Chapter 21 Topics:

The major sources of
renewable energy
 Solar
energy
 Wind energy
 Geothermal energy
 Ocean energy
 Hydrogen fuel cells
New renewables
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“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

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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

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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
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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
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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, heatabsorbing 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

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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

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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

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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.

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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
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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

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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
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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
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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

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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
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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
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Wind produces no emissions once installed, preventing
the release of CO2, SO2, NOx, 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
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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-mybackyard (NIMBY) syndrome)
Geothermal energy
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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
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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 CO2 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
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“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
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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

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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
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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
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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
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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
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Costs are high; no commercial operation as yet
Hydrogen
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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
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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 H2 and O2 by
Electrolysis – passing an electric current through
water; the environmental impact depends on the
source of the electricity
Producing hydrogen from methane releases CO2
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