Most energy sources can be consumed in two ways, directly and indirectly when converted to another form of energy. For example, wood can be burned directly as a primary energy source to heat space, or it can be burned to produce heat to generate electricity, a secondary energy source. Likewise, geothermal energy can be used directly as heat for industrial process or bathing, and indirectly to generate electricity. This article examines both uses but focuses mostly on indirect use for generating electricity and to a lesser extent liquid or gas fuels.
Some energy sources, such as hydro or wind power are mostly used to generate electricity but they can be used directly, as both sources have been used to raise water for irrigation. It is important to bear these distinctions in mind because the profiles of primary and secondary use of renewable energy are quite different. For example, renewables account for 12.8% of global energy supply, after fossil fuels, and biomass and combustible waste account for nearly 80% of primary renewable energy. Most consumption of primary renewable energy is of traditional fuels in developing countries, such as wood or animal dung. However, the generation of electricity, a secondary energy source, presents quite a different picture. Renewables account for 18.5% of total electricity generation. Hydro power accounts for 86% of renewables’ share of electricity production and biomass for only 6%.
Total renewables supply experienced an annual growth of 1.9% over the last 18 years, identical to the annual growth in TPES (Total Primary Energy Supply). However, the ‘new renewables’ such as solar photovoltaic and wind power have recorded a much higher annual growth of 42.5% and 25.1% respectively. Other renewables reporting growth above average include biogas (15.4%), liquid biomass (12.1%), solar thermal (10.1%), geothermal (3.1%) and hydro (2.3%).