control and further diode developments, solar TPV systems with efficiency in the range of
25–35% are possible.
Spectral control is of crucial importance and holds the key for TPV efficiency. The goal of
spectral control is to allow only photons above the band gap to reach the diode, as photons below
the band gap not only represent a loss of useful energy but also reduce diode efficiency because
they cause a rise in the diode temperature when being absorbed. For emission control, rare earth
and transition metal-doped ceramics, refractory intermetallic coatings, thin-film and multilayer
filters, plasmonic filters, and photonic crystals have been explored (Fleming et al. 2002; Licciulli
et al. 2003). However, high-temperature operation of the emitters poses great challenges to the
stability of the materials and structures. In comparison, filters, either stand-alone or built on the
surface of the diode, suffer less from the stability issue.
In 2002, for a 1.5-kW GaSb-based system used as a home furnace, the total cost was estimated to
be $4,200 with $2,700 for the furnace and $1,500 for the TPV generator at ~15% efficiency
(Fraas and McConnell 2002). This corresponds to $1/W. If we add in the cost of the concentrator
at $1.6/W (assuming 15% efficiency, 850 W/m^2 solar insolation, and $200/m^2 concentrator cost),
the cost is $2.6/W based on current technology (not counting other items that may be needed for
the solar TPV system). If the efficiency is doubled to 30%, reducing the cost of energy in half,
then the other major opportunity for cost reduction is the concentrator cost. This cost would need
to be reduced significantly to bring the total cost to a target cost of $1/W as for solar PV.
Concentrated Photovoltaics. Concentrated photovoltaic systems do not involve a solar thermal
process, but they share the concentrator issues of linear and central receivers. Thus, this fast-
developing field might well be considered under the “Crosscutting Areas” category. In this
method, sunlight is concentrated by using mirrors or lenses, which 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 CPV
systems may be able to sustain even higher concentration ratios. We refer the reader to the Basic
Research Challenges for Solar Electricity and Solar Electricity Technology Assessment for
details of PV development. More details on CPV systems can be found in the Solar Thermal
Technology Assessment in Appendix 1. Projections (Stoddard et al. 2005) put the long-term
installed costs of CPV with multijunction cells currently under development at about $2/W. The
present cost of systems provided by Amonix and Solar Systems Pty Ltd. are roughly $4/W; these
systems use single-junction silicon cells and are in an early commercialization stage.
Concentrated Solar Thermochemical Processes
The concentrating component of these systems is identical to that of concentrated solar thermal
processes for power generation, but the energy conversion is a thermochemical process
converting radiation-to-heat-to-chemical potential. These systems provide an effective means for
long-term storage and transportation of solar energy (e.g., in the form of fuel) and its utilization
in motor vehicles and industrial applications. The basic concept is shown in Figure 16.