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 free-
standing or integrated in the emitter or the cells.
The efficiency of a TPV power generator system
can be roughly split into several factors:
η=η ηsource spectral diode mechη ηwhere
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-film-
based 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
Figure 15 A possible set-up of a solar
TPV system (courtesy: V.M. Andreev,
IOFFE, St. Petersburg, Russia)