increases by transfer of heat from another part of the cycle. From the recuperator the gas passes to the receiver where collected heat is added and the gas temperature rises to the maximum level in the cycle. The high temperature, high pressure gas then flows to the turbine where it expands to a lower temperature and pressure to produce mechanical work. A portion of the turbine work drives the compressor and the remainder drives the alternator, producing electrical energy. After leaving the turbine, the gas passes through the low pressure side of the recuperator where it transfers a large part of its remaining heat energy to the high pressure gas from the compressor. From the recuperator, the gas passes through the heat rejection system where it gives up more heat which is radiated to space. The cold gas then returns to the compressor, completing the loop. The temperatures of the state points in the CBC have been selected so that refractory materials are not needed anywhere in the system. The gaseous working fluid is a mixture of helium and xenon with an equivalent molecular weight of 40, which results in the best combination of heat transfer and thermodynamic performance. Since the Brayton cycle is all gas, it is essentially insensitive to gravitational forces. Therefore, components and the system can be designed for space operation and the performance proven with confidence in test facilities on earth. A solid rotor, Lundell type, three phase alternator mounted on a common shaft with the turbine and compressor converts mechanical energy to electric energy as three phase power. Electronic frequency changer equipment converts the three phase electric power from the alternator to distribution quality power. The SD system must convert all of the energy collected by the concentrator since it is impractical to modulate, or control, energy collection. Therefore, variations in solar input energy (insolation) and in electric load demand are accommodated by a combination of control of the total amount of gas in the closed loop and a controllable parasitic electric load. The gas inventory in the loop is increased or decreased by valves connecting the accumulator to the compressor inlet or outlet, respectively. Description of SD Power Module and Its Components Fig. 3 is a photograph of a model of the SD power module and in Fig. 4 the main components and assemblies of the module are indicated. The module includes six bays of the common 5-metre truss structure used on Freedom. The SD functional equipment is attached to the outermost bay by a single degree of freedom gimbal (beta gimbal). The inner five bays assure adequate clearance for rotation of the equipment and to prevent shadowing of the concentrator by other power modules. The axis of rotation of this beta gimbal is at 90 degrees to the axis of the alpha gimbal. The combined operation of the alpha and beta gimbals provide the coarse pointing of the concentrator. The alpha gimbal rotates once per orbit to provide orbit-by-orbit sun tracking. The beta gimbal oscillates very slowly through an arc of about + and —52 degrees to track the sun through its periodic variation in position relative to the plane of Freedom's orbit. The major assemblies in the SD power module are: (1) the concentrator with its support structure and two-axis fine pointing gimbal, (2) the receiver, (3) the power conversion unit (PCU), (4) the heat rejection assembly, (5) the electrical equipment assembly (EEA), and (6) the beta gimbal. These assemblies are all mounted to, and tied together by, the interface structure, the seventh major assembly. Electronics for control, power conditioning, and data handling are in the electrical equipment assembly. All the SD components are designed and packaged so that two complete SD
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