The Polluted Solar Cells Made Of Crystalline Silicon

The mainstream solar cells are made of crystalline silicon. Although the cost for generating one KwH is 5-10 times of conventional fuel fossil power plants, crystalline silicon solar cells are thought to be environmentally friendly. However, crystalline silicon is typically produced from highly pure quartz that may be extracted from beach sand. White beach sand can only be produced after weathering for tens of million years due to the fact that quartz is likely the most stable common mineral. Consequently, it would require very high temperature (e.g. 1700°C) to reduce quartz to form metallurgical grade silicon. While silicon is formed from quartz, 1.5 time of carbon dioxide is released that contributes to the green house effect. Moreover, the metallurgical grade silicon must be purified, again at high temperature. Eventually, the pure silicon is melted above 1450°C to form either ingot for single crystal applications or to cast in polycrystal blocks. In all these high temperature process steps, significant amount of electricity may be needed.

The crystalline silicon for making solar cells may have a price tag as expensive as $100/Kg. For making solar cells, about 0.5 mm thick silicon including cutting kerf, is needed, so 1 kg of silicon may produce about 1 m2 of exposed area under the sun. As the solar constant is about 1 Kw/m2, and the average efficiency of silicon solar cells as measured around the clock is less than 10%, we may expect that the power generated from one kilogram silicon material is about 0.1 Kw. Assuming that the power cost is about $0.05/KwH, the above 0.1 Kw solar panel would require about 2000 KwH or energy to produce the silicon material for constructing the panel. To pay back the bill, the silicon solar cells may have to run 20,000 hours or about 2.3 years! Governments have to subsidize of the electricity produced by as much as three times of the ordinary cost of electricity derived from power plants. Consequently, crystalline silicon solar cells are neither clean, nor cheap. It is desirable to constructing solar cells by using more environmental friendly materials. One of such materials is amorphous diamond.

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