Solar Fuels and Materials

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1 Solar Fuels and Materials ALDO STEINFELD ETH Swiss Federal Institute of Technology Zurich, Switzerland ANTON MEIER Paul Scherrer Institute Villigen, Switzerland 1. Introduction 2. Thermodynamics of Solar Thermochemical Conversion 3. Solar Thermochemical Processes 4. Outlook Glossary aperture Opening of a solar cavity receiver. Carnot efficiency Maximum efficiency for converting heat from a high-temperature thermal reservoir at T H into work in a cyclic process and rejecting heat to a low-temperature thermal reservoir at T L, given by 1 T L /T H. exergy efficiency (for a solar thermochemical process) The efficiency for converting into chemical energy, given by the ratio of the maximum work that may be extracted from a solar fuel to the input for producing such a fuel. solar cavity-receiver A well-insulated enclosure, with a small opening to let in concentrated, that approaches a black-body absorber in its ability to capture. solar concentration ratio Nondimensional ratio of the solar flux intensity (e.g., in suns ) achieved after concentration to the incident normal beam insolation. solar thermochemical process Any endothermic process that uses concentrated as the source of high-temperature process heat. This article develops the underlying science of solar thermochemical processes and describes their applications for delivering clean fuels and material commodities. The thermodynamics of solar thermochemical conversion are examined, the most promising solar thermochemical processes are discussed, examples of solar chemical reactors are presented, and the latest technological developments are summarized. 1. INTRODUCTION Using only 0.1% of the earth s land space with solar collectors that operate with a collection efficiency of merely 20%, one could gather more than enough energy to supply the current yearly energy needs of all inhabitants of the planet (B kwh). Furthermore, the reserve is essentially unlimited, and its use is ecologically benign. However, solar radiation is dilute (only B1kW/m 2 ), intermittent (available only during daytime and under clear-sky conditions), and unequally distributed (mostly by the equator). These drawbacks can be overcome by converting into chemical energy carriers solar fuels that can be stored long term and transported long range, from the sunny and desert regions to the industrialized and populated centers of the earth, where much of the energy is needed. Solar fuels can be burned to generate heat, further processed into electrical or mechanical work, or used directly to generate electricity in fuel cells and batteries to meet customers energy demands. Solar process heat can also assist in the processing and recycling of energyintensive materials, thereby avoiding greenhouse gas emissions and other pollutants derived from the combustion of fossil fuels for heat and electricity generation. There are basically three pathways for making solar fuels from : * Solar electrochemical path: solar-made electricity, from photovoltaic or solar thermal systems, followed by an electrolytic process * Solar photochemical path: direct use of the photon energy * Solar thermochemical path: solar-made heat followed by a thermochemical process Encyclopedia of Energy, Volume 5. r 2004 Elsevier Inc. All rights reserved. 623

2 624 Solar Fuels and Materials A B C Receiver Receiver Receiver Concentrator Heliostats Concentrator Absorption Reactants Heat Chemical reactor Q H,T H Solar fuels FIGURE 2 Schematic of the three main optical configurations for large-scale collection and concentration of : (A) the trough system, (B) the tower system, and (C) the dish system. Fuel cell Q L,T L Transportation and power generation FIGURE 1 Schematic of conversion into solar fuels. solar radiation is used as the energy source for high-temperature process heat to drive endothermic chemical reactions toward the production of storable and transportable fuels. Combinations of these three pathways are possible, but the thermochemical path offers some intriguing thermodynamic advantages. It is this approach that is the subject of this article, whereas the other two paths are beyond the scope of the article. Figure 1 illustrates the basic idea of the solar thermochemical path. By concentrating the diluted sunlight over a small area with help of parabolic mirrors and then capturing the radiant energy with help of suitable receivers and reactors, we can obtain heat at high temperatures for carrying out an endothermic chemical transformation and producing storable and transportable fuels. These solar fuels ultimately store within their chemical bonds. Regardless of the fuel, the higher the temperature of the reaction process, the higher the efficiency of energy conversion. However, higher temperatures also lead to greater losses by reradiation from the solar receiver. The thermodynamic implications of such phenomena are examined in the following section. 2. THERMODYNAMICS OF SOLAR THERMOCHEMICAL CONVERSION The state-of-the-art technology for large-scale collection and concentration of is based on three main optical configurations using parabolicshaped mirrors. These are the trough, tower, and dish systems shown schematically in Fig. 2. Trough systems use linear, two-dimensional, parabolic mirrors to focus sunlight onto a solar tubular receiver positioned along their focal line. Tower systems use a field of heliostats (two-axis tracking parabolic mirrors) that focus the sunrays onto a solar receiver mounted on top of a centrally located tower. Dish systems use paraboloidal mirrors to focus sunlight on a solar receiver positioned at their focus. A recently developed Cassegrain optical configuration for the tower system makes use of a hyperboloidal reflector at the top of the tower to redirect sunlight to a receiver located on the ground level. Solar furnaces are concentrating facilities in which high-flux solar intensities are usually obtained at a fixed location inside a housed laboratory. The traditional design consists of using a sun-tracking, flat heliostat on-axis with a stationary primary paraboloidal concentrator; off-axis configurations have also been designed. The capability of these collection systems to concentrate is often expressed in terms of their mean flux concentration ratio C over a targeted area A at the focal plane, normalized with respect to the incident normal beam insolation I as follows: C ¼ Q solar I A ; ð1þ where Q solar is the solar power input into the target. C is often expressed in units of suns when normalized to 1 kw/m 2. The solar flux concentration ratio typically obtained at the focal plane varies between 30 and 100 suns for trough systems, between 500 and 5,000 suns for tower systems, and between 1,000 and 10,000 suns for dish systems. Higher concentration ratios imply lower heat losses from smaller receivers and, consequently, higher attainable temperatures at the receiver. To some extent, the flux concentration can be further augmented with the help of a nonimaging secondary concentrator (e.g., a compound parabolic concentrator [CPC]) that is positioned in tandem with the primary concentrating system. The aforementioned

