analysis and integration of fuel cell combined cycles for development of low-carbon energy...
TRANSCRIPT
ARTICLE IN PRESS
Energy 33 (2008) 1508– 1517
Contents lists available at ScienceDirect
Energy
0360-54
doi:10.1
Abbre
preheat
CC, Com
generat
return;
De-aera
Europea
FCCC, Fu
Generat
recover
combin
cell; PE
fuel cell
up to 10� Corr
E-m
journal homepage: www.elsevier.com/locate/energy
Analysis and integration of fuel cell combined cycles for developmentof low-carbon energy technologies
Petar Varbanov a,�, Jirı Klemes b
a Marie Curie ERG ESCHAINS, Research Institute of Chemical and Process Engineering, University of Pannonia, Egyetem u. 10, Veszprem H-8200, Hungaryb EC Marie Curie Chair (EXC) ‘‘INEMAGLOW’’, Research Institute of Chemical Technology and Process Engineering, FIT,
University of Pannonia, Egyetem u. 10, Veszprem H-8200, Hungary
a r t i c l e i n f o
Article history:
Received 28 February 2008
Keywords:
Energy efficiency
High-temperature fuel cells
Combined cycle
Power cycle integration
Total site heat integration
CO2 minimisation
42/$ - see front matter & 2008 Elsevier Ltd. A
016/j.energy.2008.04.014
viations: A, HEN ‘‘A’’; AB, Afterburner; ABC,
; B, HEN ‘‘B’’; BD, Blow down; BFW, Boiler feed
bined cycle; CFP, Carbon foot print; CHP, Com
ion; CNDIN, Condenser inlet; CNDOUT, Conde
DAF, De-aerator feed; DAG, De-aeration steam
tor outlet; DAS, De-aeration steam; DW, Dem
n community; EC, Exhaust cooling; ES, Expa
el cell combined cycle; FP, Fuel preheat; GCC
ed steam; GT, Gas turbine; HEN, Heat exchan
y steam generator; HTFC, High-temperature f
ed cycle; LTFC, Low-temperature fuel cell; MC
M FC, Proton exchange fuel cell; SG, Steam ge
; ST, Steam turbine; V, Vaporiser; mGT, Micro g
0–200 kW).
esponding author. Tel.: +36 88 424 483; fax:
ail address: [email protected] (P.
a b s t r a c t
Integrated and combined cycles (ICC, CC) traditionally involve gas and steam turbines only. The paper
analyses the further integration of high-temperature fuel cells (FC) having high electrical efficiency
reaching up to 60% compared with 30–35% for most gas turbines. The previous research on FC hybrids
indicates achieving high efficiencies and economic viability is possible. The ICC of various FC
types—their performance and the potential for utilisation of renewables—are analysed considering also
power generation capacity and site heat integration context. Further research and development with
industrial relevance are outlined focusing on CO2 emissions reduction.
& 2008 Elsevier Ltd. All rights reserved.
1. Introduction
The integrated combined cycles (ICCs) traditionally involveonly gas and steam turbines. They can be broadened to theintegration of high-temperature FC having high electrical effi-ciency reaching 40–60%, compared to 30–35% for most gasturbines [1]. There are three main CO2 pathways through fuel-based energy systems, including FC, with regard to the ambient:recycling, build-up and sequestration (Fig. 1). The extent of thecurrently developing climate change is influenced by the volumefraction of CO2 in the atmosphere. The presented diagram in Fig. 1points to three major ways of limiting CO2 emissions—improvingenergy conversion efficiency, increasing the CO2 recycling viabiofuels and CO2 sequestration. There is an extensive researchaiming at efficiency improvement of FC systems. Karvountzi et al.
ll rights reserved.
Afterburner cooling; AP, Air
water; CC, Composite curve;
bined heat-and-power
nser outlet; CR, Condensate
at generation pressure; DAO,
ineralised water; EC,
nsion steam; FC, Fuel cell;
, Grand composite curve; GS,
ger network; HRSG, Heat
uel cell; ICC, Integrated
FC, Molten carbonate fuel
neration; SOFC, Solid oxide
as turbine (i.e. with capacity
+36 88 428 275.
Varbanov).
[2] compared the integration of molten carbonate fuel cells(MCFC) and solid oxide fuel cells (SOFC) into hybrid systems. Kurz[3] emphasises on the choice of appropriate gas turbines (GT) forgiven fuel cells, and Massardo and Bosio [4] study the MCFCcombinations with gas and steam turbines. A very promisingoption is to integrate the FC with bottoming cycles to designdedicated power generation or combined heat-and-power (CHP)applications [5].
