Patent Publication Number: US-2013234069-A1

Title: Solar Receivers for Use in Solar-Driven Thermochemical Processes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Ser. No. 13/538,304, entitled “Reactor, System, and Method for Solid Reactant-Based Thermochemical Processes”, filed Jun. 29, 2012, the contents of which are incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention is in the field of solar collectors for collecting and concentrating solar radiation to produce heat to drive high temperature thermal processes. 
     BACKGROUND OF THE INVENTION 
     Thermochemical processes, such as water splitting for power generation, CO 2  capture, and/or syngas production require high temperatures, e.g., greater than 1000° C.-1400° C., to drive these processes. Solar absorption systems are means for generating heat to drive such processes. The system contains a receiver which collects and concentrates the solar radiation to generate heat. The heat can be stored in a working fluid to be delivered later to drive a process, such as a chemical reaction. However, typical modem receivers use molten-salts for heat storage and therefore cannot generate heat at the necessary temperatures to drive high temperature processes, such as those described above. For example, modern molten salt towers are typically limited to a maximum temperature of about 565° C. for the following reasons: (1) the molten nitrate salts are thermally stable only up to about 600° C. and (2) above 565° C. chromium diffuses to the grain boundaries in stainless steel which would be quickly eroded by the flowing salt. 
     Therefore, there exists a need for solar receivers that contain a heat transfer fluid which is stable at the temperatures necessary to drive high temperature thermochemical processes. There is also a need for receiver components, such as pipes, housings, valves, etc. which can withstand such high temperatures. The high temperature heat itself can also be used to directly generate electricity through a high temperature heat engine such as a Brayton or combined Brayton+Rankine cycle. 
     Therefore, it is an object of the invention to provide solar receivers that contain a heat transfer fluid or working fluid which is stable at the temperatures necessary to drive high temperature thermochemical processes and methods of making and using thereof. 
     SUMMARY OF THE INVENTION 
     Solar receivers which produce heat at very high temperatures (in excess of 1000° C.) are described herein. In some embodiments, the receiver is in a tower configuration. In some embodiments, the tower contains a cavity receiver which produces the high temperature heat and radiates the heat to a containment element (e.g., pipe) that contains a heat transfer fluid which absorbs the heat. The fluid is preferably a material which is thermally and chemically stable at the temperatures involved. The heat transfer fluid absorbs the heat and can deliver it to a reactor system to drive an endothermic reaction, such as thermochemical water splitting, CO 2  capture, and/or syngas production. Alternatively, the heat can be used to directly generate electricity through a high temperature heat engine such as a Brayton or combined Brayton+Rankine cycle. 
     The receiver can be incorporated into a solar absorber system, wherein the system contains one or more collectors which collect solar radiation and direct it to the receiver. In some embodiments, the system contains a plurality of heliostats that direct light to the receiver. The heliostat can be modified to improve efficiency, for example, by coating the mirrors with a material which converts the highest frequency photons at higher efficiency and then reflects the remaining photons to the receiver which should improve overall system efficiency. In other embodiment, the collectors are one or more parabolic mirrors which couple the light to fiber optical cables which avoids the need for a power tower. 
     The heat transfer fluid delivers the heat to a reactor system. In one embodiment, the reactor is for thermochemical water splitting. In one embodiment, the heat transfer fluid is pumped through pipes or other elements that are coated with a solid reactant material which catalyzes the water splitting reaction. Water is introduced into the reactor and upon contact with the pipes, is split into hydrogen and oxygen gases. The gases can be pumped from the reaction chamber and stored in appropriate vessels until needed for power generation or other applications. The containment elements are preferably formed of one or more refractory materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a representation of a solar receiver described herein. 
         FIG. 2  is a representation of a cavity-receiver. 
         FIG. 3  is a representation of a solar absorber system containing a tower receiver and a plurality of heliostats. 
         FIG. 4  depicts a reaction chamber showing how the pipes converge to its pump, which then distributes the flow of a working fluid to the solid reactant coated pipes (SRCP) inside the reaction chamber. 
         FIG. 5  depicts an exemplary power generation system. 
         FIG. 6A  depicts the lattice structure of an exemplary solid reactant. 
         FIG. 6B  depicts the phase diagram of a solid reactant. 
         FIG. 7A  illustrates an overview of the thermal and chemical processes involved in a thermochemical water splitting cycle.  FIG. 7B  graphically illustrates a thermochemical step  1  (TCS 1 ).  FIG. 7C  graphically illustrates a thermochemical step  2  (TCS 2 ).  FIG. 7D  graphically illustrates a thermochemical step  3  (TCS 3 ).  FIG. 7E  graphically illustrates a thermochemical step  4  (TCS 4 ) according to one embodiment. 
         FIG. 8  depicts the top view of a reaction chamber array showing the network of pipes that connect each reaction chamber with all others. 
         FIG. 9  depicts a flow diagram of a method for solid based reactant thermochemical solar power generation according to an embodiment, 
         FIG. 10  depicts a high-level block diagram of a computing device suitable for use in implementing various functions described herein. 
         FIG. 11  is representation of a cutaway of the reactor. 
         FIG. 12  is a representation of an aerial view of the reactor. 
         FIG. 13  is a representation of a close-up view of the reactor. 
         FIG. 14  is a representation of the reactor pipes. 
         FIG. 15  is a representation of an array of reactors. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     I. Definitions 
     “Heliostat”, as used herein, refers to a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun&#39;s apparent motions in the sky. 
     “Stable”, as used herein, refers to, typically refers to the absorbing medium and means that it is chemically stable (i.e., does not react with the containment materials and/or other materials in the absorbing medium and/or degrade) and thermally stable (i.e., does not boil or chemically disassociate in an undesirable way). 
     “Containment materials” or “containment element”, as used herein, refers to a component of the receiver and/or reactor system that contains the heat transfer fluid, such as a pipe, valve, pump, etc. or materials that are used to form such components. 
     “Heat transfer fluid”, as used herein, refers to the medium which absorbs the heat collected by the receiver. The fluid can be transported to a reactor system which the heat is used to drive one or more endothermic reactions. 
     II. Solar Absorption system 
     A. Receiver 
     The systems described herein contain a receiver that receivers the electromagnetic radiation (e.g., sunlight) reflected from a collector, such as a heliostat. The receiver can be in any configuration provided it is compatible with the devices for reflecting sunlight. In medium to large scale operations, the receiver may be in the form of a tower, referred to as the collector tower. The collector tower is sometimes referred to as a “power tower”. A representation of a solar receiver tower is shown in  FIG. 1 . Compared to linear concentrating solar power plants such as parabolic trough systems and linear fresnel reflectors, solar tower plants can reach higher concentrations of solar radiation and therefore reach significantly higher temperatures. 
