In a suitable location small-scale hydropower is one of the most cost-effective and reliable of the renewable energy technologies.

Why micro-hydro?

It has several advantages over wind, wave and solar power, namely:

  • a high efficiency (70 - 90 per cent), by far the best of all the technologies
  • a high capacity factor, i.e. time generating power throughout the year; (typically greater than 50 per cent compared with 10 per cent for solar and 30 per cent for wind)
  • a high level of predictability, varying with annual rainfall patterns
  • a slow rate of change; the output power varies only gradually from day to day (not from minute to minute)
  • it is a long-lasting and robust technology; systems can readily be engineered to last for 50 years or more
  • it is environmentally benign; micro-hydro is in most cases ‘run-of-river’; in other words, any dam or barrage is quite small, usually just a weir, and little or no water is stored

Hydro principles

The basic principle of hydropower is that water can be piped from a certain level to a lower level, with the resulting water pressure being used to do work. If the water pressure is allowed to move a mechanical component, then that movement involves the conversion of the potential energy of the water into mechanical energy. Hydro turbines convert water pressure into mechanical shaft power, which can be used to drive an electricity generator.


In Northern Ireland in the late 1800’s, there were approximately 1200 water powered mills, a proportion of which were hydro turbines. By the early 1900’s, these were the main source of electricity for rural communities. With the development of the National Grid around 1960 and relatively cheap electricity, most of these sites became obsolete.

Calculating hydro power

Before embarking on any hydro power generation project, it is essential to survey the proposed site to calculate the amount of available hydro power.

The two vital factors to consider are the flow and the head of the stream or river.

The flow is the volume of water which can be captured and re-directed to turn the turbine generator.

The head is the distance the water will fall on its way to the generator.

The larger the flow - that is, the more water there is, and the higher the head - i.e. the higher the distance the water falls - the more energy is available for conversion to electricity. Double the flow and double the power, double the head and double the power again.

A low head site has a head of below 10 metres. In this case you need to have a good volume of water flow if you are to generate much electricity. A high head site has a head of above 20 metres. In this case you can get away with having a lesser flow of water.

The key equation is the following:

Power = Head x Flow x Gravity

where power is measured in Watts, head in metres, flow in litres per second, and acceleration due to gravity in metres per second per second.

The acceleration due to gravity is approximately 9.81 metres per second per second - that is, each second an object is falling, its speed increases by 9.81 metres per second (until it reaches its terminal velocity).

Therefore, it is very simple to calculate how much hydro power you can generate.


Head of 12 metres

Flow of 200 litres per second

Power = 12 x 200 x 9.81 = 23,544 Watts or 23.5kW

Types of turbine design

A turbine converts energy in the form of falling water into rotating shaft power. The selection of the best turbine for any particular hydro site depends on the site characteristics, the dominant ones being the head and flow available. Selection also depends on the desired running speed of the generator or other device loading the turbine. Other considerations such as whether the turbine is expected to produce power under part-flow conditions, also play an important role in the selection.

All turbines have a power-speed characteristic. They will tend to run most efficiently at a particular speed, head and flow combination. A turbine design speed is largely determined by the head under which it operates. Turbines can be classified as high head, medium head or low head machines. Turbines are also divided by their principle way of operating and can be either impulse or reaction turbines. The range of common design types is given in the table below.

  High Head Medium Head Low Head
Impulse Turbines Pelton Turgo Cross-flow Multi-jet Pelton Turgo Cross-flow
Reaction Turbines   Francis Propeller Kaplan

Hydro facts

  • hydropower is the world’s No.1 source of renewable energy. It produces almost 20 per cent of the world’s electricity and over 90 per cent of the world’s renewable power
  • less than one third of the world’s practical hydro capacity has been developed
  • there are no direct CO2 emissions from hydro projects.
  • small hydro schemes have minimum visual impact on their surrounding environment.
  • 25 countries depend on hydropower for 90 per cent of their electricity (99.3 per cent in Norway); 12 are 100 per cent reliant on hydro. Hydro produces the bulk of electricity in 65 countries and plays some role in 155. Canada, China and the USA are the three largest generators of hydro electricity
  • hydro installations can have a useful life of over 100 years - many such plants are in existence worldwide
  • a modern hydro turbine generator can convert over 90 per cent of the energy in the available water into electricity - this is more efficient than any other form of generation

For further information contact David Trimble, Renewable Energy Technologist, CAFRE, at

(Article by Eoin McCambridge)

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