Where will our energy come from? Ch. 16 All from the Sun.
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Transcript of Where will our energy come from? Ch. 16 All from the Sun.
Where will our energy come from?
Ch. 16
All from the Sun
A problem: Dependence on imported oil• Cost to the economy: 350 billion dollars per year (2011 prices)
• Transferred to foreign (hostile) oil producers, unpredictable interruptions
25
20
15
10
5
0
1950 1960 1970 1980 1990 2000
Mill
ion
barr
els
pe
r da
y
US consumption
US production
Finding more oil
Producing oil gets more costly, riskier, dirtier (deep sea, fracking, tar
sands).
Deep Sea
MiddleEast NorthAfrica
MiddleEast
, NorthAfric
a
Tar sands
Enhanced Oil
Recovery
Oil recovery from tar sands in Alberta
Requires large amounts of hot water, leaves tar-contaminated water.
• Horizontal wells are drilled into gas-filled rock formations ( Marcellus shale ).• Explosives create paper-thin cracks in the rock, which release trapped gas.• Water, sand, and chemicals are pumped into the well (4000 gallons/minute).• The sand keeps the cracks open after the water pressure is released.• New source of gas and oil, cleaner than coal, abundant reserves. • Consumes lots of water, contaminates it with chemicals, risk of contaminating the water supply with methane where the well punches through am aquifer.
Natl. Geographic, Breaking Fuel From the Rock , December 2012, p. 90
Well meets water supply
Natural gas from
fracking
Oil from fracking
• Drilling for oil in North
Dakota • Each well contains a vertical and a horizontal part (dot and line, about 2 miles deep/long)
• Currently about 8,000 wells in North Dakota. They produce more oil than Alaska. Planned increase to
about 40,000 wells.
• One well uses 2 million gal-lons of water plus 350 barrels of chemicals over its lifetime.
• Most of the
contaminated water is
pumped back deep into the ground.
Natl. Geographic, March 2013,
p.47
Supply and demand are far apart
Wind
Dem
and
Solar
Renewable Energy
Electricity from wind power
Electricity from photovoltaics
Growing rapidly,
but still a small fraction of the
consumption ( 400 GW in the US ).
Annual Solar Cell Production (from PVNews)
Data from PVNews 4/2009, 5/2010,
5/2011
100100 km2 of solar cells could produce all the electricity for the
US.
0.4 TW
US Electricity Consumpti
on
Photovoltaics: Electricity from the Sun
Take it from the source. Electricity is fully convertible (Lect. 7, Slide
4).
TW = TeraWatt
= 1000 GigaWatt
The required area of solar cells
All the rooftops in the US could generate 0.66 TW (NREL study
44073.pdf, p.5).Incorporation into buildings eliminates the need for costly support structures.
1 kW/m2 (Incident solar power)
¼ (Useful daylight)
0.16 (Efficiency of a solar cell)
1010 m2 (100100 km2)
= 0.4 TW
= Electric power consumed in the US
Polycrystalline silicon solar cell
Most common, but requires a lot of energy to make.
Solar cell array at Nellis Air Force Base, Nevada
Thin film solar cells
• Compound semiconductors ( Cadmium Telluride, CIGS, … )
• Less material, less energy (low temperature processing)
• Print solar cells like newspaper (roll-to-roll)
Nanosolar (San Jose)
Many designs, efficiency growing slowly …
… but efficiency demands a price
Physics Today, March 2007, p.
37
1 $/WGoal
High endLow end
Low end designs are more cost-effective (less $/W).
How much would it cost to generate all the
electricity in the US by solar
cells ?
1 $/W (Price of solar cells per Watt)
0.4 TW (Electric power generated in the US)
= 0.4 T$ = 0.4 Trillion Dollars
The mechanical support structure adds significant costs. But one can design buildings to provide the
support.
Price comparison between solar and fossil energy
• Solar energy is free, while fossil fuels need to be paid for.
