From the basic definition, the thermal efficiency of a GT-HRSG based CHP system should be:
η=(Wnet+Qph)/Qcc where
Qph= Process heat generated,
Qcc= Heat generated at the combustion chamber
However, in many articles, say for instance,Article Comprehensive analysis of energy, exergy and exergo-economic...
the expression for the thermal efficiency is used in the following form:
η=(Wnet+(Ex,out-Ex,in))/Qcc
where,
Ex, out and Ex, in are the exergy flow rate inlet and outlet to the HRSG or in other words the net exergy addition to the water for process heating at the HRSG.
hi, in my opinion it is depend of the point of view. every body know about the capability of the second law to unraveling te hidden ireversibilities, but in the traditional formulation, the heat sent to the thermal process is made "similar" in quality to the output work. on the other hand when the exergy is considered, just part of the thermal energy is considered because, as all we know, not all heat can be transformed in work. personally I dont like that approach because if the thermal process need energy as heat, doesn't matter what would be its exergy. Of course the thermal application probably has a low exergetic efficiency as happen with the liquefaction process (linde or claude). Finally I like the traditional approach where heat an work are treated as equals.
The ASME PTC-46 Committee has been wrestling with this problem for years. Several of my colleagues have served on this committee and I have contributed a few ideas to it myself. There is a tendency to embrace simple answers and also ones that favor a particular technology. It's difficult to identify an entirely objective position because heat is different from work, which we can agree on is measured with a watt meter. In some sense, the value of heat is in the eye of the beholder. It is too simplistic to merely convert heat to equivalent power. We don't report shaft work for a power system, nor do we take credit for VARs. We evaluate a power system on net watts, not gross. The same sort of considerations must be included in evaluating heat.
Hi, you had better use all parameters in exergy form.
for example, for heat use Q(1-T0/T)
Both energy (thermal efficiency) and exergy (exergy efficiency and exergoeconomic) analyses are important to evaluate the system.
Don't forget to include the losses in the definition of the thermal efficiency.
Both definitions are correct. It depends on your choice what you want to evaluate.
The first one is the thermal efficiency whereas the second one is the thermal transport efficienc.
Thanks
For calaculation of heat rate of Co-generation system, first we should calculate the station or Co-gen heat rate by considering the heat input & output to the station.
That is
Cogenertion heat rate =( Fuel consumption X GCV + Heat content in make up water + Heat content in process return condensate -Heat content in steam given to process) / Powergeneration
CHR = ..kcal/KWH
Cogen eff = 860 X 100 / Heat rate
For detailed calculation of Co-gen heat rate refer following web.
https://www.powerplantandcalculations.com/2020/05/understanding-of-heat-rate-and.html
&
https://www.powerplantandcalculations.com/2020/07/power-plants-equipments-efficiency-calculation.html
If we pursue the concept of available energy the the second expression is justified. However, it is well said by many researcher here that it depends upon the point of the view. Certainly, it is not advisable to assume the shaft work (energy) similar to that of heat. But keeping the first law of thermodynamics in context, First expression is easily understandable and justified. This we also face during the co-generation through PVT Modules and we follow the first expression.
Check the equations in the following paper:
Sopian, K., Al-Waeli, A. H., & Kazem, H. A. (2020). Energy, exergy and efficiency of four photovoltaic thermal collectors with different energy storage material. Journal of Energy Storage, 29, 101245.
https://www.sciencedirect.com/science/article/abs/pii/S2352152X19312915
The two are correct, however, the second equation which involves exergy seems more detailed because primary thermal efficiency is net work divided by heat input. The exergy terms is still in terms of work which agrees with the primary definition of thermal efficiency.
In our nomenclature, ηgl=(Wnet+Qph)/Qcc represents the overall global efficiency of the cogeneration cycle.
The thermal efficiency is defined as ηth=Qph/Qcc, while the electrical one is ηel=Wnet/Qcc.
Usually you would say that the remaining thermodynamic potential of heat cooling down and thus its exergy is the correct value. It has the benefit that in industrial applications the different pressure levels in steam supply can be charged differently. This type of industry has qualified staff to do it correctly and check possible system improvements resulting from exergetic analysis. However if there are only compareable low temperature heating applications as in district heating in residential areas it does not make very much sense to calculate exactly for every household. The technical staff would mainly look on heat losses in the distrbution system and improvements there than for exergy in households. So the final decision will mainly will depend on the use of its practical application. The important point is to understand what you are doing with the equations and not so much the philosophy. The use of an electric driven heat pump heat e.g could deliver even an efficiency > 100 % if somebody would only look for the first law and thus simply look on efficiecies defined by utilized energy and heat input by fuel. (see e.g. Fig. 17 inConference Paper Fuel Cell Generators and the Second Law of Thermodynamics
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