3 Solar Fuels and Materials 625 solar concentrating systems have proved to be technically feasible in large-scale experimental demonstrations aimed mainly at the production of solar thermal electricity in which a working fluid (typically air, water, helium, sodium, or molten salt) is solar heated and further used in traditional Rankine, Brayton, and Stirling cycles. Solar thermochemical applications, although not developed to the extent that solar thermal electricity generation has been, employ the same solar concentrating technology. Solar chemical reactors for highly concentrated solar systems usually feature the use of a cavity receiver-type configuration, that is, a well-insulated enclosure with a small opening (the aperture) to let in concentrated solar radiation. Because of multiple internal reflections, the fraction of the incoming energy absorbed by the cavity greatly exceeds the simple surface absorptance of the inner walls. The larger the ratio of the cavity s characteristic length to the aperture diameter, the closer the cavity receiver approaches a black-body absorber. Smaller apertures also serve to reduce reradiation losses. However, they intercept a reduced fraction of the sunlight reflected from the concentrators. Consequently, the optimum aperture size is a compromise between maximizing radiation capture and minimizing radiation losses. The absorption efficiency of a solar reactor, Z absorption, is defined as the net rate at which energy is being absorbed divided by the solar power coming from the concentrator. At temperatures above approximately 1000 K, the net power absorbed is diminished mostly by radiant losses through the aperture. For a perfectly insulated black-body cavity receiver, it is given by Z absorption ¼ 1 st4 ; ð2þ I C where T is the nominal cavity-receiver temperature and s is the Stefan Boltzmann constant. The absorbed concentrated solar radiation drives an endothermic chemical reaction. The measure of how well is converted into chemical energy for a given process is the exergy efficiency, defined as Z exergy ¼ ndgj 298 K; ð3þ Q solar where Q solar is the solar power input, n is the products molar flow rate, and DG is the maximum possible amount of work (Gibbs free energy change) that may be extracted from the products as they are transformed back to reactants at 298 K. The second law of thermodynamics is now applied to calculate the theoretical maximum exergy efficiency Z exergy,ideal. Because the conversion of solar process heat to chemical energy is limited by both the solar absorption efficiency and the Carnot efficiency, the maximum ideal exergy efficiency is given by Z exergy;ideal ¼ Z absorption Z Carnot ¼ 1 st4 H I C ~ 1 T L T H ; ð4þ where T H and T L are the upper and lower operating temperatures of the equivalent Carnot heat engine. Z exergy,ideal is plotted in Fig. 3 as a function of T H for T L ¼ 298 K and for various solar flux concentration ratios. Because of the Carnot limit, one should try to operate thermochemical processes at the highest upper temperature possible; however, from a heat transfer perspective, the higher the temperature, the higher the reradiation losses. The highest temperature an ideal solar cavity receiver is capable of achieving, defined as the stagnation temperature T stagnation, is calculated by setting Z exergy,ideal equal to zero, yielding T stagnation ¼ I 0:25 C : ð5þ s At this temperature, energy is being reradiated as fast as it is absorbed. Stagnation temperatures exceeding 3000 K are attainable with solar concentration ratios above 5000 suns. However, an energy-efficient η exergy, ideal 1 Carnot 0.9 T optimum , , , , , ,000 1,500 2,000 2,500 3,000 3,500 4,000 Temperature (K) FIGURE 3 Variation of the ideal exergy efficiency Z exergy,ideal as a function of the operating temperature T H for a black-body cavity receiver converting concentrated into chemical energy. The mean solar flux concentration is the parameter: 1,000 y 40,000. Also plotted are the Carnot efficiency and the locus of the optimum cavity temperature T optimum.

4 626 Solar Fuels and Materials process must run at temperatures that are substantially below T stagnation. There is an optimum temperature T optimum for maximum efficiency obtained by exergy;ideal ¼ 0: Assuming a uniform power flux distribution, this relation yields the following implicit equation for T optimum : Toptimum T LToptimum 4 T LI C ¼ 0: ð7þ 4s The locus of T optimum is shown in Fig. 3. For example, for a solar concentration ratio of 5000, the optimum temperature of a solar receiver is 1500 K, giving a maximum theoretical efficiency of 75%, that is, the portion of that could in principle be converted into the chemical energy of fuels. In practice, when considering convection and conduction losses in addition to reradiation losses, the efficiency will peak at a somewhat lower temperature. The process modeling described in the following paragraphs establishes a base for evaluating and comparing different solar thermochemical processes in terms of their ideal exergy efficiencies. Figure 4 depicts a flow diagram of an ideal cyclic process that leads to a fuel, which uses a solar reactor, a quenching device, and a fuel cell. The complete process is carried out at constant pressure. In practice, pressure drops will occur throughout the system. However, if one assumes frictionless operating conditions, no pumping work is required. The reactants may be preheated in an adiabatic heat exchanger, where some portion of the sensible and latent heat of the products is transferred to the reactants; for simplicity, a heat exchanger has been omitted. The reactor is assumed to be a perfect blackbody cavity receiver. The reactants enter the solar reactor at T L and are further heated to the reaction temperature T H. Chemical equilibrium is assumed inside the reactor. The net power absorbed in the solar reactor should match the enthalpy change per unit time of the reaction, Q reactor;net ¼ ndh j Reactants@TL!Products@T H : ð8þ Irreversibility in the solar reactor arises from the nonreversible chemical transformation and reradiation losses Q reradiation to the surroundings at T L.Itis found that Irr reactor ¼ Q solar T H þ Q reradiation T L þ n : DSj Reactants@TL!Products@T H : ð9þ Products exit the solar reactor at T H and are cooled rapidly to T L. The amount of power lost during quenching is Q quench ¼ ndh j Products@TH -Products@T L ð10þ Q solar T H The irreversibility associated with quenching is Irr quench ¼ Q quench T L þ : ð11þ ndsj Products@TH -Products@T L Q reradiation T L Quench Fuel cell T H T L Q quench Q F.C. W F.C. FIGURE 4 Model flow diagram of an ideal cyclic process for calculating the maximum exergy efficiency of a solar thermochemical process. Introducing a reversible ideal fuel cell, in which the products recombine to form the original reactants and thereby generate electrical power, closes the cycle. The maximum amount of work that the products leaving the reactor could produce if they combined at T L and a total pressure of 1 bar is given by W F:C: ¼ ndgj Products@TL -Reactants@T L : ð12þ This work value is also known as the exergy of the products at ambient temperature. The fuel cell operates isothermally; the amount of heat rejected to the surroundings is Q F:C: ¼ T L ndsj Products@ TL -Reactants@T L : ð13þ

