US20100263832A1

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Publication number: US20100263832A1
Application number: US12/424,545
Authority: US
Inventors: Ralph A. Dalla Betta
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-04-16
Filing date: 2009-04-16
Publication date: 2010-10-21
Also published as: WO2010120652A1

Abstract

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 SO₃to SO₂and O₂and the subsequent oxidation of SO₂and O₂to 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 SO₂to electrical energy.

Description

FIELD OF THE INVENTION

The present invention relates to a concept and system design for thermochemical energy storage.

BACKGROUND

The 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 inequations 1 and 2 as described in published articles by Luzzi, Lovegrove and coauthors (Solar Energy 66(2) pp. 91-101 (1999)).

H₂+3N₂→2 NH_{3l +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 HN₃to H₂and N₂, The H₂an N₂are 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 H₂and N₂. This is not an ideal situation since at ambient temperatures, H₂and N₂can 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 NH₃adsorption-desorption. The reaction couple can be selected to collect energy and release energy over a variety of temperature ranges. The CaO—Ca(OH)₂couple 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 NH₃complexes, 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 SO₃into SO₂and O₂and the subsequent oxidation of SO₂with O₂to SO₃. The process described uses a catalyst in the solar energy collector to decompose SO₃to SO₂and O₂at about high temperature with the adsorption of heat, transport of the SO₂and O₂gas to the process or an energy storage unit where the SO₂and O₂is coverted to SO₃and 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.

SUMMARY

In 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 NO₂as 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, SO₂is the stable species above 700 to 800° C. and SO₃is 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 NO₂the preferred chemical species above 600° C. and NO the preferred chemical form below 300 to 400° C. SO₂and NO plus O₂provide thermochemical energy transport methods. The inventive cycles use the SO₂to SO₃interconversion combined with liquid storage of the SO₂and SO₃to provide a thermochemical energy storage system with a high energy density and low capital cost. Since the SO₂and SO₃can 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 O₂or removal and supply of the O₂are described. In addition, an inventive electrochemical generator for the electrochemical oxidation of SO₂into SO₃and 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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the equilibrium distribution of SO₃, SO₂and O₂over a range of temperatures for 1 bar pressure starting with 1 mole SO₃and 0.5 mole O₂.

FIG. 2 shows the equilibrium distribution of NO₂, NO and O₂over a range of temperatures for 1 bar pressure starting with 1 mole NO₂and 0.5 mole O₂.

FIG. 3 is a schematic diagram of thermochemical energy cycle using SO₂and SO₃where the decomposition of SO₃to SO₂and O₂adsorbs thermal energy and the oxidation of SO₂to SO₃releases the thermal energy at the point of use.

FIG. 4 is a schematic diagram of a thermochemical energy cycle including energy storage using liquid SO₂and SO₃in one embodiment of the invention.

FIG. 5 is a graph showing the effect of oxygen partial pressure on the equilibrium fraction of SO₂at10 bar pressure for temperatures of 700 and 800° C. in one embodiment of the invention.

FIG. 6 is a schematic diagram of a thermochemical energy cycle including energy storage using an oxygen pump to remove and add O₂to the process stream in one embodiment of the invention.

FIG. 7 is a schematic diagram of an electrochemical O₂pump in one embodiment of the invention.

FIG. 8 is a schematic diagram of an electrochemical O₂pump combined with a catalytic process to decompose SO₃into SO₂and O₂in one embodiment of the invention.

FIG. 9 is a schematic diagram of an electrochemical process for converting SO₂and O₂into SO₃and electric power in one embodiment of the invention.

FIG. 10 is a schematic diagram of a thermochemical energy cycle including energy storage and electrical power generation using thermal energy to generate steam and the steam to drive a turbine generator in one embodiment of the invention.

FIG. 11 is a schematic diagram of a thermochemical energy cycle including energy storage and an electrochemical generator that directly converts SO₂and O₂into electricity in one embodiment of the invention.

