SYSTEMS DEVELOPMENT FOR PLANAR
SOFC BASED POWER PLANT
ETSU F/01/00195/REP
DTI/Pub URN 02/868
Contractor
ALSTOM Research and Technology Centre
Prepared by
S H Pyke
A J Burnett
R T Leah
The work described in this report was carried
out under contract as part of the DTI
Sustainable Energy Programmes. The views
and judgements expressed in this report are
those of the contractor and do not necessarily
reflect those of the DTI.
First published 2002
© Crown Copyright 2002
Further Renewable Energy information from the Sustainable Energy
Programme, and copies of publications can be obtained from:
Renewable Energy Helpline
Tel: +44 1235 432450
Email: [email protected]
EXECUTIVE SUMMARY
Solid oxide fuel cells (SOFC) are now the subject of development
programmes world-wide. The main focus of these programmes is in stationary
power applications, although considerable investment is also being made for
mobile auxiliary power units (APUs). The reasons for this interest are based
on several key advantages that SOFC systems potentially offer:
− Very high system electrical efficiency (>50%)
− Low emissions of harmful pollutants (NOx, SOx, unburned hydrocarbons,
etc)
− Fuel flexibility (can be operated on a range of hydrocarbon fuels without
significant loss of efficiency or the need for complex processing plant)
In order to realise this potential SOFC systems must demonstrate a major
reduction in costs compared to systems demonstrated to date. This will
require the design of a compact and highly integrated balance-of-plant system
and the development of low-cost SOFC stack technology. Reduced operating
temperature (<800°C) SOFC stacks based on anode-supported cells have the
potential to achieve major cost reduction both in the stack and system.
As part of a programme to develop SOFC system technology for distributed
power applications, ALSTOM Research & Technology Centre has coordinated a European Commission Framework 5 Programme project (acronym
“ProCon”). Project partners were Forschungszentrum Jülich (Germany) and
Prototech (Norway), whose work is also reported in outline here.
The aims of the ProCon project were to investigate the design of a SOFC
system based on intermediate temperature SOFC stack technology and to
design and test the performance of a SOFC stack based on anode-supported
cells. System design was focused on a 20 kW system since this size would
have the potential to be built as a demonstrator. However, the aim was to
design a system that could demonstrate the feasibility for commercially viable
SOFC power plant, that could have a power rating in the range 100-200 kWe.
The principal design targets for the system were that it should be compact
(target maximum: 2 m x 2 m x 2 m) and have high electrical efficiency
(>45%).
To meet these targets requires a reduction in the size of key components, such
as heat exchangers, afterburner and compressors, and a high level of
integration and performance optimisation. It also implies a significant level of
internal reforming of methane in the stack. Extensive process engineering
simulations were performed to derive a flow sheet for a system that could
potentially meet these targets.
The system designed included recycle loops for both the anode and cathode
exhaust gases. Recycling of the anode exhaust gas provides both steam and
heat for the pre-reformer feed gas, which eliminates the need for a large
external steam generator and reduces the size of the fuel pre-heater. Recycling
of air from the stack lowers the necessary air-flow, which reduces the amount
of heat ejected in the system exhaust and reduces the amount of heat that must
i
be transferred in the pre-heater. The system performance for the unit with
anode and cathode recycle is summarised in table 1i.
Result
Value
Units
DC electrical power
20.06
kW
Gross AC electrical power
18.66
kW
Net AC electrical power
16.83
kW
Useful heat
12.80
kW
Heat in fuel (LHV)
34.42
kW
Net electrical efficiency
48.9
%
Thermal efficiency
37.2
%
Overall efficiency
86.1
%
Power for air compressor
-0.831
kW
Power to run control system
-1.0
kW
Power loss in power conversion
-1.40
kW
Total heat loss through insulation
-2.23
kW
Cell voltage
0.715
V
Stack voltage
186
V
Current density
299
mAcm-2
Air mass flow
21.0
gs-1
Fuel mass flow
0.772
gs-1
Specific fuel consumption
165
gkWhe-1
Specific CO2 emissions
418
gkWhe-1
Table 1i: Summary of performance of system with cathode recycle at full
power
A dynamic model of the system was developed which enabled the
investigation of system operability and transient performance, such as loadfollowing capability. The model included simulation of the control system
required to operate the SOFC unit under varying load conditions, as well as
start-up and shut-down. The model indicated that the system was controllable
over the range 50-100% of rated power and that the load following capability
was better than a priori expectations.
As well as the ‘gas balance-of-plant’, a design for the power conditioning was
also developed that would meet the requirements for connection of the system
to the public electricity supply and that would interface with the particular
characteristics of the SOFC stack. Simulations indicated that the resulting first
generation design of power conditioning has excellent performance and can
regulate the load on the SOFC stack independently of the mains supply.
The SOFC design work by Forschungszentrum Jülich focused on the design of
a 20 kW SOFC module that would integrate into the system and the detailed
engineering of a 5 kW stack based on anode-supported cells. The stack
development, including materials and cell technology was performed outside
the ProCon project. The stack design, which employs 20 cm square anodesupported cells, features an internal manifold and parallel flow arrangement.
Stack tests with this design have achieved power densities over 0.5 Wcm-2 at
800°C. To date, materials degradation problems have delayed the 2,000 hour
ii
test of a 5 kW stack of design. This test is now scheduled to be completed in
late 2002.
The design and performance modelling results clearly demonstrate the
potential to achieve a compact, low-cost and highly efficient SOFC system
based on a stack operating at temperatures below 800°C. The next phase
should now be to prepare for a complete system demonstration. Work should
address the testing of all components and sub-systems and of the control
system. Further evaluation of the stack performance is required to inform
system design, including, operation with internal reforming and transient
performance.
Although the development of the SOFC stack was outside the scope of this
project, it is clear that in order to exploit the potential of planar, intermediate
temperature SOFC technology, development of the stack should focus on
development of a cost-effective design and increasing the durability.
iii
TABLE OF CONTENTS
1.
2.
3.
4.
5.
6.
7.
8.
9.
BACKGROUND ..................................................................................................2
OBJECTIVES ......................................................................................................4
FUNCTIONAL SPECIFICATION OF SOFC SYSTEM ................................5
DESIGN AND MODELLING OF A 20 KW SOFC SYSTEM........................6
4.1 Process flow engineering ................................................................................6
4.1.1 Anode gas recycling ..............................................................................6
4.1.2 Cathode gas recycling...........................................................................7
4.2 Conceptual design of the 20 kW integrated SOFC system .........................9
4.3 Dynamic modelling of the 20 kW integrated SOFC system......................10
SOFC SYSTEM POWER ELECTRONICS ...................................................12
5.1 SOFC Electrical Characteristics .................................................................12
5.2 The power conditioning system ...................................................................14
5.3 Optimisation of the power conditioning .....................................................15
DESIGN OF THE SOFC STACK....................................................................17
6.1 Modelling of the 20 kW module...................................................................17
6.2 Design engineering of the 5 kW SOFC stack .............................................18
CONCLUSIONS ................................................................................................20
RECOMMENDATIONS...................................................................................22
ACKNOWLEDGEMENTS ..............................................................................23
1
1.
