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Ene rgy has different meanings in different contexts. In layperson’s terms energy can be defined as the measure of potential to bring changes in a system. In physics parlance, energy refers to capacity of doing work. Energy can occur in various forms: kinetic, potential, electromagnetic, sound, and so on. Energy of a moving car is kinetic energy, where as the energy of water stored in a dam is potential energy. Energy can be converted from one form to another. For example, when water falls from a dam, the potential energy of water gets converted into kinetic energy, which drives turbines to produce electrical energy. When a car crashes into a wall, the kinetic energy of the car is converted into heat and sound energy. When such a conversion occurs, some of the useful energy is lost. As a result, not all energy can be converted to useful work. However, energy can neither be created nor destroyed (First Law of Thermodynamics). In SI units, energy is measured in Joule (J), which is equivalent to the work done when one Newton force is applied to move an object by 1 meter. Maintaining vital cellular activities that are necessary for survival requires a minimum of 4,000 kJ/day; whereas 20,000 kJ/day are required for activities such as bicycle riding, jogging, or construction work.
Energy sources have been broadly categorized as renewable and nonrenewable. Renewable energy refers to the energy that can not be depleted either due to its short-time frame of regeneration (e.g., biomass, ethanol from corn) or a source itself is inexhaustible for a considerable period of time running into millions of years. Traditionally, energy from sources such as solar, wind, geothermal, and biomass are considered to be renewable. Nonrenewable energy, on the other hand, can be depleted faster than it is regenerated, which usually occurs over a geologic time frame, that is, millions of years. Examples of nonrenewable energy sources are coal, oil, and natural gas.
Energy is very critical for industry, economy, and ecosystems since without energy, none of these can function and would not have existed today. The primary energy driving the earth system is solar energy, with tidal (lunar) and crustal energies being the next two most prominent sources. One example is the hydrological cycle. Solar energy heats up the oceans evaporating water into the atmosphere. Water vapors rise up due to lower density and eventually cool down to form clouds. Precipitation in the form of ice, snow and rain occurs from the clouds, which feed rivers, lakes, and groundwater, providing much-needed fresh water for humans and other living organisms. In ecosystems, primary producers (plants, green algae, diatoms, etc.) capture solar energy through photosynthesis and store it in carbohydrates, ATP, and acetate in the form of chemical energy. This stored chemical energy meets the energy demand of all other species higher up in the food chain or at higher trophic levels, including detritus. In addition, producers also release oxygen, an important component of cellular respiration that provides energy for all life functions of living organisms including movement, growth, and reproduction. Detritus plays an important role in an ecosystem by breaking down the dead plants and animals and releasing nutrients back into the ecosystems. In doing so, detritus derives energy from dead plants and animals. These nutrients, in turn, support the growth of primary producers. Industry derives materials and energy from the ecosystems that fuel growth and economic development. Fuels such as coal, oil, and natural gases, which are predominantly used by economy, are derived from energy stored in dead plants and animals through a series of transformations over a period of millions of years. Many of valuable materials we derive from ecosystems such as medicines, timber, foods, and biofuels are all products of biochemical pathways involving solar energy.
However, energy can also be destructive. Violent meteorological processes such as lightening, tornadoes, snow avalanches, and geological processes such as earthquakes, tsunamis, volcanoes are all manifestations of highly concentrated forms of energy derived either from sunlight or from the energy stored in the mantle of the earth.
History of Energy Use
Other than sunlight, fire from biomass is probably the earliest reported use of energy by humans. In preindustrial societies, wood, straw, and charcoal were used to meet energy needs such as home heating, cooking, and ore smelting. Many such sources have been severely depleted due to their use in a nonrenewable manner. Energy required for labor was derived from the muscular power of humans and animals. For example, people used to plow agricultural fields with the help of animals. Seeding, planting and harvesting of crops used to be done manually by hand. Activities such as grain milling and transportation involved the use of cattle such as water buffaloes.
