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Unformatted text preview: Alaska, often covered in clouds and at high latitude may get
only 75 W/m2 on average • half the sphere (2 R2) projects into just R2 for the sun
• twice as much area as the sun “sees” • So 1/8 of the incident sunlight is typically available at the ground
– 171 W/m2 on average Winter 2007 49 Winter 2007 UCSD: Physics 121; 2007 Average daily radiation received 50 UCSD: Physics 121; 2007 Higher Resolution Insolation Map ranges in W/m2:
< 138
138–162
162–185
185–208
208–231
> 231
Winter 2007 Lecture 16 divide by 24 hr to get average kW/m2 51 Winter 2007 52 13 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Total available solar energy So why don’t we go solar?
don’ • Looking at average insolation map (which includes day/night,
weather, etc.), I estimate average of 4.25 kWh/day = 177 W/m2
• The area of the U.S. is 3.615 106 square miles
10 square • What would it take?
• To convert 1/500th of available energy to useful
of
forms, would need 1/500th of land at 100% efficiency
of
– about the size of New Jersey – this is 9.36 1 012 m2 • Multiplying gives 1.66 1015 Watts average available power
10 Watts
• Multiply by 3.1557 107 seconds/year gives 5.23 1022 Joules
1 0 Joules
1 0 seconds/year
every year
• This is 50 1018 Btu, or 50,000 QBtu
1 0 Btu, • But 100% efficiency is unrealistic: try 15%
– now need 1/75th of land
– Pennsylvaniasized (100% covered) • Can reduce area somewhat by placing in S.W. • Compare to annual budget of about 100 QBtu
– 500 times more sun than current energy budget Winter 2007 53 Winter 2007 54 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Making sense of these big numbers How good can it get? • How much area is this per person? • Silicon is transparent at wavelengths longer than 1.1
microns (1100 nm) U.S. is 9.36 1 012 m2
1/75th of this is 1.25 1011 m2
300 million people in the U.S.
416 m2 per person 4,500 square feet
– this is a square 20.4 meters (67 ft) on a side
– one football field serves only about 10 people!
– much larger than a typical person’s house area
–
–
–
– – 23% of sunlight passes right through with no effect • Excess photon energy is wasted as heat
– nearinfrared light (1100 nm) only delivers 51% of its photon
energy into electrical current energy
– red light (700 nm) only delivers 33%
– blue light (400 nm) only delivers 19% • rooftops can’t be the whole answer, especially in cities Winter 2007 Lecture 16 • All together, the maximum theoretical efficiency for a
silicon PV in sunlight is about 23% 55 Winter 2007 56 14 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Silicon Photovoltaic Budget The rise of photovoltaics • Only 77% of solar spectrum is absorbed by silicon
• Of what remains, 30% is used as electrical energy
• Net effect is 23% maximum theoretical efficiency Winter 2007 57 Winter 2007 58 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Future Projections A less rosy picture: • Photovoltaics are likely to become increasingly
prevalent in our world
• As fuels run out, their prices will climb relative to PV
prices
• Breakeven time will drop from 20 to 10 to 5 years
• Meanwhile, invest in PV: it’s sure to climb!
it’ • Photovoltaics don’t last forever
don’
– useful life is about 30 years
– damage from radiation, cosmic rays create crystal
imperfections • Some toxic chemicals used during production
– therefore not e ntirely environmentally friendly • Much land area is covered, with corresponding loss
of habitat
– not clear that this is worse than mining/processing and
power plant land use (plus thermal pollution of rivers) Winter 2007 Lecture 16 59 Winter 2007 60 15 Energy, Sustainability...
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