• One-time cost for solar, continuous costs for fossil energy.
• Energy payback time matters for solar energy (1-4 years).
• The price of solar cells is only about 1⁄3 of the total cost. The rest is for the support structure, the converter, labor.
$/W $/Ws
Solar thermal power plant in Spain
Convert solar energy to steam, then to electricity the conventional way. The mirrors focus sunlight onto a steam generator at the top
of the tower.
How do we use energy ?
1. Electricity
2. Fuel
3. Heat
1. Electricity is easy to use, but difficult to store.
2. Fuel is easy to store, but creates pollution.
3. Heat is easy to produce, but difficult to transport.
How does nature convert solar energy to chemical
energy ?
Convert plants into fuel: Make ethanol, diesel fuel
from sugar, corn starch, plant oil, cellulose ...
Split water into hydrogen and oxygen using sunlight. Use hydrogen as fuel. No greenhouse gases.
Still at the research stage.
Fuel from the Sun
• Photosynthesis
• Biofuels
• Water splitting (artificial photosynthesis)
Photosynthesis
Plants convert solar energy into chemical energy (here glucose, a sugar):
6 CO2 + 6 H2O + photons C6H12O6 + 6 O2
About 2% of the solar energy gets converted.
Next slide
Light-harvesting proteinsNext slide
The photosynthetic center
4 manganese atoms and 1
calcium form the reaction
center.
Biofuels
Production of ethanol fuel from corn and sugar cane:
Need energy for fertilizer, farm machinery, distilling.
(National Geographic, Oct. 2007, p. 44-47)
Output/Input = 1.3 Output/Input = 8
Poor return, competes with food Much better return
Cellulose is abundantly available in corn stalks, wood chips, switchgrass
Cellulose consists of a network of sugar molecules. If the network can be broken up into individual sugar molecules, ethanol can be produced by fermentation and distillation.
Bacteria in the gut of cows and termites break up cellulose.
Companies like Virent in Madison are producing such biofuel.
Cellulose
Large amounts of land (and irrigation water) are required for replacing gasoline with biofuel.
Algae can live in ocean water or sewage.
Discover Magazine Nov. 2011
Efficiency comparison for solar energy How far could one drive a car with the solar energy provided by 100x100 m2 (2.5 acres) of land in a year ?
Biodiesel: 21 500 km
Bioethanol 22 500 km
Biomass to liquid: 60 000 km
Photovoltaics, electric car: 3 250 000 km
(PHOTON International, April 2007, p. 106 www.photon-magazine.com)
• Solar cells are more efficient than photosynthesis (16%
vs. 2%).
• Electric motors are more efficient than combustion
engines
• Biomass to fuel conversion is inefficient.
(90% vs.
25%).
Electrical Storage
Chemical Storage
Storing energy
Energy/Weight
Energ
y/V
olu
me
0
10
20
30
0 10 20 30 40
Energy Storage Density Gasoline
Batteries
Supercapacitors
• Batteries for electric and hybrid cars, storing solar energy
overnight.• But batteries have 30-50 times lower energy density than fuels.
• Store fuel and convert it directly to electricity in a fuel cell.
Ethanol
Fuel cell
A fuel cell converts fuel directly into electricity , without creating heat by combustion. That’s why they can be 60% efficient, while the efficiency of a combustion engine is only
about 25% (Lect. 7, Slide 6).
In this example, hydrogen is combined with oxygen to form water plus energy. Usually, an explosion would result, but here the energy of the fuel drives elec- trons (e-) around an electrical circuit.
The Apollo program used fuel cells for electric power. When the oxygen tank of Appollo13 exploded, the crew sent the famous message: “
Houston we’ve had a problem “
Solar hot water
Best return on
investment in solar energy
Conserve energy instead of producing more
Infrared image
of thermal radiation reveals
leakage in the insulation. ( Red is warmer.)
See also DOE/EERE