Originally, wood was the primary heat energy source, then coal provided the energy source for western industrialization; and later petroleum and natural gas fueled society's thirst for energy. Fig ? shows world energy use since 1800. Modern conveniences have changed energy use, examples include, appliances, air conditioning, heat pumps, and (future) electric cars. Much of today's energy is supplied in the form of electricity.
Energy sources can basically be broken into renewable and depletable sources, which ultimately originate from space and terrestrial sources, respectively. The space sources include solar and lunar origins where the latter gravitational energy results in tidal flows. The sun provides directly useable energy as well as indirectly providing hydro, wind, ocean (thermal & current), and even biomass through photosynthesis. Depletable sources include fossil fuels, nuclear, and geothermal (geothermal is depletable but not always thought of in that manner).
Energy is then applied to one of the end-use sectors or the intermediate energy form of electricity. The end-use sectors include industrial, transportation, residential and commercial. The Department of Energy's (DOE) Energy Information Administration (EIA) has a nice energy flow diagram shown below from the Annual Energy Review.
Most of the electricity today is supplied by coal-fired and nuclear power plants. Nuclear power plants are much like fossil units except from where the heat originates, fission versus combustion, respectively. Another well known electricity generator is the hydroelectric plant that converts potential energy to electricity using a hydraulic turbine.
Utility demand patterns have variations on diurnal (daily), seasonal (summer vs. winter), and random (cloudy day) bases. For example, the common place use of air conditioning has changed peak load patterns from winter to summer. Individual power systems must meet their own grid demands, and hence, must plan for long-term growth far enough in advance such that construction of new plants can be completed before the future demand manifests itself. Electricity demand is pretty much a linear function of the gross national product (GNP).
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