Renewable Energy renewable energy sources scholarly articles Current and Potential Issues BioScience Oxford Academic
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Renewable Energy renewable energy sources scholarly articles Current and Potential Issues BioScience Oxford Academic
The major environmental problem associated with photovoltaic systems is the use of toxic chemicals, such as cadmium sulfide and gallium arsenide, in their manufacture . Because these chemicals are highly toxic and persist in the environment for centuries, disposal and recycling of the materials in inoperative cells could become a major problem. For many centuries, wind power has provided energy to pump water and to run mills and other machines. Today, turbines with a capacity of at least 500 kW produce most commercially wind-generated electricity. Operating at an ideal location, one of these turbines can run at maximum 30% efficiency and yield an energy output of 1.3 million kWh per year . An initial investment of approximately $500,000 for a 500 kW capacity turbine , operating at 30% efficiency, will yield an input–output ratio of 1:5 over 30 years of operation . During the 30-year life of the system, the annual operating costs amount to $40,500 . The estimated cost of electricity generated is $0.07 per kWh . Repetto R . 1992 . Accounting for environmental assets. Scientific American . 263 : 94 - 100 . International Commission on Large Dams . 1988 . World Register of Dams. Paris: ICLD. Worldwide, approximately 408 quads of all types of energy are used by the population of more than 6 billion people . Using available renewable energy technologies, an estimated 200 quads of renewable energy could be produced worldwide on about 20% of the land area of the world. A self-sustaining renewable energy system producing 200 quads of energy per year for about 2 billion people would provide each person with about 5000 liters of oil equivalents per year—approximately half of America's current consumption per year, but an increase for most people of the world . Dickson YL Yates BC . 1983 . Examination of the Environmental Effects of Salt-Gradient Solar Ponds in the Red River Basin of Texas. Denton: North Texas State University. Ethanol production in the United States using corn is heavily subsidized by public tax money . However, numerous studies have concluded that ethanol production does not enhance energy security, is not a renewable energy source, is not an economical fuel, and does not ensure clean air. Furthermore, its production uses land suitable for crop production . Ethanol produced using sugarcane is more energy efficient than that produced using corn; however, more fossil energy is still required to produce a liter of ethanol than the energy output in ethanol . Diverse renewable energy sources currently provide only about 8% of US needs and about 14% of world needs , although the development and use of renewable energy is expected to increase as fossil fuel supplies decline. Several different technologies are projected to provide the United States most of its renewable energy in the future: hydroelectric systems, biomass, wind power, solar thermal systems, photovoltaic systems, passive energy systems, geothermal systems, biogas, ethanol, methanol, and vegetable oil. In this article, we assess the potential of these various renewable energy technologies for supplying the future needs of the United States and the world in terms of land requirements, environmental benefits and risks, and energetic and economic costs. Schumer CE . 2001 . New Analysis Shows America on Precipice of an Energy Crunch. Press Release of Senator Charles E. Schumer. 26 April 2001. American Petroleum Institute . 1999 . Basic Petroleum Data Book. Washington : API. World Bank . 1992 . China: Long-Term Issues and Options in the Health Transition. Washington : World Bank. Nelson V . 1996 . Wind Energy and Wind Turbines. Canyon : Alternative Energy Institute. Dutta S Rehman IH Malhortra P Venkata RP . 1997 . Biogas: The Indian NGO experience. Delhi : Tata Energy Research Institute. Winter CJ Nitsch J . 1988 . Hydrogen as an Energy Carrier: Technologies, Systems, and Economy. Berlin: Verlag. Duncan RC . 2001 . World energy production, population growth, and the road to the Olduvai Gorge. Population and Environment . 22 : 503 - 522 . An investigation of the environmental impacts of wind energy production reveals a few hazards. Locating the wind turbines in or near the flyways of migrating birds and wildlife refuges may result in birds colliding with the supporting towers and rotating blades . For this reason, Clarke suggests that wind farms be located at least 300 meters from nature reserves to reduce the risk to birds. The estimated 13,000 wind turbines installed in the United States have killed fewer than 300 birds per year . Proper siting and improved repellant technology, such as strobe lights or paint patterns, might further reduce the number of birds killed. The energy input for making the structural materials of a photovoltaic system capable of delivering 1 billion kWh during a life of 30 years is calculated to be approximately 143 million kWh. Thus, the energy input–output ratio for the modules is about 1:7 . Peschka W . 1987 . The status of handling and storage techniques for liquid hydrogen in motor vehicles. International Journal of Hydrogen Energy . 12 : 753 - 764 . Improvements in passive solar technology are making it more effective and less expensive than in the past . Current research in window design focuses on the development of “superwindows” with high insulating values and “smart” or electrochromic windows that can respond to electric current, temperature, or sunlight to control the admission of light energy . Pimentel D Bailey O Kim P Mullaney E Calabrese J Walman F Nelson F Yao X . 1999 . Will the limits of the Earth's resources control human populations?. Environment, Development and Sustainability . 1 : 19 - 39 . US Department of Energy–Energy Information Administration . 