SOLAR POWER SATELLITE PROGRAM REVIEW GLOSSARY Absorption The process in which incident radiant energy is retained by a substance. Angle of incidence The angle at which a ray of energy or an object impinges upon a surface. It is measured between the direction of propagation of the energy or object and a perpendicular to the surface at the point of impingement or incidence. Angstrom A unit of measure equalling 10"^ meter. Automated beam builder A device that continuously produces structural beams, from coils of metal, to build structures in space. Blanket A sheet of photovoltaic cells. British thermal unit (Btu) A unit of energy defined as the heat required to raise the temperature of one pound of water one degree Fahrenheit; it is equal to 252.1 calories or 1055 joules. Beam width The angular width of a beam of radiation, measured between the directions in which the power intensity is a specified fraction, usually one-half, of the maximum. Closed Brayton cycle A sol ar-thermal power-conversion process in which helium is compressed and then heated by solar energy causing the gas to expand. The expanding gas turns turbines which generate electricity. The heated helium then goes through a radiator, which rejects the heat into space and the cycle begins again. Comparative assessment A DOE comparison of the benefits and problems of SPS, and other circa-2000 potential energy options. SPS assessment subjects include cost and performance, environmental impacts, health and safety effects, resource utilization, economic and social factors, and institutional considerations.
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