Even if implemented to the maximum, renewable energy sources would replace only about half the U.S. consumption of oil, natural gas and coal, according to an analysis by ecologists at Cornell.

Furthermore, the analysis states alternative energy systems -- such as hydroelectric, solar and wind power and biofuels -- if fully developed would occupy up to one-sixth of America's land area. And some of the so-called renewable energy sources aren't really renewable, the ecologists say, calling for energy conservation as a proven alternative.

"We wish this had turned out differently -- we really do -- but it's hard to argue with the facts," says David Pimentel, the Cornell professor of ecology who led a team of graduate students through an objective analysis of future energy needs of the United States. Their findings are reported in the December 2002 issue of the journal BioScience (Vol. 52, No. 12) as "Renewable Energy: Current and Potential Issues."

"If all the best renewable energy technologies were implemented to the fullest, those hydroelectric dams, wind farms and other installations would take up 17 percent of the land and still replace less than 50 percent of our fossil-fuel consumption. The biggest problem is our extraordinary rate of energy consumption to maintain our standard of living -- or should we say standard of burning," Pimentel explains.

"With less than 4 percent of the world population, we are emitting 22 percent of the carbon dioxide from burning fossil fuels. Developing countries use the energy equivalent of 500 liters of oil [132 U.S. gallons] per capita per year; in the United States each of us, on average, uses 8,000 liters [2,113 U.S. gallons] of oil equivalents annually -- 16 times as much. The only way to change the equation -- to make renewable energy sources go further and kick our addiction to fossil fuels -- is to reduce energy consumption, and that means conservation."

The ecologists reviewed 10 alternative energy sources but called two of these -- geothermal systems and the biofuels ethanol, methanol and vegetable oils -- not truly renewable and sustainable. The other eight alternative energy systems reviewed in the analysis are: hydroelectric; biomass energy; wind power; solar thermal conversion, including solar ponds and parabolic troughs, photovoltaic systems; hydrogen and fuel cells; and passive heating and cooling of buildings.

This recommendation was made by the ecologists: "The first priority of the U.S. energy program should be for individuals, communities and industries to conserve fossil fuel resources by using renewable resources and by reducing consumption. Other developed countries have proved that high productivity and high standards of living can be achieved with the use of half the energy expenditure of the United States."

o Hydroelectric systems, which make electricity from water-storage reservoirs and dammed rivers, currently produce 11 percent of the nation's electricity -- about 989 kWh (kilowatt hours) annually. With development of new hydroelectric sources and rehabilitation of existing dams, an additional 60 billion kWh per year could be produced. That production increase would occupy an additional 17 million hectares of land (one hectare is 2.5 acres) beyond the 26 million hectares (or 102,000 square miles) now occupied for hydroelectric production out of the total U.S. land area of 917 million hectares (3.5 million square miles). And although hydroelectric power has one of the lowest energy costs (about 2 cents per kWh, compared to 7 cents per kWh for wind power and 5 cents for nuclear power), it also leads to numerous ecological impacts and environmental problems, the Cornell study emphasizes.

o Biomass energy systems, which grow, harvest and burn woody materials for cooking, heating and conversion to electricity, require even more land -- about 75 million hectares (290,000 square miles) in the United States today and as much as 102 million hectares if biomass systems were expanded in the year 2050. The environmental effects of burning biomass are less harmful than those associated with coal but are more harmful than those from burning natural gas, the Cornell ecologists say, noting that biomass combustion releases some 200 different chemical pollutants, including 14 known carcinogens. In the United States, diseases exacerbated by wood smoke kill 30,000 people each year.

o Wind power, from high-efficiency wind turbines, now produces about 6.6 billion kWh of electricity per year in the United States. If all economically feasible land sites were developed, as much as 675 billion kWh a year could come from wind power. Offshore wind-power sites could produce another 102 billion kWh for the United States, for a potential total of 777 kWh a year, or 23 percent of the current U.S. usage of electrical energy. About 13,700 hectares (53 square miles) of land are needed for every 1 billion kWh of wind-generated electricity a year. But since each turbine's "footprint" occupies only 2 percent of the surrounding land, some crops can be grown and harvested beneath the whirling blades of the turbines, the Cornell ecologists observe. They list visual and noise "pollution" and electromagnetic interference as the main environmental impacts of wind power but say that harm to animal life is overestimated by the general public: In fact, fewer than 300 birds are killed each year by the estimated 13,000 wind turbines in the United States, they say. That number could be further reduced with the installation of strobe lights and painted patterns to warn migrating birds.