2. Efficiency of FC and combined cycles
2.1. Operating temperature and fuel cell efficiency
Most FCs use H2. One exception is the direct-methanol FCreported by Toshiba [6]. The primary fuel—mostly natural gas orbiogas, undergoes an endothermic conversion consisting ofreforming and shift reactions to generate the required H2. High-temperature FCs (HTFCs) allow heat integration between the fuelconversion and the power generation processes. In contrast, low-temperature FCs (LTFCs) do not, and additional fuel is burnt tosupply the heat for the main fuel conversion [7], resulting inelectrical efficiencies around 35% for LTFC against 41% for HTFC.Similar estimates result from MCFC integration [5] (see Table 1).
2.2. Combinations with bottoming cycles
After ensuring maximum energy recovery within the FC,integrated systems can make even better use of the fuel. Steamand the gas turbines can be used for increasing the power output,
ARTICLE IN PRESS
Nomenclature
A heat exchanger area (m2)Cp specific heat capacity (kJ kg�1
1C�1)CP heat capacity flowrate (kW 1C�1)ECHP CHP energy output of the FCCC system (kW) or (MW)m mass flowrate (kg s�1) or (t h�1)P pressure (bar), (atm) or (kPa)Q heat flow (kW) or (MW)QEvap heat consumption rate for evaporating the BFW (kW)
or (MW)QFuel energy input to the system in the form of fuel (kW) or
(MW)QPreheat heat consumption rate for preheating the BFW (kW)
or (MW)QSH heat consumption rate for superheating the steam
(kW) or (MW)QTotal total heat consumption for generating steam (kW) or
(MW)QUH useful heat generated by the FCCC system (kW) or
(MW)
R power-to-heat ratio (�) or (MW MW�1) or(kW kW�1); R ¼ P/Q
T temperature (1C)TBFW boiler feed water temperature (1C)TS supply temperature (1C)TSat saturation temperature (1C)TSh superheat temperature (1C)TT target temperature (1C)WSOFC power generated by a SOFC (kW) or (MW)WST power generated by a steam turbine (kW) or (MW)WTotal total power generated by a FCCC system (kW) or
(MW)DTMin minimum allowed temperature difference (C)
Greek symbols
ZCHP CHP efficiency (�) or (%)ZCHP,Max maximum possible CHP efficiency (�) or (%)ZE electrical (power) efficiency (�) or (%)
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–1517 1509
which can be supplemented by heat cogeneration. Only high-temperature FCs are suitable for such integration, which combinesadvantageously with the conclusion from the previous section.Several integration options have been investigated in theliterature. A summary of the most interesting works is given inTable 2. In the table, the rightmost column represents an estimateof what would the CHP efficiency of the corresponding systems beif the heat recovery from the main exhaust streams wasmaximised.
Table 1MCFC properties from Varbanov et al. [5] for 2320 MW power generation
Fuel for power generation (MW) 5002
Additional fuel (no integration) (MW) 1610
MCFC efficiency (%) 35.09
FCCC efficiency (%) 46.38
2.2.1. Fuel cell–steam cycle hybrids
The simplest way for FC integration is with steam cycles [5].The sensitivity analysis in this work for a wide range of FC capitalcosts indicates the potential benefits of fuel cell combined cycle(FCCC) systems. Using the year 2005 power prices and 1.255 $ h�1
they achieved prices as low as 40–47 $ MW�1 h�1
(32.65–38.37 hMW�1 h�1).
Fossil Fuels
Biofuels
CO2 recycling
CO2 BUILD-UP
Energy conversionprocesses
Fig. 1. CO2 pathways fo
2.2.2. Fuel cell–GT hybrids
The efficiency of FCCC with gas turbines has been investigatedelsewhere. Massardo and Bosio [4] studied the combination ofMCFC with gas turbines and the option of further adding steamturbines. The systems achieve electrical efficiencies up to 69% atpower generation capacities of 10–15 MW. Uechi et al. [8]investigated a SOFC+mGT (micro gas turbine) system with a totalpower capacity of 30 kW (23.6 kW from the SOFC and 6.4 kW fromthe mGT). Even this small system approaches 67% electricalefficiency. Oyarzabal et al. [10], Lunghi and Ubertini [11]. Bedont
CO2
Sunlight
CO2
Power + Heat
Sequestration
r energy systems.
