Systems L Blum and E Riensche, Institute of Energy Research/Fuel Cells, Ju ¨lich, Germany & 2009 Elsevier B.V. All rights reserved. Introduction The solid oxide fuel cell (SOFC) is described as a technology that enables a very high electrical system efficiency and provides, at the same time, high off-gas temperatures, which enable a large range of heat appli- cations. The first point is true if one succeeds in oper- ating a cell at quite high cell voltages using a system technology that enables high fuel utilization and low internal consumption of the produced electricity (para- sitic losses). The second point is only partly true, because a large amount of heat is used internally in the system to heat the cold incoming gases (especially air) and, in some cases, to produce the steam needed for the reforming process. Various system concepts have previously been out- lined for achieving efficient system operation. In the following, the different basic plant arrangements and their special features are described. For this purpose, first, the different types of efficiencies are described in order to explain the effect of single measures on the overall electrical efficiency. Second, an overview on how to perform energy balancing of SOFC plants is given, and in particular a detailed description of the effects of internal reforming and temperature differences across the stack on parasitic power consumption is given. Finally, six plant concepts are described to provide an insight into various design options and the advantages and dis- advantages that may be attributed to each of them. Tasks of the System – Control Loops (Plant Requirements) The various SOFC types have different requirements regarding plant configuration, mainly based on the re- quired operating temperature and on their capability of allowing on-anode reforming. In principle, the plant concepts for the SOFC are much simpler than those for low-temperature fuel cells; because no carbon monoxide poisoning occurs, a shift reactor is not needed to convert most of the carbon monoxide to carbon dioxide (as per phosphoric acid fuel cell (PAFC) nor is it necessary to reduce the remaining carbon monoxide to concentrations below 10 ppm (as per proton-exchange membrane fuel cell (PEMFC)). The possibility of internal (or integrated) reforming avoids the necessity of a reformer – only a small prereformer may be needed (see ‘SOFC Internal and External Reforming’ Fuel Cells – Solid Oxide Fuel Cells: Internal and Ex- ternal Reformation). On the contrary, especially on the cathode side, relatively large recuperators are needed to preheat the incoming gas to the required stack inlet temperature. Additionally, much more thermal insulation is required to limit the heat losses at high stack temperatures. To be able to operate an SOFC system, different control loops and steering (open loop controls) must be installed. These are as follows: • fuel flow control proportional to the electric current, • air flow control proportional to the electric current superposed by the cooling requirements of the stack (if it is air-cooled), • air inlet temperature control into the stack, • fuel inlet temperature control into the stack, • temperature control of the fuel reformer, • flow control of water or steam, • removal of product water, • heating sequence, • cooling sequence, • hot standby sequence, and • shutdown and safety shutdown. Efficiencies Relevant for the System Fuel cell systems promise high electrical efficiencies, but on the way from cell to system many obstacles exist that reduce the high efficiency values at the cell level. In designing an SOFC system, one should be aware of the different factors that can affect (and so reduce) the electrical system efficiency. These factors can be assigned to special efficiencies, the product of which will form the total efficiency. The definitions of these efficiencies are given in the following. Cell Efficiency Cell efficiency e c , is the basic efficiency of a fuel cell and it describes the relation between the specific energy, delivered under load (h ¼ zxFxV p (i )), and the reaction enthalpy of the cell reaction, related to standard con- ditions (DH ¼ zxFxV LHV ): e c ¼ V p V LHV ½199
Transcript
Systems
L Blum and E Riensche, Institute of Energy Research/Fuel Cells, Julich, Germany
& 2009 Elsevier B.V. All rights reserved.
Introduction
The solid oxide fuel cell (SOFC) is described as atechnology that enables a very high electrical systemefficiency and provides, at the same time, high off-gastemperatures, which enable a large range of heat appli-cations. The first point is true if one succeeds in oper-ating a cell at quite high cell voltages using a systemtechnology that enables high fuel utilization and lowinternal consumption of the produced electricity (para-sitic losses). The second point is only partly true, becausea large amount of heat is used internally in the system toheat the cold incoming gases (especially air) and, in somecases, to produce the steam needed for the reformingprocess.
Various system concepts have previously been out-lined for achieving efficient system operation. In thefollowing, the different basic plant arrangements andtheir special features are described. For this purpose, first,the different types of efficiencies are described in orderto explain the effect of single measures on the overallelectrical efficiency. Second, an overview on how toperform energy balancing of SOFC plants is given, and inparticular a detailed description of the effects of internalreforming and temperature differences across the stackon parasitic power consumption is given. Finally, sixplant concepts are described to provide an insight intovarious design options and the advantages and dis-advantages that may be attributed to each of them.
Tasks of the System – Control Loops(Plant Requirements)
The various SOFC types have different requirementsregarding plant configuration, mainly based on the re-quired operating temperature and on their capability ofallowing on-anode reforming.
In principle, the plant concepts for the SOFC aremuch simpler than those for low-temperature fuel cells;because no carbon monoxide poisoning occurs, a shiftreactor is not needed to convert most of the carbonmonoxide to carbon dioxide (as per phosphoric acid fuelcell (PAFC) nor is it necessary to reduce the remainingcarbon monoxide to concentrations below 10 ppm (as perproton-exchange membrane fuel cell (PEMFC)). Thepossibility of internal (or integrated) reforming avoids thenecessity of a reformer – only a small prereformer maybe needed (see ‘SOFC Internal and External Reforming’
On the contrary, especially on the cathode side,relatively large recuperators are needed to preheat theincoming gas to the required stack inlet temperature.Additionally, much more thermal insulation is requiredto limit the heat losses at high stack temperatures.
To be able to operate an SOFC system, differentcontrol loops and steering (open loop controls) must beinstalled. These are as follows:
• fuel flow control proportional to the electric current,
• air flow control proportional to the electric currentsuperposed by the cooling requirements of the stack(if it is air-cooled),
• air inlet temperature control into the stack,
• fuel inlet temperature control into the stack,
• temperature control of the fuel reformer,
• flow control of water or steam,
• removal of product water,
• heating sequence,
• cooling sequence,
• hot standby sequence, and
• shutdown and safety shutdown.
Efficiencies Relevant for the System
Fuel cell systems promise high electrical efficiencies, buton the way from cell to system many obstacles exist thatreduce the high efficiency values at the cell level. Indesigning an SOFC system, one should be aware of thedifferent factors that can affect (and so reduce) theelectrical system efficiency. These factors can be assignedto special efficiencies, the product of which will form thetotal efficiency.
The definitions of these efficiencies are given in thefollowing.
Cell Efficiency
Cell efficiency ec, is the basic efficiency of a fuel cell andit describes the relation between the specific energy,delivered under load (h¼ � zxFxVp(i )), and the reactionenthalpy of the cell reaction, related to standard con-ditions (DH¼ � zxFxVLHV):
ec ¼Vp
VLHV½1�
99
100 Fuel Cells – Solid Oxide Fuel Cells | Systems
where Vp is the (practical) cell voltage and VLHV theLHV heading voltage. In the case of operation at a cellvoltage of 0.7 V using various gases, the cell efficiencywould amount to 56% for hydrogen, 67% for methane,and 48% for carbon monoxide.