     In some embodiments, the receiver or receiver tower is a thermal receiver. The thermal receiver produces heat in the form of a heated medium, such as a high temperature liquid metal. The heat can be used for a variety of functions, including process heat to drive a chemical reaction or reactions for the production of electricity, The advantage of a solar tower with a thermal receiver is the possibility to store the heat in a thermal storage and to use the heat at a later time independently from collection of the solar radiation. This form of thermal storage is significantly less expensive than batteries, which are required for technologies producing the electricity directly such as wind turbines or photovoltaic systems. 
     In one embodiment, the receiver/tower generates heat at a temperature sufficient to drive high temperature processes, such as thermochemical water splitting, CO 2  capture, and/or syngas production. Typical receivers in the art use molten-salts for heat storage and therefore cannot generate heat at the necessary temperature to drive high temperature process, such as those described above. For example, modem molten salt towers are typically limited to a maximum temperature of about 565° C. for two reasons: (1) the molten nitrate salts are thermally stable only up to about 600° C. and (2) above 565° C. chromium diffuses to the grain boundaries in stainless steel which would be quickly eroded by the flowing salt. 
     In contrast, the receiver/tower described herein generates heat at temperatures greater than 1000° C., 1100° C., 1200° C., 1300° C., 1400° C., 1500° C. or greater, In a particular embodiment, the temperature is about 1500° C. Secondary concentrators can be used to achieve high concentration ratios (e.g., greater than 1000 suns) in order to achieve the temperatures described herein. All components of the receiver, e.g., receiver housing, pipes containing the heat transfer fluid, etc. should designed to be thermally stable up to temperature of about 1500° C. or greater. 
     At the temperatures described herein, radiation loss is increased. Therefore, the receiver should be designed to minimize radiation loss at these temperatures. In some embodiments, the receiver/tower contains a cavity receiver. A cavity receiver is a well-insulated enclosure, with a small opening to let in concentrated solar energy, which approaches a blackbody absorber in its ability to capture solar energy. 
     In some embodiments, the cavity receiver typically contains a plurality of apertures through which the solar radiation enters the receiver. A representation of a cavity receiver is shown in  FIG. 2 , The cavity can be any shape, such as spherical, circular, cylindrical, dome-shaped, heteroconical, elliptical, or conical. The receiver ( 2 ) contains a plurality of apertures ( 4 ) which allow the reflected light to enter the receiver. The amount of heat lost due to re-radiation to the atmosphere depends on the amount of surface area that is exposed (aperture size) to the atmosphere, where the sunlight is concentrated. It is therefore advantageous to minimize this area, so that losses are minimized and efficiency is maximized. In some embodiments, the aperture(s) are covered or contain a transparent window which allows light to pass through but serves as a chemical barrier between the cavity and the outside environment, e.g., an inert (low oxygen pressure) environment is maintained inside the cavity. 
     The heat transfer fluid ( 6 ) flows through the cavity receiver in a containment element, e.g. a pipe ( 8 ). The radiation that enters the receiver is absorbed by the receiver and radiated to the outer surface of the containment element. The outer surface of the containment element and the receiver are at the same temperature and radiate heat between their surfaces. The heat absorbed by the outer surface of the containment element is conducted through the pipe to the heat transfer fluid contained therein. Absorption of this radiation raises the temperature of the heat transfer fluid to the necessary working temperature. 
     The waste heat from a solar power plant, to the extent any exists, can be used to power other processes, such as solar desalination processes. Thereby, solar power plants are also used to desalinate water if installed near the sea. This is a very useful application in sunny and dry regions or islands. 
     B. Heat Transfer Fluid 
     The system generates high concentrations of solar radiation and thus very high temperatures, e.g. in excess of 1000° C., 1100° C., 1200° C., 1300° C., 1400° C., or 1500° C., The heat can be stored in a heat transfer fluid. The heat transfer fluid must be a material that can absorb heat at these temperatures without boiling or chemically disassociating in an undesirable way. In some embodiments, the heat transfer fluid can boil, and can then be condensed elsewhere to deliver the heat. Examples include, but are not limited to, lithium metal. Suitable materials include, but are not limited to, liquid inorganic materials, such as a liquid metals, liquid metal oxides, liquid mixed metal oxides, liquid organic materials, and combinations thereof. 
     In one embodiment, the heat transfer fluid is a liquid metal. Suitable metals are those that have low melting points, high boiling points, and are inexpensive. In some embodiments, the metals are liquid at, and do not boil at, temperatures greater than about 1000° C., 1100° C., 1200° C., 1300° C., 1400° C., 1500° C. or greater. In a particular embodiment, the metal is a liquid and does not boil at about 1500° C. Exemplary metals include, but are not limited to, tin, aluminum, gallium, sodium, lead, lithium, bismuth, calcium, and combinations thereof. In one embodiment, the liquid metal is tin. Tin is unreactive (e.g., doesn&#39;t react with carbon or tungsten) and is very inexpensive. One can purchase multi-ton quantities of 98% pure tin for $1-2/lb compared to $10/lb for 99.8% pure. Purity of the heat transfer fluid is generally not a concern provided it does not affect the chemical and thermal stability. In another embodiment, the material is a liquid organic or organometallic material. 
     The receiver contains one or more components which contain the heat transfer fluid. These components must be fabricated from materials which can withstand the high temperatures of the heat transfer fluid, e.g. greater than 1000° C., 1100° C., 1200° C., 1300° C., or 1400° C., preferably about 1500° C. and are not corroded by the heat transfer fluid. Exemplary materials include, but are not limited to, metal, metal oxides, mixed metal oxides, metal carbides, metal nitrides, metal borides, metal silicides, graphite, other refractory materials, and combinations thereof. “Refractory” refers to a group of materials, typically metals, that have exceptionally high melting points and are resistant to wear, corrosion and deformation. Exemplary metals include, but are not limited to, Molybdenum (Mo), Niobium (Nb), Rhenium (Re), Tantalum (Ta), and Tungsten (W). “Refractory” is also sometimes defined to include Chromium (Cr), Hafnium (Hf), Iridium (Ir), Osmium (Os), Rhodium (Rh), Ruthenium (Ru), Titanium (Ti), Vanadium (V), and Zirconium (Zr). Other materials that can be used include, but are not limited to, aluminum oxide (Al 2 O 3 ), zirconia (ZrO 2 ), and magnesia (MgO). These materials are highly corrosion resistant, and stable at high temperatures. Transition metal carbides, for example, are also known to exhibit non-wetting interactions with aluminum, gallium, and tin. 
