Power, Methods of Generating

All space vehicles, whether robotic probes or vehicles for human exploration, require electrical power. Electrical power is required to run the computers and control systems, to operate the communications system, to operate scientific instruments, and to power the life support equipment to keep humans alive and healthy in space. For missions to the surface of the Moon or the planets, power may be required to run rovers, or to process material into useful products such as fuel or oxygen. Advanced rocket engines such as ion drives also run on electrical power.

Electrical power sources can be categorized into three basic types: batteries and fuel cells , solar power systems, and nuclear power systems.

For short missions, power can be provided by batteries or fuel cells, which produce power from chemical energy. Fuel cells are similar to batteries, producing electricity from a fuel and an oxidizer, stored in separate tanks. The space shuttle power system, for example, uses fuel cells that combine hydrogen and oxygen to produce electrical power, as well as by-product water. Primary cells produce power only until the chemical feedstock that powers the reaction is used up. The space shuttle's fuel cells consume about 150 kilograms (330 pounds) of hydrogen and oxygen per day.

A battery that can be recharged with an external source of power is called a rechargeable (or "secondary") battery. A fuel cell is called regenerative if it can electrolyze the by-product water back into hydrogen and oxygen. Rechargeable batteries or regenerative fuel cells can thus be used to store energy from a solar array.

Solar Power Generation

Solar arrays produce electrical power directly from sunlight. Most long-duration space missions use solar arrays for their primary power. Most designs use photovoltaic cells to convert sunlight into electricity. They can be made from crystalline silicon, or from advanced materials such as gallium arsenide (GaAs) or cadmium telluride (CdTe). The photovoltaic cells with the highest efficiency use several layers of semiconductor material, with each layer optimized to convert a different portion of the solar spectrum. The solar intensity at Earth's orbit is 1,368 watts per square meter, and the best photovoltaic cells manufactured today can convert about a third of the solar energy to electrical power. For electrical power when the Sun is not available (for example, when a space vehicle is over the night side of Earth), solar power systems typically use rechargeable batteries for storage.

Solar power systems can also be designed using mirrors or lenses to concentrate sunlight onto a thermal receiver. The heat produced by the thermal receiver then is used in a heat engine, similar to the steam turbines used in terrestrial power plants, to produce power. Systems of this type can store power in the form of heat, instead of requiring batteries, but have not yet been used in space.

Nuclear Power

Since solar power decreases with the square of the distance from the Sun, missions to the outer planets require an alternate power source. Nuclear power systems can provide power even when sunlight is unavailable. Nuclear generators are categorized as "radioisotope" power systems, which generate heat by the natural radioactive decay of an isotope, and "reactor" power systems, which generate heat by a nuclear chain reaction. For both of these power systems types, the heat is then converted into electrical power by a thermal generator, either a thermoelectric generator that uses thermocouples to produce power, or a turbine. For radioisotope power systems, the most commonly used isotope is Plutonium-238. The plutonium is encapsulated in a heat-resistant ceramic shell, to prevent it from being released into the environment in the case of a launch accident. Such isotope power systems have been used on the Pioneer, Voyager, Galileo, and Cassini missions to the outer planets (Jupiter and beyond), where the sunlight is weak, and also on Apollo missions to the surface of the Moon, where power is required over the long lunar night.

Solar Power Satellites

Scientist Peter Glaser has proposed that very large solar arrays could be put into space and the power generated by the solar arrays can be transmitted to the surface of Earth using a microwave or laser beam. Glaser argues that such a "solar power satellite" concept would be a pollution-free source of low-cost solar power, and that by putting the solar power system above the atmosphere, 24-hour power could be produced with no interruptions by clouds or nighttime. To be practical, such solar power satellites will require a reduction in the cost of manufacturing solar cells, and new methods of low-cost launch into space.

see also Solar Power Systems (volume 4).

Geoffrey A. Landis

Bibliography

Glaser, Peter, F. P. Davidson, and K. I. Csigi. Solar Power Satellites: The Emerging Energy Option. New York: Ellis Horwood, 1993.

Green, Martin. Solar Cells: Operating Principles, Technology, and System Applications. Englewood Cliffs, NJ: Prentice-Hall, 1982.

Landis, Geoffrey A., Sheila G. Bailey, and Barbara I. McKissock. "Designing PowerSystems." In Human Spaceflight: Mission Analysis and Design, eds. W. J. Larson and L. Pranke. New York: McGraw-Hill, 1999.