Ch 20
Sustainable Energy
Ch 20 Outline
• 20.1 Conservation
– Cogeneration
• 20.2 Tapping Solar Energy
– Passive vs. Active
• 20.3 High Temperature Solar Energy
– Photovoltaic Cells
• 20.4 Fuel Cells
• 20.5 Energy From Biomass
• 20.6 Energy From Earth’s Forces
2
Conservation
• Utilization Efficiencies
– Today’s average new home uses half the fuel
required in a house built in 1974.
– Reducing air infiltration is usually the most
effective way of saving household energy.
– According to new national standards:
– New washing machines will have to use 35%
less water.
– Will U.S. cut water use by 40 trillion liters
annually and save enough electricity every
year to light all the homes in the U.S.?
3
Utilization Efficiencies
• For even greater savings, new houses can
be built with extra thick superinsulated
walls, air-to-air heat exchangers, and
double-walled sections.
– Straw-bale construction
– Home orientation so have passive solar
gains in winter and shade from trees in
summer
– Turn off appliances on standby - TV,
printer, computer
4
Standby Energy Consumption
5
Energy Conversion Efficiencies
• Energy Efficiency is a measure of energy
produced compared to energy consumed.
– Thermal conversion machines such as
steam turbines can turn no more than
40% of energy in primary fuel into
electricity or mechanical power due to
waste heat.
– We could be recapturing the heat and
using it for space heating
– Fuel cells can theoretically approach
80% efficiency using hydrogen or
methane.
6
Energy Conversion Efficiencies
• Transportation
– Raising average fuel efficiency in U.S. by
3 miles per gallon would save more oil
than the maximum expected production
from drilling in Arctic Wildlife Refuge.
– There are now more vehicles in the U.S.
than there are licensed drivers.
– In the 1970s, when oil prices rose, U.S.
doubled auto gas mileages. Reached
25.9 mpg in 1988 but now down to 22.1
mpg.
7
Energy Conversion Efficiencies
• Transportation
– For short trips, could walk or bicycle
– Could buy high efficiency mini car that gets 60 mpg like
the one shown in photo
– Could buy hybrid gasoline electric car
8
Transportation Efficiencies
• Could buy plug in hybrid car which recharges
batteries from household current at night
– Electricity costs the equivalent of 50 cents
per gallon
– Need to generate more electricity but could
capture pollutants at the plant
• Could buy diesel. A diesel sold in Europe
currently gets 150 mpg.
• A diesel plug in hybrid could make the U.S.
entirely independent from imported oil.
9
Transportation Efficiencies
• Fuel-cell powered vehicles are being
developed which use hydrogen gas as fuel.
– Produce water as their only waste product
– Will take at least twenty years to come to
market
– Most hydrogen is currently created from
natural gas, making it no cleaner or more
efficient than burning the gas directly.
– Governments in U.S. and Europe are
spending billions on this.
10
Cogeneration
• 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
U.S. came from plants also providing
industrial steam or district heating.
– By 1970’s cogeneration had fallen to
less than 5% of power supplies.
11
Cogeneration
• Interest is being renewed
– District heating systems are being
rejuvenated.
– Plants that burn municipal waste are
being studied.
– Combined cycle coal gasification plants
may be used in urban locations.
– Apartment building-sized power
generating units are being built that use
methane, diesel or coal.
12
Tapping Solar Energy
• 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.
13
Solar Energy
• Passive Solar Heat using absorptive
structures with no
moving parts to gather
and hold heat
– Greenhouse Design
14
• Active Solar Heat - pump
heat-absorbing medium
through a collector,
rather than passively
collecting heat in a
stationary object
– Water heating
consumes 15% of U.S.
domestic energy
budget. A flat panel of
5 m2 can provide hot
water for family of 4.
15
High Temperature Solar Energy
• 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.
16
Parabolic Mirrors
17
18
Solar Energy
• Only solar power tower in U.S. is in Southern
California. It generates enough electricity
for 5,000 homes at cost far below oil or
nuclear power.
