Where will our energy come from ?

Coal: 10,000 tons of coal per day (1 freight train)

Nuclear:100 tons of uranium per year

Hydroelectric: 60,000 tons of water per second

Comparison of power plants

for 1 GW

Where is solar energy ?

Although most of our energy originally comes from the Sun (except nuclear), only a minuscule amount of solar energy is being used

today.

A problem: Dependence on imported oil

Find alternative energy sources: Solar, wind, biofuel, …

• Cost to the economy: $ 700 B/yr (peak prices), $ 200 B/yr (average prices)

• Transferred to foreign (hostile) oil producers, unpredictable interruptions25

20

15

10

5

0

1950 1960 1970 1980 1990 2000

Mill

ion

barr

els

pe

r da

y

US consumption

US production

Supply and demand are far apart

Wind

Dem

and

Solar

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

• How do we store solar electricity overnight, wind electricity

when calm ?

• The range of all-electric cars is short due to poor storage by

batteries.

• Batteries have 30-50 times lower energy density than gasoline.

• Chemical energy is easy to store in fuel, but electricity is not.

Ethanol

How do we use energy ?

• Electricity

• Fuel

• Heat

• Electricity is easy to use, but difficult to store.

• Fuel is easy to store, but creates pollution.

• Heat is easy to produce, but difficult to transport.

Electricity from the Sun (photovoltaics)

100100 square kilometers of solar cells could produce all the electricity for the US. But they are

still too costly.

0.4 TW

US Electricity Consumption

The required area of solar cells

1 kW/m2 (Incident solar power)

1/4 (Fraction of useful daylight)

0.16 (Efficiency of a solar cell

16%)

100100 ·106 m2 (100100 km2)

= 4 · 108 kW (Electric power generation in the US)

Solar cell power plants

Growing rapidly almost everywhere, but the total production was less than 1 GW in 2003 (about one

nuclear power plant).

Solar cells

Polycrystalline silicon solar cell

Thin film solar cells

• Compound semiconductors: CdTe, CIGS = Cu(InGa)Se

• Less material, less energy by low temperature processing

• Print solar cells like newspaper (roll-to-roll)

Solar cell printed on plastic

Nanosolar (San Jose, Berlin)

The efficiency keeps growing slowly

PV Module Production Experience (or “Learning”) Curve

80%

2005

1976“80% Learning Curve”:

Module price decreases by20% for every doubling of

cumulative production

20102015

Silicon Wafer Technologies

• Slow power law, not exponential like Moore’s law

• Slope of the double-log plot gives the power (1/3)

But efficiency demands a price

Physics Today, March 2007, p. 37

1 $/W

Goal

High endLow end

How much would it cost to generate all the electricity in

the US by solar cells ?

1 $/W (Price of solar cells)

4 ·108 kW (Electric power generated in the US)

= 4 ·1011 $

= 400 Billion Dollars

The support structure adds substantial costs.

Solar thermalConvert solar energy to steam, then to electricity

Solar thermal (31% efficiency)

Fuel from the Sun ?

• Photosynthesis

• Biofuels

• Split Water

Plants convert solar energy to chemical energy

but the efficiency is low (1%-2%)

Convert plants to 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. (Futuristic)

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

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.

We are still searching for an efficient way to do the same.

Biofuels versus photovoltaics (PV)

How far could one drive a car with the energy produced by 100x100 m2 (2.5 acres) of land in a year ?

Biodiesel: 21 500 kmBioethanol 22 500 kmBiomass to liquid: 60 000 kmPhotovoltaics, electric car: 3 250 000 km

Solar cells are more efficient than photosynthesis.Electric motors are more efficient than combustion engines.

PHOTON International, April 2007, p. 106 (www.photon-magazine.com)

Solar water heater

Currently the best return on investment in solar energy

Conserve energy rather than produce more

An infrared image of thermal radiation reveals weak spots in the insulation.