Solar  |  Wind  |  Geothermal  |  Hydroelectric  |  Grants  |  FAQ  |  Links

Hydroelectric Energy Explained

Hydroelectric power is the energy derived from flowing water in rivers, or from man-made installations where water flows from a high-level reservoir down through a tunnel and away from the dam. Water power was used for centuries to power machinery, for example for grinding corn or in mills and factories, but was largely replaced by steam power in the Industrial Revolution. Water power is now mainly used to generate electrical energy.Hydroelectic Energy


Turbines placed within the flow of water extract its kinetic energy and convert it to mechanical energy. This causes the turbines to rotate at high speed. The turbines drive a generator that converts the mechanical energy into electrical energy. The amount of hydroelectric power that can be generated is related to the water flow and the vertical distance (known as ‘head’) through which the water has fallen.

In the smallest hydroelectric schemes, the head of water can be a few metres; in larger schemes, the power station that houses the turbines is often hundreds of metres below the reservoir.

Hydroelectric systems can be connected to the main electricity grid, or can be part of a stand-alone power system. In a grid-connected system, any electricity generated in excess of consumption on site can be ‘sold’ to electricity companies. In an off-grid hydroelectric system, electricity can be supplied directly to the user or via a battery bank.

There are three main types of hydroelectric schemes:
> storage schemes
> run-of-river schemes
> pumped storage.

In storage schemes, a dam impounds water in a reservoir that feeds the turbine and generator, usually located within the dam itself.

Run-of-river schemes utilise the natural flow of a river, where the continuity of flow can be enhanced by a weir. Both storage and run-of-river schemes can be diversion schemes where water is channelled from a river, lake or dammed reservoir to a remote powerhouse containing the turbine and generator. A canal or low-pressure tunnel transports the water to this end point and then back to the river or to another watercourse.

Pumped storage incorporates two reservoirs. At times of low demand, generally at night, electricity is used to pump water from the lower to the upper basin. This water is then released to create power at a time when demand, and therefore price, is high. Pumped storage is not a renewable application as it is reliant upon an electricity supply and energy losses are always involved when pumping the water.

Tidal barrage systems can, like run-of-river schemes, use the incoming and outgoing tidal flow, or, like pumped storage schemes, store the incoming tidal flow in a reservoir to be released at low tide.

There a number of instances where people have decided to install micro-hydro schemes and, in general, even small-scale schemes provide sufficient power for a number of houses or a small community. The geography and geology of your surrounding area will play a major role in determining the feasibility and cost of the project.

In general you will need planning permission. This is because water resources do not usually fall within an individual’s or community’s area of land. You should always contact your local district or borough council and speak to a planner before proceeding. The British Hydropower Association ( can tell you more about developments in your area.

Micro Hydro System

Greenearth Energy is an accredited installer of micro hydro power systems under the Clear Skies grant scheme.

This system is suitable for situations where there is a combination of sufficient water flow and 'head' (the height the water drops) to drive a turbine to generate electricity. The output from the system is a product of the two, with some efficiency factors to consider.

Not very much water is needed if the water drops more than 20metres (for example a mountain stream) or very large amounts are needed if the water only drops a few feet (a large river). See table here for indications of the relationship between flow and head for the same output. Please note, not all the water in a stream may be taken for hydro purposes. Typically only up to 50% is allowed and it is necessary to obtain an abstraction license from the Environment Agency.

The system requires an intake for the water, a penstock (conduit) through which the water is transported to the turbine. The turbine, generator and control gear need to be housed. Hydro can be used for off-grid or grid connected applications.

Greenearth is currently working on a number of hydro installations. The most recently commissioned hydro turbine, installed on a small farm in South Wales, produces 1.7 kilowatts continuously (equivalent to 1.7 units per hour). This equates to over 14,875 units per year - the average consumption per UK household is about 3,800 units per year.

For further information visit