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.

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 n-type 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