4 replies to this topic

[sheiko]

Dear members,

I'm looking for a simple and accurate way to estimate the thermal efficiency of a cogeneration unit (combined heat and power).

Shall I segregate between the fuel (gas and liquid) turbine efficiency and the steam boiler efficiency, or is it better to calculate a single efficiency for the whole unit?

Any idea/resource/reference?

Thanks

Edited by sheiko, 07 October 2011 - 10:46 PM.

[breizh]

Hi Sheiko ,

Let you consider this resource , you may find what you are looking for.
http://www1.eere.ene...pubs_steam.html

Hope this helps



Breizh

[sheiko]

Thank you Breizh.

I would like to have your opinion on the following:

Is it OK to calculate the cogeneration (combined heat and power) unit efficiency by the formula?

Efficiency = useful energy / energy input, with:
- useful energy = electrical production + enthalpy difference between feed water and superheated steam
- energy input = fuel combustion heat

To have consistent units, I have converted MWh (electrical production) to kcal by multiplying MWh by the factor (10 000 000 / 11.626).

The result is an efficiency of around 55-60%. The order of magnitude seems OK to me.

However, I have not found this "direct" approach in the litterature, so I would like to have some feedback.

Thanks.

Edited by sheiko, 08 October 2011 - 07:11 AM.

[breizh]

Good day sheiko ,
Additional info to validate your calculation:

http://www.epa.gov/c...s/tsd_cogen.pdf


Breizh

[kkala]

Global energy utilization is indicated by the (single) overall thermal efficiency. But electricity is more valuable than thermal power, so a power to heat ratio had better be also specified. An example of efficiency estimate can be seen in attached cogeneff.xls, even though it concerns a boiler - steam turbine cogeneration , not a plant with gas turbines. Mentioned estimate is "approximately" based on the 25 MW turbine plus boiler plant, 1st example of attached tsd_cogen.pdf, where efficiency calculation is explained (but I have not understood explanation thoroughly).
I believe that efficiency concept contains a lot of parameters to be clarified, not limited to whether it should be based on HHV or LHV of fuel, as indicated in "cogeneff.xls". So efficiency may be fine for a quick understanding, but in a requisition I would add a simplified diagram with actual flows of fuel, steam, hot water, etc (like the scheme of cogeneff.xls, not limited to enthalpies), so that vendor would avoid misunderstanding. During guarantee test runs, "corrections" would be made for any difference in weather conditions or temperatures of inlet streams. This principle has been applied in two boiler cases here, and a requisition for a cogeneration would contain something similar.
Concerning efficiencies, net (heat+electricity) would be OK as overall thermal efficiency , expressed energy wise as percentages of input fuel (using low or high heating value). Electrical efficiency would similarly correspond to net electricity given to grid (it is not so in "cogeneff.xls").
Concerning queries of the second post by sheiko, the plant has probably gas turbines and waste heat boiler (HRSG) without steam turbine. Overall efficiency formula looks precise in case when "superheated steam" is exploited to a condensate temperature equal to that of make up water (blow down neglected). See case 1 & 2 in "cogeneff.pdf".
Stated efficiency looks to be in the ballpark (http://en.wikipedia..../Combined_cycle).
1MWh=1000*3600 kWs=3.6E6 kJ = 3.6E6/4.1868 kcal = 859845 kcal, practically same as the factor assumed (860141).

Editing note: "tsd_cogen.pdf" is the file referred by breizh in his previous post, but I have not realized it before sending this post.

Attached Files

•  cogeneff.xls   28KB   109 downloads
•  tsd_cogen.pdf   108.53KB   72 downloads

Edited by kkala, 09 October 2011 - 01:40 PM.