Use of the same reference numbers in different figures indicates similar or identical elements.

DETAILED DESCRIPTIONBasic System Design

FIG. 1 illustrates the thermodynamic species balance for the oxidation of SO₂to SO₃andFIG. 2 illustrates the thermodynamic balance for the NO to NO₂couple, both at 1 bar pressure and with a starting composition of 1 mole of NO and SO₂and 1 mole of O₂.FIG. 1 shows that for the oxidation of SO₂to SO₃, equation 10, the equilibrium lies far to the left with SO₂the preferred species at temperatures below about 750° C. and SO₃the preferred species above 750° C. The oxidation of SO₂to SO₃is highly exothermic and the decomposition of SO₃into SO₂and O₂is highly endothermic. By employing a catalyst that can facilitate the oxidation of SO₂to SO₃(equation 10) or the decomposition of SO₃into O₂and SO₂(equation 9), this interconversion can form a thermochemical cycle.

SO₃→SO₂+0.5 O₂ (9)

SO₂+0.5 O₂→SO₃ (10)

FIG. 3 shows asystem300 using thermochemical energy cycle of SO₂and SO₃. SO₃is decomposed in areactor301 to SO₂and O₂at a temperature of about 900° C. withheat input302 since the process is endothermic. The SO₂can then be circulated through aheat exchanger303 to reduce the stream temperature to about 600° C. and then to asecond reactor304 which operates at a lower temperature where SO₃is thermodynamically preferred species and where the SO₂is oxidized to SO₃with the release ofheat305. To complete the cycle, the process stream fromreactor304 is circulated throughheat exchanger306 to raise the stream temperature to 900° C. and back to the SO₃decomposition reactor301. This thermochemical cycle moves energy from one site where heat is adsorbed to a second site where energy is released.

This energy transfer cycle can also be an energy storage cycle by storing the high energy species, SO₂.FIG. 4 shows asystem400 similar tosystem300 ofFIG. 3 but offers energy storage in one embodiment of the invention.System400 similarly usesreactor301 to decompose SO₃to SO₂and O₂to produceprocess stream401 which passes throughheat exchanger303 to reduce the stream temperature to 600° C. atpoint402 prior to enteringreactor304. However, a portion ofstream402 can be split off to processstream403 which then passes throughheat exchanger404 to drop the stream temperature to about 25° C. where a substantial portion of the SO₂will change phase to a liquid and be collected intank405. The actual temperature of this stream and the temperature of the liquid SO₃will depend on the system pressure and the overall process design. The temperature could range from −50 to 250° C. In addition, the O₂inprocess stream403 will pass throughtank405, through line orprocess passage406 to avessel407 containing an oxygen storage material which will adsorb or otherwise trap a substantial portion of the O₂. Such an oxygen storage material is sometimes referred to as an oxygen storage compound (OSC). In addition,407 can refer interchangeable to the oxygen storage material and the vessel containing the oxygen storage material. It should be noted that line and process passage are used interchangeably. In this manner,energy input302 can be stored by producing a large amount of SO₂and O₂that is stored in

vessels

405 and407. Such an energy storage system must also have a source of SO₃to convert to SO₂. This is also shown inFIG. 4 where liquid SO₃is stored invessel408. During this energy storage operation, the liquid SO₃invessel408 is vaporized and heated to 600° C. inheat exchanger409 and then passes throughline410 andline411 toheat exchanger306 to be heated to 800° C. and then throughline412 todecomposition reactor301 to complete the cycle. The overall process for energy storage then consists of vaporizing liquid SO₃fromvessel408, heating this stream to 800° C., passing this process stream throughreactor301 to decompose the SO₃to SO₂and O₂using a heat input source, then cooling this SO₂+O₂stream to 25° C. where the SO₂is condensed to be stored as a liquid and the O₂is stored in an oxygen storage material. This energy storage process can have a high energy density since the high energy phase, SO₂is stored as a liquid and the O₂can be stored as an absorbed phase in an oxygen storage material at reasonable pressures without the need for high pressure gas storage.

