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Energy 45 (2012) 247e255

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Energy

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Assessment of community energy supply systems using energy,exergy and exergoeconomic analysis

Audrius Bagdanavicius a,*, Nick Jenkins a, Geoffrey P. Hammond b

a Institute of Energy, Cardiff School of Engineering, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, Wales UKb Institute for Sustainable Energy and the Environment, Department of Mechanical Engineering, University of Bath, Bath, England, UK

a r t i c l e i n f o

Article history:Received 24 August 2011Received in revised form19 January 2012Accepted 22 January 2012Available online 20 February 2012

Keywords:Combined heat and powerCommunity energy supply systemDistrict heatingExergyExergoeconomicsSPECO

* Corresponding author. Tel.: þ44 29 2087 0674; faE-mail addresses: bagdanaviciusa@cardiff.ac.uk (A

cardiff.ac.uk (N. Jenkins), ensgph@bath.ac.uk (G.P. Ham

0360-5442/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.energy.2012.01.058

a b s t r a c t

Energy, exergy and exergoeconomic analysis are often used for assessing large energy conversionsystems. However exergy and exergoeconomic analysis are rarely used when small or medium scaleenergy generation systems, such as community CHP/CCHP plants or microcogeneration systems areevaluated.

In this study energy, exergy and exergoeconomic analysis of four Community Energy Supply (CES)systems has been carried out. Biomass Steam Turbine CHP (BST), Gas Turbine CHP (GT), Biomass Inte-grated Gasification Gas Turbine CHP (BIGGT) and Biomass Integrated Gasification Combined Cycle CHP(BIGCC) systems have been modelled. Modelling and energy/exergy analysis have been conducted usingthe computer programme Cycle-Tempo. Exergoeconomic evaluation of CESS has been performed usingthe Specific Exergy Costing (SPECO) approach. Exergy costs of the main products: heat and electricity,have been calculated.

The analysis shows that gasification of biomass reduces overall system efficiency due to the exergydestruction in the thermo-chemical conversion process when air is used as an oxidizer. A GT usingnatural gas as a fuel and BIGCC are the most exergy efficient systems in this study with the lowest exergycost of electricity and heat produced. The exergy cost of electricity generated in BST is the highest.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Increasing concern over climate change is leading to action toreduce CO2 emissions from power plants in a number of countries.One possible way to reduce the impact of electricity generation onthe environment is to use cogeneration e the simultaneousgeneration of electricity and heat in Combined Heat and Power(CHP) plants. However due to their location it is sometimesimpractical for large power plants to use this approach whilesmaller units can more easily use heat from generation that wouldotherwise be wasted. With such Community Energy Supply (CES)systems electricity is supplied to the local community or exportedto the grid and heat is supplied to the community using a DistrictHeating (DH) network. CES systems offer a number of advantagesover large central electricity generators and the use of individualnatural gas domestic boilers, the energy supply system presentlyused in the UK. The benefits of CES systems include that: various

x: þ44 29 2087 4939.. Bagdanavicius), jenkinsn6@mond).

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biomass fuels can be used, fuel handling is easier and CO2 emissionscan be monitored and controlled more easily than from smallindividual heating systems.

Conventional energy analysis and economic evaluation arecommonly used when making a decision on the suitability of a CHPscheme. In some cases this analysis is not sufficient and exergyanalysis along with thermoeconomic analysis can then be used.Thermoeconomics or exergoeconomics, if a combination of exergyanalysis and economics [1] is applied, has been used extensivelyover the last 30 years [2]. Two main groups of thermoeconomicmethods have been developed: cost accounting methods andoptimisation methods. Cost accounting methods, such as: ExergyCost Theory [3], Average Cost (AVCO) approach [4], Last-in-First-out(LIFO) method [5] or the Specific Exergy Costing (SPECO) [6e8]method have been used to calculate costs of exergy streams inenergy conversion systems. Other methods, such as: Thermoeco-nomic Functional Analysis (TFA) [9] or Engineering FunctionalAnalysis (EFA) [10] have been used as optimisation tools of complexenergy systems.

