Calculate the total available energy from a water stream by use of a turbine?

[bobmurray1]

Had this posted in different forum, think this is more appropriate.

I keep confusing myself on this question, basically i want to know how you would determine the amount of potential energy you could capture by using a turbine, if water is flowing down a pipe into a river.

The change in elevation from the start to end of pipe is 30 meters.
The pipe is 100 meters long.
Water is flowing at a rate of 10 liters per second.

Not assuming the efficiency of the water turbine itself, is the formula just the potential energy minus the friction in the pipe? Thus:
Power = (Mass Flowrate X Gravitational constant X Change in Elevation) - Friction losses
= ((10 L/s X 0.001 m3/L X 1000 kg/m3) X (9.81 m/s2) X (30 m)) - Friction
= (2943 W/s or 2.94 kW/s) - Friction

Also second question, I know how to calculate the friction in a pipe, but the problem i cannot get my head around is that because the pipe is nearly a meter in diameter, the pipe is not full of water, thus i dont know how to calculate Reynolds number etc? Any help here would also be appreciated,
Would all the potential energy from the water elevated 30m be dissipated as it travels down the pipe because the pipe is not full? If this is the case, how does that make sense? Surely flowing down a larger diameter if pipe would reduce friction?

Thanks Guys

 

[cvg]

typical turbine installation includes an angled or vertical penstock which will flow full, under pressure. You will need to determine the elevation of the free surface. From that point the pipe will flow full, under pressure and you can use the reynolds number to calculate pipe friction loss. Also don't forget that the discharge from the turbine will have energy also which does not get converted to mechanical energy.


Friction loss will inclue
1) friction in canal,
2) entrance loss to penstock,
3) loss in penstock,
4) loss in scroll case,
5) loss in turbine,
6) loss in draft tube.

 

[bobmurray1]

So a turbine could not be installed onto the end of a pipe which is not full of water? Could u explain why? im just not totally understanding

 

[bobmurray1]

The reason i'm asking is that i'm trying to work out whether its worthwhile adding a turbine to a drainage pipe in which used cooling water will be travelling down from a height of around 30 meters.

The only issue is that the diameter of the drainage pipe is very large, thus the pipe is not fully contained with water, only partially.

 

[rconner]

I think 10 l/s is little more than a m/s flow velocity in a full 100mm pipe (I don't know for sure but suspect with this size and your available head it would probably take a goodly bit of time to re-coup the initial cost of recovering whatever power you could get out of your little turbine!)

 

[cvg]

10 l/s is a very small flow and your penstock need not be so large. you can install a reducer on the pipe and probably run a 100 mm pipe into your turbine. Head loss due to spinning the turbine will cause your penstock to back up and flow full. You should confirm that it is designed for pressure flow. You will have close to 40 psi at the bottom if it completely fills.

 

[bobmurray1]

(The 10 L/s was a made up number as i didnt have the real figures on me, but just wanted to use it for ease of understanding )

As the drainage is constant at 10 L/s, how would i then ensure that once the pipe is backed up and full of water, that it does not overflor/backflow? If you get what i mean? Without having to install a water tank? Could you ensure that 10 L/s of water travels through the turbine? so that the pipe remains full but does not backlog?

Thanks again

 

[cvg]

you are going to have to do a hydraulic analysis using the actual flow characteristics of your system and your proposed turbine and piping, for a variety of flow rates in order to determine the elevation of the free surface in the large pipe. If you have an overflow elevation that you must maintain, than install an overflow line. It shouldnt need to be very big.

 

[ccfowler]

If your flow rate is anywhere near what you have stated, the most economically practical option would likely be to use a suitable conventional centrifugal pump as a turbine by reversing the flow direction through the pump. For estimating purposes, I would guess at a nominal "turbine" efficiency of about 50% just to get a rough idea of what may be practical.

A fairly simple level sensing arrangement at the upper elevation to keep the penstock full could be used to control a waste valve installed near the bottom of the penstock to bleed off excess water flow when the amount available is greater than what the pump/turbine can pass. This by-pass arrangement should be configured to be capable of wasting the full flow if the pump/turbine should be tripped for any reason, and the penstock could still be kept full for easy return to normal operation. Since the penstock seems to be relatively much greater in diameter than would be normal for a penstock for the expected flow, friction losses in the penstock can probably be ignored for estimating purposes, and the extra flow through the waste valve would probably have a trivial effect on generator output. This waste valve could be a simple, moderately slow opening, slow closing valve that would serve to keep the upper water level within some reasonable range between a "high" limit and a "low" limit.

With this arrangement, a nearly constant amount of power would be generated. For safety and simplicity, it will almost certainly be best to plan to use an induction generator. The use of multiple standard pipe reducer fittings in series would probably be adequate to provide suitably gentle acceleration of the flow into the pump "discharge" combined with similar use of pipe reducer fittings to gently decelerate the flow from the pump "inlet" would be significant in helping to maximize recovery of the available potential energy from the flow.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[bobmurray1]

Would there be any way of finding out roughly how much water per unit time would pass through the turbine/reverse pump? It wouldnt seem right to have say half of the water flow being diverted constantly, as the turbine can only pass half the flowrate? But obviously, again if the turbine is too large, the pipe would just drain, no?

Also, i just said 10 L/s as a guess, i think it is much larger than that,

Thanks for help

 

[ccfowler]

For the present, I will claim a "senior moment" as my excuse for a memory pause, but several pump manufacturers have developed some reasonably good performance characteristics of some of their pumps when used as turbines. A bit of Internet searching should produce results for you reasonably quickly. You would do yourself a great service in getting a credible estimate of the minimum stable flow that can be expected for your system.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[bobmurray1]

Great, thanks very much for your help

 

[LittleInch]

I can see you're new to this site so will cut you some slack, but double posting is frowned upon. Leave your original for at least a week and at the very least give a link back to the original so that people can see what has already been said. This post also appears in the chemical plant design forum if anyone wants to see the other replies..

Back to your issue. My thoughts are that you need to figure out what your average or most common flowrate if it is variable, then do some calcs in excel for various friction losses, say 1,2,3,4,5,6....m in total, work out what size of PE pipe would accommodate these friction losses– search head loss PE pipe on the internet - then see if that pipe would fit inside your existing pipe. Then channel all the water into the end of your new pipe and any excess can then run down the old pipe. PE pipe is cheap, quite smooth and easy to install. Then you can go looking for a turbine with a good idea of flow and head available. 50% overall efficiency is a good number to start with.

If when you try it out the turbine takes too much water, you may need to increase the turbine resistance. Start with a small one and add more if the water in your pipe doesn't start to fall.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[chicopee]

You may want to check out an Ossberger hydraulic turbine. I remember, a few year, having read on the Internet that Oregon University built and tested an Ossberger hydraulic turbine. That type of turbine may be suitable for your project. A lot of third world countries have these things up in them thar hills..

 

[DRWeig]

Not being facetious here, but just to illustrate what the total energy might be with a pipe that is not full:

Best to you,

Goober Dave

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