Water Power - a brief on how it works.

From the Hoover Dam to a home made small scale structure, hydro electric is one of the most popular and useful forms of clean electricity generation. Once the construction has been implemented a hydro generating plant will operate for many years with little maintenance requirements and provide a relatively steady supply of electricity compared to the more variable solar and wind generation.

What does come as some surprise to most is the amount of water required to generate a useable amount of power. This comes down to two basics. 

All forms of power need a force to generate it. With water it is down to the weight of fluid passing a turbine which will then turn a generator to produce electricity. A turbine is no more than a traditional waterwheel as you will have seen on old watermills excepting that a modern turbine is fine tuned to gain the most from the passing weight of water The weight of water can be increased or rather, the same weight of water can supply more force if the water travels faster as it gets to the turbine. 

If you hold a hammer a few millimetres above your foot and drop it, it may cause a mild discomfort at best. If on the other hand you drop a hammer from the roof of your house and let it land on your toe, it WILL hurt. Why? Because of gravity. We all realise things fall to the ground and most of us learnt that Sir Isaac Newton figured out gravity is a force acting upon the Earth. Perhaps what we do not all realise is that when you drop something, it gets faster the more it drops. Gravity accelerates at 9.81 meters per second. As we are here to look at hydro dams and not high level physics, we simplify that and say it accelerates at 10 meters per second. But that is only for the first second. The next second of travel it accelerates a further 10 meters per second so that after two seconds of falling, an object is now travelling at 20 meters per second. This scale would carry on until the dropped object comes to rest or an unsuspecting toe intervenes the dropped objects path.

Aha! Some of you have sussed out an obvious flaw. If you drop a feather it will not drop as quickly as a hammer! Why? 

To answer that involves details such as mass, volume, air resistance and a multitude of other physics which  I do not want to bore you with right now. If we are happy to accept that gravity acts on falling water at a speed of 10 meters per second for every second (10mtr/sec2) that will be fine for our purposes of obtaining the force we can get from water to generate electricity.

What we said first was that the weight of water will turn a turbine to produce electricity. If we drop that weight of water, it will speed up thanks to gravity and that extra speed imparts extra force to the same weight of water. Therefore, for the same weight of water, if dropped from a height it will create more force and enable us to produce more electricity. 

If you did not already know it, you now understand the equation F=MA and in the world of power generation that is a pretty important bit of knowledge. The equation translated is:

• 'F' is Force
• 'M' is Mass
• 'A' is Acceleration. 

From the equation we can see that Force is equal to Mass x Acceleration (no, I don't know why physicists and mathematicians do away with putting in 'x' symbols and the like but yes, it is annoying :)). If we know what our Mass and Acceleration values are and multiply them we will find our total force. Mass will be in kilos and is basically the weight (I can feel the physics masters brewing at that comment:) of the object whereas acceleration is how fast an object is moving at a given point; as it is accelerating the speed is changing so we can not say it is a continuous speed. When we multiply mass and acceleration we would be multiplying e.g. 10kgs x 5 mtrs per sec. Equalling 50 newtons which, and again physics teachers will hate me saying this but is roughly equal to a force of about 50kgs in weight.

When we want to see how much electricity we can produce from a supply of water, we need to work out how much water is flowing and the distance it is dropping. In a river this drop may be negligible when viewed from the riverbank. But if the water is coming down a hillside we would certainly be able to see the drop, or more correctly, what we call the 'head of water' or just 'head'. By knowing the rate of flow and head of water it is a simple equation to work out the amount of power contained within it.

If we take Flow x Head, which for our example is 10mtrs x 10 litres we get 100. We need to add in the force of gravity which is again, 10 (or 9.81 for the picky amongst us :)) which will give us 1000. Great but a 1000 what? Correct. It gives us a thousand watts of power.

Sounds easy doesn't it. Well yes but a minor detail here. That is 1000 watts (enough to power ten, 100 watt light bulbs) being produced for one second. 10 litres of water dropped 10 meters every second will light our ten, 100 watt light bulbs for ,,,, yes, one second. We need to keep dropping 10 litres of water every second and if we leave those ten, 100 watt lights on for a minute, it is 60 seconds of power needed. Hence, after 60 seconds we will have used up 600 litres of water. 

That's equal to about ten fill ups on the average cars fuel tank and all we have done is leave ten, 100 watt light bulbs on for one minute! Now pause for a moment and think how much water you need to power a 3 Kw heater (equal to 3,000 watts) for an evening at home and we dare not consider the TV, lights, kettle for a cuppa or PC we may use.

A point of interest. Government research in the U.S.A. in 2000, found the daily consumption of water was 1550,000,000,000 litres (408 billion U.S. gallons) or approx 5700 ltrs. per person (1500 U.S. gallons) This figure includes industry and agriculture as well as personal use. 50% of this figure went to thermo-electric production so that's 750 billion litres of water per day, every day! 

It gets worse!

You may have forgotten but back a ways, I said there are two basics we need to know about producing electricity from hydro power. The second is losses. Above we looked at how much power we can obtain from water but that is the ideal part of the story. One day, we may be able to obtain something close to those figures when getting power from water but our technology as we use in today's hydro generating plants is a long way from achieving them. 

We get losses as some of the waters power is lost in just making the turbine turn. Just to make the turbine move atall requires a degree of power. It is the same as lifting a heavy weight with the aid of a lever. You need a degree of power to move the lever alone. Put a weight on the lever and you need a good deal more power but you are getting something done or work achieved. The turbine is the lever that operates the generator (our 'weight') This is where a loss occurs. In operating the lever or turbine, we use up some of the power we had available. We get other losses in the form of friction. 

Our turbine and generator spin round on bearings and though they may be the best bearings in the world, they will still have a degree of friction to them and use up a bit more of our waters power. A penstock, which is a tube that supplies water to a turbine, causes friction with the water that is passing through it. This is in effect slowing the water down and therefore, loosing us some more power. There are many parts of the hydro generating machinery and design that all go to causing power losses and this can add up considerably.

Modern hydro plants may operate at efficiencies of 80% or more but it is not uncommon to come across those with less than 60% efficiency. What this means, is that the 100 litres of water we were using earlier is not producing 1,000 watts as we had worked out but only capable of producing between 600 watts or thereabouts. If that sounds acceptable then imagine going around your home and removing nearly half of the light bulbs. You would survive just fine but life would not be as comfortable as it is now would it.

What makes the above interesting to the engineers and inventors is how to be more efficient and eradicate some of those losses. Turbines are being refined all the time and how water is delivered to the turbine is a constant evolution of design. The bearings that turbine and generators run on have evolved a long way but there are new concepts and ideas coming along and being implemented. Magnetic bearings may one day be the norm for all applications requiring them. In a magnetic bearing the shaft and body are kept apart by magnetic forces allowing the object to rotate without and rubbing surfaces.

I never say anything is impossible so maybe one day humankind will develop hydro generation with 99.9% efficiency. I hope they never quite reach 100% or there won't be anything to improve on :)