Several attempts have been made to compare the differentthermoeconomic methods [11,12] and to facilitate unification ofnomenclature and methodology [13,14]. Despite all these efforts,

A. Bagdanavicius et al. / Energy 45 (2012) 247e255248

a variety of different methods continue to be used to analyse energyconversion technologies and new methods continue to emerge[15]. One of the best developed and comprehensive methods is theSPECO methodology presented by Lazzaretto and Tsatsaronis [6].This tool provides simple and unambiguous procedures for evalu-ating energy conversion systems and uses a matrix formulationwhich facilitates fast problem solving.

Exergoeconomic analysis is rarely applied to CHP schemes inCES systems due to its complexity. This complexity makes it diffi-cult to gather all the necessary information on various alternativesof a small scheme and encumbers decisionmaking. The objective ofthis study was to apply the exergoeconomic method to a CESsystem and to perform a comparative study of different CHP plants.The SPECO method [6] was used in this study.

2. Community energy supply systems

A CES system, based on a real project in Ebbw Vale (Wales, UK)was used as a case study. The required maximum heating energydemand of the system is 11 MW, electricity demand is 3.5 MW. TheCES systemwas designed to supply the energy required for heating.Electricity, generated by the CHP unit, can be exported or importedfrom the grid depending on the demand.

Four CES systems were studied: Biomass Steam Turbine CHPplant (BST), Gas Turbine CHP plant using natural gas (GT), BiomassIntegrated Gasification Gas Turbine CHP plant (BIGGT) and BiomassIntegrated Gasification Combined Cycle CHP plant (BIGCC). CHPplants were modelled using the Cycle-Tempo modelling tool [16]developed at Delft University. An ambient temperatureT0 ¼ 288.15 K and pressure P0 ¼ 101.3 kPa were assumed for theexergy calculations. Exergies of streams and fuels were calculatedusing Cycle-Tempo. Total exergy was used in this study, because theuse of separate forms of exergy has little effect on the final results[6]. Biomass fuel of a spruce-pine-fir mixture [17] was used for theBST, BIGGT and BIGCC systems, and natural gas was used for GT.Data used for modelling are presented in Table 1.

2.1. Biomass steam turbine CHP system (BST)

A Biomass Steam Turbine CHP system (BST) is shown in Fig. 1.The BST system consists of a steam boiler (1), steam turbine (2),district heating (DH) heat exchanger (3), deaerator (4), DH pump(6), pumps (5, 7, and 12), DH network (8), condenser (11), coolingtower (13), and cooling system pump (14).

The design steam pressure is 4.5 MPa, steam temperature is450 �C. The design supply temperature of the DH is 90 �C and thereturn temperature is 50 �C. All other design values are shown inTable 1. The system is designed to supply the required energy of

Table 1Data used for modelling.

Parameter Value

Steam turbine isentropic efficiency, % 0.80Gas turbine isentropic efficiency (natural gas), % 0.90Gas turbine/compressor isentropic efficiency (syngas), % 0.85Gasification compressor isentropic efficiency, % 0.80Mechanical efficiency of compressor, GT and ST, % 0.97Lower heating value (biomass), kJ/kg 16,700Lower heating value (natural gas), kJ/kg 37,999Turbine Inlet Temperature, �C z1130GT operating pressure (natural gas), MPa 1.5GT operating pressure (syngas), MPa 1.7Steam cycle operating temperature, �C 450Steam cycle operating pressure BST/BIGCC, MPa 4.5/4.0DH network supply/return temperature, �C 90/50

11 MW for heating. Biomass fuel is used for combustion in thesteam boiler. The steam turbine is extraction type. During theheating season part of the steam at higher temperature is deliveredto the heat exchanger (3). The system is designed so that during thepeak heating demand all the steam is extracted. During the periodsof low heat demand, the steam is delivered to the condenser (11),thus increasing the electrical power output.

2.2. Gas turbine CHP system (GT)

The gas turbine CHP system (GT) is shown in Fig. 2. The GTsystem consists of compressor (1), combustor (2), turbine (3), heatexchanger (4), DH circulation pump (5), DH network (6), and fluegas stack (7). Natural gas is used as fuel in the combustor. Thecompressed products of combustion expand in the gas turbinegenerating power and then pass to the heat exchanger (4). Exhaustgas temperatures after the turbine exceed 500 �C. The design fluegas temperature after the heat exchanger is 120 �C. Other param-eters used in modelling are shown in Table 1.

