Alternative Energy Sources
Alternative Energy Sources
Alternative Energy Sources
Nonrenewable fossil fuels—coal, petroleum, and natural gas—provide more than 85% of the energy used around the world. In the United States, fossil fuels comprise approximately 80% of the total energy supply, nuclear power provides about 7.7% and all renewable energy sources provide about 7%. (Note that these figures lump together electricity and all other forms of energy use, such as heat and transport fuel; in practice, electricity is best treated as a special category of energy consumption.) Wind power, active and passive solar systems, geothermal energy, and biomass are examples of renewable or alternative energy sources. Although such alternative sources make up a small fraction of total energy production today, their share is growing rapidly. As of 2006, wind power was the cheapest form of large-scale electric generation to install (measured by kilowatt-hours delivered)—cheaper than new coal plants, new nuclear plants, or solar cells, for example.
Scientists estimate that easily extractable fossil fuels will be largely used up during the twenty-first century: known petroleum reserves will last less than 50 years at current rates of use; the rate of discovery of new reserves is decreasing. Much larger reserves of coal exist, but could be extracted only at catastrophic environmental cost. Proponents of nuclear power are today urging that nuclear power has many virtues, including a zero output of greenhouse gasses such as carbon dioxide (released by burning fossil fuels). Opponents
of nuclear power urge that it is unacceptable because of its high cost, its vulnerability to military and terror attack, possible accidental releases of radioactivity during fuel processing and operation, and the challenge of waste disposal. Further, critics argue that nuclear-power technologies cannot be disseminated globally without spreading at the same time much of the materials and know-how for producing nuclear weapons. For example, the U.S. government was concerned in 2006 that the Iranian government might be obtaining material for a secret nuclear-weapons program from its civilian nuclear power program.
Achieving wider use of renewable sources of energy is thus, seen by many planners as key for a sustainable, affordable, and peaceful global economy. In 2002, the 15-nation European Union declared its intention to shift away from both fossil fuels and nuclear power, with an initial goal of generating 12% of its total energy and 22% of its electricity from renewable sources by 2010.
The exact contribution that alternative energy sources make to the total primary energy used around the world is not known. However, there is no fundamental physical reason why the fraction of world energy supply produced by alternative (also called “renewable”) sources should not be 100%. Indeed, in the long run, renewables (and, some argue, nuclear power) are the only options: there is only so much fossil fuel in the world, and if it is used it must someday all be gone. (The theory that Earth is continually producing new petroleum from its interior is not accepted by the scientific community.) The primary barriers to the use of renewable or alternative sources for all energy are (a) lingering high cost of most renewable sources compared to fossil fuel, despite constantly declining prices for renewables, and (b) the sheer magnitude of industrial society’s energy demands, it being difficult to harvest so much energy from natural fluxes such as sunlight and wind. A combination of increased efficiency of use and decreased prices for renewables due to economies of scale and technical advances could make it possible, eventually, for renewables to take on the lion’s share of world energy demand. This seems unlikely to happen any time within the next half-century or so, however.
Although today considered an alternative energy source, wind power is one of the earliest forms of energy harvested by humans. Wind is caused by the uneven heating of Earth’s surface, and its energy is equivalent to about 2% of the solar energy reaching the planet. The amount of energy theoretically available from wind is thus, very great, although it would be neither practical, wise, nor necessary to intercept more than a tiny percentage of the world’s total windflow.
Wind is usually harvested by windmills, which may either supply mechanical energy directly to machinery or drive generators to produce electricity.
Energy must be distinguished from electricity; electricity is not a source of energy, but a form of it. In processes that burn chemical or nuclear fuel to generate electricity, more energy is lost as heat than is delivered as electricity. A windmill, likewise, supplies less usable energy when it is used to generate electricity than when it is used to do mechanical work directly. Electricity has the positive qualities of being transmissible over long distances via powerlines and of being useful for millions of applications—lighting, motors, electronics, and so on.