     C. Collector 
     The system contains one or more collectors that collect sunlight and direct it to the receiver. Any suitable collector or collectors can be used provided it supplies a sufficient concentration of sun light, alone or in combination with a secondary concentrator, to produce the necessary high temperature heat. In some embodiments, the collectors are heliostats that continually reflect sunlight toward a predetermined target, e.g., the receiver. The receiver can be any distant from the collector, provided the receiver effectively receives the reflected light. In another embodiment, a collector or collectors, such as parabolic mirrors couple the light directly to fiber optic cables bypassing the need for a tower. 
     1. Heliostat 
     In one embodiment, the collector is a plurality of heliostats. A representation of a plurality of heliostats and a tower receiver are shown in  FIG. 3 . 
     The reflective surface of the mirror is typically kept perpendicular to the bisector of the angle between the directions of the sun and the target as seen from the mirror. In almost every case, the target is stationary relative to the heliostat, so the light is reflected in a fixed direction. 
     The heliostats can be controlled by computers, particularly for large heliostat arrays. The computer is given the latitude, longitude, and elevation of the heliostat&#39;s position on the earth and the time and date. From these, using astronomical theory, it calculates the direction of the sun as seen from the mirror, e.g. its compass bearing and angle of elevation. Then, given the direction of the target, the computer calculates the direction of the required angle-bisector, and sends control signals to motors, often stepper motors, so they turn the mirror to the correct alignment. This sequence of operations is repeated frequently to keep the mirror properly oriented (e.g., every 4 seconds to one minute). Large installations such as solar-thermal power stations include fields of heliostats containing many mirrors. Usually, all the mirrors in such a field are controlled by a single computer. 
     As discussed above, the movement of most modern heliostats employs a two-axis motorized system, controlled by computer. Almost always, the primary rotation axis is vertical and the secondary horizontal, so the mirror is on an alt-azimuth mount. 
     One simple alternative is for the mirror to rotate around a polar aligned primary axis, driven by a mechanical, often clockwork, mechanism at 15 degrees per hour, compensating for the earth&#39;s rotation relative to the sun. The mirror is aligned to reflect sunlight along the same polar axis in the direction of one of the celestial poles. There is a perpendicular secondary axis allowing occasional manual adjustment of the mirror (daily or less often as necessary) to compensate for the shift in the sun&#39;s declination with the seasons. The setting of the drive clock can also be occasionally adjusted to compensate for changes in the Equation of Time. The target can be located on the same polar axis that is the mirror&#39;s primary rotation axis, or a second, stationary mirror can be used to reflect light from the polar axis toward the target, wherever that might be. 
     The alt-azimuth and polar-axis alignments are two of the three orientations for two-axis mounts that are, or have been, commonly used for heliostat mirrors. The third is the target-axis arrangement in which the primary axis points toward the target at which sunlight is to be reflected. The secondary axis is perpendicular to the primary one. Heliostats controlled by light-sensors have used this orientation. A small arm carries sensors that control motors that turn the arm around the two axes, so it points toward the sun. (Thus this design incorporates a solar tracker.) A simple mechanical arrangement bisects the angle between the primary axis, pointing to the target, and the arm, pointing to the sun. The mirror is mounted so its reflective surface is perpendicular to this bisector. 
     There are heliostat designs which do not require the rotation axes to have any exact orientation. For example, there may be light-sensors close to the target which send signals to motors so that they correct the alignment of the mirror whenever the beam of reflected light drifts away from the target. The directions of the axes need be only approximately known, since the system is intrinsically self-correcting. 
     Conventional designs for the heliostat&#39;s reflective components typically utilize a second surface mirror, The sandwich-like mirror structure generally contains a steel structural support, an adhesive layer, a protective copper layer, a layer of reflective silver, and a top protective layer of thick glass, This conventional heliostat is often referred to as a glass/metal heliostat. Alternative designs incorporate recent adhesive, composite, and thin film research to bring about materials costs and weight reduction. Some examples of alternative reflector designs are silvered polymer reflectors, glass fiber reinforced polyester sandwiches (GFRPS), and aluminized reflectors. 
     Other heliostat or heliostat-like systems can also be used. For example, an array of small mirrors (e.g., 3 in×3 in) combined with injection molded plastic joysticks to control tracking and a multiplexing fluid control scheme may also be used. 
     In some embodiments, the mirrors can be coated with a material, such as a polymeric solar cell, which converts the highest frequency photons at higher efficiency and then reflects the remaining photons to the receiver which should improve overall system efficiency. The strategy is to convert every wavelength at the high efficiency possible using either photovoltaic (PV) or concentrated solar power (CSP) in a hybrid infrastructure. 
     2. Collector Coupled to Fiber Optic Cables 
     In another embodiment, the collector or plurality of collectors can couple light directly into fiber optic cables. For example, parabolic dishes can couple the light directly into fiber optic cables. The cables can be run to a central facility on the ground, without the need for a power tower. The light is preferably coupled into at high efficiency in order to obtain the high temperatures necessary. 
     II. Reactor system 
     The solar absorption system described above can be incorporated into a reactor system for conducting high temperature thermal process, such as thermochemical water splitting, CO 2  capture, and/or syngas production. In one embodiment, the solar absorption system is used to drive a thermochemical water splitting reaction. The water splitting reaction is shown below: 
       (H 2 O→H 2 +1/2O 2 )
 
     As discussed above, energy is collected from the sun in the form of light via a heliostat or other solar collector. It is then converted to thermal energy when the reflected light is focused onto an absorber medium, such as a liquid metal, metal oxide, mixed metal oxide, molten salt, or organic material at the focal point of the collector (e.g., heliostat). The thermal energy that is absorbed is then primarily used to supply heat for an endothermic chemical reaction and can also be used in other preheating/reheating processes within the cycle. The thermal energy is converted to chemical energy through the heat driven chemical reaction(s). The chemical energy can be stored indefinitely without parasitic leakage. The high temperature heat itself can also be used to directly generate electricity through a high temperature heat engine such as a Brayton or combined Brayton+Rankine cycle. 
     The cycle has five important advantages over other alternative energy technologies: (1) energy can be buffered and stored indefinitely in simple pressure vessel gas storage; (2) the thermal to chemical energy conversion process can reach high efficiency; (3) the system is highly scalable, and can be used for utility scale (MW) power production at both peak and base loading conditions; (4) there is no net chemical footprint on the environment; and (5) unlike thermal storage, it can be efficient at less than 10 MW/hr and storage scales can be continually expanded in a modular way. 
     The reactor system is primarily described in the context of solar driven thermochemical water splitting where the primary reactant is in the solid phase. However, the system can be optimized for other thermochemical processes, such as the conversion of carbon dioxide and water into syngas or CO 2  captured from flue gas. 
