CHEMICAL ENERGY STORAGE SYSTEM

Abstract: A system for storing chemical energy comprises first and second vessels (12, 14), the first containing a liquid solution (16) of an alkali or alkaline earth metal hydroxide, chloride, or bromide at an initial concentration of between about 30% and about 70%, by weight, the second vessel containing liquid (18), a space about the liquid in each vessel (20, 22) and conduit communicating between the spaces having a valve (26) for selectively allowing liquid vapor to pass between the spaces, means (36, 56) for heating the solution to a temperature above about 80°F, means (46, 66, 70) for cooling the liquid to a temperature below about 55°F, and heat exchange means (71, 72, 73, 75, 76) for transferring heat from the heated solution and for transferring heat to the cooled liquid. Water is the preferred liquid although lower alcohols, and polyols such as glycerol and alkylene glycol ethers may be used.

Description: The present invention is directed to a method of operating a thermal energy storage system. A system suitable for the method is known from JP-A-57-117745. With the high cost of electrical energy required to operate air conditioners or heat pumps for cooling buildings, and particularly with the heavy demands for commercial building cooling systems at peak use hours, attention has been directed to various types of thermal energy storage systems. Such energy storage is advantageous since the building cooling and/or heating and process cooling may be generated and stored during off-peak hours at night when most businesses are normally closed, with the ambient outside temperatures being cooler and municipal power requirements reduced. Most state of the art thermal energy storage systems are based on a solid to liquid Phase change using energy storage in a narrow temperature range. Water based systems using ice storage are especially desirable because of low fluid costs and availability. However, the disadvantages of such systems include low evaporator temperature requirements because thermal gradient forces evaporator temperatures to a level far below 0°C, also reducing the chiller efficiency, incomplete or low phase change, often in the order of about 50% to 55%, and low overall energy density of 186 KJ/kg. However, due to the exceptional environmental acceptability qualities and large availability, water is preferred above more corrosive, volatile, expensive and less readily available energy storage materials. JP-A-57-117745 discloses a thermal energy storage system having a first vessel containing an aqueous solution of an inorganic salt, and a second vessel containing water. The vessels are connected by a pipe having a closure valve therein. During the charging phase heat from a solar collector is used to concentrate the salt solution, steam produced from the first vessel being condensed in the second vessel. During the discharge phase water in the second vessel is heated and steam therefrom condensed in the first vessel thereby producing heat through dilution of the concentrated solution. During the charging phase water in the second vessel is cooled. It is an object of the invention to operate a system of the general kind disclosed in JP-A-57-117745 as a latent chill or cooling energy storing system. Moreover, the method utilizes a system with relatively low cost and simple apparatus for taking advantage of the energy storage combined with state of the art heat exchange equipment normally found in modern residential and commercial buildings. According to the invention there is provided a method of operating a thermal energy storage system, the system comprising a first vessel containing a liquid solution of a compound selected from the group consisting of alkali and alkaline earth metal hydroxides, lithium chloride, lithium bromide, calcium chloride, calcium bromide and mixtures thereof, and a first space above the level of said liquid solution; a second vessel containing said liquid without said compound therein, and a second space above the level of said liquid; both said first and second vessels being closed to atmosphere and capable of holding a vacuum, conduit means communicating between said first space and said second space for allowing liquid vapour and pressure changes to pass therebetween, and valve means co-operating with said conduit means for controlling communication between said spaces the method comprising charging the system by the steps of a) opening said valve means; b1) heating said solution from ambient to a temperature above approximately 29.4°C, vapour from said solution passing from said first space through said conduit means to said second space, whilst concurrently cooling said liquid from ambient to a