reaction. D Figure 2 also shows electric heating filaments (22) used to initially heat the inner vessel (4) form room temperature to approximately 1600*F. The two electrical attachment fittings (23) provide a sealed and electrically insulated connection to power the filaments. Once the metal liquefies the hydriding reaction can be used to continue the heat-up cycle. The metal/metal hydride storage cell (1) in figure 2 shows a pressure lid 5 enclosing the top of the mid tank and outer tank (3A and 3B). When the reaction tower (14) is integrated, the lid may be used to hold the metal hydride supply conduit (8). A technique to allow automatic pressure compensation between the mid tank (3B) and the outer tank (3A) utilises a filter (24) which allows hydrogen gas to flow between chambers. !0 In the preferred embodiment, the inner tank (4) is made of molybdenum with Lanthanum oxide dispersed within the metal which raises the recrystallisation temperature above the metal operating temperature. Remaining below the recrystallisation temperature is very important to the strength and toughness of the molybdenum. The outer tank (3A) is made of a low temperature stainless such as 304 or 316. The mid tank (3B) is made of ?5 a high temperature stainless such as RA330 or Haynes 230. A rhenium layer is coated on the inside of the inner tank (4), both sides of the hydride transport line (8), and the reaction chamber (9) at the braze junction locations. This rhenium layer provides a barrier between the metal or metal hydride and the braze junctions used to fabricate the inner tank (4) and hydride transport line (8). If the molybdenum parts can be fabricated 0 without seams then the rhenium coating layer is not required. The inner tank (4) is fabricated from sheet stock and is brazed together at the side, top, and bottom 14 locations. The molybdenum is machined to provide a tapered overlapping joint for brazing. A chromium layer is electroplated at the joint locations. The parts are brazed with a cobalt-palladium alloy to provide sealed junctions. Once the brazed are complete the rhenium is applied locally over the braze junction with a thickness of approximately 5 0.004 inches. Bolt flanges may be used to assemble the molybdenum components. The flanges could be brazed to the plate or tubing to allow assembly. The molybdenum could also be fabricated as single plastic formed shells. A Kovar seal may be used to support the hydride transport line (8) at tank junction 0 (14A). Supports could be added along the hydride transport line (8) to keep it centred within the reaction tower (14). Heat Recovery Systems Heat exchangers with thermal storage may be used to cool the hydrogen gas stream from temperatures of approximately 2000*F to 100*F. Thus in order to maximise the 5 heat extracted from the hydrogen gas stream, prior to arriving at the hydrogen storage system, it is preferable that a heat exchange and heat storage system between the primary hydrogen storage vessel and the reaction chamber is provided to extract and store heat from the primary hydrogen gas stream between the reaction chamber and the hydrogen storage vessel. 20 The heat exchange and storage system has a heat supply arrangement to supply heat to the hydrogen storage vessel to release hydrogen from the hydrogen storage material when demand requires. The heat exchange and storage system ideally uses heat exchangers in conjunction with heat storage vessels which utilise the specific heat of liquid salts to store the ?5 thermal energy in the hydrogen gas stream. To reduce the temperature of the hydrogen gas stream from 2000*F (1093 0 C) to 1000*F (538*C), a counter-current heat exchanger (18A) is used. This heat exchanger (18A) utilises at least one high 15 temperature liquid heat exchange material which is stored in a high temperature storage vessel (17) with two internal chambers. The high temperature storage tank (17) comprises two interconnected tank chambers with the high temperature heat exchange material entering the heat exchanger (18A) 5 from one chamber or vessel and returning through the other. The high temperature heat exchange material transfers from one chamber or vessel to the other by gravity or by an induced pressure differential. One chamber contains the high temperature heat exchange material in a low temperature state, while the other contains the high temperature heat exchange material in a high temperature state. This is so that the 0 energy extracted from the hydrogen stream can be stored. The high temperature storage tank (17) operates between 2000*F (1093*C) and 1000'F (538*C) using boron oxide as a heat transfer fluid in counter-current heat exchange with the hydrogen gas stream. Ideally the temperature of the boron oxide in one of the internal chambers of the high temperature storage tank (17) is >950*F (>510 0 C). The high temperature heat 5 exchange material undergoes heat exchange with the hydrogen gas stream. The high temperature heat exchange material is then stored in a separate internal chamber within the high temperature storage tank (17). The high temperature heat exchange material is capable of absorbing heat by heat exchange with the hydrogen leaving the reaction chamber (9). The high temperature storage tank (17) is used to cool the hydrogen 20 during charging, and can also be used to heat the hydrogen during the discharge cycle of the metal/metal hydride storage cell. To further reduce the temperature of the hydrogen gas stream, a low temperature heat exchanger system utilising at least one low temperature heat exchange material can be used. The low temperature heat exchange material undergoes heat exchange with the 25 hydrogen gas stream. The low temperature heat exchange material is stored in the low temperature storage tank (22). The low temperature heat exchange material circulates between the low temperature heat exchanger (18B) and the low temperature storage tank (22), in a manner similar to that described for the high temperature heat exchange system.

