Energy utilization evaluation of CCHP systems

Energy utilization evaluation of CCHP systems

Hui Li*,Lin Fu,Kecheng Geng,Yi Jiang

Department of Building Science,Tsinghua University,Beijing100084,PR China

Received30March2005;received in revised form5June2005;accepted17June2005

Abstract

Combined cooling,heating and power(CCHP)is getting more attention for its energy saving.However,until now it is not clear yet that how to evaluate the energy utilization of CCHP scientifically and if energy saving is intrinsic to CCHP systems.One of the reasons for this lies in the inappropriate selection of reference situations that making the comparison between the gas cogeneration and the separate production fueled by coal.Another is that the ideal efficiencies of CCHP units are often cited and the differences of various practical performances are often ignored in the comparison.In this paper,an appropriate reference situation is suggested and the practical statistical performances of various units are introduced.Then,a static calculation methodology for evaluating the primary energy consumption for CCHP and separate productions is presented.Fuel energy saving ratio(FESR)definition and the boundary conditions that have to be met by CCHP systems for being energy saving to separate productions are given.Moreover,the energy saving performances of various types and sizes systems in heating and cooling modes are analyzed.Energy saving is not intrinsic but conditional to CCHP systems.The systems with the larger electric power have the higher energy saving potential.

#2005Elsevier B.V.All rights reserved.

Keywords:CCHP;Reference situation;Energy utilization evaluation;Fuel energy saving ratio

1.Introduction

At present,most analyses of energy saving evaluation are

based on the comparison between gas cogeneration and

separate production fueled by coal[1–4],in which the

efficiencies of gas cogeneration and separate production are

often given to afixed value.In this case,a viewpoint is

presented that a30–50%energy saving would be reached if

applying CHP or CCHP systems by supposing the following

efficiencies:h e=33%,h q=80%,a e=36%and a q=50%.It

seems that energy saving is intrinsic to CHP or CCHP

systems.

As we know,however,the selection of reference

situations is very important when making a comparison.

In certain cases,some absolutely different conclusions

would be drawn out because of the different selection of

reference situations.Is it appropriate to evaluate CCHP

energy saving performance by the comparison between gas

cogeneration and separate production fueled by coal?After

all,natural gas and coal are different in nature.When

comparing the different energy utilization between different

primary energy,it is difficult to know that the difference of

energy utilization is result from the primary energy quality

or the types of energy utilization systems.

In addition,it is difficult to reflect the energy saving

potential of various practical CCHP systems when the

differences of various practical efficiencies are ignored.In

practice,efficiencies of CCHP units strongly depend on the

scale and types of various units[5].Consequently,it is

difficult to illustrate energy utilization performances of

various CCHP systems in some simply uniform styles.

Marantans[6]has analyzed the realistic energy savings

based on the practical efficiencies of CHP systems and

illustrated the importance of reference situations selection.

But the appropriate reference situation is not suggested and

the effect-factor of energy saving still has not been

discussed.

In this paper,an appropriate reference situation is

suggestedfirstly and the more practical performances of

various units are introduced.Fuel energy saving ratio

(FESR)of various types and sizes systems in heating and

www.elsevier.com/locate/enbuild

*Corresponding author.Tel.:+861062794194;fax:+861062773461.

E-mail address:lh01@mails.tsinghua.edu.cn(H.Li).

0378-7788/$–see front matter#2005Elsevier B.V.All rights reserved.

doi:10.1016/j.enbuild.2005.06.007

cooling modes is analyzed.Some conclusions about the energy utilization evaluation of CCHP systems are achieved.

2.Selection of reference situations

It is necessary to point out firstly that CCHP systems mentioned in the paper are fueled by natural gas.The purpose of the comparison evaluation is to illustrate if the CCHP systems are energy saving to other gas energy application systems and to decide which system is the optimal gas application type for the rare gas resource.Consequently,it is inappropriate to compare gas cogeneration with separate productions fueled by coal.Natural gas and coal are different energy resources with different quality.By the present technology,application systems of natural gas can achieve higher efficiencies than that of coal.It means more energy can be transformed from natural gas than from coal.For example,electric efficiencies of combined cycle plants fueled by natural gas can reach more than 50%and thermal efficiencies of gas boilers is over 90%,whereas the corresponding efficien-cies fueled by coal are 33%and 80%[7].In this case,when comparing the energy utilization performances between gas cogeneration and separate production fueled by coal,the energy saving ratio actually is not mainly resulted from the performance of CCHP systems but the difference of primary energy.

So the energy utilization evaluation should be based on the comparison of the same primary energy systems.For the

CCHP systems fueled by natural gas,the primary energy of reference situation should be natural gas rather than coal,which electricity production by combined cycle plants,heat production by high efficiency gas boilers and cool production by electric compress chiller.