This stored energy can then be utilized by vaporizing SO₂invessel405, desorbing O₂from the oxygen storage material invessel407, passing this stream throughheat exchanger415 and on toreactor304 where heat is generated. The SO₃is then passes throughline416 toheat exchanger417 where it condenses to a liquid, then throughline418 tovessel408 where the SO₃is 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 SO₂and O₂to processstream413, throughreactor304 and then on through

process stream

414 and415 back to thedecomposition reactor301 where the cycle is completed. Another alternative is to split a portion of the SO₂and O₂to each of the process streams403 and413 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 noheat input302 and just produce heat output by using the stored SO₂invessel405 flowing throughreactor304 to produce heat output.

System

400 inFIG. 4 would be useful as a solar energy storage system where solar energy can be collected and supplied toreactor301, this energy can be stored as liquid SO₂invessel405 or used inreactor304 to produce heat for any needed purpose. It can provide heat output when there is limited or no solar energy or other energy input by consuming stored SO₂fromvessel405 to produce heat and then store the product SO₃invessel408 for later use. Thus, this system can be used as a means to transfer energy from a solar collector to the point of use, store solar energy during periods when solar radiation exceeds current thermal needs, and produce thermal energy at the point of use from stored chemical energy when the solar radiation does not meet the thermal requirements.

To store energy using the process design shown inFIG. 4, the operating pressure has to be sufficiently high to allow SO₂and SO₃to liquefy at reasonable temperatures. It is desirable to store the energy at ambient temperatures, for example below about 50° C. so that it can be stored for long periods without needing energy to maintain a low temperature or to thermally insulate or heat the storage vessels to maintain a higher temperature. If the process inFIG. 4 is operated at about 10 bar total pressure, then both SO₂and SO₃will condense to form a liquid if cooled below about 50° C. Pressures from 1 bar to above 100 bar could be used. In one embodiment, the operating pressure is in the range of 3 to 20 bar. In another embodiment, the operating pressure is in the range of 5 to 15 bar.

Reactors

301 and304 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 SO₃to SO₂and O₂and for the oxidation of SO₂and O₂to SO₃are well known in the art and are discussed elsewhere in this specification. The temperature of operation ofreactor301 will depend on the level of conversion desired and on other process variables such as total pressure, partial pressure of O₂and the temperature of the

storage vessels

405 and408. At 10 bar pressure, approximately 80% conversion of SO₃to SO₂can be obtained at about 1000° C. If the oxygen partial pressure is reduced, then the temperature for 80% conversion will decrease. The operating temperature ofreactor301 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 ofreactor304 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 Pressures

system

300 or400 would be in the range of 0.01 to 3 bar. In one embodiment, the partial pressure of O₂for

system

300 or400 is in the range of 0.01 to 1.5 bar. In another embodiment, the partial pressure of O₂is in the range of 0.01 to 0.75 bar.

The O₂partial pressure could be controlled by controlling the temperature of theoxygen storage material407. This vessel containing oxygen storage material could be placed upstream ofheat exchanger404 to operate at 600° C. orheat exchanger404 could be divided into several sections, the first section reducing the temperature ofstream403 to some intermediate temperature foroxygen storage material407 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 SO₂for storage invessel405. Oxygen storagematerial containing vessel407 could also be placed in other locations in the process loop. For example it could be placed just downstream ofreactor301 inline401 for operation at very high temperature. Alternatively, theoxygen storage material407 could be divided into several vessels and placed in different locations to control the O₂partial pressure to different levels in different parts of the process stream.

