This review article develops some of the underlying science for converting concentrated solar energy into chemical fuels and presents examples of solar thermochemical processes and reactors.
©2010 Optical Society of America
1. Thermodynamics of Solar Thermochemical Conversion
Solar thermochemical processes make use of concentrated solar radiation as the energy source of high-temperature process heat to drive endothermic reactions aimed at the production of chemical fuels . Solar reactors for highly concentrated solar systems usually feature the use of a cavity-receiver type configuration, i.e. 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 surface absorptance of the inner walls. As the ratio of the cavity’s characteristic length to the aperture diameter increases, the cavity-receiver approaches a blackbody absorber. The solar energy absorption efficiency of a solar reactor, ηabsorption, is defined as the net rate at which energy is being absorbed divided by the solar radiative power coming from the solar concentrator. For a perfectly insulated cavity-receiver (no convection or conduction heat losses), it is given by :
For simplification, we assume an aperture size that captures all incoming solar power so that Qaperture = Qsolar. With this assumption, and for a perfectly insulated isothermal blackbody cavity-receiver (), Eqs. (1) and (2) are combined to yield:
The absorbed concentrated solar radiation drives an endothermic chemical reaction. The measure of how well solar energy was converted into chemical energy for a given process is the solar-to-fuel energy conversion efficiency, ηsolar-to-fuel, defined asFig. 1 as a function of TH for TL = 298 K and for various solar flux concentrations. Because of the Carnot limitation, one should try to operate thermochemical processes at the highest upper temperature possible; however, from a heat-transfer perspective, higher TH implies higher re-radiation losses. The highest temperature an ideal solar cavity-receiver is capable of achieving, defined as the stagnation temperature Tstagnation, is calculated by setting Eq. (5) equal to zero, to yield
At this temperature, ηsolar-to-fuel,ideal = 0 because energy is being re-radiated as fast as it is absorbed. Stagnation temperatures exceeding 3000 K are attainable with solar concentration ratios above 5,000. However, an energy-efficient process must run at temperatures substantially below Tstagnation. There is an optimum temperature Toptimum for maximum efficiency, obtained by setting
Assuming uniform power-flux distribution, Eq. (7) yields:
The locus of Toptimum is shown in Fig. 1, and varies between 1100 and 1800 K for uniform power-flux distributions with concentrations between 1000 and 13,000. For example, for = 5000, the maximum ηsolar-to-fuel,ideal of 75% is achieved at Toptimum = 1500 K. For a Gaussian incident power-flux distribution having peak concentration ratios between 1000 and 12,000 suns, the optimal temperature varies from 800 to 1300 K. In practice, when considering convection and conduction losses in addition to radiation losses, the efficiency will peak at a somewhat lower temperature.
In order to illustrate the use of these equations, we consider as an example a 2-step solar thermochemical process for splitting H2O using ZnO/Zn redox reaction, comprising: (1) the solar endothermal dissociation of ZnO(s) into its elements; and (2) the non-solar exothermal steam-hydrolysis of Zn into H2 and ZnO(s), and represented by 
A model flow diagram for the proposed 2-step solar thermochemical cycle is shown schematically in Fig. 2 . It uses a solar reactor, a quenching device, a hydrolyser reactor, and a H2/O2 fuel cell. All materials are recycled. The complete process is carried out at a constant pressure of 1 bar. In practice, pressure drops will occur throughout the system and pumping work will be required. The solar reactor is assumed to be a cavity-receiver operating at 2000 K. The molar feed rate of ZnO to the reactor, , is set to 1 mol/s, and is equal to that of H2O fed to the hydrolyser reactor. 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,
For = 5000, ηabsorption = 82% and Qsolar = 680 kJ/mol. Products Zn(g) and O2 exit the solar reactor at 2000 K and are cooled rapidly to 298 K. It is assumed that the chemical composition of the products remains unchanged upon cooling in the quencher. Since the quench step is required for avoiding recombination of products, no heat exchanger is used for recovering their sensible and latent heat. Thus, the amount of power lost during quenching is:
After quenching, the products separate naturally (without expending work) into gaseous O2 and condensed phase zinc. Zinc is sent to the hydrolyser reactor to react exothermally with water and form hydrogen, according to reaction (10). The heat liberated is assumed lost to the surroundings, as given by:
The cycle is closed by introducing an ideal H2/O2 fuel cell, in which the products recombine to form H2O and thereby generate electrical power. WF.C. and QF.C. are the work output and heat rejected, respectively given by:
Finally, assuming no heat recovery during quenching and hydrolysis,
The major sources of irreversibility are associated with the re-radiation losses from the solar reactor and the heat lost during quenching and hydrolysis. To some extent, the sensible heat of the hot products exiting the reactor may be recovered to pre-heat the reactants, increasing the efficiency up to 50%. ηsolar-to-fuel can be further increased with higher , e.g. by incorporating a CPC at the aperture, which results in a smaller aperture to intercept the same amount of solar power, and, consequently, lower re-radiation losses. Note that, for a given , smaller apertures intercept a reduced fraction of the incoming solar power. Thus, the optimum aperture size of the solar cavity-receiver becomes a compromise between maximizing solar radiation capture and minimizing re-radiation losses . Re-radiation losses can also be diminished by implementing selective windows with high transmissivity in the solar spectrum around 0.5 μm where the solar irradiation peaks, and high reflectivity in the infrared range around 1.45 μm where the Plank’s spectral emissive power for a 2000 K blackbody peaks.
