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.

 

[waross]

The steam turbines in pulp mills are multistage to handle higher let-down ratios.

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

 

[FacEngrPE]

Since the question was asked - how do you control the speed of an induction generator, i might get a bit wordy here.

Synchronous generators only need to be speed controlled for two cases
Generator Island-ed
Generator large enough to control the frequency of the network.
in all other cases speed regulation of individual generators is not necessary. Frequency regulation is performed at the system level by sending all of the generators a increase / decrease signal that impacts their throttles.

Induction generators require VARS from a external source to induce an a real power output, so if an induction generator is islanded it looses excitation, and the real power output rapidly decays. Once excitation is lost frequency will drift off of mains frequency.

As voltage and frequency are set by the mains excitation to an induction generator, a change in throttle position results in a change in real power output equal to the change in power input.

How this impacts rotational speed is
When an induction machine is acting as a motor, and a motor is producing torque, shaft speed is slower than synchronous speed, at nameplate speed (usually 2% or so slower than synchronous) the motor will generate full torque and consume nameplate rating amps. When the induction machine is acting as a generator, shaft speed is faster than synchronous, Torque is fed into the machine, and real power is provided to the mains.

Adding a VFD to this mix is only useful for situations where is is not possible for the ideal operating speed to be matched to mains frequency. The most common examples of this are run of stream water turbines and windmills where ideal speed is a function of wind or water velocity.

Your application - capturing a high pressure geological gas source and extraction energy sounds similar to geothermal steam power generation. You might want to look at the turbine service experience of that community.

Fred

 

[LittleInch]

It's actually nothing like steam generation as the only energy in the fluid is pressure, not heat.

The OP is looking, I think, to see if there is a way to dial in extra resistance to the expander/turbine and generator set up so that the pressure drop will increase across the expander as the flow rate increase and power goes up.

Personally I think that flow of the gas is your primary control as you indicate.

There are many other issues in trying to do this from the driver end and whilst it CAN work, nobody has yet made a commercial version tat generates sufficient power to make it worthwhile buying the machine. Plus the gas will get VERY cold as it exits the machine.

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

 

[Compositepro]

"It's actually nothing like steam generation as the only energy in the fluid is pressure, not heat."

I feel the need to nit-pick that statement. The work extracted is due to delta(PV). The role of heat is only to increase the pressure and volume of the steam compared to water. The difference between steam and water in energy content is the far greater volume of steam. I'm sure there is a more elegant way to say this.

 

[FacEngrPE]

If you care to nit pick, both turbines are First Law energy conversion devices.
If you feed a relatively cold gas stream into a positive displacement motor - like is used in an air tool, the gas stream exits the device only a bit cooler.
If you use an expander, more work is extracted, but the exiting gas stream is much colder. Significant risk of ice buildup as geologic wells almost always produce saturated gas streams.
The need to keep the gas above condensing temperature is a good reason to heat the gas before it is expanded with energy extraction = unless the goal is refrigeration or making snow.

This is a bit off course for a discussion on turbine speed control.

Fred

 

[waross]

A grid connected synchronous generator speed is controlled by the grid frequency.

The frequency of a grid connected generator is controlled by the grid frequency.

Different case, using a turbine generator to control the flow of the driving gas.
The frequency of the induction generator is controlled by the regenerative VFD output frequency.

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

 

[LittleInch]

Fred,

I knew I was missing something before - the issue here is the motive fluid for the turbine (A gas well) and how to control it.

The "snow" you refer to is gas hydrate and if you don't de hydrate the gas before you stick it through an expander, you're going to get rather a lot of it very fast. Hence another reason why it's not done.

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

 

[FacEngrPE]

My first experience with making snow was with wet air, freezing weather, and air tools. The result is about 2 minutes run time, then a trip to the heater to thaw the tool out (and the fingers). It is an experience that you don't forget.

Fred

 

[Dawsonh4]

All,

Thank you for the helpful back and forth. I think the project is now much better understood than I had originally posted. The primary goal is still to control a variable inlet pressure gas stream.

The examples of the geothermal steam turbine is applicable. As Compoitepro stated - it is a fluid dynamics problem and not so much a thermo problem. Admittedly there are thermo issues as play here as the gas will get very cold due to increased potential energy (JT effect) and there is some moisture content.

The way Fred explained things is very helpful. It sounds like the best way to do this is to start with a induction generator and a VFD. As he said - this is similar to a wind turbine where you can really control input power.

I was reading that wind turbines also might do an AC-DC-AC conversation, but I get a little lost as to the reasons behind that.

 

[waross]

I tried to explain, but I will try again.
Your goal is not to control the speed of a generator, your goal is to use a turbine generator to let down gas pressure.
You don't care what speed the generator works at as long as the system works.
Just use an induction motor.
Start to slowly admit gas to the turbine so that the motor accelerates slowly.
When the motor is close to or at synchronous speed, close the switch and go DOL. (Direct-On-Line)
Or, in other words, connect the motor to the grid.
The grid will now control the speed of the motor (induction generator).
Now the more gas flows through the turbine, the more energy will be delivered to the grid.
Your utility has strict rules and requirements for exporting power.
You may find it easier to use a scheme that supplies your own energy requirements but avoids exporting energy.
Thinking out of the box, you may use electric heaters to use some of the generated energy to re-heat the gas stream.
Disclaimer: My expertise is the electrical side. Others have and will comment on the do-ability of your project on the turbine side.

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

 

[itsmoked]

Bill's right, definitely try to use the power locally instead of exporting it. Especially in your case where the energy source will dry-up in a short period.

Use it to pump oil, or compress processed gas, or even to make valuable ice.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I think we got a two-fer going here Keith.
As well as electric energy we have negative energy in the form of very cold gas, that can be used to increase the efficiency of the ice machines.
Think pre-cooling the water and cooling the refrigeration condensers.

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

 

[LittleInch]

Study gas hydrates a bit more before you go much further. Otherwise your system will run for about 5 minutes then block up.

Very cold methane and free water is a very bad combination.

Gas wells have been around for the best part of a century. If it was possible to do this then it would be done by now.


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