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The feasibility of solar power stations orbiting the Earth and sending power to the Earth's surface, was investigated during the 1970s in response to the oil embargo of the United States. The National Aeronautics and Space Administration (NASA) and the U.S. Department of Energy (DOE) studied the concept of the Solar Power Satellites System (SPSS), which consisted of placing about 60 satellites containing large photovoltaic arrays in stationary orbits above the earth. Each satellite would have a matching receiving rectifying antenna (rectenna) on the ground. The satellites would have transmitted a fixed microwave beam to the ground station. The microwave transmission system envisioned by NASA and DOE would have had three aspects:
- The conversion of direct current (DC) power (from the photovoltaic cells on the satellites) to microwave power on satellites on geosynchronous (stationary) orbit above the earth;
- The formation and control of microwave beams aimed precisely at fixed locations on the earth's surface;
- The collection of the microwave energy and its conversion into electrical energy at the earth's surface.
Each SPSS would have been massive, measuring 6.5 miles (10.5 kilometers [km]) long and 3.3 miles (5.3 km) wide, or 21 square miles (55.7 square kilometers) in area. The surface of each satellite would have been covered with 400 million solar cells.
The transmitting antenna on the satellite(s) would have been about ½ mile in diameter (1 km) and the receiving antennae on the earth's surface would have been about 6 miles (10 km) in diameter. Massive structures such as this would have been a significant engineering challenge.
Because of their size, the satellites would have been constructed in space. The plan envisioned sending small segments of the satellites into space using the Space Shuttle. The materials would have been stored at work stations in low earth orbit, and then towed to the assembly point by a purpose-built "space tug" (such as operating the space shuttle).
Cost was the major obstacle to development of the SPSS. When the NASA-DOE report was completed in 1979, the estimated cost for building a prototype was $74 billion. Construction of an SPSS system would have taken about 30 years to complete. At the time, the United States did not appropriate funds to begin construction. Other countries, such as Japan, are currently exploring the concept of solar power stations in space.
NASA has continued research into the concept of space-based power stations under its Space Solar Power Technology Advanced Research & Development Program. The goal of the program is to conduct preliminary strategic technology research and development to enable large, multi-megawatt to gigawatt-class space solar power (SSP) systems and wireless power transmission (WPT) for government missions and commercial markets (in-space and terrestrial).
A September 2001 review of NASA research on solar power stations by the National Research Council found the research worthwhile, although it noted that the funding levels would have to be much higher to achieve its goals. The review noted that "dramatic reductions" would be needed in the cost of placing the system into Earth orbit and in the cost of the solar panels, and that significant progress is still needed in the technology to manage the power in space and beam it back to Earth.
Other countries, such as Japan, are currently exploring the concept of solar power stations in space. The National Space Development Agency of Japan (NASDA) has plans to develop a satellite-based solar power system. The concept is for a large satellite carrying solar panels placed in orbit at an altitude of 36,000 kilometers. Electricity generated by the satellite will be first transmitted in the form of a laser to an airship floating in a lower orbit at 20 kilometers above the earth, and then to ground-based antennas as micro waves or via optical fibers. The plan still has to deal with many challenges, such as launch costs for solar panel construction and technical studies of laser transmission. The project team currently expects to complete basic laser technologies in the next 10 years and has set a goal to conduct its first power transmission test in 2025.
More recently a different approach has been proposed: to use smaller, low-orbiting satellites transmitting to smaller stations scattered around the earth's surface. Some physicists have suggested that existing communications satellites can be put to use sending energy to the earth.
Credits: US Department of Energy (http://www.eere.energy.gov/consumerinfo/factsheets/l123.html)