The energy sources that are available for HVAC system operation can be classified in two groups depletable (nonrenewable) and nondepletable (renewable). Depletable energy is stored in fossil fuels: coal, oil, and gas. These sources are quite limited in supply in relation to the rate that they are being used, and therefore will be depleted within a relatively short time (estimates range from 20 – 100 years for gas and oil to 1000 years for coal). Nondepletable energy sources have an expected long life in terms of human existence, so that they can be considered everlasting. Examples are the sun’s radiation, nuclear energy, wind, ocean currents and tides, and geothermal energy (hot water or steam from deep underground). The nondepletable sources are often spread out or diffuse and therefore are very difficult to utilize. However, two principles should always be considered when selecting energy sources for HVAC systems: