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CHAPTER 16. Renewable Energy. An introduction to renewable energy. Coastal locations are known for constant, cooling wind Drawing tourists and residents Vacationers increase the population and stress the energy infrastructure
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CHAPTER 16 Renewable Energy
An introduction to renewable energy • Coastal locations are known for constant, cooling wind • Drawing tourists and residents • Vacationers increase the population and stress the energy infrastructure • A proposed wind farm off Nantucket Sound could supply 75% of the area’s electricity • There would be 130 turbines, each 417 feet tall • People opposed to the wind farm filed lawsuits • Visual pollution • Threats to tourism, navigation, fishing, and birds
Wind farms are controversial • Proponents of wind farms view concerns as a case of NIMBY (Not In My Backyard) • Opinion polls show strong support for wind farms • Every state and federal agency has approved • The wind farm may be operable by 2011 • Wind farms are appearing all over the U.S. and other countries • In 2008, wind power provided energy to 6.5 million U.S. homes
Solar, too • Solar energy provides hot water and electricity globally • Building design, insulation, and solar collection devices can reduce energy bills by 75% • “Farms” with trough-shaped mirrors boil water or synthetic oil and drive turbogenerators • A renewable-energy future is absolutely essential • It can replace limited, polluting fossil fuels and nuclear • Solar and wind energy are becoming cost competitive • Renewables provide 6.7% of U.S. and 18% of world energy
Putting solar energy to work • Solar energy originates from the Sun’s thermonuclear fusion • Solar constant: radiant (solar) energy reaching Earth • Energy ranges from ultraviolet to visible to infrared (heat) • 50% of the energy makes it to Earth, 30% is reflected, 20% is absorbed by the atmosphere • The amount of sunlight reaching Earth is almost unbelievable • Full sunlight delivers 700 watts/m2 to Earth’s surface • 700 MW of power (equals a large power plant) delivered to 390 mi2 • The Sun delivers 10,000 times the energy used by humans
Using solar energy • Using solar energy does not change the biosphere’s energy balance • Energy absorbed by water or land, in photosynthesis, or used by humans is ultimately lost as heat • Solar energy is abundant but diffuse • Varies with season, latitude, and atmospheric conditions • Using solar energy requires turning a diffuse, intermittent source into a form (fuel, electricity) that can be used • Collection, conversion, and storage of solar energy are difficult and must be cost effective
Solar heating of water • Flat-plate collector: a thin, broad box • The black bottom absorbs sunlight and heats up • Water in tubes absorbs the heat • The glass top prevents heat loss • Active systems: use pumps to move the heated water • Passive systems: use gravity and convection to move water • The collector is lower than the tank • Heated water from the collector rises through convection
Solar energy is used worldwide • The U.S. has over 1 million residential and 200,000 commercial solar hot-water systems • This is only a small fraction of hot-water heaters • The initial cost is 5–10 times higher than gas or electric heaters • Over time, the solar system is cheaper to use • In the U.S., solar thermal collection systems heat pools • China leads the world in using solar thermal systems • 10% of people get hot water from these systems
Solar space heating • Flat-plate collectors can be used for space heating • They are less expensive and can be homemade • It is necessary only to have air circulate through the collector box • Efficiency is gained if collectors are mounted to allow convection to circulate the heated air into areas to be heated
Building = collector • A building can act as a collector for solar space heating • Windows should face the Sun • Insulated drapes or shades can trap heat inside • The building should have appropriate insulation, doors, and windows to store heat • Tanks of water or rocks do not store heat economically • Awnings or shades shield windows from summer heat • Deciduous vegetation blocks summer but not winter heat • Evergreen hedges on the shady side protect from the cold
Earth-sheltered housing • Combines insulation and solar heating • Earth insulates the building • The building is oriented for passive solar energy • Earth (berms) can be built up against a conventional house • Or the building can be covered with earth • The building has south-facing windows • Walls and floors store heat during the day • Release it during the night to warm the house • Moisture must be controlled with dehumidifiers