     To obtain high efficiency in a solar driven thermochemical cycle, the thermal energy that is absorbed/captured must be utilized effectively to minimize heat loss to the surroundings.  FIG. 4  shows the recovery of heat as the solid reactant is cooled to the lower temperature for the water splitting step. At high temperatures, radiative heat transfer becomes increasingly dominant; however, it is always present even when conductive and convective channels for heat transfer are available. Therefore, direct physical contact between the two streams of solid reactant would be optimal, because radiative heat transfer will also assist. This can be achieved by using a working fluid (e.g., heat transfer fluid described above) to transfer heat between the solid reactant in the up (heating) and down (cooling) streams. A design that facilitates conductive/convective heat transfer will be more optimal than a design that relies on radiative heat transfer alone. 
     Thermochemical reactions can be strongly impacted by the temperature and pressure of the gas species. It is therefore advantageous to employ a system design that allows the gas flow during one reaction (thermal reduction) to be controlled independently of other reactions (water splitting). Using an array of sealed reaction chambers allows the gas temperature and pressure to be controlled separately during each reaction. 
     Thermochemical cycles can be most efficient when certain reactions are carried out at the highest temperatures possible. High temperatures, however, can induce significant safety risks. When machines with moving parts are maintained at high temperatures or are cycled between temperature extremes the likelihood of mechanical failure increases. Thus, for safety and reliability concerns, it is advantageous to avoid having high temperature moving parts. In the present invention, moving parts typically associated with thermomechanical cycles are avoided in favor of an extensive network of permanent piping and seals. Thus, the only moving parts are contained in the fluid pumps and computer controlled valves that drive and control the various flows within the system to transport thermal energy and separate the reactants and products. The cyclic nature of the thermochemical processes in the present design arises from the cyclic sequence of opening and closing of computer controlled valves mounted inline with each pipe. 
     It is advantageous to employ a design that allows for dynamic local temperature, pressure and composition measurements. This in turn allows the reactions to be monitored so that incomplete reactions are avoided and feedback loops can be used to automate the response of internal processes. By using an array of individual sealed reaction chambers the present invention allows for such measurements and control. 
     To maximize thermochemical efficiency, it is advantageous to cycle the maximum amount of reactant material as possible. This means that the solid reactant should be packed as densely as possible maximizing the reactive surface area to volume ratio, while simultaneously minimizing the area in thermal contact with the surroundings. Minimizing the area that must be insulated keeps conductive and convective heat losses to a minimum. For illustrative purposes.  FIG. 4  depicts a reaction chamber ( 400 , top view ( 405 ), side view ( 415 ), and front view ( 425 )) showing how the pipes converge ( 410 ) to the pump, which then distributes the flow of the heat transfer fluid ( 420 ) to the solid reactant coated pipes (SRCP) inside the reactor chamber. The depiction in  FIG. 4  shows 8 pipes. Other embodiments are contemplated that would contain fewer or more densely packed SRCPs are necessary. Such arrangements can minimize thermal losses due to conduction and convection in comparison to the amount of chemical output. Further, the volume that can be occupied by reactant gas species is minimized. These reactant gases can increase the transient response time of each reaction and can reduce the total amount of gas that must be heated and separated, and therefore translates to additional energy savings. Furthermore, since the total output is proportional to the volume of material being cycled, and the heat losses are proportional to the area exposed to the environment, it is advantageous to maximize the volume/surface area ratio for the reactant to the reactor. This ratio generally increases with the overall system size, therefore the present invention can become more efficient when scaled up to larger sizes. Other embodiments are contemplated where the means of introducing the reactant and product gases are introduced is controlled and optimized such that the best flow pattern is obtained. 
       FIG. 5  depicts an exemplary power generation system.  FIG. 5  shows a reaction chamber  101  and four (4) distinct subsystems; solar absorber subsystem  105 , thermochemical subsystem  110 , gas storage subsystem  115  and power generation subsystem  120 .  FIG. 5  further depicts the 4 subsystems ( 105 ,  110 ,  115  and  120 ) showing the thermal input and electrical output of the cycle, along with the various chemical inputs and outputs to the various subsystems. The chemical inputs and outputs show that the system takes sunlight as input and converts the sun light to electrical output without any chemical input from or output to the environment. The major benefit of using subsystem  2  (thermochemical system  110 ) as opposed to other forms of energy conversion, is that the thermochemical energy conversion process allows the energy to be stored chemically and buffered in subsystem  3  (gas storage system  115 ) so that electricity can be produced at any time and can adapt to different electrical loads on the utility grid. 
     In one embodiment, subsystem  110  also referred to herein as Subsystem  2  is the thermochemical system. In various embodiments, subsystem  110  converts the thermal energy (transferred from the absorber to the working fluid) to chemical energy, which can be stored as fuel and used whenever desired and at whatever rate is required. Although described within the context of splitting water, it will be appreciated that with minor adjustments the system can be used for other reactions. Thermochemical system  110  takes as input, heat from the solar absorber as well as water stored in subsystem  3  (i.e., the gas storage system). The outputs of subsystem  2  are H 2  and O 2  gas, which are stored in subsystem  3 . Thermochemical system  110  uses the working fluid, heated by the solar absorber (subsystem  1 ), to heat a solid reactant and drive an endothermic reaction  111 , such as the thermal reduction: 
       CeO 2 +heat→CeO 2-δ +1/2 δO 2 )
 
     which creates oxygen vacancies in the lattice of the solid reactant or metal oxide. Vacancy creation in the lattice of a material is well known in the art. 
     This now reduced solid reactant is then cooled in process  112 , and the rejected heat from process  112  is carried away by the working fluid to be used elsewhere in the cycle. Once at a lower temperature, steam  118  is introduced and the tendency for the solid reactant to reoxidize drives water molecules to dissociate (the vacancies are refilled with oxygen atoms from water molecules) as described in the following reaction: 
       CeO 2-δ +H 2 O→CeO 2 +δH 2 .
 
     This reaction generates hydrogen gas  117 . The now reoxidized solid reactant is then recycled, by reheating it with rejected heat, via the working fluid from other solid reactant being cooled—thereby forming a heat exchanger. 
     Other reactants can be used for the water splitting process other than the cerium-based reaction described above. Other suitable reactants include Ferrites, Flourites, Perovskites or other metal or mixed metal oxides. 
     In various embodiments, subsystem  115  also referred to herein as subsystem  3  is the gas storage system. The product gases (H 2    117  and O 2    118 ) from the thermochemical system  110  (subsystem  2 ), can be stored in standard pressure vessels known in the art and used whenever needed at whatever rate is required. Pressure vessel gas storage may he not he ideal for some applications therefore other hydrogen storage methods (i.e., metal hydrides) may be advantageous. Another benefit of this approach is that it generates a pure O 2  stream, which can greatly enhance the efficiency and reliability of the system that converts the chemical energy to electricity. For example a fuel cell becomes much more efficient with a pure O 2  stream as opposed to air. 
