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