Renewable Energy: Energetic, Economic, and Environmental Issues

Pimentel et al
in press, BioScience [2002 May]

The United States faces energy shortages and increasing energy prices within the next few decades (Abelson 2000, Duncan 2001). Coal, petroleum, natural gas, and other mined fuels provide three quarters of the US electricity, and 93% of other US energy needs (USBC 1999). In total, each American uses nearly 8,000 liters of oil equivalents for all purposes, including transportation, heating, and (USBC 1999). A liter of gasoline costs as much as 55 per liter and may double in the next decade. The United States now imports more than 60% of its oil at an annual cost of approximately $75 billion (USBC 1999). It is projected that the US will be importing nearly 94% in approximately 20 years based on production forecasts (Duncan 2001). The 3.6 trillion kWh of electricity produced annually at a cost of 7 to 20 per kWh are becoming insufficient as the US population of more than 280 million increases of more than 3 million people each year. Under these circumstances the future contribution of renewable energy sources will be vital.

Fossil fuel consumption is the major contributor to increasing carbon dioxide concentration in the atmosphere -- a key factor in the global warming problem (Kennedy 2000, Schneider et al. 2000). Global warming has a negative impact on agricultural production and causes other biological and social problems (Kennedy 2000, Schneider et al. 2000). The US with less than 4% of the world population emits 22% of the carbon dioxide from the burning of fossil fuels or more than any other nation, and a reduction in fossil fuel consumption may help ameliorate or delay global warming (Kennedy 2000, Schneider et al. 2000).

Diverse renewable energy currently provide only 8% of US needs and about 14% of world needs (Table 1). In addition to energy conservation, the development and use of renewable energy is expected to increase as fossil fuel supplies decline.

More than 8 different renewable technologies are projected to provide most of the energy in the future. Renewable energy technologies that have the potential to provide energy supplies include: hydroelectric, biomass, wind power, solar thermal systems, photovoltaic systems, passive energy systems, geothermal systems, biogas, ethanol, methanol, and vegetable oil. The potential of these various renewable energy technologies hold for supplying the United States and world with future needs are assessed in terms of land requirements, environmental benefits and risks, as well as energetic and economic costs.

Hydroelectric Systems

Hydropower contributes significantly to world energy, providing 6.5% of world energy supply (Table 1). Approximately 340 billion kWh(e) (11%) of the total US electricity is produced by hydroelectric plants (USBC 1999), and development and rehabilitation of existing dams in the US could produce an additional 60 billion kWh(e) per year (Table 3).

Hydroelectric plants, however, require considerable land for their water-storage reservoirs. An average of 75,000 ha of reservoir land area and 14 trillion liters of water are required per 1 billion kWh/yr produced (Gleick and Adams 2000, Pimentel et al. 1994) (Table 2). Based on regional estimates of US land use and average annual energy generation, approximately 27 million hectares of the total of 917 million ha of land area in the United States currently are covered with reservoirs (Pimentel 2001). To develop the remaining best candidate sites, assuming land requirements similar to those in past developments, an additional 450,000 hectares of land would be required for water storage (Table 3).

Despite the benefits of hydroelectric power, the plants cause major environmental problems. The impounded water frequently covers valuable, agriculturally productive, alluvial bottomland. Further, dams alter the existing plants, animals, and microbes in the ecosystem (Ligon et al. 1995, Nilsson and Berggren 2000). Fish species may significantly decline in river systems because of these numerous ecological changes (Brown and Moyle 1993). Within the reservoirs, fluctuations of water levels alter shorelines, cause downstream erosion and change the physiochemical factors, as well as change aquatic communities. A major problem is the deposition of sediments behind the dams that reduce the effectiveness of dams.

Biomass Energy Systems

Although most biomass will continue to be used principally for cooking and heating, it can be converted into electricity. Under sustainable forest conditions in both temperate and tropical ecosystems, approximately 3 tons (dry)/ha/yr of woody biomass can be harvested sustainably (Birdsey 1992, Ferguson 2000, Repetto 1992, Trainer 1995). Although this amount of woody biomass has a gross energy yield of 13.5 million kcal, approximately 33 liters (330,000 kcal) of diesel fuel/ha are expended for cutting and collecting wood for transport to an electric power plant. Thus, the energy input:output ratio for such a system is calculated to be 1:22.

