In my previous letter ("In power game you don't get anything for nothing", Your say, April 29) I mentioned the different ways in which electricity could be generated from stored water in response to Alan Wright's initial letter ("Use dam water to generate power", April 21).

Perhaps I could have given a clearer explanation of why Alan's idea wouldn't work. So let's have another go…

To generate electricity from stored water you must let the water "fall" through a given height; this will give you a flow rate.

The approximate power available in a given situation can be estimated with a very simple calculation. The power, in kiloWatts, is equal to height in metres multiplied by the flow rate in cubic metres per second multiplied by five.

In hydraulics pressures are often specified as a height of a given liquid, in this case water, with a density of about 1,000 kg per cubic metre. The pressure measured at the outlet (base) of a reservoir holding 10 million litres of water with a height of, say, 25m will be precisely the same as the pressure measured at the base of a column of water 25 metres high with any diameter you care to choose.

The pipe Alan mentions, for returning the water to the reservoir, can be equated to the column of water mentioned above. So when the valve is opened the water will flow initially and rise up the return pipe until it precisely matches the level of the open water surface of the reservoir. Then the pressures will be balanced and the flow will stop.

This can be simply demonstrated using a funnel or some other vessel and a piece of clear hose. Connect the hose to the base of the funnel or a similar vessel and hold the free end of the hose to, or above, the top of the funnel/vessel.

Now fill with water; no matter how you move the pipe relative to the funnel, assuming water is not spilling, the water levels will always balance, irrespective of the volume of water in the funnel/vessel compared to the hose.

You only get energy when there is a flow and you only get flow when there is a difference in levels, in other words, a pressure difference.

I studied engineering at both college and university and recall my fluid mechanics classes, so I am confident in my understanding of Alan's proposal and its implications.

What Alan describes would be classed as a perpetual motion system which would violate the law of conservation of energy, one of the principal physical laws.

As I said before, where energy is concerned, you don't get anything for nothing!

Matthew Ambler