More sophisticated types of energy devices such as waterwheels and windmills were introduced only toward the end of the preindustrial period. In the early eighteenth century, the renowned scientist Lavoisier designed a 1,700 degree C solar furnace that concentrated solar energy and converted it to heat.
With advent of the industrial revolution, the demand, use, and diversification of energy substantially increased. For example, per capita annual combustion of fuels was 500 kg of wood equivalent by 1850s, which is very nominal compared to the current energy consumption. Taking into account the differences in energy efficiencies, annual per capita consumption of energy in 1995 was 20 times higher than in the 1850s. From early 1990s, electricity production from fossil fuels began. In 1990, less than 1 percent of fossil fuels were devoted to electricity production, which rose to 25 percent by 1990. Hydroelectricity came into existence in mid-1890s, and its global production has been increasing ever since. However, hydroelectricity production has almost leveled off in the United States since the 1970s, as virtually all viable sources of hydroelectricity have been utilized.
In 1956, the dream of the harnessing nuclear energy through controlled atomic fission was realized when the first commercial fission reactor came into operation. Initially nuclear energy was heralded as the energy of the future and a viable substitute for fossil fuels. In a flurry of activities, several nuclear power plants were built in the 1960s and 1970s, mostly in the developed countries. However, due to high construction costs, stringent safety requirements, containment of nuclear waste and disposal, and possibility of nuclear disasters (Three Mile Island Accident), and low crude oil prices, nuclear power plants became a less-preferred option. Very few nuclear plants were commissioned after 1980, and the much-hyped nuclear energy solution failed to live up to its earlier prediction.
In last 20 years, alternative sources of energy such as solar, wind, and biomass have received more attention. In terms of electricity production, wind energy is emerging as an attractive option. Production of biofuels such as ethanol and biodiesel is gradually increasing, which find their applications in automobiles as substitutes for gasoline and diesel. Brazil leads the world in biofuel production. Brazil produced 14 million m3 of ethanol from sugarcane in 2002. In the renewable energy category, hydroelectricity still predominates. In 1997, the share of hydropower in the renewable sector was 55 percent, followed by biofuels (38 percent) and geothermal (5 percent), whereas solar and wind energy accounted for only 2 percent of renewable energy produced in the United States. Nonrenewable energy including oil, gas, and coal had the largest share (86 percent) of the total marketed energy worldwide in 2003. The total primary energy consumed worldwide in 2005 was 9800.8 million tons oil equivalent (toe).
Planetary Energy Flows
The earth receives 3.93x 1024 J/yr of solar insolation. The oceans capture 5.2 x 1019 J/yr of tidal energy resulting from gravitational forces of attraction of the sun and moon acting on the earth. The earth’s crust draws 4.74 x 1020 J/yr of the heat energy from the mantle. In addition, it derives the heat energy from the radioactive decay of radioactive elements present in the interior part of the earth that equals 1.98 x 1020 J/yr, making the total crustal heat energy 6.72 x 1020 J/yr. Solar insolation and tidal energy contribute 6.49 x 1020 J/yr of heat by passing some of the energy as compression and chemical potentials. Thus, the total heat outflow in the earth system is 13.21 x 1020 J/yr. An ecosystem captures only 1% of solar energy falling on it. When energy is transferred from autotrophs to consumers at higher trophic levels, energy gets lost. Only 10 percent of the energy entering a given trophic level gets transferred to the next trophic level.
Fossil Fuels
Fossils fuels comprising petroleum, natural gas, and coal are primary energy sources. All fossil fuels are products of a series of biological, chemical, and physical transformations of plant and animal remains over geological time frames. It is estimated that total reserves of fossil fuels in the earth was about 317,700 x 1018 J in 1999.
