Transcript Solar Energy

Solar Energy
Written for Physics 101 Fall 2008
The truly vast clean source of
energy
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Output Power of the Sun
Solar Constant S0
Reaching Surface
12 hr day
Miami (whole year)
Average US
4 x1026 Watts
1.4 kW/m2
1.0 kW/m2
500 W/m2
219 W/m2
200 W/m2
Energy Units
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1 BTU = 1055 Joule = ~ 1000 J
1 calorie = 4.184 J
1 Calorie = 4186.8 J
1 J = 107 ergs
1 kilowatt-hour = 3.6 x 106 Joules
1 watt-hour = 3600 Joules
1 megaton = 4.184 x 1015 J
Energy Equivalents
• One ton of coal
2.5 x 1010 J
• 1 gal gasoline
1.3 x 108 J
• Wabash Generating Station (600 MW) operating
for one day 5.184 x 1013 J
= 2074 tons of coal
Wabash Generating Station 600,000,000 W
(electrical) or 1800,000,000 thermal divided by
200 W/m2 = 9,000,000 m2 = roughly 3.5 mi2
(note in Terre Haute this would be more)
R Values
• The United States maintains their own definition for Rvalue, which can be spotted as being approximately six
times larger than a standard unit. The imperial R-value is
measured in degrees Fahrenheit, square feet hours per
Btu, (ft²·°F·h/Btu).
• Unfortunately this does not agree with the international
value (K·m²/W), using the SI sys. Use the conversion 1
K·m²/W ≈ 5.67446 ft²·°F·h/Btu
• R values are additive, e.g. for a brick home Rvalue(brick) + R-value(fibreglass batt) + Rvalue(plasterboard) = R value(total).
Typical R values for insulating
board
• The house insulation r value of insulating
boards are:
Expanded polystyrene 4.00 per inch
Extruded polystyrene
5.00 per inch
Polyisocyanurate &
Polyurethane
6-7 per inch
• Conduction is the reciprocity of the R
value
An example
• Taking an R value of 4, imagine a room of
10 x 10 ft times 6 sides (a cube).
• Thus the conduction is 0.25 Btu/(sq ft deg
F ) BTU/hr or
• 0.25 x 100 x 6 sq ft x 68 deg F = 10,200
BTU/hr