Referat / Aufsatz (Schule), 2001, 10 Seiten
Autor: Andreas Dimmel 1. Renewable energy
Fossil fuels are nonrenewable, they draw on finite resources that will eventually becoming too expensive or too environmentally damaging to retrieve. Renewable energy resources (wind and solar energy) are will never run out. Most renewable energy comes from the sun. Solar energy can be used for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling. The winds energy is captured with wind turbines.
Sunlight also causes plants to grow, and the organic matter that makes up those plants is known as biomass. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass is called bioenergy.
But not all renewable energy resources come from the sun. Geothermal energy taps the Earth's internal heat for electric power production and the heating and cooling of buildings. And the energy of the ocean's tides comes from the gravitational pull of the moon and the sun. 1.1. Importance of renewable energy
-Renewable energy is important because of the benefits it provides -Renewable energy technologies are clean sources of energy that have a much lower environmental impact than conventional energy technologies
-Renewable energy will not run out. Ever. Other sources of energy are finite and will some day be depleted 1.2. Environmental benefits
Renewable energy technologies are a lot friendlier to the environment than conventional energy technologies. Fossil fuels contribute to many of the environmental problems.Examples: greenhouse gases, air pollution, and water and soil contamination. Renewable energy technologies can produce heat and electricity with a very low or no amount of carbon dioxide emissions.
Energy use from fossil fuels is also a primary source of air, water, and soil pollution. Pollutants take a dramatic toll on our environment. On the other hand, most renewable energy technologies produce little or no pollution.
Both pollution and global warming pose major health risks to humans. Ultimately, renewable energy technologies could help us break our conventional pattern of energy use to improve the quality of our environment. 2. Solar energy
There are a variety of technologies that have been developed to take advantage of solar energy.
Photovoltaic (solar cell) systems: Producing electricity directly from sunlight Concentrating solar systems: Using the sun's heat to produce electricity Passive solar heating and daylighting :Using solar energy to heat and light buildings Solar hot water: Heating water with solar energy
Solar process heat and space heating and cooling: Industrial and commercial uses of the sun's heat. 2.1. Photovoltaic systems
Photovoltaic (PV) cells convert sunlight directly into electricity. PV cells are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic effect. Some PV cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. But because the lenses must be pointed at the sun, the use of concentrating collectors is limited to the sunniest parts of the country. The performance of a PV cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain energies will work efficiently to create electricity, and much of it is reflected or absorbed by the material that make up the cell. Because of this, a typical commercial PV cell has an efficiency of 15% Low efficiencies mean that larger arrays are needed, and that means higher
cost. Improving PV cell efficiencies while holding down the cost per cell is an important goal of the PV industry. The first PV cells, built in the 1950s, had efficiencies of less than 4%. 2.2. Concentrating solar power
A new generation of power plants, with concentrating solar power systems, uses the sun as a heat source. There are three main types of concentrating solar power systems: parabolictrough , dish/engine , and power tower.
Parabolic-trough systems concentrate the sun's energy through long rectangular, curved (Ushaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on a pipe that runs down the center of the trough. This heats the oil flowing through the pipe. The hot oil then is used to boil water in a conventional steam generator to produce electricity. A dish/engine system uses a mirrored dish (similar to very large a satellite dish). The dishshaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and transfers it to fluid within the engine. The heat causes the fluid to expand against a piston or turbine to produce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity.
A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower. Then, the salt's heat is used to generate electricity through a conventional steam generator. Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity. That means electricity can be produced on cloudy days or even several hours after sunset. 2.3. Passive solar heating and daylighting
Today, many buildings are designed to take advantage of this natural resource through the use of passive solar heating and daylighting.
The south side of a building always receives the most sunlight. Therefore, buildings designed for passive solar heating usually have large, south-facing windows. Materials that absorb and store the sun's heat can be built into the sunlit floors and walls. The floors and walls will then heat up during the day and slowly release heat at night, when the heat is needed most. Other passive solar heating design features include sunspaces and trombe walls . A sunspace (which is much like a greenhouse) is built on the south side of a building. As sunlight passes through glass or other glazing, it warms the sunspace. Proper ventilation allows the heat to circulate into the building. On the other hand, a trombe wall is a very thick, south-facing wall, which is painted black and made of a material that
absorbs a lot of heat. A pane of glass or plastic glazing, installed a few inches in front of the wall, helps hold in the heat. The wall heats up slowly during the day. Then as it cools gradually during the night, it gives off its heat inside the building. Many of the passive solar heating design features also provide daylighting. Daylighting is simply the use of natural sunlight to brighten up a building's interior.
