The most experienced conversion system is a boiler heat engine. Other possible heat engine types are laser and photon engines that absorbs the energy into working gas. The main problem with these engines is the lack of the appropriate window materials. Quite different group of heat engines uses energy exchanger. In these energy is directly exchanged between high temperature and low temperature fluids so wall temperature of the machine is an average of these temperatures. Temperature ratios may be even 10 using high and low molecular weight fluids as hot and cold working fluids. The theoretical efficiency for energy exchanger/binary cycle concept as been calculated to be 73%. This requires however very high temperatures that cause some problems with materials. [Beverly, 1980a] Laser Applications Laser Beamed Power To Photovoltaic Receivers One of the most interesting and promising uses of laser beamed power transmission is a laser beam on earth to transmit power to a remote photovoltaic array [Landis, 1991]. This new concept was recently proposed by Landis and Rather working independently. The laser power could be transmitted to any of several existing orbiting spacecraft such as satellites, or future space systems such as Space Station Freedom or a lunar base system. By using an existing spacecraft, the laser beaming concept could be promising for a near term and low budget demonstration. Placing the laser on earth has several obvious cost advantages. Currently, earth based lasers are easier to develop and maintain for ground operations. High power lasers can operate for only short periods of time without operator intervention. Secondly, the earth environment is less destructive to the laser system hardware than a space environment. The space environment has extreme temperature variations and ionizing radiation that can adversely affect the hardware. Finally, because power on earth is much less expensive to provide than power in space, the mass and the power efficiency of the laser system is relatively unimportant for a laser demonstration. Photovoltaics can have extremely high efficiencies for conversion of monochromatic (laser) light at selected wavelengths. The peak responses to monochromatic illumination of existing solar cells is 850 run for gallium arsenide (GaAs) and 950 nm for silicon. Using IR wavelengths in the "eyesafe" range will have considerably less stringent safety restrictions; unfortunately the efficiency of photovoltaic receivers begins to drop off considerably at longer wavelengths near the "eyesafe" range. Because of the trade-offs in wavelength selection (i.e., atmospheric attenuation and biological constraints), the best laser choice may be a free electron laser (FEL), which has been operated between about 100 Angstroms and 10 mm. The output wavelength of an FEL can be chosen by adjusting one of three macroscopic parameters: the electron energy, the "wiggler" period or the field strength. Free electron lasers (FELs) operate inherently in a pulsed mode. The pulses may be very short and the pulse rate high enough so that the laser system can achieve sufficient average power for the solar cells. For induction FELs, the peak power may be higher than the average power. Therefore, it is important that the photocells on the receiver be able to operate under high-peak power with a minimum of cell degradation. The characteristic response time of a photovoltaic cell to pulsed excitation from the laser is related to the minority carrier lifetime. The response time is often shorter than the period between laser pulses. With this characteristic, for optimum use and efficiency, the solar cells used with the induction FEL should have a lower series resistance than standard non-concentrator solar cells to minimize I2R losses (power dissipation). In conclusion, the most important technical issues which should be addressed for a high efficiency laser power transmission to photovoltaic receiver system are: Radiation Damage: The orbit selection will effect the amount of radiation damage to a solar cell. In low earth orbit (LEO), there is relatively low radiation and any solar cell type is acceptable. In geosynchronous orbit, there is moderate radiation where a standard silicon solar cell could be used with a coverglass and be subject to moderate degradation. In transfer orbit, there are high doses in the radiation belts where a standard silicon or gallium arsenide cell should be protected with a thicker cover glass for radiation protection. Pulsed Mode Operation: Solar cell operation and power management systems must be able to operate in a pulsed mode. The system efficiency can vary with the pulse peak, pulse width and duty cycle. For high peak power operation, the solar cell must be designed with high metallization coverage and a prismatic cover glass.
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