animal manure Dung and urine of animals used as a form of organic fertilizer. Compare green manure.

biofuel Gas or liquid fuel (such as ethyl alcohol) made from plant material (biomass). 

cogeneration Production of two useful forms of energy, such as high-temperature heat or steam and electricity, from the same fuel source.

energy efficiency Percentage of the total energy input that does useful work and is not converted into low-quality, usually useless heat in an energy conversion system or process. See energy quality, net energy. Compare material efficiency.

Geothermal energy Heat transferred from the earth's underground concentrations of (1) dry steam (steam with no water droplets), (2) wet steam (a mixture of steam and water droplets), or (3) hot water trapped in fractured or porous rock. 

Hydroelectric power plant Structure in which the energy of falling or flowing water spins a turbine generator to produce electricity.

hydropower Electrical energy produced by falling or flowing water. See hydroelectric power plant.

kilowatt (kW) Unit of electrical power equal to 1,000 watts. See watt. 

life cycle cost Initial cost plus lifetime operating costs of an economic good. Compare full cost.

manure See animal manure, green manure. 

Micropower systems Systems of small-scale decentralized units that generate 1-10,000 kilowatts of electricity. Examples include (1) microturbines, (2) fuel cells, and (3) household solar panels and solar roofs. 

net energy Total amount of useful energy available from an energy resource or energy system over its lifetime, minus the amount of energy (1) used (the first law of thermodynamics), (2) automatically wasted (the second law of thermodynamics), and (3) unnecessarily wasted in finding, processing, concentrating, and transporting it to users.

passive solar heating system System that captures sunlight directly within a structure and converts it into lowtemperature heat for space heating or for heating water for domestic use without the use of mechanical devices. Compare active solar heating system. 

photovoltaic cell (solar cell) Device which converts radiant (solar) energy directly into electrical energy. 

Radioactive waste Waste products of nuclear power plants, research, medicine, weapon production, or other processes involving nuclear reactions. See radioactivity. 

solar cell See photovoltaic cell.

solar collector Device for collecting radiant energy from the sun and converting it into heat. See active solar heating system, passive solar heating system.

Superinsulated house House that is heavily insulated and extremely airtight. Typically, active or passive solar collectors are used to heat water, and an air-to-air heat exchanger is used to prevent buildup of excessive moisture and indoor air pollutants.

watt Unit of power, or rate at which electrical work is done. See kilowatt. 

wind farm Cluster of small to medium-sized wind turbines in a windy area to capture wind energy and convert it into electrical energy.