Testing a PEM Fuel Cell System with Biogas Fuel

Testing a PEM Fuel Cell System with Biogas Fuel

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1

Testing a PEM Fuel Cell System with Biogas Fuel
Ralf Schmersahl and Volkhard Scholz Institute of Agricultural Engineering Bornim e.V. (ATB), Max-Eyth-Allee 100, 14469 Potsdam, Germany rschmersahl@atb-potsdam.de ABSTRACT The use of biogas in fuel cells advantageously combines a cost effective renewable energy source with a technology which promises high electrical efficiency and low environmental impact. For the first time, the suitability of biogas as a fuel for proton exchange membrane fuel cell systems (PEMFC) has been experimentally confirmed. The advantages of this particular type of fuel cell are its low operating temperatures, modular system design and moderate costs. Measurement results from a 650 Wel test plant show a cell efficiency of 58% at a power density of 0.14 W/cm2. A particularly problematic component is the steam reformer with a thermal energy efficiency of 38%. A model calculation on the basis of an optimised PEMFC system shows that an electrical efficiency of over 40% can be obtained. Keywords: biogas, CHP, energy, fuel cell, PEM, steam reforming 1. INTRODUCTION Fuel cell technology provides a promising alternative to the conventional electrodynamic use of gaseous hydrocarbons in combined heat and power plants (CHP). Fuel cells produce less noise and pollutant emissions while achieving higher electrical efficiency, particularly in partial load operation. Proton exchange membrane fuel cells exhibit a comparatively higher power density and make less material demands as a result of low operating temperatures, thus allowing for lower costs. PEMFC systems are being developed for stationary use with natural gas as fuel. As this type of fuel cell is only able to utilise hydrogen, the natural gas needs to be reformed to a hydrogen rich gas (reformate). Biogas has similar properties to natural gas and is by far the most cost effective renewable fuel. However, biogas has a lower power density, noticeably higher carbon dioxide content and exhibits various other harmful components such as sulphur compounds and ammonia. The intention of research at the ATB is to develop and test a PEMFC system as an efficient, cost effective and reliable technology for generating electrical energy from biogas. Primarily, the focus is on experimentally verifying the compatibility of biogas with PEMFC and determining the system parameters needed for optimal operation. In order to estimate the attainable electrical efficiency the most important characteristic energetic values for an optimised system are calculated. 1.1 Processing Biogas and Hydrogen Generation At present, several fuel cell systems are in development for distributed energy supply based on natural gas. The use of biogas as a fuel requires the systems to be technically adapted and optimised, as biogas differs to natural gas in several aspects (Schmersahl and Scholz, 2004). R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