Fuel Utilization
Owing to the formation of water on the anode side, thewater vapor partial pressure increases toward the gasoutlet of the cell. As a consequence, the Nernst voltagedrops markedly in the case of water content above 95%.For this fundamental reason, it is not possible to elec-trochemically react all fuel in an SOFC. The relationbetween the amount of fuel gas reacting in the cell mrea
and the amount of fuel gas min entering the system (re-spectively the stack) is defined as fuel utilization. Becauseit is not so easy to measure the amount of reacted fuel, itis much more accurate to calculate it with Faraday’s law,using the current produced by the stack, which is easy tomeasure. As a stack normally consists of several layers,the current has to be multiplied by the number of cells,connected electrically in series, to obtain Itot:
uF ¼mrea
min¼
M
z� F� Itot
min½2�
where M is the molar mass and uF the utilization of fuel.If a mixture of gases is supplied to the system, it issometimes more efficient to calculate how many elec-trons are supplied to the system by using Faraday’s lawfor each gas component:
uF ¼Itot
Iin¼ ItotP
i
mi;in
Mi
� zi � F½3�
Stack Efficiency
Sometimes it is not very clear in publicized values whichefficiency is meant. Very often it is not the system effi-ciency but the stack efficiency (also sometimes calledeffective efficiency). Stack efficiency, es, is the product ofcell efficiency ec and fuel utilization uF:
es ¼ ec � uF ½4�
In the case of operation at a cell voltage of 0.7 V and fuelutilization of 80%, the stack efficiency, using methane asthe fuel, would amount to 53.6%.
Inverter Efficiency
Because the inverter is the most important electronicplant component, it is normally considered separately.The inverter efficiency describes the loss when adapting
the direct current (DC) voltage level of the stack to thealternating current (AC) voltage level of the consumer,respectively the grid:
eInv ¼PAC;gross
PDC;gross½5�
Inverters of low power can have efficiencies as low as 85–90%, whereas the best ones with higher power outputand high input voltage can achieve 96%.
Parasitic Efficiency
Parasitic efficiency ep, describes the relation betweeneffective net power and produced gross power, because apart of the produced power is needed for operating theplant and is therefore not available for exploitation. Asthe final consumers are normally supplied with AC, theAC side of the inverter must be considered:
ep ¼PAC;gross � PAC;parasitic
PAC;gross¼ PAC;net
PAC;gross½6�
Electric Plant Efficiency
The total electric plant efficiency, eel, can be derived bycombining the efficiencies described in eqns [1]–[6]. Itcan also be described by the relation between the elec-trical energy, available for the consumer, and the energyfed to the system by the fuel gas, but this provides lessinformation about the effect of the different plant com-ponents on the overall efficiency:
eel ¼PAC;net
m� LHV¼ es � ep � eInv ¼ ec � uF � ep � eInv ½7�
In power plant technology, it is general practice to usethe lower heating value (LHV); therefore the LHV isgenerally used here in the calculations.
Thermal Plant Efficiency
As the intention is often to use an SOFC system as acombined heat and power plant (CHP), the thermal ef-ficiency, eth, is of importance. It describes the relationbetween the amount of waste heat (used by the consumerfor heating purposes) and the energy fed to the system bythe fuel gas:
eth ¼Pth;net
m� LHV½8�
where Pth, net is the net used thermal power.
Total Efficiency
As there is competition between SOFC systems andconventional CHP plants, the total efficiency, etot, is often
Fuel Cells – Solid Oxide Fuel Cells | Systems 101
used for comparison. It describes the relation between thesum of usable thermal power and electrical power andthe energy fed to the system by the fuel gas:
etot ¼PAC;net þ Pth;net
m� LHV¼ eel þ eth ½9�
Some of these efficiencies can be influenced by the sys-tem configuration and the layout of the single compo-nents. The main tasks of the system layout are to find asolution for low internal consumption and to create arobust configuration that allows high fuel utilization.
Energy Balance of an Solid Oxide FuelCell Plant
The fuel cell plant is composed of the stack and plant(BoP) components, of which the reformer plays a par-ticularly important role in the energy balance.
The energy calculations are presented to demonstratethe interdependencies between the different main plantcomponents. It is practical to use some simplifications:
• reforming and shift reaction are completely on thecarbon dioxide side, and no carbon monoxide is left(no equilibrium calculations)
• to simplify both explanation and understanding, theentering heating energy (LHV) is set to 100 kW.
These simplifications will not replace detailed flow sheetcalculations, in most cases performed using commercialcodes, but they are useful to gain understanding of theway in which parameter changes affect the system.
The following point must be borne in mind: whendoing an enthalpy balance of flows containing chemical
Reformer + Shift
Conversion rate 100%
Balancing border
mCH4,in xLHVoCH4
mH2,inxLHV
100 kW 120 kW
20kWQref
Figure 1 Control volume for balancing the changes in reaction ent
reactions, the amount of energy (reaction enthalpy) re-leased by the reaction has to be taken into account asanother source of heat. The correct result will be ob-tained only if the enthalpy scale of all media involved isrelated to the same standard conditions. Independent ofthe temperature at which the reaction really takes place,all reaction enthalpies have to be taken at standardconditions (1013 mbar, 25 1C). This is because of thedefinition of enthalpy, and reaction enthalpy in particular(Kirchhoff ’s law), as a function of temperature.
Balancing the Changes in Reaction Enthalpy
To be in a position to determine the stack efficiency (theproduct of cell efficiency and fuel utilization), one musttake into account which reaction enthalpy (heating value)is entering the system and which reaction enthalpy andelectrical power are leaving the system. For this purpose,a control volume is drawn around the reformer (in-cluding the shift reactor) and stack, as shown in Figure 1
The reaction enthalpy Hin, which enters the systemwith the fuel, is
Hin ¼ mCH4 ;in � LHVoCH4¼ 100 kW
Assuming a complete reforming and shift reaction, 4mol of hydrogen are produced out of each mole ofmethane.
Based on the LHVs of methane (803 kJ mol�1) andhydrogen (242 kJ mol�1), there is an increase in reactionenthalpy at the reformer outlet from 100 kW to
Href ;out ¼ HS;in ¼4� LHVo
H2
LHVoCH4
� Hin ¼ 1:20� 100 kW
¼ 120 kW ½10�
oH2
mH2,outxLHVoH2
QH2,rea 42.2kW
Stack
Vp= 0.7 V
uF= 80%
Hrea= 96 kW 24 kW
PDC 53.8 kW
halpy of a reformer with a stack.
102 Fuel Cells – Solid Oxide Fuel Cells | Systems
This difference of 20 kW (or increase by 20%) has to beprovided to the reformer and shift reactor as heatingenergy Qref.
In the case of a fuel utilization uF of 80%, the enthalpyHrea reacted in the stack is
Hrea ¼ mH2 ;in � LHVoH2� uF ¼ 120 kW� 0:8 ¼ 96 kW
Operating at a mean cell voltage Vp of 0.7 V and based onthe heating voltage VLHV of hydrogen of 1.25 V, theproduced electrical power PDC is determined as
PDC ¼ Hrea � ec ¼ Hrea �Vp
VLHV¼ Hrea � 0:56 ¼ 53:8 kW
Relating this to the incoming hydrogen results in a stackefficiency es of
es ¼PDC
HS;in¼ 53:8
120¼ 44:8% ¼ uF � ec;H2
This value would also be valid including the reformer, ifthe reformer had to be heated by burning additionalmethane to provide the 20 kW heating energy.
