Power from sunlight — a long-time dream of philosophers and inventors — is hecoming an engineering reality. Solar heating and cooling is beginning to undergo commercial development. A pilot plant phase has begun for groundbased solar electric plants. The ultimate solar power plant — a power station in space called powersat — is being studied by Boeing. But why a power station in space? Isn’t that prohibitively expensive and impractical? The Boeing Company does not think so, and its findings have been summarized in this report.
Two primary candidates for a means of converting solar power to electrical power in space exist: solar cells (photovoltaic) and thermal engines. Although Boeing is investigating both candidates with equal vigor, this report primarily deals with a thermal-engine concept called powersat.
The powersat will be essentially continuously illuminated by sunlight (no night, no weather) and will collect over six times the solar energy falling on any equivalent size area on Earth. Power beamed from the powersat can be coupled to a converter station sited in any part of the nation — or the world, for that matter — to provide continuous baseload electric power. In contrast, early ground-based solar plants will produce intermediate load (i.e., only daytime) power and only in sunny regions. Continuous illumination at higher intensity offers a potential economic advantage to spacebased solar power if the transportation to space can be accomplished at a sufficiently low cost. Boeing’s studies of the system economics indicate that this accomplishment is possible and that the outlook for commercially competitive electric power from satellites is promising.
The powersat envisioned by Boeing would use lightweight mirrors to concentrate sunlight into a cavity and thereby heat the cavity so that It serves as a “boiler.” The heat would then be supplied to turbine generators similar to those in use at conventional powerplants. These machines convert about a third of the input heat energy to electricity; the other two-thirds (the thermal pollution of conventional powerplants) is returned to the environment. The powersat would use space radiators to reradiate this unusable heat to space far from the Earth. The electricity would be converted to a microwave beam for transmission to a receiving antenna on Earth for commercial distribution as electric power.
The powersat would be large — many square kilometers in size — but would produce great amounts of power. A typical design requires h8.6 square kilometers (12 000 acres) of mirrors for 10 000 000 kilowatts of electric output from the ground station. Most of the satellite area consists of thin, reflective plastic film, which minimizes the weight to be transported to space.
The powersat illustrated here is the result of conceptual design studies performed at Boeing over the past year. The four power generation modules shown provide a reasonable compromise between the simplicity of a single large module and the practical considerations of transportation and operation.
Each module consists of a mainframe structure formed from fold-out trusses, a spiderweblike fill-in structure to support the plastic film mirrors, 10 000 to 12 000 lOll.7l-square-meter (0.25 acre) mirrors and their spreader frames, a cavity heat absorber surrounded by twelve 3OO-megawatt helium turbogenerators, and a heat radiator.
Attached to one of the modules on a rotating joint are the microwave generator and antenna. The electric power produced by the turbogenerators is routed to the microwave generator for conversion and transmission.
The parts of the satellite are designed as subassemblies for transportation by the space freighter. For example, one turbogenerator with its heat exchangers and accessories can be packaged on a pallet for a single-launch delivery; the pallet forms a portion of the wall of the cavity heat absorber. Hexagonal plastic film mirrors can be folded and rolled so that many reflectors can be launched together.
Located in a stationary orbit 35 405.6 kilometers (22 000 miles) above the Earth, the powersats will be illuminated by sunlight more than 99 percent of the time. They will appear to hang motionless in the sky, and a simple fixed-position array of antenna elements (dipoles) will serve as the ground-based converter for the power beam. The converter array will be approximately 8.0 kilometers (5 miles) in diameter. Its construction and appearance will resemble cyclone fencing.