The ideal location for a windmill generator is in a place with constant and relatively fast winds and no obstacles such as tall buildings or trees. An efficient modern windmill can extract about one third of the energy of the wind passing through it. The estimated cost of generating one kilowatt-hour (the amount of energy consumed by ten 100-watt light bulbs in one hour) by wind power was about 3–6 cents in 2006 as compared to about 15 cents for new nuclear power, and was decreasing rapidly. California leads the United States in utilization of wind power, producing approximately 1.5% of its electric usage in 2006 from wind, more than enough to light San Francisco. Denmark led the world in this respect in 2006, obtaining 23% of its electricity from windmills (and six more percent from other renewable sources). Germany was obtaining about 6% of its energy from wind in 2006 and Spain about 8%, with vigorous government-funded programs to build more capacity. Wind power cannot supply more than a certain fraction of an electric system’s power due to the window’s stop-and-go nature, but distributing wind turbines over geographic areas linked by transmission lines softens this effect. Ultimately, some form of energy storage would be necessary if wind were to supply the majority of a society’s electric supply. Storing energy would allow windmills to supply a steady, reliable flow of power.
Solar energy can be utilized either directly as heat or indirectly as electricity produced using photovoltaic cells or steam generators. Greenhouses and solariums are common examples of the direct use of solar energy, having glass surfaces that allow the passage of visible light from the sun but slow the escape of heat and infrared radiation. Another direct method involves flat-plate solar collectors that can be mounted on rooftops to provide energy needed for water heating or space heating. Windows and collectors are considered passive systems for harnessing solar energy. Active solar systems use fans, pumps, or other machinery to transport heat derived from sunlight.
Photovoltaic cells are flat electronic devices that convert some of the light that falls on them directly to electricity. Typical commercial photovoltaic cells convert 10–20% of the sunlight that falls on them to electricity. In the laboratory, the highest efficiency demonstrated so far is over 30%. (Photovoltaic efficiency is important even though sunlight is free; higher-efficiency cells produce more power in a limited space, such as on a rooftop.) Photovoltaics are already economic for use in remote applications, such as highway construction signs, spacecraft, lighthouses, boats, rural villages, and isolated homes, and large-scale initiatives are under way in California and other places to produce thousands of megawatts of power from rooftop-mounted photovoltaic systems. California alone was committed, as of 2006, to installing 3, 000 megawatts of new photovoltaic capacity by 2017.
Starting in the late 1990s, Germany and Japan began ambitious government programs to promote photovoltaic energy, and as of 2006 were the world’s leaders in the manufacture and installation of solar cells. Germany, the world’s sixth-largest electricity consumer, installed 837 megawatts of solar capacity in 2005 alone.
Thermal-electric solar systems have also been developed using tracking circuits that follow the sun and mirrored reflectors that concentrate its energy. These systems develop intense heat that generates steam, which in turn drives a turbine generator to produce electricity. These centralized solar plants, however, have been made less relatively economical by declining prices for photovoltaic cells. Photovoltaic cells also have extraordinarily low operating costs, since they have no moving parts (unless governed by heliostats that turn them to face the moving sun).
Geothermal energy is heat generated naturally in the interior of the earth, and can be used either directly, as heat, or indirectly, to generate electricity. Geothermal energy can be used to generate electricity by the flashed-steam method, in which high-temperature geothermal brine is used as a heat source to convert water injected from the surface into steam. The steam is used to turn a turbine and generator, which produces electricity. When geothermal wells are not hot enough to create steam, a fluid that evaporates at a lower temperature than water, such as isobutane or ammonia, can be used in a closed loop in which the geothermal heat evaporates the fluid to run a turbine and the cooled vapor is recondensed and reused. More than 20 countries utilize geothermal energy, including Iceland, Italy, Japan, Mexico, Russia, and the United States. Unlike solar energy and wind power, geothermal energy may contribute to air pollution and may raise dissolved salts and toxic elements such as mercury and arsenic to the surface.
