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

Around 80% of the electricity demand in the world is still supplied by fossil fuelledpower or nuclear, i.e. thermal generation. Wind and solar power is integrated intothe electricity generation systems to decrease the amount of carbon dioxide emissionsassociated with the generation of electricity as well as to enhance security ofsupply. Wind and solar power plants differ from thermal generation in two importantways: they have very low running costs (and high capital costs) and a generationlevel that depends on external elements. Due to the low running costs thereare strong economic incentives for the employment of wind and solar power tosupply 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 certainhour or second varies irregularly since it depends on prevailing wind speeds, solarirradiation and cloudiness.Thermal units are most efficiently run continuously at rated power. However, in amixed 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 bywind and solar power may be offset by a reduced efficiency in the operation of thethermal plants. This chapter discusses the interaction between intermittent renewablepower and thermal power, and investigates briefly the impact of including amore controllable renewable source such as hydropower in these mixed systems