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Layout of SOFC-GT cycles with el


									        Layout of SOFC-GT cycles with electric
                efficiencies over 80 %

                      Wolfgang Winkler and Hagen Lorenz
     fuel cells and rational use of energy, faculty of mechanical engineering
                           university of applied sciences
                         Berliner Tor 21, D-20099 Hamburg
           Tel. :+49-40-428 59-3137 /-4299, Fax :+49-40-428 59-2658


The thermodynamic reference cycle of any combination of fuel cells and heat
engines indicates an efficiency potential of 80 % for real cycles. The combination
of a SOFC and a gas turbine (GT) connects the air flow with the theoretical
independent heat engine supplied by the cell cooling. The waste heat extraction
of a SOFC module can be done by an intermediate expansion of the waste air in
gas turbines located after each SOFC sub-module of a divided SOFC module or
by an external cooling of the SOFC module by the flue gas cooled down by the
air and fuel heating. The combination of both principles leads to a reheat (RH)
SOFC-GT cycle that can be improved by a steam turbine (ST) cycle. The first
results of a study of such a RH-SOFC-GT-ST cycle indicate that a cycle design
with an efficiency of more than 80 % is possible and confirm the predictions by
the above mentioned theoretical thermodynamic model. The calculations show
that the influence of the system pressure decreases with the increasing efficiency
of the cycle by adding the ST cycle, caused by a better heat recovery of the ST
cycle at lower pressures. The size of the excess air has to be sufficient for the
electrochemical reaction. This indicates that the system should be operated with
an excess air of about 1,5 at temperatures of about 950 °C and at a maximal
pressure between about 15 to 20 bar to avoid higher pressures at higher
temperatures. The additional increase of the efficiency by the higher temperature
is comparable small and it seems that higher temperatures and pressures are
not satisfied. The addition of the ST cycle needs a minimum capacity of the cycle
of more than 10 MW to get the additional ST cycle commercial. Thus it can be
expected that the market entrance with small SOFC-GT units will happen without
an additional ST cycle and the RH SOFC-GT-ST cycle may become
commercially interesting later. But the RH SOFC-GT system alone might be an
interesting cycle for the market entrance because it allows an efficiency of more
than 70 % and delivers a waste gas with a very high temperature for different
industrial CHP applications without an alone power producing ST cycle.
         The thermodynamic principles of SOFC-GT design

The efficiency potential of combinations of fuel cells and heat engines has been
estimated with about 80 % by a generalised thermodynamic model already in
1993 (1). The U.S. Department of Energy has mentioned this figure for
combinations of SOFC and gas turbines in its announcement for research in
1999 (2). The principles of the design of combinations can be learned by the
generalised thermodynamic model : the generalised fuel cell - heat engine cycle
as given in (1), fig. 1.

                                                                       work         Fig. 1 The     generalised
                               heat engine                             fluid        fuel cell - heat engine
∆ϑmax                                -                                              cycle and the area and the
          fuel cell heat
                              fuel processing
                                                                                    tasks of the thermal
                                                               area of
                      preheater          waste heat
                                                          The fuel cell is only a
              fuel flue gas                               power      delivering  heat
                                                          source like a "power
    •     choice of heat engine and integration           producing burner" defined
    •     integration of fuel preparation
    •     integration of preheaters
                                                          by the ratio of delivered
    ⊗     restriction by ceramic cell : ∆ϑmax             power and heat in the area
                                                          of thermal engineering and
                                                                  W. Winkler 2000

the resulting engineering tasks are listed in fig. 1. The thermal integration of the
fuel processing is necessary to avoid entropy losses to the environment resulting
in efficiency losses in the order of 10 % as already shown in (3), (4). The
generalised cycle shows that the heat recovery process for the air heating and
the fuel heating is independent of the heat engine cycle. We could realise such a
cycle by using e.g. a Stirling engine as the heat engine. But if we use a common
gas turbine (GT) as a heat engine, we get a matching between the gas turbine
cycle and the air heating because the air flow of the fuel cell becomes a part of
the GT process as well. The design process of such a GT cycle is directly
determined by the restrictions of the thermal stresses of the SOFC. The maximal
allowable temperature difference ∆ϑmax between the inlet and outlet temperature
of the cathode e.g. 150 K delivers a very high air flow for the SOFC cooling only
by air. If we allow this, the waste gas loss will drastically increase and the system
efficiency can become lower than the cell efficiency itself.

