Renewable Energy Resources
A renewable energy resource is one that can constantly be tapped because it replenishes itself. The search for viable renewable energy sources is a matter of growing urgency as non-renewable sources of energy become more difficult and expensive to discover and exploit. The major sources of renewable energy include solar energy, wind energy, bioenergy, geothermal energy, hydropower, and ocean energy. However, these sources have limitations that make them more expensive or less efficient in situations worldwide.
Solar energy, or energy derived from the Sun, works well in areas that have a good supply of direct sun light. Certain types of elements, such as cadmium sulfide or gallium arsenide, separated by layers of crystalline silicon, when exposed to direct sunlight, will produce an electric current. Light striking these substances knocks electrons off the element's surface. If the substances are formed into photovoltaic cells, the electrons can produce an electric current as they move from the negative pole of the cell (the cathode) to the positive pole (the anode). These photovoltaic cells, placed in arrays, can produce significant amounts of electricity. Sometimes it is possible to reduce the number of cells by using lenses or concave reflectors to focus an intense beam of light onto the cells. The downside of this approach is the need to keep moving the reflectors and lenses to keep the moving sun focusing on the cells. Unfortunately photovoltaic cells are comparatively inefficient (between 15% and 30%), and they can only produce electricity at a cost of about 30 cents per kilowatt hour, and energy from other sources is currently cheaper to produce.
Wind energy is another growing source of energy in the world. High-tech windmills can be very efficient and currently are beginning to produce increasing amounts of energy in those places where the wind are reasonably strong and consistent. Although modern windmills (sometimes called wind turbines) cause less damage to the environment than some other types of renewable energy resources, they are not a universal solution. In order to be efficient, wind turbines require average wind velocities in excess of (21 km/hr) 13 mi/hr over the course of a year. Other problems associated with these devices include high initial start up costs, noise, spoiling of the visual environment, and bird deaths. Energy storage is also a problem for those times when energy is needed but the wind is not available.
The burning of wood, agricultural waste, and the organic components of domestic and industrial wastes to produce energy is increasingly common. There are several ways that biomass can be converted to energy. Chemicals can turn the biomass into fuel oil or gasoline additives. Ethanol, methanol and biodiesel are three biofuels that can be produced from the biomass. Biodiesel can be produced vegetable oils or animal fat. The generation of electricity from biomass is also possible. Burning the biomass as a supplement with coal can help reduce the use of this non-renewable resource. Biomass can also be turned directly into gas by direct application of heat or by natural decay. Some landfills are set up to produce natural gas, which is then tapped and used for heating or to produce electricity--although the process of burning can produce pollution.
Electricity can also be generated by using geothermal heat sources. In certain parts of the world raw steam emerges from vents which can be directed onto turbine blades to generate electricity. In other places very hot water emerges which can then be provided with additional heat to achieve the same result. In those places where no steam or hot water emerges it may be possible to inject a fluid deep into the hot bedrock. This scalding hot fluid can then be passed through pipes that in turn will turn water into steam. Geothermal energy is already used extensively in Iceland, where extensive volcanism makes natural steam sources common.
Mountainous regions offer another renewable energy source in the form of hydropower. In this case a dam is built to form a reservoir. The water is released from the dam to falls under gravity onto the blades of turbines that generate electricity. Clearly, the higher fall the greater the energy produced. A different type of hydropower uses pumped water. In this case water is pumped up into a high reservoir at night, when electrical power is least used. When electricity is expensive the process can be reserved and water allowed to fall back down onto turbine blades. However, hydroelectric power is of limited use except in areas with major rivers--and the dams used to generate it can cause extensive environmental damage.
Finally, the ocean can also be a source of renewable energy. The motion of the tides can be harnessed in the same way as hydroelectric generators harness the energy of flowing rivers. Incoming tides flood through hydroelectric dams, turning turbines that generate electric current. The water they bring in is then caught behind the dams and, as the tide recedes, the water can again run through the turbines, again generating electric current. Tidal generators are most efficient where tides are highest. They have been put into place in northwestern France, but such generators are expensive and few others have been built.
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