The mirrors are mechanically driven to perform sun-tracking. This mechanism (not shown in the figure) usually consists of a sun sensor/seeker and servo-mechanisms. A radiator, consisting of six or eight panels mounted on the back side of the primary mirror, rejects waste heat from the primary and secondary mirror assembly, from the reflective cone and from the solar cells. To minimize system weight and maximize primary mirror effective area utilization, the ratio of the radiator and primary mirror area should be around 1.0 [13]. Pyrolitic graphite is being considered for the space radiators because of its low density and high emittance. Some sort of a flexible loop or syphon bellows arrangement is needed to connect the mirrors' pedestals with a heat pipe. This intermediate connector may be a form of heat pipe itself. Fins can be added to the collecting mirrors to improve cooling. A coolant should also be used to cool the cells and the reflective cone. In order to adequately define the optical and thermal response of a TPV system, the entire system must be included in the analysis, from the concentrating mirror assembly to the solar cell/radiator and its associated cavity receiver. The size of the collector system can be determined from the energy balance equation; where Pin is the power into the cavity, Pout is the total power removed by the cells, Pr is the power re-radiated from the cavity and PQ is the power lost by conduction through the cavity insulation. The assumptions made in Ref. [11] can be used as a guide to complete the calculations for a given TPV configuation. The equations given in Refs [10] and [14] can be used to determine the TPV cell conversion efficiencies. In this work, no attempt was made to do any detailed economic analysis. 3. Solar-powered Gas Lasers 3.1 Introduction The invention of the laser is perhaps one of the most important technical achievements of the last three decades. This device has given scientists and engineers a new tool for generating and using coherent radiation. Due to the many potential uses of lasers in almost all areas of technology, interest in new lasers and different method of excitation continues. An electric discharge has been a common means of excitation in gas laser systems. While this method is both well understood and reliable, it does require a large regulated power supply for the excitation. For space application, solar energy can be converted into electricity by conventional methods such as Brayton cycles, solar cells, etc. to supply the power to drive laser systems. However, there has been growing interest in pumping lasers by use of sunlight without the need for an intermediate form of energy. When sunlight is used as the pumping mechanism, a simple method is to beam it directly into the laser mixture as shown in Fig. 3(a). Direct solar pumping of gas laser media (e.g. CO2, CO, N2O), while possible in principle, would be very inefficient, even if the conversion of the absorbed light to laser radiation were efficient, because the bandwidth of the absorption spectrum is narrow in relation to the sun's spectrum [15]. Admittedly, direct solar pumping of molecular systems would require a very large collector and radiator. An attractive solution to some of the limitations of direct solar pumping is one in which focused sunlight can be used to efficiently heat an intermediate blackbody cavity, as suggested by Christiansen [16]. Focused sunlight is coupled to the blackbody cavity through an entrance hole, heating the cavity which, in
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