The cost of producing a kilowatt of electricity from woody biomass is about 5.8, which is competitive with other electricity production systems (Table 2). Approximately 3 kWh of thermal energy is expended to produce 1 kWh of electricity or a ratio of 1:7 (Pimentel 2001) (Table 2).

Per capita consumption of woody biomass for heat in the US amounts to 625 kg annually. In developing nations, use of diverse biomass (wood, crop residues, and dung) resources ranges from 630 kg (Kitani 1999) per capita to approximately 1,000 kg (Hall 1992). Developing countries use only about 500 liters of oil equivalents of fossil energy per capita compared with nearly 8,000 liters of oil equivalents of fossil energy are used per capita in the US (Table 1).

Woody biomass could supply the US with about 5 quads (1.5 x 1012 kWh thermal) of its total gross energy supply by the year 2050 provided an available area of approximately 175 million ha (Table 3). A city of 100,000 people using the biomass from a sustainable forest (3 t/ha/yr) electricity would require approximately 200,000 ha of forest area, based on an average electrical demand of slightly more than 1 billion kWh(e) (860 kcal = 1 kWh) (USBC 1999) (Table 2).

Environmental impacts of burning biomass are more harmful than those associated with natural gas, but less polluting than coal (Pimentel 2001). Biomass combustion releases more than 200 different chemical pollutants, including 14 carcinogens and 4 cocarcinogens into the atmosphere (Alfheim and Ramdahl 1986, Godish 1991); however, the pollutants from electric plants can be controlled.

Globally, but especially in developing nations where people cook with fuelwood over open fires, approximately 4 billion humans suffer from the continuous exposure to smoke (WHO/UNEP 1993). Additionally, fuelwood smoke is estimated to cause the death of 4 million children each year worldwide (Bank 1992).

Wind Power

For many centuries, wind power has provided energy to pump water and run mills and other machines. Today, turbines with a capacity of at least 500 kW produce most of the commercially wind-generated electricity. These turbines operating under ideal siting will run at maximum 30% efficiency and yield an energy output of 1.3 million kWh(e) annually (AWEA 2000a). An initial investment of approximately $500,000 for a 500 kW capacity turbine (Nelson 1996), operating at 30% efficiency, will yield an input:output ratio of 1:5 over 30 years of operation (Table 2). During the 30-year life of the system the annual operating costs amount to $40,500 (Nelson 1996). The estimated cost of electricity generated is 7 per kWh(e) (Table 2).

Currently in the US, there are 2,502 MW of installed wind generators producing about 6.6 billion kWh of electrical energy annually (Chambers 2000). AWEA (2000b) estimates that the US could support a capacity of 30,000 MW by the year 2010, producing 75 billion kWh(e) per year at a capacity of 30%, or approximately 2% of the annual US electrical consumption. If all economically feasible land sites are developed, the full potential of wind power is estimated to be about 675 billion kWh(e) (AWEA 2000b). An additional 102 billion kWh(e) can be provided through the development of offshore sites (Gaudiosi 1996), making the total potential of wind power is estimated to be 777 billion kWh(e) or 23% of current electrical use.

Widespread development of wind power is limited by the availability of sites with sufficient wind (at least 20 km/h) 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 (Smith and Ilyin 1991). Based on this figure approximately 13,700 ha of land are needed to supply 1 billion kWh/yr (Table 2). However, because the turbines themselves only occupy approximately 2% of the area, most of the land can for agriculture.

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 flying into the supporting towers and rotating blades (Kellet 1990). For this reason, Clarke (Clarke 1991) suggests that wind farms be located at least 300 meters from nature reserves to reduce this risk to birds. The estimated 13,000 wind turbines installed in the US have found fewer than 300 birds killed annually (Kerlinger 2000). With proper siting and improved repellant technology, such as strobe lights or paint patterns, the number of birds killed might be reduced.

The rotating magnets in the turbine electrical generator produce a small level of electromagnetic interference that can affect television and radio signals within 2 to 3 km of large installations (IEA 1987). Fortunately, with the widespread use of cable networks or line-of-sight microwave satellite transmission, both TV and radio are unaffected by this interference. The noise caused by rotating blades is an unavoidable side effect of wind turbine operation. Beyond 2.1 km he largest turbines are inaudible even downwind. At a distance of 400 meters, the noise level is about 56 dB (IEA 1987), corresponding roughly to the noise level of a home air-conditioning unit.

