The solar-thermal conversion method of Section 2.1.1 can be modified to be applicable to solar cells. Figure 8.7 illustrates the principle.
A focussing optical system is used to concentrate the solar radiation onto an intermediate absorber which, as a result, is heated to the temperature 7. Solar cells with an energy gap Eg are placed concentrically around the intermediate absorber. They have an interference filter on their surface, which transmits all photons with £q < Йш < Eg + de without loss and reflects all other photons, which cannot be used optimally, back to the intermediate absorber. These photons, together with the photons emitted by the solar cell and transmitted by the filter, help to maintain the temperature 7д of the intermediate absorber. If the recombination is entirely radiative and the photons emitted by the cells are re-absorbed by the absorber and are therefore not lost, the cells can be operated close to their open-circuit voltage and thus
Since this is the Carnot efficiency, which was used for the efficiency of the heat engine of the solar-thermal conversion process described in Section 2.1.1, we find, for the thermo – photovoltaic conversion process, the same efficiency as for the solar-thermal conversion, thus
with a maximum value of T| — 0.85 at an absorber temperature of = 2478K.
The implementation of this concept in practice is difficult for two reasons. At the optimal temperature of the intermediate absorber of 7д = 2478 К all materials evaporate so strongly that the interference filter is quickly covered with an opaque layer. Moreover, in practice it is not possible to construct an interference filter transmitting only in a narrow energy interval and reflecting the rest of the spectrum, while also being free of absorption. With the use of, e. g., silicon solar cells, only a very small portion of the photons emitted from the intermediate absorber have the required energy йсо > Єс – Even very little absorption by the interference filter of all the other photons leads to a considerable loss. Smaller band gap materials like GaSb are more favourable for thermo-photovoltaic conversion.
In the construction principle of Figure 8.7, another drawback is hidden. The absorber must be able to emit almost as much energy as it absorbs in the narrow energy interval transmitted by the filter. Since 7д is much smaller than the temperature of the sun 7s, the emitting area must be much larger than the absorbing area. A factor of 4, as provided by the arrangement in Figure 8.7 is far from sufficient.
This problem can be solved by an even wilder idea, called thermo-photonics. Emission of only photons with йсо > Єо can be accomplished by placing a semiconductor on the emitting area of the intermediate absorber. (To prevent transmission by the semiconductor of smaller energy photons emitted by the absorber, a mirror must be placed between the semiconductor and the absorber.) When the semiconductor is supplied with membranes such as a solar cell, it can be operated as a light-emitting diode (LED). A LED is the same engine as a solar cell, only operated in reverse, just as a refrigerator or heat pump is a reversely operated heat engine. If some of the power delivered by the solar cells is used to drive the LED on the absorber, the emitted intensity is enhanced enormously. Although some of the energy emitted by the LED is Free Energy supplied by the solar cells, most of it is heat supplied by the absorber. As a result, the area and the temperature of the emitter can be reduced, provided an LED can be made, which works at about 1000 °С with close to 100% external quantum efficiency.
With the arrangement shown in Figure 8.7, however, the intermediate absorber does not have to be heated by the sun. It is also possible to heat the intermediate absorber in another way, e. g., by burning gas. Radiation losses to the environment would then not have to occur, because the cavity can be completely closed, optically. The conversion of heat into electrical energy in this way was first proposed in the Soviet Union for nuclear reactors. The basic concept was to surround incandescent reactor fuel elements with solar cells. Fortunately, no one had the courage to try this out.
Figure 8.8: Transition of an electron from a higher band to the minimum of the conduction band by impact ionization in an indirect semiconductor, resulting in the additional generation of an electron and a hole at the band edges.