– If entire U.S. used solar towers, it would
take up an area half the size of South
Dakota (but less land than will be strip
mined in next 30 years to get coal).
• Parabolic mirrors or solar box cookers can
also be used for home cooking in tropical
countries.
19
Solar Cooker
• An inexpensive
insulated box with a
black interior and a
clear plastic lid can
serve as a solar
cooker. Helps reduce
deforestation and
avoids health risks from
smoky cooking fires in
tropical countries.
20
Photovoltaic Solar Energy
• 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
– 2007 - $2.50 / watt
– 2009 - $1.00 / watt
21
Photovoltaic Cells
• During the past 25 years, efficiency of energy
capture by photovoltaic cells has increased
from less than 1% of light to more than 15%
in field conditions and over 75% in the
laboratory.
– Invention of amorphous silicon collectors
has allowed production of lightweight,
cheaper cells.
– Roof tiles with photovoltaic cells can
generate enough electricity for a home.
• At least 2 billion people now live without
electricity. This could be a solution to their
problems.
23
Storing Electrical Energy
• Electrical energy storage is difficult and
expensive.
– Lead-acid batteries are heavy (3-4 tons)
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.
24
Fuel Cells
• Fuel Cells - use ongoing electrochemical
reactions to produce electric current.
– Cathode (+) and anode (-) separated by
electrolyte which allows ions to pass,
but is impermeable to electrons
– Hydrogen passed over anode where a
catalyst strips an electron
– Electrons pass through external circuit,
and generate electrical current.
– Hydrogen ion passes to cathode where
it is united with oxygen to form water.
25
Fuel Cell
26
Fuel Cells
• 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. Oxygen
comes from air.
– Fuel cells run on pure oxygen and
hydrogen, and produce no waste
products except drinkable water and
radiant heat.
27
Fuel Cells
• Typical fuel cell efficiency is 40-45%.
• Current is proportional to the size of the
electrodes, while voltage is limited (1.23
volts/cell).
– Fuel cells can be stacked until the
desired power level is achieved. A fuel
cell stack that could provide all the
electricity for a home would be about
the size of a refrigerator.
28
Energy from Biomass
• 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.
– Biomass resources include wood,
wood chips, bark, leaves and starchy
roots.
29
Burning Biomass
• Wood provides less than 1% of U.S.
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.
– Clean burning woodstoves are
available but expensive, produces
fewer sulfur gases than coal.
30
Fuelwood Crisis
• 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.
– Gathering wood is work of women and
children and in some places it now
takes 8 hours to get to supply and
even longer to walk back with wood that
will last only a few days.
31
Fuelwood Crisis
• In cities, people must pay high prices for
wood, as much as 25% of household
income.
• By 2025, if current trends continue, the
demand is expected to be twice current
harvest rates while supply will stay
steady.
• In some African nations, demand is
already ten times the sustainable yield.
32
Dung
• Where other fuel is in short supply, people often dry
and burn animal dung.
– Downside: not returning animal dung to land as fertilizer
reduces crop production and food supplies.
33
33
Methane
• 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. This could be
burned for electricity.
34
Anaerobic Production of Methane
35
Methane
• Cattle feedlots and chicken farms are a
tremendous potential fuel source since
wastes contain more energy than all the
nation’s farmers use.
– Haubenschild dairy farm uses manure to
generate all their electricity. In January
2001, the farm saved 35 tons of coal,
1,200 gallons of propane, and made
$4,380 selling electricity.
36
Alcohol from Biomass
• Ethanol or methanol made from plant
materials or diesel made from vegetable oils
or animal fats
• Gasohol - mixture of gasoline and ethanol
– Ethanol in gasohol makes gasoline burn
cleaner and most states require that 5%
to 10% be added to gasoline.
– Most ethanol now made from grain but
can be made from any cellulosic material
such as wood chips or straw.
