co-generation(gas-turbine generator) question !

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Hi All!

I have an urgent question for you!

I am trying to model a gas-turbine generator (CHP) plant per the DES
guideline Opt2.
The CHP generates electricity using "track thermal load" and provides
recovered hot water directly to a hot water loop.

The problem is that the total electricity generated by the CHP plant is
much less than it is supposed to be.
(I put "HIR=2.5" for the gas-turbine generator as the electricity
generation efficiency is 40%. However, the output shows that the total
electricity generated is only 4% of the total fuel input)

Does anyone have some thoughts on this?

Any comment will be greatly appreciated!

Thank you!!

Kim

DongEun Kim's picture
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Kim,

The gas turbine generator is controlled by the settings in the electric meter to which it is assigned. The generator will match the required electricity consumption (if it has the capacity) if you set the COGEN-TRACK-MODE to "Track Electric Load". Setting the track mode to "Track Thermal Load" will control the generator to meet the hot water demand of the loop to which it is attached, and you only get electricity when the generator is running to meet the HW load. If your heating demand is higher than your electricity demand, the generator will produce more electricity than can be used by the UTILITY meter and the surplus power is lost unless you also attach the generator to an ELECTRIC-SALE meter. Setting the track mode to "Track Electric Load" should increase the fuel-to-electricity efficiency but then you will be wasting some of the recoverable exhaust heat.

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Regards,
Bill

William Bishop, PE, BEMP, BEAP, LEED AP

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This response will target specifically the thermal tracking option for cogeneration systems, with regards to gas turbine engines/generators (GTG).

FYI, based on data I have analyzed previously from a major GTG manufacturer (on the Southern West Coast), I think the issue is that GTG exhaust heat output is not linear with electrical load. In other words, you can drop the electrical load on the GTG by 50% and still get ~ 77% of the thermal output that you get at 100% load. Thus, I'm not surprised at the results you are getting using the thermal tracking option. The eQuest curves appear to be taking into account the non-linear relationship between GTG electrical and thermal output. However, I would step back and ask the question, does thermal tracking really work? In other words, if you have to significantly drop your electrical output in order to match the thermal load, that may work inside eQuest due to the use of a theoretical curve. However, it may not work in the real world due to operational constraints. This varies by equipment, but what I mean by this is the following. For example, manufacturers of the simple Brayton cycle gas turbine engines appear (to me) to not want the GTGs operated at less than 50% load for stability reasons, and I know they will not rate them at load conditions < 50%. It's likely that you couldn't operate the GTG at these conditions because the GTG PLC controller would be programmed to override to protect the equipment. Thus, the controller may not allow the electrical load to go less than 50%, in which case you would be forced to find a way to dump the excess heat, which is typically done via a diverter stack, which diverts exhaust gas around the Heat Recovery Steam Generator (HRSG).

To summarize, my take is what eQuest is doing is correct. Your potential solutions could be the following:

1) Generate too much electric power and thermal energy; sell the excess electricity to the grid if you can, and waste the excess thermal energy through a diverter stack that operates in conjunction with the heat recovery steam generator/heat recovery hot water generator.

2) Downsize the GTG equipment to better match electrical and thermal capacity to loads, minimizing non-utilized heat output, and minimizing electricity sold to the grid.

3) Combination of the above.

4) Use a different type of cogeneration equipment, such as a reciprocating engine or a micro steam turbine generator. Do you really have a gas turbine generator, or do you have a reciprocating engine? A recip engine may have different thermal output versus electrical output characteristics that might work better in your favor. I'm not as familiar with the recips so somebody more knowledgeable in this area would need to respond.

In practice, the sizing of cogeneration systems is quite tricky. I don't think there is a perfect solution, but you strive for the best solution you can get based on the loads you have and the available cogeneration equipment in the market, including consideration of their specific operation constraints. In general, the goal is to size the cogeneration equipment to match thermal loads, but it's not possible to get a perfect match, and therefore some heat will be wasted.

Your specific problem may be that you are trying to model a campus cogeneration system serving just the building for your project, in which case the loads versus cogeneration capacity would be a complete mismatch inside eQuest. In that case, I think you would be better off ditching eQuest to model the cogeneration system and just roll your own spreadsheet based on data supplied from whoever is operating the cogeneration plant, which is allowed per the USGBC DESv2.0 guidelines. For example, if you had monthly/annual "monitored" data on total electrical output, total gas input, total hot water output, etc. you could create a spreadsheet that satisfies the DESv2.0 guidelines. My opinion is that this approach would be easier than wrestling with eQuest on cogeneration. The simplest solution, but least accurate but perhaps more cost effective for you, is to use the default CHP efficiencies on page 27 of DESv2.0 (i.e. Appendix D).

