Controlling the rotational speed of a generator

Thread Starter

Dawsonh4

Joined Apr 11, 2017
38
Hi,

I am working on a project that would insert a generator/expander system to a pressurized stream of gas in order to generate electricity.

I would like to control the rotational speed of the generator in order to adjust the gas stream flow rate.

Would it be possible to control the rotational speed of a generator through some sort of applied resistance? I am thinking something like an eddy brake.

Additionally, would large amount of resistance applied to the generator result in more power generated?

I appreciate anyone's input.
 

nsaspook

Joined Aug 27, 2009
7,738
That seems backward IRT to efficiency.

Using a dump load on generator output will work but that will waste energy from the generator output that originally came from the energy source.
 
Either throttle the prime mover (ideal) or dissipate the excess energy as heat - be that through a friction brake, eddy brake or "dump load" resistor.

If you're talking about some sort of air/inert gas turbine then you'd be looking at an electronic throttle valve driven by a closed loop PID controller.
 

Thread Starter

Dawsonh4

Joined Apr 11, 2017
38
Thanks for the reply.

Forgive my lack of knowledge - I have more of a chemical/mechanical background than electrical.

I understand the comment on efficiency, but the current method is not harnessing any energy. So anything is an improvement.

The generator would be connected to the grid and therefore always have some amount of load on it.

Would you be able to adjust that load speed up or slow down the RPM on the generator?

In the throttle valve scenario - wouldn't you lose potential energy any time the valve would partially closed?
 

nsaspook

Joined Aug 27, 2009
7,738
The generator would be connected to the grid and therefore always have some amount of load on it.
There are some pretty big implications and liabilities to address with that. Backfeeding to the utility is a big deal. Lots of hoops to jump through and boxes to check. Designing and incorporating anti-islanding protection, getting your design listed by an independent testing laboratory, having bidirectional metering installed by the utility, seeking evaluation and approval from the utility to connect your installation to their system and then an indefinite length of time to amortize the cost of all the above. Skip any steps and you'll be opening yourself up to litigation if something goes wrong.

As for losses incurred by the use of a throttle valve - you'd have the Joule-Thompson effect to contend with (add an air-over heat exchanger to compensate if it's an issue) but otherwise it would undoubtedly be more efficient than simply wasting the excess power as heat.

Throttling the engine in your car vs leaving it pinned wide open & relying on your brakes to control your speed. Conserving unneeded gasoline (air/gas) vs wasting the difference via friction brakes/resistor grids.
 
Last edited:

MrAl

Joined Jun 17, 2014
7,849
Hi,

I am working on a project that would insert a generator/expander system to a pressurized stream of gas in order to generate electricity.

I would like to control the rotational speed of the generator in order to adjust the gas stream flow rate.

Would it be possible to control the rotational speed of a generator through some sort of applied resistance? I am thinking something like an eddy brake.

Additionally, would large amount of resistance applied to the generator result in more power generated?

I appreciate anyone's input.
Hi,

My first thought is to install a gas regulation valve electronically controlled if needed.

My second through it to install a gas bypass valve electronically controlled that can bypass some of the gas around the generator so the generator does not get the full gas supply.

Either of those methods does not involve the loss of much energy.

Third, if the generator is connected to the outside power line then you could use an electric regulator that adjusts the power going back into the line. This method does not require slowing the generator down, just adjusting the output similar to how a photo voltaic system works. Also not too much waste of power but there is some due to the high power electronic regulating circuit involved. 90 percent efficiency could probably be achieved.

Fourth, you can load the generator as you were thinking. This of course will slow it down.
Not the best choice but maybe the easiest, but it has the disadvantage that much of the energy that could have been gleaned from the gas flow will be lost, and that loss will be in the form of heat which may heat up the local area too much although it depends on what level of power we are talking about here. If we are talking 10's of killowatts then you really really have to consider the local environment heating effects, an even 1 killowatt may present a problem. 100 watts maybe not, but still something to consider.

