Renewable Energy Sources


Source: U.S. Department of Energy

Concern over the environmental impact of burning fossil fuels has helped spur interest in alternative fuels that are less polluting. And since the supply of fossil fuels is finite and diminishing, there is interest in “renewable” sources that do not deplete existing supplies. However, renewable energy sources still make up only a small share of U.S. domestic energy production (about 9%, or excluding hydropower about 5%). The major reason for this is their relatively higher cost (in some cases 2 to 4 times that of power obtained from traditional fuels). The following are the major renewable energy sources available.

Biomass is plant-derived material usable as a renewable energy source, including wood energy crops such as hybrid poplars and willow trees, agricultural crops including soybeans and corn, and animal and other wastes. Biomass is one of the two most common energy sources in the U.S. today. Forms of biomass such as wood can be burned to produce heat and generate electricity. Agricultural crops can be chemically converted into fuels such as ethanol and biodiesel; these are the only known renewable liquid energy sources, and may one day replace petroleum and fossil-fuel produced diesel. But bringing ethanol and biodiesel into wide use would require more energy-efficient methods of production and transportation. Overall, biomass fuels are much cleaner-burning than fossil fuels, though biomass fuels do produce carbon dioxide and other pollutants.

Geothermal energy is generated from heat from inside Earth. This form of energy is both clean and renewable. The technology has caught on in countries with substantial geothermal activity such as Iceland, where it accounts for 16% of electricity output and 86% of all energy used for home heating. In the U.S. the best sources for geothermal power are in the west, where there are many underground lakes of healed water; however, large-scale access would require drilling. A major goal in this field is to find a way to harness energy directly from magma (molten rock material), which has great potential because of its high temperature.

Hydrogen is the 3rd most abundant element on Earth. It does not naturally occur on Earth as a pure gas or liquid, but is always combined with other elements (such as with oxygen to form water or carbon to form methane). For energy use it is produced from hydrocarbons using heat, bacteria or algae through photosynthesis, or by using sunlight or electricity to split water into hydrogen and oxygen. Hydrogen batteries or “fuel cells” arc already used by NASA on the space shuttle. In a fuel cell, electrons are released from the hydrogen atoms in a chemical reaction and flow through an external circuit as electricity. The protons then combine with oxygen (and some of the electrons in the electric current) to make heat and water suitable for drinking. Fuel cells do not run down, but work as long as hydrogen is supplied. Some experts think hydrogen will be the power source of the future. However, an infrastructure would need to be created for safe and cost-effective transportation and storage of hydrogen.

Hydropower, or hydroelectric power is generated by water flowing through turbines. It is one of the two most common renewable energy sources in the U.S. today. A dam on a river is a common hydropower producer. No harmful greenhouse gases are produced, but the dams needed to generate the power can harm river ecosystems. Researchers are working on turbine technologies that may maximize use of hydropower and reduce adverse environmental effects.

Ocean energy is generated in two ways. Thermal ocean energy uses (he heat that the ocean absorbs from the sun to power generators, and sometimes drinkable desalinated water is a by-product. Mechanical ocean energy is generated by the movement of tides and waves through a turbine. In both cases, power generation is not very efficient with current technology. Much more research is needed to make thermal ocean energy generation a reality. Mechanical ocean energy requires large darns or breakwater-type structures called “tidal barrages” to be built, which could cause harm to coastal ecosystems.

Solar energy is generated using heat and light from the sun. Solar energy is an increasingly common source of electricity. Photovoltaic (PV) solar cells are made of semi-conducting materials that can directly convert sunlight to electricity without any harmful waste product. Solar collectors are made more efficient by using arrays of mirrors to concentrate the sun’s rays onto PV panels. Another way of using sunlight is to heat water directly. According to the DOE, homes incorporating solar heating designs can save as much as 50% on heating bills. The downside to solar energy is that it depends heavily on a range of factors including location, time of year, and weather.

Wind energy uses wind turbines to produce energy. They are perched on high towers, usually 100 feet or higher, and often placed in large groups (“farms”) to generate electricity for towns and cities. On a much smaller scale, stand-alone turbines are sometimes used by farmers and homeowners to generate supplemental electricity. In the past 20 years, government incentives in the form of tax credits to producers and incentives for homeowners have helped lower the price of wind power by 85%, making it a more feasible option. Some people object to wind farms because of their appearance or the noise the turbines make. Wind power raises few other environmental problems; but the turbines can pose a danger to birds. In addition, because weather is involved, consistent generation is a challenge.

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