5 Solar Fuels and Materials 627 The exergy system efficiency of the closed cycle is then calculated using Eq. (3) as Z exergy ¼ W F:C: : ð14þ Q solar This thermodynamic analysis can be verified by performing an energy balance and by evaluating the maximum achievable efficiency (Carnot efficiency) from the total available work and from the total power input. The energy balance should satisfy W F:C: ¼ Q solar Q reradiation þ Q quench þ Q F:C: : ð15þ The available work is calculated as the sum of the fuel cell work plus the lost work due to the irreversibility associated with the solar reactor and with quenching. Thus, Z max ¼ W F:C: þ T L Irr reactor þ Irr quench Q solar ¼ 1 T L ¼ Z T Carnot : ð16þ H Any solar thermochemical processes can be thought of in this manner, and their exergy efficiencies can be compared as one criterion for judging their relative industrial potentials. The higher the exergy efficiency, the lower the required solar collection area for producing a given amount of solar fuel and, consequently, the lower the costs incurred for the solar concentrating system, which usually correspond to half of the total investments for the entire solar chemical plant. Thus, high-exergy efficiency implies favorable competitiveness. 3. SOLAR THERMOCHEMICAL PROCESSES 3.1 Solar Production of Fuels Solar Hydrogen Five thermochemical routes for solar hydrogen production are depicted in Fig. 5. Indicated is the chemical source of H 2 :water for the solar thermolysis and solar thermochemical cycles, fossil fuels for the solar cracking, and a combination of fossil fuels and for the solar reforming and solar gasification. All of these routes make use of concentrated solar radiation as the energy source of high-temperature process heat H 2 from Although the single-step thermal dissociation of water, known as water thermolysis, is conceptually simple, it has been Solar Thermolysis Solar thermochemical cycles Solar reforming Solar hydrogen Fossil fuels (NG, oil, coal) Solar cracking CO 2 /C sequestration impeded by the need for a high-temperature heat source at above 2500 K to achieve a reasonable degree of dissociation and by the need for an effective technique to separate H 2 and O 2 to avoid recombination or ending up with an explosive mixture. Among the ideas proposed for separating H 2 from the products are effusion and electrolytic separation. Water-splitting thermochemical cycles bypass the H 2 /O 2 separation problem and further allow operating at relatively moderate upper temperatures. Previous studies performed on -splitting thermochemical cycles were characterized mostly by the use of process heat at temperatures below approximately 1300 K, available from nuclear and other thermal sources. These cycles required multiple (more than two) steps and were suffering from inherent inefficiencies associated with heat transfer and product separation at each step. During recent years, significant progress has been made in the development of optical systems for large-scale collection and concentration of capable of achieving solar concentration ratios of 5000 suns or higher. Such high-radiation fluxes allow the conversion of solar energy to thermal reservoirs at 2000 K and above that are needed for the two-step thermochemical cycles using metal oxide redox reactions: First step ðsolarþ: M x O y -xm þ y 2 O 2 ð17þ Second step ðnonsolarþ: xm þ y-m x O y þ yh 2 ð18þ Here, M denotes a metal and M x O y denotes the corresponding metal oxide. The cycle is represented Solar gasification FIGURE 5 Five thermochemical routes for the production of solar hydrogen.

6 628 Solar Fuels and Materials M x O y Solar reactor M x O y = xm + y/2 O 2 M ½ O Hydrolysis reactor xm + y = M x O y + yh 2 H Recycle M x O y FIGURE 6 Representation of a two-step water-splitting thermochemical cycle using metal oxide redox reactions. in Fig. 6. The first endothermic step is the solar thermal dissociation of the metal oxide to the metal or the lower valence metal oxide. The second, nonsolar exothermic step is the hydrolysis of the metal to form H 2 and the corresponding metal oxide. The net reaction is ¼ H 2 þ 0.5O 2, but because H 2 and O 2 are formed in different steps, the need for high-temperature gas separation is thereby eliminated. The most favorable candidate metal oxide redox pairs for this cycle are ZnO/Zn and Fe 3 O 4 / FeO. In both cases, the minimum temperature requirement for the solar step is 2300 K, and the products need to be either quenched or separated at high temperatures to prevent their recombination. Quenching introduces irreversibility and may be a factor of complexity in large-scale use. Figure 7 shows a solar chemical reactor concept for performing the thermal dissociation of ZnO that features a windowed rotating cavity receiver lined with ZnO particles that are held by centrifugal force. With this arrangement, ZnO is directly exposed to high-flux solar irradiation and serves the functions of radiant absorber, thermal insulator, and chemical reactant simultaneously. The shell of the cavity is made from conventional steel materials and is lined with the same material as are the reactants themselves. This aspect of the design eliminates the need for using expensive and difficult-to-fabricate ceramic insulating materials for ultra-high temperatures. It also offers excellent resistance to thermal shocks that are intrinsic in short start-up solar applications. The direct absorption of concentrated by directly irradiated reactants FIGURE 7 Schematic of a rotating-cavity solar chemical reactor for the thermal dissociation of ZnO to Zn and O 2 at 2300 K. It consists of a rotating conical cavity receiver (1) that contains an aperture (2) for access of concentrated solar radiation through a quartz window (3). The solar flux concentration is further augmented by incorporating a CPC (4) in front of the aperture. Both the window mount and the CPC are water cooled and integrated into a concentric (nonrotating) conical shell (5). ZnO particles are fed continuously by means of a screw powder feeder located at the rear of the reactor (6). The centripetal acceleration forces the ZnO powder to the wall, where it forms a thick layer of ZnO (7) that insulates and reduces the thermal load on the inner cavity walls. A purge gas flow enters the cavity receiver tangentially at the front (8) and keeps the window cool and clear of particles or condensable gases. The gaseous products Zn and O 2 exit continuously via an outlet port (9) to a quench device (10). Source: Paul Scherrer Institute, Villigen, Switzerland. provides efficient radiation heat transfer to the reaction site where the energy is needed, bypassing the limitations imposed by indirect heat transport via heat exchangers. We illustrate the application of the Second-Law (exergy) analysis, Eqs. (1) to (16), for calculating the theoretical maximum exergy efficiency of the twostep water-splitting solar thermochemical cycle based on the ZnO/Zn redox reaction. Figure 8 depicts the ideal cyclic process. The solar reactor is assumed to be a cavity-receiver operating at 2300 K. The reactant ZnO(s) enters the solar reactor at 298 K and is further heated to the reaction temperature at 2300 K. The molar feed rate of ZnO to the solar reactor, n; is set to 1 mol/s and is equal to that of fed to the hydrolyser reactor. After quenching, the products separate naturally (without expending work) into gaseous O 2 and condensed phase Zn. Zinc is sent to the hydrolyser reactor to react exothermally with water and form hydrogen, according to Eq. (18). The heat liberated is assumed