Oxygen Storage Materials

The oxygen storage materials used insystem400 will depend on the specific system design and operating conditions. Metal oxides and peroxides offer one type of oxygen storage material. BaO and BaO₂is one example. At about 825° C., BaO₂would have an O₂partial pressure of about 1 bar. Lower temperatures would provide lower equilibrium O₂partial 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 O₂storage 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 O₂Partial Pressure

The 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 O₂does not have to be stored within the process but can be removed from the process as excess SO₂is generated and stored invessel405 and then added back to the process when the SO₂is oxidized.FIG. 6 shows asystem600 similar tosystem400 ofFIG. 4 but replaces theoxygen storage material407 with single or multiple electrochemical cells or electrochemical oxygen pumps in one embodiment of the invention. Electrochemical oxygen pump and electrochemical oxygen cell are used interchangeably in this disclosure. Insystem600,

electrochemical pumps

601 and602 replace the oxygenstorage material vessel407. These electrochemical pumps function as O₂pumps.Cell601 pumps O₂out of the process when excess SO₃is decomposed to SO₂and O₂moving the O₂to an air purge stream and into the atmosphere.Cell602 pumps O₂into the process when the SO₂is vaporized and is to be oxidized to SO₃to generate heat. In this case, theelectrochemical cell602 pumps O₂from the air purge stream into the process.FIG. 7 is a schematic diagram of such anelectrochemical cell700 in one embodiment of the invention. InFIG. 7,electrochemical pump700 operates in the mode required foroxygen removal pump601 ofFIG. 6. SO₂and O₂pass throughchannel701. Electrode702 dissociates the O₂into oxygen ions, O²⁻ and electrons, the oxygen ions are then transported throughelectrolyte703 to theopposite electrode704. Atelectrode704 the oxygen ions are recombined with electrons to produce O₂which is then extruded into the gas phase, a flowing purge stream of air inchannel705. The electrons travel from electrode702 through

conductors

706 and708 toelectrode704. The pumping of O₂fromchannel701 to channel705 is driven by the applied voltage, V,707. The applied voltage controls the O₂pumping rate and the oxygen partial pressure that can be achieved inchannel701. By increasing the voltage, the O₂partial pressure can be driven to a very low value if desired and the pumping rate can be very high. For a given voltage, the pumping rate and the ultimate O₂partial pressure can be also be influenced by the O₂partial pressure in theair purge channel705. If the purge air is at 1 bar pressure, then the O₂partial pressure would be approximately 0.21 bar. If a vacuum is applied tochannel705, then the O₂partial pressure can be reduced to any desired value. For example, by operating theair purge channel705 at 0.25 bar, the effective O₂partial pressure would be 0.05 bar in the process flow stream.

Theelectrochemical pump602 inFIG. 6 is essentially similar to pump700 shown inFIG. 7 except that the O₂flow is in the direction to move O₂from the air purge stream to the process flow stream, pumping O₂into the process flow stream. The current flow would be in the opposite direction and the effect of the O₂partial pressure on the air purge side would be opposite. A higher air pressure in theair purge channel705 would help drive the O₂into the process flow stream and require a lower pumping voltage.

The use of an electrochemical pump to move O₂from air into the process and from the process out to air would eliminate the need to store O₂in 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 O₂reactant. 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 O₂into oxygen ions and recombine the oxygen ions to form O₂and 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 at601 and602 ofFIG. 6 with two vessels containing an oxygen storage material that would absorb and release the oxygen as the electrochemical pump removes it from or adds it to the process stream. This could provide an advantage if any SO₃or SO₂were to leak across the electrochemical cell or pump since it would be trapped within the vessels holding the oxygen storage material and would not be released to the ambient air. The oxygen storage material could be contained in two vessels, one for each

electrochemical pump

601 and602 or it could be a single vessel connected to both

electrochemical pump

601 and602.

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 O₂from 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 O₂into the process stream or out of the process stream would be the O₂partial pressure on the opposite side of the membrane. A high O₂pressure or high air pressure would drive O₂into the process and a low O₂partial pressure or air pressure would drive O₂out of the process.