2 The methane content and thus the energy density is lower and irregular. The high content of associated catalyst poisons, above all sulphur compounds, demands additional gas purification. The use of natural gas and biogas in fuel cells requires the methane to be converted. State of the art is steam reforming as used by the petrochemical industry on an industrial scale. During steam reforming, a catalytic conversion of methane with steam to hydrogen, carbon monoxide and carbon dioxide occurs. The reforming temperatures are at 650 to 850� C. As well as the steam reforming, desulphurisation, carbon monoxide conversion (CO-Shift) and carbon monoxide elimination are integrated into the fuel processor. The thermal efficiency Ref of the fuel processor can reach more than 80% (Schmitz, 2002). After the reforming process a CO-shift is carried out in one or two steps. Subsequently, the natural gas reformate exhibits a hydrogen content of 70% to 80% referring to the dry gas. The carbon monoxide content amounts to less than 1% (Krause et al., 2001). Due to the low operating temperature of [ 90� C, PEMFCs demand additional gas purification to reduce the carbon monoxide build up during reforming to below 10ppm. This is achieved by selective oxidation (selox) of carbon monoxide with atmospheric oxygen requiring a catalyst. 1.2 PEM Fuel Cells The fuel cell stack consists of several layered single cells. The membrane electrode assemblies (MEA) are separated by the bipolar plates which feed the reactant gases. Around the stack there are peripheral components designed for media supply, cooling, process control, DC regulation and voltage conversion. First experiences of PEMFC systems fueled by natural gas at the demonstration stage show a net system efficiency of 25% at 5 kWel (Koschowitz, 2003) up to 35% at 200 kWel (Pokojski, 2001)]. The optimisation of a 1 kWel system at the Universit�t Gesamthochschule Essen delivered a maximum electrical efficiency of 42% (Schmitz, 2002). 1.3 Present Research in Biogas Fuel Cells Besides those of the ATB, research and development activities for three different biogas fuel cell systems are known of in Europe. One research project aims to adapt the molten carbonate fuel cell (MCFC), namely the Hot Module of the company MTU, for use with biogas (Ott and Tamm, 2003). Another group of scientists at the Federal Research Institute for Agriculture is conducting research into steam reforming of biogas with subsequent CO2 sequestration. The hydrogen produced is used to generate electricity in a PEMFC. In Switzerland is an adapted solid oxide fuel cell (SOFC) for residential power generation on test operation with biogas (Schuler, 2001). The above named research projects are all characterised by the fact that they aim to adjust natural gas technology for use with biogas by modified gas processing. Particular emphasis is placed on the removal of the associated harmful gases prevalent in biogas. The purity requirements are largely determined by the reformer catalyst. However, the fuel cell electrodes are also damaged cumulatively and irreversibly by sulphur compounds, for example (DOE, 2000). The higher carbon dioxide load in biogas compared to natural gas calls for a change in fuel processor design. The reformate exhibits a lower hydrogen content causing a decrease in cell voltage. The fuel cell stack can be adapted by changing the flow fields and the catalyst loading of the anode (Beckmann, 2003). R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

3 2. MATERIALS AND METHODS 2.1 Supply of Raw Gas The ATBs fuel cell test stand is supplied with biogas from two solid state fermenters (Linke et al., 2001). The substrate used is a mixture of fermented manure, manure and corn silage. The resulting biogas is stored in a 16 m3 balloon gas-holder. The methane content of 60% is varied by adding manufactured gases (CH4, CO2). 2.2 Experimental System The test stand consists of the fuel processor with integrated desulphurisation and the fuel cell. Both units include a programmable logic controller (PLC). The PLC of the fuel cell assumes higher ranking functions such as initialising the start/stop procedures as well as the load demand. Media supply of the test plant secures the flow of biogas, deionised water and tap water used to feed the cooling circuits. The fuel cell unit has two independent water-cooled fuel cell stacks manufactured by Schaltand Regeltechnik GmbH Berlin. A test stack with 4 cells (150 Wel) serves to examine the effect of harmful gases, the operating stack with 14 cells (650 Wel) is intended to examine system behaviour and determine the energetic parameters. The fuel is treated by a fuel processor with reformer, two stage shift converter and selective oxidation (figure 1). The reformer load is regulated by fed process gas mass flow in the range of 20% to 100% (1.4 to 5.8 lN/min). For operation with 60% methane in the biogas, a maximum hydrogen output of 2.3 kW with a hydrogen content of 64% is specified.
TE 102 TE 103 TE 104 TE 105 TE 106

MFC

R-1

R-2
P-4

MFC

Mass flow meter

TE xxx

Temperature sensor Boiler Compressor Burner Steam reformer CO-Shift Selective oxidation (Prox) Condensate separator Activated carbon adsorber

TE 101

V-1A

H-1
TE 108 TE 107

V-2

V-1B

B-1

B-1 C-1 H-1 R-1 R-2 R-3x V-2 V-1x

& m bio, burner
C-1

R-3A

R-3B

& m Ref

Figure 1. Mass flow scheme of the fuel processor The fuel processor integrates gas compression and desulphurisation as well as steam generation and has internal heat recovery. The raw gas is compressed to 720 mbar by the compressor C-1. The gas flow is then separated. The combustion gas is fed into the R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