As the enthalpy of the nonreacted fuel leaving thestack is high enough, in our case no additional fuel isnecessary. So the balance, drawn from the whole controlvolume (stack plus reformer/shift), results in a stackefficiency of
es ¼PDC
Hin¼ 53:8
100¼ 53:8% ¼ uF � ec;CH4
AN1
AN2
AC1
Pel (AN1+AN2)
Air forcooling
Airstoichiom.
Methane
AC2 Reformer + shift
Conversion rate 100%
TR,in=TR, out= 750 °C
mH2O,R x h(TR,in)
SteamQref mN2
xh(TN2,o
mO2xh(TO2
mair,c x h(Tin)
mO2,st x h(Tin)mN2,st x h(Tin)
mCH4 x h(TR,in)
m
m
mH2mCOmH2
mH2mCH4 x LHV°CH4
�is
Figure 2 Control volume of a plant with external reforming.
One advantage of an SOFC system is that, because of thehigh operating temperature, some of the heat containedin the exhaust gases leaving the stack can be used to heatthe reformer. This, for example, happens during internalreforming. This makes it possible to further increasethe fuel utilization in an SOFC stack. Only the electro-chemical behavior of the cell prevents fuel utilizationexceeding 90%. In the case of low and mean temperaturefuel cells, fuel utilization is limited because the reformercan be heated only by burning additional fuel. Sothe maximal possible fuel utilization would be 100/120¼ 83%. Even this is only a theoretical value becausein reality there are heat losses and the gases and com-ponents have to be heated to the reaction temperature. Arealistic fuel utilization for these types of fuel cells isaround 70%.
Energy Balance of a Plant with ExternalReforming
For balancing the whole plant, in addition to the energyfluxes depicted in Figure 1, all incoming and outgoingenthalpy flows have to be added. For this purpose, theflow scheme has to be extended as shown in Figure 2.
The energy balance now has to be resolved accordingto the material carrying the remaining waste heat out ofthe system. In the case of an SOFC, this is normally theair flow on the cathode side, which is also used as acoolant.
For this purpose, the complete balance of stack andreformer has to be performed first.
Balancing border
Stack
TS,in = 750 °CTS,out = 850 °C
Vp= 0.7 VuF=80%
ut)
,out)
PDC
air,cxh(TS,in) mair,cxh(TS,out)
mN2,st xh(TS,out)
mH2,S,outx LHV°H2
mH2,S,outxh(TS,out)mCO2,S,outxh(TS,out)mH2O,S,out
xh(TS,out)
O2,st xh(TS,in)mN2,st xh(TS,in)
,S,in xh(TS,in)
2,S,inxh(TS,in)
O,S,inxh(TS,in)
,S,inx LHV°H2
Fuel Cells – Solid Oxide Fuel Cells | Systems 103
Balance of reformer
Before entering the reformer, the methane is preheatedin heat exchanger AC2 to the reforming temperature(e.g., 750 1C) together with the water vapor using the hotoff-gas. As long as this can be done by internal heattransfer, this is not relevant for the overall balance.
The energy input HCH4by methane into the reformer
is defined as
HCH4¼ m
CH4� LHVo
CH4þ cp � Tref ;in � T0
� �h i
where T0 is the standard temperature and cp the specificheat capacity. The necessary amount of water is deter-mined by the chosen steam to carbon ratio (S/C), whichis normally in the region of 2.5. This avoids the risk ofcarbon formation in the piping and components and issufficient for steam reforming and shift reaction:
Reforming CH4 þH2O-COþ 3H2
Shift COþH2O-CO2 þH2
Based on this, the mass flow of water vapor can be cal-culated as
mH2O;ref ¼ S=C� nCH4�MH2O
The enthalpy HH2O;ref that enters the reformer by thewater vapor flow is
HH2O;ref ¼ mH2O;ref � cp � Tref ;in � T0
� �
The amounts of hydrogen and carbon dioxide that arecreated by reforming and shift reaction and the re-maining amount of steam result in (the indices now referto the stack inlet, which is equal to the reformer outlet):
These calculations also provide the input data for theanode side of the stack.
Balance of stack
The outgoing flows on the anode side of the stack can bedetermined as follows.
The mass flow of nonreacted fuel can be determinedby inlet flow and fuel utilization:
mH2 ;S;out ¼ mH2 ;S;in � 1� uFð Þ
The effluent carbon dioxide mass flow corresponds to theincoming carbon dioxide mass flow (assuming that thereis no back reaction from carbon dioxide to carbonmonoxide).
mCO2;S;out ¼ mCO2 ;S;in
The outlet steam mass flow is the combination of theinlet water and the amount formed in the electro-chemical cell operation. The latter can be calculatedusing Faraday’s law if the electrical current is known orby the number of moles of reacted hydrogen:
nH2;rea ¼mH2;S;in � uF
MH2
mH2O;S;out ¼ mH2O;S;in þ nH2 ;rea �MH2O
Based on these, the outgoing enthalpies on the anode sidecan be calculated:
The stoichiometric amount of oxygen nO2;st necessaryto react with the consumed hydrogen can easily be cal-culated based on the oxy-hydrogen reaction:
nO2 ;st ¼ 0:5� nH2 ;rea ¼ 0:5� mH2 ;S;in
MH2
� uF
So the stoichiometric oxygen mass flow becomes
mO2;st ¼ nO2;st �MO2
Using the simplified assumption that air consists of onlyoxygen (23.3 wt%) and nitrogen, the correspondingamount of nitrogen is
mN2;st ¼1� 0:233
0:233� mO2 ;st ¼ 3:29� mO2 ;st
Now the inlet and outlet enthalpies on the cathode sidecan be calculated:
HO2;st�S;in ¼ mO2 ;st � cp � TS;in � T0
� �HN2;st�S;in ¼ mN2 ;st � cp � TS;in � T0
� �HN2 ;st�S;out ¼ mN2 ;st � cp � TS;out � T0
� �DHair;S;c ¼ mair;c � cp � TS;out � TS;in
� �½12�
104 Fuel Cells – Solid Oxide Fuel Cells | Systems
The enthalpy DHair;S;c that has to be removed by thecooling air results from the balanced equation. To be ableto do this, the electrical energy produced in the stack hasto be known. This results from the product of mean cellvoltage Vp and electric current I determined by thereacted hydrogen using Faraday’s law.
I ¼ mH2 ;reaz� F
MH2
So the electric power is
PDC ¼ Vp � I
The balanced equation, resolved to the enthalpy DHair;S;c
(to be removed by the cooling air), is as follows:
DHair;S;c ¼ HH2;S;in � HH2 ;S;out
� �þ HN2 ;st�S;in � HN2;st�S;out
� �þ HO2 ;st�S;in þ HCO2 ;S;in � HCO2 ;S;out
� �þ HH2O;S;in � HH2O;S;out
� �� PDC ½13�
Energy balance of the plant
Having now determined all enthalpy flows for both thestack and the reformer, the energy balance for the wholeplant can be worked out. For this purpose, all electricalconsumers in the plant have to be known. The BoPcomponent consuming the most electric power is the aircompressor (AN1þAN2).
