# Controlling Variable Turbine Rotor Speed

Discussion in 'The Projects Forum' started by Dawsonh4, Apr 11, 2017.

1. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
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1
New the the forum. I have searched extensively for an answer to the below question, but am having no luck. Most likely because of my novice experience.

My proposed project is a high pressure air powered turbine generator to feed electricity back into the grid. I have a flowing air stream in the 1,200 - 2,500 psi range that is currently being cut back to 500 psi in order to control flow via a simple gate valve.

I believe it might be feasible to attach a generator inline with the air stream in order to restrict flow in the same manner as the gate valve while generating energy that can be fed into the grid.

My concern is that the outflow must be set at 500 psi with variable flow input. This would require larger resistance on the shaft during high pressure inflow and lower resistance on the shaft during lower pressure inflows.

I was thinking that some type of variable resistance generator might solve this, allowing for larger generation of electricity as the inflow pressure increases. However, I am having no luck finding anything of the type.

Would love any input.

2. ### BR-549 AAC Fanatic!

Sep 22, 2013
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This is new. First, never throttle with a gate valve.......use a globe valve.

What are you gonna use for a volute. The metal casing around the rotor for air flow direction.

The pressure is between 500-2500 lbs. What is the air flow (the mass) with that pressure.

The air pressure is like voltage and the flow rate is like current. How much work is in that?

And when you convert that air work to electrical work....you will have some loss.

So first you need to measure air work. And see if it's worth it.

Not to mention the rotor and volute,.....and control hardware and circuitry.

3. ### crutschow Expert

Mar 14, 2008
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Sounds very inefficient.
Where is generating this compressed air?
Is it being generated during the day from a solar source (or using the sun to directly heat air and cause the pressure to rise) to be used at night for grid power?

The generator shaft "resistance" to rotation is determined by the load, which for the grid is essentially zero ohms, so the generator will absorb all the shaft power put into it until it burns out, if the input shaft power exceeds the generator rating.
So all you need to do is to adjust the airflow to keep the generator output below its rated capacity.
The generator will take care of the rest.

4. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
Thanks for the responses!

I honestly haven't thought that far in advance. I would use some type of axial/radial turbine expander. Ultimately the source would be natural gas well that are in the 10 MMcf/day range.

Doing some rough calculations I would expect about 0.5 MW.

I am still not sure on the feasibility of this, but my question remains. How would I regulate a generator connected to the grid to compensate for changing inlet pressures and flows.

5. ### BR-549 AAC Fanatic!

Sep 22, 2013
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Any energy you harvest........will have to be re-placed by the gas pumping station.

6. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
There should be no need for a pumping station as the gas coming from the ground is supplied at 1000-2000 psi flowing at a rate of 10 MMcf/day.

However, The inlet pressure and flow will slow over time and I am looking for a way to decrease generator resistance/load to allow outlet flow to remain constant. Is there a way that the load on the generator can be made variable so that in the first few years of the well producing the generator has high load and can produced larger amount of electricity while in later years the generator has low load producing less electricity as the inlet pressure drops.

7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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You gonna run raw natural gas through a turbine for electricity?

Have you found an insurance company to cover you yet? Does the well owner know? Does the gas company operating the extraction equipment know?

Wow

8. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
All theoretical at this point, but I appreciate your concern.

Currently natural gas is run through heaters and compressors with the appropriate safety precautions. How would this be any more dangerous? The UEL and LEL are pretty narrow for natural gas plus its in a pipeline at close to 100% (much higher than the 15% UEL).

9. ### crutschow Expert

Mar 14, 2008
20,744
5,896
You insist on requesting something that is not needed.
When the generator is connected to the grid, it provides a load the absorbs all the electrical energy the generator generates, there is no need to provide an adjustment for that.
A generator's output electrical power is proportional to the input (shaft) power.
If the input power drops, then so will the output power.
You appear not to fully understand how generators work, so I suggest you read some tutorials on them.

10. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
I certainly have much to learn when it comes to generators. However, I do understand that shaft power is proportional to output power. What I am having trouble communicating to you all is what I want to control.

Ideally, I would be able to control either; (a) the pressure in the pipeline immediately after the generator/turbine, or (b) the flow rate in the pipeline immediately after the generator/turbine.

In today's setup these variables are controlled by valves. As the well ages and is producing gas at a lower pressure the valve is opened up in order to keep the pressure after the valve steady. In the oil & gas industry this is called "managed pressure draw down" and it is beneficial for a number of reasons.

I believe that the energy lost from this "choke" could be turned into electrical energy and sold by replacing the "choke" with a generator. However the generator would need to be able to adjust its restriction on the flow of gas as the inlet pressure changed - keeping the outlet pressure constant. Maybe this could be done through a controller measuring outlet pressure and adjusting the load placed on the generator, but I really am not sure. Thus my post.

http://www.saudiaramco.com/content/dam/Publications/Journal-of-Technology/Winter2014/Article10.pdf

Hopefully I cleared up some of the confusion.