     In various embodiments, subsystem  120  also referred to herein as subsystem  4  is the power generation system. Subsystem  4  ( 120 ) takes as inputs H 2    117  and O 2    116  gas stored in subsystem  3  and outputs electrical power and H 2 O, which is then stored in a separate tank in subsystem  3 . The output H 2 O stored in subsystem  3  can be recycled as input to subsystem  2 , thus forming an entirely closed looped system, whereby no chemicals are released to the environment. The net result of the entire cycle (subsystems  1 - 4 ) is a system that ultimately converts sunlight into electricity, where the energy can be buffered and stored indefinitely as H 2  and O 2  in subsystem 3. 
     Several options exist for converting chemical energy into electricity. In one embodiment, subsystem  4  can contain a fuel cell, which can convert, the chemical energy into electricity at high theoretical efficiency or a heat engine or a combination of both. The advantage of using a fuel cell (e.g., solid oxide fuel cell operating at about 500-900° C.) is its potentially higher efficiency, which when combined with a heat engine such as a steam based Rankine cycle or supercritical CO 2  cycle (which uses the high temperature waste heat as input) can achieve the highest fuel to electricity conversion efficiencies. 
     One major benefit of this cycle (subsystems  1 - 4 ) is that both H 2  and O 2  are stored in subsystem  3  and serve as inputs to subsystem  4 . As a result, combustion of H 2  need not use air from the environment, which contains about 70% nitrogen. During combustion processes that use air from the environment, such as internal combustion engines in automobiles and conventional power plants, nitrogen oxides (NO x ) gases are generated in the exhaust and are harmful. Many of the difficulties associated with reducing NO x  gases in combustion exhaust, however, can be circumvented here, by simply using the pure O 2  stored in subsystem  3  through oxy-combustion. 
     In some embodiments, ceria is used as the solid reactant material.  FIG. 6A  depicts the lattice structure of the solid reactant.  FIG. 6B  is a phase diagram of the solid reactant. The structure in  FIG. 6A  is classified as a fluorite. 
     Ceria has many advantages for this application. First, ceria can support large off-stoichiometry CeO 2-δ δ max ˜0.25 while remaining in the fluorite structure. This property is not necessarily peculiar to ceria, but is notable and is responsible for its ability to be cycled many times between reaction steps  1  &amp;  2  (TCS 1  &amp; TCS  2 ) without degradation. Other materials such as perovskites may also exhibit some of the same features. Second, ceria remains in the fluorite crystal structure, from room temperature to well above 1500° C. which allows it to be cycled repeatedly without changing phase. The only changes at lower temperatures as shown in  FIG. 6B  are the formation of ordered vacancy phases. Third, the enthalpy of reduction for ceria is ˜4-5 eV, which makes it significantly reducible at 1500° C. (reaction step  1 ) and provides a strong driving force for reoxidation during reaction step  2 , Fourth, ceria has a positive entropy of reduction. This property is peculiar to ceria and it minimizes the required difference in temperature needed to cycle between reaction steps  1  &amp;  2 . Other materials with similar characteristics may also be used. 
       FIGS. 7A and 7B  depict an overview of the thermal and chemical processes involved in a thermochemical water splitting cycle, using non-stoichiometric ceria (CeO 2 -δ), with heat recovery indicated by the pictures of heat exchangers  325  and  350 , Specifically,  FIG. 7A  shows an outline of the thermal and chemical processes involved in a solar driven thermochemical water splitting cycle. In one embodiment, the cycle shown in  FIG. 7A  is a two-step thermochemical cycle based on a reduced metal oxide, such as ceria (CeO 2 -δ). In various embodiments, materials equivalent to ceria (CeO 2 -δ), i.e., materials that exhibit similar properties as ceria can also be used. 
     The term non-stoichiometric ceria (CeO 2 -δ) refers to metal oxide material ceria where the quantity δ implies the presence of a significant amount of oxygen vacancies in the crystal lattice. These oxygen vacancies are created during thermochemical step  1  (TCS 1 ) the thermal reduction step, and are refilled by the oxygen atoms contained in water (H 2 O) during thermochemical step  3  (TCS 3 ), the water splitting step. The decrease in energy or increase in entropy associated with refilling the oxygen vacancies is the driving force for the water splitting reaction. It will be appreciated that the steps may be implemented in any sequence. 
     It should be noted that the temperature at the start of each stage is equivalent to the temperature at the end of the previous stage. The temperature at the end of each stage is the average temperature of the primary reaction chamber along with the two other chambers it interacted with during that stage. During the thermal reduction step (TCS 1 ), the reaction chamber is assumed to equilibrate with the working fluid from the solar absorber at the highest temperature. The working fluid can be a liquid metal such as tin or aluminum, which remain in the liquid phase throughout the different phases of the cycle. 
       FIG. 7B  graphically illustrates thermochemical step  1  (TCS 1 ), The cycle begins by heating metal oxide  375  to a high temperature in Thermal Reduction Endothermic Reaction Heat Input  320 . Specifically, solid reactant  375  (CeO 2 ) undergoes the first reaction at high temperature, where solid reactant  375  is thermally reduced (CeO 2 +heat→CeO 2 -δ+1/2 δO 2 ). In one embodiment the high temperature ranges from ˜1200-1600° C. In various embodiments, the high temperature is determined according to the solid reactant used and the overall reaction of interest. The O 2  gas reaction product can be cooled (the rejected heat can be used for preheating or reheating elsewhere in the cycle) to room temperature and stored in a pressure vessel in subsystem  3 . The oxygen partial pressure is simultaneously reduced. Oxygen vacancies are created in the solid reactant in the process (reduced metal oxide) [CeO 2 +heat→CeO 2 -δ+½ δO 2 ] and oxygen gas  335  is removed so that the solid reactant will not re-oxidize upon cooling. The reduced solid reactant continues within the cycle to thermochemical step  2  (TCS 2 ). 
       FIG. 7C  graphically illustrates thermochemical step  2  (TCS 2 ). Specifically, the cycle continues with solid reactant CeO 2 -δ being cooled in  330  exchanging heat via heat exchanger  320  using a working fluid. The rejected heat is used to warm more solid reactant (see TCS 4 ) in preparation for the reduction step (TCS 1 ). The heat exchange continues as the solid reactant approaches the temperature needed for the water splitting reaction. In one embodiment the low temperature ranges from (˜500-800° C.). In various embodiments, the low temperature is determined according to the solid reactant used. 
     TCS 2  is primarily a thermal step (no chemical changes) where solid reactant is cooled from the high temperature to a lower reaction temperature. However, from a kinetics points of view, the two reactions may be conducted continually through the temperature swing to allow more time to react, i.e. reduction begins as soon as it is being warmed to the high temperature. Therefore, in practice, TSC 2  may not be purely a thermal step. 
