Basic Research Needs for Solar Energy Utilization

Means to store

and transport

solar energy

Figure 16 General concept of solar-driven thermochemical cycle

Water-splitting Thermochemical Cycles. In recent years, significant progress has been made
in the development of optical systems for large-scale solar concentration capable of achieving
mean solar concentration ratios exceeding 2,000 suns (1 sun = 1 kW/m^2 ), and present efforts are
aimed at reaching concentrations of 5,000 suns. Such high radiation fluxes allow the conversion
of solar energy to thermal reservoirs at 2000K and above, which are the conditions needed for
efficient water-splitting thermochemical cycles using metal oxide redox reactions. This two-step
cycle consists of first-step solar endothermic dissociation of a metal oxide and second-step
nonsolar exothermic hydrolysis of the metal. The net reaction is H 2 O = H 2 + 0.5O 2 , but since H 2
and O 2 are formed in different steps, the need for high-temperature gas separation is thereby
eliminated.

Solar Thermal Decarbonization Processes. Hybrid solar/fossil endothermic processes, in
which fossil fuels are used exclusively as the chemical source for H 2 production and
concentrated solar power is used exclusively as the energy source of process heat, offer a viable
route for fossil fuel decarbonization and create a transition path toward solar hydrogen. An
important example of such hybridization is the endothermic steam gasification of carbonaceous
materials (coal, coke, biomass) to syngas. The advantages of supplying solar energy for process
heat are threefold: (1) the calorific value of the feedstock is upgraded; (2) the gaseous products
are not contaminated by the by-products of combustion; and (3) discharge of pollutants to the
environment is avoided. A Second-Law analysis for generating electricity by using solar
gasification products indicates the potential of doubling the specific electrical output and,
consequently, halving the specific CO 2 emissions compared with conventional coal-fired power
plants.

Another approach to hydrogen production is high-temperature electrolysis of water in a one-step
process or electrolysis of another material as part of a two- to four-step water-splitting process.