Solar thermochemical processing system and method

Abstract

A solar thermochemical processing system is disclosed. The system includes a first unit operation for receiving concentrated solar energy. Heat from the solar energy is used to drive the first unit operation. The first unit operation also receives a first set of reactants and produces a first set of products. A second unit operation receives the first set of products from the first unit operation and produces a second set of products. A third unit operation receives heat from the second unit operation to produce a portion of the first set of reactants.

Claims

1. A method of providing a solar energy augment to the chemical energy content of a reactant stream, the method comprising: heating a solar reforming reactor from a solar concentrator, the reactor comprising reactor channels and product return flow channels separated by a middle plate, the middle plate providing both a wall of the reactor channels and wall of the product return flow channels; reacting the reactants in the presence of a catalyst in the reaction channels of the reactor to generate a product stream; and conveying the product stream from the reaction channels to the product return channels while maintaining thermal contact across the middle plate and between the product stream and the reactants in the reaction zone.

2. The method of claim 1 further comprising combusting the product stream in order to provide heat to a power system or for other unit operations requiring heat.

3. The method of claim 1 wherein the power system or the unit operations requiring heat is a combined cycle, fuel cell or power plant, or a factory or chemical process facility requiring heat for steam generation.

4. The method of claim 1 further comprising providing the reactants to a centerpoint of the reactor and conveying the reactants through the reaction channels to a perimeter of the reactor.

5. The method of claim 1 further comprising exchanging the heat from the product stream with the reactants prior to the reactants entering the reaction zone.

6. The method of claim 1 wherein the product stream output is in thermal contact with the reactant stream intake.

7. The method of claim 1 wherein the solar thermochemical augment is at least 20%, wherein the solar thermochemical augment is measured as the increase in Higher Heating Value in the reacting stream divided by the Higher Heating Value of the reactants, times 100%.

8. The method of claim 1 wherein the product stream comprises syngas.

9. The method of claim 1 wherein the product stream is generated at a solar-to-chemical energy conversion efficiency greater than about 60%, wherein the product stream includes syngas, and wherein the solar thermochemical augment is at least 20%, wherein the solar thermochemical augment is measured as the increase in Higher Heating Value in the reacting stream divided by the Higher Heating Value of the reactants, times 100%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a process diagram of a solar thermochemical processing system, in accordance with one embodiment of the present invention.

(2) FIG. 2 is a process diagram of a solar thermochemical processing system, in accordance with one embodiment of the present invention.

(3) FIG. 3 is a process diagram depicting the operation of a solar thermochemical processing system during periods of sunlight, in accordance with one embodiment of the present invention.

(4) FIG. 4 is a process diagram depicting the operation of a solar thermochemical processing system during periods when sunlight is unavailable.

(5) FIG. 5 is a summary of data from a day of shakedown testing with steam reforming of a solar thermochemical processing system under certain operating conditions.

(6) FIG. 6 is an exploded view of a solar reforming reactor, in accordance with one embodiment of the present invention.

(7) FIG. 7 is a radial flow receiver-reactor, in accordance with one embodiment of the present invention.

(8) FIG. 8A is a front view of a tiled receiver-reactor, in accordance with one embodiment of the present invention.

(9) FIG. 8B is a rear view of a tiled receiver-reactor, in accordance with one embodiment of the present invention.

(10) FIG. 9 is an exploded view of a counter radial flow receiver-reactor, in accordance with one embodiment of the present invention.

(11) FIG. 10 is an implementation of parabolic dish concentrator for driving endothermic hydrocarbon reforming reactions, in accordance with one embodiment of the present invention.

(12) FIG. 11 is an adaptive flow feature for use in a microchannel solar receiver, in accordance with one embodiment of the present invention.

(13) FIG. 12 is a passive flow control device, in accordance with one embodiment of the present invention.

(14) FIG. 13 shows a predicted solar flux map on receiver panels at noon on equinox as predicted by an optical design tool.

(15) FIG. 14 shows the solar flux on a dish concentrator receiver.

(16) FIG. 15A shows how a bimetallic beam amplifies the motion that can be achieved due to thermal expansion.