BACKGROUND
Of the various fuel cell types currently under development, solid oxide fuel cells
(SOFCs) appear best suited to the demands of the stationary electrical power
generation market. SOFCs will offer the highest electrical efficiency when operating
on hydrocarbon fuels (in excess of 45%), so they can be expected to have the most
competitive running costs. They will also offer fuel flexibility and security of supply
since they can potentially be operated on a range of fuels, including pipeline natural
gas and bio-mass, without a significant loss of efficiency or increase in system
complexity and cost. With these attributes, and being essentially modular, SOFC
systems will be highly attractive for the distributed power market where units can be
configured and sized to meet a particular local power generation demand. High
system efficiencies for SOFC should also make the biggest impact in reducing
emissions of carbon dioxide which, combined with very low emissions of major local
air pollutants (CO, NOx and unburned hydrocarbons), will make them an extremely
attractive generation technology as tighter emissions legislation is implemented.
In order for SOFC systems to realise their potential in the stationary power generation
market, they will have to achieve substantial cost reductions compared to systems
demonstrated to date. The major cost saving for SOFC will be achieved by exploiting
the potential for making systems simpler and more compact than is possible for other
fuel cell types. Low-temperature fuel cells (PEMFC, PAFC, etc) use noble-metal
catalyst electrodes which must be fed with a high purity hydrogen fuel. This requires
a complex and expensive balance-of-plant (BOP) in order to provide the necessary
pre-processing for hydrocarbon fuels. The energy consumed in performing this
processing also limits the efficiency of these systems. By contrast, the high operating
temperature of SOFCs gives them the possibility of steam reforming methane to
hydrogen and carbon monoxide within the fuel cell stack by means of a CO-tolerant
nickel catalyst. This means that SOFC can potentially operate on natural gas with
minimal pre-processing of the fuel.
In addition to the simplified fuel processing, optimisation of SOFC system design can
result in much higher levels of component integration, a reduction in component
dimensions and minimisation of parasitic losses (heat losses, compressor
work/pressure drops, etc) in the system. This will not only reduce the capital cost of
SOFC systems but will also result in systems of high electrical efficiency.
The cost of the SOFC stack is clearly the other major factor in the overall system cost.
Work on reducing SOFC costs is focussed on planar stack and cell designs which
promise the lowest stack production costs since high-volume, low-cost manufacturing
processes, such as tape-casting, can be employed. Efforts to reduce the
manufacturing costs and improve durability of planar SOFC have resulted in
evolutionary modifications to the designs. The use of a metallic, rather than a
ceramic, interconnect promises significantly lower manufacturing costs. However,
the problems of high-temperature corrosion and the need to use readily available and
easily formed metals, such as stainless steels, require a reduction in the stack
operating temperature.
Currently, the most feasible way of enabling efficient, intermediate temperature
operation is to base the stack on anode-supported cells. Conventionally, planar SOFC
2
cells use the electrolyte as the structural support for the electrodes, which limits
minimum electrolyte thickness to around 100 µm. By using a thick, structural anode,
much thinner electrolytes can be used with a concomitant reduction in cell resistance.
Work at ALSTOM, in conjunction with the development programmes at
Forschungszentrum Jülich and ECN, indicates that this will allow stacks based on
anode-supported cells to operate at temperatures in the range 700-800°C compared to
the 800-900°C necessary for electrolyte-supported, planar cells.
The advantages of lower temperature stack operation should also extend to the costs
of the surrounding balance-of-plant components, such as heat exchangers, piping and
afterburners, since they too will not have to be made from high-cost, heat-resistant
alloys.
In order to investigate the potential for building an efficient, low-cost system based on
SOFC stacks operating in the temperature range 700-800°C, a study involving the
detailed design and performance modelling of an SOFC unit was performed. The
major cost elements of the BOP system are heat exchangers, reformer, afterburner and
compressors, therefore the focus of the BOP design was to reduce the size of these
components.
In parallel, the development of a planar stack with performance appropriate to the
system operation was conducted. The design and analysis was based on a nominal
20 kW SOFC system since this sizing would have the potential to be built as a
pilot-scale demonstration. The design, however, was also intended to be scaleable to
a larger system of, say, 100-200 kW, which would have wide commercial
applicability. The work described in this report was performed and funded within a
European Commission Framework 5 Project (acronym: “ProCon”). Project partners
were Forschungszentrum Jülich (Germany) and Prototech (Norway).
3
2.
OBJECTIVES
The aim of this project was to investigate the design and model the performance of a
system based on a SOFC stack module using anode-supported cells and, therefore,
with an operating temperature in the range 700-800°C. In parallel, the project aimed
to demonstrate the feasibility and performance of a planar stack based on
anode-supported cells under projected system operating conditions. Specific project
objectives were:
•
•
•
•
•
To assemble a 5 kW-scale planar SOFC stack
To design and build a suitably instrumented test rig for operation on reformed
methane
To test the performance of the 5 kW-scale stack during a 2000 hour trial
To design and model a conceptual 20 kW integrated system
To perform the detailed engineering of system components
4
3.
FUNCTIONAL SPECIFICATION OF SOFC SYSTEM
At the outset of the project, a functional specification for a 20 kW SOFC system was
derived. This provided for an SOFC system that would have performance and
attributes that would be attractive for a range of distributed power applications, rather
than a detailed specification for a particular application. As well as these
specifications the system had to be scaleable, based on a notional future systems
requirement in the range 100-200 kW. Whilst no cost targets were set, the system
designed clearly also had to address the requirement for low production costs.
Key parameters for the functional specification of the 20 kW system were:
−
−
−
−
−
−
−
−
−
−
electrical load driven, with heat a (useful) by-product
maximise electrical efficiency (target: 45% for 20 kW system)
overall efficiency 70%
turndown ratio: minimum operational load 50% (possibly lower for larger
systems)
start up time: 12 hours from cold, 3 hours from hot standby
shut down: safe and non-catastrophic in the event of loss of gas supply or grid
comply with requirements, for grid connection:
− 50 Hz
− 400 V (or 415 V)
− EN-50160 Power Quality Standard
siting indoors
compact (target maximum: 2 x 2 x 2 m3)
operation on three specified gas compositions with varying sulphur and higher
hydrocarbon content
5
4.
