Chapter 21 Topics:  The major sources of renewable energy  Solar energy  Wind energy  Geothermal energy  Ocean energy  Hydrogen fuel cells

New renewables  “New” renewables are a group of alternative energy sources that include the sun, wind, geothermal heat, and ocean water  They are referred to as “new” because:  They are not yet used on a wide scale  Their technologies are in a rapid phase of development  They will play a larger role in our future energy use  New renewables provide energy for electricity, heating, and fuel for vehicles

A growing energy sector  New renewables are growing faster than conventional energy sources  Benefits of the new renewables include:  Alleviating air pollution and greenhouse gas emissions  They are inexhaustible, unlike fossil fuels  They help diversify our energy economy  They create jobs, income, and taxes, especially in rural areas

Barriers remain  Some technological and economic barriers to the greater use of new renewables exist  Recent advances suggest that these barriers are the legacy of “entrenched” policies (and thus, politics)  Conventional energy sources get more government subsidies, tax breaks, and research investment  In the absence of clear policy signals in support of new renewables, businesses (with their short-term, quarterly profit focus) are reluctant to act

Solar energy  Solar energy has been used for hundreds of years  Each square meter of Earth receives about 1,000 Joules per second (1 kilowatt) of solar energy  Passive solar = used primarily for heating buildings; uses building orientation, dimensions, and materials to allow winter sunlight into the building (and to collect its thermal energy) but block summer sunlight  Active solar = used for heating air and water, cooking, and generating electricity; uses technology to focus, collect, move, and/or store solar energy

Passive solar  Passive solar is simple and effective, conserving energy and reducing costs  Low, south-facing windows maximize heat gain in the winter; roof overhangs block summer light  Thermal mass = construction materials that absorb, store, and release heat are used in floors, roofs, and walls  Vegetation protects buildings from temperature swings

Active solar – heating air/water  Flat plate solar collectors = dark-colored, heat- absorbing metal plates mounted on rooftops  Water, air, or antifreeze is pumped through the collectors, transferring heat throughout the building  Heated liquids can be stored and used later  Can be used in isolated locations

Focusing the sun’s rays  Focusing solar energy on a single point magnifies its intensity  This concept can be used for something as simple as a solar cooker, or as complex as making electricity

Concentrated solar power  Concentrated solar power (CSP) = technologies that concentrate solar energy  The trough approach  Curved mirrors focus sunlight on pipes containing fluids  Thermal energy captured in the fluids is used to produce electricity  The “power tower” approach  An array of mirrors focus sunlight on a receiver at the top of a tall tower  Mirrors are programmed to track the sun’s movement

Photovoltaic cells  Photovoltaic (PV) cells = convert sunlight directly into electrical energy  The photovoltaic (photoelectric) effect occurs when light hits the PV cell and hits a plate made of silicon  Released electrons are attracted to the opposite plate  Wires connecting the two plates let electrons flow, creating an electric current  Small PV cells are in watches and calculators  On roofs, PV cells are arranged in modules, which comprise panels gathered into arrays

Variations on PV technology  Thin-film solar cells = PV materials are compressed into thin sheets  Less efficient but cheaper  Can be incorporated into roofing shingles, roads, etc.  Net metering = the value of the power the consumer provides is subtracted from the monthly utility bill  Feed-in tariffs pay producers more than the market price of power, so power producers turn a profit

Growth of solar power  Due to erratic funding for research & development, solar energy contributes only a very small part of U.S. energy production  Use has increased 31% per year since 1971  Solar energy is attractive in developing nations, where many don’t have access to electricity  Solar energy use should increase, due to:  Falling prices  Improved technologies  Economic incentives

Advantages  Solar technologies use no fuels, are quiet and safe, contain no moving parts, require little maintenance  Solar technologies do not emit greenhouse gases or air pollution  They allow local, decentralized control over power  PV owners can sell excess electricity to their utility  Green-collar jobs are being created

Disadvantages  Location - given current technologies, many regions do not get enough sunlight to be effective  Daily and seasonal variation also poses problems  Storage (batteries) and back-up power are needed  Costs - costs are high  Prices have dropped and efficiency has increased (20%)  Subsidies and market pricing favor fossil fuels (external costs not included)

Wind energy  Wind energy = energy derived from movement of air  Wind turbines = devices that convert wind’s kinetic energy into electric energy  Windmills have been used for 800 years to pump water  After the 1973 oil embargo, governments funded research and development  Moderate funding boosted technological progress  Today’s wind turbines look like airplane propellers or helicopters

Wind turbines  Wind blowing against the blades of the rotor turn the turbine, generating electricity  Towers are 45–105 m (148–344 ft) tall, minimizing turbulence and maximizing wind speed  Wind farms = turbines erected in groups of up to hundreds of turbines  Energy varies as the square of wind speed

Growth of wind power  Output has doubled every 3 years in recent years  Five nations produce 75% of the world’s wind power  But dozens of nations now produce wind power  Electricity is almost as cheap as from fossil fuels  A long-term federal tax credit would increase wind power even more

