Micro Hydro Power

Micro Hydro Power

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Water power

Micro-hydro turbines can be a very efficient and convenient form of small-scale renewable electricity. Suitable sites will not be that common, because the best locations will be on steep hills, with fast flowing water. Community ownership is a great way of setting up and using hydropower.

The upfront cost of hydro power can be quite high, but on a suitable site it’s a good long-term investment. On grid-connected sites you can still get feed-in tariff (FIT) payments for the electricity generated. On off-grid sites a hydro turbine should be much better in the long term than running a diesel generator for electricity.

Micro Hydro at CAT

When CAT started in the mid-1970s, it was a big help that we had a great site for harnessing water power. We installed a small second-hand turbine to provide much of the electricity we needed around the site. CAT has recently upgraded to a new turbine, which you can see if you visit. We’re reusing our old turbine as a test rig for running educational workshops on water power.

How much electricity can I generate?

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 that result, to account for losses and inefficiencies, to get an idea of potential power generation in watts.

How much electricity you can generate per year will depend on the seasonal water flows on your site. For example, a 3kW turbine running for 4,400 hours (about half of the year) will produce: 3kW x 4400 hours = 13,200 kilowatt-hours (kWh).

What sort of turbine should I use?

The available head and flow on your site would need to be assessed in order to choose a suitable turbine. As the equation above makes clear, a higher head gives you more power to harness. Also, as a high head turbine will spin very quickly, there may be no need for complex gearboxes or belts.

You usually need a drop of over 10 metres for a higher head turbine to be viable. These types, such as Pelton or Turgo turbines, are comparatively cheap, easy to install and work well in a fluctuating water flow.

For a low head sites there are Crossflow or Archimedes Screw turbines, which tend to be much bigger for the same power output because they need to deal with a larger flow rate.

Most schemes are ‘run-of-river’ – which means they don’t have a reservoir and only take water from the stream when it is available.

What will it cost?

Hydro installations are extremely site-specific. Prices vary widely depending on the type and size of system and how much work you are able to put in yourself. The basic equipment for a 1kW off-grid battery charging system might cost £5,000 to £6,000, plus installation costs. It might be possible to DIY a small scheme for under £10,000 and in some situations this could be cheaper than paying grid-connection costs. See for example the Powerspout range of turbines, for low or high head sites.

The total cost of a Pelton turbine producing 5kW from a 25m head might be £25,000 professionally installed, less if DIY. There is often an economy of scale, with a 5kW system maybe only costing 50% more than a 2kW system.

A hydro scheme may well cost much more than a solar PV array of similar kilowatt rating. However, if there’s a good water flow most of the year, then the annual output of each kilowatt of hydro power will be several times as much as each kilowatt of solar PV.

How much will it pay back?

The Feed-in tariff (FIT) scheme for renewable electricity generation can make micro-hydro an attractive option. From October 2018, you can sign up for a FIT payment of 8.04 pence for every unit (kWh) of electricity generated from a small micro-hydro scheme (under 100kW). You also get an export tariff of 5.24 p per kWh, usually set at 50% of total generation. In addition, whatever you use directly will save you about 16p per kWh on bills.

For a 5kW hydro scheme, the annual payment could be thousands of pounds, guaranteed for 20 years. To be eligible for FIT income you’ll need to seek accreditation through Ofgem’s ROO-FIT process.

Further advice

For more information on working out if micro-hydro power would suit your site, please see the questions below, or some of the courses, workshops or consultancy CAT offers.

Related Questions

How do I measure flow rate for a micro hydro scheme?

There are several methods for measuring flow rate for a potential micro-hydro scheme. The methods vary in complexity and in accuracy.

For a small stream, the easiest way is to divert the entire flow into a container of known size and time how long it takes to fill. Dividing volume by time gives a rough estimate of the flow rate.

For a larger stream, an initial estimate can be made by multiplying the speed of the water by the cross-sectional area. The speed is measured by timing a float along a measured distance of stream (preferably straight and free from obstacles) and multiplying by a factor of between 0.8 (straight smooth channel) and 0.6 (rocky stream). The area can be calculated by measuring the depth at several points across the stream, taking the average and multiplying by the width at that point.

A more accurate method of flow measurement involves building a wooden weir across the stream, with either a V-shaped or rectangular notch cut in it. If the height of water flowing over the weir is measured, then the flow rate can be worked out (see below for more). Although this method involves a lot of work initially, it is then easy to take readings from the weir over a period of time and establish the flow variation.

An entirely different approach is to calculate the catchment area of the stream from a large scale map and then calculate the expected run-off using rainfall statistics such as those available from the Met Office. You’ll need data about geology, rainfall, transpiration and evaporation. Results from this method can be extremely accurate, and they also predict how the flow will vary over the year.

Another option is ‘salt gulp’ – add a salt solution and measure conductivity downstream with a conductivity meter. This is only worth buying if you’re doing a lot of site assessments, as it costs a few hundred pounds.

