Thermochemical Energy Storage System
A system is described for the thermochemical capture of heat energy and the transfer of this energy to a point of use using a cycle decomposing SO3 to SO2 and O2 and the subsequent oxidation of SO2 and O2 to SO3. This system can store this energy in the form of chemical energy by storing liquid SO2. Embodiments are described wherein oxygen is stored by a solid oxygen storage material or removed and added to the process by selective membranes or by electrochemical pumping. In addition, an alternative embodiment uses an electrochemical generator for the direct conversion of SO2 to electrical energy.
The present invention relates to a concept and system design for thermochemical energy storage.
BACKGROUNDThe headings used herein are for illustrative purposes and do not limit the interpretation of the following specification.
Thermal solar systems generate thermal energy when the sun is shinning but when the sun sets, solar thermal energy is not available and such system must either rely on conventional stored carbon based fuels or stored energy generated during the period when solar energy is available. Energy can be stored in a variety of ways such as electrical energy stored in batteries or capacitors, pumped storage where water is pumped to an elevated storage area for later use to generate electrical energy through a water powered turbine generator, pressurized storage of compressed gas etc. Another such energy storage technique is to use the solar energy to convert a chemical substance from a low energy state to a high energy state, storing this chemical compound and later use this chemical compound to generate energy through a chemical reaction that returns the chemical to it initial state and release energy, typically heat. One such process is the ammonia synthesis reaction and ammonia decomposition reaction shown in equations 1 and 2 as described in published articles by Luzzi, Lovegrove and coauthors (Solar Energy 66(2) pp. 91-101 (1999)).
H2+3N2→2 NH3l +heat (1)
3 NH3+heat→3 H2+N2 (2)
Both of these reactions are carried out over a catalyst. This reaction can transport energy from the solar collector where energy input dissociates HN3 to H2 and N2, The H2 an N2 are then transported to the location where the heat is needed where the ammonia synthesis reaction releases heat. To store energy for use during periods when solar energy is not available, for example at night or during cloudy periods, this system has to produce and store excess H2 and N2. This is not an ideal situation since at ambient temperatures, H2 and N2 can only be stored as compressed gases since ambient temperature is above the critical temperature of these species. Storage as liquids would require additional energy input to liquefy these gases and storage at very low temperatures would have additional energy costs or require additional process equipment to deal with continual liquid evaporation. Also, storage of gas at high pressure results in high capital costs for the storage facilities and expenditure of considerable energy for compression as the gas is stored.
An alternative type of energy storage is described by Azpiazu and coauthors (Applied Thermal Engineering 23 (2003) p. 733-741) which uses the hydration of calcium oxide as the energy cycle. CaO is hydrated by reaction with water to release heat and then dehydrated using solar energy to regenerate the CaO and release the water.
Ca(OH)2+heat→CaO+H2O (3)
CaO+H2O→Ca(OH)2+heat (4)
In operation as a solar thermochemical storage system, the calcuim hydroxide is contained in a reactor as a packed bed or similar solid mass. Heat from solar energy collector is used to heat a gas or liquid heat transfer medium that transfers the heat energy to a heat exchange system installed in the calcium hydroxide bed to dehydrate the calcium hydroxide to calcium oxide and free water vapor at approximately 500° C. When heat energy is required from the stored chemical energy, the calcium oxide bed temperature is lowered to 25° C. and water added back to form calcium hydroxide and release the sorted energy. The energy is extracted by gas or fluid flow through a heat exchange system installed in the storage bed. A wide range of such adsorption-desorption reactions can be envisioned. U.S. Pat. No. 4,365,475 describes a number of such reaction couples for NH3 adsorption-desorption. The reaction couple can be selected to collect energy and release energy over a variety of temperature ranges. The CaO—Ca(OH)2 couple described above releases heat near ambient temperature so it would be a good cycle for space heating and adsorption chillers. Cycles described in U.S. Pat. No. 4,65,475 involving the formation of NH3 complexes, could release energy at higher temperatures, for example 200 to 250° C., allowing the released heat to be used to generate steam for electrical power generation using a steam turbine. One major disadvantage of these adsorption-desorption systems is that the energy storage and release requires that a massive solid bed be heated and cooled to the required cycle temperatures thus wasting significant energy. Also, the large change in volume of the solid upon hydration-dehydration or ammonia complexation-decomposition make it difficult to prepare durable solid materials that can go through many cycles. Azpiazu and coauthors describe the calcium oxide-hydroxide couple as limited to 20 cycles before performance degradation was excessive.
