Solar Power Systems
The first use of solar cells in space occurred on the satellite Vanguard 1, which was launched on March 17, 1958. Eight tiny panels were installed symmetrically around the satellite to ensure power generation during the satellite's random tumbling. They delivered fifty to one hundred milliwatts of power and provided secondary electricity for a beacon signal generator* . Each panel had six square silicon cells, measuring 2 centimeters (0.79 inch) by 2 centimeters (0.79 inch) by 0.4 centimeters (0.16 inch), with a photovoltaic* (PV) conversion efficiency of approximately 10%. The panels of solar cells were protected by a thick cover glass to avoid radiation damage from electrons and protons trapped in the Van Allen radiation belts* that surround Earth. The longevity of Vanguard 1's beacon signal surpassed all expectations—lasting until May 1964. As a result, future regulations required shutting down satellite power supplies to avoid cluttering the radio wave spectrum with unwanted signals from satellites no longer in use.
The early use of solar cells was tentative, but they eventually emerged as the primary power source for satellites that were required to operate for more than a few weeks. While radioactive materials can, and have, been used to power spacecraft, their use can be problematic since the radioactive material is inherently quite dangerous to humans and other life forms. Because of the potential harm those radioactive substances can
pose to people for long periods of time if they are accidentally released into the environment, so-called atomic batteries (also referred to by other labels, such as radioisotope generators) are not used in the vast majority of spacecraft presently in use.
Solar cells improved steadily and successfully met the unique power requirements of space travel while other possible competitive sources were proven unfeasible for most space missions. Over the years, solar cells have provided electricity to thousands of space missions that operated near Earth, on the Moon, and in planetary or interplanetary missions.
Telecommunication satellites require several kilowatts of electric power, while most other satellites require several hundred watts. The long-duration human missions, Skylab, launched in 1973, and the Russian space station Mir, launched in 1986, each used 25 kilowatts from solar cells. The International Space Station has solar PV modules with a total rated generation capacity of approximately 130 kilowatts of DC (Direct Current) electrical power. The space station's solar array* has eight “wings” and operates at 160 volts DC. The International Space Station's solar array was completed in March 2009 with a delivery of the last two solar wings aboard the space shuttle Discovery. After being fully extended, each of the
space station's eight wings is 11.6 meters (38 feet) wide by 32.9 meters (108 feet) long. In each wing there are 32,800 square silicon cells that are 8 centimeters (3.15 inches) by 8 centimeters by 0.2 centimeters (0.08 inches) thick with an average conversion efficiency of 14.2%.
The total power of the solar array of the International Space Station is more than one million times greater than that produced by the first solar panels on Vanguard 1. Current and future manned missions—whether Earth orbiting, lunar, or planetary—will require power systems that are an order of magnitude larger than that of today's largest satellite power systems.
Current Space Solar Cells and Systems
A typical solar cell is a diode illuminated by sunlight. Diodes are prepared by forming a pn single junction* in a semiconductor* such as silicon. For this purpose, an n-type region in which electrons are negatively charged majority carriers* is grown on a p-type base in which holes are positively charged majority carriers or vice versa. The early silicon cells were of p on n type. The first commercial communication satellite, Telstar 1, launched in 1962, used n on p silicon cells. The cell design and performance remained fairly static during the 1960s. During the 1970s, efficiency was increased to about 14% by improving the cell design. The improvements were achieved by forming a heavily doped* rear interface known as back surface field; applying photolithography* to create finer, more closely spaced front grid fingers; and applying a texture and antireflection coating to the cell surface. Efforts were also focused on reducing costs associated with PV array components by, for example, decreasing interconnect costs and using larger cells and lighter arrays.
In the 1980s and 1990s, gallium arsenide-based solar cells were developed. Gallium arsenide on germanium solar cells were developed to increase the size and to reduce the thickness of cells by increasing the mechanical strength. Multijunction cells using gallium arsenide, gallium-aluminum arsenide, and gallium-aluminum phosphide on germanium were developed to effectively use a larger portion of the solar spectrum. Typical efficiencies for gallium arsenide-based cells range from 18.5% for single junction diodes to a little over 30% for triple junction diodes.
Gallium arsenide cells are used for critical space missions that require high power. The manufacturing costs of gallium arsenide on germanium cells are six to nine times that of silicon cells. The overall cost is reduced, however, because of the higher efficiency of the cells. More satellites can be launched on a single rocket* because of the smaller array area. Limiting the total area of the solar array is an important factor in the viability of space power arrays because of the estimated attitude control costs, which for a ten-year Geostationary Earth Orbit* (GEO) mission amount to $48,000 per square meter. In this regard, high-efficiency multijunction arrays are more attractive for missions requiring higher power. Megawatts of power required by larger satellite constellations will be satisfied with multijunction solar cells having still higher efficiencies of greater than 30%. Iridium, a constellation of sixty-six communication satellites,
required a total power of 125 kilowatts for each of its satellites and used gallium arsenide on germanium solar cells. The spectacular performance of the Mars Pathfinder mission to Mars was made possible by the mission's gallium arsenide on germanium cells. The cells provided all the necessary power, including 280 watts for the cruise module, 177 watts for the lander, and 45 watts for the Sojourner rover.
An important consideration for satellite programs is the weight of the spacecraft. Currently, the power system typically takes up about a quarter of the total spacecraft mass budget; in turn, the solar array and support structure comprise about a third of the power system mass. Launch costs are estimated to be $11,000 per kilogram to reach low Earth orbit* and $66,000 per kilogram to attain GEO. Hence, reducing spacecraft mass has a potential added benefit of lowering launch costs.