Energy Stars • A well-designed solar-energy building can reduce bills • With added construction costs of only 5–10% • 25% of our energy use is for space and water heating • Solar design can save oil, natural gas, and coal • In 2001, the EPA extended its Energy Star Program to buildings using at least 40% less energy and meeting other criteria • By 2008, 4,100 buildings had saved $1.5 billion/yr
Criticizing solar heating misses the point • Solar heating is criticized for needing a backup heating system • Good insulation minimizes this need • Backup can come from a small wood stove or gas heater • This criticism misses the point: • The objective of solar heating is to reduce dependency on conventional fuels • Fuel demand is reduced, and the economic and environmental costs are also reduced
Solar production of electricity • Solar energy can produce electrical power • Providing an alternative to coal and nuclear power • Photovoltaic (PV) cells: a wafer of material • One wire is attached to the top, one to the bottom • Sunlight striking the wafer puts out 1 watt of power • 40 linked PV cells produce enough energy to light a lightbulb • PV cells are highly sophisticated • Light hitting the wafer causes an electron to go from the lower layer to the upper layer
Solar cells do not wear out • Solar cells convert light energy directly to electrical power • With 15–30% efficiency • Cells have no moving parts and last about 20 years • Silicon, one of the most abundant elements, is the main material used in solar cells • The cell’s cost lies mostly in its design and construction • Cells are used in calculators, watches, toys, rural homes, pumps, traffic signals, transmitters, lighthouses, satellites • Net metering: rooftop electricity is subtracted from power use from the power grid
Cost • The cost of PV power = 25 cents per kilowatt-hour • Other sources = 6–12 cents per kilowatt-hour • More efficient cells and less expensive technologies • Dramatically reduce costs • Provide incentives for more applications, sales, and markets • This industry is the fastest growing energy technology • In 2008: existing PV panels turned sunlight into 16,000 MW
Inverters: complicated yet necessary • Act as an interface between the solar PV modules and the electric grid or batteries • Convert incoming direct current (DC) to alternating current (AC) from the grid or for powering the devices • Also act as a control that detects and responds to fluctuations in voltage or current • Must be robust enough to withstand the heat of attics for rooftop PV systems • Costs have declined, but are still substantial • But they are eligible for subsidies
Utilities • Utility companies are moving toward large-scale PV use • 53 plants worldwide (2008) generating 10 MW • The largest U.S. plant is in Nevada • The most promising future for PV power: rooftop panels • Unused space (e.g., warehouse and factory roofs) • California, New York, New Jersey, Connecticut give incentives for home use • A 2 kW system gives half the energy needed for 1 year • The 2008 Emergency Economic Stabilization Act gives tax credits for 30% of system costs
New PV technologies • Costs must decline to compete with other electricity sources • New technologies are in production or close to it • Thin-film PV cells: amorphous silicon or cadmium telluride • Instead of expensive silicon • The film is applied to roofing tiles or glass • Silicon crystals are sliced to make SLIVERs • Uses 10% of the silicon in normal cells • Light-absorbing dyes transmit energy to solar cells on the edge of the glass • Light passes through to conventional solar panels
Concentrated solar power (CSP) • Technologies convert solar energy into electricity • Reflectors (concentrators) focus sunlight on a receiver, which transfers the heat to a conventional turbogenerator • CSP works well in sunny, remote areas • Solar troughs: reflect sunlight onto a center pipe • Heat-absorbing liquid is heated to high temperatures, boiling water to produce steam for a turbogenerator • Energy can be stored for release at night • The Mojave Desert generates one-third the capacity of a nuclear power plant at a cost equal to coal-fired facilities
CSP technologies • Power towers: Sun-tracking mirrors focus sunlight onto a receiver mounted on a tower at the center of the area • Heated liquid flows to a heat exchanger to drive a turbogenerator or a tank to store the heat • A California facility generates electricity for 10,000 homes • Dish-engine system: parabolic concentrators focus sunlight onto a receiver • Fluid is transferred to an engine that directly generates electricity at 30% efficiency
The future of solar energy • Solar energy: a $20 billion industry and is growing rapidly • It is hard to keep up with demand • But solar energy does have disadvantages • Technologies are more expensive than traditional energy sources (but subsidies help decrease costs) • A backup energy source, storage battery, or thermal storage for nighttime power is needed • Many areas are not sunny in winter • But other energies also have disadvantages • Mining, greenhouse gas emissions, nuclear waste, etc.