(17) FIG. 15B shows how leverage can be used to amplify the linear displacement achieved due to thermal expansion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(18) The present invention is directed to the use of high-temperature heat that is available from solar concentrators to change the chemical composition or increase the chemical energy content of a reacting stream. Embodiments of the invention include, but are not limited to, open and closed-cycle chemical loops that efficiently convert concentrated solar energy into chemical energy through the use of chemical conversion reactions, and advanced process-intensive micro- and meso-channel process technology, which enables unit chemical operations to be made compact enough to be mounted at the focal point of a dish-concentrator, a central receiver, or another solar concentrator installation.

(19) A microchannel may be of any length in the general direction of bulk flow and has one dimension, e.g., the width, that is greater than or equal to 1 micron and less than or equal to 5 millimeters.

(20) A mesochannel may be of any length in the general direction of bulk flow and has one dimension, e.g., the width, that is greater than 5 millimeters and less than or equal to 5 centimeters.

(21) It is especially useful to couple microchannel and mesochannel reactors and heat exchangers with solar concentrators, such as parabolic dish and central receiver concentrators, which by virtue of being able to concentrate direct normal solar energy by factors of 100 or 1000, or greater, provide sufficiently high fluxes and temperatures (e.g., greater than 500° C.) to enable high temperature endothermic reactions.

(22) In certain embodiments, concentrated solar energy is used to drive an efficient, high temperature endothermic chemical reaction to produce higher energy products and to enable chemical energy storage. This fuel upgrading and chemical energy storage can be coupled with a heat engine or fuel cell to produce electricity at higher capacity factors and reduced costs compared to other solar energy systems.

(23) In other embodiments, concentrated solar energy can be used to produce transportation fuels and other chemical products. For example, methane can be converted to synthesis gas (or ‘syngas’), followed by a Fischer-Tropsch reaction to produce long-chain hydrocarbons, with subsequent additional processing steps to produce gasoline, diesel fuel, jet fuel, or other fuel products in what is known as a “gas-to-liquids” process. Conventional gas-to-liquids processes would consume a portion of the chemical content and energy in the feedstock reactants in order to drive endothermic gas-to-liquids operations, with one result being that a significant portion of the carbon in the methane feedstock is emitted from the plant as CO.sub.2. Typically only about 60% of the feedstock carbon finds its way into the liquid hydrocarbon product. However, with the concentrated solar energy providing moderate- and high-temperature heat to drive endothermic unit operations, such as reforming and distillation, more effective utilization of the feedstock is realized, carbon dioxide emissions are substantially reduced, the plant produces a greater quantity of product fuel and reduced quantities of carbon dioxide, a greenhouse gas.

(24) FIG. 1 is a process diagram of a solar thermochemical processing system, in accordance with one embodiment of the present invention. In this system, reactants such as, but not limited to, water and methane—available from natural gas or biomass (e.g., the effluent of an anaerobic digester)—are preheated and then reacted in an endothermic, steam-reforming reactor. The endothermic reactor is heated by concentrated solar energy from a parabolic dish or other solar concentrator. The product stream or ‘reformate’ is cooled and then further reacted to produce methanol in a downstream, exothermic reactor with the heat of reaction being used to vaporize the liquid water feed stream. A heat pump (not shown), such as vapor-compression, sorption or thermoelectric heat pump, can be included in the system for taking in heat from the methanol synthesis reactor and providing additional heat at a higher temperature to the vaporizer. A downstream separator allows methanol and water to be recovered with unreacted syngas components (CH.sub.4, CO, CO.sub.2 and H.sub.2) being sent on to provide the heat for a heat engine through a combustion process. This enables part of the heating value of the product stream to be used immediately to produce power, while stored methanol is used for power generation at another time.

(25) FIG. 2 is a process diagram of a solar thermochemical processing system, in accordance with one embodiment of the present invention. In this embodiment, water and methane are fed to the system by positive displacement pumps and mass flow controllers, respectively, to control the steam to carbon ratio (S:C) and overall flow rate. Each stream is initially preheated through a counter-flow microchannel recuperative heat exchanger as shown, heated by the exhaust stream of the reforming operation.