Solar Thermal Conversion Systems

Solar thermal energy systems collect the sun's radiant energy and convert it into heat. This heat can be used for household and industrial purposes and also to produce steam to drive turbines to produce electricity. System complexity ranges from solar ponds to the electricity-generating parabolic troughs. In the following analyses, the conversion of thermal energy into electricity is used to facilitate comparison to the other solar energy technologies.

Solar Ponds

Solar ponds are used to capture radiation and store the energy at temperatures at nearly 100o C. Constructed ponds can be made into solar ponds by creating a layered salt-concentration gradient. These layers prevent natural convection from occurring in the pond and thus enabling the heat collected from solar radiation to be trapped in the bottom layer of brine. The hot brine from the bottom of the pond is piped out for use as heat and/or for generating electricity.

For successful operation, the salt-concentration gradient and the water levels must be maintained. Solar ponds covering 4,000 ha lose approximately 3 billion liters of water per year (750,000 liters/ha/yr) under arid conditions Note, the solar ponds in Israel have been closed because of recent difficulties.

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 (Table 2). A 100 hectare (1 km2) solar pond can produce electricity at a rate of approximately 15 per kWh (Kishore 1993).

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 leak proof and prevent contamination of the adjacent soil and groundwater with salt. The degradation of soil quality using NaCl salt can be avoided by using an ammonium salt-fertilizer (Hull 1986). Furthermore, burrowing animals must be kept away from the ponds by buried screening (Dickson and Yates 1983).

Solar Power Parabolic Troughs

Another solar thermal technology that concentrates solar radiation for large-scale energy production is parabolic troughs. The parabolic trough is similar to a half of a large drainpipe that reflects the sunlight to a central-pipe receiver that runs above the troughs. Pressurized water and other fluids are heated in the pipe and used to generate steam to drive turbo-generators for electricity production or provide industry with heat energy.

Parabolic troughs that have entered the commercial market have the potential for efficient electricity production because they are able to achieve high turbine inlet temperatures (Winter et al. 1991). Assuming peak efficiency and favorable sunlight conditions the land requirements for the central receiver technology are approximately 1,100 ha per1 billion kWh/yr (Table 2). The energy input:output ratio is calculated to be 1:5 (Table 2). Solar thermal receivers are estimated to produce electricity at approximately 7 to 9 per kWh (DOE/EREN 2001).

The potential environmental impacts of solar thermal receivers include the accidental or emergency release of toxic chemicals used in the heat transfer system (Baechler and Lee 1991). Water availability can be a problem in arid regions.

Photovoltaic Systems

Photovoltaic cells have the potential to provide a significant portion of our future electrical energy (Gregory et al. 1997). Photovoltaic cells produce electricity when sunlight excites electrons in the cells. The most promising photovoltaic cells in terms of low cost, mass production, and relatively high efficiency are those being manufactured using silicon. Because the size of the unit is flexible and adaptable, photovoltaic cells can be used in homes, industries, and utilities.

Before widespread use, however, improvements are needed to make the photovoltaic cells economically competitive. Test cells have reached efficiencies that range from 20% to 25% (Sorensen 2000). However, the durability of photovoltaic cells needs to be lengthened and current production costs reduced several times to make them economically feasible.

Currently production of electricity from photovoltaic cells costs 12 to 20/kWh (DOE 2000). Using mass produced photovoltaic cells with about 18% efficiency, 1 billion kWh/yr of electricity could be produced on approximately 2,800 ha of land, or approximately 280 square meters per person (DOE 2001) (Table 2).

The energy input for making the structural materials of a photovoltaic system capable of delivering 1 billion, during a life of 30 years, is calculated to be approximately 143 million kWh(e) . Thus, the energy input:output ratio for the modules is about 1:7 (Knapp and Jester 2000) (Table 2). The energy input for making the structural materials of a photovoltaic system capable of delivering 1 billion, during a life of 30 years, is calculated to be approximately 143 million kWh(e) . Thus, the energy input:output ratio for the modules is about 1:7 (Knapp and Jester 2000) (Table 2).

Locating the photovoltaic cells on the roofs of home, industries, and other buildings would reduce the need for additional land by an estimated 20% as well as reduce transmission costs. However, because storage systems such as batteries can not store energy for extended periods, conventional back-up systems are required with photovoltaics.

