Thursday, December 30, 2010




Hydro electricity can be one of the cheapest methods of providing off-grid renewable electricity, but it is also very site specific. The best sites are on steep hills, with fast flowing water. One advantage is that on a good site you may not need batteries or an inverter (to step from DC to AC voltage), as the turbine will produce 240 volts AC and can just be turned on when needed.

The capital cost of hydro power schemes is quite high, but if you have a suitable site it can be a good investment. As of Spring 2010, ‘feed-in tariffs’ give a good price for electricity generated - a reasonable size scheme can recoup costs in 5 years or so.

To learn more about this technology, take a look at our blogs

Related questions

Do I need permission to install a micro hydro system?

If you are planning to remove or abstract more than 20m2 (20,000 litres or approx. 4,400 gallons) of water per day from a watercourse you will need an abstraction licence, even if the water is later put back into the watercourse. This means that virtually all micro hydro projects will require such a license, as even a flow rate of 1 litre per second amounts to 86m2 per day.

The license must be sought from the Environment Agency, who will assess effects on river ecology and flooding, prior to installation. The Environment Agency recommends that you contact them as early as possible as it can take around 3 months to get the license. For further information, consult the Environment Agency's document "Abstracting water - A guide to getting your licence".

It's also worth discussing details with local planning officials, as the powerhouse ans pipework may require planning permission.

If you don't own the land involved you'll need to seek permission from the landowners.

Can I convert an old watermill to generate electricity?

Old watermill sites are not usually good for generating electricity. A large, slow-moving body of water gives a high torque (turning force) and waterwheels make use of this to operate machinery directly. Low rotational speeds makes it difficult to use them for electricity generation; it’s easier to make electricity with a fast flow of water that can be channelled to hit a turbine at high pressure. Waterwheels are also expensive to construct compared to water turbines and need lots of maintenance. However, some 8,000 mills or mill sites are recorded in Britain, and as a small number may be suitable for generating electricity, it may be worth looking into. A hydro turbine installed at Gants Mill in Somerset generates up to 12kW of electricity and feeds into the local grid.

Another example is a waterwheel adapted to generate electricity at Pedley Wood in Cheshire.

The most suitable type of waterwheel for conversion to electricity production is the overshot style, as it has the highest head. It often proves worthwhile to increase the head by raising the headrace and/or lowering the tailrace. Some types of waterwheel can operate at a very low fall of only a few metres – you’d then need large flows of water to get reasonable amounts of power out of them.

Generators operate most efficiently at high speeds. Motors or generators that run at very low rpm (revolutions per minute) are large and expensive - a 1000rpm motor is much bigger than a 1500rpm one. Therefore, it may be more practical to gear up to a faster turbine, or consider installing a micro-hydro turbine instead.

How much will a micro hydro system earn?

The feed-in tariff (FiT) scheme for renewable electricity generation can make micro-hydro a very attractive option. Under this scheme, a generator receives a certain payment (19.9 p/kWh for systems installed in 2010/11) for every unit of electricity generated from micro-hydro power, whether you use it yourself (and save on bills) or sell it to the grid (for another 3 p/kWh).

For a 5 kW hydro scheme, this could work out to annual payments of £5,000 to £10,000, guaranteed for 20 years. However, to be eligible for FiT income, the hydro scheme has to be installed by a professional accredited under, and using turbines registered with, the Microgeneration Certification Scheme.

How much electricity can a micro hydro system produce?

A good hydro site depends on the 'head' of water (the vertical drop) and the flow rate. To estimate the energy in a water source, multiply the flow (in litres per second) by the head (in metres) by 10 (acceleration due to gravity). Halve the result, to account for losses and inefficiencies, to get an idea of potential power generation (in watts).

Flow x Head x 10 x 0.5 = Potential power generation in Watts

As this equation makes clear, a greater head will provide more power. Also, as a high head turbine will spin very quickly, there may be no need for complex gearboxes or belts.

Most micro-hydro schemes are ‘run-of-river’ - they don’t have a reservoir and only take water from the stream when it is available. You usually need a drop of over 10 metres for a scheme to be viable. Highhead ‘Pelton’ turbines are comparatively cheap, easy to install and work well in fluctuating flow. Crossflow turbines are more suitable for lower heads. Other turbines are available; suitability depends on a combination of the available head and flow of water.

Free, independent and impartial advice on renewable energy and sustainable living provided by the CROATIAN CENTER of RENEWABLE ENERGY SOURCES

Željko Serdar
Head of business association

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