7 Solar Fuels and Materials 629 TABLE I Q solar T H = 2300 K Exergy Analysis of the Two-Step Water-Splitting Solar Thermochemical Cycle Using the Process Modeling Depicted in Fig. 8 C 5,000 10,000 Q F.C. lost to the surroundings, as given by ¼ ndhj ZnþH2 O@298 K-ZnOþH K : ð19þ Q hydrolyser W F.C. Fuel cell Q reradiation 298 K ½O 298K Quench Hydrolyser Zn + ½ O 2300K Zn@ 298K Q quench Q hydrolyser Thus, the irreversibility associated with the hydrolyser is Irr hydrolyser ¼ Q hydrolyser 298 K þ ndsj ZnþH2 O@298 K-ZnOþH K : ð20þ Introducing an H 2 /O 2 fuel cell, in which the products recombine to form and thereby generate electrical power, closes the cycle. W F.C. and Q F.C. are the work output and the heat rejected, respectively, given by W F:C: ¼ Z F:C: ndgj H2 þ0:5o K-@298 K ð21þ Q F:C: ¼ T L ndsj H2 þ0:5o K-@298 K ; ð22þ where Z F.C. is the efficiency of the H 2 /O 2 fuel cell, assumed to be 100% when based on the maximum work output of H 2 (also known as its exergy). Finally, the exergy efficiency is calculated using Eq. (14). Table I gives the results of the calculation for two values of the solar concentration ratio: 5,000 and 10, H 2 from Fossil Fuels Three solar thermochemical processes for H 2 production using fossil H 2 ZnO FIGURE 8 Model flow diagram of the two-step water-splitting solar thermochemical cycle used for the exergy analysis. Q solar 815 kw 662 kw Q reradiation 258 kw 105 kw Q reactor,net 557 kw 0.81 kw K 1 Irr reactor Q quench Irr quench Q hydrolyser Irr hydrolyser 209 kw 0.52 kw K 1 64 kw 0.27 kw K 1 Q F.C. 49 kw W F.C. 237 kw Z absorption 68% 84% Z exergy (without heat recovery) 29% 36% Z exergy (with heat recovery) 69% 84% Note. The ZnO and molar rates are set to 1 mol/s. fuels as the chemical source are considered: cracking, reforming, and gasification. These routes are shown schematically in Fig. 9. The solar cracking route refers to the thermal decomposition of natural gas (NG), oil, and other hydrocarbons and can be represented by the simplified net reaction: C x H y ¼ xcðgrþþ y 2 H 2: ð23þ Other compounds may also be formed, depending on the reaction kinetics and on the presence of impurities in the raw materials. The thermal decomposition yields a carbon-rich condensed phase and a hydrogen-rich gas phase. The carbonaceous solid product can either be sequestered without CO 2 release or be used as a material commodity under less severe CO 2 restraints. It can also be applied as a reducing agent in metallurgical processes. The hydrogen-rich gas mixture can be further processed to high-purity hydrogen that is not contaminated with oxides of carbon and, thus, can be used in proton exchange membrane (PEM) fuel cells without inhibiting platinum-made electrodes. From the point of view of carbon sequestration, it is easier to separate, handle, transport, and store solid carbon than gaseous CO 2. Furthermore, while the steamreforming/gasification method requires additional steps for shifting CO and for separating CO 2, the thermal cracking accomplishes the removal and separation of carbon in a single step. In contrast,

8 630 Solar Fuels and Materials Fossil fuels (NG, oil) Solar cracking reactor Feed NG Particle feed H 2 sweep O 2 H 2 C Sequestration Fuel cell Work output Porous C(gr) tube Fossil fuels (coal, NG, oil) Solar gasification/ reforming reactor Solid C(gr) absorber CO H2 Quartz envelope Shift reactor CO 2 H 2 Separation O 2 H 2 CO 2 Sequestration H 2, C, CH x Fuel cell Work output FIGURE 9 Schematic of solar thermochemical routes for H 2 production using fossil fuels and as the chemical source: solar cracking (upper box) and solar reforming and gasification (lower box). the major drawback of the thermal decomposition method is the energy loss associated with the sequestration of carbon. Thus, the solar cracking may be the preferred option for NG and other hydrocarbons with high H 2 /carbon ratios. The exergy efficiency for these routes is defined as the ratio of the work output by the fuel cell to the total thermal energy input by both the and the heating value of the reactants: W F:C: Z exergy ¼ : ð24þ Q solar þ HHV reactant where HHV reactant is the high heating value of the fossil fuel being processed (e.g., B890 kj mol 1 for NG). Assuming a 65% efficient H 2 /O 2 fuel cell, Z exergy ¼ 30%. This route offers zero CO 2 emissions as a result of carbon sequestration. However, the energy penalty for completely avoiding CO 2 reaches 30% of the electrical output vis-à-vis the direct use of NG for fueling a 55% efficient combined Brayton FIGURE 10 Scheme of an aerosol solar reactor for cracking NG. Source: National Renewable Energy Laboratory, Golden, CO. Rankine cycle. Figure 10 shows an aerosol solar reactor concept for cracking NG that features two concentric graphite tubular reactors, with the outer solid tube serving as the solar absorber and the inner porous tube containing a flow of NG laden with carbon nanoparticles that serve the functions of radiant absorbers. The steam reforming of NG, oil, and other hydrocarbons, as well as the steam gasification of coal and other carbonaceous materials, can be represented by the simplified net reaction: C x H y þ x ¼ y 2 þ x H 2 þ xco: ð25þ Other compounds may also be formed, especially with coal, but some impurities contained in the raw materials are cleaned out prior to the decarbonization process. The principal product is high-quality synthesis gas (syngas), the building block for a wide