U.S. wind potential and generation

Offshore wind potential  Wind speeds are 20% greater over water than over land and there is less air turbulence over water  Costs to erect and maintain turbines in water are higher, but more power is produced and it is more profitable  Currently, turbines are limited to shallow water  An offshore wind farm off Cape Cod will have 130 turbines

Advantages  Wind produces no emissions once installed, preventing the release of CO 2, SO 2, NO x, mercury  It is more efficient than conventional power sources with an EROI = 23:1 (nuclear = 16:1; coal = 11:1)  It is readily scaled to any size and local areas can become more self-sufficient  Farmers and ranchers can lease their land for wind farms, supplementing their farm income  Advancing technology is reducing the cost of wind farm construction

Disadvantages  We have no control over when wind will occur  Limits our ability to rely on it for electricity  Batteries or hydrogen fuel can store the energy  Wind sources are not always near population centers, so transmission networks need to be built  Turbines threaten birds and bats, which can be killed when they fly into rotating blades  Local residents often oppose them ( the Not-in-my- backyard (NIMBY) syndrome)

Geothermal energy  Geothermal energy = thermal energy from beneath Earth’s surface  Radioactive decay of elements under extremely high pressures deep inside the planet generates heat  Which rises through magma, fissures, and cracks  Or heats groundwater, which erupts as geysers or submarine hydrothermal vents  Geothermal power plants use hot water and steam for heating homes, drying crops, and generating electricity  Geothermal energy provides more electricity than solar and as much as wind

…the how…

…the where…

Advantages and disadvantages  Advantages  Each megawatt of geothermal power prevents release of 15.5 million lbs of CO 2 each year  The source of the thermal energy is “perpetual”  Disadvantages  Limited to locations with high heat flow (plate tectonics)  Water can be removed faster than it can recharge  Water can be corrosive, requiring maintenance

Enhanced geothermal systems  “Geothermal on demand” – anywhere you want  Enhanced geothermal systems (EGS)  Deep holes are drilled into the Earth  Cold water is pumped in and heats  Hot water is withdrawn to generate electricity  Heat resource below the U.S. could power the Earth’s demands for millennia  Get’s around the location limitation  But EGS can trigger minor earthquakes and is not particularly cost-effective

Low-temperature geothermal  We can take advantage of natural temperature differences between the soil and air  Soil temperatures vary less than air temperatures  Soil temperatures are nearly constant year round  Ground source heat pumps (GSHPs) = heat pumps that exchange thermal energy with the subsurface  In the winter, thermal energy is transferred from the ground to the building  In summer, thermal energy is transferred from the building to the ground

More on GSHPs  More than 600,000 U.S. homes use GSHPs  Heat spaces 50–70% more efficiently  Cool spaces 20–40% more efficiently  Reduce electricity use 25–60%  Reduce emissions up to 70%

Energy from the ocean – tidal  Kinetic energy from the natural motion of ocean water can generate electrical power  Tidal energy  The rising and falling of ocean tides twice each day moves large amounts of water  Differences in height fro low to high tide are especially great in long, narrow bays

Energy from the ocean – waves  Wave energy = the motion of waves is harnessed and converted from mechanical energy into electricity  Many designs exist; few have been adequately tested  Some designs are for offshore facilities and involve floating devices that move up and down the waves  Wave energy is greater at deep ocean sites, but transmitting electricity to shore is very expensive  Another design uses the motion of ocean currents, such as the Gulf Stream

Coastal wave energy concepts  One coastal design uses rising and falling waves, which push air in and out of chambers, turning turbines to generate electricity  No commercial wave energy facilities operate yet but demonstration projects exist in Europe, Japan, and Oregon

Energy from the ocean - thermal  Each day, tropical oceans absorb solar radiation equal to the heat content of 250 billion barrels of oil  Ocean thermal energy conversion (OTEC) = uses temperature differences between the surface and deep water  Closed cycle approach = warm surface water evaporates ammonia, which spin turbines to generate electricity  Open cycle approach = warm surface water is evaporated in a vacuum and its steam turns turbines  Costs are high; no commercial operation as yet

Hydrogen  Fuel cells (hydrogen batteries) use hydrogen and oxygen to produce electricity to power cars, homes, computers, etc. (“waste” = water)  A hydrogen economy would provide a clean, safe, and efficient energy system by using the world’s simplest and most abundant element as fuel  Electricity produced from intermittent sources (sun, wind) could be used to produce the hydrogen  Governments are funding research into hydrogen and fuel cell technology

A hydrogen fuel cell

Producing hydrogen  Hydrogen gas does not exist freely on Earth  Energy must be used to force hydrogen-bearing molecules (water, methane) to release the hydrogen  Water can be broken down into H 2 and O 2 by Electrolysis – passing an electric current through water; the environmental impact depends on the source of the electricity  Producing hydrogen from methane releases CO 2

Advantages and disadvantages  Advantages  We will never run out; it is clean and nontoxic to use  Fuel cells are silent, non-polluting and up to 90% energy efficient  Can be used to store energy from intermittent sources  If kept under pressure, hydrogen is no more dangerous than gasoline in tanks  Disadvantages  The infrastructure needed for wide-spread use does not exist and will be expensive to build  Leakage of hydrogen can deplete stratospheric ozone