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Hydro Electricity

Introduction to hydro electricity

Water was one of the first energy sources to be harnessed by humans. It seems likely that the first water power device was used in the Middle East six centuries BC. By the seventeenth century England had an estimated 20,000 working water mills.

Traditionally the power generated by hydro was used as mechanical power – to raise water, grind corn, drive machinery in mines and factories. However, once steam power came on the scene the future of hydro looked bleak.

By 1881 there had been a complete turn about and the first power station supplying public electricity opened in Godalming, Surrey. It was powered by a water wheel. Now hydro-power is the largest source of renewable energy, providing about 16 per cent of the world’s electricity

Hydro-power is not used so much at a domestic level because it needs the right site: most people have not got a stream handy. Also it is expensive to install. The term ‘micro-hydro’ is used for domestic size systems, and generally refers to those which generate between 5 and 100 kW of power.

Is hydro electricity suitable for my home?

If you have a good site, hydro-power is one of the most efficient and reliable forms of renewable energy. Micro systems tend to be between 60 and 90 per cent efficient – where efficiency measures how well the water’s power is converted into electrical power. Hydro also has good predictability, and in the UK output is generally greatest in the winter months when energy demand is highest.  

In Hydro terms a good site generally means one on a steep hill with fast flowing water. The head (or maximum vertical drop) of water is key. Less than 10m is normally classed as low head, from 10-50m medium, and more than 50m, high. If the head and flow are good enough, useful power can be generated from small streams.

The flow rate is the volume of water passing per second. For small schemes it is usually measured in litres per second. It is preferable to have more head than more flow, as you will be able to use smaller equipment. For formulae to measure the head and flow you can download the British Hydro Association’s step by step guide to mini-hydro. The European Small Hydropower Association has a Guide on How to Develop a Small Hydropower Plant available to download free. 

Somewhat surprisingly, old watermills are not necessarily suitable hydro electricity sites. Although they may have an old canal drawing water off the stream, many do not have a strong enough flow or high enough head to generate electricity effectively. This is because they were designed to operate machinery directly. It is estimated that up to 15 per cent may be suitable for upgrading.

If your site is suitable, it is also important that it is reasonably close to where the power is going to be used, and to a connection point to the grid, if you plan to export any surplus.

If you are off-grid, and have a good site, you can use power directly from the turbine, turning it on and off as you need it. Alternatively you can use batteries to store the excess.

Find a hydro electricity installer. 

How much will a hydro turbine cost?

As with everything concerning Hydro, the costs are site specific. They will depend on:

● The head available – the higher the head the smaller the turbine needed to generate the same level of power. High head machines can also be connected directly to the generator without the need for gearbox or belts.
● How much digging and building is needed to create channels for the water and foundations and housing for the turbine and how much of it you’re prepared to do yourself.
● Whether you want to connect to the grid, and, if so, distance from the connection point.

You can expect to pay at least £300, and probably more, for an initial site survey to determine whether you have a suitable location for micro-hydro.

Expect it to be a five figure sum. According to the Energy Saving Trust a typical 5kW schemes suitable for an average home might cost £20 - £25,000 including installation.

This is quite a high capital cost, but with a good site, it can pay off in 5 years or so, with help from feed-in tariffs (see below). 

Because every installation is different, all hydro installations are accredited under the ROO-FIT system rather than under MCS. You can still find an accredited hydro turbine on the MCS website.

What is the feed-in tariff?

Hydro installations are eligible for the feed-in tariff. For the period 8 February2016 - 31 March 2016, the tariffs for eligible new installations are below. 

15kW or less: 8.54 /kWh
>15kW - 100kW: 8.54 p/kWh
>100kW - 500kW: 6.14 p/kWh
>500kW - 2MW: 6.14 p/kWh
2MW+: 4.43 p/kWh

The export tariff for new installations: 4.77 p/kWh

Your eligibility date refers to the later of the following three dates: a) When a FIT supplier received a written request for FIT registration (including MCS certificate) b) When Ofgem received a request for ROO-FIT accreditation, or c) the commissioning date of the installation. 

Because every installation is different, all hydro installations are accredited under the ROO-FIT system rather than under MCS. You can still find an accredited hydro turbine on the MCS website

Due to the long time it takes to plan and get relevant permissions for a hydro project, you can apply for preliminary accreditation for the feed-in tariff.

Find a hydro electricity installer.

How does hydro electricity work?

  • Water is taken from a river or stream. Usually it comes from behind a weir or a small dam.
  • It may run down a canal (or leat) to a head tank, where it settles and fish, silt and other debris are filtered out.
  • From there the water drops down a pipe (called the penstock) to turn a turbine. The height of the drop (called the head) is one of the significant aspects of whether your site will be suitable. The greater the head, the more power is generated.
  • The turbine is located in a powerhouse with the generator, transformer and the control equipment. From there the power generated can be used directly to power your house, stored in batteries or exported to the grid. One advantage of hydro is that with a good site you may not need batteries or an inverter (to convert DC to AC voltage), as the turbine can produce a constant 240 volts when turned on. 
  • Once it has left the turbine, the water returns to the river along another canal (the tailrace).

The British Hydro Association has an animated diagram which shows clearly how a site works.

Find a hydro electricity installer.

Which type of hydro turbine is best?

An Archimedes Screw turbine on the River Dart

An Archimedes Screw turbine on the River Dart

There are two types of turbine. The impulse turbine is driven by jets of water. It is not fully submerged in the water. Pelton, Crossflow, and Turgo are the three main types of impulse turbine in use. Impulse turbines are generally most suited to high-head, low flow conditions. 

The reaction turbine is fully immersed in water, and enclosed in a pressure casing. Pressure differences across the blades create lift forces that make the blades rotate. Reaction turbines are generally more suited to higher flows and lower heads. The Francis, Kinetic and Propeller are all types of reaction turbines. Kinetic energy turbines work by using the energy in the flow rather than the head, and do not require any diversion or manmade channel. Propeller turbines including Kaplan, Tube, and Straflo, usually have a runner with three to six blades. In propeller turbines, water constantly moves over all of the blades. 

The Archimedes screw (pictured at right) is based on an ancient design that has recently become popular for hydro power in the UK. These turbines are good for sites with lower heads (as low as one meter) and large flows (they need a minimum flow of around 600 litres/second). Archimedes screws are attractive because they cost less to install than similar turbines and require minimal screening for fish or debris. 

Which is best depends on your site, and particularly the head at your site. If you have medium or high head Pelton is the most suitable for many micro sites, as it is the cheapest.

When comparing different turbines it’s important to compare their efficiency both at the flows they are designed to work best at, and at reduced flow. The British Hydro Association has a chart comparing part flow efficiencies.

Find a hydro electricity installer. 

Planning permission for hydro turbine?

Planning permission is usually needed for small micro schemes. As part of this you’re likely to have to provide an assessment of the project’s likely environmental effects.

You will also have to consult the Environment Agency to ensure that the site, and your design for it, are acceptable.

If the river or stream is used for fishing you may also need to consult your local angling club or relevant fisheries bodies.

For more information about who to consult and what kind of planning you might need, see the British Hydro Association’s Guide to UK mini hydro developments.

Find a hydro electricity installer. 

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