The two most deeply investigated SPS concepts, the Boeing and the Rockwell designs - and consequently both reference concepts - use photovoltaic devices for power conversion, although a closed cycle thermodynamic conversion is considered as a potential competitor. Boeing proposes 50 urn Silicon cells <fig. 2.1.1) without concentration. The cells are expected to perform 16.5 % efficiency at 36.5 C, AMO. This should be partially achieved by the v-shaped grooves in the c^ver glass. The blanket2unit mass is expected to be 0.427 kg/m with a cost of £ 35/n/. These cells are planned to be angealed by a laser device at 500° C (a more recent figure says 350 C, or even lower, because of severe problems expected with the higher temperature). Fig. 2.1.2: GaALAs SOLAR CELL BLANKET CROSS SECTION [Source: Rockwell ] The Rockwell team chose GaAlAs cells with a concentration ratio (CR) of 2 (fig. 2.1.2). These cells are expected to perform an efficiency of 18.2 % at 125 C (operating temperature) or 20.0 % at 28°C, AMO. The GaAlAs cells under investigation show very good annealing characteristics at operating temperature, this is called self-annealing because no extra device will be required. The specific weight of the blanket is 0.252 kg/m^ at a cost of $ 71/m . As a material of the reflectors 12.5 urn aluminised Kapton was selected with reflectivities of 90 % BOL and 83 % EOL respectively. The specific weight is 0.018 kg/m2.
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