2001 . Annual Energy Outlook with Projections to 2020. Washington : DOE/EIA. Renewable energy systems require more labor than fossil energy systems. For example, wood-fired steam plants require several times more workers than coal-fired plants . Weisz PB Marshall JF . 1980 . Fuels from Biomass: A Critical Analysis of Technology and Economics. New York: Marcel Dekker. As the growing US and world populations demand increased electricity and liquid fuels, constraints like land availability and high investment costs will restrict the potential development of renewable energy technologies. Energy use is projected on the basis of current growth to increase from the current consumption of nearly 100 quads to approximately 145 quads by 2050 . Land availability is also a problem, with the US population increasing by about 3.3 million people each year . Each person added requires about 0.4 ha of land for urbanization and highways and about 0.5 ha of cropland . Pimentel D . 2001 . The limitations of biomass energy. Pages. 159 - 171 . of Encyclopedia of Physical Sciences and Technology. San Diego: Academic Press. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide US Environmental Protection Agency . 2000 . AgSTAR Digest. Washington : EPA. Woody biomass could supply the United States with about 1.5 × 10 12 kWh of its total gross energy supply by the year 2050, provided that approximately 102 million ha were available . A city of 100,000 people using the biomass from a sustainable forest for electricity would require approximately 200,000 ha of forest area, based on an average electrical demand of slightly more than 1 billion kWh . Although most biomass is burned for cooking and heating, it can also be converted into electricity. Under sustainable forest conditions in both temperate and tropical ecosystems, approximately 3 dry metric tons per ha per year of woody biomass can be harvested sustainably . Although this amount of woody biomass has a gross energy yield of 13.5 million kcal, approximately 33 liters of diesel fuel per ha, plus the embodied energy, are expended for cutting and collecting the wood for transport to an electric power plant. Thus, the energy input–output ratio for such a system is calculated to be 1:22. Kishore VVN . 1993 . Economics of solar pond generation. Pages. 53 - 68 . in Kishore VVN. Renewable Energy Utilization: Scope, Economics, and Perspectives. New Delhi : Tata Energy Research Institute. Smil V . 1994 . Energy in World History. Boulder : Westview Press. Potential environmental problems with geothermal energy include water shortages, air pollution, waste effluent disposal, subsidence, and noise . The wastes produced in the sludge include toxic metals such as arsenic, boron, lead, mercury, radon, and vanadium . Water shortages are an important limitation in some regions . Geothermal systems produce hydrogen sulfide, a potential air pollutant; however, this product could be processed and removed for use in industry . Overall, the environmental costs of geothermal energy appear to be minimal relative to those of fossil fuel systems. British Petroleum . 2001 . British Petroleum Statistical Review of World Energy. London: British Petroleum Corporate Communications Services. Gawlik K Kutscher C . 2000 . Investigation of the Opportunity for Small-Scale Geothermal Power Plants in the Western United States. Golden : National Renewable Energy Laboratory. The first priority of the US 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 a high standard of living can be achieved with the use of half the energy expenditure of the United States . In the United States, fossil energy subsidies of approximately $40 billion per year should be withdrawn and the savings invested in renewable energy research and education to encourage the development and implementation of renewable technologies. If the United States became a leader in the development of renewable energy technologies, then it would likely capture the world market for this industry . Solar thermal energy systems collect the sun's radiant energy and convert it into heat. This heat can be used directly for household and industrial purposes or to produce steam to drive turbines that produce electricity. These systems range in complexity from solar ponds to electricity-generating parabolic troughs. In the material that follows, we convert thermal energy into electricity to facilitate comparison with other solar energy technologies. Winter CJ Meineke W Neumann A . 1991 . Solar thermal power plants: No need for energy raw material—only conversion technologies pose environmental questions. Pages. 1981 - 1986 . in Arden ME, Burley SMA, Coleman M, eds. 1991. Solar World Congress. Oxford : Per gamon Press. The United States, having consumed from 82% to 88% of its proved oil reserves , now imports more than 60% of its oil at an annual cost of approximately $75 billion . General production, import, and consumption trends and forecasts suggest that within 20 years the United States will be importing from 80% to 90% of its oil. The US population of more than 285 million is growing each year, and the 3.6 trillion kWh of electricity produced annually at a cost of $0.07 to $0.20 per kWh are becoming insufficient for the country's current needs. As energy becomes more scarce and more expensive, the future contribution of renewable energy sources will be vital . Megan Herz, Michele Glickstein, Mathew Zimmerman, Richard Allen, Katrina Becker, Jeff Evans, Benita Hussain, Ryan Sarsfeld, Anat Grosfeld, and Thomas Seidel are graduate students, in the College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853-0901. Reddy AKN Williams RH Johansson TB . 1997 . Energy after Rio: Prospects and Challenges. New York: United Nations Development Programme. Sommer SG Husted S . 1995 . The chemical buffer system in raw and digested animal slurry. Journal of Agricultural Science . 124 : 45 - 53 . US Department of Energy–Energy Information Administration . 