9 Solar Fuels and Materials 631 variety of synthetic fuels, including Fischer Tropschtype chemicals, hydrogen, ammonia, and methanol. Its quality is determined mainly by the H 2 /CO and CO 2 /CO molar ratios. For example, the solar steam gasification of anthracite coal at above 1500 K yields syngas with an H 2 /CO molar ratio of 1.2 and a CO 2 / CO molar ratio of The CO content in the syngas can be shifted to H 2 via the catalytic water gas shift reaction (CO þ ¼ H 2 þ CO 2 ), and the product CO 2 can be separated from H 2 using, for example, the pressure swing adsorption technique. The exergy efficiency for this route is defined by Eq. (24). Assuming a 65% efficient H 2 /O 2 fuel cell and HHV reactant of 35,700 kj kg 1 for anthracite coal, Z exergy ¼ 46%. This route offers a net gain in the electrical output by a factor varying in the range of 1.7 to 1.8 (depending on the coal type) vis-à-vis the direct use of coal for fueling a 35% efficient Rankine cycle. If CO 2 is not sequestered, specific CO 2 emissions amount to 0.53 to 0.56 kg CO 2 / kwh e, approximately half as much as the specific CO 2 emissions discharged by conventional coal-fired power plants. Some of these processes are practiced at an industrial scale, with the process heat supplied by burning a significant portion of the feedstock. Internal combustion results in the contamination of the gaseous products, whereas external combustion results in a lower thermal efficiency due to the irreversibility associated with indirect heat transfer. Feed gas in C L Top view 2345 mm Tubes Alternatively, using for process heat offers a threefold advantage: (1) the discharge of pollutants is avoided, (2) the gaseous products are not contaminated, and (3) the calorific value of the fuel is upgraded by adding in an amount equal to the DH of the reaction. The solar reforming of NG has been studied extensively in solar furnaces with small-scale solar reactor prototypes using Rh-based catalyst. Recently, this solar process has been scaled up to power levels of 300 to 500 kw in a solar tower facility using two solar reactor concepts: indirect irradiation and direct irradiation. Figure 11 shows a schematic of the indirectirradiated solar reforming reactor. It consists of a pentagonal cavity receiver, insulated with ceramic fibers, containing a set of Inconel tubes arranged vertically in two banks parallel to the front-side walls. The tubes are filled with a packed bed of catalyst, usually 2% Rh on Al 2 O 3 support. The tubular reactors are externally exposed to the concentrated solar radiation entering through the cavity aperture and transfer the absorbed heat to the chemical reactants. The aperture is windowless and tilted from the horizontal axis for an optimal view to the heliostat field. A matching CPC is implemented at the aperture for capturing radiation spillage, augmenting the average solar flux concentration, and providing uniform heating of the tubes. Indirect-irradiated reactors such as this one have the advantage of eliminating the need for a transparent window. The disadvantages are linked to the limitations imposed by the materials of construction of the reactor walls: limitations in the maximum operating temperature, thermal conductivity, radiant absorptance, inertness, resistance to thermal shocks, and suitability for transient operation. Figure 12 shows a schematic of the directirradiated solar reforming reactor, also referred to CPC Window Reactants inlet Side view Tubes 4500 mm Aperture 2035 mm 2675 mm Sun Concentrator Syngas out Insulated receiver FIGURE 11 Scheme of an indirect-irradiated solar reforming chemical reactor. Source: Weizmann Institute of Science, Rehovot, Israel. Catalytic absorber Products outlet FIGURE 12 Scheme of a direct-irradiated solar reforming chemical reactor. Source: Deutsches Zentrum für Luft- und Raumfahrt, Germany.

10 632 Solar Fuels and Materials as the volumetric reactor. The main component is the porous ceramic absorber, coated with Rh catalyst, that is directly exposed to the concentrated solar radiation. A concave quartz window, mounted at the aperture, minimizes reflection losses and permits operation at high pressures. Similar to the indirectirradiated reactor, a CPC is implemented at the aperture. Direct-irradiated reactors such as this one have the advantage of providing efficient radiation heat transfer directly to the reaction. Furthermore, under proper conditions, direct irradiation may photochemically enhance the kinetics of the reaction. The major drawback, when working with reducing or inert atmospheres, is the requirement of a transparent window, which is a critical and troublesome component under high-pressure and severe gas environment. The solar dry reforming of CH 4 (with CO 2 ) can be applied in a closed-loop chemical heat pipe (Fig. 13). The product of this reversible reaction is syngas that can be stored at ambient temperatures and transported to the site where the energy is needed. By the reverse exothermic reaction, stored is released in the form of high-temperature heat, which can be used, for example, for generating electricity via a Rankine cycle. The products of this reverse reaction are again recycled to the solar reactor, where the process is repeated. The dissociation/synthesis of NH 3 can also be applied in a chemical heat pipe for storage and transportation of. An optional source of H 2 is H 2 S, a highly toxic industrial product recovered in large quantities in the sweetening of NG and in the removal of organically A B A B Endothermic reactor Exothermic reactor A Power generation Storage and transport B FIGURE 13 Solar chemical heat pipe for the storage and transportation of. High-temperature solar process heat is used to drive the endothermic reversible reaction A-B. The product B may be stored long term and transported long range to the site where the energy is needed. At that site, the exothermic reverse reaction B-A is effected and yields hightemperature process heat in an amount equal to the stored solar energy DH A-B. The chemical product A of the reverse reaction is returned to the solar reactor for reuse. bound sulfur from petroleum and coal. Current industrial practice uses the Claus process to recover the sulfur from H 2 S, but the process wastes H 2 to produce low-grade process heat. Alternatively, H 2 S can be decomposed to H 2 and S 2 at 1800 K and 0.03 to 0.5 bar. The product gas mixture is quenched, and condensed S 2 is separated from H 2. In contrast to thermolysis, the quench is relatively easy and the reverse reaction between the products seems to be unimportant at temperatures as high as 1500 K Solar Metals Metals are attractive candidates for storage and transport of. They may be used to generate either high-temperature heat via combustion or electricity via fuel cells and batteries. Metals can also be used to produce hydrogen from via hydrolysis, as shown in Eq. (18). The chemical products from any of these power/h 2 -generating processes are metal oxides, which in turn need to be recycled. The conventional extraction of metals from their oxides by carbothermic and electrolytic processes is characterized by its high energy consumption and its concomitant environmental pollution. The extractive metallurgical industry discharges vast amounts of greenhouse gases and other pollutants to the environment, derived mainly from the combustion of fossil fuels for heat and electricity generation. These emissions can be reduced substantially, or even eliminated completely, by using concentrated as the source of high-temperature process heat. The thermal dissociation and electrothermal reduction of metal oxides proceeds without reducing agents, while the carbothermal reduction of metal oxides uses solid carbon C(gr) or hydrocarbons (e.g., CH 4 ) as reducing agents. The corresponding overall chemical reactions may be represented as follows: M x O y -xm þ y 2 O 2 ð26þ M x O y þ ycðgrþ-xm þ yco ð27þ M x O y þ ych 4 -xm þ yð2h 2 þ COÞ: ð28þ Table II lists the approximate temperatures at which the standard DG1 rxn for Reactions (26), (27), and (28) equals zero for various metal oxides of interest. Except for the thermal dissociation of ZnO, the required temperature for effecting Reaction (26) exceeds 3000 K. At temperatures for which DG1 rxn is positive, solar process heat alone will not make the reaction proceed; some amount of high-quality energy is required in the form of work. It may be