Thus any successful cooling strategy of the SOFC of a SOFC-GT system must
avoid a high excess air at the outlet of the total system. Fig. 2 gives an overview
over the possible strategies. One strategy is to divide the SOFC module in sub-
modules and to extract the heat of the SOFC module by cooling down the waste
air of the first sub-module to the inlet temperature of the cathode of the following
sub-module by a gas turbine and producing additional power. This process of an
intermediate expansion (INEX) can be carried on until the last GT delivers the
waste gas for the air heater and the fuel heaters (HEX). The other strategy is to
  Intermediate expansion INEX :     External cooling EXCO : cool the SOFC module by
                                                            an external cooler (EXCO)
                    ° exhaust temperature
                                                            fed with the flue gas that
                               ¬ SOFC waste heat            has been cooled by the air
                               extraction (sub-systems)
                                                            and the fuel heating.

                                                                               Fig. 2 Cooling strategies
                                                                               of SOFC modules by GT

   - pressure difference HEX walls               fuel
                                                        The SOFC module is the
   ® air inlet temperature in SOFC
                                                 air    heat source of the GT
                                                 flue gas
                                                        cycle and the air is heated
   ¯ size of HEX surfaces                                   W. Winkler 2000
                                                        by the flue gas as shown
in the generalised model. This cycle is the result of the trial to use the cold air for
the cell cooling and to use the full temperature difference between compressor
and cell outlet (5). But the direct heat transfer to the cold air by the cell would
lead to the damage of the cell. The integrated gas heater/cell cooler can be
heated by radiation and this clearly reduces the danger of a cell damage by not
acceptable temperature differences. The integrated gas heater allows an
optimisation of the temperature level of the cooling flows around the cell together
with an integrated air heater and this avoids unacceptable thermal stresses of
the cell ceramic and disturbances of the electrochemical process.

The main differences between INEX and EXCO are listed in fig. 2 and compared
in fig. 3 (6),(7). The waste heat extraction ¬ is done in one pressure level in the
EXCO design. The waste heat extraction is done in up to n pressure levels in the
INEX design, depending on the allowable temperature difference of the cathode.
                                                      The number of pressure
                        INEX :         EXCO :         levels is equal to the
                                                      number of the pressurised
  ¬ SOFC waste heat     2 - n pressure 1 pressure
 extraction (sub-systems)   levels (systems)      level (system)

 - pressure difference      maximal pressure      only pressure               Fig. 3 Comparison of the
 HEX walls                  differerence          loss                        INEX and the EXCO design
 ® limit for air inlet      gas turbine outlet    SOFC
 temperature in SOFC        temperature               The pressure difference on
                                                      the heat exchanger (HEX)
                        min. 1/2,5 of  min. 1/7 of    walls mainly of the air
 ¯ size of HEX surfaces ambient system ambient system
                                                      heater - is the maximal
 ° exhaust temperature  ~ 200 °C       500 - 600 °C   pressure at the INEX design
                                                      and only the pressure loss
                                                            W. Winkler 2000

of the module at the EXCO design. The demands of an EXCO design on the
material quality for the heat exchangers is thus comparable small. The maximal
air heater outlet temperature ® is limited by the SOFC (module outlet)
temperature at the EXCO design and by the lower gas turbine outlet temperature
at the INEX design. The size of the HEX ¯ of an INEX design is about 2,5 times
smaller than under ambient conditions caused by the pressurisation at one side,
but the EXCO design has up to about 7 times smaller HEX surfaces than under
ambient conditions caused by the pressurisation on both sides (6). The electric
efficiency of an INEX design with two turbines is about 70 % (8) similar to the
EXCO design. But the exhaust temperature ° of the INEX design is about 200
°C and of the EXCO design is about 500 to 600 °C depending on the individual
parameters. The EXCO design has thus the potential for a combination with a
steam turbine cycle (ST) that could be e.g. a Cheng cycle. This would lead to an
electric efficiency of about 75 % (5).