2002 . Annual Energy Outlook with Projections to 2020. Washington : DOE/EIA. Hydropower contributes significantly to world energy, providing 6.5% of the supply . In the United States, hydroelectric plants produce approximately 989 billion kWh , or 11% of the nation's electricity, each year at a cost of $0.02 per kWh . Development and rehabilitation of existing dams in the United States could produce an additional 60 billion kWh per year . The material and energy inputs for a hydrogen production facility are primarily those needed to build and run a solar electric production facility, like photovoltaics and hydropower. The energy required to produce 1 billion kWh of hydrogen is 1.4 billion kWh of electricity . Photovoltaic cells currently require 2800 ha per 1 billion kWh; therefore, a total of 3920 ha would be needed to supply the equivalent of 1 billion kWh of hydrogen fuel. The water required for electrolytic production of 1 billion kWh per year equivalent of hydrogen is approximately 300 million liters per year . On a per capita basis, this amounts to 3000 liters of water per year. The liquefaction of hydrogen requires significant energy inputs because the hydrogen must be cooled to about −253°C and pressurized. About 30% of the hydrogen energy is required for the liquefaction process . The efficiency of solar ponds in converting solar radiation into heat is estimated to be approximately 1:4 , assuming a 30-year life for the solar pond . Electricity produced by a 100 ha solar pond costs approximately $0.15 per kWh . Nilsson C Berggren K . 2000 . Alterations of riparian ecosystems caused by river regulation. BioScience . 50 : 783 - 792 . Giampietro M Ulgiati A Pimentel D . 1998 . Feasibility of large-scale biofuel production. BioScience . 47 : 587 - 600 . Widespread development of wind power is limited by the availability of sites with sufficient wind and the number of wind machines that the site can accommodate. In California's Altamont Pass Wind Resource Area, an average of one 50 kW turbine per 1.8 ha allows sufficient spacing to produce maximum power . Based on this figure, approximately 13,700 ha of land is needed to supply 1 billion kWh per year . Because the turbines themselves occupy only approximately 2% of the area, most of the land can be used for vegetables, nursery stock, and cattle . However, it may be impractical to produce corn or other grains because the heavy equipment used in this type of farming could not operate easily between the turbines. World Health Organization–United Nations Environment Programme . 1993 . Urban Air Pollution in Megacities of the World. Geneva : WHO/UNEP. Chambers A . 2000 . Wind power spins into contention. Power Engineering . 104 : 14 - 18 . For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Parabolic troughs that have entered the commercial market have the potential for efficient electricity production because they can achieve high turbine inlet temperatures . Assuming peak efficiency and favorable sunlight conditions, the land requirements for the central receiver technology are approximately 1100 ha per 1 billion kWh per year . The energy input–output ratio is calculated to be 1:5 . Solar thermal receivers are estimated to produce electricity at a cost of approximately $0.07 to $0.09 per kWh . US Department of Energy . 2001 . Energy efficiency and renewable energy network. . Renewable Energy renewable energy sources scholarly articles Current and Potential Issues BioScience Oxford Academic
Renewable Energy renewable energy sources scholarly articles Current and Potential Issues BioScience Oxford Academic
Alfheim I Ramdahl T . 1986 . Mutagenic and Carcinogenic Compounds from Energy Generation. Oslo : Senter for Industriforkning. Methanol can be produced from a gasifier–pyrolysis reactor using biomass as a feedstock . The yield from 1 t of dry wood is about 370 liters of methanol . For a plant with economies of scale to operate efficiently, more than 1.5 million ha of sustainable forest would be required to supply it . Biomass is generally not available in such enormous quantities, even from extensive forests, at acceptable prices. Most methanol today is produced from natural gas. Smith DR Ilyin MA . 1991 . Wind and solar energy: Costs and value. Pages. 29 - 32 . in Berg DE, Veers PS, eds. The American Society of Mechanical Engineers 10th Annual Wind Energy Symposium; 20–23 January 1991; Houston, TX. Fuel cells using hydrogen are an environmentally clean, quiet, and efficient method of generating electricity and heat from natural gas and other fuels. Fuel cells are electrochemical devices, much like storage batteries, that use energy from the chemical synthesis of water to produce electricity. The fuel cell provides a way to burn hydrogen using oxygen, capturing the electrical energy released . Stored hydrogen is fed into a fuel cell apparatus along with oxygen from the atmosphere, producing effective electrical energy . The conversion of hydrogen into direct current using a fuel cell is about 40% efficient. Bradley RL . 1997 . Renewable Energy: Not Cheap, Not “Green”. Washington : Cato Institute. US Department of Agriculture . 2001 . Agricultural Statistics. Washington : USDA. If the United States does not commit itself to the transition from fossil to renewable energy during the next decade or two, the economy and national security will be at risk. It is of paramount importance that US residents work together to conserve energy, land, water, and biological resources. To ensure a reasonable standard of living in the future, there must be a fair balance between human population density and use of energy, land, water, and biological resources. Table 1. Fossil and solar energy use in the United States and world, in kilowatt-hours and quads. Both new and established homes can be fitted with solar heating and cooling systems. Installing passive solar systems in new homes is less costly than retrofitting existing homes. Based on the cost of construction and the amount of energy saved, measured in terms of reduced heating and cooling costs over 10 years, the estimated returns of passive solar heating and cooling range from $0.02 to $0.10 per kWh . David Pimentel is a professor, in the College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853-0901. Some hazards are associated with solar ponds, but most can be avoided with careful management. It is essential to use plastic liners to make the ponds leakproof and prevent contamination of the adjacent soil and groundwater with salt. The degradation of soil quality caused by sodium chloride can be avoided by using an ammonium salt fertilizer . Burrowing animals must be kept away from the ponds by buried screening . Ferguson ARB . 