# Why do hydroelectric plants use turbines instead of an efficient 'water wheel' configuration?

I have been thinking about hydroelectric plants and I have been wondering why they are built the way they are. As in understand it, a stream of water is sprayed at a turbine, located at the bottom of a dam (Pelton Turbine.) This does not seem to be a very effective way to convert the energy. What are the factors that make this method effective?

Another method would be to place the water into buckets, then connect these buckets to a belt system and use the belt to power the generator. This method is similar to how water wheel works. What are the draw backs to this method that make it less effective than the turbine.

• You have based your question on a water wheel being more efficient than a turbine. Do you know if this is true? – hazzey Apr 1 '15 at 21:26
• "As in understand it, a stream of water is sprayed at a turbine" - this misunderstanding is probably why you don't expect it to be efficient. The turbine is fully immersed in the water, with the water moving through the turbine. – AndyT Apr 2 '15 at 11:48
• @AndyT depends on the turbine design. In a Pelton Wheel setup, as the asker describes, the turbine is not immersed. Different types of turbine are optimal for different situations. – Flyto Apr 6 '15 at 9:26
• @AndyT could you talk a bit more about those situations (and include the water wheel if possible?) This is mostly for small scale, residential situation. – Hoytman Apr 7 '15 at 1:47

Actually the turbine method is very efficient. The Turgo turbine is up to 90% efficient at extracting the energy from the water. Large-scale hydro plants using Francis Turbines can be as high as 95% efficient (see here).

Electrical generators typically use high RPMs, so you need speed as well as torque to drive it round. Low RPM generators are possible too, but less efficient. Energy available in hydro power comes from the 'head' (height drop) and the 'flow' (amount of water per second). So in low-head situations, you need much more flow to get the same energy. In these cases, you won't get high pressures (and therefore high speeds) without gearing (efficiency losses there too), so a water-wheel approach is feasible. In fact there are low-head generators similar to how you describe - the bucket-and-belt approach - as well as more traditional low-head turbines.

Low-head situations are harder to engineer (economically), because high flows are needed so the whole installation (channels, bearings, generators, etc.) must be much stronger and bigger. It's cheaper (per kW of generating capability) to use high-head situations with smaller/faster turbine wheels at higher pressures.

So these are the main reasons you see mostly turbines instead of other types of hydro plant. As technology improves and low-head locations become cheaper to exploit, we will see more of them.

• "High RPM" - more precisely 3000 RPM in Europe and 3600 RPM in the USA for best results - that way the turbine can be coupled directly to the generator and the generator will generate the power grid frequency electricity (and with most efficient design, no redundant coiling), just match phase and regulate the average voltage. – SF. Apr 3 '15 at 6:49
• @SF.: Would it be that speed, or a fraction of that? I would think that a rotor with multiple arms would have better dynamic-balance characteristics than a one which only had two. – supercat May 5 '15 at 22:59
• @supercat: dynamic-balance characteristics of what? Generated power, or mechanical? – SF. May 6 '15 at 5:51
• @SF.: Mechanical. I would expect a rotor with torque concentrated on two points 180 degrees apart would behave somewhat less rigidly than one where the torque was divided among points that were 90, 45, or 36 degrees apart (if a single generator is using one rotor and three sets of stator coils to produce three-phase power, I don't think 60 would work), and any lack of rigidity could lead to dynamic imbalance. I could be mistaken, though. – supercat May 6 '15 at 15:27
• @supercat: Possibly, but that's only a matter of engineering better bearings and firmer axis, or maybe extra (dummy) rotor weight (which is desirable anyway, as a flywheel to even out spikes in current draw). OTOH energy produced is cleaner, less harmonic components etc. Plus more kinetic energy stored = better immunity against power draw spikes again. Multi-arm generators are better when you can't afford the high quality/precision/strength construction - "home" energy production. – SF. May 7 '15 at 5:31

There are two different ways for the water flow to transmit energy. One is the pressure difference between the water entering and leaving the turbine. The other the kinetic energy change of fast flowing water entering the turbine and slow flowing water leaving it.

Your bucket idea seem to be like a http://en.wikipedia.org/wiki/Pelton_wheel which works by extracting the kinetic energy from a fast water stream and is efficient for low flow rates. If the buckets on the wheel are moving at half the speed of the water, the theoretical efficiency is 100%, but if the speeds are in a different ratio the efficiency can be much less.

On the other hand, if the water supply has a high pressure (e.g. at the bottom of a deep dam) it is more efficient to use the pressure difference directly in a design like a Francis Turbine, rather than converting the high pressure into a high flow velocity (which would waste a lot of energy in turbulence in the water and friction against the walls of the pipes) and then using the energy that is left to drive a Pelton wheel or some other "bucket" design.

• $E_k = {1 \over 2 } m v^2; E_p = m g h$ That ² means a lot. If you have water moving fast, you don't want to slow it down before extracting the power. – SF. Apr 3 '15 at 6:52