4 atmospheric burner H-1 and the process gas is fed into two activated carbon filters that are impregnated with copper and adsorb hydrogen sulphide. The purified gas passes through a mass flow controller (MFC) and, en route, is mixed with steam at a S/C ratio of 3.5 (steam to carbon ratio, carbon from methane). The mixture is preheated in a heat exchanger, induced into the steam reformer R-1 and passes through the ring shaped reforming catalyst (G-90B). Here it is heated up by the burner H-1 and by the counter current flow of reformate which is transported through the inner cylinder. Thereby a temperature of about 720 to 840� C is obtained at the outlet of the catalyst bed (TE-103). The reformate passes over a heat exchanger, is directed through both shift converter stages at a temperature of 250 to 300� C and cooled. Cooling to approximately 30� C results in condensate formation, which is trapped (V-2). Following this, the reformate is fed to the selective oxidation, consisting of the watercooled catalysts R-3A und R-3B (50 to 60� C). The reformate with a dew point of 40 to 45 �C is supplied to the fuel cell via a heated hose. Depending on the position of the valve, it is either passed into the anode chamber of one of the two fuel cell stacks or through a bypass directly to the outlet. In the fuel cell stack it is fed through the flow fields of the bipolar plates to the polymer electrolyte membrane (Primea 5621 series, 207 cm2). Operating pressure is 1.3 bar and operating temperature is 45 to 48 �C. 2.3 Gas Analysis The gas measurement equipment consists of an online gas analyser by Pronova (Berlin) and a drum gas meter to record the amount of supplied gas. Process gases are taken from three sample ports in order to analyse and record the composition of biogas, reformate and anode off-gas (H2, CH4, CO2, CO). The volumetric concentration of CH4, CO2 und CO is determined by infrared spectroscopy and the concentration of H2 is measured by a thermal conductivity detector. The total biogas volume flow is measured with a drum gas meter and the process biogas volume flow is recorded by the thermal mass flow controller of the fuel processor. 2.4 Experimental Procedure 65 experiments in total are conducted in 5 test series to evaluate the reformer, the fuel cell and the entire system (Schmersahl, 2004). To examine the reformer, the influence of selective oxidation on the composition of the reformate and the operational behaviour for biogas with different methane content are analysed. For this, the selox air volume flow and the reformer load (process gas mass flow) are varied. The methane conversion rate u of the fuel processor is calculated as follows:
u= & m CH 4 ,c & m CH 4 ,pro

(1)

where: m CH &

4 ,c

=

methane converted methane input to process

& m CH 4 ,in =

The fuel processor efficiency ref can be defined as:
Ref =
& m CH 4iin (h LHV )CH 4 & m H 2 ,ref (h LHV )H 2

(2)

R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

5 where: m H &
& m CH 4 ,in
2 , ref

= = =

hydrogen output

methane input Specific enthalpy of component i

(hLHV)i

For the fuel cell, polarisation curves for various compositions of biogas are recorded and investigations into the influence of excess hydrogen flow are undertaken. Operational behaviour and performance of the fuel cell are described by polarisation curves, indicating the relationship between voltage and current density. The current density i, relevant for physical size and costs, is defined as the ratio of the current IFC to the membrane surface area AC. The thermoneutral maximum voltage of 1.25 V refers to the lower heating value of hydrogen:
VC, rev = - (h LHV )H 2 2F = 1.25 V

(3)

where: VC,rev

(h

LHV ) H 2

)

= =

reversible maximum cell voltage specific enthalpy of hydrogen (= -241.8 kJ/mol)

F

=

Faraday constant (= 96487 As/mol)

This leads to a practical definition of cell efficiency as a function of the cell voltage VC:
C = VC 1.25V

(4)

The arrangement of individual fuel cells comprises the fuel cell stack. From the point of view of fluid dynamics, the cells are in parallel connection whereas the electric connection is in series, so that the voltage of the single cells is added to VFC. As not all of the supplied fuel is converted by the cells, efficiency loss occurs. Fuel utilisation f can be defined as:
f = & m H 2 ,u & m H 2 ,ref

(5) hydrogen utilised

where: m H &
& m H 2 ,ref

2 ,u

= =

hydrogen input

Equation 6 gives the stack efficiency BZ for a fuel cell with z individual cells:
FC = f VC 1.25V

(6)

The gross electrical system efficiency Sys is given by the ratio of effective electrical power Pel to fed fuel not accounting for auxiliary energy consumption:
Sys = Pel = Re f FC & CH 4 ,in (h LHV )CH m 4