DESIGN AND MODELLING OF A 20 KW SOFC SYSTEM
4.1
Process flow engineering
The aim of this task was to derive and model plant concepts that would meet the
functional specifications. To meet these targets requires a high level of integration
and performance optimisation and implies a significant level of internal reforming of
methane in the stack. Reduction in the size of key components such as heat
exchangers is also a key requirement.
Process engineering for concept plant focused on the balance-of-plant layout but also
considered the projected requirements for control and power conversion and stack
module design and performance. In order that each of the partners could use the
output data from process flow modelling, it was important at the outset that data could
be generated and compared reliably and reproducibly. Since the project partners used
different software for process engineering, a validation case was defined, which was
simulated by all partners to check their compatibility.
The flow sheet for the validation case is shown in figure 1. In the system defined for
the validation case, inlet natural gas is mixed with a separate steam supply and passes
through a partial reformer before being fed to the SOFC stack. On the air-side, inlet
air is preheated prior to feeding to the stack. Exhausts from the fuel and air-sides of
the stack are then fed to an afterburner. The burner exhaust in this system is split to
heat the reformer and air in parallel. Further downstream, residual exhaust heat is
used to generate steam for the reformer and any remaining useful heat is used for
space heating.
The output data files for the three partners were compared and after clarification of
certain definitions, such as fuel utilisation, air ratio, etc., only small differences (≤1%)
remained. These were agreed to be tolerable for the ongoing simulation work. As
expected, the net electrical efficiency for the validation case of 35 % (gross 47 %) was
relatively low. In order to derive a system that was potentially compact and could
meet the target electrical efficiency, operating parameters were varied and modified
flow sheet designs were investigated. In particular, it was decided to:
− increase the fuel utilisation (max. 85 %)
− increase the degree of internal reforming (with a minimum of 10% H2 at SOFC
inlet)
Flow sheet design modifications included:
− anode gas recycling with an ejector (Oxygen/Carbon ratio = 1.8 minimum)
− cathode gas recycling with an ejector
4.1.1
Anode gas recycling
Where an anode gas recycle is adopted, much of the exhaust gas from the anode side
of the stack is recycled and mixed with fresh fuel before the mixture is fed to the prereformer. The recycling, generally, would be performed using an ejector (jet pump),
6
driven by the fresh fuel. It was assumed in this system that the natural gas was
supplied at 3 bar, and therefore no compressor work was required to drive the ejector,
meaning the energy to drive it was effectively free.
The principal reason for anode gas recycling is to eliminate the need for a large
external steam generator, although a small steam supply is likely still to be required
for system start-up. This is beneficial for the system thermal efficiency, since the heat
required to supply the steam would otherwise be extracted from the exhaust gases,
which would require an expensive boiler and a supply of deionised water. In addition,
the heat in the recycled exhaust gas provides much of the preheating required for the
fresh fuel, reducing the size of, or even eliminating the need for, a fuel pre-heater.
A further potential benefit is that there may be sufficient heat in the recycled anode
exhaust to enable a low, but sufficient, degree of external reforming (including
complete conversion of higher hydrocarbons) to be achieved in a simple adiabatic prereformer. This would eliminate the need for an externally heated pre-reformer, which
is a more complex and expensive component. However, there were doubts
concerning the effectiveness of an adiabatic pre-reformer under part-load operation,
so in the final design a heated pre-reformer was adopted. However, to optimise the
system efficiency the degree of pre-reforming was still relatively low, with most of
the methane reformed internally within the stack.
The addition of anode gas recycling to the system was shown to increase significantly
both the electrical and thermal efficiency of the system, with values of net electrical
efficiency as high as 48% at full load. A generic flowsheet of a system with anode
gas recycling is illustrated in figure 2.
4.1.2
Cathode gas recycling
The flow of air carries oxygen to the cathode and also removes excess heat from the
stack. The flow required for thermal balance is usually larger than the minimum
necessary to supply oxygen in the stack. A large net air-flow should be avoided since
it carries a greater proportion of heat out of the system at the final exhaust
temperature of 80 °C. Additionally, with increasing air flow, a larger fraction of the
power generated in the stack is consumed by the air compressor and a larger heat
exchange area is required to pre-heat the incoming air.
Recycling a proportion of the hot cathode exhaust reduces both the fresh air flow and
the amount of heat that must be transferred in the pre-heater. This allows a
significantly smaller air pre-heater to be specified with a corresponding cost saving.
The penalty is that, whilst fresh air flow is reduced, the ejector is pressure-driven so
additional power is required to drive the compressor.
Simulation results showed that with the lower net air flow and reduced heat exchange
in a system with air recycle, it is feasible to operate without an air pre-heater at the
design load. However, investigation of system operation on part-load showed that
this was not practicable since there is no margin of excess air that can be used to
reduce the flow.
7
The main benefit of designing the system with cathode gas recycle was an increase in
thermal efficiency and system compactness. The effect on electrical efficiency was
less significant than for anode gas recycle. Additionally, there was some concern
over the controllability of a system with cathode recycle under part-load and start up
conditions because of the non-linear variation in performance of the air-side ejector
with changing pressure.
The predicted performances of the systems both with and without cathode gas recycle
(both systems have an anode gas recycle loop) are summarised in tables 1 and 2.
Result
Value
Units
DC electrical power
20.29
kW
Gross AC electrical power
18.87
kW
Net AC electrical power
17.05
kW
Useful heat
9.65
kW
Heat in fuel (LHV)
34.42
kW
Net electrical efficiency
49.5
%
Thermal efficiency
28.0
%
Overall efficiency
77.5
%
Power for air compressor
-0.818
kW
Power to run control system
-1.0
kW
Power loss in power conversion
-1.42
kW
Total heat loss through insulation
-2.23
kW
Cell voltage
0.723
V
Stack voltage
188
V
Current density
299
mAcm-2
Air mass flow
54.10
gs-1
Fuel mass flow
0.772
gs-1
Specific fuel consumption
163
gkWhe-1
Specific CO2 emissions
413
gkWhe-1
Table 1: Summary of performance of system without cathode recycle at full
power
8
Result
Value
Units
DC electrical power
20.06
kW
Gross AC electrical power
18.66
kW
Net AC electrical power
16.83
kW
Useful heat
12.80
kW
Heat in fuel (LHV)
34.42
kW
Net electrical efficiency
48.9
%
Thermal efficiency
37.2
%
Overall efficiency
86.1
%
Power for air compressor
-0.831
kW
Power to run control system
-1.0
kW
Power loss in power conversion
-1.40
kW
Total heat loss through insulation
-2.23
kW
Cell voltage
0.715
V
Stack voltage
186
V
Current density
299
mAcm-2
Air mass flow
21.0
gs-1
Fuel mass flow
0.772
gs-1
Specific fuel consumption
165
gkWhe-1
Specific CO2 emissions
418
gkWhe-1
Table 2: Summary of performance of system with cathode recycle at full power
4.2
Conceptual design of the 20 kW integrated SOFC system
The conceptual design of the balance-of-plant (BOP) system was performed by
Prototech. The concept system was based on the process flow modelling discussed in
the preceding section and was for a system design that included recycling of both the
anode and cathode off-gases. The BOP layout for this system is shown schematically
in figure 3. The design of the system BOP focused on the major cost and size
elements which are the heat exchangers, reformer, afterburner and compressor(s),
since a reduction in size of these components will contribute not only to major cost
reductions but also to a more compact packaged system.