ARTICLE IN PRESS
Table 2Sources on cycle integration of fuel cells
Source System/notes ZE (%) ZCHP,Max (%)
Uechi et al. [8] SOFC+mGT. Integrated GT compressor 66.5 93.0
Gunes and Ellis [9] PEM FC. Residential CHP 31.0 80.0
Oyarzabal et al. [10] PEM FC+GT. Considers CHP 39.0 73.0
Lunghi and Ubertini [11] MCFC+GT. No cogeneration 59.2 59.2
Bedont et al. [12] MCFC+GT. Integrated GT compressor 59.7 83.5
Massardo and Bosio [4] MCFC+GT+ST. 1- and 2-level HRSG 69.1 82.7
Campanari [13] SOFC+mGT 64.9 71.9
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–15171510
et al. [12] and Campanari [13] also investigated several differentFCCC arrangements where the main distinctions were in theconnection of the GT heat input and the placement of thecompressors. The options can be classified as:
(a)
Systems with indirectly heated gas turbines. They have gas–gasheat exchangers recovering the heat from the FC exhaust. TheFC and the GT components also have separate air compressors.(b)
Systems with an integrated air compressor. The GT compressoris used to raise the pressure of the inlet to the FC cathodecompartment (the air steam). After that, the stream passesthrough a post-combustor and through the GT expander,where it generates torque.Option (a) has the advantage that the working pressures in theFC and the GT are independent, allowing more flexibility. Foroption (b) the GT pressure must be lower than that in the FC,which results in lower compression ratios and in lower GTefficiencies. However, in this case the very large and costlygas–gas heat exchanger is avoided, which can also reduce physicalspace requirements.
2.2.3. Fuel cell–GT–steam cycle hybrids
These systems have not been much investigated due to theirrelative complexity and the small marginal efficiency increasecompared to fuel cell–GT hybrids. From the sources in Table 2,only Massardo and Bosio [4] investigate such a system with a100 kW MCFC. These systems achieve maximum electricalefficiency of the MCFC+GT+ST system of around 67.4% and 69.1%for the cases of integrating single-level and two-level steamcycles, respectively.
3. Fuel options and renewable energy
3.1. Major trade-offs
The fuels for FC-based systems influence the electricalefficiencies, carbon emission levels and plant economics. Withregard to emissions, using H2-rich feedstocks such as naturalgas is more advantageous since they generate much lesscarbon emissions. Biofuels lower the emissions too. The fossilfuels are still priced relatively lower even with the rise of thecrude oil prices above 120 $ bbl�1. Even with the further increaseof the oil price, the main bottleneck of using biofuels will be theirsufficient availability. This is an important though frequentlyoverlooked factor. This also calls for higher energy conversionefficiency.
A study of CH4–CO2 fuel compositions for using in SOFCs [14]suggests that maximum efficiency is achieved at CH4 volumefraction around 0.45 which falls within the usual range of biogascompositions. The main reason given in that paper is that H2 is
produced by dry reforming, where CO2 and CH4 are consumed inequimolar quantities producing H2 and CO. Consequently, wastetreatment plants can be suitably equipped with SOFC-based unitsto produce power and heat from biogas at top efficiency. Severalcompanies, including Siemens and General Electric [15], have alsostarted to develop coal-based FCs using coal synthesis gas as fuel.Using syngas from combined biomass and coal gasification mayalso be attractive.
3.2. Implications for carbon capture and sequestration
Burning biofuels is potentially carbon-neutral—see CO2 recy-cling in Fig. 1. However, the overall carbon footprint analysisreveals that actually some carbon is released into the ambient[16]. The CFP is obviously considerably higher when using fossilfuels and consequently CO2 capture and sequestration should beconsidered.
FCs keep the path of the air stream apart from that of the fueland its products. Stoichiometrically the only anode-side productsare just CO2 and water. Certain residual amounts of the fuel arealso present in the FC anode exhaust. This prompts post-combustion introducing a certain small amount of air into theexhaust. Thus, FCs offer an opportunity for very efficient CO2
capture and subsequent sequestration. SOFC systems take thisadvantage to the extreme since they can oxidise both H2 and CO[8]. A very interesting research direction is the development ofcheaper SOFCs with maximum fuel utilisation, producing mix-tures of water and CO2 only. This would potentially eliminate theneed for external CO2 capture units.
