Green Energy Guide
A Consumer's Guide to Sustainable Electricity
Sources of Electricity
Green Energy Guide: Sources of Electricity
The kinetic energy of the wind can be harnessed with turbines that, although they are quieter, sturdier, and more efficient, are not far from the age-old windmill. Once installed, turbines produce no air or water pollution beyond a negligible amount produced during occasional maintenance. Although wind energy can only be generated in windy areas, the potential for wind energy in the United States is tremendous. It is estimated that the wind energy potential in just two states, North and South Dakota, could meet 80 percent of the nation's electricity needs. A potential drawback to wind power is that it relies on a naturally variable resource that, though renewable, can not be turned on and off according to demand. But the fact that the wind is likely to be blowing somewhere has underlied findings that up to half of overall system energy could be derived from wind power before running into problems associated with variability. Wind power now contributes less than 1 percent of the electricity used in the US, but it is also the least expensive and fastest growing renewable energy source.
The solar energy falling on the Earth each day equals nearly 500,000 times the electric power capacity of the United States. This energy can be converted directly to electricity by photovoltaic cells (PVs) which produce an electric current when struck by sunlight. Because the solar energy supply is inexhaustible, its potential for electricity generation is limited only by the efficiency at which we can capture it and the amount of surface area we can devote to it. PVs are typically 99 percent silicon, an inert substance which is the second most abundant element on Earth and a primary constituent of sand. PVs work without sound, air or water emissions, moving parts, and require little maintenance and no water. Even when you consider the manufacture of photovoltaics the environmental impacts are very low, and the PVs themselves are recyclable after their useful life ends. Currently solar power contributes less than 1 percent of our electricity. Expansion has primarily been hampered by its high cost, although this has fallen significantly in the last decade. Another drawback to PVs is that they only generate electricity when the sun is shining. At a small scale, therefore, some sort of energy storage or back-up system is required. At larger scales, however, studies and field experience have shown that integrating intermittent PV-generated electricity into the electric grid provides few technical difficulties, even when considering much higher levels of solar power usage.
Biomass refers to wood, crops, harvest residues, urban refuse, or methane gas produced by landfills that are burned to spin turbines and produce electricity. Biomas is an attractive energy source because it avoids two drawbacks accompanying most other forms of renewable energy: high cost of collection and intermittency. The solar collectors are the leaves of plants, requiring much less capital than wind turbines or PV cells and providing a convenient medium for energy storage, allowing electricity to produced on-demand, and contributing no net carbon dioxide to the atmosphere. However, if biomass were used for electricity production on a large scale, the impacts would be significant. For example, commercial applications of biomass would require vast acreage of fertile land to be committed to trees or crops grown specifically for energy production. Furthermore, biomass-fueled electricity production would also be extremely water intensive. Put in perspective, to produce electricity for just one household over the course of a year with biomass as a source would require over 25,000 gallons of water and almost three quarters of an acre of land. It also produces 232 pounds of carbon monoxide per household each year -- more than thirty times the level of any other source, as well as significant amounts of other air pollutants. Combustion of wood now contributes about 1 percent of the nation=B9s electricity supply, and the electricity generated from waste about half that amount. Waste is not considered a viable option for large-scale electricity production and is not a truly renewable resource.
The kinetic energy of flowing water can also be used to spin turbines. Hydroelectricity, which currerntly generates about 9 percent of the nation's electricity, is renewable, can produce electricity on-demand, and generates electricity with few emissions. On the other hand, a dam can have devastating impacts on the ecological systems up and downstream. In the United States alone there are more than 5,500 large dams impeding our rivers, leaving less than 2 percent of our country's 3.1 million miles of rivers and streams flowing free. An important distinction to make, however, is between large and small hydroelectric projects. Large hydro projects involve constructing a large dam on a river and flooding its river basin to create a reservoir. Small hydroelectric plants generate less than 30 megawatts of electricity and have much smaller impacts than large hydro projects, though they may not always be able to provide on-demand power because they are much more susceptible to variations in river flow. Because the most promising large dam sites in the United States have either been developed or protected, and because the general public has become outspoken about the negative impacts of large dams, dam construction has been declining.
Natural gas was formed millions of years ago when buried organic matter was subjected to very high temperatures and pressures. Although the formation process continues, the rate is negligible compared to the rate of human extraction, making natural gas a non-renewable resource. Natural gas may be found along with coal or oil, or it may be found alone; off-shore drilling is becoming more prevalent, with about one-fifth of U.S. natural gas coming from off-shore sites. Once extracted and refined, the gas is burned to create steam, which then turns turbines to produce electricity. About 15 percent of US electricity comes from natural gas combustion. Like coal, natural gas is relatively cheap, the technology involved is simple and widespread, and electricity can be produced on-demand. It produces much lower levels of air pollutants such as particulates, sulfur dioxide and nitrogen oxides than coal, but significantly more than other source apart from biomass. For instance, a household supplied only by natural gas for a year would generate 24 pounds of air-borne particulate matter which has been implicated in respiratory infection and asthma. Maybe most significant, however, is the amount of the greenhouse gas carbon dioxide emitted during natural gas combustion: over 10,000 pounds per household per year if natural gas were the sole source. Touted as the cleaner alternative to coal, this claim may be true, but it doesn't make natural gas sustainable.
