How Does Hydroelectric Power Work? Height Turned Into Electricity

The available power is the product of four things: the mass of water flowing per second, the acceleration due to gravity, the vertical distance it falls, and the efficiency of the machinery. That vertical distance is called the head, and it matters as much as the flow, which is why two very different plant types exist. A high-head scheme uses a modest flow falling hundreds of metres from a mountain reservoir through a steep pipe called a penstock. A low-head scheme uses an enormous flow falling only a few metres across a wide river. Both can produce the same power. Efficiency is remarkable, with large modern turbines converting over ninety percent of the water's energy into shaft power and the generator losing only a little more, which makes a hydro plant the most efficient large-scale energy conversion in common use, considerably better than any thermal plant, which is limited by thermodynamics to far less.

Three designs dominate and each suits a particular combination of head and flow:

• •The Pelton wheel, used for very high heads and low flows, which fires a jet of water at buckets on the rim of a wheel, with each bucket split down the middle so the water is turned back almost completely and gives up nearly all its momentum
• •The Francis turbine, the workhorse for medium heads, in which water enters around the circumference and spirals inward through fixed guide vanes onto a runner, exiting along the axis
• •The Kaplan turbine, for low heads and large flows, essentially a propeller in a tube with adjustable blade pitch, which keeps efficiency high across varying flow
• •Bulb and cross-flow variants for very low heads and small installations
• •Draft tubes below the runner, which slow the exiting water gradually and recover some of the remaining energy, contributing meaningfully to overall efficiency
• •Adjustable guide vanes and blade angles, which let a plant follow demand precisely rather than running at one output

The most useful hydro application in a modern grid generates nothing on balance. A pumped storage scheme has two reservoirs at different heights, pumps water uphill when electricity is cheap and plentiful, and releases it through turbines when it is scarce and expensive. It loses roughly a fifth to a quarter of the energy in the round trip, and it remains overwhelmingly the largest form of grid electricity storage in the world, far exceeding batteries in total installed capacity, because it stores enormous quantities for hours and the equipment lasts for decades. Its value rises as variable renewables grow, since the problem shifts from generating enough energy to having it at the right moment. Conventional hydro contributes to the same problem in a different way, by providing fast response: a plant can reach full output in well under a minute, and the spinning mass of its generators provides inertia that helps hold the grid's frequency steady, a service thermal plants used to supply and inverters must now be programmed to imitate.

Large dams have consequences that are not captured in the generation cost. Reservoirs flood land, and the cumulative displacement of people by dam projects runs into tens of millions, frequently without adequate compensation and disproportionately affecting rural and indigenous communities. Sediment that previously reached downstream floodplains and deltas is trapped behind the dam, which starves those landscapes and eventually fills the reservoir, limiting its useful life. Migratory fish are blocked, and fish passes work imperfectly for some species and not at all for others, which has contributed to severe declines in salmon and sturgeon populations. Flow regulation changes the downstream ecosystem, removing the seasonal floods that many species depend on. Reservoirs in warm regions can emit significant methane from decomposing flooded vegetation, so the carbon case is not automatic. Rivers crossing borders make dams a source of international tension, with disputes over the Nile, the Mekong and the Tigris and Euphrates all active. Run-of-river and small-scale schemes avoid much of this at the cost of storage and firm output.

Hydro power multiplies flow by height by gravity, so a small flow falling far and a huge flow falling a little produce the same output, and turbines convert over ninety percent of it, better than any thermal plant. Pelton, Francis and Kaplan designs suit high, medium and low heads. Pumped storage loses around a fifth of the energy and remains the world's dominant grid storage. The costs that matter most are displacement, trapped sediment, blocked fish migration and altered downstream flows.