2001 . Biomass and Energy. Manchester : Optimum Population Trust. International Energy Agency . 1987 . Renewable Sources of Energy. Paris: IEA. Wet biomass materials can be converted effectively into usable energy with anaerobic microbes. In the United States, livestock dung is normally gravity fed or intermittently pumped through a plug-flow digester, which is a long, lined, insulated pit in the earth. Bacteria break down volatile solids in the manure and convert them into methane gas and carbon dioxide . A flexible liner stretches over the pit and collects the biogas, inflating like a balloon. The biogas may be used to heat the digester, to heat farm buildings, or to produce electricity. A large facility capable of processing the dung from 500 cows costs nearly $300,000 . The Environmental Protection Agency estimates that more than 2000 digesters could be economically installed in the United States. Table 2. Land resource requirements and total energy inputs for construction of facilities that produce 1 billion kilowatt-hours of electricity per year. Ellington RT Meo M El-Sayed DA . 1993 . The net greenhouse warming forcing of methanol produced from biomass. Biomass and Bioenergy . 4 : 405 - 418 . Bilgen E . 2000 . Passive solar massive wall systems with fins attached on the heated wall and without glazing. Journal of Solar Energy Engineering . 122 : 30 - 34 . Using solar electric technologies for its production, gaseous hydrogen produced by the electrolysis of water has the potential to serve as a renewable fuel to power vehicles and generate electricity. In addition, hydrogen can be used as an energy storage system for various electric solar energy technologies . Photovoltaic cells have the potential to provide a significant portion of future US and world electrical energy . Photovoltaic cells produce electricity when sunlight excites electrons in the cells. The most promising photovoltaic cells in terms of cost, mass production, and relatively high efficiency are those manufactured using silicon. Because the size of the unit is flexible and adaptable, photovoltaic cells can be used in homes, industries, and utilities. Natural Resources Canada . 2002 . Technologies and applications. . Bockris JOM Wass JC . 1988 . About the real economics of massive hydrogen production at 2010 AD. Pages. 101 - 151 . in Veziroglu TN, Protsenko AN, eds. Hydrogen Energy Progress VII. New York: Pergamon Press. Table 1. Fossil and solar energy use in the United States and world, in kilowatt-hours and quads. This PDF is available to Subscribers Only Processed vegetable oils from sunflower, soybean, rape, and other oil plants can be used as fuel in diesel engines. Unfortunately, producing vegetable oils for use in diesel engines is costly in terms of both time and energy . Kreutz TG Ogden JM . 2000 . Assessment of hydrogen-fueled proton exchange membrane fuel cells for distributed generation and cogeneration. Proceedings of the 2000. US Department of Energy Hydrogen Program Review; Washington, DC. NREL/CP-570-28890. Approximately 20% of the fossil energy used each year in the United States is used for heating and cooling buildings and for heating hot water . At present only about 0.3 quads of energy are being saved by technologies that employ passive and active solar heating and cooling of buildings , which means that the potential for energy savings through increased energy efficiency and through the use of solar technologies for buildings is tremendous. Estimates suggest that the amount of energy lost through poorly insulated windows and doors is approximately 1.1 × 10 12 kWh each year—the approximate energy equivalent of all the oil pumped in Alaska per year . Despite the benefits of hydroelectric power, the plants cause major environmental problems. The impounded water frequently covers valuable, agriculturally productive, alluvial bottomland. Furthermore, dams alter the existing plants, animals, and microbes in the ecosystem . Fish species may significantly decline in river systems because of these numerous ecological changes . Within the reservoirs, fluctuations of water levels alter shorelines, cause downstream erosion, change physiochemical factors such as water temperature and chemicals, and affect aquatic communities. Sediments build up behind the dams, reducing their effectiveness and creating another major environmental problem. American Wind Energy Association . 2000a . Wind energy and climate change. . US Department of Energy . 2000 . Consumer energy information: EREC fact sheets. . Production of electricity from photovoltaic cells currently costs $0.12 to $0.20 per kWh . Using mass-produced photovoltaic cells with about 18% efficiency, 1 billion kWh per year of electricity could be produced on approximately 2800 ha of land, which is sufficient to supply the electrical energy needs of 100,000 people . Locating the photovoltaic cells on the roofs of homes, industries, and other buildings would reduce the need for additional land by an estimated 20% and reduce transmission costs. However, because storage systems such as batteries cannot store energy for extended periods, photovoltaics require conventional backup systems. Godish T . 1991 . Air Quality. Chelsea : Lewis Publishers. Larminie J Dicks A . 2000 . Fuel Cell Systems Explained. Chichester : John Wiley and Sons. MacKenzie JJ . 1994 . The Keys to the Car: Electric and Hydrogen Vehicles for the 21st Century. Washington : World Resources Institute. Peschka W . 