Renewable energy sources present particular challenges for the electricity grid. For example, the performance of hydro plants is diminished in dry seasons; good wind conditions don’t generally correlate to when electricity demand is at its highest; and, of course, solar power generating technologies depend on clear sunshine.
Of the two major intermittent renewables, solar and wind, solar PV more closely matches peak demand use. However, it trails off towards the middle of the afternoon and cannot meet the second daily peak in demand after 6 pm when people arrive home from work. Furthermore, cloud cover causes rapid variability in output, which needs to be covered from other generation sources.
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 report 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%.