16 The low temperature heat storage tank (22) also comprises two interconnected tank chambers with the low temperature heat storage material entering the low temperature heat exchanger (18B) from one chamber and returning through the other. The heat storage material transfers from one chamber or vessel to the other by gravity or by an 5 induced pressure differential. The low temperature heat exchange material is capable of absorbing heat by heat exchange with the hydrogen stream after it exits the high temperature heat exchanger (18A). The low temperature storage tank (22) uses a liquid eutectic mixture of sodium nitrate, potassium nitrate, and calcium nitrate and operates between 1000OF (538*C) and 400'F (204"C). The nitrate salt mixture may be lithium 0 nitrate instead of, or in combination with, the calcium nitrate as a eutectic with the potassium and sodium nitrate salts. The preferred nitrate salt mixture operates in counter current heat exchange with the hydrogen gas stream. The temperature heat storage tanks (22 and 17) have two functions. During charging they are used to cool the hydrogen gas stream. During the discharge operation, they 5 are used to heat the hydrogen stream. In particular the low temperature heat exchange system can be used to heat the hydrogen storage material in the hydrogen storage vessel (2) which releases the hydrogen making it available to recombine with the metal in the metal/metal hydride vessel after being further heated by the high temperature heat exchange system. In both cases the heat exchange with the hydrogen gas stream O occurs without mixing between the hydrogen gas and the heat exchange material. The system can operate without either the high temperature heat exchange system or the low temperature heat exchange system. In either case, a secondary cooling loop using an external heat exchange system with a heat exchange material such as air, cooling water, or a solid heat sink .can be used. This secondary cooling system can 25 also be employed to further reduce the temperature of the hydrogen stream prior to storage in the hydrogen storage tank (2) during normal operation. Hydrogen Storage System Hydrogen storage vessel (2) is used to hold the hydrogen at low temperatures; typically room temperature to 350*F (177*C). The hydrogen storage tank (2) is a double walled 17 vessel with insulation between an inner tank and an outer vessel. The inner hydrogen storage vessel contains a solid porous material which allows rapid hydrogen transport in and out of the porous material. A chemical bond is created within the material which creates a metal hydride. The hydride storage material may be selected from hydrides of 5 a number of metal or metal alloy systems such as magnesium nickel hydride, lithium aluminium hydride, magnesium iron hydride, lanthanum nickel aluminium hydride, calcium nickel hydride, titanium iron hydride, or magnesium hydride. Sodium aluminum hydride is chosen for this application as a medium temperature low cost material. Over 5% hydrogen by weight is absorbed when the hydride is created. Hydrogen is absorbed 0 at room temperatures and released when heated to approximately 350*F (177*C). The hydrogen pressure varies with temperature. At room temperature the pressure is near 1 atmosphere. As the temperature is raised, the reaction equilibrium shifts to the release of hydrogen from the hydrogen storage material and the pressure rises to several atmospheres. 5 Two heat exchangers may be used in association with the hydrogen storage vessel (2) and the low temperature storage tank (22) as the hydrogen movement results in either an exothermic or endothermic reaction within the reaction chamber. During the charge operation, hydrogen gas is absorbed. This reaction is exothermic, and so heat exchanger (19B) is used to cool the hydrogen storage vessel (2). During the discharge O operation, the first heat exchanger (19A) uses the stored heat from the low temperature storage tank (22) to heat the hydride material in the hydrogen storage vessel (2) which releases hydrogen gas. The hydrogen lines are attached as follows: 1) Hydrogen outlet (11A) and hydrogen inlet (11C) are connected with two valves 25 (V1) and (V2) in line and a hydrogen line (11D) teeing off the common line between the two valves. 2) The hydrogen line (11D) connects to the lower end of the counter-current high temperature heat exchanger (1 8A).