                   The layout of a reheat (RH) SOFC-GT-ST cycle

The first ideas of the EXCO design included a reheat cycle (5) but with an
additional heat exchanger within the SOFC module. This design didn't seem very
easy to realise. But a comparison of the INEX and the EXCO design shows that
the benefit of the EXCO design to reduce the excess air in one process step at
one pressure level with small HEXs can be combined with the benefit of the
INEX design to allow a simple cascading of gas turbine cycles as needed for a
reheat GT- cycle. This led to the following proposal of the reheat SOFC-GT cycle
with a bottomed steam turbine (ST) cycle - the RH SOFC-GT-ST cycle - as
                                                         shown in fig. 4.
 compressor                            LP-SOFC module
                                                                   Fig. 4 The layout of the
                                                                   RH SOFC-GT-ST cycle

    HEX                               flue gas           The EXCO design is the
               HP-SOFC module                            first stage of the RH
     fuel gas            waste air                       SOFC-GT-ST cycle. The
     air                 turbine                         first GT, after the high
     waste air 1 stage                                   pressure section (HP-
     flue gas              steam cycle
     steam /water                                        SOFC module) as the first
                                                         stage, is named as "waste
                       Target : ηel ≥ 80 %               air turbine" to show that
                                                 W. Winkler 2000
                                                         the waste air is used as
the combustion air of the second stage. The second stage, the low pressure (LP)
section, doesn't need any external gas cooler because the comparable small LP-
SOFC module is cooled by a comparable high waste air flow coming from the
HP-SOFC module. The waste gas boiler of the ST cycle is supplied with the flue
gas of the last GT - the "flue gas turbine" - to use the waste heat of the cell in a
most efficient way.
This cycle has been calculated by a PC based SOFC-GT model with methane as
the fuel and a cell efficiency of 55 %. Some results are presented in the following
figures to give a first impression of the performance of this design. The
examinations of the RH SOFC-GT-ST cycle will be continued. Fig. 5 shows the
electric efficiency of the system (produced power related on LHV) of the RH
SOFC-GT cycle and of the RH SOFC-GT-ST cycle as well.

                             0,85                             SOFC module
                                                                                          Fig. 5 The electric efficiency of the
                                                              temperature in °C           RH SOFC-GT cycle and of the RH
                                                                                          SOFC-GT-ST cycle
   electric efficiency

                                                          The efficiency of the RH SOFC-GT
                                        RH SOFC-GT-ST cycle              1000
                                                          cycle depends more on the HP-
      0,75                                 1050
                                                          SOFC module pressure than the
                                                          efficiency of the RH SOFC-GT-ST
                                                          cycle. The results of the RH SOFC-
                                                          GT cycle are similar to the results
               RH SOFC-GT cycle                           of a RH GT cycle cooling the SOFC
      0,65                                                module as published in (5) in a
             5 10 15 20 25 30                             combination with a Cheng cycle.
                                                          But the new design shows a more
      HP-SOFC module pressure in
                                                          stable performance if the system
                                                          pressure is changed. It seems to be
                                                          a benefit to separate the RH-
ζsteam = 0,8                    H.Lorenz, W. Winkler 2000
                                                          SOFC-GT cycle and the ST cycle.
The effect of the addition of the ST cycle is that the optimal HP-SOFC module
pressure is reduced and the differences between the efficiencies at different HP-
SOFC module pressures at one certain temperature level are reduced as well.
However the influence of the SOFC module temperature is comparable small.
Thus near 80 % seems to be a border, as predicted.

The reason why the ST cycle reduces the differences in the system efficiency
                                                    can be easily understood
    1000                                            by fig. 6. Fig. 6 shows the
outlet temperature of flue

                                  SOFC module
                                                    influences on the outlet
     gas turbine in °C

     800                          temperature in °C

     600                             950
                                                    temperature of the flue gas
                                     1000           turbine that is the outlet
     400                             1050           temperature of the RH
     200                                            SOFC-GT system as well.
                                    5      10     15     20    25        30                                 Fig. 6 The outlet tempera-
                                        HP-SOFC module pressure in bar
                                                                                                            ture of the flue gas turbine
                                                                                                            depending on the HP-
                                                                                                            SOFC module pressure
                                                                                H.Lorenz, W. Winkler 2000
The outlet temperature of the RH SOFC-GT system allows a higher heat
recovery of the ST cycle for lower HP-SOFC module pressure levels than for
higher pressure levels at all temperature levels. The importance of the ST cycle
increases with a decreasing HP-SOFC module pressure.