(26) Additional heat exchangers are shown in FIG. 2, compared to FIG. 1, providing preheating of reactants. The steam and methane are mixed before entering the high temperature heat exchanger or recuperator, where the combined stream is preheated, to the extent practical, to a temperature that is close to the operating temperature of the reforming reactor. The combined and preheated stream then enters the reforming reactor where the mixture passes over a methane steam reforming catalyst, absorbs solar heat at temperatures up to or higher than approximately 650° C., more preferably above 800° C. and is converted to syngas according to the methane reforming reaction and the water gas shift reactions, equations 1 and 2, respectively.
CH4+H2O.fwdarw.CO+3H2  (1)
CO+H2O.Math.CO2+H2  (2)

(27) Because the combined reaction is highly endothermic, solar energy is effectively converted to chemical energy within the reformer. The hot syngas exiting the reactor immediately flows into the high-temperature recuperator where it preheats the combined methane/steam stream before feeding the low-temperature recuperator(s) mentioned above. Upon leaving the recuperator(s), the syngas stream flows through other heat exchangers and an air-cooled radiator where it is further cooled before flowing through a vapor-liquid separator (VLS) where condensed water is removed. The relatively dry syngas exiting the VLS is characterized in terms of flow-rate and composition, after which it is ready for use, either in a combustion-driven system or in a chemical process such as methanol synthesis.

(28) FIG. 3 is a process diagram depicting the operation of a solar thermochemical processing system, in accordance with one embodiment of the present invention. During periods of sunlight, solar energy is efficiently converted to chemical energy, through the steps of methane reforming and methanol synthesis, with power generation occurring through the exothermic remethanation of a portion of the unconverted synthesis gas. Methanol and H2 or syngas are also stored during these operations.

(29) FIG. 4 is a process diagram depicting the operation of a solar thermochemical processing system of FIG. 3 during periods when sunlight is unavailable. When sunlight is unavailable stored methanol and H2 or synthesis gas is remethanated, producing heat for power generation, with water and methane being stored. Since the heat engine is not made integral to the dish-concentrator, the number and optimization of heat engines is decoupled from the number and optimization of dish-concentrators.

(30) The heat engines in FIGS. 3 and 4 can be any of several types of thermodynamic cycles for heat engines, such as Rankine, Stirling or Brayton cycles.

(31) FIG. 5 is a summary of data from a day of shakedown testing with steam reforming of a solar thermochemical processing system of FIG. 2 under certain operating conditions. On a previous date, the system was operated at low temperatures with a nitrogen-hydrogen stream for the initial reduction of the steam reforming catalyst and for trials of the control system. For steam reforming, three solar operating conditions were explored, one condition in which a screen was placed in front of the aperture to block roughly half of the solar flux (“screen condition”), therefore limiting the incoming solar flux during initial trial operations, a second condition in which the dish was moved in circular motion to “spill” a portion of the solar flux away from the aperture (“circle tracking”), and a third condition where complete solar flux was directed into the aperture of the receiver unit (“full sun”).

(32) System startup began with the use of the screen, allowing the reactor to be started with less than the full heat available from the solar concentrator. Circle tracking was used after the screen was removed, again to limit the solar flux on the reactor, with testing eventually progressing to full solar energy (“full sun”). The results of this day of testing are illustrated in FIG. 5, including conditions with the screen, with circle tracking, and with full sun. FIG. 5 also shows a period of testing, near the end of the day, where the dish was partially shadowed by a nearby tree.

(33) The initial test condition, as shown in FIG. 5, was in screen mode and occurred between 9 a.m. and 10 a.m., where the solar flux was high (868±4 W/m.sup.2) and the screen provided about 50% attenuation of the solar flux to the reactor. Under this condition, the reactor inlet and outlet were at 373° C. and 610° C., respectively, which are highly non-ideal conditions for the reaction. Not surprisingly, conversion was low (45-52%), the change in higher heating value was on the order of 4 kW.

(34) Due to significant mid-day cloud cover, the system was shut down in order to remove the screen and perform some system diagnostics. By about 2 p.m., the cloud cover had passed and the system was restarted.

(35) The next few steady state values were obtained using circle tracking mode at increasing methane feed rates. Unfortunately, water pump issues caused the steam-to-carbon ratio to drift during this time. Nonetheless, the data obtained during this circle-tracking period yielded some useful results. Methane conversion increased during this time period as the reactor temperature (and product outlet temperature) continued to rise. Methane conversion relative to equilibrium conversion (at reactor outlet temperature), referred to as “conversion approach” also continued to rise, reaching as high as 75%.

