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 interruptions Million barrels per day US consumption US production

Supply and demand are far apart Wind Demand Solar

Electrical Storage Chemical Storage Storing energy Energy/Weight Energy/Volume 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 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) 100  100 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/m 2 (Incident solar power)  1/4(Fraction of useful daylight)  0.16(Efficiency of a solar cell  16%)  100  100 ·10 6 m 2 (100  100 km 2 ) = 4 · 10 8 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% “80% Learning Curve”: Module price decreases by 20% for every doubling of cumulative production 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 $/W Goal High end Low end

How much would it cost to generate all the electricity in the US by solar cells ? 1 $/W (Price of solar cells)  4 ·10 8 kW (Electric power generated in the US) = 4 ·10 11 $ = 400 Billion Dollars The support structure adds substantial costs.

Solar thermal Convert 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 ) 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 m 2 (2.5 acres) of land in a year ? Biodiesel: km Bioethanol km Biomass to liquid: km Photovoltaics, electric car: km Solar cells are more efficient than photosynthesis. Electric motors are more efficient than combustion engines. PHOTON International, April 2007, p. 106 (

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.