4. Application of FC-based energy conversion
4.1. Combined supply chains for energy, food and waste-
management
Increasing the share of renewables is highly dependent on theability to develop economically viable solutions. It is clear thatrenewable energy production is in competition for primaryresources with food production. Food and other industries needto manage large volumes of solid and liquid waste of organicorigin—e.g. manure, vegetable residues, black liquor, etc.
FCCC systems can be integrated into combined energy–food–waste supply chains with other processes. See Fig. 2, as presentedby Beamon [17]. The benefits of the FC-based systems can comefrom:
�
Cost sharing between the biofuel and the other biomass-basedproducts. � In some selected cases, waste streams can be used as fuel rawmaterials which they cannot release directly into the environ-ment. Using these resources for FC fuel generation would result
ARTICLE IN PRESS
AgriculcturalProducers
FoodProcessing
Othersproducing
organic waste
TreatmentProcesses
Transportationlinks
FCCC systems
ByproductsWaste
Fuels:Biogas, H2,Syngas, etc.
Fig. 2. Combined energy–food–waste supply chains.
0
200
400
600
800
1000
1200
0 10 20 30 40 50 60 70Q (kW )
T (°
C)
Small driving forcesHigh capital cost
Hot CompositeCold Composite
Fig. 3. Composite curves for the heat integration of the 30 kW SOFC+GT system in
Ref. [8].
0
200
400
600
800
1000
0 1000 2000 3000 4000Q (kW )
T (°
C)
Larger driving forcesLower capital costCHP possible too
Hot CC
Cold CC
Fig. 4. Composite curves for the MCFC heat-and-power integration [5].
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–1517 1511
in virtually negative fuel prices due to the fees to be chargedfor the waste treatment.
4.2. Types of applications and power-to-heat ratio
Another important indicator for evaluating the performance ofenergy systems is the power-to-heat ratio (R). Generally, thecogeneration efficiency decreases when the value of R isincreased. This is used to classify the demands and energyconversion technologies. Energy users differ widely by the scaleand the power-to-heat ratio (R) of the demands. Residentialapplications feature daytime RDay410 and RNightE1. The R-valuesof industrial energy demands also vary. An EC-project report [18]quotes values in the range 0.4–0.6. Grid supply power stations arealso a promising application, where district heating CHP variantscovering R 0.10–0.49 [19] are put at strong advantage by thelegislation in most industrialised countries.
The cogeneration efficiencies for the reviewed systemsare given in Table 2. They can serve a wide range of applicationswith essentially any practical value of R. For energy demands withR41 (e.g. mechanical processing, grid-dedicated power plants), FChybrids can be applied directly. For smaller R some of the systemcomponents such as the GT can be discarded. For very small valuesof R, e.g. smaller than 0.2, a CHP plant with R40.2 may bedesigned and the excess power can be sold to the grid, providedthis is contractually and physically possible.
An interesting direction is the design and operation of FC-basedCHP systems serving large industrial sites. In oil refineries, there areoften large amounts of chemically low-quality hydrocarbon feed-stocks (currently burned) suitable for reforming and use as FC fuels.
4.3. Heat integration options
The heat integration has been a well-developed methodologyfor minimising the energy consumption, increasing the efficiencyand minimising emissions [20]. It has been described in detailelsewhere [21,22], offering various options to properly integratethe ICCs. The recovery of thermal energy between the componentsin FC systems has been analysed for different arrangements.
Figs. 3 and 4 show the composite curves (CCs) [21] for tworepresentative cases—integration of a SOFC with GT and that of aMCFC with a steam cycle, respectively. CCs are a very useful toolfor representation of heat exchange systems and can be beneficialfor selecting the integration of the component utilities into hybridsystems. Comparing the two cases in Figs. 3 and 4 leads to acouple of interesting conclusions:
(i)
In the SOFC+GT arrangement [8], the components are moretightly integrated. This leads to high efficiency, but also tosmaller driving forces, which would tend to increase thecapital costs.
(ii)
For the MCFC+ST [5] combination, higher efficiency is stillpossible, but the driving forces are much larger whichindicates potentially smaller capital costs for the heatrecovery.To illustrate the potential for generating steam the grandcomposite curves (GCC) corresponding to the same SOFC+GTand MCFC systems are shown in Figs. 5 and 6. Fig. 6 indicatesthat the MCFC+ST arrangement allows significant gene-ration of any level steam for heating to be used on-site or soldfor profit.