The major environmental problem associated with photovoltaic systems is the use of toxic chemicals, such as cadmium sulfide and gallium arsenide, in their manufacture (Bradley 1997). 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 issue.

Hydrogen And Fuel Cells

Using solar-electric technologies for its production, gaseous hydrogen produced by the electrolysis of water has the potential to serve as a renewable fuel for vehicles and electric generation. In addition, hydrogen can be used as an energy-storage system for various electric solar-energy technologies (MacKenzie 1994, Winter and Nitsch 1988).

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 (th) of hydrogen is 1.4 billion kWh of electricity (Ogden and Nitsch 1993). Current photovoltaic cells (Table 2) require 2,800 ha/1 billion kWh (e), therefore a total of 3,920 ha would be needed to supply the equivalent of 1 billion kWh (th) of hydrogen fuel. The water required for electrolytic production of 1 billion kWh (th)/yr is approximately 300 million liters/yr (Voigt 1984).

Liquid hydrogen-fuel occupies about 3 times the volume of an energy equivalent of gasoline. Storing 25 kg of gasoline requires a tank weighing 17 kg, whereas the storage of 9.5 kg of hydrogen requires a tank weighing 55 kg (Peschka 1987, 1992). Although the hydrogen storage vessel is large, hydrogen burns 1.33 times more efficiently than gasoline in automobiles (Bockris 1988). 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 (MacKenzie 1994).

The liquefaction of hydrogen requires significant energy inputs because the hydrogen must be cooled to about -253oC. and pressurized About 30% of the hydrogen energy is required for the liquefaction process (Peschka 1992, Trainer 1995).

At present, commercial hydrogen is more expensive than gasoline. One kilogram of gasoline sells for about 56, whereas liquid hydrogen with the same energy equivalent sells for about $1.68 (MacKenzie 1994).

Fuel cells using hydrogen are an environmentally clean, quiet, and efficient method of transforming energy into electricity and heat. Fuel cells are electrochemical devices, much like storage batteries, that utilize energy from the chemical synthesis of water to produce electricity. The fuel cell provides a way to synthesize water from hydrogen and oxygen, capturing the electrical energy released (Larminie and Dicks 2000).

Stored hydrogen is fed into a fuel-cell apparatus and oxygen is fed from the atmosphere, thus producing effective electrical energy (Larminie and Dicks 2000). The conversion of hydrogen into DC electricity using a fuel cell is about 40% efficient.

The major costs of fuel cells are the electrolytes, catalysts, and storage. Phosphoric acid fuel cells (PAFCS) and proton exchange membrane fuel cells (PEMS) are the most widely used and most efficient. PAFCS have an efficiency of 40%-45%, compared to diesel engines of 36%-39%. However, PAFCS are complex and have high costs because they operate at temperatures of 50o to 100oC (DOE 1999). A fuel cell PEM engine costs $500/kW, compared to $50/kW for a gasoline engine (DOE 1999), leading to a total price of approximately $100,000 for an automobile running on fuel cells (Ogden and Nitsch 1993)..

Hydrogen has serious explosive risks because it is difficult to contain within steel tanks at 930 atmospheres. Mixing with 4% or more oxygen will result in intense flames that will take place more rapidly than gasoline and diesel fuels (Peschka 1992). Other environmental impacts are all associated with the solar electric technologies used in hydrogen production. Water for the production of hydrogen also may be a problem in the arid regions of the US and world.

Passive Heating And Cooling Of Buildings

Approximately 20% (19 quads) of fossil energy yearly in the United States is used for space heating and cooling of buildings and for heating hot water (USBC 1999). 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 (Table 3). Tremendous potential exists for substantial energy savings through increased energy efficiency and by using solar technologies for buildings. Estimates suggest that the amount of energy lost through poorly insulated windows and doors is approximately 3.8 quads each year - approximately equal to the total oil pumped in Alaska per year (EETD 2001).

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 range from 2 to 10/kWh (Balcomb 1992).

Improvements in passive solar technology are making it more effective and less expensive than in the past (Bilgen 2000). In window designs, current research is focused on the development of super windows with high-insulating values and smart or electrochromic windows that can respond to electric current, temperature, or incident sunlight to control the admission of light energy (DOE 2000, Roos and Karlsson 1994). Use of transparent insulation materials makes window designs that transmit from 50% to 70% of incident solar energy while at the same time providing insulating values typical of 25 cm of fiber glass insulation (Chahroudi 1992, Twidell et al. 1994). Such materials have a wide range of applications beyond windows, including home heating with transparent, insulated collector-storage walls and integrated storage collectors for domestic hot water (Wittwer et al. 1991).