Coal: Coal, which led to the growth of fossil-fueled civilization, is a solid black or brown mass obtained from the arrested decay of the metamorphosed remains of plants that were buried in marshes and bogs millions of years ago. First plant debris got converted to peat through bacterial and chemical transformation. Thereafter, a series of actions involving heat and pressure converted peat into various types of coal. Coals are not identical because of differences in original vegetation, the extent of transformation, the magnitude and duration of pressures, and temperatures. Coal primarily consists of carbon and small amounts of sulfur, nitrogen, and ash. Good-quality coals-anthracite and bituminous coal-were obtained from the wood of large, scaly barked trees that grew in large coastal swamps about 2 million years ago. The low-quality coals-lignites-are the youngest, and soft with a brown tinge. Due to appreciable amounts of moisture, sulfur, and ash, they have low heat content and emit substantial amounts of oxides of sulfur and nitrogen. Coal is extracted either by surface mining or underground mining.
Petroleum: Petroleum, also known as rock oil, is a liquid present in the upper earth crust. Like coal, petroleum is derived from biological, chemical and physical transformations of plant and animal debris over millions of years. Petroleum is a complex mixture of hydrocarbons with a varying molecular weight and physical and chemical attributes. Petroleum, being a hydrocarbon, primarily consists of carbon and hydrogen with small amounts of nitrogen and sulfur and a few metals. Petroleum is processed and refined to produce gasoline, diesel, jet fuel, methyl tertiary butyl ether (MTBE), tar, and other products.
Natural Gas: Natural gas occurs in the underground reservoirs of porous rocks. It also occurs as a mixture with petroleum and is recovered by petroleum refineries. Natural gas consists of methane as a major component (70-90 percent by volume) with smaller amounts of ethane, propane, butane and other paraffins. In addition, natural gas consists of inert gases such as nitrogen and carbon dioxide along with hydrogen sulfide. Natural gas is distributed to consumers (industrial, commercial, residential) via pipelines. Natural gas is also liquefied and transported by special tankers.
Biomass Energy
Biomass refers to plant-based organic products such as wood, corn, soybean, crop residues, and organic wastes. Biomass can be burned directly to produce heat and electricity or converted to liquid fuels and gas through pyrolysis, fermentation, and anaerobic digestion. Biomass is an important part of the renewable energy supply in developing countries and is used primarily for heating and cooking. It is gaining importance in developed countries for electricity production and automobile transport. Biomass has chemical energy stored in carbohydrates and other complex organic compounds that can be harnessed for different uses. Biomass as an energy raw material is attractive due to its wide distribution, availability, and renewability, which make it possible to develop decentralized energy production and distribution systems. However, its low energy content, the need for drying, transportation, and competition for land with other uses such as food, wood, and shelter undermine its advantages.
Biomass, including wood, switchgrass, and bagasse has been used to generate process heat, steam, and electricity either through direct burning or gasification. The biomass gasifiers yield gaseous products whose composition varies depending on the nature of feedstock and reactor conditions. Short-rotation woody crops such as sycamore, poplar, and eucalyptus have been studied for use in electricity production through cofiring or gasification. Biomass can also be converted to gaseous products and coke through pyrolysis that can be used for space and water heating, cooking, and process heat. Ethanol is derived from sugarcane and corn by fermentation, and has been used as a transportation fuel. In the United States, the total ethanol production from corn topped 4 billion gallons in 2005. Recent studies suggest that ethanol can also be produced from cellulosic feedstock, expanding the possibility of ethanol production. It is estimated that the global potential for ethanol production from crop residues and wasted crops could be 442 GL/year. Biodiesel has been synthesized from a variety of oilseed plants, including soybean, jatropha, rapeseed, and sunflower through transesterification.
Biomass such as municipal wastes and cattle manure is used to produce methane through anaerobic digestion. Methane digesters are more popular in developing countries. Thousands of homes have benefited from installations of methane digesters. These digesters mainly rely on cattle manure for energy feedstock. A typical digester consists of a digester chamber, where an anaerobic reaction occurs; a metal dome with a pipe to collect biogas (methane); an inlet to feed the digester with manure; and an outlet connected to overflow tank that collects digested slurry. The digested slurry is applied to farmlands to improve soil fertility and increase productivity. Biogas is mainly used for cooking and lighting. Since biomass burns more cleanly than fuel wood, it has reduced indoor air pollution and respiratory diseases, particularly for women, since women normally prepare meals for their families in developing countries.
