SPACE SOLAR POWER SYSTEMS

INTRODUCTION

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.

GENERAL DESCRIPTION

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.

ECONOMIC CONSIDERATIONS

Cost and economic analyses have indicated promise of commercially competitive electric power costs even i f the development of the powersat i s amortized by operational revenues. A busbar cost of 6.9 mills ($0.0069) per megajoule (25 mills ($0.025) per kilowatt hour) has been used in the Boeing studies. This rate is about equal to projected busbar costs for nuclear power in the 1990′s; it is slightly more than that for most current coal-fired electric power generation but is cheaper than that for current oil-fired power. The 25-mill busbar cost enables an allotment of $60 billion for development and $13 billion per satellite, with use of an 8-percent discount rate, an investment horizon of 30 years beyond beginning of service of the first satellite, and addition of one satellite (10 000-megawatt ground output) per year to the system. Cost studies indicate that the $13 billion per satellite should be divided roughly as $8 billion for satellite hardware (including orbital assembly costs ) , $4 billion for space transportation, and $1 billion for the ground station. Preliminary estimates of powersat system costs fall within these targets.

Technical feasibility of the powersat concept is not in question. It could undoubtedly work. The issue is implementation at an acceptable cost, such that an economically feasible project is the result. Boeing’s studies have outlined practical approaches to an economically promising system.

Closed-cycle turbogenerators offer efficiency, compactness, light weight, and low cost for conversion of heat energy to electricity. Helium turbines are under development for Earth-based applications — the largest yet operated has a 50-megawatt rating. Scale-up to the projected powersat size of 300 megawatts would be a straightforward development; this size has been studied for nuclear reactor applications. The closedcycle helium gas turbine provides compatibility with desirable cycle-limit temperatures and enables use of high temperatures without corrosion or oxidation.

A simple geometric principle was used by Boeing to construct and test intrinsically flat mirrors of metallized plastic film during a heliostat research project for groundbased solar power. These mirrors are very light in weight and do not require precision parts. The test mirrors had 2.5 square meters (27 square feet) of reflector and weighed approximately 453.6 grams (l pound) each The powersat would use thousands of mirrors and similar design, approximately 1114.8 square meters in size (12 000 square feet or approximately 0.25 acre). Each would be individually steered by a small servo system to direct its energy to the central cavity. The use of steered, flat mirrors provides the requisite high concentration of sunlight at light weight and low cost and eliminates the need for a large, massive high-precision structure. In space, the protective plastic bubble is not needed.

The 54 431.1-megagram (60 000 ton) satellite cannot be transported to orbit in a single flight. It will be assembled in space from subassemblies of manageable size. The National Aeronautics and Space Administration (NASA) Skylab program demonstrated the effectiveness of men working in space. The space assembly Job is analogous to the construction of powerplants on Earth, with transportable elements at a field location.

Transmission of the converted power to Earth was early recognized as a key problem. Microwave transmission studies and experiments by the NASA, Jet Propulsion Laboratory (JPL), and Raytheon have demonstrated the principle of efficient transmission. Efficiencies of about 90 percent for each of the three transmission steps (electric-to-microwave conversion, antenna-to-antenna transmission, and microwave-to-electric conversion) are indicated, and these values lead to a predicted transmission link efficiency of 70 percent or better.

System economics are dependent on low-cost space transportation. Flight to orbit is not intrinsically expensive; that is, the basic energy cost is not high. The cost of propellants for a large rocket (Saturn or the projected’ space freighter) is less than $11.02/kg ($5/lb) of payload. The history of rocket development from Vanguard to Saturn shows reduced cost through Increased efficiency and size. By partial reuse, the shuttle will eliminate a large part of the highest cost element of Saturn. Through complete reuse, efficiency that results from large size, and efficiency that results from using launch crews at high launch rates, the space freighter can keep costs below $44.09/kg ($20/lb), according to Boeing studies.

SETUP AID OPERATION METHODOLOGY

Powersat configuration.

Powersat configuration.

The size of the power sat is awesome, primarily because normally space payloads are throught of as being fully assembled on Earth and then launched into space. Skyscrapers, dams, or nuclear powerplants would be equally awesome if they were thought of as being transported, fully assembled, from one place to another on Earth.

Powersat parts and subassemblies would be transported to a low Earth orbit, similar to the Skylab orbit, in hundreds of flights of a low-cost space freighter, shown in comparison with the Apollo-Saturn moon rocket. The freighter would burn nonpolluting hydrogen fuel as did the Saturn upper stages. Its “fat Albert” shape provides a large-volume payload bay and enables controlled aerodynamic return to Earth and a rocket-arrested soft landing. The freighter shown has been sized for 226 796.2 kilograms (250 tons) of payload and would use space shuttle engine technology and a watercooled heat shield to achieve total reusability and a ground handling time between flights of less than 1 day.

The destination of the space freighter is an assembly station in space, manned by a construction crew. In the zero gravity of orbit, the lightweight structure of the powersat can be safely deployed with a minimum of effort and all the parts can be installed, connected, and checked out.

Completed power generation modules would be efficiently transferred to a stationary orbit at 35 405.6 kilometers (22 000 miles) by electric rockets drawing electric power from the modules. The low thrust of the electric rockets would not overstress the lightweight structure and could effect the transfer in about 3 months. Small conventional rockets would assist the electric rockets in keeping the power module pointed at the Sun. The electric rocket engines and their controls would be returned to the assembly station to be used again and again. Final assembly of the powersat occurs in the operational orbit by joining the modules together and attaching the microwave transmitter, transported by one of the power generation modules, at its operating position.

The power transmission system must efficiently move the electric energy obtained from the Sun over 35 405.6 kilometers (22 000)miles of space to the Earth’s surface for use. Efficiency is critical. To maintain the same useful power output on the ground if transmission efficiency is reduced 10 percent, the entire powersat must be made 10-percent larger.

Wireless transmission of power, rather than just signals, has been foreign to our thinking. However, the possibility was originally recognized by Nikola Tesla about 75 years ago. Recent experiments at Raytheon and the JPL have demonstrated overall link efficiencies greater than 50 percent. Still to be accomplished are the major engineering developments of precision electronic phase control for beam forming and further conversion element efficiency Improvement to reach the projected 70-percent transmission
efficiency.

The transmission system would use amplitron radiofrequency generators much like microwave oven tubes, and a wavelength of approximately 12.7 centimeters (5 inches). Maximum beam strength at the center of the receiving antenna is approximately 484.4 W/m2 (45 W/ft2 ). At the antenna edge, the beam strength is well below public exposure standards.