(36) The day's experiments concluded by operating the system with direct sun. By this time, the direct normal incidence (DNI) was beginning to wane, but the results are quite encouraging relative to our stated goals. For instance, our approach to equilibrium conversion continued to rise and reached a level of about 99% and overall methane conversion exceeded 90%. In addition, the overall solar-to-chemical energy conversion, calculated as the ratio of the increase in the Higher Heating Value of the reacting stream to the direct normal sunlight that was incident upon the dish concentrator, was calculated to be 63±4%, which represents the highest values of which we are aware. These exceptional results were enabled by the highly effective thermal recuperation provided by the microchannel heat exchangers, resulting in significant preheat of the reactant stream entering the reactor and allowing the majority of the concentrated solar energy to be used to drive the endothermic steam reforming reaction.

(37) FIG. 6 is an exploded view of a solar reforming reactor, in accordance with one embodiment of the present invention. The reactor is capable of extended high temperature and high pressure operation. In FIG. 6, the collector plate is the concentrator-facing plate of the assembly and also contains the reaction channels and steam reforming catalyst wedges. In one embodiment, the outer diameter and thickness of the collector plate are approximately 27.3 cm. and 1.9 cm. respectively, and the individual channels are mesochannels, approximately 0.6 cm. deep. It should be noted that the collector plate is not limited to these dimensions. A triangular-shaped “dead zone” is located within the collector plate and corresponds to an area where reduced solar flux will be received due to the design of the dish concentrator.

(38) Use of a highly active catalyst is an enabling factor in the deployment of compact reactors, in which heat and mass transfer resistance has been minimized. Under such conditions, the reforming catalyst used here greatly outperforms standard base-metal reforming catalysts in terms of activity and coking. The combination of device architecture and catalyst selection enables process intensification, a key to capturing concentrated solar energy in a chemical process.

(39) Also in FIG. 6 are the other portions of the solar reforming reactor, including a depiction of an individual “catalyst wedge”, for insertion within a reaction channel, the middle plate, manifold plate and header for collection and routing of the reaction products, and the reactor inlet and reactor outlet. Exiting gases from the reaction channels/catalyst wedges flow out of the collector plate channels by way of the circular array of holes on the middle plate, through the product return flow channels, are collected and passed through the manifold plate and to the reactor outlet through a tapered channel on the back of the manifold plate. Note also that the reactor inlet tube is welded to the center hole of the middle plate, thus ensuring that the reactants and products do not mix.

(40) Reaction channel geometries were developed that were more suited to the circular receiver geometry used on circular parabolic dish solar concentrators.

(41) Radial flow is a natural choice for incorporating reaction channels into circular solar receiver geometry. FIG. 7 is a radial flow receiver-reactor 700, in accordance with one embodiment of the present invention. The reactor 700 includes a front plate 710, a back plate 720, support ribs 750, and a porous catalyst support 740 through which reactants flow. Reactants flow enter at the center 760 of the back plate 720, splits into multiple flow paths 730, flows toward the periphery of the device through the catalyst support 740, and exits through periphery manifolds (not shown). The flow of reacting gas would be either from the center 760 to the periphery of the reactor 700.

(42) Another approach for incorporating reaction channels into a circular geometry is to divide the geometry up into smaller sections containing inlets, outlets and catalyst channels. These reactor “tiles” can be combined to fill the circular receiver area. An advantage of the tiled geometry is that the length of a given flow channel can be decreased making the cool inlets in closer proximity to hot outlets, and better enabling thermal conduction in the top plate to decrease the temperature gradients seen at the surface and in the top plate of the receiver-reactor. This advantage creates the ability to compensate for hot spots on the receiver surface due to aberrations or imperfections of the parabolic solar concentrator. With individual control of the flow through each tile section, areas receiving a higher solar flux could be fed more methane and steam.

(43) FIGS. 8A and 8B show a tiled receiver-reactor 800 incorporating metal heat conduction paths through a catalyst layer 860. FIG. 8A is a front view of a tiled receiver-reactor 800, and FIG. 8B is a rear view of the tiled receiver-reactor 800, in accordance with one embodiment of the present invention. The tiled receiver-reactor 800 include a front plate 810, a back plate 820, inlets 830, outlets 840, metal pieces 850, and the catalyst layer 860 where the reactants flow through.

(44) There can also be a dead spot in the flow field. The catalyst in this area contributes little to the reaction. Although maximum temperatures are similar to the radial flow simulations, the smaller tile dimensions decrease temperature gradients and produce a more uniform temperature at the receiver surface.