temperature below approximately 12.7°C; c) terminating said heating and cooling when the concentration of said solution is between approximately 6% and 25% greater than the initial concentration of said solution; and terminating said pumping; and d) closing said valve means, the method further comprising discharging the system by the steps of e) opening said valve means thus permitting vapour to pass from said second space to said first space; f1) exposing said solution to first heat exchange means to recover heat therefrom whilst concurrently exposing said liquid to second heat exchange means for utilizing the thermal energy benefit of the cooled liquid to provide cooling for air conditioning, heat pump or refrigerant equipment; g) closing said valve means and terminating said pumping as the solution and liquid approach ambient temperature. The invention also provides apparatus used in the method and including a first vessel or chamber containing a liquid solution of a compound selected from the group consisting of alkali and alkaline earth metal hydroxide, lithium chloride, lithium bromide, calcium chloride and calcium bromide, and mixtures thereof said solution having an initial concentration of between about 30% and about 70%, by weight of said compound, said liquid selected from the group consisting of water, an alcohol having between 1 and 8 carbon atoms and, glycerol and glycol ethers, and a first space above the level of said liquid solution; a second vessel or chamber containing said liquid without said compound therein, and a second space above the level of said liquid; both said first and second vessels being closed to atmosphere and capable of holding a vacuum, conduit means communicating between said first space and second space allowing liquid vapour and pressure changes to pass therebetween, and valve means cooperating with said conduit means and controlling communication between said spaces, said apparatus further comprising first heat exchange means cooperating with said first vessel and through which said solution passes, second heat exchange means cooperating with said second vessel and through which said liquid passes, first pumping means pumping said solution to distribution means and directing said solution into said first space; and second pumping means pumping said liquid to distribution means and directing said liquid into said second space. The invention is more fully described with reference to the accompanying drawings in which: Fig. 1 is a schematic sectional view of a first embodiment of the system of the invention used in direct heat transfer; Fig. 2 is a schematic sectional view of the system of the invention using a direct expansion system; and Fig. 3 is a schematic sectional elevation illustrating a system for use in an indirect heat transfer system. The energy storage system is charged by heating an aqueous salt solution and cooling water in separate vessels to a liquid temperature differential of at least 16.6°C and preferably 27.7°C or more up to about 100°C differential. The vessels are separated although the space above each liquid in the respective vessels are in communication until the desired vapor mass transfer is completed. Thereafter the spatial communication is terminated until it is desired to discharge the energy stored in the system as will be described hereinafter. In Fig. 1 there is shown an illustration of a first embodiment of the chemical thermal storage system. The apparatus illustrated schematically comprises a container 10 having two cavities or vessels 12 and 14 separated by a baffle or wall 24. The container must be air-tight so that the two cells or cavities can selectively maintain different vapor pressures. It will be appreciated that although a single container 10 is shown having the two vessels, different containers may be used, the important consideration being that two vessels or cavities are required for holding two different liquids of the system with means for transferring water vapor and communicating vapor pressure differentials between the vessels. In the preferred embodiment shown, a valve 26 communicates between the spaces of the two vessels above the liquid levels. The valve, conveniently located in wall 24, is preferably a butterfly valve with an orifice of several inches and associated with convenient means for opening and closing the valve. A first liquid composition 16 is located in first vessel 12 and fills the cavity only partially leaving a space 20 above the liquid level. Similarly, in second vessel 14, liquid 18 only partially fills the cavity leaving a second space 22 above the liquid level. It is the spaces 20 and 22 which are in communication via valve 26 which can be selectively opened or closed