This lecture will help you understand:
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Major sources of renewable energy
Solar energy
Wind energy
Geothermal energy
Ocean energy sources
Hydrogen fuel cells
Central Case: Iceland Moves Toward a Hydrogen
Economy
• Iceland pioneered geothermal power years ago, but relies on imported fossil
fuels and emits many greenhouse gases.
• An activist professor convinced politicians and businesses that Iceland should
now pioneer a hydrogen economy.
• In 2003 Iceland’s buses began converting to hydrogen; a filling station
opened—initial steps in a historic transition.
“New renewables”
Commonly referred to as “new” because:
• They are not yet used on a wide scale.
• They are harnessed using technologies that are still in a rapid phase of
development.
• It is widely believed that they will play a large role in our energy use in
the future.
They provide only 0.5% of our global energy supply.
“New renewables”
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Solar: from the sun’s rays
Wind: from the wind
Geothermal: from heat and heated water beneath the ground
Ocean sources: from the tides and from waves
Hydrogen: fuel and fuel cells that store renewable energy in usable form
Renewable sources for electricity
Renewable sources presently generate 9% of electricity in the U.S.
The vast majority of this is from hydropower.
Growth of renewable sources
Renewable sources, esp. wind and solar, are growing at an astounding rate.
These figures are percentage growth per year.
Renewable sources: Outlook
The outlook for renewable sources is good.
• But will governments raise subsidies to the level offered to fossil fuels?
• Will research and development proceed fast enough?
• Will companies have incentives to invest in developing these sources?
• Will consumers choose alternative energy sources?
Solar energy
• Use of energy from the sun
• Huge potential: Each day Earth receives enough sunlight to power human
consumption for 27 years, if we could somehow capture it all.
• Technology not new:
• First thermal solar collector: 1767
• First commercial solar water heaters: 1890
Solar energy
• Passive solar = designs buildings to maximize capture of sunlight in
winter, but keep buildings cool in summer
• through window placement, absorbent materials, and thermal mass
materials that absorb, store, and release heat
• Active solar = uses technological devices to focus, move, or store solar
energy
• solar panels: dark, heat-absorbing metal plates in glass-covered boxes,
often mounted on roofs
Solar energy: Active solar
Numerous mirrors focus sunlight on a receiver atop a “power tower” in the
California desert. This facility was the first to generate much solar power
commercially.
Solar energy: PV cells
• Photovoltaic cells (PV cells) convert solar energy directly into electrical
energy by making use of the photoelectric effect:
• When sunlight strikes a charged metal plate, electrons migrate to
another plate, and electric current is produced.
• In PV cells, light strikes negatively charged phosphorus, and electrons
migrate through silicon to positively charged boron.
Solar energy: PV cells
Electrons move from the phosphorus side of the silicon plate to the boron
side, creating electric current. PV cells are arranged in modules, panels,
and arrays.
Solar power
• Is little used, but fast growing
• Currently only 0.06% of primary energy supply in the U.S.
• Growing at 33% per year
• Cheaper technologies are taking off in developing countries.
• More expensive technologies are growing more slowly in developed
countries.
Pros and cons of solar power
PROS:
• Renewable as long as sun keeps on shining
• Sun’s energy abundant, if technology can capture it
• Allows for local control over power
• Solar cookers in developing nations lessen workloads.
• No direct greenhouse gas emissions
CONS:
• Not everywhere is sunny enough
• Up-front investment cost is high; takes years to pay for itself
Solar power
Wind power
• Takes kinetic energy of wind and converts it to electrical energy
• Fastest growing power source today
• Technology = wind turbines, machines with turning blades that convert
energy of motion into electrical energy by spinning a generator
• Windmills have been used for centuries.
• First wind turbine for electricity: late 1800s
Wind power: Wind turbines
Wind spins the blades, which turn the gearbox, which turns the generator to
produce electricity.
Wind power
Most wind power so far is concentrated in a few nations.
Wind power: Wind turbines
Turbines are often located in groups (“wind farms”) at sites with
exceptionally good wind conditions.
Wind power
Wind speeds vary tremendously from place to place.
Windiest in the U.S. are mountainous areas and parts of the Great Plains.
Wind power
So it’s not surprising that wind power has been most developed so far in
those states with the best wind resources.
Pros and cons of wind power
PROS:
• Renewable as long as wind blows
• No emissions after equipment made, installed
• Can allow local decentralized control over power, and local profit from
electricity sales
• Costs low after initial investment; costs dropping
CONS:
• Not everywhere is windy enough.
