Hydropower plants can be divided into three macro categories, according to the type of system used: run-of-river power stations, conventional hydropower stations and pumped storage power stations.
In run-of-river power stations, the natural flow of a river is used, on two different levels. The water is conveyed through a diversion channel without the aid of penstocks and reaches the turbines. The plant’s power mainly depends on the speed of the water through the passage from one level to the other, the so-called jump, and on the flow of the river.
In conventional hydropower stations, an upstream basin is used, the surge chamber, that can be natural – like a lake – or created with a dam. Penstocks convey the water from the dam toward hydraulic turbines, which generate mechanical energy by turning. This energy is then converted into electrical energy by a rotating electric generator.
There is a pool downstream, where the turbulent water that just went through the turbines is calmed before being returned to the normal flow of the river. The existence of an upstream reservoir, unlike run-of-river systems, allows for the control of water flows and therefore of the linked renewable electrical production.
Pumped storage power stations have two reservoirs at different altitudes, one upstream and one downstream, the second of which serves as an energy reserve. During times of lower energy demand, the water is brought up from the downstream basin to the upstream basin through a pumping station, allowing the system to deal with higher energy demand safely. In some plants, it’s possible to use the reversibility of Francis turbines to convert them into pumps and return the water to the upstream basin.