11 Solar Fuels and Materials 633 TABLE II Approximate Temperatures (K) for Which DG 1 rxn of Reactions (26), (27), and (28) Equals Zero Metal oxide DG 1 rxn 26 ¼ DG1 rxn 27 ¼ DG1 rxn 28 ¼ 0@ a Fe 2 O Al 2 O MgO ZnO a TiO a SiO CaO a Fe 2 O 3,TiO 2, and SiO 2 decompose to lower valence oxides before complete dissociation to the metal. Reactants inlet Off-gas outlet supplied in the form of electrical energy in electrolytic processes or in the form of chemical energy by introducing a reducing agent in thermochemical processes. An example of a solar electrothermal reduction process that has been demonstrated experimentally is the electrolysis of ZnO using an electrolytic cell housed in a solar cavity-receiver. At 1000 K, up to 30% of the total amount of energy required to produce Zn could be supplied by solar process heat. Other interesting candidates for solar high-temperature electrolysis are MgO and Al 2 O 3. If one wishes to decompose metal oxides thermally into their elements without the application of electrical work, a chemical reducing agent is necessary to lower the dissociation temperature. Coal (as coke) and NG (as methane) are preferred reducing agents in blast furnace processes due to their availability and relatively low price. In the presence of carbon, the uptake of oxygen by the formation of CO brings about a reduction of the oxides at much lower temperatures. Although Reactions (27) and (28) have favorable free energies above the temperatures indicated in Table II, a more detailed calculation of the chemical equilibrium composition shows that only the carbothermic reductions of Fe 2 O 3, MgO, and ZnO will result in significant free metal formation. The carbides Al 3 C 4, CaC 2, SiC, and TiC are thermodynamically stable in an inert atmosphere; the nitrides AlN, Si 3 N 4, and TiN are stable in an N 2 atmosphere. Using NG as a reducing agent, as indicated in Eq. (28), combines in a single process the reduction of metal oxides with the reforming of NG for the coproduction of metals and syngas. Thus, CH 4 is reformed in the absence of catalysts and, with proper optimizations, may be made to produce high-quality FIGURE 14 Schematic of a two-cavity solar chemical reactor concept for the carbothermal reduction of ZnO. It features two cavities in series, with the inner one functioning as the solar absorber and the outer one functioning as the reaction chamber. The inner cavity (1) is made of graphite and contains a windowed aperture (2) to let in concentrated solar radiation. A CPC (3) is implemented at the reactor s aperture. The outer cavity (4) is well insulated and contains the ZnO carbon mixture that is subjected to irradiation by the graphite absorber separating the two cavities. With this arrangement, the inner cavity protects the window against particles and condensable gases coming from the reaction chamber. Uniform distribution of continuously fed reactants is achieved by rotating the outer cavity (5). The reactor is designed specifically for beam-down incident radiation, as obtained through a Cassegrain optical configuration that makes use of a hyperbolical reflector at the top of the tower to redirect sunlight to a receiver located on the ground level. Source: Paul Scherrer Institute, Villigen, Switzerland. syngas with an H 2 /CO molar ratio of 2, which is especially suitable for synthesizing methanol a potential substitute for petrol. Thermal reductions of Fe 3 O 4 and ZnO with C(gr) and CH 4 to produce Fe, Zn, and syngas have been demonstrated in solar furnaces using fluidized bed and vortex-type reactors. These reactions are highly endothermic and proceed to completion at temperatures above approximately 1300 K. Zinc production by solar carbothermic reduction of ZnO offers a CO 2 emission reduction by a factor of 5 vis-à-vis the conventional fossil fuelbased electrolytic or imperial smelting process. Furthermore, the use of for supplying the enthalpy of the reaction upgrades the calorific value of the initial reactants by 39%. Two examples of solar chemical reactor concepts for producing Zn by Reactions (27) and (28) are shown in Figs. 14 and 15, respectively: the twocavity solar reactor based on the indirect irradiation