Electrochemical SO₃Decomposition Reactor

The decomposition of SO₃, equation 9, is limited by thermodynamics at the selected operating temperature. This is shown inFIG. 1 for a pressure of 1 bar as one embodiment of the invention. At the operating pressure of 5 to 15 bar, these curves would be shifted to the right and the equilibrium limited conversion to SO₂can be low as shown inFIG. 5. Reducing O₂partial pressure allows a higher equilibrium conversion. One approach to reducing the O₂partial pressure is to make thereactor301 inFIG. 4 a combined O₂cell to pump oxygen and SO₃decomposition reactor.FIG. 8 shows such areactor800 that can be placed in the location occupied byreactor301 inFIG. 4 in one embodiment of the invention. The SO₃enters reactor throughchannel801. One wall of the reactor is formed by theelectrode802 that is coated with acatalyst layer803. The SO₃reacts on thecatalyst803 to decompose into SO₂and O₂. The O₂diffuses to electrode802, is dissociated into oxygen ions atelectrode802, is pumped throughelectrolyte807 to theopposite electrode804 where it is recombined to form O₂and passes intochannel805 where it is swept away in the flowing air purge stream. By controllingvoltage806, the O₂partial pressure at the interface between thecatalyst layer803 andelectrode802 can be controlled to the desired value. The O₂partial pressure in the catalyst layer could be lowered to a very low value and as shown inFIG. 5, the equilibrium SO₂conversion can be very high. This could also allow thereactor800 to operate at a lower temperature ranging from 650 to 800° C.

Direct Electrochemical Conversion of SO₂to Electrical Power

The process shown inFIG. 6 produces energy by oxidation of the stored SO₂oxidation with O₂to produce heat inreactor604. This heat can then be used to produce steam which would power a steam turbine to produce electricity. Direct conversion of the SO₂into electrical energy would be more desirable and could be substantially more efficient.FIG. 9 shows asystem900 with direction conversion of SO₂into electrical energy in one embodiment of the invention. This device can be referred to as an SO₂fuel cell or an SO₂electrochemical generator. For the anode of the SO₂fuel cell, SO₂enterschannel901 and reacts onelectrode902 with oxygen ion, O²⁻, to form SO₃and 2 electrons, 2e⁻. The O²⁻ is supplied through theelectrode902 fromelectrolyte903. For the cathode of this fuel cell, O₂enterschannel904 and reacts atelectrode905 to receive 2 electrons per O atom and form O⁻²at the electrode. The electrons flow from theanode902 throughelectric line906 to theload907 andelectric line908 to thecathode905. Theelectrolyte903 must transport O²⁻ from thecathode905 to theanode902. In one embodiment of the invention, this is done by the reactions shown inFIG. 9. At the cathode, O²⁻ reacts with 2H⁺ in the electrolyte to form H₂O. The H⁺ is transported from the anode and H₂O is transported to the anode through theelectrolyte903. At the anode, H₂O dissociates to 2H⁺ and O²⁻ thus supplying the H⁺for transport to the cathode and oxygen ion, O²⁻, for the anode reaction to oxidize SO₂to SO₃. An appropriate electrolyte for the SO₂fuel cell ofFIG. 9 would be sulfuric acid, H₂SO₄or any electrolyte that transports H⁺ and H₂O. The sulfuric acid could range from a dilute sulfuric acid to a nearly 100% concentrated sulfuric acid. The electrolyte could also be formed of a membrane or other porous material that is impregnated or filled with sulfuric acid. Alternatively the electrolyte could be a membrane or porous medium with the ability to transport H₂O and H⁺. One such embodiment would be Nafion, a sulfonated tetrafluoroethylene copolymer. The transport of H⁺is equivalent to the transport of hydronium ion, H₃O⁺. The overall electrochemical process shown inFIG. 9, identical to reaction equation 10, has a standard Gibbs free energy of reaction of −70.9 kJ/mol at 25° C. Using the Nernst equation, this equates to a standard cell potential of +0.37 volts. The operating temperature of this cell in one embodiment of the invention can vary over a wide range from 0° C. to 300° C. A lower temperature will provide a higher driving force and a larger cell voltage. In one embodiment, the operating temperature is in the range of 25° C. to 200° C.