Coal production has been increasing since the 1950s, and today the United States extracts huge quantities of coal (over 1 billion short tons in 1998). Currently, coal contributes over half of the nation=B9s electricity, and over 90 percent of the coal produced is used for electricity generation. Besides being cheap and abundant, the only thing that coal has to recommend it is that is can provide power on-demand. Coal mining has major impacts on terrestrial and aquatic ecosystems. In many cases, whole mountaintops are removed for coal extraction, and valleys are filled in with the waste rock (tailings). Whether it is mountain-top removal, open-pit, or underground mining, however, a major problems stems from rain filtering through the coal mine and tailings. Some of the sulfur in the coal dissolves into the water, turning it acidic; this "acid mine drainage" has impacted thousands of stream miles across the country. The combustion of coal also produces many gaseous wastes, some of which are "scrubbed" out of the emission stream in smokestacks, but many are not, including carbon dioxide. A single household being supplied solely from coal-produced electricity would generate over 61 pounds of sulfur dioxide, 60 pounds of nitrogen oxide, 30 pounds of particulates, 6 pounds of carbon monoxide, 2 pounds of volatile organics, and 17,000 pounds of carbon dioxide, and require over 7,000 gallons of water.
While every other source of electricity is basically solar energy in one form or another, nuclear power harnesses the power contained within the nuclei of atoms. Consequently, risks and impacts involved are unique. With its low emissions and low land use, nuclear seems falsely attractive at first glance, but when you begin to look at its entire lifecycle, the potentially devastating human health and safety concerns are clear. The nuclear fuel cycle begins with the mining of uranium ore (a non-renewable resource), releasing radon (radioactive gas) and creating large amounts of radioactive waste rock (tailings). The uranium is then processed in a highly energy-intensive process and fabricated into fuel rods. Nuclear power plants produce energy through either fission reactions (when an atom of a radioactive element such as uranium or plutonium collides with a neutron, splitting the element apart) or fusion reactions (where two elements collide at high speed, forming one or more heavier elements). In both cases, a large amount of heat is released which is used to create steam to turn turbines and generate electrical energy.
Along with heat, a significant amount of radiation is produced. Radiation is extremely dangerous to people and biological systems, causing acute illness or death at high doses, or cancer or genetic mutations at lower doses. Although most of this radiation is not released into the atmosphere, it does not disappear. It is contained instead in large amounts of radioactive waste, which remains hazardous for many thousands of years. No permanent facility for the storage of high-level waste exists, and even if one is built, it may adequately contain the waste for only 1,000 years. Routine plant operation produces upwards of 3.64 tons of low-level waste per GWh of electricity, and about 30 to 40 tons of high-level waste over the course of a power plant year. The largest volumes of nuclear waste, however, stem from the decommissioning of plants at the end of their life spans. Current decommissioning practices of reactors consist of processing some of the waste, but "mothballing" the reactor core and much of the rest of the reactor in the hopes that better methods of dealing with its might be found in the future.
One of the most frightening propositions in the modern world is the event of an accident involving a nuclear power plant or radioactive waste. The U.S. Nuclear Regulatory Commission (NRC) has estimated that there is a 45 percent chance of a severe accident occurring in the next twenty years at one of the 109 U.S. reactors. The possibility of an accident and the unavoidable reality of the nuclear waste highlights a moral issue: what are we leaving for our children and all future generations? The risks that stem from the air pollution emitted during fossil fuel combustion are significant, but they have short-term impacts only. The same people (or at least generation of people) that are suffering the costs are also receiving the benefits of that electricity. In contrast, the legacy of the nuclear electricity being produced and consumed today will continue to pose a threat for many generations to come.
Nuclear power plants multiplied rapidly during the late sixties and early seventies, but interest declined for various reasons including high operating costs, and public disfavor over the risk of serious accidents after the Three Mile Island incident in 1979. Currently, nuclear provides the United State with a little less than 20 percent of its electricity. More than half of the current reactors are scheduled to retired in the next 20 years and no new ones are scheduled to be built. Consumers can send a loud message to the electricity industry by refusing to buy power that includes nuclear in the mix.
Table 1. Pros and Cons of Electricity Sources