Controlling the rotational speed of a generator 2

[Dawsonh4]

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.

 

[BrianPetersen]

Any sort of "resistance" implies throwing away some of the hard work that your prime mover (engine/turbine) did in the first place, by throwing away some of the electricity that you just generated (and turning it into heat), so that's not a preferred choice.

What is the nature of the load that you are connecting the generator to? Is it a single light bulb (resistive load), or is it a motor (electromotive load - at which point you also have to ask about the nature of the load that said motor is driving), or is it a battery charger (fixed, or almost-fixed, voltage), or is it the local electrical grid (essentially your generator is a little pee going into an ocean, and it's essentially powerless to do anything about the voltage or frequency), or something else?

What is the nature of load/output-torque control of your prime mover?

Normally in a turbine application, you would use some sort of variable inlet guide vane to redirect the gas flow and/or change the effective area of the inlet nozzle and/or some means of varying the upstream amount of pressurised gas being generated (e.g. by varying the amount of coal you throw on the fire), and let the generator generate what it wants to generate. In other words, backwards to your way of thinking. Use varying upstream pressure/flow by some suitable means to vary the amount that the generator generates, as opposed to somehow trying to work it backwards.

 

[waross]

Brian has covered it fairly well.
It may help you to understand if I say somewhat the same things with different words and examples.
A classic example is a recovery boiler and turbo generator.
Recovery boilers run at very high temperatures and pressures.
The steam pressure is too high to be used directly so it is sent to a two stage turbo generator.
In the first stage, some of the steam exits at about 125 PSIG and supplies some of the steam needs of the mill.
In the second stage the steam is dropped to about 30 PSIG and used in the drier heaters.

The generator is tied to the grid and the grid sets the speed and/or frequency.
The output of the turbo generator depends on the amount of steam needed.
As the steam demand varies, the Watts output of the generator varies.
The speed stays the same.

I suggest that you look for ways to control the flow of gas apart from the generator, and use the generator to regain the available energy.

But, that said:
If your project is quite small, and controlling gas flow with variable loading on a turbine is an accepted technique in your field,
this may work:
Connect a load bank permanently to the generator.
Use a PID controller and a suitable transducer to vary the excitation of the generator.

At this point at least one of my friends is pulling his hair out, so I will say what he wants to say:

In place of the generator use an induction motor running as an induction generator.
Connect this to the grid with a fully regenerative VFD.

Vary the speed of the generator by varying the set point of the VFD.
THe induction motor/generator, when over-driven by gas flow will stay within a few RPM of the set point.

You will be able to control the speed quite precisely and recover almost all of the energy available.
(Have I made any silly typos, Keith?) grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Negatory! Right-on! Power to da people!


I'd also say that Dawsonh4 is set up to make a large amount of expensive silly errors from that vague description.

Dawsonh4; You should fill us in a great deal more so we can zero in on a cost effective solution.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Dawsonh4]

Thanks for the reply.

Currently this is just a theory that I am trying to test the efficacy of. I have more of a chemical/mechanical background than electrical - I am out of my element. Any and all input is welcome.

The load would be the grid. Selling electricity to the grid much like a back pressure turbine you see at paper mills.

I have researched a few different ways to control the feed going into the turbine/expander, i.e. guide vains, regulators, JT valves, etc. However, all those ways result in energy lost through the mechanical throttle. I was hoping to find a way to waste less energy through the control of the rotational speed of the generator/turbine system. I do understand there will always be a need for control valves.

Waross - thanks for the input. Could you elaborate on the major differences between using a controller to vary the excitation level of the generator vs using the induction generator with a regenerative VFD?

 

[waross]

That would be synchronous generator versus induction generator.
Cost and availability.
Depending on the size, the VFD and Induction motor/generator solution is available off the shelf.
The VFD and Induction motor/generator solution easily lends itself to 4-20 ma control.
Pole slip may be an issue with a synchronous generator at low loading.
The induction generator depends on pole slip for re-generation.

The speed of the synchronous generator will be locked to the grid.
The output of the synchronous generator will depend on the energy supplied to the turbine.
Hence some added flow control devices which you do not wish to use.

What size of machine do you anticipate?
As you get into sizes at which synchronous motors are readily available, the cost advantage of the VFD becomes less.

One thing that I do not know, and there may not be enough information given for anyone to answer:
Will a turbo-expander do what you want to do?
What will be the operating limits of the turbo-expander?