Solar Energy
Due to widespread availability of solar power, people have been harnessing solar energy in many ways since time immemorial. An example is the passive energy system utilized by the homes of the Anazasi Indian Tribes in the southwest United States. The solar energy technologies can be broadly categorized into four groups: passive heating, active heating, solar-thermal electric, and solar photovoltaic.
Passive solar technology relies on design and placement of windows and walls to optimize heat collection, and retention in buildings. The characteristics of building materials such as cement, clay, stones are also taken into consideration. The term passive implies the absence of moving parts and controls. Active solar technology, on the other hand, is designed to capture greater amount of available solar energy by utilizing collectors and a circulating coolant that transfers heat from the collector to the point of use. The term active implies that it has moving parts and controls. A solar water heater is one example of active technology. There are several types of collectors that have been used: flat-plate collectors, focusing collectors, evacuated tube collectors, and parabolic dish solar collectors. Of these, flat-plate collectors are most widely used, mostly in homes.
The principle of a solar-thermal electric system is same as that of active solar heating. The only difference is that the heat captured from the coolant is used to heat a primer fluid that can be pressurized water or compressed air that drives a turbo generator unit to produce electricity. Focusing or parabolic dish solar collectors are used for such an application.
A solar photovoltaic system is based on the principle of the photoelectric effect. A photovoltaic system converts solar energy to electrical energy when solar rays fall on the p-n photvoltaic device, causing the release and migration of electrons from an n-type semiconductor to a p-type semiconductor. Silicon dioxide, cadmium telluride, and copper indium diselenide are the commonly used semiconductors in photovoltaic cells. Features such as simplicity, low maintenance requirements, absence of moving parts, and scalability make it attractive. Nonetheless, the main concerns of photovoltaic have been the cost and efficiency. efficiency of solar-electricity conversion has improved from a few percent to 20 percent, and costs have decreased from $250/W to $2.5/W or less. These numbers are still not good enough for large-scale commercial production and adoption, especially considering low costs of fossil fuels.
Wind Energy
Wind power technology utilizes the energy of the sufficiently strong winds to drive turbines and produce electricity. The differential heating of the earth’s land and sea surfaces produces winds by creating a pressure gradient. Air moves from the area of high pressure to low pressure, creating a wind. Wind electricity production is basically the extension of traditional windmills. Wind power plants are usually installed in the areas that experience regular and reasonably strong winds with speeds greater than 5.5 m/s. Theoretically, it is possible for the earth’s winds to provide 5,800 quadrillion BTUs of energy per year, which is 15 times more than the present world energy demand.
The worldwide wind-electric capacity has been increasing steadily. The world wind power capacity was 58,982 megawatts (MW) in 2005, which was less than 1 percent of the worldwide electricity supply. In 2005, Germany was leading wind power production with 18,428 MW capacity, followed by Spain, The United States, India, and Denmark. Although wind energy is relatively economical among renewable energy alternatives, concerns about aesthetics and noise pollution, failures of some product lines, remoteness of suitable sites from highly populated areas requiring high voltage transmission systems, and daily and seasonal variation in wind speed are obstructing its rapid expansion.
Geothermal Energy
Geothermal energy is the energy extracted from the porous and permeable hot rocks with or without fluid present in the earth’s crust, a few miles below the surface. The upward conduction and convection of the heat from the mantle, and the heat energy produced by radioactive disintegration of radioactive elements heat up the rocks. Occasionally, magma also intrudes the earth crust, transferring heat to the rocks. At places where subterranean faults and cracks are present, rainwater and snowmelt seep underground and come in contact with the hot rocks, where the water is heated and returns back to the surface in the form of hot springs, geysers, and mud spots. If the heated water cannot rise to the surface due to the presence of impermeable rock above, it fills the pores and cracks of the hot rocks below, creating geothermal reservoirs. The temperatures of water in geothermal reservoirs are far greater than those of hot springs, reaching more than 350 degrees C. The hot water can exist as a supercritical liquid or saturated steam, and can be extracted by drilling and used for electricity generation or space heating. The hot water either rises to the surface naturally or has to be pumped up. Generally, shallower geothermal reservoirs with lower temperatures (41-149 degrees C) are used for heat ing in spas, greenhouses, industry, and homes.