       FIG. 7D  graphically illustrates thermochemical step  3  (TCS 2 ). TCS 3  is a thermochemical step, where water is split into H 2  and O 2 , where the O 2  reoxidizes the solid reactant (CeO 2 -δ+δH 2 O→CeO 2 +δH 2 ). The exothermic heat of reaction along with the gaseous H. 2  reaction product can be used to preheat the steam, via heat exchanger. The reoxidized solid reactant (CeO 2 ) then continues within the cycle to thermochemical step  4  (TCS 4 ). Specifically, water is taken from a reservoir (water storage tank  310 ) in subsystem  3 , heated, and allowed to react with the reduced solid reactant in the Water Splitting Exothermic Reaction Heat Output  355 . As the solid reactant approaches the temperature required for the water splitting reaction steam is introduced and reacts with the reduced metal oxide to re-oxidize it. In one embodiment the temperature required for the water splitting reaction ranges from (˜500-800° C.). In various embodiments, the temperature required for the water splitting reaction is determined according to the solid reactant used. 
     In this phase of the process, the presence of the oxygen vacancies within the solid reactant serves as a driving force for the reaction. As a result, water molecules disassociate so that the oxygen vacancies in the solid reactant can be refilled by the oxygen atoms in water. During this process, hydrogen atoms combine to form H 2  and desorb from the solid reactant surface as H 2  gas  340 . The net reaction is then CeO 2δ +δH 2 O→CeO 2 +δH 2 , where the oxygen vacancies in CeO 2 -δ are refilled by the oxygen atoms obtained from disassociating water molecules. The product H 2  is then removed with a sweep gas, such as excess H 2 O, which can be separated elsewhere by condensing H 2 O into the liquid phase (400K) depending upon the pressure). The product H 2  gas can then be stored in subsystem  3  (gas storage system). 
       FIG. 7E  graphically illustrates thermochemical step  4  (TCS 4 ). TCS 4  is primarily a thermal process, where the temperature of the solid reactant CeO 2  is raised from the lower reaction temperature (800-1000° K) to the high temperature (1500-1800° K). The output of TCS 4 , reoxidized CeO 2 , continues to TCS 1 , where the cycle begins again-forming a chemically closed loop, where the net chemical reaction is (H 2 O→H 2 +1/2O 2 ). Specifically, the solid reactant (CeO 2 ) is reheated. A major portion of the heat input is taken from the heat rejected from other solid reactant undergoing TCS 2 . However, due to imperfect heat exchange (2nd law of thermodynamics), fully reheating the solid reactant will require supplementary heat, which is taken from the solar absorber. 
       FIG. 4  depicts reaction chamber  400  comprising top view  405 , side view  415 , front view  425  and piping panel  410  showing how the pipes converge to its pump, which then distributes the flow of the working fluid to the solid reactant coated pipes (SRCP) inside the reaction chamber. Specifically, piping panel  410  is where all the pipes between a single reaction chamber and all other reaction chamber converge. Inside the piping panel, there are fixtures, permanent seals and computer controlled valves that are not shown. The computer controlled valves determine the pipes through which the working fluid circulates. The valves are toggled on and off at different points in the cycle for heat recovery purposes. 
     Top view  405  of reaction chamber  400  shows in more detail how the pipes converge to its pump, which then distributes the flow to the solid reactant coated pipes (SRCP) inside the reaction chamber. Arrows indicate the direction of the working fluid flow circulating between the reaction chambers. 
     Side view  415  of reaction chamber  400  shows its pump and how the 8 stacks of inlet and outlet pipes converge onto its pump. The pump then distributes the flow to the four layers of SRCP inside the reaction chamber. The red arrow indicates working fluid pumped directly from the solar absorber (˜1500-1600° C.), which is used during the high temperature reduction (step  1 ). Arrows indicate gas flow pipes that transport the sweep, reactant and product gases in and out of the reaction chamber. 
     Pipe orifice  420  provides an entry point for working fluid heated in the solar absorber. A computer controlled valve is turned on during the reduction step (TCS 1 ) so that the SCRP is heated to the highest temperature. 
     Front view  425  of reaction chamber  400  shows the four layers of the SRCP being used. 
       FIG. 8  depicts an array of individual reaction chambers  500 . Each reaction chamber ( 505 ,  510 ,  515 ,  520 ,  525 ,  530 ,  535  and  540 ) contains pipes within it that are coated with the solid reactant material and are thus termed solid reactant coated pipes (SRCP). The convention used to designate a pipe connecting two reaction chambers is denoted as x:y where x represents the source chamber and y represents the destination  1 : 8 ,  8 : 1  connects reaction chamber  1  to reaction chamber  8  and represents pipe  1 : 8  and pipe  8 : 1 . As indicated in the  6  design criteria, the present design separates the thermal processes and chemical processes in an elegant way, by using sealed reactors. 
     With the solid reactant coated on the outside of the SRCP that run through the reaction chamber walls (see  FIG. 4 ), all chemical processes occur on the outer surfaces of these pipes within the permanently sealed chamber. The thermal processes are achieved by circulating a working fluid through the inner walls of the SRCP, where convection from the working fluid transfers heat to the inner walls of the SRCP, which is then conducted through the pipe walls to the solid reactant coated on its outer walls. In various embodiments, the pipe walls are as thin as possible to minimize thermal resistance. As a result, chemical processes occur on the outer walls of the SRCP while thermal processes occur on the inner walls of the SRCP. In other embodiments, more optimal SCRP geometries are used. 
     The heat recovery (heat exchange) between TCS 2  and TCS 4  is achieved by circulating the working fluid from one reaction chamber to other reaction chambers through a network of pipes. When the fluid is circulated, heat is exchanged between the solid reactant contained within the reactors until they approach thermal equilibrium. The circulation of the working fluid can be dictated by computer controlled valves on each of the pipes within the network. The sequence of which valves are opened and closed at different time intervals, controls the cyclic nature of the thermochemical processes and avoids moving parts associated with moving the solid reactant itself. In one embodiment, the design comprises 8 reaction chambers. In other embodiments, any suitable number of arrays of reaction chambers may be used. Further, in one embodiment N reaction chambers are assembled in a concentric arrangement. In other embodiments, other assembly geometries are also possible. 