The complete system is illustrated schematically in figure 4, where it is separated into
three main sections: the stack module, the BOP module and the control and power
conditioning module. A photo-rendered impression of the packaged unit is shown in
figure 5. The system is designed for siting indoors, with a key objective therefore
being to make it compact. The unit designed has a total footprint of 1.6 m x 0.8 m =
1.3 m2 which is within the initial specification, of 2 m x 2 m.
Heat exchangers and the pre-reformer were designed for durable and stable operation,
and are therefore conservatively oversized in the concept shown with respect to the
number of plates and plate spacing. The BOP module and the stack module are
insulated to minimise heat losses by means of a 100 mm layer of high performance,
microporous insulation; total heat loss with this configuration was calculated to be
835 W at the nominal full electrical power output of 20 kW.
9
The criterion for the sizing of the pipework was that the gas velocity was generally
kept below 20 m/s to avoid unnecessary pressure drops. The heat exchangers were
dimensioned using the CFD software package Star-CD. The analysis included
temperature dependent density, viscosity and conductivity and was performed with a
3D mesh representing the real 3D geometry. The heat exchangers are of plate type
with dimensions 500 mm x 200 mm and a thickness of 1 mm.
For the case with no cathode gas recycle, the heat exchange is 380 W per plate,
requiring a total of 120 plates with a total area of 12 m2 (9.6 m2 net). For the system
with cathode gas recycle, because of the much higher temperature difference between
the streams and the significantly lower flow, the heat exchange is 1.3 kW per plate.
This requires a total of 3 plates with a total area of 0.3 m2. This large reduction in
size, and therefore cost, of the air pre-heater is the key advantage of employing a
cathode gas recycle loop. A photo-rendered impression of the smaller heat exchanger
is shown in figure 6 and the calculated temperature distribution is shown in figure 7.
4.3
Dynamic modelling of the 20 kW integrated SOFC system
A dynamic model of the 20 kW system was developed and tested by ALSTOM, using
a commercially available dynamic simulation tool, gPROMs. Two system concepts,
based upon the steady-state simulations by Prototech, were modelled; one system that
included cathode gas recycle and one without. The model includes simulation of the
control scheme required to operate the system under varying load conditions, as well
as start-up and shut-down.
To this end, the model developed is more detailed than the steady-state models, and
gives a greater insight into the operability of the system, particularly at part-load.
This is an important consideration, since it is quite possible to design a system which
works very efficiently on paper at maximum power, but which would be inoperable in
practice since it would be impossible to start.
Control loops, which relate the fuel supply to the applied stack current, regulate the
anode gas recycle loop and control the stack temperature, were simulated. These are
generally PI controllers, with a simulated supervisory PLC controller which modifies
their set-points according to the required power output. The stack temperature is
regulated by changes, either to the temperature or flow, of the air supply. The use of
the air supply to control stack temperature may cause difficulties in the control of the
system with cathode recycle at part-load, due to certain non-linearities in the recycle
ejector performance.
The following control objectives were specified:
− maintain constant stack temperature from 50 to 100% rated power
− fuel utilisation to remain constant at all power outputs
− oxygen / carbon (in the fuel) ratio at the pre-reformer inlet always to exceed 1.8 in
order to avoid the potential for carbon deposition
− water temperature to remain constant at 80°C
− exhaust temperature not to fall below 50°C
− oxygen (in air) / fuel ratio, lambda, at the stack inlet always to exceed 2.0
10
With both systems it was possible to meet these objectives, although the method for
stack temperature control differs. Using the model, it has been demonstrated that it
should be possible to regulate the stack temperature to within a few degrees over the
range 30-100% power by careful regulation of the air supply. This is largely as a
result of the high thermal mass of the stack, which means that any temperature
changes occur slowly. This indicates that the dynamic performance of the SOFC
generator system can be predicted to be relatively fast. In particular, the dynamic
performance of a planar SOFC system is predicted to be significantly better than a
tubular type system since, in this planar design, the stack is much more compact, and
therefore has a smaller inventory of reactants. Also the thermal conductivity of the
metallic, planar stack components is much higher than the ceramic components of a
tubular stack, which should help to prevent large temperature gradients occurring
within the stack.
Simulations have also been performed on a system cold start, and to this end
additional plant items (a steam generator, an external heat supply to the stack and
various bypass valves) have been provided.
Further simulations have also been performed to investigate the control and
component requirements for putting the system into ‘hot standby’. This is important,
for example, during a temporary loss of the mains supply, since it would avoid the
need for a complete shut-down of the system and would allow it to re-start exporting
power with only a short interruption.
It is intended to submit a patent to cover much of this work, so the details provided
here are limited. However, typical results are illustrated in figures 8 to 12. Figures 8
and 9 show the predicted electrical and thermal power outputs of the systems with and
without cathode gas recycle during a simulation where the load was reduced in steps
from 100% to 50% power. It can be seen that the electrical power output is predicted
to closely follow changes in current. This is largely because the control enables the
stack to be maintained at a constant temperature, as discussed earlier. The thermal
power output is shown to take longer to react to load changes. It can also be seen that
the thermal power output of the system with cathode recycle is somewhat higher than
that without recycle, since less heat is rejected in the exhaust.
Figure 10 shows the changes in electrical and thermal efficiencies with reduced power
output. Down to 50% of rated power output, electrical efficiency is seen to rise as a
result of the lower losses when operating the stack at lower current density and,
therefore, higher voltage (figure 11). Figure 12 shows the results of a start-up
simulation.
11
5.
SOFC SYSTEM POWER ELECTRONICS
The power conditioning for an SOFC power generation scheme sets significant
technical challenges in order for it to interface with the unique characteristics of the
SOFC stack and to achieve the required performance within a compact and low-cost
solution. The power conditioning also represents the for the SOFC system
owner/operator’s interface to the grid, therefore an understanding of the electricity
generating and distribution system can usefully serve to identify and guide the
complete equipment requirements towards a design that meets the end applications.
