A number of multistep thermal processes and high-temperature electrolysis methods have been
developed in an attempt to enable this reaction.
An overview of solar thermochemical processes is provided by Steinfeld and Palumbo (2001).
Some of the topics addressed by recent studies are an evaluation of novel processes for fuel
synthesis (Möller et al. 2002; Dahl et al. 2004) and material production (Murray et al. 1995;
Wieckert et al. 2004); development of novel solar reactors (Anikeev et al. 1998; Osinga et al.
2004); and catalyst development for solar-driven high-temperature gas-gas reactions (Berman
and Epstein 1997).
Solar fuel productions will probably be two to three times more expensive than present high-
emission industrial methods. However, various studies predict that solar fuels production can be
competitive if carbon emission cost is considered.
Concentrated Photovoltaic Systems
Although not strictly a solar thermal process, we discuss here the fast-developing field of
concentrated photovoltaic systems — systems that rely on optical configurations similar to those
used for concentration in solar thermal processes. In this method, sunlight is concentrated by
using mirrors or lenses that are much cheaper than PV panels, and the concentrated light is
focused onto the PV cells. The required cell area is therefore reduced by the concentration factor,
which in present systems can be as high as 500; future cells may be able to sustain even higher
concentration ratios. The higher the concentration is, the smaller is the cell area, and the
influence of cell cost on overall system cost is diminished. This approach enables the use of the
most efficient PV cells available because when the cell area is very small, the cost reduction due
to the increase in cell efficiency outweighs the cost increase associated with high-performance
cells. Mature high-concentration PV systems should cost about 40–60% of standard flat PV
systems and provide 10–20% more energy than standard PV systems with the same installed
power rating.
Some concentrated PV (CPV) systems have one-axis trackers, similar to that shown in Figure 74,
and they concentrate the light in one direction, onto a line focus. The concentration ratio of one-
axis CPV systems is commonly 10–50. High-concentration PV (HCPV) systems use two-axis
trackers and concentrate in two directions; their concentration ratio is typically 200–500. The
two-axis HCPV systems are more efficient and less costly.
Recent developments demonstrated the technological feasibility of several CPV and HCPV
configurations. These advances include an increase in HCPV cell efficiency (Sherif et al. 2004).
Designs of the cell’s dense array, used as the power conversion component in some HCPV
systems, have been improved (Lasich 2004).
Methods have been developed to provide a uniform high solar flux distribution on the solar cells
(Ries et al. 1997b). The performance and durability have been upgraded for Fresnel lenses,
which are used as sunlight concentrators in some HCPV systems (Diaz et al. 2004).