2.3. Biomass integrated gasification gas turbine CHP system(BIGGT)

The Biomass Integrated Gasification Gas Turbine CHP system(BIGGT) consists of three subsystems: biomass gasification system,gas turbine and DH network (Fig. 3).

A pressurised circulating fluidized bed (CFB) type gasifier usingair as oxidant, operating at 18 bar pressure and 850 �C temperaturewas simulated. The gasification process was modelled based on thedata taken from an existing gasification combined cycle CHP plantin Värnamo, Sweden [18]. Biomass gasification modelling usingCycle-Tempo is not trivial and very often more complicated modelsneed to be considered in order to obtain syngas composition closeto experimental results [19]. In this study a simple biomass gasifi-cation model was applied. Syngas composition simulated in Cycle-Tempo is slightly different compared with the real data, but thissimplification has little effect on energy and exergy analysis.

Biomass is supplied to the gasifier (1). The main gas turbinecompressor (5) is used to deliver air to the combustion chamber (4).About 10% of total air supplied by the compressor (5) is used in thegasification process. The auxiliary compressor (2) pressurises air tothe required 18 bar pressure. Syngas leaves the gasifier and iscooled down from 850 �C to 400 �C in the heat exchanger (3).Cooled high pressure gas is supplied to the combustor (4). Hot fluegas expands in the gas turbine (6) generating electricity. Then hotflue gas is cooled in the heat exchanger (7). Heat exchangers (3 and7) are used to heat DH water. The design DH supply temperature is90 �C, return temperature is 50 �C.

2.4. Biomass integrated gasification combined cycle CHP system(BIGCC)

The Biomass Integrated Gasification Combined Cycle CHPsystem (BIGCC) consists of a number of subsystems: gasifiersystem, two heat recovery steam generator (HRSG) systems, gasturbine system, steam turbine system and DH network (Fig. 4).

The BIGCC model was designed based on a biomass gasificationCHP plant in Värnamo [18]. The gasification process is identical tothat of the BIGGTsystem. However in the BIGCC hot syngas is cooledfrom 850 �C to 400 �C in the HRSG (3), where steam is produced.Cooled gas is delivered to the combustor (4), expands in the turbine(6) and cools to 120 �C in the HRSG (7, 71, 72) and heat exchanger(12). Steam produced in HRSG (3 and 7, 71, 72) at 40 bar pressureand 450 �C temperature is supplied to the steam turbine (8).Exhaust steam at 1.1 bar pressure is delivered to the heat exchanger

Fig. 1. BST system.

A. Bagdanavicius et al. / Energy 45 (2012) 247e255 249

(13) where it condenses. District heating water is heated by theexhaust the flue gas from the gas turbine in the heat exchanger (12)and by the condensing steam in the heat exchanger (13).

3. Methodology

The Specific Exergy Costing (SPECO) method was applied in thisstudy. It is a systematic methodology for calculating exergy relatedcosts in thermal systems [6].

Fig. 2. GT system.

The SPECO method consists of three steps:

- Identification of exergy streams;- Definition of products and fuels;- Construction of cost equations.

The first step was carried out using Cycle-Tempo software.Energy and exergy streams in each model were calculated. Totalexergy was used in this study, because the use of separate forms ofexergy, such as: thermal, mechanical or chemical, only marginallyimproves calculation accuracy [6]. Fuels and products of eachsystem were found using the definitions proposed by Tsatsaronis[1,20].

Finally the construction of cost equations was carried out basedon the SPECOmethod [6]. This consists of two parts. First the exergycost equation for each component of the system is composed. Thegeneral form of this equation for the k-th component is:X

e

�ce _Ee

�k þ cw;kWk ¼ cq;k _Eq;k þ

X

i

�ci _Ei

�k þ _Zk (1)

Here ce, ci, cq, cw are the average costs per unit of exergy; _Ee, _Ei, _Eq,Wk are the exergy streams and _Zk is the sum of capital investmentsand operating and maintenance expenses.

