Intermittent renewables, thermal power and hydropower - complements or competitors?

Around 80% of the electricity demand in the world is still supplied by fossil fuelled power or nuclear, i.e. thermal generation. Wind and solar power is integrated into the electricity generation systems to decrease the amount of carbon dioxide emissions associated with the generation of electricity as well as to enhance security of supply. Wind and solar power plants differ from thermal generation in two important ways: they have very low running costs (and high capital costs) and a generation level that depends on external elements. Due to the low running costs there are strong economic incentives for the employment of wind and solar power to supply the electricity demand once the capacity has been put in place. However, the share of the load that can be supplied by wind and solar power in a certain hour or second varies irregularly since it depends on prevailing wind speeds, solar irradiation and cloudiness. Thermal units are most efficiently run continuously at rated power. However, in a mixed renewable-thermal system they may have to compensate for fluctuations in wind and solar generation. Thus, depending on the characteristics of the renewable- thermal system, part of the decrease in fuel costs and emissions realised by wind and solar power may be offset by a reduced efficiency in the operation of the thermal plants. This chapter discusses the interaction between intermittent renewable power and thermal power, and investigates briefly the impact of including a more controllable renewable source such as hydropower in these mixed systems