Crystalline silicon and gallium-arsenide-based solar cells, currently employed in space solar PV power arrays, are rigid and fragile. Therefore, the PV arrays employ a honeycomb core with face sheets of aluminum, or alternatively very lightweight Kevlar®* or graphite fibers. The PV array blanket is folded in an accordion style before placement in a canister. Deploying the array can pose problems. This happened in December 2006, with the large solar array on the International Space Station. STS-116 astronaut Robert Culbeam had to make an unscheduled spacewalk in order to manually unfurl a solar array after the automatic procedure did not work.
Manufacturing costs for solar arrays are an important consideration for the total spacecraft budget. The array manufacturing costs for a medium-sized, five-kilowatt satellite can exceed two million dollars. Current single-crystal technology can cost more than $300 per watt at the array level and possess a mass of more than one kilogram per square meter equivalent to a specific power* of about 65 watts per kilogram.
Future missions would include very large solar power satellites as well as very small satellites. Some long-term plans envision swarms of very small, distributed, autonomous satellites called microsats (short for microsatellites), nanosats (nanosatellites), and picosats (picosatellites) to perform specific tasks. In all these missions, reducing the total system cost would become increasingly more important. Highly efficient gallium arsenide-based multi-junction cells, concentrator systems, and thin-film cells are being developed, and will undoubtedly find uses in future space missions. Copper-indium-gallium selenide-sulfide or amorphous hydrogenated silicon thin-film solar cells may be able to reduce both the manufacturing cost and the mass per unit power by an order of magnitude from current levels. Moving to a thin-film technology could conservatively reduce array-manufacturing costs to less than $500,000 from the current cost of $2 million for a medium-sized five-kilowatt satellite. For small satellites, increasing the solar array specific power from a current typical value of 65 watts per kilogram will allow for either an increase in payload* power or payload mass, or both.
Weight benefits of higher efficiency cells are decreased and high costs become less affordable in the case of flexible thin-film blanket arrays that can be easily rolled out. Nonrigid cells also have an advantage in stability. For example, flexible amorphous hydrogenated silicon solar arrays have continued to function after being pierced by tiny meteorites* .
Solar Electric Propulsion
Some missions will use solar electric propulsion instead of chemical rocket engines. In solar electric propulsion, electric power obtained from sunlight is used to ionize a gas and then to accelerate and emit the ions. The spacecraft is propelled in the forward direction as a reaction to the emission of ions going in the opposite direction. This technology has been successfully demonstrated in the Deep Space 1 (DS1) mission. Because of the low initial velocities and steady acceleration, however, solar electric propulsion satellites must spend long periods in intense regions of trapped radiation belts. Studies since the year 2000 have clearly shown that copper-indium-gallium selenide-sulfide solar cells are superior to the conventional silicon and gallium arsenide solar cells in the space radiation environment. The potential for improved radiation resistance of thin-film solar cells relative to single-crystal cells could extend the mission lifetimes substantially. Large-area amorphous silicon modules were successfully demonstrated on flexible substrates* on the Russian space station Mir. The efficiency was relatively low but the solar cells remained stable in the space environment.
Studies since 1999 have shown that thin-film cells would start to become cost-competitive in GEO and LEO missions at an efficiency of 12.6%. Significant technological hurdles remain, however, before thin-film technology could be implemented as the primary power source for spacecraft. A large-area
fabrication process for high-efficiency cells on a lightweight substrate has not been demonstrated. Research efforts are being concentrated on the development of a large-area, high-efficiency thin-film solar cell blanket on a lightweight, space-qualified substrate that will survive severe mechanical stresses during launch, then operate for extended periods in the space environment.
Books and Articles
Bailey, Sheila G., and Dennis J. Flood. “Space Photovoltaics.” Progress in Photovoltaics: Research and Applications 6 (1998):1–14.
Iles, Peter A. “From Vanguard to Pathfinder: Forty Years of Solar Cells in Space.” Proceedings of Second World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, Austria (1998):LXVII–LXXVI.
———. “Evolution of Space Solar Cells.” Solar Energy Materials and Solar Cells 68(2001):1–13.
Karam, Nasser H., et al. “Development and Characterization of High-Efficiency Ga0.5In0.5P/GaAs/Ge Dual-and Triple-Junction Solar Cells.” IEEE Transactions on Electron Devices 46 (1999):2,116–2,125.
Kurtz, Sarah R., Daryl R. Myers, and Jerry M. Olsen. “Projected Performance of Three- and Four-Junction Devices Using GaAs and GaInP.” Proceedings of Twenty-Sixth IEEE Photovoltaic Specialists' Conference (1997):875–878.
Patel, Mukund R. Spacecraft Power Systems. Boca Raton, FL: CRC, 2005.
Ralph, Eugene L., and Thomas W. Woike. “Solar Cell Array System Trades: Present and Future.” Proceedings of Thirty-Seventh American Institute of Aeronautics and Astronautics Aerospace Sciences Meeting and Exhibit (1999):1–7.
Knier, Gil. How Do Photovoltaics Work? National Aeronautics and Space Administration. <http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/ > (accessed October 23, 2011).
Solar Energy Technologies Program. U.S. Department of Energy. <http://www1.eere.energy.gov/solar/ > (accessed October 23, 2011).
Space-Based Solar Power as an Opportunity for Strategic Security. National Security Space Office. <http://www.nss.org/settlement/ssp/library/final-sbsp-interim-assessment-release-01.pdf > (accessed October 23, 2011).