(45) The metal 850 in the center of the tile dramatically improves heat conduction to the back side of the catalyst channels 860. Compared to the previous tile geometry, this geometry with discrete flow channels and heat conduction paths to the back plate 820 requires significantly less catalyst material to achieve the same amount of methane conversion. These metal conduction paths also function as structural supports and greatly increase the strength of the reactor.

(46) One of the issues with accomplishing a reforming reaction in a solar receiver-reactor is the uniformity or nonuniformity of the heat flux provided by a solar concentrator. It would be expected that the reforming reaction is most active at the inlet where methane concentrations are highest and lowest where the methane is depleted. However, if a constant heat flux per area on the receiver-reactor surface is accomplished, the reaction rate will be limited by the heat supplied and the temperatures achieved, and a more constant reaction rate in the channel may be realized. This happens as areas with a high methane concentration are cooler and areas with low methane concentration are hotter such that the higher catalyst activity (at higher temperatures) compensates for the low methane concentration. As a result, uniform heat flux on the receiver-reactor contributes to the temperature gradients and the maximum temperatures seen in the reactor structure. Lower maximum temperatures are desirable in that the strength and creep rupture resistance of the metal used to construct the reactor are higher, and catalyst deactivation lower at lower temperatures. In short, while higher temperatures may contribute to higher solar-to-chemical energy conversion efficiencies, running a portion of the receiver-reactor at excessively high temperatures will decrease the lifetime of the unit. A possible solution to this problem that can reduce excessive reactor temperatures is a geometry with counter flowing channels. In a counter-flow geometry the methane rich portion of one channel (the inlet portion) is placed adjacent to the methane depleted portion (outlet) of other channels, as shown in FIG. 9. This results in lower reactor temperatures in that heat is more easily channeled to areas within the reactor with high methane concentrations and high rates of reaction.

(47) FIG. 9 is an exploded view of a counter radial flow receiver-reactor 900, in accordance with one embodiment of the present invention. The reactor 900 includes a front plate 910, porous catalyst pieces 920, an inlet header 930, an outlet header 940, a back plate 950, ribs 960, and flow channels 970. Also shown is a heat exchanger having an inlet tube 980 for preheating reactants before entering the reactor 900 and an outlet tube 980 for cooling products exiting the reactor 900.

(48) In one embodiment, half of the reactants enter at the center of the reactor 900 and flow through the catalyst sections 920 to the periphery of the reactor 900; the other half of the reactants enter at the periphery and flow toward the center.

(49) A counter-flow radial geometry, similar to FIG. 9, was simulated to investigate how the geometry could decrease the temperature at the receiver surface and reduce temperature gradients within the reactor. This geometry contains channels that enter the center of the receiver-reactor and flow in a radial direction toward outlets at the periphery of the circular reactor. Adjacent to these outward flowing channels are inward flowing channels that enter at the periphery and exit toward the center of the receiver-reactor. In the simulation only two such channels were modeled taking into account the symmetry of the geometry (only half of each channel was modeled in that a plane of symmetry runs down the center of each channel). As with the modified tiled geometry with a central metal section, this geometry benefits from the metal ribs that separate the flow channels. These ribs allow for heat conduction to the back plate of the reactor and help achieve more uniform temperatures through the catalyst. Although the headers shown in FIG. 9 at the periphery of the device connect all inflow channels and outflow channels together, if individual headers were used the flow to each channel could be controlled. This would enable some spatial control of the rate of reforming, and similar to the tiled receiver-reactor designs help maintain more uniform receiver temperatures with a non-uniform heat flux.

(50) In the simulation, the maximum temperature at the receiver surface was approximately 60 to 80° C. cooler compared to the tiled or radial flow reactor geometries. The lower temperatures achievable with counter-flow geometry will have a significant beneficial impact on reactor strength and lifetime. However, the flow channels are more complex when compared to designs with radial flow in one direction, and this could increase the difficulty or cost of fabricating or assembling the reactor.

(51) The present invention describes embodiments of several reactors with smaller catalyst channel thickness and geometries that incorporate metal ribs or other structures improve heat conduction which benefits reactor performance. Counter-flow geometry is beneficial in reducing the maximum temperature and temperature gradients. Excessively high reactor temperatures can decrease lifetime and the strength and creep rupture resistance of the reactor housing, and degrade catalyst performance.