3.Characteristics of CCHP systems

At present,the prime movers types in CCHP applications

mainly include gas engines and gas turbines.And the power capacity ranges from several 10kW to several megawatts.The difference of compression ratio and air–fuel ratio results in the variety of performances.Electric efficiencies of units vary from 14%to 45%,which strongly depend on various systems sizes and types.So it is necessary to consider the practical efficiencies of various systems.3.1.Gas engines

Fig.1shows the electric and thermal efficiencies of gas engines based on statistic data [8].It appears the electric efficiencies for most CCHP systems are situated between 30%and 45%.In general,larger gas engines have the higher electric efficiencies.

The thermal efficiency of CCHP systems with gas engines strongly depends on the application.There are four main heat sources for gas engines:exhaust gases cooling,jacket water cooling,oil cooling,inter-cooling or after-cooling,which result in the complexity in the recovery of the waste heat.And the contribution of these heat sources in the total heat utilization depends on the design and the desired temperature level of CCHP users [6].To base on the same situation of waste heat application,the exhaust gases temperature is supposed to 1808C after the waste heat recovery,while the return temperature of cooling water used to drive single-effect absorption chiller and heating are supposed to 908C and 508C,respectively.

H.Li et al./Energy and Buildings 38(2006)253–257

254CCHP combined cooling,heating and power CHP combined heating and power

COP coefficient of performance of chillers FESR fuel energy saving ratio

power capacity of gas engines or gas turbines

Greek letters a efficiencies of CCHP systems h efficiencies of separate systems t ratio of heating hours to whole year operation

hours Subscripts a absorption chiller c cooling mode e electric ec electric compress chiller h heating mode i lower temperature level ii higher temperature level t

thermal

Fig.1.Efficiencies of CCHP systems based on gas engines at return temperature 508C.

Efficiencies of CCHP systems based on gas turbines are shown in Fig.2.It appears clearly that small gas turbines (<5MW)have electric efficiencies lower than30%,larger gas turbines could reach efficiencies of30–40%,and gas turbines with high electric efficiencies have lower thermal efficiencies.

Contrary to the gas engines,gas turbines have only one but a high temperature heat resource,exhaust gases,which can pass into the generator of absorption chillers or heat exchangers directly to produce cool or heat.And the exhaust gases temperature,1808C,is also supposed after the exhaust waste heat recovery.

4.Energy utilization evaluation

4.1.Calculation methodology

The‘‘static calculations methodology’’means that the variations in time of energy demands and efficiencies are not taken into account.In the ideal situation,the energy demands of buildings are equal to the energy supply of the CCHP system.

Fuel energy saving ratio means,to meet the same energy supply,the ratio of energy saving that CCHP systems relative to separate production systems,which is defined by: Fuel energy saving ratioðFESRÞ¼

F separateÀF CCHP

F separate

(1)

where F separate is the total primary energy used in the separate systems,which is electricity production by com-bined cycle plants,heat production by gas boilers and cool production by electric compress chillers and F CCHP is the total primary energy used in the CCHP systems.The scheme for comparison of CCHP and separate production system is shown in Fig.3.

According to the FESR definition and the scheme,FESR expressions are shown as follows:

Heating mode:

FESR h¼

a e=h eþa t=h tÀ1

a e=h eþa t=h t

(2) Cooling mode:

FESR c¼

a e=h eþ

ða t;iÂCOP t;iþa t;iiÂCOP t;iiÞ=COP ec=h eÀ1

a e=h eþ

ða t;iÂCOP t;iþa t;iiÂCOP t;iiÞ=COP ec=h e

(3) where efficiencies of CCHP are shown in Figs.1and2.As far as some conditions of the reference situation,h e=50% and h t=90%are supposed,which are the typical efficien-cies of combined cycle plants and gas boilers.Coefficients of performance of single-effect absorption,double-effect ab-sorption and electric compress chillers are supposed as f-ollowing:COP a,i=0.7,COP a,ii=1.2and COP ec=5.0.The thermal efficiencies of heat sources used to drive single-effect and double-effect absorption chillers are written as a t,i and a t,ii,respectively.

Substituting above conditions of the reference situation into equations(2)and(3),the boundary conditions that have to be met by CCHP systems for being energy saving to separate productions can be achievedfirstly.To be energy saving,FESR>0should be met and efficiencies of gas

H.Li et al./Energy and Buildings38(2006)253–257

255

Fig.2.Efficiencies of CCHP systems based on gas turbines at return

temperature508

C.