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0
200
400
600
800
1000
1200
0 5 10 15Q (kW )
T (°
C)
Fig. 5. Grand composite curve for the heat integration of the 30 kW SOFC+GT
system in Ref. [8].
0
200
400
600
800
1000
0 500 1000 1500 2000 2500Q (kW )
T (°
C)
Fig. 6. Grand composite curve for the MCFC heat-and-power integration [5].
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–15171512
5. Case study: energy integration of a SOFC
5.1. Definitions
This case study analyses the energy efficiency of severalpossible integrated system options measured in several ways.Firstly, the total power produced by the system can be written as:
WTotal ¼WSOFC þWST. (1)
Additionally, the CHP energy output of the system is defined asthe sum of the total power and the useful heat outputs:
ECHP ¼WTotal þ QUH. (2)
Two following efficiency definitions are used in the study—
electrical and CHP efficiency.
5.1.1. Electrical (or power) efficiency
This is the fraction of the generated electrical power relative tothe heat flowrate input to the system in the form of fuel:
ZE ¼WTotal
QFuel. (3)
5.1.2. CHP efficiency
This is the fraction of the total useful energy output relative tothe energy input of the fuel:
ZCHP ¼ECHP
QFuel¼
WTotal þ QUH
QFuel. (4)
5.2. Case study description
The case study is based on data derived from the exampledescribed by Chan and Ding [23]. This represents a SOFCgenerating around 3000 kW of electrical power at 50.90%efficiency. The main FC system operating conditions include:
�
pressure 1 atm (1.01325 bar); � 70% fuel utilisation rate; � the fuel and the air streams enter the FC at 900 and 700 1C.The system flowsheet is shown in Fig. 7.
5.3. Heat integration data and targeting
After an analysis of the flowsheet in Fig. 7, a number of processstreams which need thermal change have been identified andextracted for the heat integration study (Table 3). The reformeroperation has been extracted as a two-segment stream as its Cp
and consequently CP ¼ CP �m should be linearised by at least twosegments (Reformer-1 and Reformer-2).
The heat integration problem identified is a threshold one [22],where under DTMin ¼ 32.2 1C there is no need for a hot utility.Above this value, there is a need for some hot utility. The heatrecovery targets for DTMin ¼ 30 1C are shown in Figs. 8 and 9. Theminimum cold utility required for this case is 1197.08 kW.
By analysing Figs. 8 and 9, it is obvious that there is asignificant thermodynamic potential for either heat cogeneration(stream to stream heat recovery) or increased power generationfrom the referenced system. Cooling water or air can readily beused to satisfy the process stream cooling requirements, but thiswould generate only a heat waste.
5.4. Generating hot water
The main purpose of this case study is to investigate theoptions and scope for integrating a SOFC with other energytechnologies. A number of heat exchanger networks (HENs) havebeen generated for recovering maximum heat from the variousprocess streams (Table 3). The simplest option is to use the excess1197.1 kW waste heat as hot water.
The cogeneration is considered for feeding hot water tohouseholds located close to the power generation plant. Theusual temperature of the hot water used in households is between45 and 50 1C. To cover the heat losses during transportation of thewater from the power plant to the users, the generationrequirement for hot water is set at 65 1C. The resulting HENtopology A is shown in Fig. 10. In this case all the waste heat fromthe fuel cell system is used for heating the water and neither hotnor cold utility is needed.
The network in Fig. 10 manages to utilise the whole 1197.1 kWtargeted by the heat integration pinch analysis (see Figs. 8 and 9).This requires 140.3 m2 total heat exchange area. As a result, theoverall efficiency of the combined CHP system (SOFC+HEN)becomes 70.8%.
5.5. Increased power generation using a steam cycle
For designing the steam cycle generating steam at variouspressure levels coupled with a condensing steam turbine cycle hasbeen evaluated. The simplified flowsheet of the steam cycle isgiven in Fig. 11. This arrangement can be modified by slightlyraising the exhaust pressure of the steam turbine and using it fordistrict heating again.