The majority of geothermal power plants under operation today are flashed steam plants. When the hot water is suddenly released from the reservoir pressure, it boils and produces steam, which drives the turbines and generates electricity. To maintain the reservoir pressure and recharge the reservoir, cold water is recycled back to the reservoir. Since the installation of the first geothermal system at Larderello, Italy in 1904, its use has increased worldwide, with the current production capacity standing at 8,000 MW.
Nuclear Energy
Nuclear power has a share of 17 percent of the world’s electricity supply and contributes about 7 percent of the world’s energy supply. Nuclear energy is primarily derived from the atomic fission of heavy isotopes such as 235U and 239Pu. When fission of heavy isotopes occurs upon neutron bombardment, it is accompanied by the release of more neutrons and a net mass loss. This lost mass manifests itself in the form of energy according to the famous equation, E = MC2. When a fission reaction occurs uncontrollably in a critical mass, a massive amount of energy is released, which is the basis of atomic bombs used in the World War II. However, if the fission reaction is controlled and kept in a steady state by using control rods such as cadmium that capture neutrons, thus preventing them from causing more fission reactions, heat energy can be generated in a sustained manner. This heat energy is used to produce steam that drives turbines to produce electricity. The first constructive application of nuclear energy was the nuclear driven submarine that used a small boiling water reactor.
There are different types of nuclear reactors used for producing electricity. They are: light-water reactors (LWR), pressurized-water reactors (PWR), boiling water reactors (BWR), large tube type reactors (RBMK), heavy-water cooled reactors (also known as CANDU), gas-cooled reactors (GCR), and liquid metal reactors (LMR).
An important component of nuclear energy is the fuel itself. The fuel, mainly uranium, is extracted from earth as uranium-bearing ore by surface mining. Uranium is separated from its ore in a chemical leaching fatility as U3O8. This is followed by its conversion to gaseous uranium, UF6, which facilitates its enrichment via gaseous diffusion or centrifuge-based process. The enriched UF6 is converted into UO2 and fabricated into rods for use in nuclear reactors. The enriched uranium contains 3% or more of 235U. When the fuel rods no longer become usable, they have to be removed and stored as spent fuels to avoid radioactive contamination in the surrounding environment. The long-lasting containment of the spent fuel is one of the concerns afflicting the nuclear power technology.
Hydropower
Hydropower is the largest renewable energy used in the world and contributes 20 percent of the worldwide electricity production. Hydropower comes from moving or falling water that drives turbines and generates electricity. In the process the potential energy of water is converted into electric energy. This has been possible due to the hydrologic cycle, which is driven by the solar energy. The amount of power that can be extracted from water is a function of the head (difference in height between the water’s outflow and the turbine), volumetric flow rate of water, and efficiency of the turbine.
Energy stored in water is tapped in three different ways: creating a reservoir by dam construction, diversion hydropower in which a part of the river is diverted through a canal and made to fall from a suitable location that provides adequate head, and pumped storage, wherein power in off-peak hours is used to pump the water from the source to the reservoir located at the higher elevation and its energy is subsequently tapped during the peak hours. Hydropower installations are economical and known for robustness and durability. Some hydropower plants are operating even after 100 years. Canada is the largest producer of hydropower, which meets 70 percent of the total electricity demand. Virtually all of Norway’s electricity comes from hydropower. Iceland meets 83 percent of its electricity demand from hydropower. Overall, it is estimated that it is economically feasible to harness more than 7,300 TWh/yr of hydropower worldwide.