11. ### crutschow Expert

Mar 14, 2008
20,744
5,896
Okay.
I misunderstood the problem.
You might be able to do what you want with a generator that has an adjustable rotor field, (similar to an automobile alternator).
That will allow you to control the load on the turbine.

If you want to use an induction generator to allow automatic synchronism to the mains frequency, then you should be able to control the output by inserting a variable resistance in the rotor to control the induced rotor current.
This would likely require a custom designed generator.

Have you determined how much energy you can actually generate with this scheme?

12. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
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Awesome.

Now I really have no idea what is going on. I will need to do some research on an adjustable rotor field and induction generators tonight. Ideally the added resistance on the generator due to larger input pressure/flows would result in larger electricity production.

In the mean time I believe power generated could be in the range of 0.75 MW to 5 MW depending on the well production and efficiency.

Available Energy = R To [ln(P1/Po)-1+(Po/P1)]

13. ### shortbus AAC Fanatic!

Sep 30, 2009
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They do it small scale now. Many wells do it for their production chart drives. But they are small and only run the measurement and charting devices.

14. ### BR-549 AAC Fanatic!

Sep 22, 2013
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If you try to use that pressure drop............two things will happen.

The amount of free flow product from the head will drop...AND you will pay more for pipe pump-age from well head.

The choke uses the pressure drop to ratio these two aspects. You can't use it. It's already being used.

It will probably cost big bucks per kilowatt in the long run.

15. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
shortbus,

Do you have any more info on the small scale product? (Manufacturer/setup)

16. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
BR-549,

I am not sure I follow. There will definitely be a loss of product on a daily basis due to the choke, but that is the point of "managed pressure draw down". It is believed that if you allow a well to free flow then the dramatic changes in pressure will cause fractures down hole to collapse and hurt the long-term performance of the well.

The generator would be replacing the choke in this scenario and should incur no significant long term costs. Obviously there would be maintenance, but the bulk of cost should come from the upfront capital.

Anyway, back to researching Crutschow's comments.

17. ### Dawsonh4 Thread Starter New Member

Apr 11, 2017
29
1
Looks like Crutschow is hitting the nail on the head.

A wiki excerpt reads "An induction generator or asynchronous generator is a type of alternating current (AC) electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.

Also, I found an article discussing adjustable speed generators as they pertain to windmills. I figured windmills would be a close example of what my proposal is as they have to adjust to ever changing wind conditions.

http://web.mit.edu/kirtley/binlustuff/literature/wind turbine sys/DFIGinWindTurbine.pdf

in the "direct ASG system" from the link above would it be able to connect to a controller that would adjust resistance of the shaft depending on input (outlet pressure or flow in my case)/

18. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
You stated you want 500 psi at the delivery end.

Put the turbine before the pressure regulator. Whatever your pressure is on the intake will be used for power and the turbine will exhaust into a 500 psi delivery line.

The induction motor will be connected across the grid power lines, and started via grid power. The gas flow is then started and it tries to drives the turbine faster than synchronous speed. The motor becomes a generator at this point, and every bit of kinetic energy above 500 psi will be extracted and turned into either heat or grid power. If pressure falls on the intake, the only change would be a drop in power generation.

19. ### shortbus AAC Fanatic!

Sep 30, 2009
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I have two directional well heads on my property. I'll try to catch the well tender and see if he'll let me get some information off of the setup. They keep the fence around the tank farm looked and he doesn't have a set schedule of when he comes. I don't think the electrical out put is AC though. There are no mains wires going to the chart recorders or the well head, but there is a wire coming from the generator to the recorder.

The turbine is in a pipe that is shunted off of the discharge pipe. A regulated amount of gas is sent through the turbine and then back in to the discharge pipe. No gas is 'lost', only a small amount of kinetic energy of the gas flow is taken out to run the turbine.

What BR-549 was saying(I think) doesn't even make sense. That's like saying a hydro electric or wind generator takes water out of the downstream river, or the air down wind of the generator.

20. ### shortbus AAC Fanatic!

Sep 30, 2009
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One other thought. At least on my wells the output isn't constant. It cycles. In the wells there is something the drillers called the 'rabbit'. The rabbit is a piston/weight that goes up and down the pipe. The gas builds up pressure and pushes it up, discharges the pressurized gas into the distribution line then falls back down in the well. They do that in this area because they are 'wet wells' some oil and some other liquid comes out with the gas. The rabbit wipes the inside of the pipe and keeps it open. The 'tank farm' has two big tanks 8' around by 12' high, that collect the liquid, and a tanker truck comes every year or so to empty the tanks.