An alternative embodiment of the invention to the process shown inFIG. 9 where the electrolyte transports protons, H⁺, and water, H₂O, the electrolyte could transport only protons and use water supplied with the SO₂flow inchannel901. In this case the reaction of water to form H⁺ and O²⁻, H₂O→2H⁺+O²⁻, would occur on the electrode inchannel901 using water supplied in the stream flowing throughchannel901.

Thermal Solar Energy Storage System

The process and system design described above can be used as system for the capture and storage of solar energy.FIG. 10 illustrates one such of asystem1000 in one embodiment of the invention. Insystem1000,solar energy1001 is captured in thecollector1002 as high temperature heat. SO₃flows intocollector1002 throughstream1003.Collector1002 is filled with a catalyst for the conversion of SO₃to SO₂and O₂and captures the thermal energy from solar radiation. The SO₂and O₂flow throughprocess stream1004 and then is split intosection1005 or to processstream1006. Insection1005, the SO₂and O₂are stored for use during low solar energy periods. The SO₂and O₂that flow throughprocess stream1006 flow intoreactor1007 where the SO₂and O₂react to produce SO₃and release thermal energy, heat.Reactor1007 could be a heat exchange reactor such as a tube and shell reactor containing a catalyst for the conversion of SO₂and O₂into SO₃and heat. Water or low temperature steam could flow into reactor throughline1008 to produce high temperature steam that exitsreactor1007 throughline1009 tosteam turbine1010 where the energy is extracted to driveelectric generator1011 to produceelectric power output1012. The steam or condensed water flows out of thesteam turbine1010 tostorage vessel1013 and tocirculation pump1014 and back toreactor1007. The SO₂and O₂storage section1005 and the SO₃storage section1015 are not shown in detail as they can take any of the forms described in other parts of this specification and other forms that can be implemented by those familiar with the art.

The system shown inFIG. 10 is one example of a solar energy generation system using the aspects described in this specification. This system uses the solar energy to generate high temperature steam that is used to drive a steam turbine to produce electrical power. This system is only described for illustration purposes and the system can take many forms. In other embodiments of the invention, the solar energy collection components can be a central tower structure with surrounding solar reflectors that concentrate the solar energy on the central energy collector. An alternative embodiment is a parabolic trough collector that concentrates the solar energy on a linear tube reactor. A third embodiment would be a circular parabolic mirror that concentrates the solar energy on a central thermal collector. The SO₂and O₂storage system could be a system that liquefies the SO₂and stores the SO₂as a liquid at temperatures near ambient. The O₂can be stored by absorption on an oxygen storage material or by adsorption on the surface of an adsorbent. The gaseous O₂can be compressed at high pressure to store the O₂in a pressure vessel. An alternative embodiment is to use an oxygen transfer membrane to extract O₂from the process stream and move it into the surrounding air. Such a membrane could take the form of an electrochemical cell or pump that transfers the O₂as oxide ions such as stabilized ZrO₂membranes operating at high temperatures. The electrical power generation process can also take many forms. It can take the form of a thermal process wherein the oxidation of SO₂to SO₃produces heat that is used to produce steam that then drives a turbine and electric generator as shown inFIG. 10. Alternatively the heat from the decomposition reactor can be used to heat a heat transfer fluid which is then used to generate steam in a boiler or to heat other fluid to drive a turbine or other engine to produce electrical power. The engine can take the form of a turbine, reciprocating engine, sterling engine etc.