(52) Excessively cold temperatures are also to be avoided, as reaction kinetics and conversions are directly proportional for endothermic reactions. Thus there is a need to control localized reaction temperatures within a preferred temperature range.

(53) FIG. 10 is a parabolic dish concentrator including endothermic and exothermic reactors, in accordance with one embodiment of the present invention. The parabolic dish concentrators provide heat to drive endothermic hydrocarbon reforming reactions, such as methane steam reforming.

(54) The solar flux on a receiver surface is not uniform. The flux can vary with position, and maximum solar flux on a portion of the receiver can be 2 or 3 times the average flux. The receivers of the present invention are designed to cope with this nonuniform flux. Flow instabilities can also occur in receivers with multiple flow paths when large changes in the working fluids density and/or viscosity accompany the heat transfer process. Importantly, to maximize receiver efficiency it is desirable to have each portion of the receiver operating at close to an optimum flow rate that minimizes the temperature differences between the receiver's surface and working fluid and that maintains acceptable operating temperatures. Adaptive flow control may be used to accomplish this.

(55) Passive flow control features can be used that exploit differences in thermal expansion to increase the flow to hotter sections of the receiver, minimizing hot spots and providing more uniform heating of the working fluid. In one embodiment, such flow control features consist of inserts of a refractory metal, such as tungsten, with a different thermal expansion coefficient than the receiver's body. The difference in thermal expansion is used to open up a flow feature, such as an orifice, increasing working fluid flow in hotter sections of the receiver and reducing fluid flow in colder sections. These adaptive flow features are designed to adjust the flow within an appropriate range based on working fluid properties and solar fluxes expected at the receiver's surface.

(56) The use of adaptive flow control enhances flow stability and optimizes working fluid flow as a function of the solar flux incident on the receiver's surface. This type of adaptive flow control is desirable in solar receivers where the incident solar flux varies as a function of position. The desire to adaptively control the mass flow rate entering different portions of a heat exchange structure is somewhat unique to solar receivers. The control features of the present invention use differences in thermal expansion to increase the working fluid flow in hotter sections of the receiver, compensating for increased solar flux.

(57) The terms “passive flow control” and “adaptive flow control” are used interchangeably. The term “adaptive” describes how the control device adjusts the flow depending on the temperature. The term “passive” emphasizes that the device self-adjusts without the intervention of an outside user or signal.

(58) FIG. 13 shows the predicted solar flux map using DELSOL (an optical design tool) for a molten salt receiver panels at noon on equinox. From this map it can be seen that the solar flux varies almost an order of magnitude over the receiver surface; however the flux on the majority of the surface is within a factor of 3. It should also be noted that the solar flux distribution will vary as a function of time (time of day), and other environmental variables such as cloud cover. The spatial distances of the solar flux variability are large compared to the channel length used in a microchannel solar receiver.

(59) FIG. 14 shows the solar flux on a dish concentrator receiver. In this case the spatial variability in the solar flux occurs over distances that are comparable to the length of flow channels used in a micro- or meso-channel receiver. Both central and parabolic dish receivers benefit from an adaptive flow control system able to increase flow in high flux regions of the receiver. Adaptive flow control allows the same receiver panel to work at any location in a central receiver. In the dish concentrator systems, adaptive flow control can be used to cope with the large solar flux variations over relatively small distances. In this case the adaptive control allows each flow channel to operate close to the design temperature to obtain maximum performance. The adaptive flow control should be able to throttle the flow a factor of 3 or 4 over a temperature range of 50-100° C.

(60) FIG. 11 illustrates one such adaptive flow feature, although other geometries and mechanisms are possible. In the FIG. 11, the adaptive flow is accomplished using an insert constructed from a refractory material such as tungsten with a low thermal expansion coefficient compared to the receiver body. Other materials besides tungsten can include, but are not limited to, ceramics, silicon and other metals. As the temperature of the receiver increases the channel length increases, and the length of the insert increases by a smaller amount. This opens up an orifice used to control the flow of hot working fluid exiting the heat transfer channel.

(61) The adaptive flow control structure is designed to alter the flow—at the designed pressure drop—through the orifice or control feature changes in a way that compensates for the local heat flux, minimizing the differences in working fluid temperature exiting different portions of the receiver. FIG. 11 shows one adaptive flow feature of the present invention.

