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Weighing in on Renewable Energy Efficiency
David Pimentel

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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.

Funding green power

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.

Federal subsidies include a tax credit of 1.5 cents per kilowatt-hour for wind power, given to utility companies that have invested in the technology (public utilities get a corresponding payment, as they do not pay taxes, but that requires an appropriation by Congress). Federal production incentives, mortgages and the EnergyStar Program are among the many financial carrots disbursed by the U.S. government to encourage renewables.

At the local level, these kinds of subsidies, tax credits and grant programs vary dramatically by state and even by city governments. In California, which has a personal tax credit for individuals who use wind power or photovoltaics of $4.50 per watt at peak, the city of Palo Alto offers an additional $4-per-watt rebate for photovoltaic installations. In North Carolina, private utilities offer the opportunity to buy “green power,” but the state has no incentives for corporations or individuals to install renewable energy technologies, according to the Database of State Incentives for Renewable Energy.

The federal government also provides research and development dollars, but these fluctuate too much for industry to count on, says Frank Laird, a policy specialist at the University of Denver, Colorado. Laird notes that in addition to direct subsidies and research and development, “one type of policy that people don’t think about” is procurements, where governments decide to buy into a technology and create a market. “That’s hugely important policy,” Laird says, giving the examples of transistors and integrated circuits, which, when the government “bought them by the boatload provided a really good stream of revenue” to companies trying to establish themselves.

“In fairness, everybody gets subsidies,” Laird says, including the coal and oil industries. And for renewable energies, “in the short-term, you need some subsidies.”

Naomi Lubick

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Pimentel is professor of ecology and agricultural sciences at Cornell University in Ithaca, N.Y. E-mail: dp18@cornell.edu.

Links:
"The Sustainable Hydrogen Economy," Geotimes, August 2005
"Hooking into Local Clean Energy: Wind crop in Iowa," Geotimes, August 2005 Print Exclusive

References:

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Kreutz, T.G. and J.M. Ogden. 2000. "Assessment of hydrogen-fueled proton exchange membrane fuel cells for distributed generation and cogeneration." Proceedings of the 2000 U.S. Department of Energy Hydrogen Program Review; Washington, DC. NREL/CP-570-28890.
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Pimentel, D., A. Pleasant, J. Barron, J. Gaudioso, N. Pollock, E. Chae, Y. Kim, A. Lassiter, C. Schiavoni, A. Jackson, M. Lee and A. Eaton (2004). "U.S. energy conservation and efficiency: benefits and costs." Environment Development and Sustainability 6: 279-305.
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