the sequestration step, these solar-driven processes offer significant reduction in CO 2 emissions
compared with conventional combustion-based processes. A Second-Law analysis for generating
electricity indicates the potential of doubling the specific electrical output and, consequently,
halving the specific CO 2 emissions compared with conventional fossil-fuel power plants (von
Zedtwitz and Steinfeld 2003). These processes proceed endothermically in the 800–1,500K
range. The advantages of supplying solar energy for process heat are threefold: (1) the calorific
value of the feedstock is upgraded by adding solar energy in an amount equal to the enthalpy
change of the reaction; (2) the gaseous products are not contaminated by any combustion by-
products; and (3) the discharge of pollutants to the environment is avoided. The solar chemical
reactor technology for these processes includes a vortex-type and an aerosol-type flow reactor
for solar natural gas decomposition (Hirsch and Steinfeld 2004; Dahl et al. 2004; Kogan et al.
2005), a catalytic porous-ceramic absorber for NG reforming (Moeller et al. 2002), and a
fluidized bed reactor for coal gasification (Müller et al. 2003; Trommer et al. 2005). The
experimental demonstration of solar reactor prototypes points toward developing solar chemical
technology to an industrial megawatt scale.
SCIENTIFIC CHALLENGES
Radiative Exchange In Chemically Reacting Flows
Fundamental research, both theoretical and experimental, is in radiation heat transfer of
multiphase chemical-reacting flows. The analysis of thermal radiative transport coupled to the
reaction kinetics of heterogeneous chemical systems, in which optical properties, species
composition, and phases vary as the chemical reaction progresses, is a complex and challenging
problem to be tackled in the design of high-temperature thermochemical reactors. Of special
interest is the radiative exchange within absorbing-emitting-scattering particle suspensions,
applied in thermochemical processes such as thermal cracking, gasification, reforming,
decomposition, and reduction processes.
Directly Irradiated Solar Chemical Reactors
The direct absorption of concentrated solar energy by directly irradiated reactants 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. Spectrally selective windows
can further augment radiation capture and absorption. The use of nanoparticles in gas/solid
reactions augments the reaction kinetics and heat/mass transfer.
Materials for High-temperature Solar Chemical Reactors
Materials for construction of solar chemical reactors require chemical and thermal stability at
temperatures >1,500°C and solar radiative fluxes >5,000 suns. Advanced ceramic materials and
coatings are needed for operating in high-temperature oxidizing atmospheres and for
withstanding severe thermal shocks occurring in directly irradiated solar reactors. The ability to
develop electrolysis processes at high temperatures depends on the development of stable