18 3) The top of the counter-current high temperature heat exchanger (18A) has a hydrogen line which connects to the bottom of the counter-current low temperature heat exchanger (18B) with line (11 B). 4) The top of the high temperature heat exchanger (18A) also connects, through a 5 valve (V3), to the hydrogen storage tank (2). The top of the low temperature heat exchanger (18B) connects, through a valve (V4), to the hydrogen storage tank (2). The common hydrogen storage vessel feed line is (11 F). 5) A third counter flow heat exchanger (18C) is attached to the low temperature storage tank (22). The low temperature heat exchanger (22) is directly coupled to 0 a heat exchanger (19A) within the hydrogen storage tank (2). A pump (P1) is used to move a fluid, such as Paratherm LR, between the two heat exchangers. 6) The heat exchanger (19B) is used to cool the hydrogen storage tank (2). This can be run through an external air cooled closed loop system with Paratherm LR used for the heat transport fluid. 5 Operation The system operates in a charging mode and a discharging mode and will be described with reference to a calcium/calcium hydride system but as described earlier other metal/metal hydride systems can be used without departing from the inventive concept. 20 Charqinq mode: During the daylight hours solar energy is available for thermal storage. Figure 4 is a flow diagram of the overall process of hydrogen storage during charging. The reaction of the calcium hydride breaking apart to form calcium and hydrogen is endothermic and absorbs the focused heat from the sunlight. A series of reflectors focuses the suns 25 energy through the quartz window (15) and onto the reaction chamber (9). The calcium and calcium hydride located in the inner tank (4) is maintained at 2000*F (1093 0 C). Approximately 6 atmosphere of argon (7) is used to pressurise the outer tank (3A). Maintaining the same pressure in vessels (3A), (3B) and (4) eliminates the stress on the wall of vessels (4) and (3B) due to the pressure. The outer tank (3A) is at low 19 temperature and provides the structure for the argon pressure. The reaction chamber (9) pressure is lowered to one atmosphere by reducing the temperature of the hydrogen storage tank (2) using the heat exchanger (19B). The lower pressure in the reaction chamber (9) causes the liquid calcium hydride to rise up the hydride transport line (8) 5 until it reaches the reaction chamber (9). Varying the hydrogen pressure in outlet line (11a), using a valve (V1), allows the calcium hydride level to be controlled within the reaction chamber (9). With the calcium hydride at 2000*F (1093*C) and 1 atmosphere an endothermic reaction occurs releasing hydrogen and calcium. The hydrogen exits through the hydrogen outlet (11A). The calcium exits through the calcium outlet (12). A 0 liquid metal pump (13) pumps the calcium from the reaction chamber (9) into the inner vessel (4). The hydrogen exiting the reaction chamber (9) is at 2000*F (1093*C) and as such, contains a substantial amount of thermal energy. The hydrogen flows through an open valve (V1) and into line (11D). The valve (V2) to the line (11C) is closed during this 5 mode of operation. The hydrogen flows through the counter-current high temperature heat exchanger (18A) and is cooled by the high temperature heat exchange material, in this case boron oxide, which is flowing between the two internal chambers within the high temperature storage tank (17). The hydrogen temperature drops from 2000*F (1093'C) to 1000'F (538'C) while the boron oxide is heated from 1000*F (538 0 C) to 40 2000*F (1093 0 C) through the high temperature heat exchanger (18A). The hydrogen is cooled further through a low temperature heat exchanger (18B) using a low temperature heat exchange material consisting of a liquid eutectic mixture of sodium nitrate/ potassium nitrate/ calcium nitrate which melts at approx 300'F (149*C). An air cooled heat exchanger cools the hydrogen to room temperature where it enters 25 the hydrogen