Tidal Energy
Since ocean tides embody vast amounts of energy, they have become a part of an effort to harness earth’s renewable energy dating back to medieval periods. Tidal waves offer possibilities of harnessing energy in two ways: kinetic energy that results from the currents between the high (surging) and low (ebbing) tides, and potential energy due to the head between high and low tides. However, all tidal energy installations at present exploit the potential energy of tides, even though harnessing kinetic energy also looks feasible. High and low tides are due to the earth’s rotation and gravitational force of attraction between the moon and earth, and the sun and earth.
To capture the potential energy, barrages consisting of sluices and turbines are built to trap the ocean water in the basin during the high tides. During the low tides, the head is created between the water levels inside and outside the barrage. Due to this head, water flows back to the ocean when sluices are opened, thereby driving turbines and generating electricity. This mode of operation is called ebb generation. Alternatively, tidal energy can be captured through a flood generation method. A barrage is built to hold back the incoming high tides that create a head difference across the barrage. As water flows into the basin, turbines rotate producing electricity. This is a less efficient mode of operation A tidal power plant cannot provide continuous electricity, because high and low tides occur only twice a day. Typically, a conventional tidal plant generates electricity for 6-10 hours a day irrespective of a mode of operation. Today the worldwide tidal power capacity stands at about 11,000 MW.
Energy Use
Energy finds its use in every facet of human and ecosystem activity, such as transportation, industry, and commercial and residential buildings.
Transportation is an important component of our daily life and, on average, a person spends 10-15 percent of their income for transportation. Energy consumption in the transportation sector accounts for one-fourth of the total national energy use in the developed countries. Most of this energy goes into driving personal vehicles and heavy trucks. Automobiles require 5,874 Btu of energy per mile per vehicle. Sport utility vehicles and light trucks consume even more energy per mile (7,247 Btu/mile). Air transportation consumes about 10,481 Btu/passenger-mile. Almost all personal vehicles and other means of transport are fueled by petroleum although renewable energy is finding its way in. For example, natural gas constitutes 2.5 percent of energy consumption in the transportation sector whereas the electricity accounts for 1.2 percent in the United States.
Industry accounts for the largest energy use in the world. In the United States, the industry sector consumes about 35 percent of the total national energy output. Worldwide it accounted for 33 percent of the total energy consumption in 2003. The most energy-intensive industries are paper, chemicals, primary metals, and petroleum. Fossil fuels are used in the petroleum and petrochemical industries not only as fuels, but also as feedstock. Since the ma jority of energy used in industry comes from fossil fuels, carbon dioxide emissions from the industries is significant. Industry contributes about 20 percent of the total air pollution.
The reason why commercial and residential buildings consume a substantial amount of energy is that energy is required not only for their construction but also for operation and maintenance. Production of building materials are highly energy intensive. To produce 1 ton of aluminum requires 150-220 GJ of energy, whereas 1 ton of steel needs 25-45 GJ. However, other building materials such as brick, concrete, and wood consume far less energy. In residential buildings, space and water heating alone accounts for 80 percent (worldwide average) of the total energy use in buildings. Refrigeration and lighting accounts only 9 percent of energy consumption in buildings.
Environmental Impacts
Whether energy is renewable or nonrenewable, impacts of energy on the environment at all stages, from the cradle to grave, is inevitable. Even seemingly benign technologies such as solar and wind have indirect impacts on the environment. Manufacture of components used in wind turbines and blades require fossil fuels, which emit greenhouse gases and other air pollutants. Wind power plants have been criticized for their impacts on natural aesthetics and threat to certain bird species. Solar cells and batteries use toxic chemicals that need to be disposed off carefully or recycled. Production of biofuels requires agrochemicals, fossil fuels, and capital equipment that emit harmful pollutants into the environment directly or indirectly. Agrochemicals such as pesticides and fertilizers impact surface water bodies (eutrophication, aquatic toxicity) whereas fossil fuel use emits greenhouse gases and other air pollutants including PM10, volatile organic compounds, sulfur dioxide, carbon monoxide, etc. Large-scale hydropower developments alter river and riverside habitats, disrupt sediment flow and natural fish migration, submerge large lands, and displace local communities.
