Other Fuels and Sources of Energy
Solar energy is used, mostly passively, in the world today to perform smaller residential jobs like using flat panel collectors for converting the Sun's energy into hot water, or photovoltaic cells, converting solar energy into electric energy.
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Photovoltaics
Concentrating Solar Power
Solar Thermal
Tidal energy uses the weight of the water captured, usually by closing a bay with a dam. At high tide, the dam is closed to hold the water, then at low tide, the stored water is released through a turbine to generate electric power. This form of energy can only be utilized when tides fall, and few bays offer terrain to accommodate such dams.
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Wind power is used for turning windmills and moving the occasional ship, although the latter is usually for pleasure or show. Although very clean, wind power is not yet very practical since it requires strong and constant winds.
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Chemical energy is most commonly related to generating electrical power in batteries. Since some batteries are rechargeable they are more likely be used in conjunction with another form of alternate power, such as windmills or solar collection. As storage devices, batteries compete with hydrogen fuel cells as methods to store electricity. A battery can store and produce at a later time as much as 90% of the electricity charged into it, whereas a hydrogen fuel cell will lose 30% of the electricity through electrolysis (creating the hydrogen from electricity) and another 30% when passing the hydrogen back through the fuel cell (converting the hydrogen back into usable electricity). Therefore a chemical battery can be 90% efficient, vs. a hydrogen fuel cell which is only about 50% efficient. Batteries are also much cheaper than fuel cells. But a hydrogen and fuel cell system can weigh far less than batteries, which limits the utility of batteries, especially for automotive use. The newest lithium batteries can store up to 120 watt-hours per kilogram, over twice that of lead acid batteries, but these batteries are still very expensive. Who will win the battle between batteries and hydrogen fuel cells to store electricity in the future is still a very open question.
Geothermal power is generated below the Earth's surface wherever water comes in contact with heated rocks. When the heated rocks are located close enough to the surface, we can drill and pump the hot water and steam to the surface where it is used to generate power. Engineers will pump water into the ground if heated rocks are present, but no water. Bolivia, Iceland, Italy, Japan, the Philippines, New Zealand, and the United States have developed and use geothermal power plants to produce energy.
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Fuel cells can be likened to batteries in which gas or liquid fuels are combined to chemically generate electric energy. Spacecraft use fuel cells to produce electricity by combining hydrogen and oxygen. It is important to emphasize, however, that fuel cells convert a fuel such as hydrogen or natural gas into electricity - they don't create energy, they convert energy. Moreover, fuel cells are extremely expensive, and they are fragile and degrade relatively quickly.
Solid and liquid wastes can also provide energy. By burning trash we can produce heat energy and electric energy and reduce landfill mass at the same time. Many paper and lumber mills use the waste wood that is not useful to fuel boilers which in turn generate steam for running machinery in the plants. Liquids like sewage can be used to produce methane, and the use of a process called bioconversion converts organic animal and plant waste into liquid fuels such as methanol.
What's in the future?
Magnetohydrodynamic (MHD) generators are used to convert fuel directly into electric energy. An MHD generator produces hot, ionized (electrified) gases by burning coal or other fuels at high temperatures. The ionized gases are then forced through a duct in a magnetic field, where they produce an electric current which is drawn off by electrodes. MHD generators are more efficient than typical steam powered devices, also once the gas passes through the collection ducts it can be used to run a turbine to produce still more electricity. Until the technology is refined, we will not see MHD generators in common use.
Hydrogen may very well replace both gas and oil as the fuel we use for all energy. Hydrogen burns easily and releases vast amounts of heat, best of all, there is but one by-product created by the process, useful water. The process to remove hydrogen from water is called electrolysis where an electric current runs through the water. Electrolysis, however, is very costly in that it requires an enormous amount of electric power extract the hydrogen. In essence hydrogen is a useful way to store electricity but hydrogen is not itself a primary fuel. Hydrogen must either be extracted from other hydrocarbons such as natural gas or oil, or it must be manufactured using electricity and water. Furthermore, because hydrogen is the lightest element in the universe, in order for meaningful amounts of hydrogen to be stored, it must be placed either under extreme pressure or at a temperature so cold it liquifies. Both of these methods are very expensive. The fact that hydrogen must be manufactured from other fuel, and the fact that hydrogen is very difficult to store, makes hydrogen a promising energy alternative, but not a sure bet to be the clean energy of the future.
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Oil shale and bituminous sands may also become energy sources in the future. Bituminous sands (tar sands) are deposits of sand covered with an oil-producing substance, and oil shale is a type of rock that can be processed to make natural gas and crude oil. Today this process is far more expensive than obtaining the fossil fuels we use now, but as the current reserves run out, oil shale and bituminous sands may become an option.
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