Another system that has been described in U.S. Pat. Nos. 3,972,183 and 3,997,001 is the conversion of SO3 into SO2 and O2 and the subsequent oxidation of SO2 with O2 to SO3. The process described uses a catalyst in the solar energy collector to decompose SO3 to SO2 and O2 at about high temperature with the adsorption of heat, transport of the SO2 and O2 gas to the process or an energy storage unit where the SO2 and O2 is coverted to SO3 and heat and the heat stored. These references describe the storage of energy as sensible heat in a thermal mass such as a bed of hot rocks or heat of phase change in a molten salt. Storage of energy as a hot thermal mass or as a hot molten salt requires insulation to retain the heat and also results in a slow loss of the stored energy through loss of heat.
SUMMARYIn embodiments of the invention, thermochemical energy storage cycles are provided that use the a reaction couple of a gaseous species that is catalytically decomposed to a less oxidized species and free oxygen with the adsorption of heat to store thermochemical energy followed by the catalytic oxidation of this less oxidized species to release energy. One embodiment of such the cycle is shown in equations 5 and 6.
SO3+heat→SO2+0.5 O2 (5)
SO2+0.5 O2→SO3+heat (6)
Another embodiment of the cycle employs NO and NO2 as shown in equation 7 and 8.
NO2+heat→NO+0.5 O2 (7)
NO+0.5 O2→NO2+heat (8)
For the SOx couple of equations 5-6, SO2 is the stable species above 700 to 800° C. and SO3 is the stable species below about 600° C. Thus, reaction 5 would be operated at 700 to 1000° C. with energy input from a solar or other energy source supplying heat and reaction 6 at 600° C. or lower with energy output to the target process. This would allow the production of high quality steam for power generation or process heat. The NOx cycle is similar with the NO2 the preferred chemical species above 600° C. and NO the preferred chemical form below 300 to 400° C. SO2 and NO plus O2 provide thermochemical energy transport methods. The inventive cycles use the SO2 to SO3 interconversion combined with liquid storage of the SO2 and SO3 to provide a thermochemical energy storage system with a high energy density and low capital cost. Since the SO2 and SO3 can be stored at ambient temperature as a liquid, this eliminates the need for thermal energy storage at high temperature, eliminates the need for insulation to retain this high temperature stored heat and eliminates any loss of energy through slow loss of heat with time. In addition, inventive methods for the storage of the O2 or removal and supply of the O2 are described. In addition, an inventive electrochemical generator for the electrochemical oxidation of SO2 into SO3 and electricity is described.
A number of other aspects of these cycles will be described and systems described for energy transfer from the energy source (solar) to the process and for high density energy storage and release.
Use of the same reference numbers in different figures indicates similar or identical elements.
DETAILED DESCRIPTION Basic System Design
SO3→SO2+0.5 O2 (9)
SO2+0.5 O2→SO3 (10)
This energy transfer cycle can also be an energy storage cycle by storing the high energy species, SO2.
This stored energy can then be utilized by vaporizing SO2 in vessel 405, desorbing O2 from the oxygen storage material in vessel 407, passing this stream through heat exchanger 415 and on to reactor 304 where heat is generated. The SO3 is then passes through line 416 to heat exchanger 417 where it condenses to a liquid, then through line 418 to vessel 408 where the SO3 is stored as a liquid.
It should be noted that in operation, this process can operate in a variety of modes. All of the heat input from solar or other heat sources can be converted to heat output by directing all of the SO2 and O2 to process stream 413, through reactor 304 and then on through process stream 414 and 415 back to the decomposition reactor 301 where the cycle is completed. Another alternative is to split a portion of the SO2 and O2 to each of the process streams 403 and 413 so that some of the heat input is used to provide heat output and some of the energy input is stored as liquid SO2. Of course the third operating mode is to have no heat input 302 and just produce heat output by using the stored SO2 in vessel 405 flowing through reactor 304 to produce heat output.
