Generator Start Up vs. Full Speed Operation

[AN1]

I am sorry to pose the following basic question. I am not an EE.

I have a general question regarding the efficiency in or cost of starting a power generator cold to full speed/full load. I suspect the efficiency/cost of this can be broken down into fuel, and wear.

Are there any rules of thumb for determining the impact of this type of start up vs. regular operation at full speed/full load?

Also, is there a rule of thumb in terms of cost or efficiency for running at full speed/no load to going to full speed/full load?

Finally, I suspect the situation would be similar between hydro power and fossil fuel power except for the cost of the fuel?

Thanks a lot.

 

[itsmoked]

Starting any internal combustion engine is where a great deal of its wear comes from. Starting an internal combustion engine directly into full output while done all the time is probably where and when they break the most. This along with other points makes your question a tricky one to answer.

As for efficiency... If this is an emergency generator efficiency has little to do with anything based on the supposition that life, safety, corporate, or business loss, will be avoided by use of the generator. The numbers of which utterly eclipse the expense of 'a start' efficient or otherwise.

Further a generator that is in standby waiting to start into full load without even a one or two minute warm up will normally be kept hot with electricity. This might be anywhere from 100W to 1000W of electrical heat. These days that represents a sizable piece of change that constantly dribbles off the bottom line. (reducing efficiency)

There will be no comparison between running an unloaded generator constantly as a trade off to a rapid start/full load. Running a generator unloaded constantly will destroy it. Will cost 10% fuel usage constantly, and is flat illegal in some locals due to air quality. It will be less expensive to rapid start.

The difference between hydro and fossil are so vast as to render the question.. um..er... does not compute.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

If you have a special operation that requires "blink free" power, and so much power, that a UPS is not viable, there are special generators that are kept in motion; engine turning, oil pressure up, and engine at operating temperature. On power loss the controller energizes the fuel system and the engine takes on the load with mostly just a cylinder head temperature increase and exhaust production.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

What is the prime mover for the generator? You are really asking prime mover questions, not generator questions. For reciprocating engine driven generators, you can bring the generator up as fast as you can bring the engine up, emergency units do it in less than 10 seconds while 30 seconds to 1 minute will be more fuel efficient. That 10 second start requires full full from the get go, even though the first few strokes of the engine can't use all the fuel, a longer ramp allow the fuel to ramp up as the engine can use it. Hydro can be very fast as it generally has a very simple prime mover. Gas turbine units will have startup times in minutes, and steam turbine units may have start up times in significant fractions of days. The steam turbine needs that long to allow the unit to reach thermal equilibrium at operating temperature before it can safely produce power.

In all cases, the generator itself will respond much faster than the prime mover, no need to worry about the generator.

 

[davidbeach]

Just caught something in the title of the thread - the generator will only be connected to the load/system and producing power at full speed. Operation is ALWAYS at full speed, plus or minus a very small percentage. Operation, can however, range from zero power output to full power output, but always at full speed.

 

[waross]

What is the fuel source of your engine? Gasoline, natural gas, propane, diesel?
respectfully

 

[davidbeach]

Interesting what people read between the lines. The OP does not mention the type of prime more, nor does he mention the type of use. Keith goes after the emergency generation issues and waross assumes an engine. I saw the bit about hydro and figured something rather larger, but hydro can also be quite small. So, we really need AN1 to come back and provide many more details. Is it an emergency unit, is it a thermal base load unit, is it something in between? Inquiring minds need to know.

 

[ScottyUK]

Some gas turbine OEMs equate a fired start as being equivalent to a number of equivalent baseload running hours. They apply similar a principle to operation on, say, distillate instead of natural gas, and to any trip under load.

Inspection is typically required after a given number of starts or equivalent baseload hours.


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I don't suffer from insanity. I enjoy it...

 

[waross]

Hi davidbeach;
Your point is well taken. I may have meant "What is the energy source of your prime mover?"
Another assumption I jumped to was that "Hydro" power meant utility power. I am now in an area where the word "Hydro" is used to denote either the utility company or power from the utility company.
Respectfully

 

[jraef]

I find it interesting that everyone seems to have a different take on the OPs question. As I see it, he is asking from the standpoint of the generator itself, irregardless of the prime mover, evidenced by the last sentence. That makes me think he is asking if the generator coils have a different power generating efficiency / operating cost when hot or cold.

