Hydro is a Greek word that means water, so hydroelectricity literally means ‘water electricity’. In common usage, hydroelectricity is generally understood to mean generating electricity using the flow of water.

Tidal flows in seas and oceans around shorelines are also used to generate electricity, but the word hydroelectricity is not used to refer to this kind of generation. Hydroelectricity specifically means using the flow of water in rivers (usually channeled using a dam and internal pipes) to spin turbines that generate electricity.

Additionally, there is a process called ‘pumped hydroelectricity’ which is not technically generation, but rather a form of energy storage.

Hydroelectricity has a long heritage stretching back to the very dawn of commercial electricity. Thomas Edison patented his incandescent light bulb in 1879, and the Vulcan Street Plant, the world’s first hydroelectric power plant, began operation three years later in 1882 in Wisconsin, USA(1). 

Hydroelectricity is a renewable form of electricity, since no fuel is used up to generate the electricity. Consequently, hydroelectricity is zero-carbon at point of generation, thereby gaining favor around the world as countries attempt to combat climate change by reducing carbon emissions.

The rate at which hydroelectric plants are being built has been increasing over the past 5 years or so, and it is expected that the total number of hydroelectric plants in operation worldwide will double over the next decade(2)

Currently in operation around the world (as of Summer 2021), there are over 190 large scale hydroelectric power plants(3). “Large scale” means generation capacity of 1 Gigawatt (GW) or over. However, there are thousands more smaller hydroelectric plants also in operation.

The largest and most powerful hydroelectric power plant currently operational in the world is the Three Gorges Dam across the Yangtze River in China. This has a maximum generation capacity of 22.5GW(4).

From a purely practical aspect, any large, fast-flowing river that drops a reasonable height over a short distance (for example over a waterfall) is a good location candidate for a hydroelectric installation.

To begin with, a dam is built, which floods the area around the high-level river, creating a reservoir.

The dam is built with internal pipework that feeds water from the reservoir at the top of the dam, to turbine generators at the bottom (see illustration). The turbine generators are spun by the water as it flows past, generating electricity which is fed into the national electricity supply just like a conventional power station. The water then continues its journey downriver as before.

The more energy there is in the water, the more electricity can be generated from it. The amount of energy in the water is determined by two things: head and flow. To achieve the most amount of energy, the head and the flow need to be as high as can practically be engineered.

Head is the height distance between the top of the reservoir and the turbine itself (see illustration). The larger this distance, the more water there is ‘pushing down’ on the water as it passes through the pipes, forcing it through the turbine with greater pressure.

Flow is the volume (or mass) of water that passes through the turbine per second. A faster flowing, wider river will create more flow.

An ideal hydroelectric power plant would be fed by a very wide, deep, fast flowing river that falls a large distance. However, finding such a location is rare, as large fall distances are usually found in more mountainous, upstream locations, where the river has a smaller flow.
And vice-versa: large, high-flow rivers are generally found in flatter lowlands, where there are few falls, and any falls there are, are usually small. However there are a few notable exceptions to this rule, like the Niagara falls, which plays host to a number of hydroelectric power plants, due to its high flow rate and large head height(5).

For most hydroelectric installations, compromises must be made, and river locations identified that have an acceptable balance of head and flow.

Pumped hydroelectricity is very similar to regular hydroelectricity, but rather than being used for energy generation, it is used for energy storage. Much like a gigantic rechargeable battery, a pumped hydro plant can be ‘charged up’ using electricity from the national grid, and then, when the energy is needed, it is ‘discharged’, providing power to the national grid. These two different modes of operation are described below as ‘Charging the battery’ and ‘Discharging the battery’.

Charging the battery

When there is excess electricity supply on the national grid (for example overnight), a pumped hydro plant goes into ‘charging the battery’ mode. It draws electricity from the grid to power pumps that pump water from the lower reservoir to the upper reservoir (see illustration).

Discharging the battery

Then, when demand for electricity on the grid increases, the pumped hydro plant flips into its other mode of operation, ‘discharging the battery’. The water in the upper reservoir is allowed to flow back down to the lower reservoir under gravity, turning the turbines and generating electricity that is sent to the grid just like regular hydroelectric power.

Of course, there is no actual battery – energy is stored as potential energy in the water as it is moved to the higher reservoir.

Note that, unlike a regular hydroelectric power plant, the dam is not placed across a river, it is placed between two static reservoirs, an upper and a lower one, and the same water goes round and round the system, pumped up and then flowing back down again. This means the environmental impact of pumped hydro is much less significant than regular hydroelectric generation as it does not impede natural river flows.

In some cases a dam is not required because the natural geography of the land presents two nearby lakes, one higher than the other, and these can be connected with pipework, like the Dinorwig pumped hydro plant in North Wales, UK(6).

Since hydroelectricity usually requires damming a river, there is controversy over its environmental credentials. For example, some countries are building dams to flood environmentally sensitive locations.

It is desirable to build a taller dam to create a higher head reservoir, as this means more electricity can be generated. However, a taller dam will mean a larger area of land around the high level river will become flooded which has significant environmental consequences. For example, when the Three Gorges Dam was built, 1500 cities towns and villages along the river Yangtze were deliberately flooded, displacing over a million people from their homes(7).

Damming rivers to create head reservoirs for hydroelectric power plants continues to be a source of environmental concern. Reports by organizations such as International Rivers have highlighted the plight of people who have suffered as a result of damming rivers. For example, in 2018, a saddle dam in Southern Laos collapsed, submerging entire villages, destroying crops and killing many people and livestock(8).

Another issue that has arisen from hydroelectric dams is that they cause deterioration in the water quality. Water downstream of the dam is depleted of oxygen, damaging fish populations and other aquatic life. The head reservoir water becomes vulnerable to algal blooms, and can also leach toxic metals from submerged soil, poisoning the water that flows downstream, harming people and livestock who drink the river water(9).

As the world’s thirst for renewable, low carbon electricity generation increases, so the number of hydroelectric plants is increasing, and these environmental issues need to be seriously addressed for hydroelectricity to be able to maintain its claim as an environmentally friendly source of energy.