Sustainable Energy
Chapter 22
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Outline:
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Conservation
 Cogeneration
Tapping Solar Energy
 Passive vs. Active
High Temperature Solar Energy
 Photovoltaic Cells
Fuel Cells
Energy From Biomass
Energy From Earth’s Forces
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
CONSERVATION
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Utilization Efficiencies
 Most potential energy in fuel is lost as
waste heat.
- In response to 1970’s oil prices, average
US automobile gas-mileage increased
from 13 mpg in 1975 to 28.8 mpg in
1988.
 Falling fuel prices of the 1980’s
discouraged further conservation.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Utilization Efficiencies
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Today’s average new home uses half the fuel
required in a house built in 1974.
 Reducing air infiltration is usually the
cheapest, quickest, and most effective way
of saving household energy.
According to new national standards:
 New washing machines will have to use
35% less water in 2007.
- Will cut US water use by 40 trillion liters
annually.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Energy Conversion Efficiencies
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Energy Efficiency is a measure of energy
produced compared to energy consumed.
 Thermal conversion machines can turn no
more than 40% of energy in primary fuel
into electricity or mechanical power due to
waste heat.
 Fuel cells can theoretically approach 80%
efficiency using hydrogen or methane.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Energy Conversion Efficiencies
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Net Energy Yield - Based on total useful
energy produced during the lifetime of an
entire energy system, minus the energy
required to make useful energy available.
 Expressed as ratio between output of
useful energy and energy costs.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Negawatt Programs
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It is much less expensive to finance
conservation projects than to build new power
plants.
 Power companies investing in negawatts of
demand avoidance.
- Conservation costs on average $350/kw
- Nuclear Power Plant: $3,000 - $8,000/kw
- Coal Power Plant: $1,000/kw
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Cogeneration
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Cogeneration - Simultaneous production of
both electricity and steam, or hot water, in
the same plant.
 Increases net energy yield from 30-35% to
80-90%.
- In 1900, half of electricity generated in
US came from plants also providing
industrial steam or district heating.
 By 1970’s cogeneration had fallen to
less than 5% of power supplies.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
TAPPING SOLAR ENERGY
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A Vast Resource
 Average amount of solar energy arriving
on top of the atmosphere is 1,330 watts
per square meter.
- Amount reaching the earth’s surface is
10,000 times more than all commercial
energy used annually.
 Until recently, this energy source has
been too diffuse and low intensity to
capitalize for electricity.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Average Daily Solar Radiation
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Solar Energy
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Passive Solar Heat - Using absorptive
structures with no moving parts to gather and
hold heat.
 Greenhouse Design
Active Solar Heat - Generally pump heatabsorbing medium through a collector, rather
than passively collecting heat in a stationary
object.
 Water heating consumes 15% of US
domestic energy budget.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Solar Energy
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Eutectic Chemicals are also used to store
large amounts of energy in a small volume.
 Heating melts the chemicals and cooling
returns them to a solid state.
- Most do not swell when they solidify and
undergo phase changes at higher
temperatures than water and ice.
 More convenient for heat storage.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
HIGH TEMPERATURE SOLAR ENERGY
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Parabolic mirrors are curved reflective
surfaces that collect light and focus it onto a
concentrated point. Two techniques:
 Long curved mirrors focused on a central
tube containing a heat-absorbing fluid.
 Small mirrors arranged in concentric rings
around a tall central tower track the sun
and focus light on a heat absorber on top
of the tower where molten salt is heated to
drive a steam-turbine electric generator.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Promoting Renewable Energy
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Proposed Energy Conservation Policies:
 Distributional Surcharges
- Small fee levied on all utility customers.
 Renewable Portfolio
- Suppliers must get minimum percentage
of power from renewable sources.
 Green Pricing
- Allows utilities to profit from conservation
programs and charge premium prices for
renewable energy.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Photovoltaic Solar Energy
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Photovoltaic cells capture solar energy and
convert it directly to electrical current by
separating electrons from parent atoms and
accelerating them across a one-way
electrostatic barrier.
 Bell Laboratories - 1954
- 1958 - $2,000 / watt
- 1970 - $100 / watt
- 2001 - $5 / watt
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Energy Costs
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Photovoltaic Cells
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During the past 25 years, efficiency of energy
capture by photovoltaic cells has increased
from less than 1% of incident light to more
than 10% in field conditions.
 Invention of amorphous silicon collectors
has allowed production of lightweight,
cheaper cells.
- Currently $100 million annual market.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Storing Electrical Energy
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Electrical energy storage is difficult and
expensive.
 Lead-acid batteries are heavy and have
low energy density.
 Metal-gas batteries are inexpensive and
have high energy densities, but short lives.
 Alkali-metal batteries have high storage
capacity, but are more expensive.
 Lithium batteries have very long lives, and
store large amounts of energy, but are very
expensive.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
FUEL CELLS
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Fuel Cells - Use on-going electrochemical
reactions to produce electric current.
 Positive electrode (cathode) and negative
electrode (anode) separated by electrolyte
which allows charged atoms to pass, but is
impermeable to electrons.
