From what I've read it seems that hydro turbines can theoretically achieve 100% energy conversion from kinetic to electrical energy. However, I just wanted to double check if this was true.

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    $\begingroup$ Links to what you have read would be helpful. $\endgroup$
    – Fred
    Sep 20, 2016 at 6:06
  • $\begingroup$ Even in theory I doubt 100% is possible - because the speed of the water after the turbine is effectively carrying away some lost energy (loosely speaking). You'd need a second low pressure turbine of a different design to recover some of that, and the water would be exiting it even slower meaning there's a little more energy to reclaim, etc. $\endgroup$
    – Andy
    Sep 20, 2016 at 6:39
  • $\begingroup$ Further, you need to define what energy you're converting. Certainly you're not removing the heat energy (leaving the output as ice at 1mK :-) ). $\endgroup$ Sep 20, 2016 at 14:21

3 Answers 3


Something can only be 100% efficient when a lot of generous assumptions are taken into account.

If you assume:

  1. No friction (turbine surface, turbulence, nozzle, or bearings)
  2. Perfect cup design (100% 180degree jet reversal)
  3. A dry wheel (no water is spun off)
  4. Not counting the gravitational potential to drop the water away from the wheel in the efficiency calculation.

Then a Pelton wheel is 100% efficient.

I worked in alternative energy for several years and, 90% is the best is have seen (Pelton, oversized penstock, ~400ft head). 50% would probably be a good across-the-board average for currently installed microturbines in the western United States. The lower the head(upstream pressure), the larger and more expensive a turbine and supporting components have to be to achieve the same efficiency. Also, the energy to remove the water from the turbine becomes appreciable. So generally, the lower the head the lower the efficiency.

Below is a graph of different turbine efficiencies. Pelton is a common choice for variable flow streams because a single turbine has a relatively high efficiency across a wide range of flowrates. Efficiency is very design and resource dependent so use this graph for a general understanding only (image source).

A Pelton wheel is a good turbine to start learning the mechanics before you look at more complex turbines. Pelton Turbine - Working & Design Aspects enter image description here


Yes, it's true in the sense that there's no other theoretical upper limit on conversion that's below 100%.

There are such theoretical limits that are strictly less than 100% for PV; one for single-junction (the Shockley Queisser limit), a higher one for two junction, and so on.

And there is a theoretical limit on heat engines - the Carnot efficiency.

Hydro has no such theoretical limit other than the standard hard 100% limit: all hydro is strictly less than 100%.


I guess if the design is like a wind turbine, like a propeller sat in the flow with no ducting then the betz limit would apply. See: https://energyeducation.ca/encyclopedia/Betz_limit

  • $\begingroup$ Welcome to Engineering. You may not have noticed that this question was last active about two years ago, and it already has an accepted answer. On a question like this, the community's expectation is that future answers will provide more detail than previous answers and that the new answer touches upon an area that hasn't already been addressed. This answer appears to be a link-only answer, which forces future readers to click through on the hopes that there is relevant information. It's better to explain here in the answer why the link is relevant and which portion addresses the question. $\endgroup$
    – user16
    Sep 2, 2018 at 20:21

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