The compressor power consumption PC can be de-termined by
PC ¼ mair � cp � T2 � T1ð Þ ¼ mair;c þ mair;st
� �� cp �
T
eis
� p2
p1
� �k�1k�1
0@
1A ½14�
This makes it clear that the air mass flow necessary forcooling is the main driver for the compressor power. Thesecond key parameter is the pressure increase p2 neces-sary to overcome the flow resistance in the system –which is again influenced by the mass flow of air. Basedon fixed geometries, this is more than proportional to theair flow.
There are two methods for minimizing the necessaryamount of air for cooling. One is to increase the tem-perature difference across the stack between the inletand outlet. The other is to use internal reforming.Both methods carry some risks with regard to thermo-mechanical stress resulting from excessive temperaturegradients; however, these are issues for stack designersand material scientists. In the following sections, howthese two methods will influence the amount of air nec-essary for cooling is analyzed.
To be able to do this, two assumptions are made:
• there is no cooling taking place by the media flow onthe anode side (all media having the same inlet andoutlet temperature) and
• there is no heat loss to the environment.
With these assumptions, the calculated air flow will besomewhat too high (in the range of 10–20%), but thevalues obtained will be useful to illustrate how cell effi-ciency, fuel utilization, temperature difference, and in-ternal reforming interact.
Air flow as a function of cell efficiency and
temperature difference across the stack
First of all, the surplus air flow coefficient l needs tobe defined. The coefficient of surplus air flow, l, whenmultiplied with the stoichiometric air flow (necessaryto react the consumed fuel) gives the total air flow;l of 1 means no additional air for the cooling and l of 2means the same amount for cooling as for the electro-chemical reaction. In this case, the oxygen content at theend of the electrochemical active cell would be half, thatis, about 10 vol%. To ensure the safe operation of thecathode, the oxygen content should not be reduced belowthis value.
As there is no general rule, some refer to l to use thetotal fuel flow entering the system; so this l is a factor ofuF smaller than the one used here.
Coolant Flow without Internal Reforming
To show the dependency of l on cell efficiency andtemperature difference, one has to derive a relation be-tween these parameters. This can be done as follows:
Then, from a defined relation between fuel consumptionand stoichiometric air flow
nair;st ¼1
0:21� 1
2� nH2 ;rea
Fuel Cells – Solid Oxide Fuel Cells | Systems 105
relation [16] can be described as follows:
l ¼ 0:42�LHVo
H2
cp;air
� �� 1
DTair1� ecð Þ ½17�
Equation [17] describes the necessary surplus air flowwithout internal reforming (no heat loss, no cooling viaanode gas flow).
As can be seen from Figure 3, the necessary air flowincreases markedly in the case of only 50 K temperaturedifference. This would result in an unacceptably highpower consumption by the air compressor. Without in-ternal reforming (or operating with a fuel gas containingonly hydrogen and carbon monoixde), the temperaturedifference permitted across the stack should be at least150 K or better 200 K, especially if one has to operate at alower cell voltage to increase the power density of thestack.
Figure 4 illustrates the influence of compressor powerconsumption on the system efficiency. If one assumes amoderate flow resistance of the cathode side of the sys-tem (air preheater, stack, off-gas components, and piping)of 100 mbar at a l of 4, a linear increase of flow resistancewith flow rate (all laminar, no deflections, etc.), and acompressor efficiency of 50%, already at a l of 10 thecompressor would consume 20% of the stack power.
Coolant Flow with Internal Reforming
To assess the cooling effect created by internal re-forming, one must first derive a correlation between theenergy to be removed on the cathode side (heat pro-duced) QH2;rea, the energy consumed by the reformingprocess Qref , and the fuel utilization uF:
Figure 3 Surplus air flow for cooling as a function of cell voltage in th
(without internal reforming).
In the case of a system where hydrogen does not enterthe stack as a fuel but is formed from methane fuel as aresult of internal reforming, eqn [18] becomes
QH2 ;rea ¼ 1� ecð Þ � uF � HCH4;S;in þ Qref
� �½19�
Now HCH4;S;in can be replaced by Qref because of therelation
HCH4þ Qrefð Þ ¼ HH2
¼ 1:2� HCH4) HCH4
¼ 5� Qref
This leads to
QH2 ;rea ¼ 1� ecð Þ � uF � 6� Qref ½20�
So the part of reaction heat that can be removed by theendothermic reforming process can be described by
Qref
QH2 ;rea¼ 1
6� uF � 1� ecð Þ ½21�
This means that low fuel utilization takes out a largerproportion of the reaction heat and that a lower cellvoltage requires higher additional air flow for cooling.
These relations are pointed out in Figure 5. Thisdiagram shows that it will not be possible to operate asystem at a cell voltage of 0.8 V and fuel utilization of50%, because nearly all the heat would be removed bythe reforming process, and gas flows and heat loss wouldcool down the stack below the operating temperature. Inthe case of high fuel utilization and low cell voltage, thecooling effect by the reforming process falls below 50%.Within a reasonable range of operating parameters(because of the goal of high system efficiency), thecooling rate by internal reforming should be in the rangeof 40–60%.
0.8 0.9 1 1.1
ltage (V)
Δ T = 50 K
Δ T= 100 K
Δ T= 150 K
Δ T= 200 K
Δ T= 300 K
Lower limit
e case of different temperature difference values across the stack
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 4 8 10 12 14 16 18 20
Stoichiometric air flow coefficient (�)
Com
pres
sor
pow
er in
rel
atio
n to
st
ack
pow
er
62
Compressor efficiency 50%
Flow resistance increases proportional to air flow (laminar)
Flow resistance at � = 4 is set to 100 mbar
Figure 4 Compressor power related to stack power as a function of stoichiometric air flow coefficient l.
0%
20%
40%
60%
80%
100%
120%
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
Cell voltage (V)
Qre
f/QH
2,re
a
uF
= 50%
uF
= 60%
uF
= 70%
uF
= 80%
uF
= 90%
Reasonable range of operation
Figure 5 Ratio of cooling by internal reforming and reaction heat as a function of cell voltage in the case of different fuel utilization
rates.
106 Fuel Cells – Solid Oxide Fuel Cells | Systems
Using relation [21], eqn [17] can be expanded by in-tegrating internal reforming:
l ¼ 0:42�LHVo
H2
cp;air
� �� 1
DTair��ð1� ecÞ �
1
6� uF
�½22�
Now the necessary air flow can be calculated in caseswhere there is internal reforming for different fuelutilizations.
The comparison of the necessary air flow with andwithout internal reforming in the case of fuel utilizationof 60% (Figure 6) and 80% (Figure 7) reveals twothings: (1) with internal reforming, fuel utilization has asignificant impact on air flow rate and (2) the tempera-ture difference across the stack should still be at least
100 K. In the case of low-rated cell voltage, it should beeven higher.
System Concepts
There are various system concepts that can potentiallymeet the operational requirements and control loopsdescribed above. These vary in terms of complexity andalso provide different levels of efficiency.
In the following the most common concepts are pre-sented in a very basic way, describing the specific featuresand their pros and cons.