- Electrons pass through external circuit,
and generate electrical current.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuel Cells
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuel Cells
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Fuel cells provide direct-current electricity as
long as supplied with hydrogen and oxygen.
 Hydrogen can be supplied as pure gas, or
a reformer can be used to strip hydrogen
from other fuels.
 Fuel cells run on pure oxygen and
hydrogen produce no waste products
except drinkable water and radiant heat.
- Reformer releases some pollutants, but
far below conventional fuel levels.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuel Cells
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Typical fuel cell efficiency is 40-45%.
Current is proportional to the size of the
electrodes, while voltage is limited to about
1.23 volts/cell.
 Fuel cells can be stacked together until the
desired power level is achieved.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuel Cell Types
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Proton Exchange Membrane - Design being
developed for use in automobiles.
 Lightweight and operate at low temps.
 Efficiency typically less than 40%.
Phosphoric Acid - Most common fuel design
for stationary electrical generation.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuel Cell Types
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Carbonite - Uses inexpensive nickel catalyst,
and operates at 650o C.
 Good heat cogeneration, but difficult to
operate due to the extreme heat.
Solid Oxide - Uses coated zirconium ceramic
as electrolyte.
 High operating temperatures, but highest
efficiency of any design.
- Still in experimental stage.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
ENERGY FROM BIOMASS
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Plants capture about 0.1% of all solar energy
that reaches the earth’s surface.
 About half the energy used in metabolism.
- Useful biomass production estimated at
15 - 20 times the amount currently
obtained from all commercial energy
sources.
 Renewable energy resources account
for 18% of total world energy use, and
biomass makes of three-quarters of
that supply.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Burning Biomass
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Wood provides less than 1% of US energy,
but provides up to 95% in poorer countries.
 1,500 million cubic meters of fuelwood
collected in the world annually.
- Inefficient burning of wood produces
smoke laden with fine ash and soot and
hazardous amounts of carbon monoxide
(CO) and hydrocarbons.
 Produces few sulfur gases, and burns
at lower temperature than coal.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuelwood Crisis
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About 40% of world population depends on
firewood and charcoal as their primary
energy source.
 Of these, three-quarters do not have an
adequate supply.
- Problem intensifies as less developed
countries continue to grow.
 For urban dwellers, the opportunity to
scavenge wood is generally
nonexistent.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Fuelwood Crisis
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Currently, about half of worldwide annual
wood harvest is used as fuel.
 Eighty-five percent of fuelwood harvested
in developing countries.
- By 2025, worldwide demand for
fuelwood is expected to be twice current
harvest rates while supplies will have
remained relatively static.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Wood Harvest
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Dung
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Where other fuel is in short supply, people
often dry and burn animal dung.
 Not returning animal dung to land as
fertilizer reduces crop production and food
supplies.
- When burned in open fires, 90% of
potential heat and most of the nutrients
are lost.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Methane
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Methane is main component of natural gas.
 Produced by anaerobic decomposition.
- Burning methane produced from manure
provides more heat than burning dung
itself, and left-over sludge from bacterial
digestion is a nutrient-rich fertilizer.
 Methane is clean, efficient fuel.
 Municipal landfills contribute as
much as 20% of annual output of
methane to the atmosphere.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Anaerobic Fermentation
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Wind Energy
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Estimated 20 million MW of wind power
could be commercially tapped worldwide.
 Fifty times current nuclear generation.
- Typically operate at 35% efficiency
under field conditions.
 When conditions are favorable (min.
24 km/hr) electric prices typically run
as low as 3 cents / KWH.
 Standard modern turbine uses only
two or three blades in order to
operate better at high wind speeds.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Wind Energy
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Wind Farms - Large concentrations of wind
generators producing commercial electricity.
 Negative Impacts:
- Interrupt view in remote places
- Destroy sense of isolation
- Potential bird kills
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Geothermal Energy
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High-pressure, high-temperature steam fields
exist below the earth’s surface.
 Recently, geothermal energy has been
used in electric power production,
industrial processing, space heating,
agriculture, and aquaculture.
- Have long life span, no mining needs,
and little waste disposal.
 Potential danger of noxious gases and
noise problems from steam valves.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Tidal and Wave Energy
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Ocean tides and waves contain enormous
amounts of energy that can be harnessed.
 Tidal Station - Tide flows through turbines,
creating electricity.
- Requires a high tide / low-tide differential
of several meters.
 Main worries are saltwater flooding
behind the dam and heavy siltation.
 Stormy coasts with strongest
waves are often far from major
population centers.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Tidal Power
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Ocean Thermal Electric Conversion
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Heat from sun-warmed upper ocean layers is
used to evaporate a working fluid, such as
ammonia, which has a low boiling point.
 Gas pressure spins electrical turbines.
- Need temperature differential of about
20o C between warm upper layers and
cooling water.
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.
Summary:
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•
•
•
•
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Conservation
 Cogeneration
Tapping Solar Energy
 Passive vs. Active
High Temperature Solar Energy
 Photovoltaic Cells
Fuel Cells
Energy From Biomass
Energy From Earth’s Forces
Cunningham - Cunningham - Saigo: Environmental Science 7th Ed.