Cost streams _C associated with the corresponding exergystreams are calculated using equations:

_Ce ¼ ce _Ee; _Ci ¼ ci _Ei (2)

However there are more streams than devices, therefore auxil-iary equations have to be formulated. F and P principles [6] are usedto compose auxiliary equations. According to the F principle, thespecific cost (cost per exergy unit) associated with the removal ofexergy from the fuel stream is equal to the average specific cost atwhich the removed exergy is supplied to the same stream in theupstream components [6]. In this way one auxiliary equation iscreated for each removal of exergy. The P principle assigns the sameunit cost for the added exergy to every exergy stream belonging tothe product [6]. Equations associated with the exergy streams

Fig. 3. BIGGT system.

Fig. 4. BIGCC system.

A. Bagdanavicius et al. / Energy 45 (2012) 247e255250

Table 3Total capital costs of CES systems.

BST GT BIGGT BIGCC

5.14 MEUR 3.46 MEUR 17.01 MEUR 21.57 MEUR

A. Bagdanavicius et al. / Energy 45 (2012) 247e255 251

supplied to the systems from outside and auxiliary equationsformulated based on F and P principles provide the requirednumber of equations in order to find the specific costs of thestreams.

The external exergy streams are fuel, air or electricity. The costflow rate of the fuel exergy stream is calculated from the fuelenergy price. Biomass fuel exergy was calculated from the LowerHeating Value (LHV) using the equation provided in [21]. Theexergy of natural gas was calculated within Cycle-Tempo. The costflow rate of the air stream was set equal to zero.

In CES systems, electricity is used for pumps and auxiliarydevices. In this study it was assumed that electricity generated inthe CHP plant was used for the pumps and auxiliary compressors,therefore electricity exergy streams were considered as internalstreams. Auxiliary equations for the different CES systems arepresented in Table 2.

To solve the system of linear equations the capital investmentsand operating and maintenance expenses ( _Zk) for each device werecalculated as shown by Kotas [21]. Purchased Equipment Costs(PEC) were calculated using functions presented by Silveira andTuna [22] and interpolating data from the estimating chartsprovided in [23]. The PECs of gasifiers were calculated using thecost functions provided by Bridgwater et al. [24]. The ChemicalEngineering Plant Cost Index (CEPCI) was used to calculate equip-ment costs at the reference year (2009, CEPCI ¼ 521.9). All costswere converted to Euro using a fixed currency conversion rate. Thetotal costs of CES systems are presented Table 3.

It was assumed that the life-time of CHP is 20 years; interest rateon the capital e 10%; operation time of plant e 7000 h/year. Theenergy price 0.0205 EUR/kWh of natural gas for industrialconsumers in UK in 2009 was taken from EU Energy Portal [25]. Abiomass fuel price 90 GBP/t was taken from the E4Tech report [26].

4. System analysis

First exergy analysis was carried out using the Cycle-Temposoftware. The exergy of the product and fuel of the CES systemcomponents were calculated. Exergy efficiency was calculatedusing Equation (3) [1]:

ε ¼ _EP= _EF ¼ 1� �_ED þ _EL

��_EF (3)

Here _EP is the exergy of the product of the component; _EF is theexergy of the fuel of the component; _ED is the exergy destructionrate of the component and _EL is the exergy loss rate of thecomponent. In this study it was assumed that _EL ¼ 0. Exergy

Table 2Auxiliary equations for different CES systems.

BST GT BIGGT BIGCC_C14/ _E14 ¼ _C14/ _E14 _C12/ _E12 ¼ _C11/ _E11 _C19/ _E19 ¼ _C23/ _E23 _C26/ _E26 ¼ _C31/ _E31_C15/ _E15 ¼ _C13/ _E13 _C10/ _E10 ¼ _C11/ _E11 _C20/ _E20 ¼ _C23/ _E23 _C30/ _E30 ¼ _C31/ _E31_C16/ _E16 ¼ _C13/ _E13 _C2/ _E2 ¼ _C3/ _E3 _C21/ _E21 ¼ _C23/ _E23 _C27/ _E27 ¼ _C32/ _E32_C1/ _E1 ¼ _C2/ _E2 _C9/ _E9 ¼ _C3/ _E3 _C7/ _E7 ¼ _C8/ _E8 _C28/ _E28 ¼ _C32/ _E32_C1/ _E1 ¼ _C3/ _E3 _C5/ _E5 ¼ _C6/ _E6 _C2/ _E2 ¼ _C3/ _E3 _C29/ _E29 ¼ _C32/ _E32_C4/ _E4 ¼ _C5/ _E5 _C8/ _E8 ¼ _C16/ _E16 _C7/ _E7 ¼ _C8/ _E8_C10/ _E10 ¼ _C11/ _E11 _C9/ _E9 ¼ _C15/ _E15 _C10/ _E10 ¼ _C11/ _E11

