Hydroelectricity captures the natural kinetic energy arising from the gravitational fall of water to generate electrical energy. In 2014, over 1,005 GW of hydroelectric power was installed globally. There are two main forms, run-of-river and dam based applications.
Run of river systems divert all or a portion of a river system through a turbine to capture the kinetic energy associated with a river flowing downstream. Projects range from small applications, capturing energy from minor diversions of part of a stream, to large projects diverting the whole of large river systems.
Hydro projects can alternatively involve the construction of a dam, flooding a portion of a river valley upstream and creating a hydraulic head (i.e. pressure) at the base of the dam that can be used to power turbines.
Hydroelectric power has the advantage of being relatively cheap when compared with other sources of renewable electricity, as well as being a reliable source of baseload electricity (subject only to patterns of rainfall). It is also highly flexible, with short ramp-up and ramp-down times. The main challenges associated with the construction of hydroelectric power, in particular conventional dam systems, are social and environmental. Flooding large areas of river valley can result in the displacement of population and destruction of ecosystems, as well as create social and political issues associated with the control of water downstream (particularly where this crosses national boundaries or regions). Methane emissions from large reservoirs have also been cited as a negative environmental impact of large dams, due to the anaerobic breakdown of organic matter in flooded areas.