This kind of process modeling establishes a base for evaluating and comparing different solar thermochemical processes for ideal, closed cyclic systems that recycle all materials. For open materials cycles, in which fuels are the reactants being solar-upgraded (see next section: cracking, reforming, gasification), the solar-to-fuel energy conversion efficiency is calculated as:
2. Solar Thermochemical Processes and Reactors
Five thermochemical routes for solar fuels production are depicted in Fig. 3 . Indicated is the chemical feedstock: H2O and/or carbonaceous feedstock (e.g. natural gas, oil, coal, biomass). All of these routes are highly endothermic processes that proceed at high temperatures and make use of concentrated solar radiation as the energy source of process heat .
The single-step thermal dissociation of water, known as water thermolysis, although conceptually simple, has been impeded by the need of a high-temperature heat source at above 2500 K for achieving a reasonable degree of dissociation, and by the need of an effective technique for separating H2 and O2 to avoid recombination or ending up with an explosive mixture. Among the ideas proposed for separating H2 from the products are effusion and electrolytic separation .
Water-splitting thermochemical cycles bypass the H2/O2 separation problem and further allow operating at relatively moderate upper temperatures. Previous studies performed on H2O-splitting thermochemical cycles were mostly characterized by the use of process heat at temperatures below about 1200 K, available from nuclear and other thermal sources. These cycles required multiple steps (>2) and were suffering from inherent inefficiencies associated with heat transfer and product separation at each step. In recent years, significant progress has been accomplished in the development of optical systems for large-scale collection and concentration of solar energy capable of achieving solar concentration ratios of 5,000 suns and higher. Such high solar radiation fluxes allow the conversion of solar energy to thermal reservoirs at 1500 K and above which are needed for the more efficient two-step thermochemical cycles using metal oxide redox reactions [4–17]. The cycle is depicted in Fig. 4 .
The first, endothermic step is the solar thermal dissociation of the metal oxide to the metal or the lower-valence metal oxide. The second, non-solar, exothermic step is the hydrolysis of the metal to form H2 and the corresponding metal oxide. The net reaction is H2O = H2 + 0.5O2, but since H2 and O2 are formed in different steps, the need for high-temperature gas separation is thereby eliminated. The second hydrolysis step can be accomplished on demand at the H2 consumer site, as it is decoupled from the availability of solar energy. Alternatively, CO2 can be co-fed with H2O to react with the metal and produce syngas, which can be further processed to liquid fuels [15–17]. One promising redox system is ZnO/Zn (see Eqs. (9-10). Figure 5 shows a solar chemical reactor configuration for performing the thermal dissociation of ZnO that consists of a windowed rotating cavity-receiver lined with ZnO particles. With this arrangement, ZnO is directly exposed to high-flux solar irradiation and serves simultaneously the functions of radiant absorber, thermal insulator, and chemical reactant [18,19].
Spinel ferrites of the form MxFe3-xO4, where M generally represents Ni, Zn, Co, Mn, or other transition metals, have been shown to be capable of splitting water according to the two-step cycle shown below:
It has recently been shown that the reduction reaction (18) is thermodynamically limited to about 60% conversion and that metal oxide solutions play a critical role in the overall process . Hence, for a nickel based ferrite, the best that one can hope for is:20]. The process is assumed to cycle between 1673 K for reduction and 1273 K for oxidation. The solid products of the reduction reaction are stored adiabatically at 1273K and then oxidized with steam to generate H2 at 300 psig. The 100,000 kg H2/yr can be achieved with five 265 m tall towers utilizing 2.46 x 106 m2 of heliostats. Each of the five receivers provides for 295 Mwthermal energy to drive the reduction reaction. System design uses a secondary concentrator providing 3868 suns concentration. Overall solar efficiency for a plant in Daggett, CA is 0.414 and overall efficiency for the process to produce H2 (no work) is 0.178 .
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:
Other compounds may also be formed, depending 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 CO2 release or used as material commodity under less severe CO2 restraints. It can also be applied as 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 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 CO2. Assuming carbon sequestration, Eq. (17) yields ηsolar-to-fuel = 0.55 . Figure 6 shows a scheme of a solar reactor that features a continuous flow of CH4 laden with μm-sized carbon black particles, confined to a cavity receiver and directly exposed to concentrated solar irradiation . The carbon particles fed serve the functions of radiant absorbers and nucleation sites for the heterogeneous reaction.