_C5/ _E5 ¼ _C6/ _E6 _C10/ _E10 ¼ _C12/ _E12_C11/ _E11 ¼ _C12/ _E12 _C9/ _E9 ¼ _C25/ _E25

_C12/ _E12 ¼ _C13/ _E13_C8/ _E8 ¼ _C9/ _E9_C18/ _E18 ¼ _C17/ _E17_C2/ _E2 ¼ _C3/ _E3_C5/ _E5 ¼ _C6/ _E6_C22/ _E22 ¼ _C23/ _E23

efficiency shows the percentage of product exergy that is found inthe fuel exergy of a component.

In order to find which components are responsible for theexergy destruction in the system an exergy destruction ratio wascalculated using Equation (4) [1]:

yD;k ¼ _ED;k= _ED;tot (4)

The exergy destruction ratio shows the share of the exergydestroyed in the k-th component compared with the total exergydestruction in the system.

Using the SPECO method cost rates of exergy streams ( _C) werecalculated. From the cost rates associated with exergy stream, costrates associated with the fuel ( _CF) and the product ( _CP) werecalculated. The average cost per exergy unit of fuel and product forthe component k was defined using the Equation (5) [1]:

cF;k ¼ _CF;k=_EF;k; cP;k ¼ _CP;k=

_EP;k; (5)

To conduct exergoeconomic analysis exergoeconomic variablesof the component k, such as: cost rate of exergy destruction _CD,k,relative cost difference rk and exergoeconomic factor fk werecalculated.

The cost rate of exergy destruction _CD,k were calculated usingEquation (6) [1]:

_CD;k ¼ cF;k _ED;k (6)

Here cF,k is the unit cost of fuel and _ED,k is the exergy destructionrate.

The relative cost difference rk indicates the relative increase inthe average cost per exergy unit between the fuel and product ofthe component. It was calculated using Equation (7) [1]:

rk ¼ �cP;k � cF;k

��cP;k (7)

Here cF,k is the unit cost of fuel and cP,k is the unit cost of product.The exergoeconomic factor fk combines non-exergy costs

(capital investment and operating and maintenance costs _Zk) withexergy destruction costs _CD,k. It was calculated using the Equation(8) [1]:

fk ¼ _Zk=�_Zk þ _CD;k

�(8)

The exergoeconomic factor shows the contribution of non-exergy related cost (capital cost) to the total cost increase. A lowvalue indicates that the larger cost of the component would beacceptable if the exergy destructionwere reduced. A high value of fkindicates the cost of the component should be reduced, even if theexergy efficiency of the component decreases.

4.1. BST system

The BST exergy efficiency and exergoeconomic parameters areshown in Fig. 5. The largest exergy destruction (white column ofFig. 5a) is observed in the boiler. The combustion process is thelargest contributor to exergy destruction. The boiler is responsiblefor more than 85% of all exergy destruction in the system. Theexergy destruction in the other components, such as: turbine and

Fig. 6. GT system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.Fig. 5. BST system analysis: (a) exergy efficiency and (b) exergoeconomic parameters.

A. Bagdanavicius et al. / Energy 45 (2012) 247e255252

heat exchanger (3) is below 10%. The exergy destruction in thedeaerator and pumps is insignificant (Fig. 5a).