to allow for water vapor to pass between the two spaces thereby maintaining vapor pressure differential between the two vessels when the valve is closed. In the first vessel 12 is a liquid solution of an alkali or alkaline earth metal hydroxide or lithium or calcium chloride or bromide or mixtures thereof having initial concentrations of between about 30 and about 70%, and preferably between about 40% and 60%, by weight. In the second vessel 14 is liquid 18. The preferred solvent in vessel 12 and liquid in vessel 14 is water although lower alcohols, glycerol, alkylene, glycol ethers and aqueous solutions or mixtures thereof may also be used. Lower alcohols are those having between 1 and 8 carbon atoms with methanol and ethanol being preferred. Suitable glycol ethers include ethylene glycol dimethyl ether, diethylene glycol diethyl ether, etc. Most preferred salts are the alkali or alkaline earth metal hydroxides especially those of sodium, potassium, cesium, magnesium, lithium, strontium and calcium. Mixtures of these hydroxides may also be used together with lithium chloride or lithium bromide or calcium chloride as well as nitrate salts of those metals as corrosion suppressing additives, in systems where corrosion may be a problem. Organic additives, such as long chain alcohols having 7-12 carbon atoms or glycol and its derivatives may be used to increase the sorption rates, increase the differential pressure and serve as freeze point suppressants in vessels 12 and 14. For this purpose, plastic vessels or containers which are resistant to the aqueous hydroxide solutions are to be used as are plastic tubing or conduits. Other materials that are lined or coated with compositions which are not susceptible to corrosion when exposed to the strong hydroxide compositions may be used. The apparatus also includes suitable conduits, pumps and spray nozzle systems for handling the liquids in the respective cavities. Thus, conduits 30 and 32, and pump 34 direct aqueous solution 16 from vessel 12 to a heat exchanger, for example, a condenser of a heat pump or chiller or a waste heat source (not shown) for heating the solution to a desired temperature of at least 16.6°C and preferably at least 27.7°C higher than the temperature of water in second vessel 14. Preferred solution temperatures are between about 29.4°C and about 54.4°C although higher temperatures may be used. The heated solution is then directed to first space 20. A preferred method of returning the heated aqueous solution to the first vessel incorporates a nozzle or spray nozzle system 36 which simply sprays the heated aqueous composition in the form of droplets or a fine mist into first space 20 above the surface of liquid 16 in vessel 12. Any suitable spray nozzle means may be used for this purpose. Similarly, in second vessel 14, water 18 is pumped to a heat exchanger by pump 44 via line 42 where it is cooled, for example, by an air conditioning or heat pump evaporator, cooling tower or other evaporative cooling means or an air to air means after which the cooled water is directed via pipe 40 to second space 22 above the water level using a spray nozzle means 46. In operating the above-described apparatus preferably, during the night, or otherwise at relatively low ambient temperature conditions and when area or municipal use loads are at below peak or high requirements, aqueous solution 16 is heated with condenser heat from a building heat pump system, or otherwise heated conveniently to a temperature of above about 29.4°C up to about 54.4°C At the same time, water 18 is cooled to a temperature of below about 12.7°C and preferably below 16°C using an evaporator from a building heat pump or air conditioning system. Because the vapor pressure of the aqueous solution, for example 42% NaOH, by weight, at 48.8°C is higher than the vapor pressure of water at 1.1°C the solution will desorb water in the form of water vapor via open valve 26 into second space 22 which condenses into the liquid water 18. This process continues until a solution concentration of approximately 52% of NaOH, by weight, is achieved. At that point, the system is charged, and valve 26 is closed to separate the first and second spaces in the two vessels and maintain vapor pressure differential which thereby allows the charged system to remain stored for an indefinite period of time. When it is desired to utilize the stored energy in the system created by the above-described charging process, the cold water is circulated to a heat exchanger for the building, for example, passed through a cooling coil in an air handler for cooling the building. During this discharge period, again, the water is pumped via pipe 42 using pump 44 to the building air handler or