2.4. Solar hot water
The shallow water of a lake is usually warmer than the deep water. That's because the sunlight can heat the lake bottom in the shallow areas, which in turn, heats the water. It's nature's way of solar water heating. The sun can be used in basically the same way to heat water used in buildings and swimming pools.
Most solar water heating systems for buildings have two main parts: a solar collector and a storage tank. The most common collector is called a flat-plate collector. Solar water heating systems can be either active or passive, but the most common are active systems. Active systems rely on pumps to move the liquid between the collector and the storage tank, while passive systems rely on gravity and the tendency for water to naturally circulate as it is heated.
2.5. Solar process heat and space heating and cooling
Commercial and industrial buildings may use the same solar technologies - photovoltaics, passive heating, daylighting, and water heating - that are used for residential buildings. These nonresidential buildings can also use solar energy technologies that would be impractical for a home.
Many large buildings need ventilated air to maintain indoor air quality. In cold climates, heating this air can use large amounts of energy. A solar ventilation system can preheat the air, saving both energy and money. This type of system typically uses a transpired collector , which consists of a thin, black metal panel mounted on a south-facing wall to absorb the sun's heat. Air passes through the many small holes in the panel. A space behind the perforated wall allows the air streams from the holes to mix together.
The heated air is then sucked out from the top of the space into the ventilation system. Solar process heating systems are designed to provide large quantities of hot water or space heating for nonresidential buildings. A typical system includes solar collectors that work along with a pump, a heat exchanger, and/or one or more large storage tanks. The two main
types of solar collectors used - an evacuated-tube collector and a parabolic-trough collectorcan operate at high temperatures with high efficiency. 3. Wind energy
The windmill's modern equivalent - a wind turbine - can use the wind's energy to generate electricity.
Wind turbines, like windmills, are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more aboveground, they can take advantage of the faster and less turbulent wind. Turbines catch the wind's energy with their propeller-like blades. Usually, two or three blades are mounted on a shaft to form a rotor.
When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is actually much stronger than the wind's force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity. Wind turbines can be used as stand-alone applications, or they can be connected to a utility power grid or even combined with a photovoltaic system. Stand-alone wind turbines are typically used for water pumping or communications. 4. Bioenergy
We have used bioenergy ever since people started burning wood for keep warm. And today, wood is still our largest biomass resource for bioenergy. But many other sources of biomass can now be used for bioenergy, including plants, residues from agriculture or forestry, and the organic component of municipal and industrial wastes. Even the fumes from landfills can be used as an energy source.
The use of bioenergy has the potential to greatly reduce our greenhouse gas emissions. Bioenergy generates about the same amount of carbon dioxide as fossil fuels, but every time a new plant grows, carbon dioxide is actually removed from the atmosphere. Bioenergy applications:
Biochemicals : Converting biomass into chemicals to generate electricity Biofuels : Converting biomass into liquid fuels for transportation
Biopower : Burning biomass directly, or converting it into a gaseous fuel, to generate
electricity 4.1. Biochemicals
Heat can be used to chemically convert biomass into a fuel oil. The chemical conversion process is called pyrolysis. After pyrolysis, biomass turns into a liquid - called pyrolysis oilwhich can be burned like petroleum to generate electricity.
Pyrolysis oil has its advantages: it's easier to transport and store than solid biomass material, and it can be refined in ways similar to crude petroleum oil.
A chemical called phenol can also be extracted from pyrolysis oil. Phenol is used to make wood adhesives, molded plastic, and foam insulation.
Other industrial uses of biochemicals are being researched, including the use of bioenergy feedstocks to cost-effectively manufacture high-volume chemical building blocks. 4.2. Biofuels
Unlike other renewable energy sources, biomass can be converted directly into liquid fuelsbiofuels - for our transportation needs (cars, trucks, buses, airplanes, and trains). The two most common types of biofuels are ethanol and biodiesel .
