Harvesting
the energy of a piece of wood in a tree requires cutting the tree down and then
into pieces. Then the wood pieces must be transported home and eventually placed
into a furnace or fireplace. That harvesting may take, say, 10,000 kilocalories
of energy, including the gasoline and oil used to operate the chainsaw. The
fuel wood placed in the furnace might contain 220,000 kilocalories (there are
30,000 kilocalories in a gallon of gasoline). Thus, for every 1 kilocalorie
invested in harvesting wood for the furnace, about 22 kilocalories of wood energy
was harvested. This calculation translates to an energy efficiency ratio of
1:22.
This type of “input versus output”
analysis is applicable to all energy resources, as all energy production requires
energy in the first place. In this balancing act, some resources fare better
than others.
Wood biomass, burned for heat, accounts for 2.7 percent of U.S. energy needs.
It is renewable, but like all energy sources requires energy inputs, like the
power from the machinery to cut the trees. Courtesy of Warren Gretz.
The development of new energy sources to replace the dwindling supply of fossil
energy is urgent, yet diverse renewable energy sources provide only 8 percent
of U.S. needs. Although many different renewable energy technologies exist,
the following are projected to provide the United States with most of its renewable
energy in the future: biomass (including ethanol and biodiesel), hydroelectric,
wind power, photovoltaics and hydrogen.
In considering the future viability of these sources, it is important to weigh
the benefits against their costs. This type of evaluation often takes the form
of energy efficiency analysis — a measure of the amount of energy that
must be invested to obtain a quantity of energy for use.
Burning biomass
Most biomass
burned in the United States is for providing thermal energy. In calculating
energy efficiency for biomass production, the most important input arguably
is nitrogen fertilizer. All living plants require nitrogen nutrients for growth,
including trees, but some require more than others, changing the efficiency
equation for various types of biomass.
A researcher for the National Renewable Energy Laboratories prepares wood chips
for fuel conversion into ethanol. Ethanol comes from corn, soybeans, sunflower
seeds, wood and grasses, and requires many energy inputs to create energy. Courtesy
of Warren Gretz.
Wood biomass is burned in furnaces and fireplaces for heat energy. Using wood
as biomass is renewable, like hydropower, and provides about 2.7 percent of
U.S. energy needs. Under sustainable forest conditions with adequate rainfall,
approximately 3 dry tons per hectare (1 hectare is about 2.5 acres) per year
of woody biomass can be harvested, with minimal inputs of nitrogen fertilizer.
With the diesel fuel per hectare required for cutting and collecting wood for
transport to a powerplant for use, the energy input per output ratio for such
a system is calculated to be 1:22, as described earlier.
If the wood is converted into electricity, the energy input per output declines
to a ratio of 1:7. The price per kilowatt-hour is estimated to be about 6 cents.
A city of 100,000 people using biomass from a sustainable forest (3 tons per
hectare per year) for electricity would require approximately 200,000 hectares
(about 770 square miles — close to 10 times the area of Washington, D.C.)
of forest area, based on an average electrical demand of slightly more than
1 billion kilowatt-hours of electricity per year for the community.
In addition to burning wood for energy, other biomass resources include corn,
soybeans, sunflower seeds, wood and grasses — all of which can produce
both ethanol and biodiesel. Approximately 1.3 liters of oil equivalents are
required to produce 1 liter of 99.5 percent ethanol that costs 42 cents per
liter, making ethanol a relatively inefficient energy source.
Getting that ethanol energy source in the first place requires 14 energy inputs,
including fertilizers and pesticides for corn production, and 10 inputs in the
fermentation and distillation process. Some of the major inputs in production
are fertilizers and farm machinery, while inputs in the fermentation and distillation
process include the corn grain and steam generation required for the distillation
process.
The environmental impacts from producing ethanol are also significant. Corn
causes more soil erosion than any other crop grown. Also, pollution is considerable
because corn production uses more nitrogen fertilizer, insecticides and herbicides
than any other crop grown. And 13 liters of sewage effluent are produced per
1 liter of ethanol produced because of the water that has to be added to the
ground corn grain for the fermentation process.