other heat exchange cooling means, and returned via pipe 40 to second vessel 14 using the spray nozzle device 46. The water thus becomes heated as it absorbs or picks up building heat through the heat transfer system during this discharge phase. Concurrently, hot aqueous solution 16 is pumped via pipe 32 and pump 34 to outside air heat exchangers or coolers such as evaporative coolers, cooling towers and the like thereby cooling the aqueous solution which is then returned via pipe 30 and discharged into first space 20 using a spray apparatus 36. During this discharge cycle, valve 26 must be opened and because of the difference in vapor pressure between the first and second spaces, 20 and 22 respectively, water is evaporated in second space 22 to provide substantial cooling of the liquid in second vessel 14. At the same time, the evaporated water is passed into first space 20 where it is absorbed into solution 16, which causes a heat of condensation and solutions in first vessel 12, which heat is again exchanged by the outside cooling means (not shown) previously discussed. Alternatively, the heat of condensation and solutions in first vessel 12 may be used for heating purposes, for example in a dual temperature storage capability for building heating and cooling. In Fig. 2 there is illustrated another variation or embodiment of a system in which a coolant, such as a refrigerant may be directly cooled or condensed at low temperature during discharge of the energy storage system of the invention. Again, a container 10 having two vessels or cavities as illustrated in Fig. 1 may be used with a divider 24 separating the two cavities. The liquid solution 16 and water 18 in the respective containers are substantially like that previously described as is the change of vapor pressure and exchange of water vapor between the cavities 20 and 22, respectively, through valve 26, in both the charging and discharging phases of the operation of the system. In this embodiment, the water is cooled during the charging phase by direct exposure to evaporator coils of an air conditioning system evaporator. For example, the air conditioning system of the building may utilize a cooling conduit 66 which is exposed directly in second cavity 22 through which cold refrigerant is directed during the charging phase. At that time, water 18 will be pumped via pump 60 and conduit 58 and sprayed through spray nozzle apparatus 46 over the cold pipe or coil 66 to be cooled. Concurrently, with valve 26 open during the charging phase, aqueous solution 16 is heated by pumping the solution via pump 54 and conduit 52 through spray nozzle apparatus 36 over heated pipe or coils 56 from a condenser 75 or other heating means including a heat exchanger of the HVAC (heating ventilation air conditioning) equipment condensing the refrigerant. In that case, hot refrigerant may be directed into pipe or coil 56 via pump 74 from the heat exchanger 75. Once the vapor mass transfer between the liquid solution 16 and water 18 is achieved during this charge phase, further heating and cooling, respectively is terminated and valve 26 is closed thereby again maintaining the energy charged in respective liquids stored until its use is desired. During discharge, valve 26 is opened, and the respective liquids are pumped through their respective nozzle sprayers over the heat exchange conduits present in the spaces in the respective vessel. Refrigerant directed via conduit or coil 66 is cooled by evaporation of water as water is sprayed over the coil in space 22, the water gradually becoming heated at it picks up heat as it cools the refrigerant from heat exchanger 72. Similarly, in this discharge phase of operation, heat from solution 16 is removed by heat exchanger 75 which similarly pumps a coolant via coil 56 present in space 20. A third apparatus configuration utilizing an indirect heat transfer system is illustrated in Fig. 3. In this apparatus, substantially like that described in Fig. 2, a heat exchanger 76 is used for directly heating and cooling liquid 16 in the charge and discharge phases, respectively of the operation of the system by pumping a heat transfer fluid with pump 74. The heat exchanger illustrated utilizing water with the system is somewhat different in that pump 77 will direct a heat transfer material such as water, ammonia, methanol, glycol-water mixtures, and the like through remote heat exchanger 71, 73 and 79 at various locations throughout a building in which the system is to operate. Pump 77 simply pumps the secondary cooling material via pipe 70 to the respective heat exchangers, with the pipe 70 being exposed in space 22 over which water is sprayed as previously discussed regarding the apparatus shown in Fig. 2.