storage tank (2) through the inlet/outlet (11F). If the low temperature storage tank (22) is not used then the hydrogen can be cooled with the air cooled heat exchanger prior to entering the hydrogen storage vessel (2). The Stirling engine (20) extracts heat directly from the inner tank (4) which contains the calcium and calcium hydride. A heat pipe (21) connects to the hot side of the Stirling 30 engine and passes through the outer tank (3) and into the inner tank (4). Sufficient heat 20 pipe area is extended within the inner tank (4), or on the external surface of the inner tank (4), to prevent the calcium hydride from solidifying around the heat pipe while the engine is extracting thermal energy With the sterling engine removing excess heat energy during the day, the temperature 5 of the calcium/calcium hydride in the inner tank (4) is maintained at 2000*F (1 093*C) by a combination of the heat energy accompanying the hydrogen from reaction chamber (9) into the inner tank (4) and the heat generated by the exothermic reaction of hydride formation. With the pressure at approximately 6 atmospheres the hydrogen reacts exothermically with the calcium and sinks to the calcium hydride region of inner tank (4). 0 During the day a small pump (P2) is required to move the hydrogen into the inner tank (4) directly from the outlet line (11 a) by throttling the hydrogen flow through valve (V2) into the pump (P2). This in combination with the heat generated by the exothermic reaction of calcium and hydrogen maintains the temperature within vessel 4 during the day.. 5 Discharge mode: During evening operation, the Stirling engine (20) continues to extract heat directly from the inner tank (4) which contains the calcium and calcium hydride. Figure 3 is a flowchart of the operation of the hydrogen storage, heat exchange and heat storage ?0 systems during discharge of the thermal cell. The rate of hydrogen flowing out of the hydrogen storage tank (2) is controlled by heating the hydrogen storage tank (2) using thermal energy stored in the low temperature storage tank (22). The heat exchanger (19A) utilises a fluid, such as Paratherm LR, to transfer thermal energy between the low temperature storage tank (22) and the hydrogen storage tank (2). During evening ?5 operation the hydrogen storage tank (2) can be heated to replace the heat removed by the sterling engine and provide a hydrogen pressure of several atmospheres. Hydrogen flows from the hydrogen storage tank (2) into the counter-current high temperature heat exchanger (18A). The high temperature heat exchange material, in 21 this case boron oxide, is pumped back into the empty upper chamber within the high temperature storage tank (17). The increased hydrogen pressure from line (11B) can be used to pressurise the lower chamber in high temperature storage tank (17) driving the boron oxide through the high temperature heat exchanger (18B) and into the upper 5 tank. The heat exchanger (18A) heats the hydrogen to approximately 2000*F (10930C) when it flows through line (11D) and into the hydrogen inlet line (11C). The valve (V1) to the reaction chamber (11A) is closed and the valve (V2) to the hydrogen inlet line (11C) is open. The temperature in the inner tank (4) is maintained at 2000*F (10930C) by controlling the hydrogen flow rate out of the hydrogen storage tank (2). During night 0 time operation the pressure in the reaction chamber (9) is raised to approximately the pressure of inner tank (4) so that the calcium hydride can return to the inner tank (4). While the system has been described with reference to operating at 2000*F (1093*C), the system can be operated over a wider temperature range such as between 1200'F (6490C) to 2500*F (1371*C). Additionally, the system can operate with the temperature 5 in the inner tank (4) below 1800*F (9820C) so that the calcium hydride is a solid during the discharge mode. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations 20 constitute various alternative aspects of the invention.