1992 . Liquid Hydrogen: Fuel of the Future. New York: Springer Verlag. Per capita consumption of woody biomass for heat in the United States amounts to 625 kilograms per year. In developing nations, use of diverse biomass resources ranges from 630 kg per capita to approximately 1000 kg per capita . Developing countries use only about 500 liters of oil equivalents of fossil energy per capita, compared with nearly 8000 liters of oil equivalents of fossil energy used per capita in the United States. Biofuels: Ethanol, methanol, and vegetable oil Environmental Energy Technologies Division–Lawrence Berkeley National Laboratory . 2001 . Home energy saver. . Voigt C . 1984 . Material and energy requirements of solar hydrogen plants. International Journal of Hydrogen Energy . 9 : 491 - 500 . Brown LR Moyle PB . 1993 . Distribution, ecology, and status of the fishes of the San Joaquin River drainage, California. California Fish and Game . 79 : 96 - 114 . Another solar thermal technology that concentrates solar radiation for large-scale energy production is the parabolic trough. A parabolic trough, shaped like the bottom half of a large drainpipe, reflects sunlight to a central receiver tube that runs above it. Pressurized water and other fluids are heated in the tube and used to generate steam, which can drive turbogenerators for electricity production or provide heat energy for industry. Hull JR . 1986 . Solar ponds using ammonium salts. Solar Energy . 36 : 551 - 558 . Tabor HZ Doran B . 1990 . The Beith Ha'arva 5MW solar pond power plant : Progress report. Solar Energy . 45 : 247 - 253 . Hall DO . renewable energy sources scholarly articles 1992 . Biomass. Washington : World Bank. Federal Energy Regulatory Commission . 1984 . Hydroelectric Power Resources of the United States. Washington : FERC. US Bureau of the Census . 2001 . Statistical Abstract of the United States. 1999 . Washington : USBC. Hartman H Angelidake I Ahring BK . 2000 . Increase of anaerobic degradation of particulate organic matter in full-scale biogas plants by mechanical maceration. Water Science and Technology . 41 : 145 - 153 . Balcomb JD . 1992 . Introduction. Pages. 1 - 37 . in Balcomb JD, ed. Passive Solar Buildings. Cambridge : MIT Press. Birdsey RA . 1992 . Carbon Storage and Accumulation in United States Forest Ecosystems. Washington : USDA Forest Service. Jenkins BM . 1999 . Pyrolysis gas. Pages. 222 - 248 . in Kitani O, Jungbluth T, Peart RM, Ramdani A, eds. CIGAR Handbook of Agricultural Engineering. St. Joseph : American Society of Agricultural Engineering. US Department of Energy . 1999 . Fuel Cells for the 21st Century: Collaboration for a Leap in Efficiency and Cost Reduction. Washington : DOE. Baechler MC Lee AD . 1991 . Implications of environmental externalities assessments for solar thermal power plants. Pages. 151 - 158 . in Mancini TR, Watanabe K, Klett DE, eds. Solar Engineering 1991. New York: American Society of Mechanical Engineers. The amount of biogas produced is determined by the temperature of the system, the microbes present, the volatile solids content of the feedstock, and the retention time. A plug-flow digester with an average manure retention time of about 16 days under winter conditions produced 452,000 kcal per day and used 262,000 kcal per day to heat the digester to 35°C . Using the same digester during summer conditions but reducing the retention time to 10.4 days, the yield in biogas was 524,000 kcal per day, with 157,000 kcal per day used for heating the digester . The energy input–output ratios for the digester in these winter and summer conditions were 1:1.7 and 1:3.3, respectively. The energy output of biogas digesters has changed little over the past two decades . Shute N . 2001 . The weather: Global warming could cause droughts, disease, and political upheaval. US News & World Report, 11 November 2001, pp. 44 - 52 . An additional complication in the transition to renewable energies is the relationship between the location of ideal production sites and large population centers. Ideal locations for renewable energy technologies are often remote, such as deserts of the American Southwest or wind farms located kilometers offshore. Although these sites provide the most efficient generation of energy, delivering this energy to consumers presents a logistical problem. For instance, networks of distribution cables must be installed, costing about $179,000 per kilometer of 115-kilovolt lines . A percentage of the power delivered is lost as a function of electrical resistance in the distribution cable. There are five complex alternating current electrical networks in North America, and four of these are tied together by DC lines . Based on these networks, it is estimated that electricity can be transmitted up to 1500 km. The cost of producing 1 kWh of electricity from woody biomass is about $0.058, which is competitive with other systems for electricity production . Approximately 3 kWh of thermal energy is expended to produce 1 kWh of electricity, an energy input–output ratio of 1:7 . American Wind Energy Association . 2000b . Wind energy: The fuel of the future is ready today. . In developing countries such as India, biogas digesters typically treat the dung from 15 to 30 cattle from a single family or a small village. The resulting energy produced for cooking saves forests and preserves the nutrients in the dung. The capital cost for an Indian biogas unit ranges from $500 to $900 . The price value of one kWh of biogas in India is about $0.06 . The total cost of producing about 10 million kcal of biogas is estimated to be $321, assuming the cost of labor to be $7 per hour; hence, the biogas has a value of $356. Manure processed for biogas has little odor and retains its fertilizer value . US Department of Energy–Energy Efficiency and Renewable Energy Network . 2001 . Solar parabolic troughs: Concentrating solar power. . Cassedy ES . 