Solar—Thermal Process

Schematic of thermally enhanced solar water-splitting. (Courtesy of Elsevier Ltd). 

An alternative approach is to employ the thermal energy from the solar concentrator to generate electricity directly by means of a PV cell based on a semiconductor with a low band gap e.g., 0.6 eV) that is sensitive to infrared radiation. Such thermo-photovoltaic cells are already in small-scale specialized use by the military, although with the heat provided by a propane burner, rather than from the Sun. This is an alternative to the thermoelectric effect for converting heat to electricity. 

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Fletcher EA (2001) Solar thermal processing A Review. J Solar Energy Eng 123 63-74... 

Thermochemical splitting of water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Unfortunately the decomposition of water does not proceed until temperatures around 2500 K are reached. This and other thermal routes are discussed in Chapter 5. Solar thermal processes are handicapped by the Carnot efficiency limits. On the other hand, solar photonic processes are limited by fundamental considerations associated with band-gap excitation these have been reviewed in Refs.32 and 33. 

Demonstrated suitable materials of construction for a high-temperature solar-thermal process - reactor assembly constructed of exterior quartz and interior porous and solid graphite. 

This is the final year of a three-year project and no future research is plaimed. However, DOE is currently considering a process extension proposal to support a solar-thermal process to rapidly dry reform waste landfill gas to hydrogen and carbon black. Independent of the outcome of that proposal, the following issues need to be addressed ... 

Sunlight can be concentrated with mirrors and used to achieve ultra-high temperatures that are otherwise only achievable using electricity or nuclear power. The use of such renewable solar-thermal processing has significant potential in the desert southwest United States (AZ, CA, CO, NM, NV, and UT) for heating specialized chemical reactors to produce H2 from NG, biogas, or waste landfill gas. 

For the process scenario discussed, the solar-thermal process avoids 277 MJ fossil fuel and 13.9 kg-equivalent C02/kg H2 produced as compared to conventional steam-methane reforming and furnace black processing. 

The solar-thermal process is feasible there are no technical show stoppers and there are no materials concerns. The fluid-wall reactor design allows continuous operation without inside wall deposition of carbon black. Reaction rates at the demonstrated ultra-high temperatures are enormous. 

The solar-thermal process is enviromnentally friendly. The most environmentally friendly option is when selling carbon black into the tire carbon black market as the energy and pollution associated with normal carbon black production are avoided. If carbon is fed to a carbon conversion fuel cell, the total green house gas emissions are still 60% of those of a steam reformer and the bulk of the released CO2 is in a pure form so it can be easily sequestered. 

The process is potentially economical in the desert southwest United States. However, an outlet for the carbon black is an integral part of the overall economics. Compared to photovoltaic conversion and electrolysis of water to produce H2, the solar-thermal process requires 40 times less heliostat surface, and the heliostats are lower cost mirrors rather than expensive photovoltaic cells. The solar-thermal process can produce HCNG at high rates in one step by efficiently and cleanly removing carbon from fed NG. 

Dahl, JK, Tamburini, J, Weimer, AW, Eewandowski, A, Pitts, R, and Bingham, C, "Solar-thermal Processing of Methane to Produce Hydrogen and Syngas," Energy and Fuels, 15, 1227-1232 (2001). 

Weimer, AW, "Solar-thermal Process for Rapid Natural Gas Dissociation," presented to the Department of Chemical Engineering - Graduate Seminar, University of Maryland (October, 2001). 

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