Solar cell interconnections are assumed to be aluminum. Detailed design of the interconnection pattern is beyond the scope of this report. The cell backing provides thermal contact and electrical insulation between the cells and the cooling system. 1-mil Kapton was used by Horne(4), but lunar materials such as glass might be used instead. The radiator is a thin sheet of fabric woven from tungsten wire. Evaporation limits the maximum operating temperature to less than 2300 K.(5) An early version of the TPV design included a thick radiator with enough heat capacity to provide power during eclipses. This was rejected because it gives a mass penalty to the TPV concept and would make it difficult to directly compare the TPV concept with other power conversion systems. However, this option could make TPV more attractive if power during eclipses becomes a high priority. The major problem with TPV conversion in space is cooling the cells. There are two facets to this problem: massive cooling systems, and solar cells which are too opaque at long wavelengths. High specific mass in cooling systems is a common problem in spacecraft. The poor transparency of cells at long wavelengths produces heat in the cells without producing power (see TPV Technical Discussion, section 2.3.3), so the cooling load is increased. Three solutions are possible: find a cooling system which has a low specific non-lunar mass, develop a cell which is more transparent at long wavelengths, or develop a cell which is more tolerant of high temperatures. All three options are being investigated in industry and academe, so the status of TPV should improve. 2.3.3 TPV Technical Discussion The primary characteristic of a photovoltaic (PV) cell is the band-gap energy, E g, of the semiconductor material in the cell. For example, silicon has a band-gap of 1.13 eV. A photon’s energy, E_p, may have any value. For a particular photon striking the cell, if E_p > E g, then the photon will probably be converted to an electron-hole pair. From each electron-hole pair produced, at most E g energy can be converted to electricity; the remainder of the photon’s energy will appear as heat in the cell. Infrared photons with E_p < Eg will usually pass through the cell without interaction, though some will be absorbed as heat energy in the cell. Typically PV cells are used in normal sunlight, which is similar to the spectrum of a blackbody at about 6000 Kelvin. Most of the power of sunlight is carried by photons of about 2.7 eV. Thus, since only E__g/E_p of a photon’s energy can be converted to electricity, most of the power of sunlight is wasted by typical PV cells. In a TPV converter, sunlight is used to heat a radiator which illuminates PV cells. The radiator’s temperature is much lower than 6000 K, so much of its radiated power is carried by photons whose energies are only slightly above the cells’ band-gap. Thus, more of the power can be converted to electricity.
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