Alternative energy sources

Definition: Sources of energy other than the dominant fossil and mineral fuels

Energy sources that offer alternatives to the burning of fossil fuels such as coal and petroleum are urgently needed to address the rising demand for energy in ways that will not contribute to air pollution and climate change. The ideal alternative energy source is renewable or inexhaustible and causes no lasting environmental damage.

Both the extraction and the burning of fossil fuels have caused severe and growing damage to the environment, contributing to such problems as air pollution, the release of greenhouse gases (which retain heat and contribute to climate change), and sulfuric acid in rainfall. Nuclear energy sources are very limited in supply and expensive, require extreme amounts of processing, and produce long-lasting radioactive waste. In the long term, energy release from nuclear fusion has been proposed as a limitless supply of power, but industrial-scale production of fusion power continues to pose large and uncertain obstacles and hazards.

Solar Power

The sun powers winds, ocean currents, rain, and all biomass growth on the earth’s surface. Because the availability and extraction means for each of these secondary sources of solar power are diverse, each forms a different field of alternative energy technology. Where solar power is extracted and converted to energy directly, the capture can be by means of flat-plate receivers that collect at the incident intensity but can operate in diffuse light, or by means of concentrators that can achieve intensities of several hundred suns but work poorly in diffuse light.

In solar photovoltaic power (PV) technology, solar radiation is directly converted to useful power through PV cell arrays, which require semiconductor mass-production plants. PV cell technologies have evolved from using single-crystal silicon to using thinner polycrystalline silicon, gallium arsenide, thin-film amorphous silicon, cadmium telluride, and copper indium selenide. The needed materials are believed to be abundant enough to meet projected global growth. The process of purifying silicon requires large inputs of energy, however, and it generates toxic chemical waste. Regeneration of the energy required to manufacture a solar cell requires about three years of productive cell operation.

Solar cell technology continues to evolve. Broadband solar cell technologies have the potential to make cells sensitive to as much as 80 percent of the energy in the solar spectrum, up from about 60 percent. High-intensity solar cells could enable operation at several hundred times the intensity of sunlight, reducing the cell area required when used with concentrator mirrors and enabling high thermal efficiency.

Direct solar conversion is another option. Laboratory tests have shown 39 percent conversion from broadband sunlight to infrared laser beams using neodymium-chromium fiber lasers. Direct conversion of broadband sunlight to alternating-current electricity or beamed power through the use of optical antennae is projected to achieve 80 to 90 percent conversion. Such technologies offer hope for broadband solar power to be converted to narrowband power in space and then beamed to the earth by satellites.

Another way of harnessing solar power is through solar thermal technology. Solar concentrators are used with focal-point towers to achieve temperatures of thousands of kelvins and high thermal efficiency, limited by containment materials. The resulting high-temperature electrolysis of water vapor generates hydrogen and oxygen in an efficient manner, and this technology has demonstrated direct solar decomposition of carbon dioxide (CO₂) to carbon monoxide (CO) and oxygen.

Wind Power

Winds are driven by temperature and pressure gradients, ultimately caused by solar heating. Wind energy is typically extracted through the operation of turbines. Power extraction is proportional to the cube of wind speed, but wind-generated forces are proportional to the square of wind speed. Wind turbines thus can operate safely only within a limited range of wind speed, and most of the power generation occurs during periods of moderately strong winds. Turbine efficiency is strongly dependent on turbine size and is limited by material strength. The largest wind turbines have reached 8 megawatts in capacity. Denmark, the Netherlands, and India have established large wind turbine farms on flat coastal land, and Germany and the United Kingdom have opted for large offshore wind farms. In the United States, wind farms are found in the Dakotas, Minnesota, and California, as well as on Colorado and New Mexico mountain slopes and off the coasts of Texas and Massachusetts. By 2023, the United States Wind Turbine Database (USWTDB) tracked more than 72,000 turbines spread across forty-three US states, Guam, and Puerto Rico.

Because of wind fluctuations and the cubic power relation, wind power is highly unsteady, and means must be established for storing and diverting the power generated before it is connected to a power grid. In addition, offshore and coastal wind farms must plan for severe storms. Smaller wind turbines are sometimes used for power generation on farms and even for some private homes in open areas, but these tend to be inefficient and have high installation costs per unit of power. They are mainly useful for pumping irrigation water or for charging small electrical devices.