o Solar thermal conversion systems, including solar ponds that store heat in a bottom layer of brine, and parabolic troughs that concentrate reflected sunlight onto central, liquid-filled tubes, require less land. About 1,100 hectares (4 square miles) are needed to make a billion kWh per year of electricity with solar troughs -- and solar thermal systems can be quite efficient: While electricity from solar ponds costs about 15 cents per kWh, parabolic troughs and steam-driven turbines can generate electricity for as little as half that price -- about 7 cents per kWh. Barring leaks, the solar troughs are environmentally benign, the Cornell ecologists say, but they note that solar ponds require continual infusions of fresh water (as well as more salt) and are losing favor in arid regions, such as Israel.

o Photovoltaic systems, producing electricity from sunlight and silicon, are more flexible and efficient than at one time but still represent an emerging technology. "The durability of photovoltaic cells must be lengthened and production costs reduced several times to make their use economically feasible," the Cornell ecologists report. Once photovoltaic cells reach 18 percent efficiency, the electrical needs of 100,000 Americans (1 billion kWh a year) could be met with cells covering 2,800 hectares (11 square miles) -- or less if cells are installed atop homes, industries and other buildings. However, the manufacture of some photovoltaic cells requires toxic materials (including cadmium sulfide and gallium arsenide), and the disposal of inoperative cells "could become a major problem," they warn.

o Hydrogen, as a storage medium for electric solar technologies, and hydrogen-based fuel cells might be clean and "green," but their economic star has yet to rise, the ecologists conclude. Commercially produced hydrogen today is more than twice the cost of an energy-equivalent amount of gasoline in the United States -- and a hydrogen fuel cell car, with all the special equipment, would cost $100,000. Prices should decline with mass production, the ecologists say, noting: "There is high demand for fuel cell-equipped vehicles in the United States."

o Passive heating and cooling of buildings includes the more conventional hot-water heating, as well as technologically advanced "superwindows," with high insulating values, and "smart windows," that respond to changing conditions. Installation in new buildings is easier, but retrofitting new equipment in existing buildings is desperately needed, the ecologists say. Every year the poorly insulated doors and windows of America lose the energy equivalent of all the oil pumped in Alaska, the analysis notes.

o Biogas, the burnable methane produced in digesters by microbes that eat animal dung, can be used to generate electricity, heat farm buildings and keep the digesters at operating temperatures, the ecologists note. They cite an Environmental Protection Agency forecast that more than 2,000 digesters could be economically installed in the United States. But for now, biogas production is thriving in India, where a digester to handle dung from a 50-cow family farm costs less than $1,000 to build and produces biogas at an electricity price equivalent of 6 cents per kWh. "And the residue from biogas production has little odor and retains all the fertilizer value of manure," Pimentel says.

o Biofuels, such as ethanol, methanol and vegetable oil, are attractive alternatives to imported fossil fuels, but the numbers don't add up, the ecologists conclude. Although biofuels burn cleaner than gasoline and diesel fuels, their production (including farming and processing) consumes more energy than the biofuels yield. Thus, biofuels should not be regarded as "renewable," the ecologists say.

o Geothermal energy might not be "renewable" either, if some predictions are to be believed, the study notes. Commercially exploited hot springs, geysers and dry hot rock tend to decline over 40 to 100 years, the Cornell ecologists say after reviewing reports from 1997 and 2000, and "existing drilling opportunities for geothermal resources are limited to a few sites in the United States and the world." They also cite environmental liabilities for geothermal systems, including air pollution (by hydrogen sulfide) and sludge residues from deep in the earth, including arsenic, boron, lead, mercury, radon and vanadium.