12 634 Solar Fuels and Materials of ZnO þ carbon and the vortex solar reactor based on the direct irradiation of ZnO þ methane. 3.2 Solar Production of Material Commodities ZnO + CH 4 5 Zn + 2H 2 + CO FIGURE 15 Schematic of a vortex solar chemical reactor concept for the combined ZnO reduction and CH 4 reforming. It consists of a cylindrical cavity (1) that contains a windowed aperture (2) to let in concentrated. Particles of ZnO, conveyed in a flow of NG, are continuously injected into the reactor s cavity via a tangential inlet port (3). Inside the reactor s cavity, the gas particle stream forms a vortex flow that progresses toward the front following a helical path. The chemical products, Zn vapor and syngas, exit the cavity continuously via a tangential outlet port (4) located at the front of the cavity, behind the aperture. The window (5) is actively cooled and kept clear of particles by means of an auxiliary flow of gas (6) that is injected tangentially and radially at the window and aperture planes, respectively. Energy absorbed by the reactants is used to raise their temperature to above approximately 1300 K and to drive Reaction (28). Source: Paul Scherrer Institute, Villigen, Switzerland Solar Metals, Metallic Oxides, Carbides, and Nitrides The extractive metallurgical industry is a major consumer of process heat and electricity and is responsible for approximately 10% of global anthropogenic greenhouse gas emissions. A solar thermal process would drastically reduce these emissions. For example, solar processing of aluminum could reduce the CO 2 emissions per ton of aluminum from 44 to 5. The temperature required to reduce aluminum ores with carbon is in the range of 2400 to 2600 K, that is, at the upper limit for practical process heat addition from a combustion source alone. Preliminary tests performed in a concentrating solar facility suggest two feasible solar thermal routes. First, at approximately 2400 K, a mixture of Al 2 O 3, SiO 2, and carbon results in an Al Si alloy that can be purified after its production. Second, at above 2500 K, Al 2 O 3 and carbon results in an Al Al 3 C 4 mixture of varying compositions. As described in the previous subsection, Zn can be produced from ZnO by a solar thermal, electrothermal, or carbothermal reduction process. Other examples of solar thermal reduction processes that have been demonstrated in solar furnaces exploratory runs are the production of Zn by dissociation of its natural ore willemite (Zn 2 SiO 4 ), the combined ZnO reduction and biomass pyrolysis, and the production of Mg and Fe by carbothermal reduction of their oxides. Metallic carbides and nitrides can be produced by the solar carbothermic reduction of metal oxides. The reaction is generally given by M x O y þ Cgr ð ÞfþN 2 g- M x 0C y 0; M x 00N y 00 þ CO: ð29þ Exploratory experimental studies were conducted in solar furnaces in which nitrides (AlN, Si 3 N 4, TiN, and ZrN) were formed for the systems run in N 2 and carbides (Al 4 C 3,CaC 2, SiC, TiC) were formed for the systems run in inert atmosphere. These ceramics are valuable materials for high-temperature applications due to their high hardness, excellent corrosion resistance, high melting points, and low coefficients of thermal expansion. CaC 2 is well known as the feedstock for the production of acetylene. The nitrides and carbides AlN, Fe 3 C, and Mn 3 C may also be used in cyclic processes as feedstock to produce hydrogen and hydrocarbons or may serve as intermediaries in the production of the metal. The hydrolysis of AlN yields NH 3, the hydrolysis or acidolysis of Fe 3 Cyields liquid hydrocarbons, and the hydrolysis of the various carbides of manganese yields H 2 and hydrocarbons in different proportions. Thus, Reaction (29) may be incorporated into cyclic processes of the type shown in Fig. 16, in which the metal oxides that result from the hydrolysis are recycled to the solar reactor, and CO from Reaction (29) may be shifted to H 2 via the water gas shift reaction Solar Fullerenes and Carbon Nanotubes Fullerenes and carbon nanotubes have become a major field in condensed matter physics and chemistry. However, the large-scale production of these materials with high yield and selectivity is still a

13 Solar Fuels and Materials 635 Gas inlet C-source Solar reactor M M x O x O + C(gr) M y x' C y' + CO y M x O + C(gr) + N y 2 M x" N y" + CO Metal carbide Metal nitride Hydrolysis reactor M x' C y' + C x''' H + M y''' x O y Syngas Hydrogen Hydrocarbons Ammonia Solar flux Graphite target Heat exchanger Gas outlet Recycle M x" N y" + NH 3 + M x O y M x O y FIGURE 16 Schematic of a two-step thermochemical process for the production of syngas, hydrogen, hydrocarbons, and ammonia using metal oxides and. In the first (endothermic) step, the metal oxide is carbothermally reduced to a metal carbide or nitride using solar process heat. Subsequently, the carbide or nitride is reacted with water to produce hydrocarbons or ammonia, and CO is water gas shifted to syngas, whereas the metal oxide is recycled to the solar step. M, metal; M x O y, metal oxide; M x 0C y 0, metal carbide; M x 00N y 00, metal nitride. crucial problem. Conventional methods for the synthesis of fullerenes, such as electric arc discharge and laser ablation, fail when the process is being scaled up to higher power levels. The solar vaporization process seems to be more promising using the solar reactor shown in Fig. 17, in which a graphite rod contained under vacuum pressure behind a hemispherical quartz window is directly irradiated with peak solar concentration ratios exceeding 7000 suns and the vaporized carbon (at temperatures above 3300 K) is swept out by Ar, quenched, and collected in a filter bag. The key parameters characterizing this process are the carbon soot mass flow rate and the desired product yield. The former is a function of the target temperature, whereas the latter is a function of specific reactor variables such as fluid flow patterns, residence times, concentration of the carbon vapor in the carrier gas, target temperature, and temperature distribution in the cooling zone. Catalytic filamentous carbon (CFC) can be produced by the solar thermal decomposition of hydrocarbons in the presence of small metal catalyst particles. Solar furnace experiments confirmed that nanotubes can be obtained using Co/MgO catalyst for CO and CH 4 þ H 2 and that nanofibers can be obtained on Ni/Al 2 O 3 catalyst for CO, CH 4, CH 4 þ H 2, and C 4 H 10. Figure 18 shows a solar chemical reactor used for such experimental runs. It consists of a quartz tube containing a fluidized bed of catalyst and Al 2 O 3 grains. A secondary reflector, composed of a two-dimensional CPC FIGURE 17 Scheme of a solar reactor for fullerene synthesis. Source: Institut de Science et de Génie des Matériaux et Prodédés, Centre National de la Recherche Scientifique, Font-Romeu, France. Involute + two-dimensional CPC solar radiation Gaseous products to gas chromatograph CH 4 + Ar Quartz reactor Fluidized bed (catalyst + Al 2 O 3 ) Quartz filter FIGURE 18 Scheme of a solar reactor for CFC production. Source: Paul Scherrer Institute, Villigen, Switzerland. coupled to an involute, provides uniform irradiation on the tubular reactor. CFC formed typically has the following properties: surface area 100 to 170 m 2 /g, pore volume 0.4 to 0.8 cm 3 /g, micropore volume to cm 3 /g, and average pore diameter 10 to 40 nm Solar Lime and Cement The thermal decomposition of calcium carbonate (limestone) to calcium oxide (lime), CaCO 3 - CaO þ CO 2 ; ð30þ is the main endothermic step in the production of lime and cement at 1300 K. Substituting concentrated in place of carbonaceous fuels, as the source of high-temperature process heat, is a means to reduce the dependence on conventional