The steam-reforming of hydrocarbons (e.g. NG, oil, and others), and the steam-gasification of carbonaceous materials (e.g. coal, coke, biomass, and other C-containing solids) can be represented by the simplified net reaction:
Other compounds may also be formed (e.g. H2S), depending on the reaction rate and on the impurities contained in the raw materials. The principal product is high-quality synthesis gas (syngas), the building block for a wide variety of synthetic fuels including Fischer-Tropsch type chemicals (liquid fuels), hydrogen, ammonia, and methanol. The syngas quality is determined mainly by the H2:CO and CO2:CO molar ratios. For example, the solar steam gasification of anthracite coal at above 1500 K yields syngas with a H2:CO molar ratio of 1.2 and a CO2:CO molar ratio of 0.01.
Some of these processes are practiced at an industrial scale, with the process heat supplied by burning a significant portion of the feedstock. Alternatively, using solar energy for process heat offers a three-fold advantage:
- • higher energetic value of the syngas produced; i.e. higher syngas output per unit of feedstock, as the calorific value of the feedstock is upgraded by the solar energy input in an amount equal to the enthalpy change of the reaction.
- • higher quality of the syngas produced, especially in terms of the low CO2/CO molar ratio, as no syngas contamination by combustion byproducts occurs;
- • elimination of the air-separation system needed for supplying a pure oxygen stream in the autothermal gasification.
For the solar steam-reforming of natural gas and the steam-gasification of coal followed by syngas processing to H2 (assuming water-gas shift and H2/CO2 separation unit based on the pressure swing adsorption technique at 90% recovery rate and power consumption of 9.17 kJ/mol H2), Eq. (17) yields ηsolar-to-fuel = 0.71 . A solar reactor for the solar reforming of NG that uses a reticulate porous ceramic foam coated with Rh-based catalyst has been scaled-up to power levels of 300 kW in a solar tower facility . The solar steam-gasification of petroleum coke and coal were studied in fluidized-bed, vortex-flow, and aerosol-flow solar reactors [24–29]. Thermal irradiation of these particle suspensions was found to be an effective means of heat transfer directly to the reaction site, leading to extremely fast heating rates (~1000 K/s) and enhanced kinetics. A packed-bed solar reactor that can accommodate a wide range of carbonaceous feedstock is shown in Fig. 7 . It consists of two cavities separated by a radiant emitter plate, with the upper one serving as the solar absorber and the lower one containing the reacting packed bed that shrinks as the reaction progresses [30,31]. If biomass is used as feedstock, the syngas produced is CO2-neutral. A 1 MWthermal solar gasification pilot plant based on solar tower technology has recently been commissioned to convert wood waste and other forms of biomass into syngas that can be further processed into gasoline .
Solar thermochemical processes offer an efficient path for storage and transportation of solar energy. The products are renewable chemical fuels for delivering clean and sustainable energy services. There is a pressing need to develop greenhouse gas mitigation options that can be applied in the mid-term. Solar cracking, reforming, and gasification processes for upgrading and converting fossil fuels to syngas conserve fossil fuels, reduce emissions, and could become an important transition path towards solar liquid fuels. The production of solar hydrogen via 2-step thermochemical cycles using metal oxides redox reactions has favorable long-term potential, warranting further development and large-scale demonstration.
Aaperture area of aperture
C solar flux concentration ratio
I normal beam insolation
molar flow rate
HHV high heating value
Qaperture incoming solar energy intercepted by the aperture
QFC heat rejected to the surroundings by an ideal fuel cell
Qquench heat rejected to the surroundings by the quenching process
Qreactor,net net energy absorbed by the solar reactor
Qsolar total solar energy coming from the solar concentrator
T nominal solar reactor temperature
Tstagnation maximum temperature of a blackbody absorber
Toptimum optimal temperature of the solar reactor for maximum ηsolar-to-fuel
WFC work output by an ideal fuel
αeff effective absorptance of the solar cavity-receiver
εeff effective emittance of the solar cavity-receiver
ΔG Gibbs free energy change
ΔH enthalpy change
ηabsorption solar energy absorption efficiency
ηCarnot efficiency of a Carnot heat engine operating between TH and TL
ηsolar-to-fuel solar-to-fuel energy conversion efficiency
σ Stefan-Boltzmann constant (5.6705 × 10−8 Wm−2K−4)
References and links
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29. P. Lichty, C. Perkins, B. Woodruff, C. Bingham, and A. W. Weimer, “Rapid High Temperature Solar Thermal Biomass Gasification in a Prototype Cavity Reactor,” J. Sol. Energy Eng. 132(1), 011012 (2010). [CrossRef]
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