Hope this helps.

Thanks! :)

Regards,

JAH

James A. Hess, PE, CEM, BEMP

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Thank you very much Bill and James for your insights !

What I still can't understand is...
when I modeled the gas turbine generator, I specified the HIR(I put 2.5 as
the actual CHP plant data suggests 40% of electric generation
efficiency) and "Frac Input Recoverable from Exhaust" at the 'Loop
attachment.' And I chose "track thermal load(this building uses District
Hot Water from a offsite CHP plant for its heating, and I am applying Opt2
of DES guideline, directly applying generator in the model)."
The fraction of total thermal recovery(report PS-A) to the total
fuel consumed by generator matches the fraction I put in the "Frac Input
Recoverable from Exhaust." However, the fraction of total electricity
generated(report PS-C) to the total fuel is about 4% (even though I put 2.5
for the HIR)..which is too small even considering the non-linear
relationship between GTG electrical and thermal output. Also, I applied a
flat curve for the generator per DES guideline, Monitoring. There even were
some engineers who advised me to put 0.25 for the HIR for the generator to
get the 40% of electricity generation to the total fuel input.

Anyhow, I followed your leads, and tried to size the generator to have the
capacity(KW) that exactly matches the building thermal(heating)
peak load(PS-H). Then I made generator electric meter mode to be "run at
maximum" to achieve higher efficiency for elec generation as well as
thermal recovery efficiency.
Well..luckily.. It gives me the right thermal recovery efficiency(total
heat recovered/total fuel input) and electricity generation
efficiency(total electricity generation/total fuel input) as a result.
And I don't think it will be problematic for the generator to 'run at
maximum' because I have to re-calculate the electricity and the heat supply
to the building according to the DES guideline App.D anyway.

What do you think about the idea of running the appropriately sized
generator at "maximum" mode to achieve high, steady elec generation
efficiency?

I need your insightful remarks again :)

Thank you in advance!

Kim

2013/6/18 James Hess

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Hi Kim,
I haven't done a CHP eQuest project yet so I may be off base here. I
think the way you are trying to split the electrical generation and heat
produced is going to cause you problems. eQuest is a thermodynamic
box. You should have either external systems like district plants or
internal systems like boilers. Trying to split this CHP generator up
the way you are trying to do is going to be difficult as you are
experiencing but I also think your results have a major likelihoodof
giving you invalid numbers. Your CHP is a district plant and should be
treated as such.
I'm not sure of your ultimate goal, the building itself or the plant, I
assume it is the buildingmainly, then showing the advantages of
integrating it into a CHP district plant. This idea is one which should
be pursued more. Typical electrical generation mostly has an efficiency
below 40% due to all the low grade heat being dumped. If you put all
this heat to work you can get your efficiency into the 60 to 80% range
depending on how well you get everything integrated. This is the true
problem, loading the CHP plant effectively, which will determine your
overall efficiency.
I would do the building separately, it is stand alone anyway and eQuest
will function properly and give you good results. If you want to do the
plant as well then you have to look at it's "box" of which your building
is one load of a few or many. Your building could become a process load
as part of the CHP system with an electrical schedule and a heat
coolschedule. We do know how to make cold water with a heat flow so your
heating and cooling loads could load the CHP waste heat side year
round. I don't think you would get a proper analysis of the CHP system
unless you looked at all the loads and schedules on it. The CHP plant
overall efficiency is totally dependent on how evenly you load it and
how well you match the electrical load and the heat rejectionconsumption.
If your buildings are all similar you could use the results from your
one building to model the other ones on the CHP system. The area you
will have trouble with will be the building shoulder seasons when
economizers can take care of most or all of the buildings heating and
cooling needs. With cheaper hot and cold water you may tighten the
economizers operating range and load the CHP better. The penalty on the
building might be more than made up for in gain of CHP plant.
CHP plantsmost times don't operate in isolation. You may find it an
advantage to sell some of the power outside the CHP box to balance the
electrical and waste heat better. That is what a model is for,
exploring possibilities. CHP plants are a great extension of using gas
to generate electricity if they are built close to where the power is
being consumedand if you do it right you only need half the plants.
Bruce Easterbrook P.Eng.
Abode Engineering

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