IV. Program Engineering: Space Power & Infrastructure Power A space solar power system can consist of four subsystems: collection, conversion, transmission and reception. Current plans call for collection subsystems that are either photovoltaic or solar dynamic. Similarly, planned conversion, transmission, and reception subsystems for space solar power tend to employ either microwaves or lasers. In both cases the major trade-off to consider is power output vs. mass. Recent technology developments in space solar power systems have focused on improving these subsystems by changing the method of incorporating microwaves and lasers into the total space solar power system. New studies integrate sub-system level components into one unit, such as the combination of a transmitter and a solar cell. The advantages of integrated systems for space systems applications include: reductions in mass, size, and thermal losses; an increase in efficiency; and most importantly, a significant decrease in the size of solar power satellites. Traditionally, microwave beaming at 2.45 GHz has been the power transmission frequency chosen for space solar power systems; this frequency was extensively studied as part of the NASA/DOE SPS reference system. Consequently, power beaming technology is most developed at 2.45 GHz. However, it may be wise to consider space solar power transmission at 35 GHz instead. Aside from the non-technical advantages discussed earlier, beaming at 35 GHz allows the delivery of power to a rectenna approximately 200 times smaller in area than the one required by 2.45 GHz beaming, assuming the same power level, antenna size, and transmitting distance (alternatively, one can reduce the size of the antenna, increase the distance of transmission, or any combination thereof). Also, beaming at 35 GHz only heats the ionosphere to l/200th the level of 2.45 GHz beaming. Therefore, by using 35 GHz instead, one can transmit with power densities substantially greater than the 23 mW/cm^ limit on 2.45 GHz beaming used by the NASA/DOE study. Of course, there is the problem of increased atmospheric attenuation at higher frequencies, which is indeed quite serious since not just precipitation but even the mere presence of cumulus clouds will attenuate 35 GHz transmissions. But for space to space applications, for which there are no atmospheric losses, higher frequencies such as 35 GHz are preferable. Therefore, we recommend research and development target improvements at 35 GHz beaming technology, and in particular the development of high-power 35 GHz phased array transmitters. Phased array transmitters, which utilize electronic as opposed to mechanical “steering,” are superior to ordinary reflector transmitters because of new advances in solid state electronic amplifiers which permit the use of high efficiency, low mass and high power antennas for high precision beam pointing and control. Thus, we strongly suggest the space-based testing of phased array antennas and the ground-based fabrication of larger, more powerful phased arrays, especially at 35 GHz. For this reason, three of the design examples that we have considered in depth incorporate phased arrays, and two of these three operate at 35 GHz. In principle, lasers are an even more attractive technology than 35 GHz for power beaming. The divergence of laser beams is minimal, only about a few arc seconds, and theoretically requires much smaller transmitters and receivers, allowing for greater power densities. However, the efficiency of state of the art laser technology is actually quite low. Additionally, the atmospheric attenuation for available laser wavelengths causes a problem for space to earth power transmission. Also, the mass and volume of available lasers need to be reduced by at least two orders of magnitude before this beaming technology can become viable for space solar power. However, when these reductions do occur, the ramifications for space solar power will be immense. Hence we have identified the advent of affordable laser technology as one of the “high leverage” issues raised in question Q5 (see Section I), and it will be more fully discussed in Section VI. Presently, silicon photovoltaic cells are widely used to collect and convert solar energy in orbit with about 15% efficiency. These cells will probably be replaced soon for certain missions by more efficient (close to theoretical limit of 30%) GaAs or InP cells. However, it is our view that Solar Dynamic Systems (SDS) hold considerable promise for the future of power generation, offering potentially higher overall efficiencies in the range of 20-30% at reduced mass and cost when compared to photovoltaic systems. But before SDS can be incorporated into a space solar power system, they need to be space-qualified. In order to solve this problem, we recommend the launch of a solar dynamic power demonstration mission. Towards this end, the successful proposed installation
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