Ethanol is an alcohol, the same found in beer and wine. It is made by fermenting any biomass high in carbohydrates through a process similar to brewing beer. Ethanol is mostly used as a fuel additive to cut down a vehicle's carbon monoxide and other smog-causing emissions. But flexible-fuel vehicles, which run on mixtures of gasoline and up to 85% ethanol, are now available.
Biodiesel is not an alcohol. It's an ester, which is similar to vinegar. Many vegetable oils, animal fats, algae, or even recycled cooking greases are used to produce biodiesel. It is used as a diesel additive to reduce vehicle emissions or in its pure form to fuel a vehicle. Other biofuels include methanol and reformulated gasoline components. Methanol is produced through the gasification of biomass. After gasification, the resulting hot gas is sent through a tube and then converted into liquid methane. 4.3. Biopower
Biopower is the use of biomass to generate electricity. There are four major types of biopower systems: direct-fired , cofiring , gasification , and small, modular .
Most of the biopower plants in the world use direct-fired systems. They burn bioenergy feedstocks directly to produce steam. This steam is usually captured by a turbine, and a generator then converts it into electricity.
Many coal-fired power plants can use cofiring systems to significantly reduce emissions, especially sulfur dioxide emissions. Cofiring involves using bioenergy feedstocks as a supplementary energy source in high efficiency boilers.
Gasification systems use high temperatures and an oxygen-starved environment to convert biomass into a gas (a mixture of hydrogen, carbon monoxide, and methane). The gas fuels what's called a gas turbine, which is very much like a jet engine, only it turns an electric generator instead of propelling a jet.
The decay of biomass in landfills also produces a gas - methane - that can be burned in a boiler to produce steam for electricity generation or for industrial processes. To release the methane, wells are drilled into a landfill. Then pipes from each well carry the gas to a central point where it is filtered and cleaned before burning. 5. Geothermal energy
Many technologies have been developed to take advantage of geothermal energy - the heat from the earth. Geothermal applications:
Geothermal electricity production : Generating electricity from the earth's heat. Geothermal direct use : Producing heat directly from hot water within the earth. Geothermal heat pumps : Using the shallow ground to heat and cool buildings. 5.1. Geothermal electricity production
Most power plants need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power plants use steam produced from reservoirs of hot water found a couple of miles or more below the Earth's surface. There are three types of geothermal power plants: dry steam, flash steam, and binary cycle. Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a
Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator.
Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.
Binary cycle power plants operate on water at lower temperatures of about 225°-360°F (107°-182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions. 5.2. Geothermal direct use
Geothermal reservoirs of hot water, which are found a couple of miles or more beneath the Earths surface, can also be used to provide heat directly. This is called the direct use of geothermal energy.
Geothermal direct use dates back thousands of years, when people began using hot springs for bathing, cooking food, and loosening feathers and skin from game. Hot springs are still used as spas.
In modern direct-use systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical systempiping, a heat exchanger, and controls - delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface.
Geothermal hot water can be used for many applications that require heat. Its current uses include heating buildings, drying crops, heating water at fish farms, and several industrial processes, such as pasteurizing milk. 5.3. Geothermal heat pumps
The shallow ground, the upper 10 feet of the Earth, maintains a nearly constant temperature between 50° and 60°F (10°-16°C). Like a cave, this ground temperature is warmer than the air
above it in the winter and cooler than the air in the summer. Geothermal heat pumps take advantage of this resource to heat and cool buildings.
Geothermal heat pump systems consist of basically three parts: the ground heat exchanger, the heat pump unit, and the air delivery system The heat exchanger is basically a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.
In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water. Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution. 6. Hydropower
Flowing water creates energy that can be captured and turned into electricity. This is called hydropower.
The most common type of hydropower plant uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. But hydropower doesn't necessarily require a large dam. Some hydropower plants just use a small canal to channel the river water through a turbine.
Another type of hydropower plant - called a pumped storage plant - can even store power. The power is sent from a power grid into the electric generators. The generators then spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the power is stored. To use the power, the water is released from the upper reservoir back down into the river or lower reservoir. This spins the turbines forward, activating the generators to produce electricity. 6.1. Ocean energy
The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.
Oceans cover more than 70% of Earth's surface, making them the world's largest solar
collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world.
Ocean thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems. Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices. A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.
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