System 400 in
To store energy using the process design shown in
Reactors 301 and 304 may be thermal reactors or catalytic reactors. To obtain a fast rate of conversion from the input species to the output species, a catalyst may be used. Catalyst for the decomposition of SO3 to SO2 and O2 and for the oxidation of SO2 and O2 to SO3 are well known in the art and are discussed elsewhere in this specification. The temperature of operation of reactor 301 will depend on the level of conversion desired and on other process variables such as total pressure, partial pressure of O2 and the temperature of the storage vessels 405 and 408. At 10 bar pressure, approximately 80% conversion of SO3 to SO2 can be obtained at about 1000° C. If the oxygen partial pressure is reduced, then the temperature for 80% conversion will decrease. The operating temperature of reactor 301 is in the range of 600 to 1200° C. In one embodiment, the operating temperature is in the range of 700 to 1000° C. In another embodiment, the operating temperature is in the range of 800 to 1000° C. Similarly, the operating temperature of reactor 304 is in the range of 300 to 800° C. In one embodiment, the operating temperature is in the range of 500 to 700° C. In another embodiment, the operating temperature is in the range of 500 to 600° C.
Operation at Reduced Oxygen Partial PressuresAt 10 bar pressure with excess O2 in the circulating streams, reactor 301 must operate at quite high temperatures to obtain a high level of conversion from SO3 to SO2. A high operating temperature may not be desirable for thermal solar systems since this would reduce efficiency and increase cost. One strategy to reduce the temperature of reactor 301 is to decrease the oxygen concentration.
The O2 partial pressure could be controlled by controlling the temperature of the oxygen storage material 407. This vessel containing oxygen storage material could be placed upstream of heat exchanger 404 to operate at 600° C. or heat exchanger 404 could be divided into several sections, the first section reducing the temperature of stream 403 to some intermediate temperature for oxygen storage material 407 placed in this location downstream of this first heat exchanger section. A second heat exchanger section would then reduce the temperature of this process stream to the temperature required to liquefy SO2 for storage in vessel 405. Oxygen storage material containing vessel 407 could also be placed in other locations in the process loop. For example it could be placed just downstream of reactor 301 in line 401 for operation at very high temperature. Alternatively, the oxygen storage material 407 could be divided into several vessels and placed in different locations to control the O2 partial pressure to different levels in different parts of the process stream.
Oxygen Storage MaterialsThe oxygen storage materials used in system 400 will depend on the specific system design and operating conditions. Metal oxides and peroxides offer one type of oxygen storage material. BaO and BaO2 is one example. At about 825° C., BaO2 would have an O2 partial pressure of about 1 bar. Lower temperatures would provide lower equilibrium O2 partial pressures. Cerium oxides, manganese oxides, cerium and palladium oxides and mixed oxides can also provide embodiments of oxygen storage materials. One embodiment of a mixed oxide O2 storage material is the mixed oxide REBaCo4O7+ as described by Motohashi et. al. (Materials Science and Engineering B 148 (2008) 196-198) where this material can store up to 3% of the oxygen storage material's weight as oxygen in the temperature range of 200 to 400° C. RE is one of the rare earth elements, in particular Y, Dy, Yb, and Lu (elements of the periodic table).
Electrochemical Control of O2 Partial PressureThe storage of oxygen is a critical aspect of the inventive process since storage as a gas would not be cost effective since it would require very large vessels or compression to high pressures which would consume energy and reduce efficiency. A significant observation is that the O2 does not have to be stored within the process but can be removed from the process as excess SO2 is generated and stored in vessel 405 and then added back to the process when the SO2 is oxidized.
The electrochemical pump 602 in
The use of an electrochemical pump to move O2 from air into the process and from the process out to air would eliminate the need to store O2 in the process equipment and reduce the overall equipment size for energy storage. The only stored chemical is the liquid SO2. Ambient air becomes the source and sink for the required O2 reactant. The design of the electrochemical oxygen pump can take many forms. A possible form is a zirconium oxide solid electrolyte cell such as that described in U.S. Pat. No. 5,378,345 and U.S. Pat. No. 4,877,506 which are incorporated into this application in their entirety. These cells use a stabilized zirconium oxide electrolyte that can transport oxygen ions and porous electrodes that act to dissociate O2 into oxygen ions and recombine the oxygen ions to form O2 and also act as the electron conductor. A typical electrode material is a porous platinum layer. Other platinum group metals or mixtures of platinum group metals could be used.