AN1,
If that was your question, then the answer would be technically yes, but practically no. There is a difference in the winding resistance with different temperatures. For the most part, stator coils have a negative coefficient of resistance, meaning as the temperature goes up, the resistance goes down. Less winding resistance equates to better efficiency. You should also know however that winding resistance plays only a small part in overall generator efficiency, and the difference between hot and cold resistance will not be that great. So overall the generator efficiency on a cold start vs a hot start or continuous run is very very slight, probably not worth considering. A cold generator will warm up very quickly as well.

http:/

http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[AN1]

Hello All:

Thank you for the responses.

I should have been clearer in my question related to power plant generation.

I am trying to determine the added cost of starting up a natural gas turbine/generator to full speed and full load versus its normal operating cost. I was wondering in general terms what the added cost for fuel consumption was and if there was any factor for added wear. Also, I was wondering if this generator was spinning at 3600 rpm but no load was put on it, and then the load was initiated, what added costs there would be.

Finally, for my general knowledge, I was wondering the same for a hydroelectric turbine (Of course, there would be no fuel cost.).

I am a construction manager with a structural engineering background trying to understand the risks of a project. Once this project moves forward, I will consult an EE for the actual numbers for this job.

Thanks a lot.

 

[itsmoked]

This question is barely improved.... How can we give any real numbers that could mean anything?

Is this in Nigeria? New York? Singapore?

If the generator is on standby doing no useful work for 2 years before loading it.. I can guarantee it would be cheaper to start it!

If it is needed every 20 minutes then it would be cheaper to leave it run.


All the same goes for hydro. Plus umpteen other details. Spilling water may be needed for run off or may cause a later shortage.

What do you want for an answer?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

Gas turbines have a poor turn down ratio. It takes 30% to 50% of full load fuel to run one unloaded; the compressor uses a lot of the turbine output before anything can go to the generator. Every start of a gas turbine counts as so many hours of runtime toward the life of the machine and the repairs that need be done every so many hours. From a fuel standpoint, you are probably better off shutting down the turbine if you won't need the power for longer than the shutdown/startup time (could be 10s of minutes) and you have to allow for the maximum number of starts permitted per hour or per day. From a machine life standpoint, you should probably continue to run it if the down time would be less than the number of hours that a start would count against the life of the machine. If a start is the equivalent of 4 hours of run, shutting it down for 8 hours would make sense but a 2 hour shutdown probably wouldn't unless fuel costs were extremely high.

Lots of project, site, and machine specifics that would have to be evaluated. One question that would have to be thoroughly worked out is the amount of advance warning for the need of the generated power; you won't be able to rush the startup process.

For a single unit, hydro probably has the ability to add load faster than any other type of prime mover. It would depend on how fast you can open the wicket gates. The first step would be getting the machine up to rated speed, then run up the load.

I think it is probably time to find a consultant who can address the project specifics rather than depending on generalities that may or may not apply for the specific project.

 

[waross]

Hi all;
The fuel consumption figures shouild be available from any manufacturer of turbine sets. Just a phone call away.
You can get a fuel consumption curve vs loading for the set in question.
I don't yet see a size for the set. I am under the impression that the larger the gas turbine, the more of an issue will be the light loading and frequent startups. The manufacturer should also be able to give a "Run time equivalent" for each start. I suspect that the figure will also be dependant on the size of the set.
Gas turbine generators achieve their best economy when a combined cycle is implemented. The heat of the exhaust gas is used to generate steam which powers a steam turbine. If you are asking about frequent starts and light loading, I doubt that you will be able to utilize a combined cycle.
For a project that will require light loading and frequent starting, I suggest diesel up to about 2 Megawatts. Above that size is out of the range of my experience and I'll leave that call to someone with hands on experience with larger sets.
davidbeach, Hydro with a short penstock may beat the diesel response with a short penstock. With a long penstock, the diesel may be able to pick up a block load faster than the water in the penstock can accelerate. Shedding load with a long penstock is an issue.
That said, I think that both hydro and diesel have excellent response and I'm just splitting hairs a little.
In regards to hydro power, I used to live near a series of hydro electric plants that were quite elderly. The utility found that the most economical way to operate the plants was to keep them online at about 10% or less load at all times. This kept the windings warm and dry and kept the bearings lubricated and avoided babbit rubbing, which may occur during startup and which may be the major cause of wear of the bearings. When the system needed more power at peak consumption hours, the central dispacher would phone the plant operator who would open the gates until the machines were developing about 90% power. They never were stopped.
I realize that this control method is not suitable for an islanded installation.
respectfully