2000 . Prospects for Sustainable Energy. New York: Cambridge University Press. The major costs of fuel cells are the electrolytes, catalysts, and storage. Phosphoric acid fuel cells and proton exchange membrane fuel cells are the most widely used and most efficient. PAFCs have an efficiency of 40% to 45%, compared to diesel engine efficiency of 36% to 39%. However, PAFCs are complex and have high costs because they operate at temperatures of 50° to 100°C . A fuel cell PEM engine costs $500 per kW, compared to $50 per kW for a gasoline engine , leading to a total price of approximately $100,000 for an automobile running on fuel cells . These prices are for specially built vehicles, and the costs should decline as they are mass-produced. There is high demand for fuel cell–equipped vehicles in the United States . Simpson JR McPherson EG . 1998 . Simulation of tree shade impacts on residential energy use for space conditioning in Sacramento. Atmospheric Environment . 32 : 69 - 74 . Megan Herz, Michele Glickstein, Mathew Zimmerman, Richard Allen, Katrina Becker, Jeff Evans, Benita Hussain, Ryan Sarsfeld, Anat Grosfeld, and Thomas Seidel are graduate students, in the College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853-0901. The immediate priority of the United States should be to speed the transition from the reliance on nonrenewable fossil energy resources to reliance on renewable energy technologies. Various combinations of renewable technologies should be developed, consistent with the characteristics of the different geographic regions in the United States. A combination of the renewable technologies listed in table 3 should provide the United States with an estimated 45 quads of renewable energy by 2050. These technologies should be able to provide this much energy without interfering with required food and forest production. Hydroelectric plants, however, require considerable land for their water storage reservoirs. An average of 75,000 hectares of reservoir land area and 14 trillion liters of water are required per 1 billion kWh per year produced . Based on regional estimates of US land use and average annual energy generation, reservoirs currently cover approximately 26 million ha of the total 917 million ha of land area in the United States . To develop the remaining best candidate sites, assuming land requirements similar to those in past developments, an additional 17 million ha of land would be required for water storage . A sixfold increase in installed technologies would provide the United States with approximately 13.1 × 10 12 kWh of energy, less than half of current US consumption . This level of energy production would require about 159 million ha of land . This percentage is an estimate and could increase or decrease, depending on how the technologies evolve and energy conservation is encouraged. We thank the following people for reading an earlier draft of this article and for their many helpful suggestions: Louis Albright, Cornell University, Ithaca, NY; Allen Bartlett, University of Colorado, Boulder, CO; Richard C. Duncan, Institute on Energy and Man, Seattle, WA; Andrew R. B. Ferguson, Optimum Population Trust, Oxon, United Kingdom; Tillman Gerngross, Dartmouth College, Hanover, NH; O. J. Lougheed, Irkutsk, Siberia; Norman Myers, Oxford University, United Kingdom; Marcia Pimentel, Cornell University, Ithaca, NY; Nancy Rader, California Wind Energy Association; Kurt Roos, US Environmental Protection Agency, Washington, DC; Frank Roselle-Calle, King's College, London; Peter Salonius, Canadian Forest Service, Fredericton, New Brunswick, Canada; Jack Scurlock, Oak Ridge National Laboratory, Oak Ridge, TN; Henry Stone, Ionia, NY; Ted Trainer, University of New South Wales, Australia; Mohan K. Wali, Ohio State University, Columbus, OH; Paul B. Weisz, State College, PA; William Jewell, Cornell University, Ithaca, NY; Walter Youngquist, Eugene, OR. Sorensen B . 2000 . Renewable Energy: Its Physics, Engineering, Use, Environmental Impact, Economics, and Planning Aspects. San Diego: Academic Press. Vesterby M Krupa S . 2001 . Major Uses of Land in the United States, 1997. Washington : Resource Economics Division, Economic Research Service, USDA. Statistical Bulletin No. 973. . The potential environmental impacts of solar thermal receivers include the accidental or emergency release of toxic chemicals used in the heat transfer system . Water scarcity can also be a problem in arid regions. Kitani O . 1999 . Biomass resources. Pages. 6 - 11 . in Kitani O, Jungbluth T, Peart RM, Ramdami A, eds. Energy and Biomass Engineering. St. Joseph : American Society of Agricultural Engineers. David Pimentel is a professor, in the College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853-0901. US Environmental Protection Agency . 2002 . Wood smoke. .
Osburn L Osburn J . 2001 . Biomass resources for energy and industry. . The United States faces energy shortages and increasing energy prices within the next few decades . Coal, petroleum, natural gas, and other mined fuels provide 75% of US electricity and 93% of other US energy needs . On average, every year each American uses about 93,000 kilowatt-hours , equivalent to 8000 liters of oil, for all purposes, including transportation, heating, and cooling . About 12 kWh costs as much as $0.50, and this cost is projected to increase significantly in the next decade . The noise caused by rotating blades is another unavoidable side effect of wind turbine operation. Beyond 2.1 km, however, the largest turbines are inaudible even downwind. At a distance of 400 m, the noise level is about 56 decibels , corresponding roughly to the noise level of a home air-conditioning unit. Hos JJ Groenveld MJ . 1987 . Biomass gasification. Pages. 237 - 255 . in Hall DO, Overend RP, eds. Biomass. Chichester : John Wiley and Sons. Petroleum, essential for the transportation sector and the chemical industry, makes up approximately 40% of total US energy consumption. Clearly, as the supply diminishes, a shift from petroleum to alternative liquid fuels will be necessary. This analysis focuses on the potential of three fuel types: ethanol, methanol, and vegetable oil. Burned in internal combustion engines, these fuels release less carbon monoxide and sulfur dioxide than gasoline and diesel fuels; however, because the production of most of these biofuels requires more total fossil energy than they produce as a biofuel, they contribute to air pollution and global warming . Casazz JA . 