Although none of the passive heating and cooling technologies requires land, they are not without problems. Some indirect land-use problems may occur, such as the removal of trees, shading, and rights to the sun (Simpson and McPherson 1998). 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.

Geothermal Systems

Geothermal energy sources, which come from heat sources in the Earth's interior, are divided into 3 categories: hydrothermal, geopressured-geothermal, and hot dry rock. Geothermal energy uses natural heat present in the interior of the earth. Examples are geysers and hot springs, like those at Yellowstone National Park. The hydrothermal system is the simplest and most commonly used for electricity generation. The boiling liquid underground is produced from wells or through high internal pressure drive or by pumping the liquid. Currently in US, nearly 3,000 MW of installed electric generation comes from hydrothermal resources and is projected to increase by 1,500 MW (DOE/EIA 1991, DOE/EIA 2001).

Most of the geothermal sites for electric generation are located in California, Nevada, and Utah (DOE/EIA 1991). Electrical generation costs for geothermal plants in the west estimated to be about 8/kWh and range from 6 to 30/kWh (Gawlik and Kutscher 2000), suggesting that this technology offers potential to produce economic electricity. Projections by DOE/EIA (DOE/EIA 1991, DOE/EIA 2001) are that geothermal electric generation may grow 3 to 4 fold during the next 20 to 40 years. However, other investigations are not as optimistic and, in fact, suggest that geothermal is not a renewable energy system because the sources tend to decline over 40 to 100 years (Bradley 1997, Cassedy 2000, Youngquist 1997). Existing drilling opportunities for geothermal resources are limited to a few sites in the US and world, (Youngquist 1997; W. Youngquist, Consulting Geologist, Eugene, Oregon, personal communication, 2000).

Biogas

Wet biomass materials can be effectively converted into usable energy using 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 (65%) and carbon dioxide (35%) (Pimentel 2001). The biogas is collected in a flexible liner that is stretched over the pit that inflates like a balloon. The biogas is then used to heat the digester, heat farm buildings, a produce electricity. A large facility capable of processing the dung from 500 cows costs nearly $300,000 (EPA 2000). EPA (EPA 2000) estimates that more than 2,000 digesters could be economically installed in the United States.

The amount of biogas produced is determined by the temperature of the system, microbes present, the volatile solids content of the feedstock, and retention time. A plug-flow digester with an average manure retention time of about 16 days under winter conditions (-17.4 oC) can produce 452,000 kcal/day and uses 262,000 kcal/day to heat the digester to 35 oC. (Jewell et al. 1980). Using the same digester during the summer conditions (15.6 oC) but reducing the retention time to 10.4 days, the yield in biogas was 524,000 kcal/day and used 157,000 kcal/day for heating the digester (Jewell et al. 1980). The energy input:output ratios for these winter and summer conditions for the digester were 1:1.7 and 1:3.3, respectively. The energy output of biogas digesters rare similar today (Hartman et al. 2000, Sommer and Husted 1995).

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 (Kishore 1993). The price value of a kWh (th) biogas in India is about 6 (Dutta et al. 1997). The total cost of producing about 10 million kcal of biogas was estimated to be $321, assuming the cost of labor to be $7/hr, thus the biogas has a value of $356. Processed manure for biogas has fewer odors and retains its fertilizer value (Pimentel 2001).

Biofuels: Ethanol, Methanol, And Vegetable Oil

Petroleum, essential for the transportation sector as well as the chemical industry, makes up approximately 40% of total US energy consumption. Clearly, a shift from petroleum, as supply diminishes, to alternative liquid fuels will be essential. This analysis is focused on the potential of 3 fuel types: ethanol, methanol, and vegetable oil. These fuels when burned in internal combustion engines release less carbon monoxide and sulfur dioxide than gasoline and diesel fuels, however, because the production of most of these biofuels require more total fossil energy than is produced as a biofuel, they contribute to air pollution and global warming (Pimentel 2001).

Ethanol production in the United States using corn is heavily subsidized by public tax money (Pimentel 2001). 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 insure clean air. Furthermore, its production uses land suitable for crop production and causes environmental degradation (Pimentel 1991, Pimentel 2001, Weisz and Marshall 1980, Youngquist 1997).

