Water Power Project

In April 0f 2011 local resident and Parish Councillor Derek Colley came to speak to the CRAG group about the possibilities of harnessing the power of the River Wye - which runs past the edge of the village - in order to generate electricity. This isn't a project that's currently underway - though it's certainly something that some of us are thinking about - but Derek's notes are reproduced here to provide a very useful outline of the issues involved. Bear in mind that they are Derek's notes, and not something written specially for this web site, but they're still easy to follow and make very interesting reading.

Flow rate of the Wye in spate is 700 cu.mecs

There is plenty of power in the river but can we effectively harness any?

Yes but there are limitations and also some other options away from the river itself.

What we see when we look at the river flowing by is KINETIC ENERGY and the massive horizontal movement is impressive.
This kinetic energy can be utilised but not as effectively as the POTENTIAL ENERGY  of  water where it is available to fall vertically and gain the acceleration due to gravity.

To convert the energy of moving water into electricity we need to use it to create ROTATION and to gear up the speed of rotation to 625 rpm or more.

The obvious and well tried means of creating rotation from water is to feed it onto water wheels-----but this requires the water to arrive at the wheel at a suitable height.
In the river this can only be achieved by creating a weir or dam in order to form an abrupt change in level to harness the water supply as you saw at Monmouth on the River Monnow.

The Archimedes Screw is a very effective converter of water power where a wear situation is available.
 But it would be unlikely to be a feasible option on the Wye which has such a large seasonal variation in flow rate and consequent increase in  level by 5 to6 m.

Where a consistent change in level of 1.5 to 4m exists without a risk of being overtopped by flooding, then hydraulic screws can produce up to about 30kW.

Let us digress for a while from the main course of the River Wye and consider available options of utilising lesser streams which feed into it.

There are 2 types of water wheel

a)    Overshot wheel    3 to 4m in dia    Needs 3 to 4m head to produce up to 55kW

b)    Undershot wheel  3 to 8m in dia.    Needs 0.5 to2m head to produce up to 45kW

The old mill at Mordiford is an example of an Overshot wheel

In working mode 4m wide 2.9m dia.  26kW

Overshot wheels are driven by the weight of water introduced at the top of the wheel which is carried down until it discharges at the bottom of the rotation.

The Capacity of any water wheel to generate electricity is determined by the rate of delivery of water to the wheel.

In the works& before water 4.73m dia.  16kW

It is no good building a large wheel with big buckets unless there is sufficient water being fed to them. So it is important at the out set of planning an installation to establish the likely flows in the feeder stream throughout the year.

 It can then be established whether to design for a consistent lower output throughout the year or plan for a larger output during a limited period.
A less obvious type of wheel is the Undershot Wheel

This type of wheel is less demanding on the height of fall required and can operate with as little as half a metre of fall but it is not too difficult usually to create up to 2m with a little ingenuity in most stream valleys.

There is no limitation on the height of an undershot wheel so they are built up to 8m in diameter to create a useful 4m lever arm.
This increased leverage compensates for the lesser weight of water on the wheel compared with the overshot wheel which is able to carry water over the full height of its wheel.

Comparison of Wheel applications  -  note that the undershot wheel copes with higher flows

The site works for an undershot wheel are less complex than for an overshot wheel and yet the power output potential is much the same.

I can’t help but notice the suitability of the Tan House stream on the Parish Land adjacent to the Fownhope Fire Station as being ideally suited to an undershot wheel!!
Before I get too involved with the development of that idea let me return at last to the main stream of the River Wye

The one appealing characteristic of the river is its continuous and swift current.

One is naturally tempted to think of an immersed turbine as the way to extract power  and indeed this could be the case but for small scale generation such a sophisticated option would not be economically viable. Our generally shallow river would not in any case be able to provide the depth required and canoeists might object to being minced up if they tangled with one.


Just imagine immersing a brute like this in our lovely river !!

In conclusion, ( and please excuse the pun ) and to whet your appetite I would like to present to you a concept of how I believe there could be a viable way of borrowing the principle of the undershot wheel and applying it to a floating wheel which would be secured to the riverbank such that it could be turned by the current at all states of the river level.