                                                                                                  The above discussed analysis of
                                                                                                  the cycles is purely directed on
                                                                                                  the demands of heat cycles. But
                                2                                      SOFC module                it is still necessary to discuss the
                                                                       temperature in °C
                                                                                                  consequences of the optimisation
 Excess air

                        1,5                                               950                     of the process flows on the
                                                                          1000                    oxygen supply of the SOFC. Fig.
                                1                                         1050                    7 shows the excess air as a
                                                                                                  function of the HP-SOFC module
                                                                                                  pressure and the SOFC module
                        0,5                    impossible area
                                0                                                                 Fig. 7 The excess air depending
                                        5     10   15   20   25   30                              on HP-SOFC module pressure
                                        HP-SOFC module                                            and SOFC module temperature
                                         pressure in bar
                                                          The results showing an excess
                                H.Lorenz, W. Winkler 2000
                                                          air < 1 are impossible. However
                                                          the Nernst voltage is influenced
by the excess air too and thus the excess air in real plants must be clearly higher
than 1 (7). We see that the SOFC module temperature of 950 °C is a reasonable
figure for values of the excess air > 1,5. If we consider that the mentioned excess
air is after the waste gas burner and the excess air in the last SOFC is thus
higher we could assume that a SOFC module temperature of about 1000 °C
might be possible. This will depend on the mass transfer within the SOFC too.
 percentage of produced power

                                                                                                                The technical possibility of
                                                                                                                a design finally depends
                                                                                                                on the size of the unit. Fig.
                                40                                                                              8 shows the percentage of
                                                                                                                the produced power by the
             in %

                                                                                                                different power generating
                                20                      14,45               14,22                               components of the system.
                                10                                5,52
                                                                                                                Fig. 8 The percentage of
                                                                                                                power generation by the
                                            HP-SOFC LP-SOFC waste air       flue gas       steam                system components
                                            module  module  turbine         turbine        cycle
ζsteam = 0,8
                                                     The     SOFC      modules      H.Lorenz, W. Winkler 2000

deliver more than two third of the produced power in the case of the RH SOFC-
GT-ST system and about more than three quarters of the RH SOFC-GT system.
This shows that the optimisation of the steam cycle is an important part of the
system design to reach an efficiency target of more than 80 %. But if we need a
ST cycle we need a minimal size of capacity of the total system > 10 MW. Only
the demand of a very high efficiency > 73 % leads to an additional ST system
including water treatment and boiler operation etc. Thus it can be expected that
the market entrance with small SOFC-GT units will happen without an additional
ST cycle and the RH SOFC-GT-ST cycle may become commercially interesting
later. But the RH SOFC-GT cycle can be built for smaller capacities, depending
on the available gas turbines, with an efficiency over 70 % and the benefits of
delivering a waste gas with a very high temperature for different process
applications in CHP units for industrial applications and operating at a
comparable low SOFC module temperature.


The results of a study of a RH-SOFC-GT-ST cycle indicate that a cycle design
with an efficiency of more than 80 % is possible and confirm earlier predictions
by theoretical thermodynamic models. The calculations show that an additional
optimised ST cycle is necessary to boost the RH SOFC-GT technology. This
technology seems too costly for the market entrance with SOFC-GT systems
with a comparable small capacity as needed for a distributed generation. The RH
SOFC-GT-ST technology can be used later for units with a capacity > about 10
MW for a more centralised generation. But the RH SOFC-GT system alone
allows an efficiency of more than 70 % and delivers a waste gas with a very high
temperature for different industrial CHP applications at a comparable low SOFC
module temperature but without an only power producing ST cycle and it may be
interesting for a market entrance as well.


This work was done within the project : "Einsatz von Hochtemperaturbrenn-
stoffzellen (SOFC) in der Energietechnik " (aFuE - FKZ 1701998) funded by the
German Bundesministerium für Bildung und Forschung (BMBF). The authors want
to thank for this funding.


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