As anticipated the largest relative cost difference and the sum ofdestruction and capital cost rate ( _Zk þ _CD,k) are observed in theboiler (Fig. 5b). The low exergoeconomic factor suggests thatdestruction cost rate is much higher than investment (capital) costrate. However improvement in the boiler efficiency would notreduce exergy destruction appreciably. The relative cost differenceof pumps (5) and (6) is large however the sum of capital cost flowrate and exergy destruction rate ( _Zk þ _CD,k) is negligibly small,which means that pumps do not contribute to the total exergydestruction in the system. Exergy destruction and capital cost rate( _Zkþ _CD,k) in the turbine and heat exchanger are considerably lowerthan in the boiler.

4.2. GT system

GTexergy efficiency and exergoeconomic parameters are shownin Fig. 6. Four main components in the GT system contribute toexergy destruction (Fig. 6a). The combustor and heat exchanger (4)are the main components where about 85% of total exergydestruction occurs. The combustion process causes large exergydestruction. Exergy efficiency of the turbine and compressor arerelatively high (z90%) and the exergy destruction ratio of thesecomponents is below 10%. Exergy efficiency of the heat exchanger(4) is low (<30%). This is due to the large temperature difference inthe heat exchanger, where high temperature exhaust gas is used to

heat low temperature district heating water. The efficiency of theheat exchanger can be improved by changing temperatures in thesystem.

The largest sum of exergy destruction and capital cost rate( _Zk þ _CD,k) are observed in the combustor and heat exchanger (4)(Fig. 6b). It is related to the large exergy destruction in thesecomponents. The high cost rate ( _Zk þ _CD,k) and relative costdifference (rk > 2) of the heat exchanger (4) suggest that theperformance of this component should be improved. Oneway to dothat is to use the potential of the high temperature flue gas after thegas turbine, by implementing an additional Rankine cycle, forinstance. Preheating of the air before combustion may improve theperformance of the combustor.

Large relative cost difference is also observed in the pump (5)(Fig. 6b). However the sum of exergy destruction and capital costrate in the pump (5) is very low. The contribution of the pump tothe total cost of the system is insignificant. The low exer-goeconomic factor of the combustor and heat exchanger (4) indi-cates that the increase of capital costs of these components wouldbe justified if better efficiency were achieved.

4.3. BIGGT system

BIGGT exergy efficiency and exergoeconomic parameters areshown in Fig. 7. Six main components in the BIGGT systemcontribute to the exergy destruction in the plant (Fig. 7a). Gasifier,heat exchanger (7) and combustor are the main components where

Fig. 8. BIGCC system analysis: (a) exergy efficiency and (b) exergoeconomicparameters.

Fig. 7. BIGGT system analysis: (a) exergy efficiency and (b) exergoeconomicparameters.

A. Bagdanavicius et al. / Energy 45 (2012) 247e255 253

about 80% of total exergy destruction occurs. As it is anticipatedcombustion and gasification are the processes where exergydestruction ratio is high.

The exergy efficiency of the turbine and compressor are rela-tively high (z90%) and the exergy destruction ratio of thesecomponents is below 10% (Fig. 7a). The exergy efficiency of the heatexchanger (3) is lower (z25%) than that of the heat exchanger (7).However, the exergy destruction ratio in this component is lower(<10%) than that of the heat exchanger (7) (z24%). The reason forlow exergy efficiency is the large temperature difference of fluids inthe heat exchangers (3) and (7). High temperature syngas ata temperature of above 850 �C in the heat exchanger (3) and fluegas above 550 �C in the heat exchanger (7) are used to increase thetemperature of the district heating water from 50� to 90�. There-fore, a large quantity of exergy is destroyed. Much larger heatenergy (z8800 kW) is transmitted from the exhaust gas to thedistrict heating water in the heat exchanger (7) compared with theheat transmitted from the syngas in heat exchanger (3)(z2100 kW). Therefore, the exergy destruction ratio in the heatexchanger (7) is higher than that of the heat exchanger (3).

Exergoeconomic analysis shows that the gasifier, heatexchanger (7), combustor and gas turbine are themain componentswhere the cost rate ( _Zk þ _CD,k) is high (Fig. 7b). The cost rates( _Zk þ _CD,k) of compressor (5), heat exchanger (3), compressor (2)and pump (8) are much smaller. The relative cost difference of thecompressor (2), heat exchanger (3) and heat exchanger (7) is high.