     Each of the N reaction chambers has pipes ( 545 ) connecting it to each of the other N−1 reaction chambers, A plurality of pipes  545  runs between each individual reaction chamber and all other N−1 reaction chambers. Although the top view shows many places where the pipes cross, it is important to note that none of the pipes actually intersect each other within the circle. The pipes do converge onto a piping panel in front of each reaction chamber, but the flow in each pipe is distinct and does not mix with the flow in any other pipes, even though it appears to cross other pipes in  FIG. 8 . This non-intersection of the pipes can be achieved by simply layering the pipes at different heights above ground. There are a number of geometric variations for how this can be accomplished. In one embodiment, the pipes are stacked in 8 distinct layers, where layer  1  is located at the top of the stack (highest from the ground) and layer  8  is at the bottom (closest to the ground). The pipe naming convention corresponds with the direction of flow, so that  1 : 8  indicates a pipe carrying fluid from reaction chamber  1  to reaction chamber  8  and  8 : 1  carries working fluid from reaction chamber  8  back to reaction chamber  1 . The two pipes together allow the working fluid to be circulated between the reaction chambers. 
     A heat transfer/working fluid (e.g., tin, aluminum) that remains liquid at the highest temperatures flows within all the pipes shown. This working fluid carries the heat stored in the contents of one reaction chamber to the contents of another reaction chamber. Each set of pipes has a computer controlled valve that determines whether or not the working fluid will flow or remain stagnated. The valves (not shown) are closed (no fluid flow) in all the pipes at all times, except when they are opened between two reaction chambers to allow for circulation and heat transfer. Each reaction chamber also contains another pipe that connects to the fluid being directly heated by the solar absorber. This fluid is used during the reduction step (TCS 1 ) to heat the contents of the reaction chamber to the target reduction temperature. Each reaction chamber also has an inlet and outlet channel so that sweep, reactant and product gases can be added/removed from the reaction chamber. Each reaction chamber has a number of pipes running through it that are rigidly connected to its walls. These pipes are coated with the solid reactant material (e.g., CeO 2-δ ) and are therefore termed solid reactant coated pipes (SRCP). The working fluid can flow through these SRCP to heat the reactant material from the inside, transferring heat to the inner pipe walls via convection and transferring heat to the solid reactant through the pipe wall via conduction. The sweep and reactant gases can enter the reaction chamber and interact directly with the solid reactant on the outer walls of the SRCP. The flow through all pipes is driven by an array of pumps. The way in which these pumps are arranged is flexible. In one embodiment, a configuration whereby each reaction chamber has its own pump ( 506 ,  511 ,  516 ,  521 ,  526 ,  531 ,  536  and  541 ) and there are also assumed pumps (not shown) that drive the sweep, reactant and product gas flows, as well as pump(s) (not shown) to drive the working fluid to the solar absorber and into each reaction chamber during its reduction step (TCS 1 ). It is noted that all pipes are heavily insulated with radiation and convective shielding to minimize thermal loss to the surroundings. In addition, the valves on each set of pipes are computer controlled and determine whether or not the working fluid will flow. The opening and shutting off of these valves require minimal energy and can be automated through actuators while controlled by an external computer or programmed circuit. It is through the sequence of valve openings and closings that the cycle repeats and one advantage of the present design is that the timing sequence can change dynamically based on feedback from each reaction chamber via different cycle phases. 
     In the present design, TCS 1  of the process shown in  FIG. 7 , is accomplished by pumping high temperature working fluid through the SRCP of one reaction chamber, delivering heat to the solid reactant, which is coated on the pipe&#39;s outer walls. Simultaneously, the partial pressure of oxygen can be lowered by either removing oxygen directly with a vacuum pump, or using a sweep gas such as Argon or H 2 O (H 2 O is essentially inert with respect to CeO 2-δ  at such high temperatures (i.e., no water splitting). In the present design a sweep gas (H 2 O) is assumed and can flow in and out of the sealed reaction chamber through the pipes, driven by a gas pump (not shown). During step  2  when the solid reactant in one reaction chamber is cooled, working fluid is circulated between it and other reaction chambers in the circle that contain solid reactant at a lower temperature. This fluid driven heat exchange between individual reaction chambers occurs through the network of pipes shown in  FIG. 8 , by sequentially switching valves accordingly. The circulation of working fluid between two reaction chambers in the circle continues until the content of the two reaction chambers approach thermal equilibrium (the same temperature). A major advantage of the present design is that the time it takes to approach equilibrium between two reaction chambers can be controlled by the fluid Slow rate (more pumping power=faster heat exchange). Therefore, if the heat transfer processes are the rate limiting portion of the cycle, the flow rate can be optimized to yield maximum total system efficiency. After the thermal equilibrium between two reaction chambers is achieved, the valves switch again allowing fluid to circulate with the next reaction chamber in the circle. 
     Thermochemical Cycle Sequence 
     The following sequence of 8 stages describes how the reaction chamber  505  would traverse TCS 1 -TCS 4 . At the beginning of the cycle, reaction chamber  8  ( 540 ) has just finished its reduction step (TCS 1 ) at ˜1500° C. by receiving working fluid pumped from the solar absorber. At this stage reaction chamber  1  ( 505 ) is preparing for the reduction step (TCS 1 ) and exchanges working fluid with reaction chamber  8  ( 540 ) via pipes  1 : 8  and  8 : 1 . This helps to raise the temperature of chamber  1  ( 505 ) closer to 1500-1600° C. and lower the temperature of reaction chamber  8  ( 540 ) so that it can begin cooling towards the low temperature step at ˜700-800° C. At the midpoint of stage  1 , after reaction chambers  1  ( 505 ) and  8  ( 540 ) equilibrate, the valves for pipes  1 : 8  and  8 : 1  are closed and reaction chamber  1  ( 505 ) begins receiving working fluid from the solar absorber to drive the thermal reduction reaction. By the end of stage  1 , reaction chamber  1  ( 505 ) has reached 1800° C., the temperature of the working fluid pumped from the solar absorber. At the end of stage  1 , the partial pressure of oxygen has also been reduced substantially by using a sweep gas or vacuum pump to remove oxygen from the reaction chamber. Reaction chamber  1  ( 505 ) then begins its cool down sequence (stages  2 ,  3  and  4 —TCS 2 ), by exchanging working fluid with reaction chambers  2 - 7  ( 510 - 535 ) in a predetermined sequence. At each stage the valves between each pair of reaction chambers are opened and working fluid is circulated between the two reaction chambers. For example, at the beginning of stage two, pipes  1 : 2  and  2 : 1  are opened so that reaction chambers  1  ( 505 ) and  2  ( 510 ) can circulate their working fluid. In the second half of stage  2 , pipes  1 : 2  and  2 : 1  are closed and pipes  1 : 3  and  3 : 1  are opened for circulation and so on. 
     At the beginning of stage  5 , reaction chamber  1  ( 505 ) begins circulating working fluid again with reaction chamber  8  ( 540 ) through pipes  1 : 8  and  8 : 1 , which just completed its water splitting reaction at 700-800° C. (TCS 3 ). Half way through stage  5 , the valves of pipes  1 : 8  and  8 : 1  are closed and reaction chamber  1  ( 505 ) is exposed to steam at 700-800° C., which starts the water splitting reaction (TCS 3 ), By the end of stage  5  the solid reactant has been re-oxidized and it has equilibrated with the 700-800° C. flow of reactant steam. During stages  6 - 8 , reaction chamber  1  ( 505 ) is reheated (TCS 4 ) in preparation for the high temperature reduction step (TCS 1 ). 