Fig.3.Scheme for comparison of CCHP system and separate production system.engines or gas turbines have to meet following conditions: Heating mode:a eþ0:56a t>0:5(4) Cooling mode:a eþ0:12a t;iþ0:24a t;ii>0:5(5) FESR of CCHP systems in whole year depends on the ratio of operation hours in two modes.The expression can be written as:

FESR whole year¼FESR hÂtþFESR cÂð1ÀtÞ(6) where the variable t means the ratio of heating hours to whole year operation hours.When the part load efficiencies are taken into consideration,operation hour should be equivalent operation hours at full load.According to the definition,t ranges between0and1.Specially,t=1means

the system is not CCHP but CHP system without cooling mode.

In addition,to illustrate the difference of reference situations,another reference condition fueled by coal is also supposed:h e=33%and h t=80%,which are cited as the reference situations by most analyses[4].

5.Results and discussion

According to the practical efficienciesfigures and the FESR definition,FESR of various CCHP systems are achieved and shown in Figs.4and5,respectively.

For CCHP systems based on gas engines,Fig.4appears clearly the situations of FESR in heating and cooling mode. It is obvious that CCHP systems with the larger electric power have the higher energy saving.Moreover,the energy saving performances of CCHP systems are widely different in heating and cooling modes.FESR changes from15%to 25%in heating mode whileÀ40%to5%in cooling mode.It shows that CCHP systems based on gas engine almost always save energy in heating mode while waste energy in cooling mode.The energy saving performances of CCHP systems are not as good as we thought before.

As far as FESR in whole year,an example is adopted to illustrate the effect-factor t.The efficiencies of a gas engine, a e=34%,a t,i=25%and a t,ii=24%,are supposedfirstly. Substituting the efficiencies into equations(2)and(3),FESR in heating and cooling modes,FESR h=19%and FES-R c=À16%,can be achieved.Then,FESR in whole year equals to9%when t=0.7is assumed in equation(6).In this case,the CCHP system is energy saving.However, FESR whole year decreases toÀ6%when t equals to0.3, which means an energy-waste system.Consequently,the increase of t is helpful to the improvement of energy saving performances.

Similar to CCHP systems based on gas engines,the systems based on gas turbines with the larger electric power have the higher energy saving.Fig.5shows that FESR shifts from5%to20%in heating mode whereasÀ40%to0%in cooling mode.From Figs.4and5,it can be observed that FESR of CCHP systems based on gas turbines is lower than that of gas engines with the same electric power.It is resulted from the fact that,in general,the electric and total efficiencies of gas engines are higher than the efficiencies of gas turbines with the same electric power.

According to the previous discussion,it is obvious that the energy utilization performances of CCHP systems are not as good as we thought before.In fact,it is easy to understand that performances of reference situation are strong improved and the required performances of CCHP systems being preferable to separate systems have to be tightened.

If the reference situation,the separate productions,is not fueled by natural gas but fueled by coal,Fig.6shows that CCHP systems have higher energy saving ratio in heating mode and cooling mode.It seems energy saving is intrinsic to CCHP systems,which are cited by most analyses. However,in this case,energy saving does not owe to the performance of CCHP systems but the higher quality of natural gas.So it is not correct to evaluate the energy utilization performances of CCHP systems by the compar-ison of different primary energy systems.Otherwise,it would overrate the fuel energy saving ratio of CCHP systems.

H.Li et al./Energy and Buildings38(2006)253–257

256

Fig.4.FESR of CCHP systems based on gas

engines.

Fig.5.FESR of CCHP systems based on gas turbines.6.Conclusions

In this paper,practical statistical performances of various units are introduced and analyzed.An appropriate reference situation is suggested and a static calculation methodology is presented.According to analysis and discussion,some conclusions are achieved.

(1)For the CCHP systems fueled by natural gas,the

primary energy of the reference situation should be natural gas rather than coal,where electricity is produced by combined cycle plants,heat is produced by high efficiency boilers and cool is produced by electric compress chiller.Otherwise,it would overrate the fuel energy saving ratio of CCHP systems.

(2)It is not right to simply confirm that it is energy saving or

not when using CCHP systems.Energy saving is not intrinsic but conditional to CCHP systems.In general, CCHP systems almost always save energy in heating mode while the fuel energy saving ratio is not as high as we thought before.In cooling mode,CCHP systems almost always waste energy.

(3)FESR of CCHP systems based on gas engines changes

from15%to25%in heating mode whileÀ40%to5%in cooling mode.The systems with the larger electric

power have the higher energy saving.CCHP systems based on gas engines have more energy saving potential than systems based on gas turbines with the same electric power.

(4)FESR of CCHP systems in whole year strongly depend

on the ratio t.The increase of t is helpful to the improvement of energy saving performances of CCHP systems.

However,the part load performance of CCHP systems and more practical load are not yet taken into consideration in the present study.Further studies on the energy utilization of CCHP systems still are necessary. Acknowledgments

Funding for this research is provided by the Yingdong Education Fund(94014)and Beijing Science and Technol-ogy Plan(H021*********).

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Fig.6.FESR of CCHP systems with respect to the reference situation fueled by coal.