Direct Electrochemical Solar Energy System

FIG. 11 illustrates an alternative solar energy system in one embodiment of the invention.System1100 is similar tosystem1000 inFIG. 10 but with a modification in the method of electric power generation from the oxidation of SO₂to SO₃. The SO₂generated in solarenergy collection reactor1101 will flow to SO₂

storage system

1102 or to the O₂removal section1103 andelectrochemical generator1104 which produces anelectrical power output1110.Electrochemical generator1104 could be similar to that described inFIG. 9. The SO₂stream flows to theanode side1105 of the cell and an oxygen containing stream such as air flows to the cathode side of thecell1106. The SO₃oxidation product from theelectrochemical generator1104 is either recycled to thesolar energy collector1101 or stored in SO₃

storage section

1107 where the SO₃is stored for use during low solar energy periods. Thecathode side1106 of theelectrochemical generator1104 can be supplied with oxygen by a purge airstream using blower1109. Other oxidants could be used. The O₂removal section1103 could be similar to the oxygen storage material or the electrochemical pump described elsewhere in the specification and shown inFIG. 7. Alternatively, it could be a membrane based system that uses oxygen partial pressure difference to extract O₂fromstream1111. The level of oxygen removal required will depend on the sensitivity of theanode1105 to O₂. In general, the O₂level does not have to be extremely low since any oxygen present at the anode will probably just react directly with the SO₂to form SO₃thus bypassing the electrochemical conversion and lead to a lower electrical generation efficiency. It is not expected that low levels of O₂at the anode will inhibit the anode electrochemical reaction.

In one embodiment of this invention, catalysts for the oxidation of SO₂to SO₃would 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 SO₃to SO₂and O₂would be done on similar catalysts as for the oxidation as described above.

Anode materials for the electrochemical generator described inFIG. 9 would be similar to the catalysts used for the catalytic oxidation of SO₂and described above. A preferred catalyst would be a porous platinum layer since this would act as an electron conducting layer as well as a catalyst for the oxidation of SO₂using oxide ions transported by the electrolyte.

Cathode catalysts for the electrochemical generator ofFIG. 9 would be similar to those described for the hydrogen oxygen fuel cell and are well described in the literature and well known.

Alternative System Design

One alternative embodiment is shown inFIG. 12. In this embodiment,system1200 shows a solar tower or parabolic trough solar collector1201 located proximate to a countercurrent heat exchanger1202 that reduces the

process flow streams

1203 and1204 to a temperature near ambient temperature so that the process streams connecting the solar collectors to the generating plant will be near ambient temperature. This provides the advantage of reducing cost and complexity of the piping system by eliminating the need for high temperature insulation and high temperature materials. The countercurrent heat exchanger1202 can be located at each solar collector unit or at groups of closely spaced solar collectors depending on collection system design and cost requirements.Heat exchanger1202 could be combined with solar collector1201 as a single unit or an integrated unit.

Stream

1204 from the solar collector field will then either split a portion of the flow to the SO₂and O₂storage section1205 or to processstream1206 and then toheat exchanger1207.Heat exchanger1207 could also be a counter current heat exchanger which would increase the temperature of theprocess stream1206 to the required temperature for the SO₂oxidation reactor1208 where the chemical energy is converted to heat. This temperature ofreactor1208 would be in the range of 350 to 600° C. The SO₃leaving the SO₂oxidation reactor1208 flows throughprocess stream1210 back toheat exchanger1207 where the temperature is adjusted to near ambient temperature for SO₃storage or return to the solar collector field throughprocess stream1203.

In one embodimentsteam generator section1209 could consist of a steam turbine and an electric power generator. In another embodiment the heat release and electric power generating section shown asreactor1208 andsteam generator1209 could be replaced with thedirect electrochemical generator1104 shown inFIG. 11.

In other embodiments, the heat exchangers shown inFIG. 12 could be co-current heat exchangers or combinations of heat exchangers. Also, the heat exchangers could be separated into several heat exchangers to provide an intermediate temperature for SO₂, O₂and SO₃storage or for other purposes.