     In other embodiments, a feedback loop may be implemented to control the cycle thereby taking into account periods when solar energy is either diminished (e.g., cloudy days) or absent (hurricane). Such feedback loop would use temperature, pressure and chemical composition of the fluid adaptively. In such embodiments, conversion of light to thermal energy from the conversion of thermal energy to chemical energy is decoupled; therefore, a control system can be used to buffer energy at various points in the cycle. This arrangement is implemented in response to intermittent sun light, intermittent output or in response to demand/load variations. The control is executed by varying the cycle time, temperature, pressure, inlet and outlet gases, the rate of heat recuperation and the like. 
     Other embodiments provide for storing or buffering H 2  for use over longer period when solar energy is unavailable, The amount of H 2  needed is application or plant specific. However, that amount can be calculated knowing the variables for each application. 
       FIG. 9  depicts a flow diagram of a method for solid based reactant thermochemical solar power generation according to an embodiment, The cycle begins at step  605 . Metal oxide or solid reactant is heated to a high temperature. The solid reactant (CeO 2 ) undergoes the first reaction at high temperature, where the solid reactant is thermally reduced (CeO 2 +heat→CeO 2-δ +½δO 2 ). In one embodiment the high temperature ranges from ˜1200-1600° C. In various embodiments, the high temperature is determined according to the solid reactant used. The O 2  gas reaction product can be cooled (the rejected heat can be used for preheating or reheating elsewhere in the cycle) to room temperature and stored in a pressure vessel in subsystem  3 . The oxygen partial pressure is simultaneously reduced. Oxygen vacancies are created in the solid reactant in the process (reduced metal oxide) [CeO 2 +heat→CeO 2-δ +½δO 2 ] and oxygen gas  335  is removed so that the solid reactant will not re-oxidize upon cooling. The reduced solid reactant continues within the cycle to thermochemical step  2  (TCS 2 ). 
     At step  615 , the cycle continues with solid reactant CeO 2-δ  being cooled in exchanging heat via heat exchanger using a working fluid. The rejected heat is used to warm more solid reactant (see TCS 4 ) in preparation for the reduction step (TCS 1 ). The heat exchange continues as the solid reactant approaches the temperature needed for the water splitting reaction. In one embodiment the low temperature ranges from (˜500-800° C.). In various embodiments, the low temperature is determined according to the solid reactant used. TCS 2  is typically, though not always, a thermal step (no chemical changes) where solid reactant is cooled from the high temperature to a lower reaction temperature. 
     At step  620 , water is taken from a reservoir (water storage tank  310 ) in subsystem  3 , heated, and allowed to react with the reduced solid reactant. As the solid reactant approaches the temperature required for the water splitting reaction, steam is introduced and reacts with the reduced metal oxide to re-oxidize the solid reactant. In one embodiment, the temperature required for the water splitting reaction ranges from (˜500-800° C.). In various embodiments, the temperature required for the water splitting reaction is determined according to the solid reactant used. 
     In this phase of the process, the presence of the oxygen vacancies within the solid reactant serves as a driving force for the reaction. As a result, water molecules disassociate so that the oxygen vacancies in the solid reactant can be refilled by the oxygen atoms in water. During this process, hydrogen atoms combine to form H 2  and desorb from the solid reactant surface as H 2  gas  340 . The net reaction is then CeO 2-δ +δH 2 O→CeO 2 +δH 2 , where the oxygen vacancies in CeO 2-δ  are refilled by the oxygen atoms obtained from disassociating water molecules. The product H 2  is then removed with a sweep gas, such as excess H 2 O, which can be separated elsewhere by condensing H 2 O into the liquid phase (˜250-200° C.). The product H 2  gas can then be stored in subsystem  3  (gas storage system). 
     At step  630 , the temperature of the solid reactant CeO 2  is raised from the lower reaction temperature (500-800° C.) to the high temperature (1200-1600° C.). 
     At step  640 , the solid reactant (CeO 2 ) is reheated. A major portion of the heat input is taken from the heat rejected from other solid reactant undergoing TCS 2 . However, due to imperfect heat exchange (2nd law of thermodynamics), fully reheating the solid reactant will require supplementary heat, which is taken from the solar absorber. The output of TCS 4 , reoxidized CeO 2 , continues to TCS 1 , where the cycle begins again—forming a chemically closed loop, where the net chemical reaction is (H 2 O→H 2 +1/2δO 2 ). 
       FIG. 10  depicts a high-level block diagram of a computing device suitable for use in implementing various functions described herein. 
     As depicted in  FIG. 10 , computer  700  includes a processor element  702 , a memory  704  (e.g.,  123 , random access memory (RAM), read only memory (ROM), and the like), a cooperating module/process  705 , and various input/output devices  706  (e.g., a user input device (such as a keyboard, a keypad, a mouse, and the like), a user output device (such as a display, a speaker, and the like), an input port, an output port, a receiver, a transmitter, pressure transducers, flow meters, thermocouples and storage devices (e.g., a tape drive, a floppy drive, a hard disk drive, a compact disk drive, and the like)). 
       FIGS. 11-15  are representations of a reactor system ( FIGS. 11 and 15 ), an array of reactors ( FIG. 12 ), and a close up of the pipes that connect the reactors (Figures  13  and  14 ). In  FIG. 11 , the heat transfer fluid travels through the pipes in the reactor (from right to left in the figure). Water is introduced with from the bottom of the reactor where it contact the solid reactant coated pipes and is split to form hydrogen and oxygen. 
     It will be appreciated that the functions depicted and described herein may be implemented in software and/or hardware, e.g., using a general purpose computer, one or more application specific integrated circuits (ASIC), and/or any other hardware equivalents. 
     In one embodiment, the various processes  705  may be loaded into memory  704  and executed by processor  702  to implement the functions as discussed herein. Thus, various processes  705  (including associated data structures) may be stored on a computer readable storage medium, e.g., RAM memory, magnetic or optical drive or diskette, and the like. 
     It is contemplated that portions of the functions/elements described herein may be implemented as a computer program product wherein computer instructions, when processed by a computer, adapt the operation of the computer such that the methods and/or techniques described herein are invoked or otherwise provided. Instructions for invoking the inventive methods may be stored in fixed or removable media, and/or stored within a memory within a computing device operating according to the instructions. Further, the system may be controlled by a network of computers or similar arrangement. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs, Publications cited herein and the materials for which they are cited are specifically incorporated by reference. 
     Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein, Such equivalents are intended to be encompassed by the following claims.