Hydroelectric power plants make it possible to capture part of the energy of flowing water. Based on the nature of water flow, hydroelectric power plants are usually divided into the following types:
- Run-of-river plants without pondage – their energy output depends on the amount of water flowing in the river at a given time;
- Hydroelectric power plants which can regulate water flow (with pondage) – there is a pondage accumulating water in front of the power plant;
- Pumped-storage power plants – water circulation is driven artificially by pumping water successively from the lower- to the higher-elevation reservoir, and then flowing it back through the plant to the lower reservoir.
Due to the force of gravity, water in natural watercourses moves from upland areas to lowland estuaries. Its flow is caused by the difference in potential energy of water between the upper and lower course of a river. As a physicist would put it, the potential energy is converted into kinetic energy of flowing water. If a turbine is placed on its way, part of that energy can be captured and exploited.
A water turbine is a device directly akin to a water wheel. Thanks to a rotor complete with blades, it converts water energy into rotary movement. In the case of a hydroelectric power plant, a turbine drives a generator which produces electricity (a kind of power generator).
A water turbine can directly capture energy stored in water of a river (or other watercourse). This is how small hydro plants (SHP) work. Larger plants with pondage use dammed up water (serving at the same time as flood control systems and flow regulators). When the energy system reports demand for more electricity, more water is released from the reservoir.
A pumped-storage power plant is an interesting type of a hydro plant. It can supply energy to the power grid or store it. The basic components of such a plant are two reservoirs (upper and lower), which are connected by huge pipes. At times when electricity demand is low and electricity is much cheaper (e.g. at night or in summertime), water is pumped from the lower to the upper reservoir. Electric power is thus converted into potential gravity energy. When more power is needed (and when it is more expensive), the process is reversed – water is released from the upper reservoir to the lower one through a reversible turbine assembly. The devices can be used as pumps (moving water from the lower to the upper reservoir) or as ordinary water turbines.
Despite energy losses occurring within the system, the use of pumped-storage power plants is economically viable due to fluctuations in electricity prices on an annual and diurnal basis.