• Windy sites can be far from population centers.
• Some people object to aesthetics.
• Blades kill birds, bats.
• High start-up costs
Geothermal energy
• Radioactive decay of elements deep in Earth’s core creates heat that rises
toward the surface.
• This heats magma of volcanoes, and also underground water.
• Sometimes water spurts through to the surface in geysers.
• Geothermal power plants use the energy of naturally heated water to
generate electricity.
Geothermal energy
Underground heat warms water, and steam turns turbines and generators.
Condensed steam is reinjected into the aquifer to keep up pressure.
Geothermal energy
• Iceland uses geothermal energy to heat water for 86% of its homes.
• Heat pumps using surface heat can also be very efficient.
• Geothermal energy is growing 9% annually.
Pros and cons of geothermal power
PROS:
• Renewable as long as water is heated naturally
• Much lower greenhouse gas emissions than fossil fuels
• Can be inexpensive in areas where geothermal heating naturally occurs
CONS:
• Heated water may give out after a while—hotspot moves or aquifer
pressure drops
• Salts in water can corrode equipment, shorten lifespan
• Limited to geographic areas where geothermal heating naturally occurs
Ocean energy sources
Three sources from oceans:
• Tidal power: The twice-daily flow of tides (rising and falling of seas due
to the moon’s gravitational pull) creates energy of motion that can be
converted to electricity.
• Wave power: Motion of waves at ocean shores creates energy of motion
that can be converted to electricity.
• Thermal power: Exploits differences in warm and cold water. Not yet
commercially developed.
Tidal energy
Bulb turbines spin as tidal flow passes through them.
Wave energy
There are several designs for wave energy stations.
In this one, air is compressed in a chamber with each incoming wave, driving a
turbine to spin a generator.
Ocean thermal energy
• Warm surface water is piped into facilities where it evaporates volatile
substances (e.g., ammonia) to turn turbines.
• Cold, deeper water is then used to condense the substances and start the
cycle again.
• Ocean Thermal Energy Conversion (OTEC) project research has been
run in Hawaii and elsewhere, but there are no commercial operations yet.
Pros and cons of ocean power sources
PROS:
• Renewable as long as oceans behave as they always have
• No greenhouse gas emissions
CONS:
• Development could take up large portions of coastline valuable for other
uses
• Could interfere with ecology of estuaries and intertidal shorelines
• OTEC not yet commercially feasible
Hydrogen
• Hydrogen = simplest and most abundant element in universe
• Could potentially serve as basis for clean, safe, efficient energy system
• How it would work:
• Electricity generated from intermittent renewable sources like wind or
solar can be used to produce hydrogen.
• Fuel cells can then use hydrogen to produce electrical energy for
power.
Production of hydrogen fuel
• Hydrogen gas (H2) does not exist freely on Earth.
• We need to make it.
• Electrolysis is the cleanest way:
Split water into hydrogen and oxygen:
2 H2O  2 H2 + O2
• This can potentially be very clean, releasing no
emissions.
greenhouse gas
Production of hydrogen fuel
However, cleanliness of hydrogen production depends…
• If the source of electricity needed for electrolysis is
not clean (e.g., from coal), then greenhouse emissions will still occur.
• Besides electrolysis, hydrogen can also be produced from organic
molecules like fossil fuels. This entails greenhouse emissions.
• Also, it is possible that hydrogen production could have environmental
impacts of its own.
Fuel cells
In a fuel cell, hydrogen gas is used to produce electricity.
• The reaction is simply the opposite of electrolysis:
2 H2 + O2  2 H2O
How it works:
• Hydrogen molecules are stripped of electrons.
• H+ ions move through a membrane.
• Electrons complete a circuit, creating electricity.
Fuel cells
Pros and cons of hydrogen
PROS:
• We will never run out of hydrogen.
• Can be clean and nontoxic, with no greenhouse gas emissions
• Fuel cells potentially convenient, safe, and efficient
CONS:
• Depending on way hydrogen is produced, it may not be environmentally
clean
• Emission of hydrogen to atmosphere might have unforeseen impacts
Conclusion
• Renewable energy sources with sustainability promise include solar, wind,
geothermal, and ocean.
• By using electricity from renewable sources to produce hydrogen fuel, we
may be able to use fuel cells to produce electricity when and where it is
needed.
• Most renewable energy sources have been held back due to lack of
funding and artificially cheap market prices for nonrenewable resources.
• Despite this, renewable technologies have progressed far enough to offer
hope for a shift away from fossil fuels.