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Lecture IV.

Solar Energy - The Basics

The Sun --> Always there; lots of Energy
What Makes the Sun Shine?
How many photons (energy) reach the surface of the Earth on Average?

Note that most of the energy is in visible (yellow-green) photons

What is Photon Scattering
How much energy from the sun reaches the surface of the Earth on Average?

Note that we measure energy in units of Watt-hours. A watt is not a unit of energy, it is a measure of power. Energy equals

POWER x TIME

1 Kilowatt Hour = 1KWH = 1000 watts used in one hour = 10 100 watt light bulbs left on for an hour
Incident Solar Energy on the ground:
Average over the entire earth = 164 Watts per square meter over a 24 hour day
8 hour summer day, 40 degree latitude --> 600 Watts per sq. meter
So over this 8 hour day one receives:

8 hours x 600 watts per sq. m = 4800 watt-hours per sq. m which equals 4.8 kilowatt hours per sq. m
This is equivalent to 0.13 gallons of gasoline
For 1000 square feet of horizontal area (typical roof area) this is equivalent to 12 gallons of gas or about 450 KWH But to go from energy received to energy generated requires conversion of solar energy into other forms (heat, electricity) at some reduced level of efficiency.
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