Although there are several ways of utilizing energy from the oceans, the most promising are the harnessing of tidal power and ocean thermal energy conversion. The power of oceanic tides is based on the difference between the (usually) twice-daily high and low water levels. In order for tidal power to be effective, this difference in height must exceed about 15 ft (3 m). There are only a few places in the world where such differences exist, however, including the Bay of Fundy in Canada and a few sites in China.
Oceanic thermal energy conversion utilizes temperature differences rather than tides. Ocean temperature is commonly stratified, especially near the tropics; that is, the ocean is warmer at the surface and cooler at depth. Thermal energy conversion takes advantage of this fact by using a fluid with a low boiling point, such as ammonia. Vapor boiled from this fluid by warm surface water drives a turbine, with cold water from lower depths being pumped up to condense the vapor back into liquid. The electrical power thus generated can be shipped to shore over transmission lines or used to operate a floating industrial operation such as a cannery.
It is unlikely, however, that tidal energy will ever make a large contribution to world energy use; the number of suitable sites for tidal power is small, and the large-scale use of thermal energy conversion would likely cause unacceptable environmental damage. However, a number of projects are under way attempting to develop cost-effective devices for harvesting the energy of surface waves.
Biomass—wood, dried animal dung, or materials left over from agriculture—is the oldest fuel used by people, and was initially utilized for space-heating and cooking food. These uses are still major energy sources in developing countries, especially in rural areas. Biomass can also be combusted in a boiler to produce steam, which can be used to generate electricity. The biomass of trees, sugar cane, and corn can also be used to manufacture ethanol or methanol, which are useful liquid fuels.
Other sources of alternative energy, some experimental, are also being explored. Methane gas is generated from the anaerobic breakdown of organic waste in landfills and in wastewater treatment plants; this methane can be collected and used as a gaseous fuel for the generation of electricity. With the cost of garbage disposal rapidly increasing, the burning of organic garbage is becoming a viable option as an energy source. Incinerators doing this are sometimes known as “waste to energy” facilities. Adequate air pollution controls are necessary, however, to prevent the emission of toxic chemicals to the environment—a “landfill in the air” effect.
Fuel cells, although a not a source of primary energy but a way of using energy more efficiently, are another rapidly developing technology. These devices oxidize hydrogen gas, produce electricity, and release only water as a waste product. Experimental vehicles (including buses) and medium-sized generating units are already running using this promising technology. Like electricity itself, however, hydrogen is not an energy source; hydrogen gas (H2) does not occur naturally on Earth in significant quantities, but must be manufactured using energy from fossil fuel, solar power, or some other source. Fuel cells have the advantage of producing electricity at their point of end-use from a concentrated fuel that does not produce pollution; if their fuel can be produced by non-polluting means, such as from solar energy, then they can become part of a renewable, nonpolluting energy economy.
Although not in the strictest sense an alternative source of energy, conservation is perhaps the most important way of reducing society’s dependence on nonrenewable fossil and nuclear fuels. Improving the efficiency of energy usage is a way of meeting energy demands without producing pollution or requiring changes in lifestyle (though lifestyle changes may also ultimately be necessary, as nonrenewable energy stocks decline). If a society needs, for example, to double the number of refrigerators it uses from 10 to 20 it is far cheaper to engineer and manufacture 20 refrigerators with twice the efficiency of the old ones than to manufacture 10 refrigerators of the old type and double the amount of electricity produced. New electric generation facilities of any type are expensive, and all—even alternative or renewable generators—impose some costs on the environment. Experts have estimated that it is still possible, as of the early 2000s, to double the efficiency of electric motors, triple the efficiency of light bulbs (or better, with light-emitting diodes), quadruple the efficiency of refrigerators and air conditioners, and quintuple the gasoline mileage of automobiles. Several European and Japanese automobile manufacturers are already marketing hybrid vehicles with high gasoline mileage (40–70+ miles per gallon) and ultra-low emissions, and these by no means reflect the upper limit of efficiency possible.
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