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.