Claims

1. A solar energy collection and storage system including a device for focusing solar energy onto a reaction chamber for the conversion of metal hydride to liquid metal and hydrogen, 5 a metal/metal hydride vessel containing a metal/metal hydride mixture, a liquid transport conduit extending from the metal/metal hydride chamber to the reaction chamber to transport liquid metal hydride to the reaction chamber, a return conduit for transporting liquid metal to the metal/metal hydride vessel, and 0 a hydrogen storage system.

2. The system of claim 1 wherein the metal is selected from the group of calcium, magnesium, strontium, barium, lithium, sodium, potassium, titanium, and zirconium.

3. The system of claim 1 or 2 wherein the hydride is selected from the group of metal boro-hydrides of lithium, sodium or potassium. 15

4. The system of claim 1 wherein the reaction vessel is located above the metal/metal hydride pressure vessel and the liquid transport conduit extends from the metal hydride pressure vessel to the reaction chamber.

5. The system of claim 1 wherein the reaction vessel is the focus of a heliostat or field of solar mirrors. 20

6. The system of claim 1 wherein the metal/metal hydride pressure vessel is located within an outer pressure vessel and an annular space exists between the metal hydride pressure vessel and the inner wall of the outer pressure vessel. 23

7. The system of claim 1 wherein a primary gas stream extends from the reaction chamber to a hydrogen storage vessel and a secondary conduit in fluid communication with the annular space of the outer pressure vessel extends from the primary conduit.

8. The system of claim 1 wherein hydrogen storage system comprises a hydrogen 5 storage vessel containing a hydrogen storage material, the hydrogen storage material comprising a metal or metal alloy capable of reacting with or absorbing hydrogen.

9. The system of claim 8 wherein the hydride storage material is at least one metal hydride selected from the group of magnesium nickel hydride, lithium aluminium hydride, magnesium iron hydride, lanthanum nickel aluminium hydride, calcium nickel 0 hydride, titanium iron hydride, and magnesium hydride.

10. The system of claim 1 wherein the hydrogen storage system is hydrogen stored as a compressed gas or liquid

11. The system of claim 1 wherein a heat exchange and heat storage system is provided to recover heat from the primary hydrogen gas stream between the reaction 5 chamber and the hydrogen storage vessel.

12. The system of claim 11 wherein the heat exchange and storage system has a heat supply arrangement to supply heat to the hydrogen storage vessel to release hydrogen from the hydrogen storage material.

13. The system of claim 11 wherein the heat exchange and storage system 20 comprises a first high temperature heat exchanger utilising at least one high temperature liquid heat exchange material.

14. The system of claim 13 wherein the high temperature heat exchange material undergoes heat exchange with the hydrogen gas stream, the first heat exchange material being stored in a first heat storage vessel and the material circulating between 25 the first heat exchanger and the first heat storage vessel.

15. The system of claim 14 wherein the heat exchange material is boron oxide. 24

16. The system of claim 11 to 13 wherein the heat exchange and storage system further includes a second low temperature heat exchanger utilising at least one second low temperature heat exchange material.

17. The system of claim 16 wherein the second low temperature heat exchange 5 material undergoes heat exchange with the hydrogen gas stream, the second heat exchange material being stored in a first heat storage vessel and the material circulates between the second heat exchanger and the second heat storage vessel.

18. The system of claim 17 wherein the second heat exchange material is a nitrate salt mixture selected from the group of alkaline metal nitrates such as potassium and 0 sodium nitrate.

19. The system of claim 11- 19 wherein the heat from the low temperature heat exchanger material provides heat to heat the hydrogen storage material.

20. The systems of any one of claims 11 -18 wherein the heat exchange with the hydrogen gas stream occurs without mixing between the hydrogen gas and the heat 5 exchange material.