The total energy input to produce 1,000 liters of ethanol in a large plant is 8.7 million kcal (Pimentel 2001). However, 1,000 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 1,000 liters of ethanol produced. Put another way, about 70% more energy is required to produce ethanol than the ethanol contains (Pimentel 2001).

Methanol can be produced from a gasifier-pyrolysis reactor using biomass as a feedstock (Hos and Groenveld 1987, Jenkins 1999). The yield from 1 ton of dry wood yields about 370 liters of methanol (Ellington et al. 1993, Osburn and Osburn 2001). For a plant with economies of scale to operate efficiently, more than 1.5 million ha of sustainable forest would be required to supply the plant (Pimentel 2001). Biomass generally is not available in such enormous quantities even from extensive forests and at acceptable prices. Most methanol today is produced from natural gas.

Processed vegetable oils from sunflower, soybean, rape, and other oil-plants can be used as a fuel in diesel engines. Unfortunately, the energetic and economic aspects of producing vegetable oils for use in diesel engines is negative (Pimentel 2001).

Transition To Renewable Energy Alternatives

Despite some environmental and economic benefits, the transition to large-scale use of renewable energy presents some difficulties. Renewable energy technologies, all of which require land for collection and production, will compete with agriculture, forestry, and urbanization for land in the US and world. The United States is at maximum use of its prime cropland for food production per capita today, but the world has only half of the cropland per capita that it needs for a diverse diet and adequate supply of essential nutrients (USBC 1999, USDA 1998). In fact, more than 3 billion people are already malnourished in the world (WHO 1996, WHO 2000). With the world and US populations expected to double in the next 50 and 70 years, respectively, all the available cropland and forest land will be required to provide vital food and forest products (PRB 2000).

The labor input for 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 (Giampietro et al. 1998, Pimentel et al. 1988).

The growing US and world population will demand increased electricity and liquid fuels. However, constraints like land availability and high investment costs will restrict the potential development of renewable energy technologies. For instance, about a 6 fold increase in installed technologies would provide the US with approximately 45 quads (thermal) of energy, less than half of current US consumption (Table 1). This level of energy production would require about 159 million ha of land (17% of US land area). While this land is currently available in the US, population growth, land use changes and increased energy consumption will probably make this development impossible in the future and diminish the contribution of renewable energy production and supply in the United States.

Worldwide, approximately 408 quads of all types of energy are used by the population of more than 6 billion people (Table 1). Using available renewable energy technologies, an estimated 200 quads of renewable energy could be produced worldwide on about 20% of the world land area. A self-sustaining renewable energy system producing 200 quads of energy per year for about 2 billion people (Ferguson 2001) would provide each person with about 5,000 liters of oil equivalents per year - approximately half of America's current consumption per year but an increase for most people of the world (Pimentel et al. 1999).

Additional complications in the transition to renewable energies exist are the relationship between the location of ideal production sites and large population centers. Ideal locations for renewable energy technologies often are remote, such as deserts of the American southwest or wind farms located kilometers offshore. While 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 $120,000 per kilometer 230 kV lines (FERC 1989). A certain percentage of the power being delivered is lost as a function of electrical resistance in the distribution cable. Currently, there are 5 complex AC electrical networks in North America and 4 of these networks are tied together by DC lines (Casazz 1996). Based on these networks, it is estimated that electricity can be transmitted up to 1,500 km.

The first priority for the US energy program should be for individuals, communities, and industries to conserve fossil fuel resources and ideally reduce consumption. Other developed countries have proven that high productivity and a high standard of living can be achieved with the use of half the energy expenditure of the United States (Pimentel et al. 1999). In the United States, fossil energy subsidies of approximately $50 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 US became a leader in the development of renewable energy technologies, then the US is likely to capture the world market for this industry (Shute 2001).

Conclusion

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. The United States would have to be committed to the development and implementation of non-fossil fuel technologies and energy conservation. The implementation of renewable energy technologies will reduce many of the current environmental problems associated with fossil fuel production and use.

The immediate priority is to speed the transition from the reliance on non-renewable 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 several renewable technologies as listed in Table 3, should provide the US 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.

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 adversely affected. It is paramount 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 utilization of energy, land, water and biological resources.

References