Size matters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Could you elaborate on the major differences between using a controller to vary the excitation level of the generator vs using the induction generator with a regenerative VFD?

"vary the excitation level of the generator and a resistor bank"
Not easily compatible with the grid.
"the induction generator with a regenerative VFD?"
Close to "Plug and Play".

There was a scheme back in the late 70s, early 80s in the early days of the energy crisis.
Part of the advantage of this scheme was the tax advantages allowed for an energy saving system.
Wound rotor induction motors driving blowers were speed controlled to vary the output volume of the blowers.
Rather than using resistors in the rotor circuit to control the speed, the current in the rotor circuit was controlled by rectifying it to DC and then inverting it back to AC at the proper voltage, frequency, and phase angle to return energy to the grid.
HPs varied from 60 HP to 400 HP.

The concept worked, but the implementation was the stuff that the "Failures and Disasters" forum is made of.
The designer was on site.
A wise old electrician foreman (No, not me. I wasn't that old then. This became part of my education.) suggested to the designer that his choice of a main contactor may be undersized.
The designer scorned and chastised the foreman somewhat brutally, citing his PHD in electrical engineering and the testing that they had done.
("You should know your place and stay in your place.")
Ahh, hubris and too little field experience.

When the contactor in question failed catastrophically, the arcs destroyed the control panel.
The coupling joining the motor to the fan exploded, and parts were found all over the machine room.
The motor shaft was bent.
The fan shaft was bent.
The old foreman (and his crew) got his payback in actual cash, working through a weekend to repair the damage, (at double time wages).

The point of this?
The system was sound and went into service with an appropriate sized contactor.
There is more than one way to harvest energy.
You could spend a lot of money (more than you are thinking) to hire engineers to design electronics for a special case,
or
You can use a regenerative VFD which does all of that, out of the box.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

What are you expanding?
How much of it are you expanding?
What kind of pressure are you starting with?
Is this expansion to atmosphere or must it remain contained?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Dawsonh4]

Thanks again for everyone's help.

It seems to me that there are no clear advantages with a synchronous generator when compared to and induction.

A turbo expander would likely be best for the application. It would have to be appropriately sized to the pressure, flow, and generator variable.

The expansion would be a hydrocarbon gas stream. The volume and pressure can vary greatly lets says an average of 14,000 MMSCFD at 7,000 psi.

The expansion must be contained and knocked down to approximately 800 psi.

 

[waross]

What determines the flow?
What is the maximum expected flow?
As I remember the pulp mill turbo expanders, there was a flow control valve integral to the turbine that varied the flow to provide a relatively constant outlet pressure.
How much energy is released when that volume is dropped from 7,000 psi to 800 psi?
If the energy to be harvested is in the range of MegaWatts or tens of MegaWatts, a pulp mill type of turbo expander with a synchronous generator may be the way to go.
Your application may be beyond the capability of off the shelf VFDs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Dawsonh4]

Flow and pressure is determined by the underground reservoir. It starts high and begins to decline.

I will work out the math on the energy released.

Would you be able to point me in the direction of a company that manufactures these pulp mill turboexpander generators?

 

[Strong]

https://www.bing.com/search?q=pulp+mill+turboexpander+generators&FORM=ANCMS9&PC=U531

http://turboexpanders.com/

http://turboexpanders.com/energy-recovery-turboexpanders/

https://laturbine.com/turboexpanders/expander-generator/

Walt

 

[waross]

Thanks for the help, Strong.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LittleInch]

14 BILLION SCFD????

Please check your units.

And pressure.

This sounds like you're trying to use the energy fron a well head??

I think you'll find out soon why no one does it this way.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dawsonh4]

14 MMSCFD

@Stron. Thanks! I spoke to LAturbine, but their expanders have issues with anything over 2.5% water.

@LittleInch. Yes from a well head as stated above. Care to elaborate why it won't work?

 

[FacEngrPE]

Note on the induction generator. The use of a VFD is an unnecessary complication.

If a motor is driven so it's rotation speed = line frequency + slip, electric power to the mains supply will equal motor rating. As the motor is excited from the mains loss of mains supply will result in rapid voltage decay.

However loss of mains supply creates a problem - now the turbine is still powering the motor shaft, but the mains are not absorbing any energy. The result is a very rapid increase of speed.