21. A method of solar energy collection and storage comprising the steps of supplying a metal hydride from a metal/metal hydride vessel to a reaction vessel , focusing solar energy onto the reaction vessel containing metal hydride to disassociate the hydrogen from the metal hydride to produce hydrogen gas and liquid ?0 metal, returning the liquid metal to the metal/metal hydride vessel, and storing the hydrogen in a hydrogen storage system.

22. The method of claim 21 wherein the metal is a metal forming hydride selected from the group of calcium, magnesium, strontium, barium, lithium, sodium, potassium, 25

23. The method of claim 21 wherein the metal hydride is a metal boro-hydrides selected from the group of lithium, sodium and potassium boro-hydride.

24. The method of claim 21 further includes the step of passing the hydrogen to a hydrogen storage vessel containing a hydrogen storage material which reversibly 5 absorbs hydrogen.

25. The method of claim 24 wherein the hydrogen storage material is selected from the group consisting of magnesium nickel hydride, lithium aluminium hydride, magnesium iron hydride, lanthanum nickel aluminium hydride, calcium nickel hydride, titanium iron hydride, and magnesium hydride. 0

26. The method of claim 21 wherein the hydrogen is stored as a compressed gas or liquid.

27. The method of claims 21 to 26 further including the steps of passing the hydrogen gas through one or more heat exchange systems where the hydrogen gas stream exchanges heat with one or more secondary heat exchange fluids without 5 mixing the hydrogen and heat exchange fluids.

28. The method of claim 27 wherein the heat exchange fluids are used to store heat which is later used to desorb hydrogen from the hydrogen storage material when a supply of hydrogen is required to convert liquid metal to metal hydride in the metal/metal/hydride vessel. 20

29. A hydrogen storage system comprising a hydrogen storage vessel ; a heat exchange and heat storage system to recover heat from the primary hydrogen gas stream to the hydrogen storage vessel, the heat exchange and heat storage system having a heat supply arrangement to supply heat to the hydrogen storage vessel to 25 release hydrogen from the hydrogen storage material. the heat exchange and storage system comprising 26 a first high temperature heat exchanger utilising at least one high temperature heat exchange material, the high temperature heat exchange material undergoing heat exchange with the hydrogen gas stream, the first heat exchange material being stored in a first heat storage vessel and the material circulating between the first high 5 temperature heat exchanger and the first heat storage vessel; and a second low temperature heat exchanger utilising at least one second low temperature heat exchange material, the second low temperature heat exchange material undergoing heat exchange with the hydrogen gas stream subsequent to heat exchange with the first high temperature heat exchange material and prior to storage in the 0 hydrogen storage vessel, the second heat exchange material being stored in a second heat storage vessel and the second heat exchange material circulating between the second heat exchanger and the second heat storage vessel.

30. The system of claim 29 wherein the hydrogen storage vessel contains a hydrogen storage material, the hydrogen storage material comprising a metal or metal 5 alloy capable of reversibly reacting with or absorbing hydrogen to release heat and releasing hydrogen when heated.

31. The system of claim 30 wherein the hydride storage material is at least one metal hydride selected from the group of magnesium nickel hydride, lithium aluminium hydride, magnesium iron hydride, lanthanum nickel aluminium hydride, calcium nickel 20 hydride, titanium iron hydride, and magnesium hydride.

32. The system of claim 29 wherein the hydrogen storage system is hydrogen stored as a compressed gas or liquid

33. The system of claim wherein the high temperature heat exchange material is boron oxide. ?5

34. The system of any one of claims 29 to 33 wherein the second heat exchange material is a nitrate salt mixture selected from the group of alkaline metal nitrates such as potassium and sodium nitrate. 27

35 The system of any one of claim 29 to 34 wherein the heat from the low temperature heat exchanger material is able to be provided to the hydrogen storage material to release hydrogen.

36. The systems of any one of claims 29 to 35 wherein the heat exchange with the 5 hydrogen gas stream occurs without mixing between the hydrogen gas and the heat exchange material.