Measuring potential

To establish the potential of a site, the head and flow of the water need to be measured.

Head

The head is the height the water falls – taken from the point where the water will be taken from the river, to the turbine. Less than 10 metres is low head, 10 – 50 metres medium head and above 50 metres is high head.

With hydro it is very important to get as much head as you possibly can, as more head means more power for not much more cost. Depending on how much flow you have, the minimum amount of head required for a viable hydro system varies. If you have low head and low flow, then installing a hydro system won’t be very cost effective. Typically a head in excess of 1m is the minimum requirement.

The head of water available can be determined by measuring the difference between the water surface at the proposed intake and the level at the point where the water will be returned.

There are various ways of doing this:

• From a large scale ordnance survey map, simply by counting the contours between inlet and discharge points – each contour is 10 metres. This is really only suitable for higher head schemes.

• A Theodolite or Dumpy or builders level is the usual method of measuring the height. This is an optical instrument used in surveying to measure or set horizontal levels.

Flow

This is the volume of water which can be taken from the river and redirected through the turbine. It is usually measured in cubic metres per second but in small scale schemes, it is often measured in litres per second (where 1000 litres/second equals 1 cubic metre/second).

When taking water from a river, only a portion will be taken. This is so that the ecology and appearance of the river between the intake and outflow will not dramatically change. The amount of water left in the river is called the compensation flow. The Environment Agency will decide what this should be.

Obtaining the Flow Data

The Environment Agency has a number of gauging stations on the most significant rivers. A map of where these are, along with the data, can be seen at the Centre for Ecology and Hydrology's website at: www.nwl.ac.uk/ih/nrfa/. This information can be used to assess the flow rate at a given site.

If there is no gauging station near the proposed site, then there are other methods for assessing the information. The long term rainfall on the catchment area can be calculated or, the most accurate way is to measure the flow rate at the weir.

This can be done by constructing a "notched weir" and measuring the water which flows through it. It is possible to also use sand bags to construct a weir to measure from. Since the amount of water in a stream varies, then measurements should be taken over as long a period of time as possible, in all conditions.

Flow Duration Curve

Once measurements have been taken, you can then calculate the “flow duration curve” – showing the variation of flow over a year. This will then show the level of flow exceeded for 95% of the year and will then be taken as the characteristic value of river flow at the site.

A flatter, rather than steeply sloping, line is better as it means the power will be more spread over the year.

Once these figures have been obtained, the maximum amount of power at a site can be very roughly calculated as:

Power (Watts) = Head (metres) x Flow (litres per second) x 9.81kN/m3 (specific weight of water) x (in)efficiency.

A typical efficiency of a system is about 70% 'water to wire' and so the figure should then be multiplied by this (0.7) to obtain the final power potential.

Power from the Landscape can help with compiling flow duration charts as we now have the software needed to be able to calculate it. For details contact Pete Hill.

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