Solar Cells Operation: A solar cell must be chosen to match the wavelength of the laser. Operation in the eyesafe range (1700 nm) requires the use of PV material such as Si and InGaAs. The thermal environment will also affect the solar cell response. Therefore, solar cells with adverse response to high temperatures may require better thermal management systems than their current standard systems. Conclusion Laser technology is a good alternative for power beaming in principal. The divergence of the laser beam is minimal, only few arc seconds. The efficiency of state of art laser technology is however quite low. There is some possibilities to increase this efficiency. Another problem is their large mass, typically few tons for lasers capable to produce high power beams. Improvements in solid state technology may also reduce the mass of overall system below 300 kg. The atmospheric attenuation and instability for available laser wavelengths causes a problem for space to earth power transmission. Space to space transmission might become very feasible because of small scale transmission and receiving units. Taking into account these requirements even a demonstration using laser technology is not feasible at the moment. Laser power transmission may however become very feasible for space to space power transmission in few decades. Very promising technologies at moment seem to be the Free Electron Laser and the Solid State Laser. Especially Diode Array may be used at same manner as a phase array antenna for microwaves. 7.3 Receiver Location The choice of receiver location is heavily dependent both on the type of transmission that is used and on the orbit of the power beaming satellite. While receiving microwaves requires a huge area for the rectenna, but is relatively independent of the weather, receiving laser beams does not require very much space, but on the contrary it is sensitive to the weather conditions. The altitude and inclination of the satellite orbit will determine the maximum latitude at which a receiver site can be built. To make space solar power attractive for the electricity companies, it is important to have the rectenna site as close as possible to the consumers and to maximize the use of existing infrastructure for the distribution. Depending on whether the receiver receives power continuously or not, energy storage may have to be accommodated at the receiver site. Another important aspect of the receiver location is the amount of ground preparation that is required. For a microwave rectenna it is necessary to have available an area with a diameter of more than 10 km of plain ground. Although a certain degree of irregularity could be accepted, keeping the construction costs down would exclude building the rectenna in mountain areas. If the ground is swampy or the weather is extremely windy special measures would be required as well. To guarantee that no people in the vicinity of the receiver site are exposed to radiation above what is a permissible level, a buffer zone will have to be added around it. In the case of microwave transmission, the size of the buffer zone will depend on how much of the beam that is intercepted by the rectenna as well as of the size and intensity of the side lobes. Due to economical reasons, it is not worthwhile to build a rectenna that covers all of the incoming beam, so some of the energy will be wasted. The buffer zone would have to be large enough so that people will not be able to come close to where the beam intensity exceeds the safety limits for microwave exposure. In the case of laser transmission, the size of the buffer zone will probably be determined by the pointing accuracy of the laser and the time to turn it off if it should become misdirected. For both microwave and laser transmission it may be advisable to extend the buffer zones to the air space around the receiver sites. It is desirable, and it could be a requirement, that the impact of the receiver on the environment is as small as possible, especially considering the size of the receiver in the case of microwave transmission. If wire reflectors are used to focus the radiation onto the rectenna elements, the rectenna becomes almost transparent to sunlight. Then the effect of the rectenna on the vegetation beneath it would be rather limited and potentially the ground could be used for agriculture. From a maintenance aspect, it is important to also consider the impact of the
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