Other vegetable oils also can be converted into biodiesel and they work well
in diesel engines. For these biodiesels, the production of sunflower oil is
relatively energy inefficient and costly compared to soybean oil.
Although soybeans contain less oil than sunflower seeds, about 18 percent oil
for soybeans compared to 26 percent oil for sunflower seeds, soybeans can be
produced with almost no nitrogen fertilizer. This makes soybeans advantageous
for the production of biodiesel. The yield of sunflower seeds is also lower
than soybeans: 1,500 kilograms per hectare compared with 2,668 kilograms per
hectare respectively, according to the U.S. Department of Agriculture.
With an 18 percent yield for oil production, 5.6 kilograms of soybeans are required
to produce about 0.92 liters of oil. The two largest inputs in the production
of soy biodiesel are the soybeans and steam in processing, giving soybean production
a net loss of 33 percent in energy. The price per kilogram of soy biodiesel
is $1.21; however, taking a tax credit for the soy meal would reduce this price
to 92 cents per kilogram of soy oil (nearly 1 liter). Although more efficient
and less expensive than sunflower oil, soy oil still costs about 2.8 times more
than traditional diesel fuel.
Water power
Hydropower — one of the oldest forms of alternative energy, which uses
water turbines to create electricity — contributes significantly to the
U.S. energy supply, providing 2.7 percent of total energy or 11 percent of the
nation’s electricity, according to the U.S. Census Bureau. The cost of
1 kilowatt-hour is only 2 cents, the lowest of any renewable energy system.
That’s because it requires the smallest investment to produce valuable
electricity.
The major “input” for hydroelectric plants is land for water storage
reservoirs. An average of 75,000 hectares (290 square miles) of reservoir land
area and 14 trillion liters of water are required to produce 1 billion kilowatts
per hour per year. (If this amount of water were to cover Manhattan, it would
rise to just above half the height of the Empire State Building.) This electricity
is sufficient to serve a city of 100,000 people for one year. The average hydropower
plant invests 1 kilocalorie and gets in return 28 kilocalories. The water power
station at Niagara Falls has been in operation since 1883 and is one of the
most successful water power plants in the United States.
In the wind
For many centuries, wind power has proven to be a valuable technology. Most
of the electrical energy produced from wind power uses turbines with at least
500 kilowatts’ capacity. In an ideal location, a turbine can run at a maximum
of 30 percent efficiency. The construction of the equipment requires the equivalent
of 1 kilocalorie of fossil energy, and the turbines yield about 5 kilocalories
of electrical energy (a 1:5 ratio). The estimated cost of electricity generated
is 7 cents per kilowatt-hour. The best estimate is that wind power could produce
about 2 percent of total U.S. electrical energy.
The prime limitation is favorable wind sites. The wind should be blowing at
least 20 kilometers per hour (about 13 miles per hour). The North Dakota region
is a great place, for example, for wind farms. Other areas include Iowa, where
school districts are now using wind power (see story, this
issue).
In addition to having suitable winds, a community needs backup power for when
the wind is not blowing. The backup comes in the form of conventional fossil-fuel
electrical generating stations. This requirement has been estimated to add a
20 percent greater energy need than the total energy generated by the wind turbine
itself. Another option is to have the wind turbines connected to a very large
electrical grid. With suitable winds and turbines operating in many locations,
widely distributed wind machines can serve as a backup to one another.
Connecting the wind machines and locations with an electrical grid would not
be inexpensive, however. For instance, networks of distribution cables must
be installed, costing about $179,000 per kilometer with 115-kilovolt lines.
A percentage of the power delivered is lost as a function of electrical resistance
in the distribution cable. The best means of shipping electricity is using D/C
power; it can be converted back to A/C power at a slight cost. Based on current
electrical networks, it is estimated that electricity could be transmitted about
1,500 kilometers.
