hydro plant project question

[alibaba]

a hydro site has been calculated to be making 2280Nm of torque and 100 rpm at the shaft continuous power. this calculates at 23kw/h. if i were to setup a low-rpm 23kw PMG at 93% efficiency , does that mean that i will be able to get close to 20kw from the site?  what i don't understand is that the pmg generates 23kw/h at 250 rpm - i only have 100 rpm, but my site has been calculated to generate 23kw - if i was to setup a 2,5:1 gear and achieve the 250 rpm (which will lead to a 2,5 decrease in the torque) will this work and make my site efficient close to 93% of the estimated 23kw?

thank you

[dnix71]

Your PMG needs to turn 250 rpm to put out 23 kw/h. As long as the mechanical power in exceeds that (minus gear box losses) then you would get somewhere near 20kw/h out.

Is there an advantage to using a gearbox instead of a reducing v-belt pulleys?

[alibaba]

i will be using a gear box in this project. what i can't understand though is : if i achieve the 250rpm with a gear box, will there be enough torque to turn the generator and achieve the 20+kw/h. i don't understand how a project can be calculated at 23kw and still i cann't produce them effectively..

[TomW]

al;

Well, thats how "calculating" and "doing" differ, I guess. Might be worthwhile to take a step or three back and see if you can reproduce the values used originally.

Things like head, flow volume, etcetera.

It could simply be that some prior "expert" either miscalculated or was not much of an "expert"

Quoting like you did leads me to believe that person was inexperienced.

I am no expert in anything but that looks like something a college professor would write up rather than saying "23KW of energy is available".

Its your baby so do what suits you.

Tom

[Flux]

Power is torque x speed. If your torque for 23kW is 2280Nm at 100 rpm the generator will require 2280 x 100/250 Nm to produce 23kW.

Torque is decided by the speed for a given power. Each stage of conversion is less than perfect so you will loose a little in the gearbox so your input to the generator may be down to 22kW. Efficiency of 93% for a 20kW alternator is good going if you can manage it.

How accurate the 23kW calculation is is another matter I can only take that as given.

Flux

[alibaba]

2000Nm at 100 rpm makes 20kw...so does 800Nm at 250 rpm..so no change due to gear ratio..

[dnix71]

Are you familiar with torque vs horsepower curves for car engines?

Here is a simple explanation of gearing down (shifting gears) to keep the engine at the top of it's horsepower curve (not torque curve).

http://www.allpar.com/eek/hp-vs-torque.html

You don't want to be at the top of the torque curve for your turbine, you want the top of the horsepower (in this case electric output) curve for you generator head. You are running steady state, so you should want the max power out, not the maximum theoretical braking torque. In your case 250 rpm is apparently the max power point for the generator head.

You need to measure the brake hp at the output shaft of your gearbox coupled to the turbine, when the output shaft is turning 250 rpm.

http://www.land-and-sea.com/dyno/dyno.htm    The link goes to a business that has portable dynamometers. Knowing the brake hp at 250 rpm, you will know the maximum power available to run the generator head, and if it is enough to produce the electricity you have been told it will.

[Flux]

" i don't understand how a project can be calculated at 23kw and still i cann't produce them effectively.."

Can you explain what that is meant to mean?

Have you tried this and found that you can't get near the 20kW or is just some speculation where you are still confused about the issue of changing torque with speed.

As I said before I can only take your figures for the predicted 23kW. If you really have 23kW available at 100 rpm then you have 23kW available at 250 rpm with reduced torque.

As I see it from your original statement you have 2000Nm at 100 rpm ,if so you have 20kW. If you mean a static torque of 2000Nm and a speed of 100rpm then you don't have the details to calculate power.

Unless you can explain properly what you have and  what you are bothered about there is little hope of help.

Flux