Looking at the block diagrams of concepts 1 to 6which follow, one can see that a high-temperature fuel
0
5
10
15
20
25
30
35
40
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
Cell voltage (V)
Sur
plus
air
flow
(�)
Δ T= 50K
Δ T= 100K
Δ T=150K
Lower limit
Δ T=50K
Δ T= 100K
Δ T= 150K
Fuel utilization 60%
Without internalreforming
Figure 6 Surplus air flow for cooling as a function of cell voltage in the case of different temperature difference values across the stack
(with and without internal reforming): fuel utilization 60%.
0
5
10
15
20
25
30
35
40
0.4 1 1.1
Cell voltage (V)
Sur
plus
air
flow
(λ)
Δ T=50K
Δ T=100K
Δ T= 150K
Lower limit
Δ T= 50K
Δ T=100K
Δ T=150K
0.5 0.6 0.7 0.8 0.9
Fuel utilization 80%
Without internalreforming
Figure 7 Surplus air flow for cooling as a function of cell voltage in the case of different temperature difference values across the stack
(with and without internal reforming): fuel utilization 80%.
Fuel Cells – Solid Oxide Fuel Cells | Systems 107
cell system does not necessarily imply that the off-gastemperature is also very high. As seen above, this canresult if a large proportion of the energy content in thestack or burner off-gas is needed to preheat the cold airand/or to produce the steam needed on the fuel side.
Concept 1 – Basic Arrangement with a CatalyticBurner
The plant components necessary for fulfilling the re-quirements for safe stack operation are depicted in theblock diagram in Figure 8. This diagram represents onepossible basic arrangement using a catalytic waste gasburner.
At the fuel side, there is a gas supply system, whichcontains a blower or compressor (depending on theavailable feed pressure and the required operation pres-sure in the system), flow control, valves for start up andshutdown, and safety valves for emergencies. Dependingon the type of gas and its source, special cleaning stepsare necessary. In most cases, sulfur compounds will bepresent. These will result from the natural source or (inthe case of natural gas) from the sulfur-based odorants,which are added for safety reasons. Sulfur is liable topoison or react with nickel in nickel-cermet-based anodematerials, and various claims have been made aboutpurely oxide anode materials, but for now it is prudent toerr on the side of caution. Thus, a desulfurization deviceis needed to reduce the sulfur content to as low a value as
Interface to the central supply and environment:System border line for energy balance
Hot water steam
Steamgeneration
Waste heatexploitation
Wastegas
PreheatingSOFC Stack
Housing/insulation
Catalytic burner
Preheating
Prereforming
Gas cleaning/ humidification
Gas supplysystem
Signals
InverterInternalconsumption
Air supplysystem
Measurement& control
Electric output AC AirFuel
Figure 8 Block diagram of a simple arrangement using a catalytic burner. SOFC, solid oxide fuel cell; AC, alternating current.
108 Fuel Cells – Solid Oxide Fuel Cells | Systems
is reasonably possible. Estimates as to the tolerable sulfurcontent vary, but for long-term operation sub-ppm valuesare probably more appropriate. At room temperature,this can be zeolites or activated carbon impregnated withmetallic promoters or, at increased temperatures of about300–350 1C, zinc oxide can be used. Other critical com-ponents can be tars or silane, which might be present invarious biogases. These have to be removed or crackedinto less critical compounds by additional cleaning steps.
For carbon-containing fuel gases, a certain amount ofwater vapor has to be added to avoid carbon formationduring heating up by disproportionation of carbonmonoxide via the so-called Boudouard reaction
2CO-Cþ CO2 ½I�
or by pyrolysis reactions such as
CH4-Cþ 2H2 ½II�
Here the most critical temperature range is between 500and 700 1C, at which methane has its highest carbonactivity, as can be seen from Figure 9. This requires thatthe S/C ratio should be higher than the nominal value of
1.5. Normally, a value of 2–2.5 is chosen to be on the safeside. A higher value would require more energy for va-porization and would further reduce the Nernst poten-tial, which means lower cell efficiency.
After humidification, hydrocarbon gases should be fedto a prereformer unit. This prereformer can be directlyheated or the gas can be preheated using the anode off-gas. A prereformer is absolutely necessary in cases wherethe gas contains higher hydrocarbons, as these have astrong tendency to crack when they are heated above500–600 1C even at higher S/C ratios. This means thatsoot is formed, which can block catalytic active surfacesor even block piping or channels inside the stack. Inaddition, cell tests at Siemens have shown that at least alow amount of hydrogen is necessary at the stack inlet;otherwise, the nickel anode tends to get destroyed in thegas inlet area. A prereforming rate of 10–20% was foundto be sufficient. This rate of reforming occurs at a tem-perature of about 450 1C, as can be seen from the equi-librium calculation presented in Figure 10. Owing to thekinetic behavior of the reforming process, the actual re-forming rate will always be somewhat lower than the onecalculated by thermodynamics.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
350 400 450 500 550 600 650 700 750 800
Temperature (°C)
Com
posi
tion
CH4
H2O
CO2
H2
CO
Figure 10 Equilibrium composition of methane/steam mixture (S/C¼2.5) as a function of temperature.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
100 200 300 400 500 600 700 800 900 1000 1100
Temperature (°C)
S/C
p =1 bar
Critical range for carbon formation
(carbon activity >1)
Figure 9 Critical steam to carbon ratio in the case of methane as a function of temperature and operating pressure.
Fuel Cells – Solid Oxide Fuel Cells | Systems 109
When leaving the reformer at about the reformedoperation temperature, whether or not an additionalanode gas heating stage is necessary is a question of stackdesign and of the stack’s mechanical properties. Owingto the low heat capacity of the anode gas, it should bepossible in most cases to avoid an additional heat ex-changer. After the prereformer heating stage, exhaustgases can pass on to the catalytic after-burner to combustany remaining fuel in the gas mixture.
At the air side of Figure 8, there is the air supplysystem, which includes the input-blower/compressor(depending on the required operation pressure in thesystem), flow control, valves for start up and shutdown,and safety valves. A standard air filter, normally used infront of a fan, is sufficient for cleaning the ambient air. In
special environments, e.g., high salt content at the seaside, additional measures may be required, but little workhas been done on this. One of the most bulky and costlycomponents in the SOFC system is the recuperative airpreheater. Using the cathode off-gas directly to preheatthe incoming air to the required inlet temperature re-quires a large heat exchanger, as only the temperaturedifference across the stack is available to heat up the coldair. An additional point to note is that the mass of cathodeoff-gas available to transfer heat is smaller than the inputmass; as a proportion of the oxygen has been removedfrom that gas inside the cell during cell functioning, thissmaller mass exacerbates the problem. These elementswill probably result in a large air preheater component,parameterized by the permissible temperature gradient
110 Fuel Cells – Solid Oxide Fuel Cells | Systems
across the stack. The advantage of this configuration isthat the highest temperature of the heat exchangeris determined by the stack outlet temperature, which isnormally lower than the outlet temperature of the after-burner, as described in concept 3. This lower temperaturereduces the thermomechanical and corrosion stress to theheat exchanger.