1996 . Transmission access and retail wheeling: The key questions. Pages. 77 - 102 . in Einhorn M, Siddiqui R, eds. Electricity Transmission Pricing and Technology. Boston: Kluwer Academic. This assessment of renewable energy technologies confirms that these techniques have the potential to provide the nation with alternatives to meet approximately half of future US energy needs. To develop this potential, the United States would have to commit to the development and implementation of non–fossil fuel technologies and energy conservation. The implementation of renewable energy technologies would reduce many of the current environmental problems associated with fossil fuel production and use. The rotating magnets in the turbine electrical generator produce a low level of electromagnetic interference that can affect television and radio signals within 2 to 3 km of large installations . Fortunately, with the widespread use of cable networks or line-of-sight microwave satellite transmission, both television and radio are unaffected by this interference. Population Reference Bureau . 2001 . World Population Data Sheet. Washington : PRB. Kellet J . 1990 . The environmental impacts of wind energy developments. Town Planning Review . 91 : 139 - 154 . Pimentel D . 1994 . Renewable energy: Economic and environmental issues. BioScience . 44 : 536 - 547 . Table 3. Current and projected US gross annual energy supply from various renewable energy technologies, based on the thermal equivalent and required land area. Organisation for Economic Co-operation and Development . 1998 . Environmental Impacts of Renewable Energy. Paris: OCED. Passive heating and cooling of buildings Trainer FE . 1995 . Can renewable energy sources sustain affluent society?. Energy Policy . 23 : 1009 - 1026 . Geothermal energy uses natural heat present in Earth's interior. Examples are geysers and hot springs, like those at Yellowstone National Park in the United States. Geothermal energy sources are divided into three categories: hydrothermal, geopressured–geothermal, and hot dry rock. The hydrothermal system is the simplest and most commonly used one for electricity generation. The boiling liquid underground is utilized through wells, high internal pressure drives, or pumps. In the United States, nearly 3000 MW of installed electric generation comes from hydrothermal resources, and this figure is projected to increase by 1500 MW within the next 20 years . Ligon FK Dietrich WE Trush WJ . 1995 . Downstream ecological effects of dams: A geomorphic perspective. BioScience . 45 : 183 - 192 . Gregory J Silviera S Derrick A Cowley P Allinson C Paish O . 1997 . Financing Renewable Energy Projects: A Guide for Development Workers. London: Intermediate Technology Publications. Table 2. Land resource requirements and total energy inputs for construction of facilities that produce 1 billion kilowatt-hours of electricity per year. Fossil fuel consumption is the major contributor to the increasing concentration of carbon dioxide in the atmosphere, a key cause of global warming . Global warming reduces agricultural production and causes other biological and social problems . The United States, with less than 4% of the world population, emits 22% of the CO 2 from burning fossil fuels, more than any other nation. Reducing fossil fuel consumption may slow the rate of global warming . Despite the environmental and economic benefits of renewable energy, the transition to large-scale use of this energy presents some difficulties. Renewable energy technologies, all of which require land for collection and production, must compete with agriculture, forestry, and urbanization for land in the United States and the world. The United States already devotes as much prime cropland per capita to food production as is possible, given the size of the US population, and the world has only half the cropland per capita that it needs for a diverse diet and an adequate supply of essential nutrients . In fact, more than 3 billion people are already malnourished in the world . According to some sources, the world and US population could double in the next 50 and 70 years, respectively; all the available cropland and forest land would be required to provide vital food and forest products . Liquid hydrogen fuel occupies about three times the volume of an energy equivalent of gasoline. Storing 25 kg of gasoline requires a tank weighing 17 kg, whereas storing 9.5 kg of hydrogen requires a tank weighing 55 kg . Although the hydrogen storage vessel is large, hydrogen burns 1.33 times more efficiently than gasoline in automobiles . In tests, a Plymouth liquid hydrogen vehicle, with a tank weighing about 90 kg and 144 liters of liquid hydrogen, has a cruising range in traffic of 480 km with a fuel efficiency of 3.3 km per liter . However, even taking into account its greater fuel efficiency, commercial hydrogen is more expensive at present than gasoline. About 3.7 kg of gasoline sells for about $1.20, whereas 1 kg of liquid hydrogen with the same energy equivalent sells for about $2.70 . Most of the geothermal sites for electrical generation are located in California, Nevada, and Utah . Electrical generation costs for geothermal plants in the West range from $0.06 to $0.30 per kWh , suggesting that this technology offers potential to produce electricity economically. The US Department of Energy and the Energy Information Administration project that geothermal electric generation may grow three- to fourfold during the next 20 to 40 years. However, other investigations are not as optimistic and, in fact, suggest that geothermal energy systems are not renewable because the sources tend to decline over 40 to 100 years . Existing drilling opportunities for geothermal resources are limited to a few sites in the United States and the world . Kerlinger P . 2000 . Avian Mortality and Communication Towers: A Review of Recent Literature, Research, and Methodology. Washington : US Fish and Wildlife Service, Office of Migratory Bird Management. Clarke A . 