You could mention the power levels you would be dealing with. That will help make the decision a good one. Depending on the power level you may even want to store some of the excess energy in a battery of some type, perhaps a large bank of batteries, if you are into that, and that is again a ripoff idea from a photo voltaic system. Going to extreme, you could even store the heat itself that is generated and use it for heating assistance in the winter (involves some significant plumbing installations).
 
Last edited:

Thread Starter

Dawsonh4

Joined Apr 11, 2017
38
Throttling the engine in your car vs leaving it pinned wide open & relying on your brakes to control your speed. Conserving unneeded gasoline (air/gas) vs wasting the difference via friction brakes/resistor grids.
Great example Sparky. I think you would have to change it slightly to mirror the dynamics on a pressure let down reservoir. You would want the car to be traveling at a set speed (consuming a set amount of fuel). However, the fuel delivered to the engine decreases with time (reservoir depletion) and fuel is free (pressure from reservoir is free).

With those two thoughts why not leave the throttle open and add regenerative brakes?
 

Thread Starter

Dawsonh4

Joined Apr 11, 2017
38
First, nothing is free but if it was free why worry about regenerative energy returns?
Ha! Fair. There is certainly a cost of capital, but pressure is currently controlled by a valve. Why not put a turbo expander in its place?

Second - why not increase ROI?
 

nsaspook

Joined Aug 27, 2009
7,738
Ha! Fair. There is certainly a cost of capital, but pressure is currently controlled by a valve. Why not put a turbo expander in its place?

Second - why not increase ROI?
Purely in terms of entropy (order or disorder, greater the disorder the higher the entropy) things are more stable in the lowest energy state. Now, pressure is being released to maintain perfect internal disorder ie equilibrium. If you try to capture that pressure, perfect internal disorder will be disrupted and equilibrium will be lost.

Is that really what you want to do? :eek:
 

Thread Starter

Dawsonh4

Joined Apr 11, 2017
38
Hi,

My first thought is to install a gas regulation valve electronically controlled if needed.

My second through it to install a gas bypass valve electronically controlled that can bypass some of the gas around the generator so the generator does not get the full gas supply.

Either of those methods does not involve the loss of much energy.

Third, if the generator is connected to the outside power line then you could use an electric regulator that adjusts the power going back into the line. This method does not require slowing the generator down, just adjusting the output similar to how a photo voltaic system works. Also not too much waste of power but there is some due to the high power electronic regulating circuit involved. 90 percent efficiency could probably be achieved.

Fourth, you can load the generator as you were thinking. This of course will slow it down.
Not the best choice but maybe the easiest, but it has the disadvantage that much of the energy that could have been gleaned from the gas flow will be lost, and that loss will be in the form of heat which may heat up the local area too much although it depends on what level of power we are talking about here. If we are talking 10's of killowatts then you really really have to consider the local environment heating effects, an even 1 killowatt may present a problem. 100 watts maybe not, but still something to consider.

You could mention the power levels you would be dealing with. That will help make the decision a good one. Depending on the power level you may even want to store some of the excess energy in a battery of some type, perhaps a large bank of batteries, if you are into that, and that is again a ripoff idea from a photo voltaic system. Going to extreme, you could even store the heat itself that is generated and use it for heating assistance in the winter (involves some significant plumbing installations).
Very helpful post. Thanks MrAl! Your feedback on my thoughts below is much appriciated

Your first thought it how things are currently done. The downside is that there is no harvesting of exergy

Your second thought is the one currently employed by different suppliers trying to harvest the exergy. Since there is a bypass around the generator there is still exergy not being harvest. Correct??

The third thought is getting to what I am thinking. However, if you can no control he speed of the generator you would still need some by pass in order to control the outlet pressure of the gas stream.

The fourth thought, as you stated, is what I am proposing. I understand there will be energy lost from heat, but I don't see where it would have been harvested else where. Plus the JT effect could help off set this with a heat exchanger.

Thanks again for the helpful input.
 
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