Cathode off-gases will probably leave the heat ex-changer at a temperature below 200 1C (dependent onDT across the stack) and are fed to the waste gas burner.In managing the air stoichiometry in the after-burner toextract as much energy as possible from the unspent fuelin the anode gases, a certain amount of the cathode off-gas can be bypassed and can directly fed into the heatingsystem. The relatively low temperature of the post heatexchanger cathode gases and the low calorific value of thedepleted anode off-gas mean that a catalyst needs to beemployed in the after-burner to assist ignition. The ad-vantage of this design is that the off-gas will always burn,independent of conditions and composition. This cata-lytic solution, however, has to be weighed against thecosts of using a noble metal catalyst material. In somecases, it might be advantageous to condense water out ofthe anode off-gas before sending it to the after-burner.This could increase the thermal efficiency of the system,if the condensation enthalpy of the condensed water canbe transferred into the heating system. This will bedependent on the actual temperature values in variousparts of the system.
The energy content of the off-gas leaving the burnercan be used to produce the steam necessary for re-forming. The remaining energy can be transferred to thedistrict heating system, producing hot water and/orsteam.
The produced DC current has to be transferred intogrid-compatible AC current using an inverter. For con-trol of the system and for data acquisition, appropriatecomponents are necessary.
Concept 2 – Basic Arrangement with a ThermalBurner
Figure 11 shows a modification of concept 1, using athermal burner instead of a catalytic burner. ‘Thermalburner’ here means that the gases are mixed at a tem-perature above the ignition point of hydrogen (560 1C),methane (595 1C), and carbon monoxide (620 1C), sothere is no need for a catalyst. However, a catalyst maystill be employed to enable cold start up.
The main advantage of this configuration is that theoutlet temperature of the thermal burner can be higherthan the stack outlet temperature (especially in the caseof planar stack technology, which aims at reduced op-erating temperature). The burner outlet temperature canvary between 850 and 1000 1C depending on the surplus
flow of air and fuel utilization. Combined with the in-creased mass flow, this results in a smaller size of airrecuperator. For example, the size can go down to one-third in the case of a hot temperature of 850 1C instead of700 1C, heating the cold air to 600 1C. A disadvantagemay be the increased corrosion stress inside the heatexchanger and the thermal burner itself.
Concept 3 – Anode Gas Recycling
An important part of a system’s complexity is the needfor steam production for the steam reforming reaction.This requires additional components and consumes quitea large amount of heat, most of which cannot be re-covered by the waste heat exploitation system. Thus, it isstrongly advised that systems use water, which is alreadyavailable in the system, namely that in the anode off-gas.This requires that a part of the anode off-gas is recycledand mixed with the cold fuel supplied to the system. Thisis illustrated in Figure 12.
As a second effect, this also means that a part of thefuel leaving the stack is fed back to the reaction zone,which increases the fuel utilization in the system. Howthe recycling ratio influences the fuel utilization can bederived from the following equation:
uF;sys ¼uF;S
1� R 1� uF;S
� � ½23�
where the recycling rate R is defined as the ratio betweenthe mass flow of the recycled fuel mF;R and the mass flowof the fuel mF;S;out leaving the stack:
R ¼ mF;R
mF;S;out½24�
Figure 13 shows the resulting system fuel utilizationuF,sys as a function of the recycling rate R in the case ofdifferent stack fuel utilizations uF,S. For reasonable stackfuel utilizations from 50 to 70% (which means a low riskfor too low fuel partial pressure somewhere inside thestack due to inhomogeneous flow distribution), systemfuel utilization in the range of 80–90% can be achievedusing an anode off-gas recycling rate between 65 and75%. For example, the Siemens 125 kW system em-ployed a recycling rate of 65% combined with a stackfuel utilization of 65% resulting in a total utilization ofabout 84%. This generates a significant improvement inoverall system efficiency.
It is not straightforward to source suitable componentsfor the recycling system due to the high temperature ofthe fuel off-gases. One possibility is to use a jet pump,driven by pressurized fuel entering the system. Siemensemploys this method, operating the jet pump at about700 1C. The advantage of a jet pump is that there are nomoving parts, which makes it quite robust and suitablefor high temperature. The disadvantages are a poor
Interface to the central supply and environment:System border line for energy balance
Steamgeneration
Waste heatexploitation
Hot water/steam
Wastegas
Preheating
Thermal burner
Housing/insulation
SOFC Stack
InverterInternalconsumption
Air supplysystem
AirElectric output ACFuel
Gas supplysystem
Gas cleaning/humidifications
Prereforming
Preheating
Signals
Measurement& control
Figure 11 Block diagram of a simple arrangement using a thermal burner. SOFC, solid oxide fuel cell.
Fuel Cells – Solid Oxide Fuel Cells | Systems 111
efficiency and a poor controllability under part-loadconditions. Another option is to cool down the anode gasfor recycling to a level at which rotary-type devices canbe operated. However, the disadvantage with this optionis that an additional heat exchanger is necessary, imply-ing extra cost and bulk.
Concept 4 – Pressurized Hybrid
As the after-burner off-gas temperature is above 800 1C,it is possible to use it to drive a turbine generatingadditional electricity. The corresponding block diagramis shown in Figure 14, where the off-gas of the after-burner is directly fed to the turbine that drives thecompressor for the air supply. The turbine off-gas is thenused to heat the incoming compressed air to the requiredstack inlet temperature. A directly coupled SOFC/gasturbine plant has to be operated at increased pressure toensure a sufficiently high SOFC off-gas pressure as itenters the turbine stage. To suit the operating conditionsof microturbines, a pressure increase of about 2–2.5 bar
should be used. There is an additional reason for thispressure level found in the overall thermodynamics. Ascan be seen from Figure 15, starting from 835 1C, ex-pansion in the turbine leads to an outlet temperaturebetween 660 and 730 1C, depending on the isentropicefficiency eis, of the turbine. Because the off-gas is thenneeded to heat up the cold incoming air to about 600–700 1C (depending on the stack design), a much higherpressure is forbidden. The hot off-gas expands in themicroturbine and drives the compressor and generator,which provides the additional electrical energy. As-suming a microturbine generator (MTG) efficiency of30% (which is the upper limit for such small low-pres-sure machines), the increase in electrical system effi-ciency as a function of cell voltage is shown in Table 1(based on rough estimations concerning parasitic con-sumption, which is comparably low, because the aircompressor is driven by the turbine).
A coupled SOFC/gas turbine system can yield anincrease in efficiency of between 15%-points (at 600 mV)and 9%-points (at 850 mV), whereas the total efficiency
40%
50%
60%
70%
80%
90%
100%
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Recycling rate (R)
Sys
tem
fuel
util
izat
ion u F
,sys
uF,S
= 50%
uF,S
= 60%
uF,S
= 65%
uF,S
= 70%
uF,S
= 75%
uF,S
= 80%
uF,S
= 90%
uF,S
= 100%
Figure 13 System fuel utilization uF,sys as a function of the recycling rate R in the case of different stack fuel utilizations uF,S.
Interface to the central supply and environment:System border line for energy balance
Hot water/steam
Wastegas
Waste heatexploitation
Thermal burner
PreheatingSOFC Stack
Housing/insulation
InverterInternalconsumption
Air supplysystem
Air
Preheating
Prereforming
Gas cleaning
Gas supplysystem
Measurement& control
Signals
Electric output ACFuel
Figure 12 Block diagram of an arrangement incorporating anode gas recycling.
112 Fuel Cells – Solid Oxide Fuel Cells | Systems
decreases with a lower cell voltage (from 68 to 57%).Also in this context, it is still an important research targetto further improve the cell behavior such that it cangenerate a high power density at high cell voltage.
