Solar TPVs are
similar to solar cells in that they convert photon energy into electricity. The
fundamental difference from other PVs is that the photon source comes from a terrestrial
thermal radiation emitter rather than directly from the sun. The radiation emitter
can be heated by thermal sources such as fuel combustion or by concentrated solar
radiation. Solar TPVs have a theoretical system efficiency of >30% for a concentration
ratio of >10,000. Compared with nonconcentrated solar PVs, the radiation emitted
by the emitter has a higher power density. Thermal radiation from the heated radiation
emitter has a longer wavelength and correspondingly, the PV cells used in a TPV
system often have lower band gaps. Figure 15 shows a possible set-up of a solar
TPV system. Concentrated solar energy raises the temperature of a solid thermal
radiation emitter to a high temperature, typically to the range of 1,000– 2,000°C.
The efficiency of TPV systems depends critically on spectral filtering to avoid absorption of photons with energies below the band gap of the PV cells by parts
of the system other than the emitter. The filtering elements can be freestanding or integrated in the emitter or the cells. The efficiency of a TPV power generator
system can be roughly split into several factors: η=ηsourceηspectralηdiodeηmech
where ηsource - efficiency of the conversion of the energy source (fossil, solar, nuclear) into thermal radiation from the emitter
ηspectral - combined efficiency of the emitter and filter that represents the fraction of photon energy above the band gap reaching the PV cell among all
photon energy emitted , ηdiode - efficiency of the PV cell converting the photon energy above the band
gap into electricity, ηmech - efficiency of converting PV cell electrical power output to the system
power output that includes the energy lost in the pumping systems for fuel injection and thermal management.
Thermophotovoltaic energy conversion emerged in 1950s through the work of Henry Kolm at the Massachusetts Institute of Technology Lincoln Laboratory and a series of lectures given by
Pierre R. Aigrain of the École Normale Superieure (Nelson 2003). The focus of past work was on diode development and spectral control. Most diodes are built on antimony-based III-V
materials with a band gap in the range of 0.4–0.7 eV (Wang 2004). One mature example is GaSb-based TPV diodes, which were also used in high-efficiency tandem solar cells. Thin-filmbased
diodes based on InGaAsSb are also extensively studied. The band gap of such thin-film materials can be tailored to match the heat-source temperature for optimum performance.
Heterostructures can be used to further improve the cell performance. A 27% diode efficiency and 20% combined radiator-diode has been reported (Brown et al. 2003). With proper spectral 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/m2 solar insolation, and $200/m2 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.