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Solar Radiation • 1367 W/m2 this is called the solar constant • About half is visible radiation, and most of the rest is infra red. • A good deal of radiation is absorbed by the atmosphere, even when the sky is clear. • A cloud can reflect from 40% to 90% of the incident radiation. • At noon on a clear day on the equator, solar radiation at the earth’s surface can be as high as 1 KW General observations • Solar power is free, at least to those on whom the sun shines • A mean taken over the whole of the earth’s surface that averages out the variations between day and night and Summer and Winter is of little use. • Even under the most favourable circumstances, solar power is available only in day time so it is best used for day time tasks such as heating water, operating air-conditioning or charging batteries. • It is of little use in high latitudes (the arctic Winter) or where there is a great deal of cloudy weather Ways of using Solar Energy • Passive solar heating; the sun helps to keep us warm whenever it shines through the window. • Domestic heating, of the house, or of water, requires no elaborate technology, and should be capable of using almost all the energy that reaches the surface. • Solar chimney. Heating air in a tall chimney produces an upward air current that drives a turbine. • Solar powered heat engine • Direct generation of electricity Solar Cells (Photo-Voltaic cells) • First Solar Cell made from selenium in 1883 and used for a photographic exposure meter • Silicon with its crystal lattice disrupted (doped) by small quantities of impurities with what in my youth would have been called different valency • Part of a cell is doped with phosphorus, giving an ntype region (negative) Another part is doped by Boron, giving a P (positive) type region. • Electric field • Light creates more free electrons Theoretical Limits to Efficiency • Quantum effect • The quantity of energy (band gap) needed to liberate an electron depends on the material, • 1.1 ev for silicon. (wavelength 1128x10-9m. Infrared) (between .5 and .6 volts on open circuit) • Radiation with a quantum lower than that has no effect • Radiation with greater quantum will be partly wasted. • About 70% of incident energy is wasted for one or another of those reasons, setting a theoretical maximum efficiency of 30%. 25% achieved in lab, 15% common in practice • Lower energy reduces voltage • Optimum is 1.4 ev (wavelength 8886x10-9m. Near infrared) Storage of Power • Batteries • Electrolytic hydrogen • Liquid nitrogen (Stirling engine used as cooler) • Compressed air • Flywheels: 40000 rpm: can hold power for up to a week: danger of explosive shattering • Pumped storage hydro-electricity • Superconducting Magnetic Energy Storage Various Possibilities • Multi layer solar cells • Rollable solar panels at $400 for 14 Watts (max) at 12V; are about 0.44 m2. About 32 Wm-2 Prices • $9 per watt, $32 000 for a Solar House in North Carolina, does not use batteries and supplies about half electricity required, selling some to the grid. • Useful life 20 years. Assuming interest rate of 5%, capital cost about $2050 p.a. • Solar cells now available for $4 per watt