How to Estimate How Much Power You Have
A straightforward approximate equation to use is:
Power = Flowrate × Head × 6
kW = m3s–1 × m × 6
That is the easy bit. But first you need to work out these two things...
Head – The vertical distance from the surface of the water at the highest point at which you can access it (this might be the water level in an old supply to the top of a water wheel, or the surface level of water in a stream at the top end of your land) to the water level at the lowest point you can release it (perhaps the river water level at the point where it flows off your land, or the bottom of an existing wheel pit (in the case of an ex-mill site). Find this out in metres. Typically old mills sites have two to ten metres head, but hydro is often most economic with many tens of metres of head. There are machines which can use as little as one metre head. If you don't really have any head, and want to use the energy of the moving water in the river, this is something which is often asked about. “Is there something I can put into the stream to use the speed of the water to turn it?” The short answer is no, clearly you could get it to move something, but it is never worth it, unfortunately.
Flowrate – the amount of water you have. This is definitely the hard bit to work out. There are various methods to work this out, discussed also the BHA micro-hydro manual.
General methods to physically measure this are listed below, and 1, 2 and 3 are quite easy for anyone to achieve:
- Try to catch all the water, and use it to fill a container of known capacity, and time how long that takes (OK for small flows flowing over a lip or waterfall).
- Install a weir with a notch – either triangular or rectangular – into the flow, so that water flows over it and pours freely into the next bit of stream – forming effectively a waterfall. There is a relationship between the depth and the flowrate. This is a really good method for small streams, and can be left so that you can measure continually over an extended period. You can use the same equations to estimate flows over existing weirs knowing only the length of the weir and the depth of water.
- If you have a straight, square sided section of channel, measure the depth and width, to work out the cross sectional area. Measure out a length (say 10 metres) and time a marker in the middle of the stream to see how long it takes to travel 10 metres. Work out the speed (in metres per second), multiply this by the cross sectional area (in metres squared) and then multiply by a correction of 0.7, and this is a reasonable approximation to the flowrate. The 0.7 correction is not exact – it will overestimate if you have a very slow flow, and underestimate if you have a very fast flow – but it will suffice for a first estimate.
- Salt gulp method – a good method for larger water flows, fairly accurate, requires you to have a conductivity meter. This is likely to be the only way to work out flows in larger irregular section flows, like normal rivers.
- Get a flow meter – perhaps one with a propeller – and measure flow speeds at a large number of points across a cross section. This is one of the best ways to get an accurate reading, but requires a flow meter and access across the whole waterway. It is also slow, and you’d not want to do it more than once or twice really!
Measuring a flow gives you a snapshot of the flowrate at one given time, and gives an idea of the rough power output you might expect. For more detailed design you will need to know what water is available over the whole year, if you want to make a good selection of a turbine. The flow over the whole year is given in a ‘flow duration curve’ (FDC) which is a graph of flowrate, against percentage of time which that given flowrate is exceeded. Each river has a shape of FDC which is specific to it. There are lots of things which can affect this – the vegetation cover in the catchment area, the rainfall, the geology, the catchment size, groundwater flows, other abstractions, etc.
To find out or make an FDC is not particularly simple – it is a key thing we do for pre feasibility work – but if you want, you may be able to work it out (more or less) for yourself.
To actually derive this properly you just need to measure the flowrate lots, perhaps daily over a year, using one method from above.
Alternatively you can calculate it; the EA have lots of gauging stations on main rivers. If your site is fairly near a gauging station you can get this data. There will be a catchment area given at this point, you can work out the catchment area of the river upstream of your site, and scale the EA's FDC using the ratio of gauging station catchment area to your catchment area, and get a fairly accurate FDC.
More straightforwardly computer software is available to work this out. WRE Ltd will do this as part of our design and feasibility work.
Once you have a flowrate, make sure it is in cubic metres per second (or “cumec”). One cubic metre per second is 1000 litres per second.
As a bit of an approximate guide (and I stress approximate), a small stream (e.g. that you can step across in one step) might have 10 litres per second (0.01 cubic metres per second). A big stream might have 100 l/s (0.1 cumec). A small river (the sort which might have a name on a map, but still be ‘wade across in a pair of wellies in the summer’ size) might be 1000 l/s (1 cumec). A decent river (the sort of thing that people know the name of, and if you had big trees on either side of, the branches wouldn’t meet) may have 10,000 l/s (10 cumec).
Stick the resulting figures into the equation at the top, and you have a rough output. Any output over 1kW probably could be exploited by some commercially available turbine. Options for finding out about what type of system is right for your site are either employ a consultant or hydro engineering firm (that’s us), or if you are really keen on doing it yourself as a project, a good suggestion is “Micro hydro design manual” ISBN 1 85339 103 4, a fairly comprehensive design manual for the technical side, along with the BHA guide for UK specific regulatory issues.