Conceptual diagram of 1980 Solar Power Satellite "Reference Design" [3]. 

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... the increases in energy cost and recent interest in finding ways to produce energy with reduced emission of greenhouse gasses, there has been renewed interest in the concept of producing power using solar panels in space, and then beaming this power downward to provide electrical power for use on the Earth. This concept, called the "Solar Power Satellite," was first proposed by Peter Glaser in 1968 [1], and, in revised and updated form, has been proposed many times since [2-5] as a possible solution to the energy crisis. It is the purpose of this paper to examine this concept on a physics basis, reviewing the basic concepts in order to assess the current and future feasibility without adopting either a advocating or critical stance. The Satellite Power System was studied in the late 1970s by NASA in collaboration with the Department of Energy, producing a conceptual "reference" design [3] for a system. This design was analyzed and critiqued by an Office of Technology Assessment study [4]. The concept has gone by several different names and acronyms, starting as the "Solar Power Satellite" (SPS) or "Satellite Solar Power System" (SSPS), and more recently studied under the name "Space Solar Power" (SSP). Figure 1 shows a summary of the 1980 "Reference Design" for such a solar power satellite. The baseline satellite concept produces about 10 GW of electrical power at the Earth, using a large (10 km by 15 km) solar array located in Geosynchronous orbit. The power is transmitted to the Earth by a microwave beam at 2.45 GHz, and a large (approximately 100 square kilometers) rectifying antenna (or "rectenna") array at Earth receives the beamed microwave power and converts it into DC electrical power. The early study also looked at several alternate technologies for both the energy conversion and the power transmission, and made attempts to predict the In 1995, NASA headquarters initiated a "Fresh Look" study of solar power satellites, which did not revise the original concepts, but started with a clean sheet of paper to re-think the basic concepts and come up with a new design [5]. Some of the initial concepts examined included use of low Earth orbit instead of geosynchronous orbit, gravity gradient stabilized structures, sun-synchronous orbits, and use of large- area Fresnel lenses to focus light onto panels of concentrator cells. Later evolution of the Fresh Look study (which evolved into the Space Solar Power Exploratory Research and Technology (SERT) program [6-8] in the 1999-2000 time frame) returned to the geosynchronous orbit design, but continued to look at new options for satellite design, including alternative power transmission methods such as laser beaming. In addition to these NASA studies, there have been several non-NASA studies of the concept [9], as well as a number of studies that have been proposed to use power-beaming technology for other applications, including in- space applications such as satellites, electric-propulsion, and lunar bases [10]. It's worth noting that in 2004, the North American electrical energy generation was 4730 Terawatt-hrs, about 45 percent of which was generated by coal-fired plants. At a typical production cost of 5 cents per kW-hr, this represents a revenue of 230 billion dollars per year for the potential market in North America alone. Thus, although the concept requires mega-engineering on a scale that would dwarf all previous space projects, the potential revenue from it is extremely large. The ability to use solar power generated in space for terrestrial use requires beaming the power from the source, in space, to the user on the ground. This is done by converting the electrical power to electromagnetic radiation, beam the radiation across free space, and collecting it at a receiver that converts electromagnetic radiation to electrical power. In the 1980 study [3] (and most of the subsequent studies), it was proposed to do this using radio-frequency ("RF") beams at a frequency of 2.45 GHz (that is, microwaves in the Industrial, Scientific and Medical Band). This was chosen because of the high demonstrated conversion efficiency of electrical power to microwave energy, and because of the transparency of the atmosphere to microwave radiation. In the "ideal" case, RF power beaming efficiency can be quite high. At these frequencies, magnetron tubes can convert DC to RF efficiency at transmitter efficiency of 90% or better, and a rectenna array (consisting of an array of GaAs Schottky diodes and quarter wave antennas) can convert RF power back to DC at receiver efficiency that has been demonstrated as high as 86%. The product of these two efficiencies yields an overall "potential" transmission efficiency, DC in to DC out, of ~77% In the real world, however, it is very easy to produce much degraded efficiency. For example, Neville Marzwell estimated the following potential losses in a “Real World” estimated RF Link efficiency ...