The mechanical rotation can be generated directly from the primary energy (e.g. hydroelectric, windmill) or by using a thermodynamic conversion (e.g., turbine). In the following the different principles of rotation generation are described (direct dynamic conversion, thermodynamic conversion, direct conversion). Direct dynamic conversion uses primary energy that is already available in the kinetic form. Normally the direct conversion process into rotation is usually chosen. The following sources are mainly used: • Wind energy: usually horizontal or vertical wind propellers are used. Horizontal propellers have a higher efficiency rate, but they have to be oriented into the changing wind direction. Since this is not necessary for vertical axis rotators their design is much simpler. Therefore they are used more and more despite their lower efficiency. Main problems with wind generation are the high investment per installed power and the variations of wind speed. Moreover only certain areas have sufficient wind speed. However wind electricity generation is considered to have a large future potential. • Hydroelectric power (hydraulic turbine): dependent on the water pressure and volume different principles are used. Usually a water stream or ray is used to propel shovels that are mounted on the turbine's axis. Efficiency rates are about 90%. Hydroelectricity can be subdivided into reservoir power stations and river water power stations. Whereas the later delivers energy on a constant basis (dependent on the season), reservoir stations can deliver power on request. This makes them an interesting part of a power grid. Moreover hydroelectric power stations are used to a great extent to store of superfluous electric energy by pumping water uphill. Hydroelectric energy is regarded to be a very clean energy. However its use can not be expanded much further because the majority of its potentials is already used. The potentials that have not used yet are mainly located in remote areas (northern Canada and Russia, South America). Improvements in electric energy transportation (high voltage DC transmission, power relay satellites, super conductive power lines, hydrogen) could change that situation. Thermo dynamic conversion is the process that converts thermal energy into kinematic energy (movement). All major processes use gas expansion. The two main conversion processes are the combustion engine and the gas turbine. The combustion engine is only used in small power stations and was already described in the section “Energy Use in Transportation”. The gas turbine transfers the expansion of heated gas (usually water vapor) into kinetic energy. In contrast to the combustion engine the combustion process and the kinetic transformation are locally separated. This allows more control on the burning process. Therefore the efficiency rate is higher and less undesired exhaust is produced. In many countries laws enforce the additional treatment of the exhaust by filters and spray washing. In 1990 an exhaust reduction of sulfur and nitrogen oxide down to 20% and more can be achieved. Since the additional investment for the coal power station accounts for about 30% of the overall costs, exhaust treatment might not be implemented in threshold and developing countries. The efficiency rate of thermodynamic processes depends on the temperature difference the gas is undergoing in the expansion process and is theoretically limited by the law of Carnot. In modem power stations rates of 40% are achieved. The remaining 60% of the fuel energy is transferred into heat that warms up the area surrounding the power station (especially the river). The impact of a big power station on micro climate can be considerable. It is possible to use the abundant heat for remote heating of buildings. Due to the high cost this is usually not done yet. Also the heat can be used for industrial processes. Unfortunately the high temperature usually required reduces the efficiency rate of the electric conversion. Thermodynamic generation of electric energy can be used in combination with fossil energy (oil, coal, gas), nuclear energy, geothermal energy and solar energy. While fossil energy is limited and the political future of nuclear energy is unclear, a main difficulty in the usage of geothermal and solar energy is the small temperature difference. Instead of using the turbine principle, the Stirling motor (or similar devices) is applied. Energy generation is more costly for systems based on small temperature differences since less power is produced for a given amount of equipment. This makes geothermal energy production cost effective at only a few locations. To reach high temperature differences with solar energy, effective concentrators (mirrors, etc.) need to be used. This requires a mechanism to follow the changing elevation and azimuth of the sun. This mechanism is the main cost driver in current experimental thermodynamic solar power stations. Direct conversion principles are now under development to overcome the main problems of thermodynamic conversion. Principles discussed here are photovoltaic, thermoelectric conversion, magneto-hydraulic and chemo-electric:
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