From the light
Photovoltaic
(PV) cells — solar energy panels — have the potential to provide a
major portion of U.S. electrical needs. The most promising aspect of PV cells
is that they can be placed on the roofs of buildings and are generally adaptable
to a variety of situations (see story, this issue). The
current test cells provide from 10 to 20 percent efficiency in the collection
of sunlight. However, the lifespan of the PV cells must be improved. The cells
deteriorate over time because of the effects of sunlight.
The cheapest solar cells are made of amorphous silicon, and the first manufacturing
step is to grow a single silicon crystal, a cylinder that is then sliced into
thin discs. The computer chip market influences the price of silicon for solar
uses, and right now it’s quite expensive. Courtesy of National Renewable
Energy Laboratories.
The installation of PV cells and production of electricity is relatively expensive.
Currently the cost per kilowatt-hour ranges from 20 to 30 cents. PV cells are
attractive, however, because instead of requiring 200,000 hectares to provide
electricity using a sustainable forest, the same quantity of electricity for
a city of 100,000 people could be provided on about 3,000 hectares. Roughly,
the energy input-output ratio is 1 kilocalorie input of fossil energy for 7
kilocalories of electricity produced, putting it up there with wind power in
terms of efficiency.
Fueling hydrogen
Hydrogen gas can be produced using wind power, PV systems, hydropower or similar
systems that produce electricity and provide an opportunity for the electrolysis
of water — separating H2O into its individual elements (see Comment).
Under intense pressure, hydrogen can be liquefied and stored for use as an energy
storage system or to produce electricity, but it is likely to remain an inefficient
process.
The material and energy inputs for a hydrogen production facility are primarily
those needed to build and run a solar electric facility, like PV or hydropower.
According to Tom Kreutz and Joan Ogden of Princeton University, the energy required
to produce 1 billion kilowatts per hour of hydrogen is 1.4 billion kilowatts
per hour of electricity. The water required for electrolysis to produce 1 billion
kilowatts per hour of hydrogen is 300 million liters of water per year. On a
per capita basis, producing the electrical energy required in the United States
per person per year using electrolysis would require 3,000 liters of water (an
average person uses 380 liters a day for indoor residential use).
Making hydrogen into a liquid from a gas also requires significant energy inputs
because the hydrogen must be cooled to about minus 253 degrees Celsius and pressurized.
About 30 percent of the hydrogen energy is required for the liquefaction process.
Even after liquefaction, liquid hydrogen occupies about three times the volume
equivalent of an energy equivalent of gasoline, and requires a strong, heavy
tank: Thus to store an energy-equivalent amount of hydrogen liquid as gasoline,
the tank roughly would need to weigh about four times that used for gasoline.
About 3.7 kilograms of gasoline sells for about $1.20, whereas 1 kilogram of
liquid hydrogen with the same energy equivalent sells for about $2.70, or more
than twice the cost of gasoline.
Hydrogen also has serious explosive risks and it is difficult to contain even
within steel tanks. Because it is an extremely small molecule, hydrogen can
slowly leak through a steel tank, and mixing with oxygen can result in intense
flames because hydrogen burns quickly. Additionally, water for the production
of hydrogen may be especially problematic in arid regions of the United States.
A new lifestyle
Europeans on the whole have a good lifestyle, while utilizing half as much energy
as we do in the United States. The average European consumes about one-half
the energy that an American does because Europeans live in smaller homes and
apartments, drive smaller automobiles, and consume fewer goods and other materials.
Employing the five different energy technologies mentioned earlier plus adding
solar thermal passive energy systems and biogas, the United States could produce
about 46 percent of its total energy. Although such usage would not cut back
on transportation fuel use, it could replace other fossil fuel sources, such
as coal used in electricity production. The relative usage of those sources
should depend in large part on their individual energy efficiencies and costs.
At the same time, however, the U.S. population needs to take an example from
the Europeans and reduce their energy consumption through conservation and smart
energy usage. There is no simple solution, but there are lots of great places
to start.
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Last March, the U.S. Department of Agriculture announced that $22.8 million
in grants would be available to small farms and rural businesses to invest
in renewable energy technologies, from biomass digesters to small wind-power
installations. This program is one of many small and large incentives
to promote the use of clean energy, at a federal as well as local level. Naomi Lubick |
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