Matching demand • 70% of electrical demand is during the day – we can use solar • We can rely on conventional sources at night • Air-conditioning, the second largest power user (after refrigeration), is well-matched to energy from PV cells • Solar and wind can replace coal and nuclear electrical power • Can be planned and built in months, not years • Can be installed in small increments • Have less financial risk for utility companies • Are less vulnerable to terrorist attacks • Can provide energy for the 1.6 billion in poor nations
Indirect solar energy • Energy from the Sun is the driving force behind dams, firewood, windmills, sails • The force of falling water can turn paddle wheels • To grind grain, saw logs, do other tasks • Hydropower: hydroelectric dams use water under high pressure to drive turbogenerators • In the U.S., it generates 6% of electrical power • 300 large dams mainly in the Northwest and Southeast • Worldwide, dams generate 19% of electrical power • It is the most common form of renewable energy
Trade-offs: benefits of dams • Dams, especially large ones, have benefits and serious consequences • Benefits • They eliminate the cost and environmental impacts of fossil fuels and nuclear power • Dams provide flood control and irrigation water • Reservoirs provide recreational and tourist opportunities • Pumped-storage power dams can pump water to a higher reservoir for later use during peak times
Trade-offs: disadvantages of dams • Reservoirs drown farmland, wildlife habitats, towns • Land of historical, archaeological, or cultural value • Reservoirs displace rural populations • In the last 50 years, 40–80 million have been displaced • Dams impede or prevent migration of fish • Even if ladders are provided • Fish habitats are suffering • Dams wreak havoc downstream • Decreased sediment reaches the river’s mouth
More dams? • The U.S. and other developed nations have brought their hydropower to maximum capacity • The U.S. has 75,000 dams 6 feet or higher • Two million smaller dams • The Wild and Scenic Rivers Act (1968) protects most of the last 2% free-flowing rivers • Many dams that impede river flow are being removed • Worldwide, dams are controversial • The projected benefits may not justify the ecological and sociological effects
The Nam Theun 2 Dam • A 50-m-tall dam being completed on the Mekong River in Laos • Is projected to help Laos develop • The project has displaced 6,200 people • It will damage or destroy 174 square miles of sensitive environments • It is being built to sell power to Thailand • Critics say the government cannot manage this project • Money will not be used to help the poor
Dam report • The Three Gorges Dam on the Yangtze River in China • Was completed in 2006 • Is the world’s largest dam (1.4 miles long) • Displaced 1.2 million people • Will generate electricity and control floods • Produces electricity equal to 12 coal-fired plants • The World Commission on Dams 2000 report found dams are a mixed blessing • They should be built only as a last option • Many more dams will be built in China, Brazil, India, etc.
Wind power • The Horse Hollow Wind Energy Center in Texas • The world’s largest wind farm • 304 turbines on 47,000 acres produce electricity for 260,000 homes • The U.S. is the world leader in wind energy • Germany is second • Wind capacity has increased 28%/yr • It is economically competitive • It could provide 12% of the world’s electricity by 2020
Design • The most practical design is using wind-driven propellers • The propeller shaft is geared directly to a generator • Wind turbine: a wind-driven generator • Reliability and efficiency have reduced electric costs • Wind farms: arrays up to several thousand turbines • Produce pollution-free power competitive with traditional sources • The amount of wind that can be tapped is immense • Wind farms in the Midwest could meet U.S. needs • Farmers are paid well to put turbines on their land
Drawbacks • Wind is intermittent, so backup or batteries are needed • Wind farms can be visually unappealing • Turbines can be hazardous to birds • When placed on migratory routes or in critical habitat • But far fewer die than from cars, traffic, and windows • Offshore wind farms use dependable, strong winds • They have less of a visual impact • Land does not need to be bought • New turbines in 2008 displaced emissions from 40 coal plants
Biomass energy • Burning firewood for heat: the oldest form of energy • Renamed “utilizing biomass energy” • Biomass energy: energy derived from present-day photosynthesis • Leads hydropower in U.S. renewable energy • Most is used for heat • Produced by burning wood or municipal waste, generating methane, or producing alcohol