ARTICLE IN PRESS
CH4
FP Reformer
SOFC
Reformate
PowerHeat loss
Afterburner
EC
Loss
AirAP
SGCogenerated heat(water or steam) –varied in the analysis
0.12 kg/s25 °C
Fuel heat6001.7 kW
25.0 °C2.28 kg/s
0.36 kg/s25 °C
700.0 °C
0.49 kg/s
100.0 °C850.0 °C
900.0 °C
3055.0 kW
1639.6 kW
173.6 kW
1227.9 kW reaction403.4 kW heating
985.0 °C2.77 kg/s
932.2 °C
V
1000.4 kWWater/steamP=1 atm
303.6 kW130.0 °C
3119.1 kW
Exhaust
1664.6 kW
Cooling2677.0 kW
Water (demineralised)
Fig. 7. SOFC flowsheet for the case study.
Table 3Heat integration data for the case study
Stream Type Ts (1C) Tt (1C) Q (kW) CP (kW 1C�1)
Air preheat Cold 25.00 700.00 1664.58 0.2466
Fuel preheat Cold 99.97 850.00 1000.37 0.1334
Reformer-1 Cold 850.00 850.01 1227.90 12279
Reformer-2 Cold 850.01 900.00 403.38 0.8069
Vaporiser Cold 25.00 99.97 303.64 0.4050
Exhaust cooling Hot 932.22 130.00 3119.09 0.3889
Afterburner cooling Hot 932.22 932.00 2677.00 1205.8
0
200
400
600
800
1000
0 2000 4000 6000Q (kW )
T (°
C)
Hot CC Cold CC
Fig. 8. Composite curves for the SOFC integration case study.
0
200
400
600
800
1000
0 500 1000 1500 2000 2500 3000Q (kW)
T (°
C)
Fig. 9. Grand composite curve for the SOFC integration case study.
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–1517 1513
The steam specifications are tracked by the correspondingsaturation temperatures (Table 4). All resulting HEN topologies forthe steam generation case are the same. As an example, thenetwork for the 120 bar steam (TSAT ¼ 324.68 1C) is shown inFig. 12, designated as Network B, where the main difference fromNetwork A is the size of the heat exchanger area. The performanceof all networks of type B (those with steam generation) is given inTable 5.
Several observations stem from the case study as discussedbelow.
5.5.1. Observation 1: efficiency trends
The electrical (power) and CHP efficiencies feature opposite trends
with increasing the steam generation pressure. These are plotted inFig. 13. It can be seen that, by increasing the steam generationpressure, the electrical efficiency increases and the CHP efficiencydecreases simultaneously. This can be easily explained usingthermodynamic reasoning. At fixed steam exhaust pressure, byincreasing the inlet one, the driving force for energy conver-sion—the pressure differential between the steam turbine inter-faces, grows. It is clear that the larger this driving force is, thelarger is the potential to generate power and at the same time theenergy dissipation (losses) also increases due to the increasing‘‘irreversibility’’ of the process.
The overall energy efficiencies of the system are bound bythe performance of two extreme designs. This is shown again inFig. 13 where the SOFC standalone efficiency is plotted as abaseline serving as a lower bound on the efficiency. Similarly, theCHP efficiency of the integrated system for hot water generation
ARTICLE IN PRESS
Fig. 10. HEN topology A—for generating hot water.
HEN - steamgeneration
Deaerator
ES: Steam for expansionat generation pressure
SteamTurbine
Condenser
P = 1 atm =1.01325 bara
P = 1.2 bara
DAG: Deaerationsteamat generationpressure
DAS:DeaerationsteamP = 1.2 bara
Boilerfeedwater at
generationpressure
DAO:DeaeratorOutletP = 1.2 bara
BD: Blowdown water(ZERO)
GS: Generated steam
CNDIN : Condenser inletP = 0.12 bara
CNDOUT: Condenser outletP = 0.12 bara
CR: Condensatereturn
BFW:BoilerFeed-Water
DW:Demineralisedwater
DAF: Deaerator feedP = 1.2 bara
DV: Deaerator vent
Fig. 11. Steam cycle topology for increasing power generation.
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–15171514
forms an upper bound. While both bounds are quite obvious inthe light of the laws of thermodynamics, their significance forfuture design of FCCC integrated systems needs to be outlined.Identifying the bounds would allow establishing targets todetermine the thermodynamic capabilities of this kind of systemsand might provide the engineer a tool for judging the potentialtechnological, resource and economic viability of the systems.
There are two very important features of the system, bothconsidered to be resulting from the high temperature of the
SOFC—poor utilisation of the SOFC exhaust temperature potentialand the need for potentially expensive heat exchangers.