Even if your arrangement does not need to a governor to regulate generator speed / frequency, it still needs something to monitor rotational speed, and must be able to rapidly close the turbine steam supply valve. Failure to close the steam supply valve will result in rapid disassembly of the system, likely with parts exiting via holes in the roof. Don't ignore the need for an overspeed trip.

https://www.nrc.gov/docs/ML0635/ML063560418.pdf

https://www.aig.com/content/dam/aig...m-turbine-valve-risk-exposure-09-22-22017.pdf

 

[LittleInch]

Dawsonh4,

My other fellow posters are much more informed about the generator end of this.

My interest is in the driver end. Your desire to use this energy is laudable, but given that people have been drilling gas wells for 100 years and no one does it this way, kind of tells you there is an issue, mainly it costs too much.

Well fluids are a mixture of gas, saturated water vapour, free water and liquid hydrocarbons. These do not go through high speed axial turbines very well.

Is your 7,000 psi a flowing well head pressure or reservoir pressure or what?
Either way, there is a practical limit on pressure ratio to get to your 800 psi, i.e. too much expansion of the gas. I'm not sure about axial turbines for drives, but in compressors (reverse of turbines), there is a max pressure ratio of about 2.1 So 7000 to 800 in one go is just too much.

Well head equipment is there to control and reduce pressures to that which makes the rest of the system much cheaper to build. Your device may work when flowing, but not when the well is shut in / starting up / controlling flow or pressure, so anything else is extra cost to the equipment already there. High pressure equipment costs a lot of money.

The amount of pressure energy in a gas well is actually quite small, so the generation and money you can get from this generation won't pay for the equipment, especially when, as you say, the pressure declines over a relatively short time.

But the key issue is the extra fluids which come up the well pipe wrecking your turbine. Also as you drop pressure, you drop temperature and even more water and liquids drop out. Turbo expanders are used in CPFs to lower temperature, drop out liquids and then the end of the expander connects to a compressor which pumps the gas pressure back up, just not to the same level or it needs a bit of help from a motor. But this is downstream of filters, separators etc which remove all free sand, dirt and liquids. If you have to stick a separator onto each well head before the turbine that's even more money.

Lots of people have tried to do this and not succeeded.

This lot

http://www.energyrecovery.com/isoboost-isogen/

make a good unit for liquid pipelines, but no one so far has succeeded in getting sufficiently large amounts of power out of the hundreds if not thousands of gas pressure let down stations across the world where high pressure gas in transmission lines is let down to lower distribution pressures to make it worthwhile to install the equipment.

Especially at well heads, you can't mess around with the equipment - failure leads to catastrophic damage.

Whilst doing a quick search I cam accross this form 2009 -

https://www.theguardian.com/environment/2009/jan/06/gas-energy-pressure

A key paragraph is below and then silence - never happened.
"The idea is not completely new. US companies experimented with turbo expanders in the 1980s and Mercer said a handful of similar efforts have already been set up in Europe. "But this isn't a cheap way to generate electricity. The reason it hasn't really taken off is that it's expensive." "

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dawsonh4]

Thanks FracEngr. If there is no VFD could you explain in simple terms how the rotational speed would be controlled?

 

[waross]

In this application, the generator, whether synchronous or induction will be tied to the grid.
The grid will lock the speed of the generator and turbine.
This will reclaim the let-down energy from an electrical point of view (subject to turbine suitability).
This will not control the flow.
The greater the flow, the more energy will be reclaimed.
You must still either let the system run full open or control the flow externally.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Dawsonh4]

LittleInch.

I am in the same boat as you. Very familiar with the mechanical end - less so with the electrical.

There are people that have tried to do this, but you are correct. Most have not succeeded. Calnetix is a company that may be able to accomplish it.

As you state there are a few issues from a mechanical and cost stand point that need to be overcome:

1.) There are liquids and debris in the flow stream that will need to be dealt with
2.) Pressure limits and knock down ability
3.) Energy harvesting and ROI

Here are my thoughts on the above.

1.) Using an Expander rather than a Turbine would help some of the concerns. Also, still using a JT valve to knock down some of the pressure and a filter/liquid drop system before hand.
2.) The 7,000 psi would be the casing pressure. There are expanders out there that can do a 5 to 1 ratio. If more is needed you can run in series.
3.) I think that a solution to this would be a "skid" that went to wells for the first few months of flow. This is when there is the largest amount of pressure drop and energy to harvest. I believe pay back could be less than two years if power lines are in close proximity.