37
Alcohol from Biomass
• Brazil is world’s leader in alcohol from
biomass, mostly sugarcane waste.
• Ethanol production growing rapidly in the
U.S. but use of corn for fuel has increased
corn prices by 50%. Since corn is used as
animal feed, meat, milk and egg prices have
risen.
• U.S. has 5 million flex fuel vehicles now
• Increasing fuel economy by 12% would
reduce oil consumption just as much as use
of ethanol and save $10 billion in subsidies.
39
Alcohol from Biomass
• Energy crops - such as switch grass,
cattails and hybrid poplars could be
grown on marginal lands specifically as
energy source.
– Low-input high-diversity fuels - mix of
native prairie perennial species which
grow well in dry, low nitrogen conditions
and which could be harvested for fuel
40
Fuel from Biomass
• Water is a worry when using ethanol as a
biofuel.
– It takes 3 to 6 liters of water to produce
a liter of ethanol and in many of plains
states there is not enough water to
produce both food and fuel.
• Biodiesel can be made from almost
anything organic such as fat from meat or
vegetable oil. European Union already
consumes 1 billion gallons of biodiesel.
41
Energy from Earth’s Forces
• Hydropower
– In 1925, falling water generated 40% of
world’s electric power.
– Hydroelectric production capacity has
grown 15-fold but fossil fuel use has
risen so rapidly that hydroelectric only
supplies 20% of electrical generation.
– Untapped potential for hydropower in
Latin and Central America, Africa,
India and China
42
43
43
Dams
• Much of hydropower in recent years has
been from enormous dams
– Human Displacement
– Ecosystem Destruction
– Wildlife Losses
– Large-Scale Flooding due to Dam
Failures
– Sedimentation
– Herbicide Contamination
– Evaporative Losses
– Nutrient Flow Retardation
44
Dams
– Rotting of submerged vegetation kills
fish, acidifies water, produces
greenhouse gases
– Schistosomiasis - human disease
caused by parasitic fluke that lives in
snails, which like the slow moving water
behind dams
– Indigenous peoples lose their lands
45
Dam Alternatives
• Low-Head Hydropower - extract energy
from small headwater dams
• Run-of-River Flow - submerged directly in
stream and usually do not require dam or
diversion structure
• Micro-Hydro Generators - small versions
designed to supply power to single
homes
– Government subsidies for small scale
hydropower resulted in abuse of water
resources e.g. diverting small streams
46
Wind Energy
• Estimated 80 million MW of wind power
could be commercially tapped worldwide.
– Five times total current global electrical
generating capacity
– Typically operate at 35% efficiency
under field conditions
– When conditions are favorable,
electric prices typically run as low as
3 cents / kWh.
47
Wind Power
• no fuel costs or
emissions
• generates income for
farmers who rent land
for turbines or sell
electricity BUT
• intermittent source
• not enough wind
everywhere
• bird mortality
• power lines needed to
transmit the electricity
48
Wind Resources in the U.S.
49
Geothermal Energy
• Geothermal Energy - tap energy from hot
springs, geysers
• Few places have geothermal steam, but
can use Earth’s warmth everywhere by
pumping water through buried pipes using
heat pumps
• Deep wells for community geothermal
systems are being developed.
• Heat from Earth’s crust is never
exhausted
50
Geothermal Energy
51
Tidal and Wave Energy
• 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
– Pelamis wave power generator - snakelike
machine points into waves and undulates
up and down, which pumps fluid to
hydraulic motors that drive electrical
generators. Cables carry power to shore.
52
• The world's first commercial-scale and gridconnected tidal stream generator – SeaGen – in
Strangford Lough.[8] The strong wake shows the
power in the tidal current
53
Pelamis Wave Converter
54
Ocean Thermal Electric
Conversion
• 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.
– Cold water is then pumped from the
depths to condense the ammonia
again.
– Need temperature differential of about
20o C between warm upper layers and
cooling water.
55
Ideal Scenario for World Energy
Consumption 2100
56