• Photovoltaic conversion: it is usually based on semiconductive crystals (silicon, gallium arsenide, cadmium sulfide, etc.) that convert sunlight directly into electricity. Since they are one major component of space solar power stations they are described in the power chapter of this report. Main problem of photovoltaic conversion is the high price for the solar cells. However a potential of significant cost reduction exists. • Thermoelectric conversion: it is based on the different velocities of charge carriers at a junction between two different metals or semiconductors (the Seebeck effect, also known as inverse Peltier-Effect). With current materials the efficiency rates achieved are lower than for thermodynamic conversion. Since the converter is quite simple, it is often applied for small nuclear power generators in space. • Magneto hydraulic generator: it transforms the kinetic energy of a fluid or gas into electricity without using mechanical parts. Its principle is based on the induction of voltage in the fast gas flow under the presence of a strong magnetic field. To get the maximum amount of electric power, the gas must have a high velocity (generated by thermal expansion) and a low electrical resistance. In combination with gas turbines efficiency rates of 50% and more are reached at Russian experimental plants. Even though this technology is expensive it is likely that is will be used in future thermodynamic power stations. • Chemo-electric (fuel cells): they are built quite similar to an electric battery. They transfer the chemical energy of the fuel directly into electricity. The technology to convert hydrogen and oxygen into water and electric energy is most developed, but it is also demonstrated that oil products can be used. Nowadays fuel cells are mainly used to generate electricity for vehicles (also in space). A future application is the usage of hydrogen for transportation and storage of energy. Features of Electric Generators To describe a whole energy supply system, many features of the power source have to be taken into account. These features are often dependent on daytime, season, locations of the power sources, etc. Moreover the way the different power sources in a power grid fit together might be a non-linear function. This means that a power grid should be described in terms of a non-linear equation system. Based on this the optimal combination of power sources can be derived. Due to the complexity of such an approach, a much simpler model is being used here. It is based on the following three assumptions: • The cost of the power source is not included (this factor is described in the financing chapter). • The 3 main features we have taken into account are: Peak power performance (P), Local availability (L) and Mobility (M). The parameter P describes how well the power source can provide additional power on request and how predictable its delivery is. • A number between 0 and 10 is allocated to all the above features. Larger numbers represent advantages. Only two power sources are compared with each other to keep the complexity low. However, for a more precise evaluation the interaction of all power sources used must be considered. A measure for how good two power sources complement each other has been established. We have called it "Value of Combination" (VOC). This factor compares two different power sources. It is a combination of the aforementioned features (its value is between 0 and 10). Higher values indicate bigger differences between the power sources that corresponds to a better combination. The formula for VOC is arbitrarily defined as a linear combination of P, L & M. The formula is based on the assumption that the highest priority for the combination lies in P. The weighting of the different factors depends of the type of power grid. The numbers chosen here might be typical for a large power grid. Figure 2.6 shows the features for different classes of power generators and the VOC between them. A possible feature (that needs to be evaluated further) of a geostationary solar power satellite is its high mobility in terms of power delivery. A few hours after a distress (e.g., a typhoon) a deployable rectenna could be brought to the location and set up for medium size emergency power (e.g., for a hospital). To get the power to the site, only the power satellite's microwave beam must be directed towards the location. If a higher power demand has to be fulfilled, a large rectenna can be installed (the installation of a large rectenna requires much less time than the construction of a power plant). The figure below demonstrates the big differences in the way different power sources complement each other. Under the above assumptions the figure shows that fossil fuel, hydroelectric and biomass power complements best with a solar power station in an equatorial low earth orbit. A
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