Environmentalists have raised some concerns about large wind turbines. The machinery on wind farms causes objectionable noise levels, and many assert that the wind turbine towers themselves constitute a form of visual pollution. Disturbances to wildlife, particularly deaths and injuries among bird populations, are another area of concern. In addition, the construction of wind farms often requires the building of roads through previously pristine areas to enable transportation of the turbines’ large components.

Hydroelectric Power and Tidal Power

Large dams provide height differences that enable the extraction of power from flowing water using turbines. Hydroelectric power generated by dams forms a substantial percentage of the power resources in several nations with rivers and mountains. However, the building of large dams raises numerous technical, social, and public policy issues, as damming rivers may displace human inhabitants from fertile lands and may result in the flooding of pristine ecosystems, sometimes the habitats of endangered species. Increased incidence of earthquakes has also been associated with the existence of very large dams.

In some of the world’s remote communities, micro hydroelectric (or micro hydel) plants provide power, generating electricity in the 1–30 megawatt range. Very small-scale systems, known as pico hydel, extract a few kilowatts from small streams; these can provide viable energy sources for individual homes and small villages, but the extraction technology has to be refined to bring down the cost per unit of power.

Although tidal power is abundant along coastlines, the harnessing of that power has been slow to gain acceptance, in part because of the difficulties of building plants that can survive ocean storms. Tidal power is extracted in two principal ways. In one method, semipermeable barrages are built across estuaries with high tidal ranges, and the water collected in the barrages is emptied through turbines to generate power. In the second, offshore tidal streams and currents are harnessed through the use of underwater equivalents of wind turbines.

Tidal power plants typically use pistons that are driven up and down by alternating water levels or the action of waves on turbines. A rule of thumb is that a tidal range of 7 meters (23 feet) is required to produce enough hydraulic head for economical operation. One drawback is that the 12.5-hour cycle of tidal operation is out of synchronization with daily peak electricity demand times, and hence some local means of storing the power generated is desirable. In many cases, impellers or pistons are used to pump water to high levels for use when power demand is higher.

China, Russia, France, South Korea, Canada, and Northern Ireland all have operational tidal power stations. The first US tidal power station became operational in 2012 in Cobscook Bay, near Eastport, Maine.

Biomass Power

Biomass, which consists of any material that is derived from plant life, is composed primarily of hydrocarbons and water, so it offers several ways of usage in power generation. Combustion of biomass is considered to be carbon-neutral in regard to greenhouse gas emissions, but it may generate smoke particles and other pollution.

One large use of biomass is in the conversion of corn, sugarcane, and other grasses to ethyl alcohol (ethanol) to supplement fossil petroleum fuels. This use is controversial because the energy costs associated with producing and refining ethanol are said to be greater than the savings gained by using such fuel. It is argued that subsidies and other public policies and rising energy prices entice farmers to devote land to the production of ethanol crops, thus triggering shortages and increases in food prices, which hurt the poorest people the most. Brazil has advanced profitable and sustainable use of ethanol extracted from sugarcane to replace a substantial portion of the nation’s transportation fossil-fuel use.

Jatropha plants, as well as certain algae that grow on water surfaces, offer sources of biodiesel fuel. Biodiesel from jatropha is used to power operations on several segments of India’s railways, and vegetable oil from peanuts and groundnuts, and even from coconuts, has been used in test flights of aircraft ranging from strategic bombers to jetliners.

Biogas and Geothermal Energy

Hydrocarbon gases from decaying vegetation form large underground deposits have been exploited as sources of energy for many years. Technology similar to that used in extracting energy from these natural deposits, which are not considered a renewable energy source, can be used to tap the smaller but widely distributed emissions of methane-rich waste gases from compost pits and landfills. Creating the necessary infrastructure to capture these gases over large areas poses a difficult engineering challenge, however. In addition, care must be taken to avoid the release of methane from these deposits into the atmosphere, as methane is considered to be twenty times as harmful as carbon dioxide as a greenhouse gas.

Geothermal energy comes from heat released by radioactive decay inside the earth’s core, perhaps augmented by gravitational pressure. Where such heat is released gradually through vents in the earth’s surface, rather than in volcanic eruptions, it forms an abundant and steady, reliable, long-term source of thermal power. Hot springs and geothermal steam generation are used on a large scale in Iceland, and geothermal power is used in some American communities and military bases.

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