THERMOPHOTOVOLTAICS
Thermophotovoltaics (TPVs) are similar to solar PVs in that they convert photon energy into
electricity. The fundamental difference is that their photon source is a terrestrial heat source
rather than the sun. Compared with the sun, a terrestrial heat source has a higher power density
and can provide power on rainy days and at night. Thermal radiation from a terrestrial heat
source has a longer wavelength, thus, the cells used in a TPV system often have lower band gaps,
although silicon-based TPV systems are also of interest. Figure 78 shows the key components of
a typical TPV system. A heat source, which can be a fossil fuel combustor, concentrated solar
energy, or nuclear reactor, raises the temperature of a solid thermal-radiation emitter to a high
temperature, typically to the range of 1,000–1,800°C. Some systems employ filters to reflect
photons below the band gap of the PV cells, which are maintained at near room temperature by a
thermal management system. The efficiency of a TPV power generator system can be split into
several factors:
η=η ηsource spectral diode mechη η , (10)
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 into the system
power output, which includes the energy lost in the pumping systems for
fuel injection and thermal management.
The TPV diode efficiency has reached 27% (Brown et al. 2003). However, the system efficiency
has been relatively low. A combustion-based TPV system has a maximum efficiency of 4.5%
because much of the heat carried by the combustion product below the emitter temperature is
wasted. As the emitters operate at high temperatures, recuperation of the thermal energy of the
combustion products with temperature below the emitter temperatures is essential in improving
the system efficiency.
Current consensus among researchers is that TPVs are best suited for cogeneration systems,
where solar PV provides energy during sunny days while at night and on rainy days, TPVs
provide energy from decentralized home heating systems or energy-intensive industrial systems
where high temperatures are available. For example, it has been projected that an installed
1.5-kW home furnace generator costs $4,200 ($2,700 for the furnace and $1,500 for the
generator) (Fraas and McConnell 2002). The estimated payback time is four years for a home