Primary Energy Sources, Transformations, and Efficiency

Every device that does useful work draws on a primary energy source — an energy resource found in nature that has not yet been converted into another form. Primary sources are traditionally grouped into two families. Non-renewable sources, such as coal, oil, natural gas, and nuclear fuels, exist in finite quantities and are consumed far faster than natural processes can replace them. Renewable sources, such as solar radiation, wind, flowing water, tides, and geothermal heat, are replenished continuously on human timescales. A useful way to trace the origin of most of these is to notice that the overwhelming majority ultimately derive from the Sun: fossil fuels store ancient sunlight captured by photosynthesis, wind arises from uneven solar heating of the atmosphere, and hydroelectric energ