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 SO₃into SO₂and O₂;

a second reactor at the second location converting the chemical energy to a second thermal energy by oxidizing SO₂with O₂to produce SO₃, the second reactor evaporating water into the steam for the steam turbine;

liquid SO₂storage of; and

liquid SO₃storage of;

wherein an output of the first reactor is coupled to both an input of the second reactor and the liquid SO₂storage, and an output of the sec reactor is coupled to both an input of the first reactor and the liquid SO₃storage;

wherein a flow split of SO₂and O₂from the first reactor to the second reactor and the liquid SO₂storage is variable, and a flow split of SO₃from the second reactor to the first reactor and the liquid SO₃storage is variable.

2. The system ofclaim 1 wherein the thermal source is a solar concentrator that focuses sunlight on a receiver to produce a temperature high enough to decompose SO₃into SO₂.

3. The system ofclaim 1 wherein at least one of the first reactor and the second reactor comprises a catalyst for converting SO₃into SO₂and O₂or SO₂and O₂to produce SO₃.

4. The system ofclaim 1 wherein the first reactor converts SO₃into SO₂and O₂at a temperature in the range of 600 to 1200° C.

5. The system ofclaim 1 wherein the second reactor converts SO₂and O₂to produce SO₃at a temperature below about 800° C.

6. The system ofclaim 1 further comprising an oxygen storage material that chemically binds and stores a substantial portion of O₂produced by the first reactor converting SO₃into SO₂and O₂.

7. The system ofclaim 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 ofclaim 7 wherein the pressures are maintained in the range of 5 to 15 bar.

9. The system ofclaim 6 wherein at least one of the oxygen storage material and a temperature of the oxygen storage material is chosen to adsorb and release O₂at an O₂partial pressure in the range of 0.01 to 3 bar.

10. The system ofclaim 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 O₂formed 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 O₂from the ambient air into the process stream to be used by the second reactor.

11. The system ofclaim 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 O₂formed decomposing SO₃into SO₂and O₂from the process stream and pumps it into the vessel containing the oxygen storage material;

the second electrochemical cell pumps O₂from the other vessel containing the other oxygen storage material into the process stream to be used in oxidizing SO₂and O₂to produce SO₃.

12. The system ofclaim 1 wherein the first reactor includes an electrochemical cell that removes oxygen from the first reactor to allow a higher conversion of SO₃to SO₂.

13. The system ofclaim 1, wherein the second reactor includes an electrochemical cell that produces electrical power from the conversion of SO₂and O₂to 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 SO₃into SO₂and O₂;

converting the chemical energy to one of thermal energy or electrical energy in a second reactor at the second location by oxidizing SO₂with O₂to produce SO₃;

using stored SO₃liquid for the first reactor and storing a portion of SO₂from 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 SO₂liquid for the second reactor and storing SO₃liquid 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 SO₂liquid, and sorting SO₃liquid comprise:

controlling a flow split of SO₂and O₂from the first reactor to the second reactor and a liquid SO₂storage; and

controlling a flow split of SO₃from the second reactor to the first reactor and a liquid SO₃storage.

15. The method ofclaim 14, further comprising using a solar concentrator to focus sunlight on a receiver to produce a temperature high enough to decompose SO₃into SO₂.

16. The method ofclaim 14 wherein at least one of the first reactor and the second reactor comprise a catalyst for converting SO₃into SO₂and O₂or SO₂and O₂to produce SO₃.

17. The method ofclaim 14, further comprising operating first reactor at a temperature in the range of 600 to 1200° C.

18. The method ofclaim 14, further comprising operating second reactor at a temperature below about 800° C.

19. The method ofclaim 14, further comprising chemically binding at least a portion of O₂produced in the first reactor by the decomposition of SO₃into SO₂and O₂in an oxygen storage material.

20. The method ofclaim 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 ofclaim 20 wherein the pressures are maintained in the range of 5 to 15 bar.

22. The method ofclaim 14, further comprising:

pumping, with a first electrochemical cell, O₂produced in the first reactor by the decomposition of SO₃into SO₂and O₂to one of ambient air and a vessel containing an oxygen storage material;

pumping, with a second electrochemical cell, O₂from one of the ambient air and another vessel containing another oxygen storage material to react with SO₂in the second reactor to produce SO₃.