As part of the detailed investigation of the 20 kW SOFC system, a paper design for a
power conditioning system has been created and modelled to identify the major issues
and their proposed solutions. The modelling work includes detailed representations of
the power electronics elements, the transformer and the public electricity supply
(PES) to which the system is connected. The SOFC has been modelled as an
approximation of the predicted V-I characteristics as there is insufficient actual
dynamic electrical performance data to model it in detail at the present time. A
complete set of studies has been performed on the model to demonstrate the
performance of the system supplying into a PES with local load. This is considered
the most demanding case in terms of the design of the power conditioning and
performance has been found to be in accordance with the required specifications.
The key findings of the work are presented here. The current understanding of the
SOFC characteristics is discussed briefly since this imposed certain constraints on the
approach taken for designing the power conditioning. The power conditioning is
described and a discussion presented on the available system options and why the
chosen configuration was adopted.
5.1
SOFC Electrical Characteristics
The ionic transport occurring within a SOFC has led to an analogy being drawn with
the operation of a battery and, whilst this is a helpful concept, the SOFC differs
significantly in its own specific characteristics. The dynamic performance of the
SOFC is limited (not withstanding the discussion in section 4) and requires that the
current drawn must carefully regulated and must not rise or fall too rapidly.
The active cell area determines the current capacity of a stack with a typical
operational current density being of the order of 0.5 Acm-2. Each cell layer of a stack
will produce an open circuit voltage of 1.05V that falls in a broadly linear manner
with increasing current down to a set to a minimum that is typically 0.6V. Operation
below this minimum voltage should be avoided (certainly for extended periods) since
the mass transport limitations of the electrochemical reaction would tend to cause the
anode of the cell to be electrochemically re-oxidised which will degrade cell
performance and ultimately shorten stack-life. The planar stack geometry and the cell
area under investigation in this project resulted in a 20 kW stack module that
produced similar orders of voltage and current output at the maximum power point.
This is considered as a relatively low voltage and high current source for conventional
power electronics.
The SOFC balance-of-plant (BOP) has a control bandwidth (of the order of seconds)
that is much lower than the fluctuations in power demanded from the connection to
12
the PES (100s µs) and it is important that the BOP control is maintained in such a way
as to protect the SOFC module within it. Therefore the electrical power drawn from
the stack must be controlled so that it is kept within the transient capabilities of the
BOP. Constraining increased PES demands in power is a matter of control but extra
power conditioning plant is required to absorb the excess SOFC output power for
periods when the PES is unreceptive so that the load change can be smoothed out to
the BOP capabilities.
Operation of a SOFC stack in the laboratory is usually performed with a load that
draws a steady d.c. current. However, typical power conditioning equipment contains
switching elements that will draw the current in pulses. The variation imposed on the
d.c. current by the pulsing, termed ripple current, is expressed as a percentage of the
full load d.c. current. Therefore for a SOFC stack connected to power conditioning
equipment, the direct effect of the ripple current would be that it becomes
superimposed on the SOFC stack voltage and causes the cell voltage to transiently
peak and dip above and below the level normally required for the given output power.
It is known that a large ripple current will reduce the maximum power output
available from the SOFC stack but apart from this little is known about the dynamic
electrical performance, particularly with regard to long-term effects. For the purposes
of this study the power conditioning was designed to minimise the ripple current that
was drawn from the SOFC stack. A further consideration was that back flow of
power into the SOFC stack is also potentially damaging therefore the power
conditioning had to be designed to prevent this occurring.
The specification for a 20 kW SOFC stack that formed the basis of the design
requirements for the power conditioning is shown in table 3. It can be seen that the
SOFC as a power source imposes specific requirements on the design of the power
conditioning. The ripple current requirement is particularly onerous and will
currently preclude the use of most of the compact, high frequency power converters.
1
2
3
4
5
6
7
8
9
10
11
12
Fuel cell dimensions
Number of cells in the stack
Output power
No load voltage
Operating voltage
Operating current
Maximum permissible ripple current
Maximum rate for a large change in current
Maximum step of current
Nominal system parasitic losses
Additional losses for hot standby
Minimum power for stack self heating
Table 1: SOFC specification for power conditioning
13
20x20cm
260
20kW
220V
185V
108A
3.9A
1A/s
5A
1.28kW
2.4kW
3.68kW
5.2
The power conditioning system
Figure 13 is a block diagram showing the basic SOFC power conditioning system that
will connect the SOFC to the three-phase PES. The form is not dissimilar to that used
for a photovoltaic power generating plant where the key component is the d.c. to
three-phase 50 Hz static invertor. There are several ways in which the power
conditioning system can be configured. The reasons for the system topology chosen
are summarised as follows:
− The voltage output from the 20 kW power level SOFC module requires a voltage
step up to reach the PES voltage of 415 V a.c. rms
− Connecting the invertor to the SOFC stack directly will not comply with the
SOFC stack requirements of ripple current or power back feed
− The ‘power system matching block’ was therefore added between the two and
selected to optimise the loading requirements for the SOFC stack
− This restricts the operation of this converter such that it cannot currently provide
isolation or the full voltage step up
− The best compromise is therefore to step up to an intermediate voltage suitable for
the optimum utilisation of the invertor semiconductors
− A 50 Hz transformer was added downstream of the invertor to provide the balance
of the required step up to the PES
The system described above was designed as a first-generation system that is most
certain of providing the correct performance. It is not the only configuration possible
and lacks some of the optimisation that could be achieved with other more integrated
solutions. In this design, the positioning of the transformer enables it to: provide the
required galvanic isolation for safety; help with filtering out the electrical harmonic
noise from the invertor and block any d.c. current passing from the invertor into the
PES, which are all important legal requirements. However, the major disadvantage is
that 50Hz transformers are large and heavy. Indeed, if the power semiconductors are
water-cooled then the transformer becomes as big as the rest of the power
conditioning.
Since the SOFC output power is not capable of varying rapidly, then the main
disturbances to the system will come from the PES. The supply of power to the PES
can be made to operate at an appropriate bandwidth, so to account for small variations
is a matter of control. The problem occurs where a significant voltage dip occurs in
the PES or the supply becomes disconnected from the PES and it will, therefore,
become unreceptive to power. This requires a load to absorb the excess power from
the SOFC during the interruption or until the SOFC output power can be ramped
down. Energy storage can be used but batteries are not appropriate because they
cannot absorb the power at a sufficient rate. New ultra capacitor technology can
provide sufficiently high capacitance at an appropriate volume but is currently too
expensive. Therefore, the only viable alternative is a resistive load-bank producing
waste hot water. This is has been incorporated into the system model and found to
operate satisfactorily.