However, only the heat exchanger (7) plays an important role in theproduct exergy cost formation process.

High exergoeconomic factors of the compressor (2), gasifier(z0.75) and pump (8) indicate that the capital costs of thesecomponents should be reduced in order to decrease the cost rate( _Zk þ _CD,k), even if the exergy efficiency were reduced. On thecontrary very low exergoeconomic factors of heat exchanger (7)and combustor suggest that the efficiency of these componentsmay be improved. It can be done by modifying the plant design byimplementing an additional Rankine cycle as suggested for the GTplant.

4.4. BIGCC system

BIGCC exergy efficiency and exergoeconomic parameters arepresented in Fig. 8. Similar results are observed for the BIGCC as forBIGGT. Gasifier and combustor are those components where exergydestruction ratio is the largest (Fig. 8a). These devices are respon-sible for about 60% of the total exergy destruction. It is important tonote that the effect of heat recovery steam generators (HRSG3 andHRSG7) and heat exchangers (12 and 13) on exergy destructionratio is considerably less compared with the heat exchangers in theBIGGT system. The reason is that much lower temperature differ-ence of working fluids (syngase steam, flue gase steam, flue gas ewater, steam e water) is used in the BIGCC. It increases the exergyefficiency of the heat exchangers. The exergy destruction ratio in

Fig. 9. Comparison of CESS: (a) energy consumption, generation and efficiency and (b)exergy costs of electricity and heating.

A. Bagdanavicius et al. / Energy 45 (2012) 247e255254

the gas turbine (6) is larger than in the steam turbine (8), althoughthe exergy efficiency of the steam turbine (8) is lower. This is due tothe fact that about 70% of total electrical energy is generated in thegas turbine. The effect of the gas turbine compressor (5) on the totalexergy destruction is very small, and the effect of auxiliarycompressor (2) is negligible.

Exergoeconomic analysis shows that the gasifier, combustor, gasturbine (6), compressor (5) and steam turbine (8) are the maincomponents where the cost rate ( _Zk þ _CD,k) is high (Fig. 8b). Thecontribution of the heat recovery steam generators (HRSG3 andHRSG7) and heat exchangers (12 and 13) to the increase of the costrate ( _Zk þ _CD,k) is smaller in the BIGCC (z17%) compared with theBIGGT system (z31%). It infers that the use of heat transfer devicesis more efficient in the BIGCC due to the lower temperaturedifferences between the working fluids.

The relative cost difference in the devices, which largelycontributes to the increase of exergy destruction cost flow rate,such as: gasifier, combustor, turbine (6), compressor (5) and turbine(8) is relatively small compared with compressor (2) and heatexchanger (12). However these components play a pivotal role inthe process of the formation of the product exergy cost.

As in the BIGGT case high exergoeconomic factor of the gasifier(z0.75) is observed, which suggest that the capital cost of thiscomponent should be reduced. In contrast a very low exer-goeconomic factor of the combustor and the heat transfer devicesindicates that capital cost flow rate is much less than the exergydestruction cost flow rate.

5. Comparison of CES systems

The exergy and exegoeconomic analysis of four CES systemsshows the differences between the chosen energy conversiontechnologies (Fig. 9). The five columns in Fig. 9a represent fuelenergy input, fuel exergy, electricity generation, heat energy outputand heat exergy output from CHP plant. The solid line indicatesenergy efficiency and the broken line represents exergy efficiencyof the plants.

It is seen that the energy efficiency is almost the same for all fourCES systems (Fig. 9a). The exergy efficiency of GT and BIGCCsystems is highest (z40%) and BIGGT efficiency is lower at z 32%.The exergy efficiency of the BST system is the lowest at z 24%. Thegasification process leads to large exergy destruction. Therefore, thetotal exergy efficiency reduces in the BIGGT and BIGCC comparedwith the GT system. However, this reduction is compensated by theuse of combined cycle in BIGCC. Generally speaking the GT systemis the most energy and exergy efficient system. However, its impacton the environment is undoubtedly the highest.