An alternative embodiment using an electrochemical oxygen pump to remove oxygen from and add oxygen to the process stream would replace the ambient air purge at 601 and 602 of
It should also be noted that while an electrochemical oxygen pump or cell is described herein, an alternative embodiment is to use an oxygen permeable membrane to selectively remove O2 from the process stream to an external purge stream. Oxygen could also be added back using a similar oxygen permeable membrane. The driving force for movement of O2 into the process stream or out of the process stream would be the O2 partial pressure on the opposite side of the membrane. A high O2 pressure or high air pressure would drive O2 into the process and a low O2 partial pressure or air pressure would drive O2 out of the process.
Electrochemical SO3 Decomposition ReactorThe decomposition of SO3, equation 9, is limited by thermodynamics at the selected operating temperature. This is shown in
The process shown in
An alternative embodiment of the invention to the process shown in
The process and system design described above can be used as system for the capture and storage of solar energy.
The system shown in
In one embodiment of this invention, catalysts for the oxidation of SO2 to SO3 would be platinum, palladium or other platinum group metals supported on a oxide or other high surface area support such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide or mixtures of these oxides with or without additional additives. Vanadium is also a preferred catalyst, again supported on oxides such as described for the platinum group metal catalysts. An optimal catalyst used commercially for this reaction is vanadium oxide supported on silica support containing potassium, sodium and aluminum oxide additives.
The decomposition of SO3 to SO2 and O2 would be done on similar catalysts as for the oxidation as described above.
Anode materials for the electrochemical generator described in
Cathode catalysts for the electrochemical generator of
One alternative embodiment is shown in
Stream 1204 from the solar collector field will then either split a portion of the flow to the SO2 and O2 storage section 1205 or to process stream 1206 and then to heat exchanger 1207. Heat exchanger 1207 could also be a counter current heat exchanger which would increase the temperature of the process stream 1206 to the required temperature for the SO2 oxidation reactor 1208 where the chemical energy is converted to heat. This temperature of reactor 1208 would be in the range of 350 to 600° C. The SO3 leaving the SO2 oxidation reactor 1208 flows through process stream 1210 back to heat exchanger 1207 where the temperature is adjusted to near ambient temperature for SO3 storage or return to the solar collector field through process stream 1203.
In one embodiment steam generator section 1209 could consist of a steam turbine and an electric power generator. In another embodiment the heat release and electric power generating section shown as reactor 1208 and steam generator 1209 could be replaced with the direct electrochemical generator 1104 shown in
In other embodiments, the heat exchangers shown in
Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.
Claims
1. A thermochemical system that transports energy from a first location to a second location where the energy is used to generate steam for a steam turbine, comprising:
- a thermal source providing a first thermal energy;
- a first reactor at the first location converting the first thermal energy to chemical energy by decomposing SO3 into SO2 and O2;
- a second reactor at the second location converting the chemical energy to a second thermal energy by oxidizing SO2 with O2 to produce SO3, the second reactor evaporating water into the steam for the steam turbine;
- liquid SO2 storage of; and
- liquid SO3 storage of;
- wherein an output of the first reactor is coupled to both an input of the second reactor and the liquid SO2 storage, and an output of the sec reactor is coupled to both an input of the first reactor and the liquid SO3 storage;
- wherein a flow split of SO2 and O2 from the first reactor to the second reactor and the liquid SO2 storage is variable, and a flow split of SO3 from the second reactor to the first reactor and the liquid SO3 storage is variable.
2. The system of claim 1 wherein the thermal source is a solar concentrator that focuses sunlight on a receiver to produce a temperature high enough to decompose SO3 into SO2.
3. The system of claim 1 wherein at least one of the first reactor and the second reactor comprises a catalyst for converting SO3 into SO2 and O2 or SO2 and O2 to produce SO3.
4. The system of claim 1 wherein the first reactor converts SO3 into SO2 and O2 at a temperature in the range of 600 to 1200° C.
5. The system of claim 1 wherein the second reactor converts SO2 and O2 to produce SO3 at a temperature below about 800° C.
6. The system of claim 1 further comprising an oxygen storage material that chemically binds and stores a substantial portion of O2 produced by the first reactor converting SO3 into SO2 and O2.
7. The system of claim 1 wherein pressures in the first reactor and the second reactor are maintained in the range of 1 bar to 100 bar.