14 636 Solar Fuels and Materials Solid feed Insulating case solar radiation Exhaust gas stream Insulating case SiC crucible T5 Radiation duct Solid feed Melt film Air T4 T3 T2 T1 solar radiation Refractory lining Product outlet FIGURE 19 Schematic of a direct-irradiated rotary reactor for the solar production of lime. Source: Paul Scherrer Institute, Villigen, Switzerland. Absorber tubes Products outlet Reactants inlet Preheating chamber Rotating cavity Ceramic insulation FIGURE 20 Schematic of an indirect-irradiated rotary reactor for the solar production of lime. Source: Paul Scherrer Institute, Villigen, Switzerland. energy resources and to reduce emissions of CO 2 and other pollutants. Figures 19 and 20 show two solar reactor concepts for the solar calcination process based on direct irradiation and indirect irradiation, respectively. The direct-irradiated solar reactor (Fig. 19) consists of a refractory-lined conical rotary kiln operated in a horizontal position. Because of the rotational movement of the kiln, the reactants are transported within the conical reaction chamber from the preheating zone in the back (feeding side) to the high-temperature zone in the front (discharging side). The indirect-irradiated reactor (Fig. 20) consists of a multiple-tube absorber and a preheating chamber, both made from SiC. The well-insulated rotary reactor is tilted and works in continuous mode of operation Solar Thermal Recycling of Hazardous Waste Materials Solid waste materials from a wide variety of sources (e.g., municipal waste incineration residuals, discharged batteries, dirty scraps, automobile shredded residue, contaminated soil, dusts, and sludge, other by-products from the metallurgical industry) contain hazardous compounds that are usually vitrified in a Melt T6 Heating FIGURE 21 Scheme of a rotary kiln for aluminum melting. Source: Deutsches Zentrum für Luft- und Raumfahrt, Germany. nonleaching slag and are finally disposed of at hazardous waste storage sites. However, limited storage space, increasing storage costs, and environmental regulations have led the need for developing technologies that recycle these toxic materials into useful commodities rather than deposit them in dump sites for an undetermined period of time. Thermal processes are well suited for the treatment of these complex solid waste materials. Waste materials containing carbonaceous compounds can be converted by thermal pyrolysis and gasification into syngas and hydrocarbons that can be further processed into other valuable synthetic chemicals. Those containing metal oxides may be converted by carbothermal reduction into metals, nitrides, carbides, and other metallic compounds. The commercial recycling techniques by blast, induction, arc, and plasma furnaces are major consumers of fossil fuelbased electricity and heat. offers the possibility of converting hazardous solid waste material into valuable commodities for processes in closed and sustainable material cycles. Examples of recycling processes demonstrated in solar furnaces are the processing of electric arc furnace dust using the two-cavity reactor depicted in Fig. 14 and the melting of Al scrap using a SiClined rotary kiln shown in Fig. 21, which can be tilted for discharging molten metal and slag into molds. Other recent applications in the area of solar thermal processing of materials that are being explored include hardening of carbon steel, production of expanded natural graphite (ENG) by exfoliation of graphite intercalated compounds, synthesis of advanced ceramics, and purification of metallurgicalgrade silicon for photovoltaic applications Economics Economic assessments indicate that solar thermochemical processes for the production of hydrogen and other solar fuels have the potential of becoming economically competitive vis-à-vis alternative paths Melt

15 Solar Fuels and Materials 637 for producing solar fuels such as via electrolysis using solar-generated electricity. These studies further indicate that the heliostat field is responsible for approximately half of the total investment costs for the entire chemical plant, whereas the cost of the solar reactor represents 10 to 15%. However, for a fixed product throughput, the solar reactor s efficiency dictates the size of the heliostat field. Thus, reaching high solar reactor efficiencies and reducing the cost of the heliostats per unit area will have a significant impact on reducing the unit cost of the solar fuel. Application of credit for CO 2 mitigation and pollution avoidance will further enable the solar thermochemical technologies to compete favorably with fossil fuel-based processes. The weaknesses of these economic evaluations are related primarily to the uncertainties in the viable efficiencies and investment costs of the various components due to their early stage of development and their economies of scale. 4. OUTLOOK Solar thermochemical processes have favorable longterm prospects because they avoid or reduce costs for CO 2 mitigation and pollution abatement. The products are renewable fuels and material commodities for delivering clean and sustainable energy services. Further development and large-scale demonstration are warranted. SEE ALSO THE FOLLOWING ARTICLES Alternative Transportation Fuels: Contemporary Case Studies Hydrogen Production Materials for Solar Energy Photosynthesis, Artificial Solar Cells Solar Cooling, Dehumidification, and Air-Conditioning Solar Detoxification and Disinfection Solar Distillation and Drying Solar Heat Pumps Solar Ponds Solar Thermal Energy, Industrial Heat Applications Solar Thermal Power Generation Solar Water Desalination Further Reading Fletcher, E. A. (2001). Solarthermal processing: A review. J. Solar Energy Eng. 123, Steinfeld, A., and Palumbo, R. (2001). Solar thermochemical process technology. In Encyclopedia of Physical Science and Technology. (R. A. Meyers, Ed.), vol. 15, pp Academic Press, San Diego.

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