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.

• 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.

There’s also some information about measuring flow rates on the Appropedia website.

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 charges in England increased greatly in 2022, and it can now cost a few thousand pounds for an abstraction licence. In other parts of the UK it may be much cheaper.

The abstraction license must be sought from the Environment Agency in England, Natural Resources Wales, or SEPA in Scotland. They 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.

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.

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. See also the page New hydropower scheme: apply to build one.

For more on the costs, see the charging scheme document for England at Environmental permits and abstraction licences: tables of charges. Table 3.3 on page 119 has a table for hydroelectric power charges. That page also gives links for licensing in Wales, Scotland, and Northern Ireland.

On the Natural Resources Wales page about licences it mentions proposed increases for 2023 that would increase an abstraction licence cost by several thousand pounds.

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

Old watermill sites can be difficult to adapt for generating electricity, although it’s not impossible. A watermill is an effective way of generating mechanical (rather the electrical) power, to operate machinery directly.

A large, slow-moving body of water gives a high torque (turning force). Waterwheels therefore operate at low rotational speeds (less than 15rpm) and produce very high torque. This makes it difficult to generate electricity.

It’s very challenging to connect a waterwheel with an rpm of about 15rpm to a generator that needs about 15,000 rpm. Motors or generators that run at very low rpm are very large and expensive; even a 1000rpm motor is much bigger than a 1500rpm one.

It’s easier to generate electricity where there’s a fast flow of water that can be channelled to hit a turbine at high pressure. The large size of waterwheels also makes them expensive to construct compared to a water turbine, and they need a lot of maintenance.

It can be possible to adapt a waterwheel. but it may be better to instead look at a dedicated low-head turbine such as a crossflow or Archimedes screw. Or for a small site, the low-head turbine made by Powerspout.

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 then needs to be cubed (raised to the power of 3).
• p is the density, which is about 1000 kg per cubic metre for water.
• A is the cross-sectional area swept by the rotor, in square metres.

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.

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.

We have a ram pump at CAT. It uses the water flow leaving our hydro turbine, and pumps about 10% of this water up about 3 metres into the air above the lake.

The ram pump is not a new technology. It was first developed around 1800 by one of the Montgolfier brothers – famous as the inventors of hot-air balloons. Ram pumps have been manufactured in the UK ever since, and became very common in rural parts of Britain in the early 20th century.

The increasing availability of mains water and cheap power (for pumping) during the last century led to a decline in their use in the UK, but they remain a reliable, cost-effective, and environmentally friendly option in the right situation. In remote and hilly areas they offer a reliable water pumping option.

For more detailed information, books on Hydraulic Ram Pumps include “Manual On the Hydraulic Ram” (by S B Watt) and “Hydraulic Ram Pumps: a guide to ram pump water supply systems” (by T Jeffrey).

It is possible to adapt certain types of water pump so that they effectively act in reverse, and can be used to generate electricity. This has the potential to be much cheaper – for an output of a few kilowatts it could save several thousand pounds.

The book “Pumps as Turbines: A users guide” (by Arthur Williams) gives practical, technical information on the use of standard pump units as a low-cost alternative to conventional turbines. The focus is on providing stand-alone electricity generation in remote locations.

There’s a recent example of a pump as turbine (PAT) at Tŷ Mawr Wybrnant. The 4kW turbine is expected to feed about 19,000 kWh of electricity into the grid each year. This is a National Trust site and the installation was through Dŵr Uisce – a joint research project of Trinity College Dublin and Bangor University.