The same is true for fuel utilization. The ratio ofturbine power and SOFC power compared to the totalpower is shown in Figure 16 in the case of fuel utilization
of 85% and in Figure 17 in the case of fuel utilizationof 70%.
The comparison of the total power output in the caseof different fuel utilizations shown in Figure 18 illus-trates that, in a pressurized hybrid system, the totalpower output decreases with lower fuel utilization,although the power of the MTG increases.
Interface to the central supply and environment:System border line for energy balance
Thermal burner
PreheatingSOFC Stack
Housing/insulation
Pressure vessel
InverterInternalconsumption
Air supplysystem
Air
Preheating
Prereforming
Gas cleaning
Gas supplysystem
Measurement& control
Signals
Electric output ACFuel
Ele
ctric
out
put A
C
Microturbinegenerator
Waste heatexploitation
Wastegas
Hot water/steam
Air compressor
Figure 14 Block diagram of an arrangement using direct coupling with a microturbine (pressurized hybrid). SOFC, solid oxide fuel cell.
Figure 18 Pressurized hybrid – electric system power related to gas input as a function of cell voltage in the case of fuel utilization of
70% and 85%.
Interface to the central supply and environment:System border line for energy balance
Hot water/steam
Wastegas
Thermal burner
PreheatingSOFC Stack
Housing/insulation
InverterInternalconsumption
Air supplysystem
Air
Preheating
Prereforming
Gas cleaning
Gas supplysystem
Measurement& control
Signals
Electric output ACFuel
Microturbinegenerator
Heat exchanger
Ele
ctric
out
put A
C
Waste heatexploitation
Figure 19 Block diagram of an arrangement using indirect coupling with a microturbine (atmospheric hybrid). SOFC, solid oxide fuel
cell; AC, alternating current.
Fuel Cells – Solid Oxide Fuel Cells | Systems 115
116 Fuel Cells – Solid Oxide Fuel Cells | Systems
A further positive effect of pressurized operation isthe increase of power density of the SOFC up to about20% (dependant on operating conditions). However, atthe same time, this pressurized hybrid operation requiresa higher effort/cost for the system components (e.g.,pressure vessel with hot feedthroughs) and system con-trol, because of coupling two systems with completelydifferent dynamic behaviors. Special measures have to betaken to avoid a rapid pressure drop because of the risk ofturbine failure that this poses.
The concept described as concept 5 tries to avoidthese problems.
Concept 5 – Atmospheric Hybrid
As illustrated in Figure 19, the gas turbine is not inte-grated into the gas loop of the fuel cell but is coupledindirectly via a heat exchanger. The hot off-gas leavingthe after-burner heats the driving gas for the turbine viathis heat exchanger. This enables an atmospheric SOFC
Measurement& control
signals
Preheating
Prereforming
Gas cleaning
Gas supplysystem
Housing/in
SOFC
Pressure
Thermal
Microtugenera
Interface to the central supply and environment:System border line for energy balance
Ele
ctric
out
put A
C
Fuel Electric outpu
Figure 20 Block diagram of an arrangement using direct coupling
SOFC, solid oxide fuel cell.
operation (which means a comparably simple system)and there is no direct influence of the turbine behavioron the fuel cell system. On the one hand, this gives moreflexibility in the layout of the turbine (e.g., the pressureratio can be higher) and, on the other hand, an additionalair fan (with additional power consumption) for theSOFC is needed and there is some additional tempera-ture loss from the heat exchanger. These effects lead to areduced gain in electrical efficiency, which is only halfthat of the pressurized hybrid.
For efficient utilization of the energy content of thewaste gas to produce electricity, the design conceptshown in Figure 20 can be adopted. In this case, wastegas leaving the air preheater is not used for districtheating but to produce steam, which in turn drives an
Inverter
sulation
Stack
vessel
burner
rbinetor
Steam turbineWaste
gas
waterElectric output AC
Preheating
Air compressor
Air supplysystem
Internalconsumption
t AC Air
with a microturbine and a steam turbine (pressurized hybrid II).
Table 2 Effect of coupling solid oxide fuel cell (SOFC) with a microturbine generator (MTG) and a steam turbine (ST) as a function
of cell voltage
Cell
voltage
(V)
Electrical
efficiency
SOFC
(%)
Heating energy
in off-gas
(behind
burner) related
to system input
(%)
Electrical
power of
MTG
related to
system
input (%)
Total
electrical
efficiency
(%)
Power
output MTG
in case of
1000 kW
gas input
(kW)
Heating
energy in off-
gas behind
MTG related
to system
input (%)
Electrical
power of
ST related
to system
input (%)
Total
electrical
efficiency
(%)
Power
output ST
in case of
1000 kW
gas input
(kW)
0.600 44.3 51.0 15.3 59.5 153 35.7 7.1 66.7 71
0.650 47.9 46.9 14.1 62.0 141 32.8 6.6 68.6 66
0.700 51.6 42.8 12.8 64.5 128 30.0 6.0 70.5 60
0.750 55.3 38.7 11.6 66.9 116 27.1 5.4 72.4 54
0.800 59.0 34.6 10.4 69.4 104 24.2 4.8 74.2 48
0.850 62.7 30.5 9.2 71.9 92 21.4 4.3 76.1 43
0.900 66.4 26.4 7.9 74.3 79 18.5 3.7 78.0 37
Input data: efficiency of ST 20%; efficiency of MTG 30%; fuel utilization of SOFC 85%; parasitic efficiency of SOFC 95%; inverter efficiency of SOFC 95%.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95
Cell voltage (V)
Ele
ctric
al p
ower
rela
ted
to g
as in
put
Total power
SOFC power
MTG power
uF = 85%
ST power
Figure 21 Pressurized hybrid combined with steam turbine (ST) – electric power of the different plant components related to gas input
as a function of cell voltage in the case of fuel utilization of 85%. SOFC, solid oxide fuel cell; MTG, microturbine generator.
Fuel Cells – Solid Oxide Fuel Cells | Systems 117
ST. Owing to the low waste gas temperature behindthe air preheater (probably less than 300 1C), even in thecase of a 30 1C condensation temperature the Carnotefficiency is only around 40%. Being an optimistic, an STefficiency of 20% can be predicted. The simplified cal-culation giving the latter number only indicates the ap-proximate potential of this system configuration. It ispossible to design more effective but complex arrange-ments; however, the resulting efficiency gains will besomewhere in the range shown in Table 2. An increase ofabout 4–7%-points is feasible, meaning a total electricalsystem efficiency of about 70–75% (see Figure 21). Whatcan also be seen from Table 2 is the size of the ST in thecase of 1 MW gas input. This would result in an STpower of about 40–60 kW. Realistically, the coupledSOFC/MTG/ST system is viable only for plants with agas input above 10 MW.
Conclusions
In this review, some basic arrangements of SOFC systemsare described, starting with atmospheric systems using acatalytic burner or a thermal burner and anode gas re-cycling. For illustrating the potential electrical efficiencyof SOFC systems, the combinations with a gas turbineand also with an ST are described. To be able to evaluatethe potential of the different systems, first the essentialefficiencies relevant for fuel cell systems are defined andthen the basics for calculating energy balance are illus-trated. Equations are given to describe, for example, theeffect of fuel recycling on system fuel utilization or theeffect of internal reforming on the necessary air flow forcooling the stack.