Claims: A method of operating a thermal energy storage system, the system comprising a first vessel (12) containing a liquid solution (16) of a compound selected from the group consisting of alkali and alkaline earth metal hydroxides, lithium chloride, lithium bromide, calcium chloride, calcium bromide and mixtures thereof, and a first space (20) above the level of said liquid solution; a second vessel (14) containing said liquid (18) without said compound therein, and a second space (22) above the level of said liquid; both said first and second vessels (12,14) being closed to atmosphere and capable of holding a vacuum, conduit means communicating between said first space (20) and said second space (22) for allowing liquid vapour and pressure changes to pass therebetween, and valve means (26) co-operating with said conduit means for controlling communication between said spaces (20,22) the method comprising charging the system by the steps of a) opening said valve means (26); b1) heating said solution (16) from ambient to a temperature above approximately 29.4°C, vapour from said solution passing from said first space (20) through said conduit means to said second space (22), whilst concurrently cooling said liquid (18) from ambient to a temperature below approximately 12.7°C; c) terminating said heating and cooling when the concentration of said solution (16) is between approximately 6% and 25% greater than the initial concentration of said solution; and terminating said pumping; and d) closing said valve means, the method further comprising discharging the system by the steps of e) opening said valve means (26) thus permitting vapour to pass from said second space 22 to said first space (20); f1) exposing said solution (16) to first heat exchange means to recover heat therefrom whilst concurrently exposing said liquid (18) to second heat exchange means for utilizing the thermal energy benefit of the cooled liquid to provide cooling for air conditioning, heat pump or refrigerant equipment; g) closing said valve means (26) and terminating said pumping as the solution and liquid approach ambient temperature. A method according to Claim 1 and including the intermediate steps of: b2) circulating said solution (16) and said liquid (18) to respective first and second distribution means (36,46) located respectively in said first space (20) and said second space (22), and spraying said solution and said liquid from said respective distribution means (36,46); and f2) circulating said solution (16) and said liquid (18) to respective first and second distribution means (36,46) located respectively in said first space (20) and said second space (22), and spraying said solution and said liquid from said respective distribution means (36,46). A method according to Claim 2 and including the further intermediate steps of: b3) spraying said solution and said liquid over respective heat exchange means (56,66,70); and f3) spraying said solution and said liquid over respective heat exchange means (56,66,70). Apparatus used in the method of any of Claims 1-3, said apparatus including a first vessel or chamber (12) containing a liquid solution (16) of a compound selected from the group consisting of alkali and alkaline earth metal hydroxide, lithium chloride, lithium bromide, calcium chloride and calcium bromide, and mixtures thereof said solution having an initial concentration of between about 30% and about 70%, by weight of said compound, said liquid selected from the group consisting of water, an alcohol having between 1 and 8 carbon atoms and, glycerol and glycol ethers, and a first space (20) above the level of said liquid solution (16); a second vessel or chamber (14) containing said liquid (18) without said compound therein, and a second space (22) above the level of said liquid; both said first and second vessels (12,14) being closed to atmosphere and capable of holding a vacuum, conduit means communicating between said first space (20) and second space (22) allowing liquid vapour and pressure changes to pass therebetween, and valve means (26) cooperating with said conduit means and controlling communication between said spaces (20,22), said apparatus further comprising first heat exchange means cooperating with said first vessel (12) and through which said solution passes, second heat exchange means cooperating with said second vessel (14) and through which said liquid passes, first pumping means (34) pumping said solution (16) to distribution means (36) and directing said solution into said first space (20); and second pumping means (44) pumping said liquid (18) to distribution means (46) and directing said liquid into said second space (22). Apparatus according to Claim 4 wherein said first heat exchange means including evaporative or air to air cooling means (75;76) and first supplemental heat transfer means comprising a first conduit loop having heat transfer liquid therein and a pump (74) cooperating therewith and pumping said heat transfer liquid through said first conduit loop, said first conduit loop extending between said first space (20) and said evaporative or air to air cooling means (75;76). Apparatus according to Claim 4 or Claim 5 wherein said second heat exchange means includes a heat pump evaporator (71,73,79;72) and second supplemental heat transfer means comprising a second conduit loop having a heat transfer liquid therein and a pump (77) cooperating therewith and pumping said heat transfer liquid through said second conduit loop, said second conduit loop extending between said second space (22) and said evaporator (71,73,79;72).

Inventors: ROCKENFELLER, UWE
Applicants: ROCKENFELLER, UWE
Application year: 1988
Main classification: F28D20
IPC codes: F28D20, F28D5, F25B17