The control was configured to set the invertor to pass the power from the SOFC to the
mains. The power system matching block has to regulate the d.c. link and is therefore
transparent in terms of the power transfer process, i.e. the power out is the same as
power in (less losses) but at a fixed voltage. This transfer must be carried out in such
14
a way that the SOFC voltage does not dip excessively and without causing any rapid
changes of current. A control scheme was derived to use the current drawn by the
invertor in combination with the feedback from the load bank to regulate the SOFC
terminal voltage against a desired set point.
Figure 14 shows an example case where regulation is taking place. In this case, the
system was started up and then a 90% voltage dip of 10% of the mains cycle was
created on the PES after 50 ms, in order to disturb the system. It can be seen that this
created slight repetitive “notches” in the SOFC current and voltage waveforms but
was otherwise insignificant because the control instantly diverts the SOFC power that
cannot be absorbed into the load bank. The start-up also shows how well the SOFC is
controlled because the slopes of the waveforms are smooth gradients that are within
the BOP limits. It should be noted that the initial uncontrolled transient is a function
of the modelled system and will not be present in practice.
Work on the dynamic model of the BOP, described in section 4, included results of a
simulation to put the system into hot standby to avoid a complete system shutdown in
the event of temporary interruption of the load. The requirements for controlled ramp
down of the power output of the SOFC to a hot standby in the absence of the mains
power supply have been investigated in the design and modelling of the power
conditioning. The results indicate that, regardless of the PES state, the power
conditioning can be successfully integrated with the BOP to supply the parasitic
system loads (compressors, pumps, control electronics, etc) and maintain the
appropriate operating conditions for the SOFC stack and BOP.
5.3
Optimisation of the power conditioning
The design basis, as set out earlier, for the power electronics was set by certain
assumptions about the performance and protection requirements for the SOFC stack.
With better understanding, which can only be obtained in the light of practical
verification, it may be possible to relax some of the requirements for the power
conditioning to produce a lower-cost system. The justification for, and design of, the
power system matching block hinges on the electrical impedance of the SOFC stack
so a key priority is to confirm this by performing a practical evaluation of the ripple
current and transient performance of the stack.
The requirements for a more compact power conversion system have also been
considered and are briefly discussed. A more compact position for the transformer
would be within the power system matching block because it can operate at high
frequency and therefore would typically offer a 50% reduction in size and an 80%
reduction in weight but would require a more ambitious converter design. Since
smaller stacks are unlikely to reach the voltage for direct driving to the PES then
development of this improved converter would be particularly attractive for a compact
SOFC system.
Much larger SOFC stacks have the potential to achieve the voltage level for driving
the PES directly but may still require the isolation provided by a transformer and
therefore a high frequency option is still attractive. It should also be noted that the
high frequency transformer technology is only available up to 40 kW per device so
interleaving of multiple stages will certainly be required for the larger power ratings.
15
Recommendations for further work
− Evaluate the SOFC stack source impedance and incorporate the findings into the
model to reassess the proposed design
− Carry out the detailed circuit design of the power electronics elements
− Bread board test each of the sections to confirm correct operation
− Detailed design of the controls including process synchronisation and PES
interfacing protocol
− Bread board test the complete system to confirm the operation
16
6.
DESIGN OF THE SOFC STACK
Work within ProCon by Forschungszentrum Jülich addressed both the design and
performance modelling of the SOFC module for integration into the 20 kW system
(this may comprise more than one stack and includes the necessary gas connections to
the stack or stacks) and also the engineering of the 5 kW stack. Development of the
cell and stack technology was performed in parallel programmes.
6.1 Modelling of the 20 kW module
Stack modelling was performed first for one single stack based on the SOFC stack
concept using anode-supported thin electrolyte layer cells, in order to show the
feasibility of this technology in attaining stacks operating below 800 °C with
reasonable power densities. Figure 15 shows the resulting current density and
temperature distribution in one layer of this stack and a summary of the main results
of the calculations. The stack was assumed to be operated on 30% pre-reformed
methane fuel, i.e. 70% of the methane is internally reformed, with air and fuel inlet
temperatures of 700°C. For a fuel utilisation of 70%, the stack attained an average
power density of 0.21 W/cm², where the maximum temperature did not exceed 800°C.
The 20 kW SOFC module was designed to comprise several smaller stacks. Options
for combining these stacks in parallel and/or in series were investigated. One of these
options was a so-called cascade of stacks. In the cascade, the fuel flows through a
series of stacks: the exhaust fuel from two stacks in one stage is combined and fed to
one single stack in the next stage. Fresh air is supplied in parallel to all stacks in all
stages. An advantage of this arrangement is that the fuel utilisation in the stack
remains low, whereas the overall fuel utilisation still can reach values above 60%.
However, the overall power output of the cascade arrangement was not substantially
increased in comparison with a single stack operating under the same conditions.
Similar results were obtained for another option where both air and fuel flowed
through a series of stacks. This latter arrangement also showed the disadvantage of
high temperature air being fed into stacks in the next stage, raising the temperature
levels to undesirably high values. Moreover, the series connection of gas flows leads
to higher pressure differences, which would require more work by the compressors,
leading to higher electrical power losses in the system.
For these reasons it was decided to make the arrangement for the 20 kW module with
four stacks of 5 kW each, which are all fed with air and fuel in parallel.
Additional modelling of the 5 kW stack was performed, in particular at partial load
levels. The 5 kW stack was design to contain 65 cells of 20 cm x 20 cm each
(effective electrode area 19 x 19 = 361 cm²). For these calculations the fuel gas
composition exiting the adiabatic pre-reformer (derived from the process flow
modelling) was taken as input for the stack at a temperature level of 650°C. The
results of the calculations are shown in figure 16. In the first series the air
stoichiometry was kept constant at a value of 3.7, in the second series it was gradually
lowered with decreasing load, but kept at or above the minimum value of 2. The
results indicated that the minimum stack temperature was always in the range of 720
to 740°C, more or less independent of the load and the air stoichiometry. The
maximum stack operating temperature was in most cases close to 800 °C, except for
17
the cases with 50% partial load and 25% partial load, in which the cooling effect of
the air flow was evident.
6.2
Design engineering of the 5 kW SOFC stack
The cell and stack technology for the 5 kW SOFC stack was developed in parallel by
Forschungszentrum Jülich in the ongoing 'ZeuS'-project, funded by the German
Ministry for Economic Affairs (BMWi), in which ALSTOM also co-operated in the
DTI-supported project F/01/00195.
The established Forschungszentrum Jülich stack design was a block comprising
substrate cells and interconnect plates that was placed inside a metallic housing
constituting the external manifolds. This design allows the stacks to be operated in a
cross-flow configuration only. To counter the problems originating from the severe
demands on the sealing materials, the design was modified. In the new design with an
internal manifold, the sealing surfaces are all in planes perpendicular to the stacking
direction. A number of short-stacks were tested in this so-called D-design with crossflow configuration.