1991 . Wind energy progress and potential. Energy Policy . 19 : 742 - 755 . Gleick PH Adams AD . 2000 . Water: The Potential Consequences of Climate Variability and Change. Oakland : Pacific Institute for Studies in Development, Environment, and Security. For successful operation of a solar pond, the salt concentration gradient and the water level must be maintained. A solar pond covering 4000 ha loses approximately 3 billion liters of water per year under arid conditions . The solar ponds in Israel have been closed because of such problems. To counteract the water loss and the upward diffusion of salt in the ponds, the dilute salt water at the surface of the ponds has to be replaced with fresh water and salt added to the lower layer. US Department of Energy–Energy Information Administration . 1991 . Geothermal Energy in Western United States and Hawaii: Resources and Projected Electricity Generation Supplies. Washington : DOE/EIA. Roos A Karlsson B . 1994 . Optical and thermal characterization of multiple glazed windows with low U-values. Solar Energy . 52 : 315 - 325 . Jewell WJ Dell'orto S Fanfoli KJ Hates TD Leuschner AP Sherman DF . 1980 . Anaerobic Fermentation of Agricultural Residue: Potential for Improvement and Implementation. Washington : US Department of Energy. Hydrogen has serious explosive risks because it is difficult to contain within steel tanks. Mixing with oxygen can result in intense flames because hydrogen burns more quickly than gasoline and diesel fuels . Other environmental impacts are associated with the solar electric technologies used in hydrogen production. Water for the production of hydrogen may be a problem in arid regions of the United States and the world. Table 3. Current and projected US gross annual energy supply from various renewable energy technologies, based on the thermal equivalent and required land area. Gaudiosi G . 1996 . Offshore wind energy in the world context. Renewable Energy . 9 : 899 - 904 . Youngquist W . 1997 . GeoDestinies: The Inevitable Control of Earth Resources over Nations and Individuals. Portland : National Book Company. Transition to renewable energy alternatives Schneider SH Easterling WE Mearms LO . 2000 . Adaptation: Sensitivity to natural variability, agent assumptions, and dynamic climatic changes. Climatic Change . 45 : 203 - 221 . Solar ponds are used to capture radiation and store the energy at temperatures of nearly 100 degrees Celsius . Constructed ponds can be made into solar ponds by creating a layered salt concentration gradient. The layers prevent natural convection, trapping the heat collected from solar radiation in the bottom layer of brine. The hot brine from the bottom of the pond is piped out to use for heat, for generating electricity, or both. The total energy input to produce 1000 liters of ethanol in a large plant is 8.7 million kcal . However, 1000 liters of ethanol has an energy value of only 5.1 million kcal and represents a net energy loss of 3.6 million kcal per 1000 liters of ethanol produced. Put another way, about 70% more energy is required to produce ethanol than the energy that ethanol contains . The environmental effects of burning biomass are less harmful than those associated with coal, but more harmful than those associated with natural gas . Biomass combustion releases more than 200 different chemical pollutants, including 14 carcinogens and 4 cocarcinogens, into the atmosphere . Globally, but especially in developing nations where people cook with fuelwood over open fires, approximately four billion people suffer from continuous exposure to smoke . In the United States, wood smoke kills 30,000 people each year . However, the pollutants from electric plants that use wood and other biomass can be controlled. World Health Organization . 1996 . Micronutrient Malnutrition: Half of the World's Population Affected. Geneva : WHO. Press release. In the United States, 2502 megawatts of installed wind generators produce about 6.6 billion kWh of electrical energy per year . The American Wind Energy Association estimates that the United States could support a capacity of 30,000 MW by the year 2010, producing 75 billion kWh per year at a capacity of 30%, or approximately 2% of the annual US electrical consumption. If all economically feasible land sites were developed, the full potential of wind power would be about 675 billion kWh . Offshore sites could provide an additional 102 billion kWh , making the total estimated potential of wind power 777 billion kWh , or 23% of current electrical use. Although none of the passive heating and cooling technologies requires land, they are not without problems. Some indirect problems with land use may arise, concerning such issues as tree removal, shading, and rights to the sun . Glare from collectors and glazing may create hazards to automobile drivers and airline pilots. Also, when houses are designed to be extremely energy efficient and airtight, indoor air quality becomes a concern because of indoor air pollutants. However, well-designed ventilation systems with heat exchangers can take care of this problem. Ogden JM Nitsch J . 1993 . Solar hydrogen. Pages. 925 - 1010 . in Johansson TB, Kelly H, Reddy AKN, Williams RH, eds. Renewable Energy: Sources for Fuels and Electricity. Washington : Island Press. However, photovoltaic cells need improvements to make them economically competitive before their use can become widespread. Test cells have reached efficiencies ranging from 20% to 25% , but the durability of photovoltaic cells must be lengthened and production costs reduced several times to make their use economically feasible. Pimentel D . 1991 . Ethanol fuels: Energy security, economics, and the environment. Journal of Agricultural and Environmental Ethics . 4 : 1 - 13 . Pimentel D Warneke AF Teel WS Schwab KA Simcox NJ Evert DM Baenisch KD Aaron MR . 1988 . Food versus biomass fuel: Socioeconomic and environmental impacts in the United States, Brazil, India, and Kenya. solar energy news article