5.5.2. Observation 2: poor utilisation of the SOFC exhaust
temperature potential
Another important insight into the thermodynamics of theCHP system can be gained from Fig. 14. There the grand compositecurves for thee of the investigated systems are superimposed to
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Table 4Parameters of the steam levels
TSAT (1C) P (bar) TSH (1C) TBFW (1C) QPreheat (kW) QEVAP (kW) QSH (kW) QTotal (kW)
110.00 1.43 310.00 110.00 - 1013.27 182.96 1196.23
120.00 1.99 320.00 110.00 19.12 993.58 183.53 1196.23
140.00 3.62 340.00 110.00 56.81 954.02 185.40 1196.23
160.00 6.18 360.00 110.00 93.89 913.93 188.41 1196.23
180.00 10.03 380.00 110.00 130.55 872.97 192.71 1196.23
200.00 15.55 400.00 110.00 167.03 830.72 198.47 1196.23
220.00 23.19 420.00 110.00 203.59 786.67 205.97 1196.23
240.00 33.47 440.00 110.00 240.55 740.13 215.54 1196.23
260.00 46.92 460.00 110.00 278.34 690.21 227.68 1196.23
300.00 85.88 500.00 110.00 358.79 574.56 262.88 1196.23
324.68 120.00 524.68 110.00 414.75 484.98 296.50 1196.23
Fig. 12. HEN topology B—for the case of steam generation.
Table 5Parameters of the resulting HEN designs
TSAT (1C) P (bar) A (m2) Steam generation (t h�1)
110.00 1.43 137.75 1.64
120.00 1.99 138.16 1.62
140.00 3.62 139.64 1.60
160.00 6.18 141.41 1.58
180.00 10.03 142.64 1.56
200.00 15.55 143.80 1.54
220.00 23.19 144.90 1.52
240.00 33.47 145.94 1.51
260.00 46.92 146.46 1.50
300.00 85.88 158.86 1.47
324.68 120.00 149.59 1.46
50
55
60
65
70
0 5 10 15 20Steam pressure (bar)
Effic
ienc
y (%
)
Power generation CHPSOFC 51%standaloneefficiency
CHP efficiency hot water
Fig. 13. Overall FCCC cycle efficiency.
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–1517 1515
track the change of the temperature–heat–flowrate profile ofthe system with the evolving heat recovery temperatures. Thebottommost curve depicts the case of hot water generation, themiddle one shows steam generation at absolute pressure of
15.55 bar (200 1C saturation temperature), and the topmostone—generation of 120 bar (absolute pressure, saturation tem-perature 324.68 1C) steam. It is obvious that, when generating hot
ARTICLE IN PRESS
00
200
400
600
800
1000
1000 2000 3000Q (kW)
T (°°
C)
Steam at120 bar(a)
Steam at
15.55 bar(a)
Hot water 65 °C
Fig. 14. Superimposed GCCs for different SOFC integration options.
0
50
100
150
200
250
0 50 100 150Steam pressure (bar)
Pow
er (M
W)
0.70
0.75
0.80
0.85
0.90
0.95
1.00
Dry
ness
, -
Power generation Dryness Fraction
Minimum feasible dryness
Fig. 15. Steam cycle performance.
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–15171516
water, the temperature potential of the SOFC exhaust is extremelypoorly utilised despite the excellent CHP efficiency. Even generat-ing steam at 120 bar results in only modest utilisation.
The main conclusion from this observation is that energyrecovery from an SOFC or any other HTFC using HENs is suitablemainly for a specific range of power-to-heat ratios (R). In thecurrent case study, the highest R ¼ 4.751 for generating 15.55 barsteam, while the lowest is R ¼ 2.552 for generating hot water.
5.5.3. Observation 3: potentially expensive heat exchangers
Another consequence of the high FC exhaust temperature isthat the heat exchangers may need to be built of special materialsand/or with advanced designs. This may contribute to highercapital costs and thus to be economically suitable only toapplications with high profit margins and/or large scales, wherethe returns on the investment can be faster realised.
Two possible solutions can be suggested. One is to exploit thehigh FC exhaust temperature directly by adding gas turbines asdiscussed previously [2–4,10–13]. These machines already usespecial materials and are intended for power generation, wherethe profit margins are generally much higher than for heating orCHP. Moreover, such an addition also fits nicely to the problemwith efficiently utilising the exhaust temperature potential fromthe thermodynamic point of view.