Adopting an internal manifold design opens the possibility to change the gas flow to a
parallel flow (either counter-flow or a co-flow) configuration, which was given the
designation E-design. Modelling calculations show that temperature distributions
across the cells are more symmetrical and temperature gradients are lower in this
layout. For the E-design with internal manifold and counter-flow configuration,
extensive modelling of the gas flow was performed to optimise the geometry and the
dimensions of the feed tubes, the manifolds and the gas channels in the interconnect
plate.
A schematic view of the stack design is shown in figure 17. In the counter-flow
arrangement, air and fuel gases enter via the base plate of the stack through the two
outer tube sections. In each stack layer, gases enter an enclosed plenum and are then
distributed across the entire surface of the anode or cathode of the single cell before
flowing to the exhaust plenum from where the gases exit the stack via the central pipe
section. The anode and cathode facing sides of the interconnect plate are identical.
The feasibility of this stack design was first demonstrated with short stack tests using
10 cm x 10 cm cells. The stack design for the ProCon project was based on a
20 cm x 20 cm cell size. Forschungszentrum Jülich has performed a series of tests
with this stack design. For example, a test has been performed with a 10-cell stack.
At an operation temperature of 800°C this stack gave a power output of 1.6 kW
(220 A @ 7.30 V; i.e. 0.61 A/cm² and 0.44 W/cm²) with humidified hydrogen at a
fuel utilisation of 44%. At 880°C the power output was 2.4 kW (340 A @ 7.14 V;
i.e. 0.94 A/cm² and 0.68 W/cm2) at a similar fuel utilisation of 47%. Performance of
this stack design has also been demonstrated on methane. The results demonstrate
that the stack design can meet performance requirements for the system with internal
reforming.
To date, however, in these tests, it has been difficult to achieve stack lifetimes better
than 700 hours with the existing ferritic stainless steel interconnect material (DIN
1.4742). This is principally believed to be a result of oxidative degradation at the
interconnect/cathode interface. Plasma-sprayed anti-corrosion coatings on this
18
stainless steel produced by the German national aerospace research institute, DLR,
have not resulted in the expected decrease in degradation. As a result, the proposed
2,000 hour test of a stack on reformed methane has been delayed. Parallel
investigation of ferritic stainless steels is ongoing and a candidate material has been
developed which should provide the necessary increase in stack lifetime. It is
intended that this material will now be used to make the interconnect plates that will
be used for the 2,000 hour test.
19
7.
CONCLUSIONS
The complete conceptual design of a 20 kW system based on a SOFC module with a
maximum operating temperature of 800°C has been developed and its performance
modelled. The design and the performance meet the functional specification set for
the system. Key achievements of this design are:
− Compact (footprint for the packaged unit = 1.6 m x 0.8 m)
− Potentially low-cost
− High electrical efficiency (49% at full load, up to 55% at half rated power output).
Given the data and assumptions used, it is reasonable to expect that when scaledup to a power range 100-200 kWe a system of this design could attain an electrical
efficiency exceeding 55%.
The targets were reached by designing a highly integrated system with a high level of
internal reforming. Recycling of anode and cathode exhaust gases was integrated into
the balance-of-plant and enabled significant reductions in heat exchange area and
eliminated the need for a separate steam generator for fuel reforming (other than for
start-up). Designing for low pressure-drops across the entire system minimised the
parasitic losses and contributed to the high system efficiency.
A realistic dynamic model of the SOFC generator has been developed using suitable
software. The model has been used to simulate the performance of systems with and
without cathode gas recycle. The results of the dynamic model show close correlation
with steady state simulations.
The dynamic model has enabled the testing of different control strategies, and allowed
the simulation of the dynamic response of the system to external changes. The results
indicate that the proposed system is controllable over the specified range of 100-50%
electrical power, and that the dynamic response to load changes is fast. It is also
possible to control closely the stack temperature, minimising thermal stresses on the
stack during dynamic operation.
It has also been demonstrated that at least one of the proposed systems can be easily
and relatively quickly started from cold, an important aspect for any practical system.
A hot standby mode has also been demonstrated.
It has been shown that a system with cathode recycle gives a more efficient and
compact overall system but the control is more complex.
A design for the power conversion has been developed that allows for the particular
attributes of the SOFC and meets the requirements for connecting the SOFC system to
the grid.
A converter system has been designed to provide power conversion into the mains
supply for a solid oxide fuel cell. This has required both the specifications for the fuel
cell and the specifications for the mains supply to be identified so that a power
conversion system could be devised that can successfully marry the two. The
following conclusions can be drawn from this work:
20
The design of a planar SOFC stack based on anode-supported cells has been
completed. The planar stack design, which incorporates an internal manifold
structure and a parallel flow arrangement, has been tested with 20 cm x 20 cm cells.
Outputs over 0.5 Wcm-2 have been reached at 800°C on hydrogen / 3% steam.
Internal reforming with this stack has also been demonstrated.
The degradation rate of the stacks is, currently, relatively high. The probable
explanation for this is corrosion of, and Cr-evaporation from, the ferritic stainless
steel interconnect plate. Deterioration in the performance of the cathode current
collection is also a problem. As a result of this degradation the testing of a 5 kW
stack for 2,000 hour on reformed methane was delayed. It should be noted that
investigation and prevention of performance degradation lay outside the scope of this
project.
21
8.
RECOMMENDATIONS
The design and performance modelling results clearly demonstrate the potential to
achieve a compact, low-cost and highly efficient SOFC system. The logical next
phase is the building of a complete system demonstration. In preparation for the
system demonstration phase, the following areas should be addressed.
− Build and test balance-of-plant components and sub-systems to verify their
performance
− Implementation of the balance-of-plant system control developed in the project
− Detailed circuit design of power electronics elements
− Detailed design of the controls including process synchronisation and public
electricity supply interfacing protocol
Evaluation of stack performance with respect to predicted system operating
conditions, in particular:
− Operation with internal reforming
− Transient performance
− Evaluate stack source impedance to inform power conversion design
Finally, although the development of the SOFC stack was outside the scope of this
project, it is clear that in order to exploit the potential of planar, intermediate
temperature SOFC technology, development of the stack should focus on:
− Cost-reduction
− Durability
− Mechanical reliability
22
9.
ACKNOWLEDGEMENTS
The work of the SOFC team at ALSTOM Research & Technology Centre in support
of this work is acknowledged.
The work reported here includes the input of the project Partners who are thanked for
their co-operation throughout the course of the project. In particular the work of the
following individuals is acknowledged: from Forschungszentrum Jülich; Bert de
Haart, Ludger Blum, Ernst Riensche and Dieter Froening and from Prototech; Arild
Vik and Paal Bratland.
