1 and 2; loop 1 is the potassium loop and loop 2 a steam loop. In each of the energy- exchanger/binary-cycle units, the potassium vapor flows first through an energy exchanger where it undergoes an adiabatic expansion, cooling to 1065 K. The expanded potassium then passes through a heat exchanger where it rejects heat to loop 2 and, in the process, condenses to liquid potassium. The liquid potassium is then pumped up to a high pressure and sent back to the solar cavity for reheating. Loop 1 utilizes a modified Rankine cycle with superheat and loop 2 a high pressure Rankine cycle with superheat (see Ref. 27). The energy exchanger consists of tubes mounted on a rotor, with the tube axes parallel to the rotor axis. In each tube a hot, high pressure gas (the driver gas) enters from (say) the left and pushes a cool, low pressure gas (the driven gas) out on the right. Because the two gases are in contact at the contact surface, there is some intermixing, and a gas separator is required. Once the driver gas has filled up the entire tube, much of its energy has been transferred to the driven gas, and its pressure has dropped to a level at which it is readily pushed back to and out of the left side of the tube by a fresh charge of driven gas entering from the right. Shortly after the contact surface has moved to the left end of the tube, a fresh charge of driver gas is introduced at the left to repeat the cycle. It is worth emphasizing that energy is exchanged by doing work, not by the transfer of heat, which is negligible. Work is done by the driver gas on the driven gas. In the Lockheed system, potassium is the driver gas and a helium-xenon mixture is the driven gas. The input potassium is at 3400 K and the output helium-xenon at 1250 K (1790 °F). Due to metallurgical limitations, the 3400 K input gas is too hot for a turbine to handle. However, in the view of Lockheed engineers, the 1250 K output gas is cool enough to be accepted by a turbine. The energy exchanger can accept a very hot input driver gas, because the energy exchanger sees the average of the hot driver and cool driven gas temperatures. The efficiency of the energy exchanger is estimated to be 85%~95% (28). The mirror of the Lockheed power satellite is a parabola having rotational symmetry about an axis (horizontal in Fig. 14) that connects chamber (A) with the center of the mirror. The turbines in loops 2 and 3 drive generators that produce alternating current. Their output voltages are transformed, rectified, and filtered and then fed into the lasers. The Lockheed system uses 20 lasers, two for each of the 10 energy-exchanger/ binary-cycle units. The lasers are CO2 electric discharge lasers with output radiation at 10.6 p.m. The 20 individual laser beams are then phase-locked by sending the beams through a phasing array in which the optical path lengths are varied so as to compensate for any initial differences in the phases of the entering beams. As a result of this compensation, the beams are all in phase, i.e., phase-locked, when they leave the system. These beams, which are parallel as well as phase-locked, are directed from chamber (A) in Fig. 14 down along one of the power satellite's structural members to a transmitting aperture. If direct line-of-sight transmission is possible, the beam is directed from LEO to a relay in GEO. This beam has a diameter of 31.5 m near the power satellite. Over the — 40,000-km distance from the satellite to the GEO relay, the beam diameter increases to only 42 m. This increase is due to pointing jitter and diffraction. From the GEO relay, the laser beam radiation travels to the ground station (see Fig. 14). The beam diameter increases from 31.5 m at the GEO relay to about 42 m at the ground station, approximately the same increase over the approximately same distance as for the transit from the power satellite to the
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