5.5.4. Observation 4: trends in the steam cycle when varying the
steam pressure
The last observation concerns the performance of the steamcycle alone (Fig. 15). The power generation from steam firstasymptotically increases, then features a peak and slowlydecreases, with increasing the steam pressure. In parallel, thedryness of the turbine exhaust decreases monotonically and thehighest-pressure steam cycle, which is feasible, is the one with15.55 bar (see Fig. 15). These trends can be explained with themonotonic increase in the total energy extraction from the steam,consuming increasing fractions of the steam superheat and part ofthe steam condensation heat.
This trend reveals an important design limitation of systemsintegrating HTFCs (or any other source of high-temperature wasteheat) with steam turbines—there is an upper limit on the pressuredifferential of a single turbine stage. One possible solution, if evenlarger power generation is required, is to either employ a multi-level steam cycle with reheat/letdown and another is to switch toHTFC+GT based hybrid cycles.
The former option (multistage steam power cycle) wouldrequire additional steam turbine machines or a single multi-stageturbine, which would be more expensive and of much higher scale
than the SOFC considered here. Many combinations are possible.For instance, a larger steam turbine plant can be combined withmore HTFCs [4] (e.g. 10–20 units), or possibly in the future HTFCswill be capable of generating 20–30 MW of power, which would fitthe scale of larger industrial steam turbines.
The latter option (HTFC+GT) seems more attractive since mGTsare already available commercially and they fit the present HTFCprototypes by both the thermodynamic profile and the scale of thecapacity.
6. Conclusions and future work
Studies of the trends and the potential benefits of fuel cellsheat integration based on the Heat Integration (Pinch) Methodol-ogy have been performed. It has been found that the focus shouldbe on high-temperature FC. Combining FC with either GT or ST isvery efficient. Integration with both bottoming cycles provides nosignificant benefits in terms of overall efficiency.
An obvious observation is that lowering the FC cost whilepreserving their high efficiency is needed. The emphasis should beput on the CHP rather than electrical efficiency, which has thepotential to greatly increase the overall fuel utilisation efficiency.
Waste treatment and biogas plants can be suitable fuelsuppliers for FC-based CHP systems. Gasified biomass or coalcan be attractive too. With regard to fuel supply, the need forcombined energy–food–waste supply chains has been revealed.Clean coal power plants should be based on SOFC with CO2
sequestration.In terms of designing systems which both maximise the fuel
utilisation rate and are economically viable, several directions forfurther research and development can be outlined:
�
The considered energy systems are complex and heteroge-neous process systems. Designing them would require toolswith a significant strength for solving combinatorially inten-sive problems with heavy optimisation loads. A methodefficiently handling the combinatorial nature of such pro-blems is the P-graph framework developed by Friedlerand co-workers for heat exchange network synthesis [24],district energy system [25] and combined heat exchange andseparation [26]. � There is continuously a need for novel advanced design forheat transfer and power generation equipment reducing theircost. Currently special alloys and ceramic materials are used inconstructing the high-temperature devices—gas turbines andSOFCs. A possible cost reduction solution can be to lowerthe SOFC working temperatures down to 800 1C [27] and
ARTICLE IN PRESS
P. Varbanov, J. Klemes / Energy 33 (2008) 1508–1517 1517
eventually lower, which would allow the use of less expensivealloys for the heat exchangers, which defines a trade-off withthe system efficiency. On the one hand, the lower the SOFCworking temperature, the cheaper are the materials in general.On the other hand, the lower the temperature, the smaller isthe potential for efficient integration of gas turbines. In thiscase too, heat integration can offer solutions which optimisethe energy gain under the modified conditions.
� In parallel with the energy and the greenhouse effect, theindustrial significance of other resources—most notablywater—rapidly increases. The list of resources to simulta-neously account for usually also includes fuel, chemicals,labour, finances and, of course—time. All these resources areavailable in limited amounts and they cannot be considered inisolation. This calls for more sophisticated frameworks fordesign and optimisation of all process systems, includingenergy conversion facilities. A recent study by Zhelev andRidolfi [28] suggested one potential way to perform this taskby introducing the combination of the pinch analysis and aunified representation of the various resources—the emergy.
Acknowledgements
The financial support from the EC projects Marie Curie Chair(EXC) MEXC-CT-2003-042618 INEMAGLOW and Marie CurieReintegration Grant MERG-CT-2007/46579 ESCHAINS is gratefullyacknowledged.
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