Notched weirs for assessing water flow

Please Note: you’ll need to use either a computer spreadsheet or scientific calculator to calculate these equations, because of the exponential functions.

Notched weirs for hydro power survey
Notched weirs for hydro power survey
  • Q is the flow rate in cubic metres per second
  • h (in metres) is the height of water above the base of the notch
  • L is the width of the rectangular notch, and must be at least 3 x h

The distance from the outer edge of the notch to the stream edge or bank should be at least 2 x h for both types of weir.

For a v-notch (90 degrees) weir: Q = 1.4 x h^2.5 (i.e. h to the power of 2.5, then multiplied by 1.4)

For a rectangular weir: Q = 1.8 x ( L – 0.2 x h ) x h^1.5

The weirs need to be watertight, with accurate angles and sharp edges, and be mounted vertically and at right-angles to the water flow. The weir should be ventilated by air under the crest of falling water. A ‘stilling pond’ just upstream will help to keep the velocity of water before the weir to no more than 0.2 metres per second.

There is much greater detail about these methods and other ways of measuring flows, catchment areas, etc, in the book ‘Going with the Flow’ (by Langley & Curtis). Unfortunately this book is currently out of print, but you may be able to find it through a library or second-hand.

What permissions do I need to install a micro hydro system?

If you are planning to divert more than 20 cubic metres (20,000 litres or about 4,400 gallons) of water per day from a watercourse then you will need an abstraction licence – even though the water is going back into the watercourse after going through a turbine.

Virtually all small hydro projects will require such a license, as even a flow rate of 1 litre per second amounts to 86 cubic metres per day.

The abstraction license must be sought from the Environment Agency (in England and Wales, or SEPA in Scotland), who will assess effects on river ecology. It’s worth contacting them in good time, as the process could take a few months.

Certain environmental protection measures will generally be linked to the permission given. These include leaving a certain flow in the river (i.e. abstracting only a proportion) and putting in place screening and perhaps a fish pass or ladder.

For information about abstraction licences, and the bodies involved in giving permission in different parts of the UK, see the UK government webpage on the environmental management of water.

Larger schemes may involve additional permissions, for example if a reservoir is included.

You’ll also need to contact the local planning office, as planning permission is usually required (unless perhaps an old hydro scheme is being refurbished) for the installation of the structures and pipework involved.

The British Hydro Association website has a guide to micro hydro, including more detail on these planning considerations.

What if there is no head for a micro hydro scheme?

Where there is virtually no drop at all, harnessing water power effectively will be very difficult and may well not be worthwhile.

It is possible to place a ‘zero head’ turbine in a river, or tow one behind a boat, but the output will tend to be very low. Perhaps enough for trickle-charging a 12 volt battery, and maybe cost effective if the alternative is a diesel engine.

The power in a flow

The relevant calculation for the power contained in a flow through a rotor is:

Power (in watts) = ( V^3 ) x 0.5 x p x A

  • V is the water flow speed in metres per second -this needs to be cubed
  • p is the density, which is about 1000 kg per cubic metre for water.
  • A is the cross-sectional area swept by the rotor

The efficiency of the turbine will then be a big factor in how much of that power can actually be converted into electricity (or into mechanical power for an undershot waterwheel).

For fixed or towed turbines, the efficiency tends to be quite low. An example of power output is about 100 watts in a flow of 4 metres per second from a rotor diameter of about 30cm. The maximum power in that flow is about (4^3) x 0.5 x 1000 x 0.07 = just over 2000 watts. So the efficiency is just under 5% (100/2000).

A standard micro hydro system (where water is channelled in a pipe) should have at least 50% overall efficiency, after all losses. A small low-head turbine could generate about 1 kilowatt (1000 watts) from a flow of 100 litres per second dropping through 2 metres. So much more energy from a smaller flow, as long as a small head can be created by channeling the water flow over a small distance.

Other issues with a zero head turbine

It will not simply be a matter of placing a propeller-type turbine into a stream. There will be various issues that need to be addressed to ensure a long operating life:

  • Debris coming down the stream when it is in spate, and damaging the turbine or mountings.
  • Some ground work may have to be done in small streams to make sure there that there is sufficient depth to submerge the turbine at low flows.
  • At 12 or 24 volts DC output, heavy cable will be needed to run from the generator to the house, and this can be costly (e.g. if more than 20 metres).
  • The turbine should be easily accessed to remove weed build up.
  • Flow rates will be very seasonal, so you may well need other power sources to meet electricity needs in the drier months.
What is a hydraulic ram pump?

A hydraulic ram is a water-powered pump that uses the energy in a large flow of water dropping through a small height to lift a small percentage (about 10%) up to a much higher level.

A ‘compound’ ram pump uses this power to pump a separate body of water. They are very site-specific, but with only two moving parts can be a reliable and cost-effective solution on a suitable site, at around £1,000.

For more detailed information, there are books on Hydraulic Ram Pumps available in CAT’s online shop.


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