8. The system of claim 7 wherein the pressures are maintained in the range of 5 to 15 bar.
9. The system of claim 6 wherein at least one of the oxygen storage material and a temperature of the oxygen storage material is chosen to adsorb and release O2 at an O2 partial pressure in the range of 0.01 to 3 bar.
10. The system of claim 1, further comprising:
- a first electrochemical cell being located downstream of the first reactor, the first electrochemical cell being exposed on one side to a process stream and on the other side to the ambient air, the first electrochemical cell removing O2 formed by the first reactor from the process stream and pumps it into the ambient air; and
- a second electrochemical cell being located upstream of the second reactor, the second electrochemical cell begin exposed on one side to the process stream and on the other side to the ambient air, the second electrochemical cell pumping O2 from the ambient air into the process stream to be used by the second reactor.
11. The system of claim 1, further comprising:
- a first electrochemical cell being located downstream of the first reactor, the first electrochemical cell being exposed on one side to a process stream and on the other side to a vessel containing an oxygen storage material; and
- a second electrochemical cell being located upstream of the second reactor, the second electrochemical cell being exposed on one side to the process stream and on the other side to an other vessel containing an other oxygen storage material wherein: the first electrochemical cell removes O2 formed decomposing SO3 into SO2 and O2 from the process stream and pumps it into the vessel containing the oxygen storage material; the second electrochemical cell pumps O2 from the other vessel containing the other oxygen storage material into the process stream to be used in oxidizing SO2 and O2 to produce SO3.
12. The system of claim 1 wherein the first reactor includes an electrochemical cell that removes oxygen from the first reactor to allow a higher conversion of SO3 to SO2.
13. The system of claim 1, wherein the second reactor includes an electrochemical cell that produces electrical power from the conversion of SO2 and O2 to produce SO3.
14. A method of transporting energy from a first location to a second location where the energy is used to generate steam for a steam turbine, the method comprising:
- converting the thermal energy at the first location to chemical energy in a first reactor by decomposing SO3 into SO2 and O2;
- converting the chemical energy to one of thermal energy or electrical energy in a second reactor at the second location by oxidizing SO2 with O2 to produce SO3;
- using stored SO3 liquid for the first reactor and storing a portion of SO2 from the first reactor as a liquid when more energy is available at the first location then is required at the second location; and
- using stored SO2 liquid for the second reactor and storing SO3 liquid from the second reactor when more energy is needed at the second location then is available at the first location;
- wherein using stored SO3, storing a portion of the SO2, using stored SO2 liquid, and sorting SO3 liquid comprise: controlling a flow split of SO2 and O2 from the first reactor to the second reactor and a liquid SO2 storage; and
- controlling a flow split of SO3 from the second reactor to the first reactor and a liquid SO3 storage.
15. The method of claim 14, further comprising using a solar concentrator to focus sunlight on a receiver to produce a temperature high enough to decompose SO3 into SO2.
16. The method of claim 14 wherein at least one of the first reactor and the second reactor comprise a catalyst for converting SO3 into SO2 and O2 or SO2 and O2 to produce SO3.
17. The method of claim 14, further comprising operating first reactor at a temperature in the range of 600 to 1200° C.
18. The method of claim 14, further comprising operating second reactor at a temperature below about 800° C.
19. The method of claim 14, further comprising chemically binding at least a portion of O2 produced in the first reactor by the decomposition of SO3 into SO2 and O2 in an oxygen storage material.
20. The method of claim 14, further comprising maintaining pressures in the first reactor and the second rector are in the range of 1 to 100 bar.
21. The method of claim 20 wherein the pressures are maintained in the range of 5 to 15 bar.
22. The method of claim 14, further comprising:
- pumping, with a first electrochemical cell, O2 produced in the first reactor by the decomposition of SO3 into SO2 and O2 to one of ambient air and a vessel containing an oxygen storage material;
- pumping, with a second electrochemical cell, O2 from one of the ambient air and another vessel containing another oxygen storage material to react with SO2 in the second reactor to produce SO3.
Type: Application
Filed: Apr 16, 2009
Publication Date: Oct 21, 2010
Inventor: Ralph A. Dalla Betta (Saratoga, CA)
Application Number: 12/424,545
International Classification: F28D 15/00 (20060101);