23
Figure 1: Process flowsheet for the 20 kW SOFC system validation case.
24
Figure 2: Flowsheet for a SOFC system with anode recycle loop.
25
air pre-heater
air compressor
steam
generator
pre-reformer
afterburner
water heater
Figure 3: Balance of plant layout for 20 kW SOFC system
26
air compressor
pre-reformer
air heater
4 stacks
afterburner
water heater
steam generator
power conversion
and system control
Figure 4: Schematic layout of 20 kW SOFC system
Figure 5: Photo-rendered impression of packaged 20 kW unit with footprint
1.6 x 0.8 m2.
27
Figure 6: Photo-rendered impression of air-side heat exchanger for system with
cathode gas recycle.
Figure 7: Temperature profile in heat exchanger. Cold side on the left, hot side
on the right.
28
Pow er output at 100-50% D C pow er
25
100%
20
Power / kW
80%
71%
15
62%
51%
10
5
0
0
500
1000
1500
2000
2500
3000
3500
Tim e / s
Stack D C power
System gross AC power
System netAC power
U sefulheat
Figure 8: Predicted power output of the system with no cathode recycle during
load-change simulation.
Pow er output at 100-50% D C pow er
25
100%
20
Power / kW
80%
71%
15
63%
51%
10
5
0
0
500
1000
1500
2000
2500
3000
3500
Tim e / s
Stack D C power
System gross AC power
System netAC power
U sefulheat
Figure 9: Predicted power output of the system with cathode recycle during
load-change simulation.
29
Efficiencies at 100-50% D C pow er
60
51%
62%
71%
50
Efficiency (LHV basis
80%
100%
40
30
20
10
0
0
500
1000
1500
2000
2500
3000
3500
Tim e / s
N etelectricalefficiency
Therm alefficiency
Figure 10: Changes in electrical and thermal efficiencies for a system without
cathode recycle, for a load-change simulation over the range 100-50% DC power
Cellvoltage and current density at 100-50% D C pow er
350
0.860
51%
0.840
62%
0.820
Cell voltage/
250
71%
0.800
200
80%
0.780
150
0.760
100
0.740
100%
50
0.720
0.700
0
0
500
1000
1500
2000
2500
3000
3500
Tim e / s
C ellvoltage
M ean currentdensity
Figure 11: Changes in cell voltage and stack current density for a system without
cathode recycle, for a load-change simulation over the range 100-50% DC power
30
-2
Current density/ mAcm
300
Pow er output/use during the startup procedure
25
8
20
7
5
6
Power/ kW
15
3
4
10
2
5
1
0
0
500
1000
1500
2000
2500
-5
Tim e /s
Stack D C power
System netAC power
Power for steam generator
Power to furnace
Figure 12: System electrical power output/demand during the start-up
procedure
Figure 13: Block diagram of a fuel cell power conditioning system
31
3000
Figure 14: Example from power conditioning simulation of SOFC voltage and
current regulation
32
Stack with 265 anode substrate cells (20 cm x 20 cm)
average current density : 300 mA/cm 2
λ=6
30% pre-reformed methane
counter-flow
fuel utilisation: 70%
Conditions:
heat-transfer
by radiation
on 2 sides
current density distribution / mA/cm²
temperature distribution
air
15
400
300
10
200
5
100
fuel
0
5
800
air
15
740
5
720
700
0
10
15
20
cell length x / cm
0
5
10
15
20
cell length x / cm
Stack with 265 anode substrate cells (20 cm x 20 cm)
Temperatures
°C
Energy balance / per cell
W
Fuel gas composition
fuel inlet
700
electric power output
73.5
in
out
air inlet
700
heat transfer by air
23.0
mol%
mol%
fuel outlet
731
heat transfer by fuel
18.58
air outlet
782
heat transfer by radiation
maximum
800
Operation parameters
1.3
H2
28.63
13.9
CO
1.99
3.10
CO 2
5.31
15.00
Electrical efficiency
%
CH 4
17.10
0.13
related on converted fuel
65
H 2O
46.97
63.19
related on input fuel
43
N2
0.00
0.00
fuel utilisation
%
air stoichiometry
mol/ mol
6
current density
mA/ cm²
300
Pressure loss
cell voltage
mV
679
in air channels
6
power density
mW/ cm²
204
in fuel channels
<1
70
mbar
Figure 15: Modelled temperature distributions for a single stack layer and
overall for the 20 kW module
33
780
760
10
fuel
0
0
/ °C
20
500
cell width y / cm
cell width y / cm
20
air and fuel
inlet temperature:
700 °C
Conditions:
fuel: pre-reformed natural gas
with anode gas recycle
fuel utilisation: 69%
electrical load:
100%
50%
10
5
fuel
10
5
0
air
15
5
5
fuel
0
10
15
20
cell length x / cm
0
5
10
15
20
cell length x / cm
0
λ
=
3.7
10
5
fuel
15
λ
=
2.7
10
5
fuel
0
5
15
λ
=
2.0
10
5
fuel
810
0
10
15
20
cell length x / cm
0
5
10
15
20
cell length x / cm
fuel: pre-reformed natural gas
with anode gas recycle
fuel utilisation: 69%
electric load
air stoichiometry
(stack, 100% conversion)
maximum stack
operation temperature
minimum stack
operation temperature
maximum temperature
gradient
air temperature increase
in the stack
fuel temperature increase
in the stack
10
15
20
cell length x / cm
air
0
0
5
20
air
cell width y / cm
15
cell width y / cm
20
air
λ
=
3.7
10
0
0
cell width y / cm
λ
=
3.7
fuel
20
air stoichiometry
variable
air
15
temperature / °C
λ
=
3.7
20
cell width y / cm
air
15
air and fuel
inlet temperature:
650 °C
25%
20
cell width y / cm
cell width y / cm
air stoichiometry
constant
20
Conditions:
65 cells 20 cm x 20 cm
counter-flow
adiabatic
%
0
65 cells 20 cm x 20 cm
counter-flow
adiabatic
100
75
5
10
15
20
cell length x / cm
air and fuel
inlet temperature:
650 °C
50
25
3.7
3.7
3.2
3.7
2.7
3.7
2.0
°C
809
793
803
769
789
739
764
°C
722
727
741
728
743
718
738
K/mm
1.08
0.97 1.02 0.78 0.83 0.46 0.49
°C
111
98
103
81
95
69
89
°C
73
77
91
78
103
71
101
Figure 16: Modelled temperature distributions and gas flows for a 5 kW stack at
partial load
34
790
770
750
730
710
35
fuel
air
fuel
air
fuel
air
Figure 17: Schematic diagram of the internally manifolded FZJ ‘E-design’
SOFC stack.