Exergy costs of the fuel and products: electricity and heating ofCESS are shown in Fig. 9b. It is seen from the graph that the exergycost of biomass fuel and natural gas are almost the same. Theexergy cost of electricity and heat are almost identical in the BSTsystem. Electricity exergy cost is considerably lower than heatexergy cost in the GT, BIGGT and BIGCC systems with the gasturbines. The lowest electrical exergy cost is observed in the GTsystem. The increase of exergy costs in BIGGT and BIGCC is mainlyrelated to the additional exergy destruction in the gasifier and largecapital investments. In the BIGCC system the electricity is generatedin the gas turbine and steam turbine. Therefore, two electricityexergy costs have been calculated. Electricity exergy cost generatedin the gas turbine is 0.0649 EUR/kWh and the exergy cost generatedin the steam turbine is 0.1002 EUR/kWh. The weighted averagevalue is presented in Fig. 9b.

Large heat exergy cost in BIGGT system is associated with thelarge exergy destruction in heat exchangers, where high tempera-ture syngas and exhaust gas are used to heat water in DH system.

This issue is circumvented when steam Rankine cycle is imple-mented in the BIGCC system. The heat exergy cost in the BIGCC issimilar to that of in BST system.

In this study steady state energy, exergy and exergoeconomicanalysis has been carried out assuming fixed heat consumption. Inorder to understand CESS behaviour under varying energy demandconditions more detailed transient analysis is required. Addition-ally an environmental analysis would be beneficial.

6. Conclusion

Four different CES systems: BST, GT, BIGGT and BIGCC werestudied using energy, exergy and exergoeconomic analysis. Theresults are summarised below.

1. Energy analysis shows that the energy efficiency of all the CESsystems is almost the same. Energy analysis alone is notcapable of revealing all the attributes of energy conversionsystems. It is a necessary but insufficient tool for performingoverall plant analysis.

2. Exergy analysis shows that gas turbine technology has a higherexergy efficiency compared to the steam turbine. Exergy anal-ysis is capable of indicating the elements of the system wherethe exergy destruction is largest. Large exergy destruction ratesare observed in the gasification and combustion processes. Ina GT systemwhere the fluids with large temperature differenceare used in the heat transfer devices, large exergy destruction

A. Bagdanavicius et al. / Energy 45 (2012) 247e255 255

rates are observed. A reduction of temperature differenceacross elements decreases exergy destruction rate.

3. Exergoeconomic analysis using the SPECO method shows thatgasification, combustion and heat transfer processes contributemost to formation of the exergy cost. The lowest product costare observed in GT and BIGCC systems, which shows that theadvantage of gas turbine technology against steam turbine.Exergy costs of electricity generated in steam turbine in BSTand BIGCC are considerably higher compared with thosegenerated in systems with gas turbines.

This study has shown that exergy and exergoeconomic analysisprovides much useful information which can be used to assess thesuitability of energy conversion technology. The application ofthese techniques along with traditional energy and economicalanalyses is necessary to facilitate the development and utilisationof more sustainable energy conversion technologies.

Acknowledgments

This research formed part of the programme of the UK EnergyResearch Centre and was supported by the UK Research Councilsunder Natural Environment Research Council award (NE/G007748/1) (Phase II) and by the Higher Education Funding Council forWales(HEFCW).

Nomenclaturec Average cost per unit of exergy (EUR/kJ)_C Cost rate (EUR/s)fk Exergoeconomic factor of the k-th component_E Exergy transfer rate (kJ/s or kW)r Relative cost differenceW Exergy transfer rate associated with power (kJ/s or kW)yD Exergy destruction ratio_Z Capital cost rate (EUR/s)ε Exergy efficiency of the component

SubscriptsD Destructione Stream exiting the componentF Fueli Stream entering the componentk The k-th componentL LossP Productq Stream associated with heat transfertot Totalw Stream associated with work

AbbreviationsBST Biomass steam turbineBIGCC Biomass integrated gasification combined cycleBIGGT Biomass integrated gasification gas turbineCES Community energy supplyCHP Combined heat and powerDH District heatingGT Gas turbine

HEX Heat exchangerHHV Higher heating valueHRSG Heat recovery steam generatorLHV Lower heating valuePEC Purchased equipment costsSPECO Specific exergy costing

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