EPSc 116: Resources of the Earth

Lecture 15 on Ch. 6: Renewable Energy

Focal Points

What are the sources of renewable energy?

How do we extract the energy from them and convert it to our use?

What are the specific advantages of using each type of renewable energy?

What are the specific challenges to upscaling the production of this energy,

i.e., for use of it and reliance on it at a national scale?

Efficiency of energy conversion, i.e., % made available

Cost of energy production

Intermittence of energy production

History of (lack of) success

Wind Power

Solar Photovoltaic

Solar Thermal

Geothermal energy

Ocean Energy (wave, thermal, tidal)

Hydroelectric Power

Biomass (burning, anaerobic digestion to "fuel")

Nuclear fusion

Sources of Renewable Energy

Text, Fig. 6.1

Energy flow to Earth’s Surface

1) Solar (SW)

2) Tidal

3) Heat flow from below

SOURCES

Solar Energy

Low-quality solar, e.g., to heat water passively to less than 100°C

High-quality solar, i.e., concentrate sun’s rays to produce high temperatures

Solar Thermal

Text, Fig. 6.22

Solar PhotovoltaicLight interacts with a semiconductor to cause electrons to flow within it, thereby producing electricity

Use of photovoltaic cells = solar cells; array

Text, Fig. 6.23

P-N junction solar cells are moving toward 33% efficiency; routinely 10-25%

Polymer-based cells may reach 10%

Some novel cells 44%

About 25% efficiency of conversion

Text, Fig. 6.24

Satellite Solar Power Station

Large arrays of photovoltaic cells orbiting the earth capture the sun’s energy, convert it to electrical energy, and beam it back to the earth via a microwave beam.

Schematic only

Wind Energy

Wind = movement of air caused by (unequal) heating by the sun

Windmills (individual or in wind farms) convert movement of air to operation of a turbine, which produces electricity.

Various designs of windmills to account for absolute wind speed (high vs. low), (variable) direction of wind, intermittence of wind.

Theoretical limit to conversion efficiency is 59%. Usually < 50%.Images from http://www.darvill.clara.net/altenerg/wind.htm

Hydroelectric PowerEnergy of sun and gravity

Electricity generated from the energy captured from the flow of water.

Usually produced by large dams (capture water in a reservoir)

Mechanical energy directly drives turbines

80-90% efficiency of energy conversion; only a 2-step process

Compare to 35-40% for coal-fired and 30% for nuclear power plants

Pumped water storage also possible, as at Taum Sauk in Missouri

Text, Fig. 6.26

Text, Fig. 6.28

Principal countries producing electricity from hydroelectric dams

Wave PowerWind, which blows due to solar heating, causes waves to form

Waves vs. wind of same velocity: Waves have 800x more energy, because of the much greater density of water (a lot more mass is moving in water)

Tremendous amounts of wave energy are “used up” along coasts – erosion

Multiple designs (not very successful) to capture wave energy:

Flexible air bags mounted along spine of reinforced concrete

Floating rafts or tubes that transmit mechanical energy

Problems of corrosion and storms

Ocean PowerUsually refers to Ocean Thermal Energy Conversion (OTEC)

Sun warms surface of the water (by energy absorption), which leads to a gradient (difference) in temperature between surface and depth

Creates basis for a heat engine, using the (only) 20°C difference in temp.

Only about 2-3% efficiency of energy conversion, BUT there is a huge amount of total energy. (Small %)x(Huge #) = BIG #

Text Fig. 6.32a

Typical temperature variation with depth in equatorial ocean

Text Fig. 6.32bHeat engine via thermal gradient

Tidal EnergyNot derived from sun, but rather from gravitational attraction of Moon and Earth

Cycle of the tides causes fluctuation in height of water level

Rise and fall can drive a water-powered turbine to generate electricity

Best sites are where tidal fluctuation > 10 m (30 ft)

Bay of Fundy Nova Scotia Patagonian coast of Argentina

Bristol Channel, UK Murmansk coast, Barents Sea

Rance Estuary, French coast

Turbine operates during both ebb and inflow of tide

Geothermal EnergyComes from below (geothermal gradient) due to original heat plus heat from

decay of naturally radioactive elements

Geothermal gradient is 15 - 75°C/kilometer (i.e., 25 - 120°F/mile)

Vast amount of heat available (but dispersed) at depth

Usually select an area of very high heat flow near surface: volcanoes, spas,geysers, hot springs, hot dry rocks. Not truly renewable.

