Although fossil fuels (petroleum, coal, and natural gas) are still being created at a geological rate, they are being consumed at a much faster rate. Hence, fossil fuels are not renewed fast enough to be replenished. That is why fossil fuels are considered nonrenewable. Presently on Earth, renewable sources of energy are due to the sun's energy and its direct and indirect effects on the earth (solar radiation, wind, falling water, and various plants, i.e. biomass), gravitational forces (tides), and the heat of the earth's core (geothermal). Renewable energy sources (RES) will replenish themselves within our lifetime and may be used with suitable technology to produce predictable quantities of energy when required. Additionally, renewable resources are more evenly distributed than fossil and nuclear resources on the earth. The most important benefit of renewable energy systems is the decrease of environmental pollution.
The main RES and their usage forms are shown in Table 3.1. Some of these energy sources have been used by humans for over five thousand years. During the preindustrial era, RES were primarily used for heating and simple mechanical applications that did not reach high energy efficiency. In the industrial era, the energy use shifted from low energy value RES to much higher energy value coal and petroleum. In fact it is said that RES such as water and wind power probably would not have provided the same fast increase in industrial productivity as fossil fuels (Edinger and Kaul, 2000).