It is obvious that electrical efficiency strongly dependson cell voltage and fuel utilization. If cells that operate
118 Fuel Cells – Solid Oxide Fuel Cells | Systems
with a high fuel utilization at cell voltages of 800 mV areavailable, a net electrical efficiency above 55% can beachieved. The combination in a pressurized systemwith a gas turbine enables efficiencies of up to 70%and combining this system with an additional ST allowsefficiencies of up to 75%. However, as an investigationof the size of these STs shows, such combined systems(SOFC/MTG/ST) make sense only above a gas inputof 10 MW.
Nomenclature
Symbols and Units
cp
specific heat capacity at constant
pressure (J mol� 1 K)
F
Faraday constant: 96 485 As mol� 1
h
specific enthalpy (J mol� 1 or J g� 1)
H
enthalpy (J)
H
enthalpy of mass flow (J s� 1)
HCH4
enthalpy flow entering the reformer
(J s� 1)
H in
enthalpy flow entering the system
(J s� 1)
Hrea
enthalpy flow reacted in the stack
(J s� 1)
Href;out
enthalpy flow leaving the reformer
(J s� 1)
HS;in
enthalpy flow entering the stack (J s� 1)
IDC
current produced by the stack (A)
Iin
current that could be produced by the
gas entering the system (A)
Itot
current produced by the reaction
(current per cell� number of cells) (A)
LHV1
lower heating value at standard
conditions (J mol� 1)
M
molar mass
Mi
molar mass of component i (g mol� 1)
m
amount mass flow of fuel gas
mF;R
mass flow of recycled fuel
mF;S;out
mass flow of fuel leaving the stack
mi;in
mass flow of fuel component i entering
the system (kg s� 1)
min
mass flow amount of fuel entering the
system (kg kg s� 1)
mrea
mass flow amount of fuel reacted in the
cell (kg s� 1)
n
molar flow (mol s� 1)
p
pressure (bar)
PAC,gross
gross power behind the inverter (W)
PAC,net
net power behind the inverter (W)
PAC,parasite
electric power consumed by the system
(feeder behind the inverter) (W)
PC
compressor power consumption
PDC
DC electric power
PDC,gross
gross power in front of the inverter
(stack power) (W)
Pel
electric power
Pth,net
net used thermal power (W)
Q
heat (J)
Qref
heat flow to be supplied to the reformer
(J mol� 1)
QH2 ;rea
heat flow produced by the
electrochemical reaction of H2
(J mol� 1)
R
recycling rate
T
temperature (K or 1C)
T0
standard temperature (25 1C)
uF
utilization of fuel (%)
uF,S
stack fuel utilization
VLHV
LHV heating voltage (V)
Vp
(practical) cell voltage (V)
zi
equivalent number of component i
DH
reaction enthalpy (J mol� 1)
DT
temperature difference (K)
ec
cell efficiency
eel
electrical efficiency
eInv
inverter efficiency
eis
isentropic efficiency
ep
parasitic efficiency
es
effective or stack efficiency
eth
thermal efficiency
etot
total efficiency
k
coefficient of surplus air flow
Abbreviations and Acronyms
AC
alternating current
AN
compressor No. in flow scheme
BoP
Balance of Plant
CHP
combined heat and power plant
DC
direct current
LHV
lower heating value
MTG
microturbine generator
PAFC
phosphoric acid fuel cell
PEMFC
proton-exchange membrane fuel cell
SOFC
solid oxide fuel cell
ST
steam turbine
See also: Fuel Cells – Solid Oxide Fuel Cells: Internal
and External Reformation.
Further Reading
Blum L, Peters R, David P, Au SF, and Deja R (2004) Integrated stackmodule development for a 20 kW system. In: Mogensen M (ed.)Proceedings of Sixth European Solid Oxide Fuel Cell Forum, pp.173–182. Lucerne: European Fuel Cell Forum.
Finkenrath M (2005) Simulation und Analyse des dynamischenVerhaltens von Kraftwerken mit oxidkeramischer Brennstoffzelle
Fuel Cells – Solid Oxide Fuel Cells | Systems 119
(SOFC), vol. 44, ISBN 3-89336-414-5. Julich, Germany:Forschungszentrum.
Finkenrath M, Lokurlu A, Blum L, and Stolten D (2005) Modelling thedynamic behaviour of a planar SOFC CHP system. In: Mogensen M(ed.) Proceedings of Sixth European Solid Oxide Fuel Cell Forum, pp.569–578. Lucerne.
Gubner A (1996) Modelling of High Temperature Fuel Cells: TheThermal, Chemical, Electrochemical and Fluidmechanical Behaviourof Solid Oxide Fuel Cells Operating with Internal Reforming ofMethane. Thesis, University of Portsmouth, UK.
Gubner A, Landes H, Metzger J, Seeg H, and Stubner R (1997)Investigations into the degradation of the cermet anode of a solidoxide fuel cell. In: Stimming U, Singhal SC, Tagawa H, and LehnertW (eds.) Proceedings of the Fifth International Symposium on SolidOxide Fuel Cells (SOFC-V), pp. 844–850. Aachen: TheElectrochemical Society, Inc.
Hartvigsen JJ, Elangovan S, and Khandkar AC (2003) System design.In: Vielstich W, Lamm A, and Gasteiger HA (eds.) Handbook of FuelCells – Fundamentals, Technology and Applications, ISBN 0-471-49926-9, pp. 1070--1085. John Wiley & Sons, Ltd.
Larminie J and Dicks A (2000) Fuel Cell Systems Explained. ISBN0-471-49026-1. John Wiley & Sons Ltd.
Lundberg WL and Veyo SE (2001) Conceptual design and performanceanalysis of a 300 MWE LNG-fueled pressurized SOFC/gas turbine
power plant. In: Yokokawa H and Singhal SC (eds.) Proceedings ofSeventh International Symposium on Solid Oxide Fuel Cells, 3–8June 2001, Tsukuba, Japan.
Lundberg WL, Veyo SE, and Moeckel MD (2001) A High-EfficiencySOFC Hybrid Power System Using the Mercury 50 ATS Gas Turbine.ASME Paper 2001-GT-521.
Palsson J (2002) Thermodynamic Modelling and Performance ofCombined Solid Oxide Fuel Cell and Gas Turbine Systems. ISBN 91-628-5247-7. Lund Institute of Technology.
Sriramulu S, Teagan P, Stratonova M, and Thijssen J (2003) MW-classhybrid power system based on planar solid oxide stack technology.Proceedings of Seventh International Symposium on Solid OxideFuel Cells, 3–8 June 2001, Tsukuba, Japan.
Unverzagt G (1995) Energetische und wirtschaftliche Optimierung vonFestoxid-Brennstoffzellen-Kraftwerken; Jul-3159, ISBN 0944-2952.
Veyo, SE, Litzinger KP, Vora SD, and Lundberg WL (2002) Status ofPressurized SOFC/Gas Turbine Power System Development atSiemens Westinghouse. ASME Paper 2002-GT-30670.
Veyo SE, Lundberg WL, Vora SD, and Litzinger KP (2003) Tubular Sofchybrid power system status. Proceedings of ASME Turbo Expo2003: Power for Land, Sea, and Air. Atlanta, Georgia, USA. ASMEPaper 2003-GT-38943.