Wikipedia

Can directly tap either steam or super-heated water that can be allowed to “flash” into steam as it rises and depressurizes.

Also can pump cold water down into “hot dry rock”; let water rise to surface and drive turbines to generate electricity.

Text Fig. 6.37a

Steam from geothermal well drives turbines to produce electricity.

Text Fig. 6.38

Pump cold water down into hot, dry rocks. Retrieve it.

Generating Electricity from Geothermal Energy

Iceland**

Italy

New Zealand

Only ~1% efficiency in recovery of energy

Another Face to Geothermal Energy

From: http://climatelab.org/Geothermal_Energy

The Direct Application of Geothermal Energy

Energy from Biofuels and Waste Products

Much of the developing world relies on readily available combustibles: renewable and widespread wood, grain stalks, dried animal dung

Known as biofuels because they come from recently living organisms

Can lead to deforestation – erosion, loss of fertile soil, water depletion

“Closed carbon cycle”: grow, burn, release CO2, absorb CO2 in new growth

Fermentation process called anaerobic digestion (special bacteria) can convert discarded biomass into methane. C.f. coal gasification

Pyrolysis (controlled heating) produces liquid fuels from biomass: gasohol

Not just from waste products:

Brazil’s remarkable production of biofuels from sugar (cane, beet), cassava

Single-celled algae grown in ponds, harvested, fermented -- methane

Hydrogen for Energy

Hydrogen is extremely abundant on earth, but usually bonded with other elements, as in H2O (water) and CH4 (methane = natural gas)

Hydrogen burns easily by combining with oxygen and forming water

H2 is abundant, burns to produce much heat, only water as combustion waste

Challenges:

How to produce large amounts of pure H2

How to transport and handle H2 (explosive)

Pass electric current thru H2O

Keep H2 under pressure, as a cool liquid

Hydrogen-powered cars (?): fuel cellsSchematic of a fuel cell

Photocatalytic Conversion: CO2 to Hydrocarbons

Roy, S., et al. 2010. Toward solar fuels: Photocatalytic conversion of carbon dioxide to hydrocarbons: ACS Nano, v. 4, 1259-1278.

Nuclear Fusion: Ultimate Energy Source? It’s good enough for the sun!

Nuclear fusion, in which nuclei of lighter elements combine to form larger nuclei, i.e., heavier elements, with the release of huge amounts of energy

Fusion of 2 deuterium (“heavy hydrogen”) atoms produces helium + ENERGY

2H + 2H 3He + n + energy (3.2 meV)1 1 2

Problems: Lots!

Have fused deuterium + tritium to form helium, but explosive & uncontrolled

Thermonuclear bomb, AKA a hydrogen bomb

Must reach 100 million °C to form a plasma. How do you contain this?!

Doughnut-shaped magnetic field; inertial confinement (compression)

Pay-off: HUGE amount of energy without any radioactive waste

Possibly Re-Visit the Nuclear (fission) Option?

See: “The Waste Problem” by Bill Lee. In Materials Today, March 2006, vol. 9, no. 3, p. 64

See:

“Fukushima and the Future of Nuclear Power” by William Tucker. In The Wall Street Journal, March 6, 2012, p. A19.

“Coming clean about Nuclear Power” by the Editors. Scientific American, June 2011, p. 14.

Some Final Thoughts

We want sources of energy that are reliable, available, affordable, & clean.

There is no reason to believe that everyone, everywhere should or will use the exact same energy source or even tap similar energy sources in the same way.

Particular challenges to “green” renewable energy sources:

How much space will they require to tap necessary amounts of energy?

How can we deal with the problems of intermittent access to the energy?

How can we balance the use of huge amounts of land for energy retrieval against 1) other needs for that land and 2) the alternative of nuclear energy?

Reasons for making these big choices:

Desire to retain the environmental integrity of the earth

Determination to be more self-reliant rather than dependent on others

Depletion of our historical fossil fuels, especially petroleum