Hydropower: Head, Flow, and the World's Largest Store of Energy
Hydropower is the oldest of the modern renewables and still the largest. It is also the only one that comes with storage built into the resource: a reservoir is a battery that was filled by weather. That second property has become more valuable, not less, as grids fill with sources that cannot be scheduled.
Head and Flow
The energy in stored water is gravitational potential energy: mass times gravity times height. Converting it to power means multiplying by the rate at which the water descends. Everything about a hydro scheme follows from those two variables - head and flow - and they trade off against each other.
A high-head scheme drops water hundreds of metres through a pipe to a turbine at the bottom. The flow can be modest because each cubic metre carries so much energy. Norway and the Alps are built this way. A low-head scheme takes an enormous flow through a small drop, which is what a large river barrage does - the Three Gorges Dam works with about 80 metres of head and a river that carries it.
Turbine design follows. Pelton wheels, which are essentially buckets struck by a jet, suit high head and low flow. Francis turbines, where water spirals inward through guide vanes, cover the broad middle and are the most common type worldwide. Kaplan turbines resemble ship propellers with adjustable blades and handle low head and high flow.
The efficiency of this conversion is remarkable. Modern turbines exceed 90 percent, and large installations reach 95 percent. No heat is involved, so none of the thermal limits that cap a combustion or nuclear plant apply. Water goes in with energy, the runner turns, and almost all of it comes out as electricity.
Three Kinds of Scheme
Reservoir hydropower dams a river and stores water behind the wall. This is the form that provides dispatchable power: the operator decides when to generate, and a large reservoir can hold months of energy. That controllability is why hydropower has historically been used to follow demand rather than simply to supply baseload.
Run-of-river schemes divert part of a river's flow through turbines and return it downstream without significant storage. They have far smaller environmental and social footprints, since they flood little or no land, but they generate what the river gives them at that moment, which makes them seasonal rather than dispatchable.
Pumped storage inverts the whole idea. Two reservoirs at different heights are connected by a tunnel and a reversible pump-turbine. When electricity is cheap or abundant the machine pumps water uphill; when it is scarce the water runs back down and generates. It consumes more energy than it returns - round-trip efficiency is 70 to 85 percent - and it is not a generator at all but a store.
That distinction is worth keeping clear. A pumped-storage plant produces no net energy. It moves energy in time, which in a grid full of weather-dependent wind and solar is a service worth paying for on its own terms.
The World's Battery
The scale of pumped storage is routinely underestimated because it is old, quiet and unfashionable. Globally it accounts for more than 90 percent of all grid-scale stored electricity, with installed capacity around 180 gigawatts and energy capacity in the hundreds of gigawatt-hours. Every grid-scale lithium-ion battery on the planet together holds a small fraction of that.
The physics is unglamorous and that is the point: the storage medium is water and the container is a valley. Cost per unit of stored energy is very low once the site exists, and the equipment lasts 50 to 100 years, where a battery is typically replaced within 15.
The limitation is again geography. A pumped-storage scheme needs two sizeable reservoirs with a substantial height difference, reasonably close together, in a place where the construction is acceptable. Such sites are not evenly distributed and the best ones in Europe and North America are largely used. Closed-loop designs that do not sit on a river, and proposals using abandoned mines, are being pursued to widen the options.
This role in the grid is one that hydropower shares with any source able to deliver on demand rather than on weather, which is the axis along which energy storage technologies are usually compared.
Limits, Impacts and What Is Left to Build
Large dams are the most consequential renewable projects ever built, in both directions. They provide flood control, irrigation and navigation alongside power, and they have displaced tens of millions of people over the past century. The Three Gorges project alone relocated around 1.3 million.
Ecologically, a dam turns a river into a sequence of lakes. Migratory fish are blocked unless fish passes are provided and used, sediment that would have reached a delta accumulates behind the wall, and the flow regime downstream changes permanently. Reservoirs in tropical regions can emit significant methane from decomposing flooded vegetation, which narrows the climate advantage in some specific cases.
Hydropower is also climate-exposed in a way other renewables are not. Drought reduces output directly, and several recent years have seen hydro generation fall sharply in Europe, China and Latin America, in each case at a time when electricity was most needed.
Remaining potential is unevenly spread. Europe and North America have developed most of their economically viable sites. Africa has developed a small fraction of its technical potential, and parts of South Asia and the Andes retain substantial capacity. Meanwhile a large share of investment in mature markets now goes into refurbishing existing plants, where adding turbine efficiency to a century-old dam is often the cheapest new generation available.
Frequently asked questions
Why is hydropower so efficient?
Because it converts mechanical energy directly to mechanical rotation, with no heat stage. The thermal limits that cap combustion and nuclear plants around 33 to 45 percent do not apply. Water is also dense and effectively incompressible, so it transfers energy to a turbine runner with very little loss. Large machines exceed 90 percent.
Is pumped storage a source of energy?
No, it is a store. It consumes more electricity pumping water uphill than it returns when the water comes back down, with a round-trip efficiency of 70 to 85 percent. Its value is in moving energy from times of surplus to times of scarcity, which is a different service from generation.
How much of the world's electricity comes from hydropower?
Around 15 percent, which is more than all other renewable sources combined and roughly half of renewable generation. It is the largest renewable source by output and has been for the entire history of electricity grids.
What is the difference between reservoir and run-of-river?
A reservoir scheme stores water behind a dam and can generate on demand, which makes it dispatchable. A run-of-river scheme uses the flow as it arrives, with little or no storage, so its output follows the river and the season. Run-of-river floods far less land but gives up the control.
Do reservoirs emit greenhouse gases?
Some do. Vegetation flooded when a reservoir fills decomposes and can release methane, and the effect is most pronounced in warm shallow tropical reservoirs. Life-cycle emissions of hydropower remain low on average and comparable to wind, but the range between individual projects is wide.
