Download Solar 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 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