The magnitude of environmental impacts can become severe with nonrenewable energy. For example, underground mining in the Appalachian regions of the United States has severely contaminated local water supplies, rivers and streams mainly from acid mine drainage. Surface mining of coal left thousands of hectares in Appalachia and midwest denuded that they could not be restored or reclaimed for other uses. Also, acid drainage and spoils banks were more severe.
Moreover, when coal is finally combusted in power plants or other industry, it releases carbon dioxide (CO2), nitrogen oxides, sulfur oxides, and other air pollutants. Sulfur and nitrogen oxides are formed from sulfur and nitrogen present in the coal as impurities. Acid rain caused by sulfur-containing coal burning especially from power plants has been documented, which has had serious impacts on some lakes and streams of northeastern United States and Canada. In addition, coal combustion can cause serious health problems, such as respiratory diseases and irritations. The deadly smog that killed thousands of people in London in 1952 was associated with coal combustion. Coal power plants are also blamed for mercury emissions.
Use of coal and other fossil fuels in electricity production (diesel, gasoline, natural gas, transportation and industry) has dramatically increased since the last century, leading to an unprecedented rise in carbon dioxide levels. Carbon dioxide is believed to be a major culprit behind global climate change. It is estimated that 22 gigatons (Gt) of CO2 are released into the atmosphere every year from combustion of fossil fuels. Consequences of climate change can be serious, such as polar ice melting causing a rise in sea levels and subsequent submerging of coastal cities, extreme weather patterns such as extended drought, heavy rainfalls, and hurricanes, loss of species, and emergence of new tropical diseases.
Nuclear energy also has its share of environmental impacts and critical health and safety issues. Since the fuel, uranium, used in the nuclear reactor has to be mined, it presents similar environmental problems as other mining activities, which include destruction of the local habitats and contamination of water bodies. Workers working in uranium milling can be exposed to harmful radiation. In addition to the safety issues that arose in the context of the Chernobyl and Three Mile Island accidents, the long-term disposal of spent nuclear fuels is another unresolved problem. These spent fuels continue to emit harmful radioactive rays even for thousands of years due to long half-lives of radioactive isotopes. The spent fuels have to be isolated and stored in a safe and remote place, which creates unique technological and institutional problems.
The Future
What kind of energy mix we will have in the future is largely determined by the availability of fossil fuels and their cost, as well as the energy needs and corresponding energy policies of individual countries and their collective global strategies. Costs of renewable energy technologies such as solar and wind have come down significantly over the decades. However, renewable technologies, except hydropower, are still expensive and cannot survive without subsidies and incentives. Further decrease in costs is possible, but will not occur immediately. As long as costs of fossil fuels remain low, expansion of renewable energy technologies is likely to occur at a slow pace, and the current energy mix may remain unaltered for the near future. Intermittent or variable energy production and diffuse nature of renewables make them unsuitable for distribution over large areas. The proximity of renewable resources to the major population centers may improve their appeal since they require little investments in transmission and distribution networks. Renewables are an attractive option for decentralized energy production.
Bibliography:
- S. Cassedy and P. Z. Grossman, Introduction to Energy: Resources, Technology, and Society, 2nd ed. (Cambridge University Press, 1998);
- Kim and B.E Dale, “Global Potential Bioethanol Production from Wasted Crops and Crop Residues,” Biomass and Bioenergy (vol 26, 2004);
- Levenspiel, Understanding Engineering Thermo (Prentice-Hall, 1996);
- Oak Ridge National Laboratory (ORNL), Transportation Energy Data Book, 18th (U.S. Department of Energy, 1998);
- V. Smil, Energies: An Illustrated Guide to the Biosphere and Civilization (The MIT Press, 1999);
- W. Tester, E. M. Drake, M.J. Driscoll, M. W. Golay, and W. A. Peters, Sustainable Energy: Choosing Among Options (The MIT Press, 2005).
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