(62) For a quantitative calculation, assume the receiver temperature is kept within 100° C. of target—assume it is in the range of 700 to 800° C. with a target of 750° C., and to accomplish this it is necessary to throttle the flow in different portions of the receiver from zero flow to full flow. In this case the adaptive flow control is needed to go from completely shut (zero flow) to fully open with a 100° C. temperature change. Microchannels with height dimensions of a few thousands of an inch may be used. The flow control passage will have similar dimensions and will assume a 0.001″ relative motion between the two dissimilar metals is required to turn off the flow. Tungsten has a thermal expansion coefficient of approximately 4.7×10.sup.−6 to 5.0331×10.sup.−6 in/(in ° C.), and Hastalloy (refractory metal used to build the microchannel) has a thermal expansion coefficient of approximately 14×10.sup.−6 in/(in ° C.). The difference in the thermal expansion between these materials is about 9×10.sup.−6 in/(in ° C.). For a 100° C. temperature change we can achieve a difference in length of about 9×10.sup.−4, so to achieve a relative motion of 0.001″ an adaptive flow control element with a length of 1.11″ is required.

(63) It is also possible to build flow control features that use leverage to amplify the relative motion achieved from the thermal expansion. FIGS. 15A and 15B show some devices that amplify the motion due to thermal expansion. In these approaches the motion is increased at the cost of lower forces. Amplification of the maximum displacement can easily be increased at least one order of magnitude compared to the differences in thermal expansion of the different materials. These calculations and arguments show that the needed relative motion can be achieved using the differences in thermal expansion of Tungsten and Hastalloy without using leverage to amplify the motion. Actuator geometries are available that can amplify the motion, and these concepts can be adapted to actuate a flow control device.

(64) To achieve the greatest actuation for a given temperature rise it is desirable to make the passive flow control device out of a material with a thermal expansion significantly different from the thermal expansion of the flow channel. For example, Tungsten with a thermal expansion coefficient of approximately 5×10.sup.−6 cm/(cm ° C.) could be used to build a passive control device for use in a flow channel constructed from Hastalloy with a thermal expansion coefficient of approximately 14×10.sup.−6 cm/(cm ° C.). One implementation of this concept is shown in FIG. 12.

(65) FIG. 12 shows one implementation of a passive flow control device. In the flow channel, fluid flows from left to right and exits through the outlet orifice. The flow control device is made from a material with lower thermal expansion compared to the flow channel. The flow control device is attached to the channel walls at the ends of two different length legs and part of this device is an orifice cover that partially blocks the outlet orifice. As the fluid temperature increases, the flow channel and flow control device heat up. Differences in thermal expansion cause the control structure to rotate up, uncovering the outlet orifice and increasing the flow in the flow channel.

(66) The device of FIG. 12 makes use of leverage to achieve a larger displacement of the orifice cover and greater opening of the outlet orifice, compared to what could be achieved due to thermal expansion alone. For example, consider a passive flow control device constructed from tungsten placed in a flow channel constructed from Hastalloy. The longer leg of the passive control device may be approximately 1.5 cm long, and the shorter leg may be 0.5 cm long. A 50° C. increase in temperature will result in a relative difference of about 4.5×10.sup.−6 m for the flow channel length compared to the control structure over the 1 cm distance where the flow control structure is pinned to the flow channel. However, this modest 4.5×10.sup.−6 m difference can result in the orifice cover moving upward by up to about 9.5×10.sup.−4 m, amplifying the displacement over 100 times. Other geometries that amplify the displacement of an orifice cover are also possible.

(67) The present invention also describes systems and methods of efficiently converting solar energy into chemical energy. Applications include thermochemical energy storage for concentrating solar power plants, which otherwise would be unable to produce electricity when sunlight is not available, and the production of synthetic transportation fuels from natural gas and/or biomass. High methane conversion was accomplished as well as relatively high solar-to-chemical energy conversion.

(68) Also, the methanol synthesis reactor only partially converts syngas to methanol, so that the products of the system include both methanol and unconverted synthesis gas. In addition, the methanol synthesis reactor provides a substantial portion of the heat that is needed for water vaporization. It is through this integration of thermal components that high overall solar-to-chemical energy conversion can be obtained.

(69) The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. As such, references herein to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made in the embodiments chosen for illustration without departing from the spirit and scope of the invention.