(7)

R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

6 3. RESULTS 3.1 Fuel Processor The PEMFC requires a fuel largely free of carbon monoxide. In order to attain a CO content of < 10 ppm, the reformate is purified by selective oxidation with atmospheric oxygen. Reformate entirely free of carbon monoxide is obtained by adding a volumetric air flow rate of more than 5% of the reformate flow rate. Stochastic CO maximum values of > 250 ppm occur when less than 2.5% air is added. Increasing the air flow rate decreases the hydrogen and carbon monoxide content in the reformate. The slight decrease in CO2 content can be explained by dilution. The strong reduction in hydrogen concentration is caused by the reaction of hydrogen with surplus atmospheric oxygen. The air flow control of the fuel processor does not give a fixed reformate/air ratio. At lower reformer load, increasing excess air causes a minimally inferior reformate quality. For the further operation of the test stand, an air supply of 5% of the reformate volume flow is adjusted. A high hydrogen output requires a high methane conversion rate. Measurement results show a decrease in methane conversion rate with increasing reformer load indicating insufficient catalyst activity. The lowest methane conversion rate of 74% for biogas is acquired at full load with 65% methane. At 30% load, the conversion rate increases to about 90% for all compositions of biogas. Reasons for the low conversion rate are assumed to be the relatively low reforming temperatures of 700 � 740 � C (figure 2).
Hydrogen content (a) Methane conversion (a) Hydrogen output (a) 100 Hydrogen content (b) Methane conversion (b) Hydrogen output (b) Hydrogen content (c) Methane conversion (c) Hydrogen output (c)

Hydrogen content (%) Methane conversion (%)

80

1.5

70

1.0

60

0.5

50 30 40 50 60 70 80 90

0.0 100

Reformer load (%)

Figure 2. Methane conversion and hydrogen output for different methane content in biogas Measurement results are represented as an average value over 5 minutes. The reformate composition is not constant. The concentration of hydrogen fluctuates by a maximum of 4.5 percent by volume within 5 minutes. Reforming results in different concentrations of hydrogen in the reformate depending on the methane content of biogas. Measurements show an average hydrogen content of 53% (55% R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

Hydrogen output (kW)

90

(a) : 55% methane (b) : 60% methane (c) : 65% methane

2.5

2.0

7 CH4), 55% (60% CH4) and 56% (65% CH4). The peak value for biogas with 60% methane was obtained at a higher reforming temperature of 840 � C at full load. 3.2 Fuel Cell The performance of fuel cells is characterised by polarization curves. Increasing current density lowers the voltage and hence decreases cell efficiency. This leads to difficulties in determining the rated power, since there is a trade-off between high output and efficiency. The characteristic curves of the entire operating stack for different compositions of raw gas (55% to 65% CH4 content) show currents barely deviating from each other at current densities of up to 0.25 A/cm2 (figure 3). With increasing current densities the different hydrogen concentrations in the reformate, caused by different methane concentrations in the biogas, result in increasing voltage differences. Thus, the test stand attains a maximum power of more than 600 Wel with biogas containing 65% methane. At a methane content of 55% only 500 Wel is obtained, then the voltage of the weakest individual cell falls to below 450 mV leading to an automatic breakdown of load operation.

V (a) 13 12

V (b)

V (c) (a) : 55% methane (b) : 60% methane (c) : 65% methane

P (a)

P (b)

P (c) 700 600

10 9 8 7 6 0.00

400 300 200 100 0 0.45

0.05

0.10

0.15

0.20

0.25

0.30
2

0.35

0.40

Current density (A/cm )

Figure 3. Polarisation curves and power output for different methane content in biogas The characteristic curves of the single cells display significant differences. A comparison of the most efficient cell 7 with the weakest cell 2 shows a difference in cell voltage increasing over the entire range of performance. At a current density of 0.2 A/cm2, the difference amounts to approximately 80 mV. At a current density of 0.35 A/cm2 the difference increases to more than 100 mV, corresponding to a difference of more than 8 percent in efficiency. This is assumed to be due to irregular flow conditions (figure 4).

R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

Power output (W)

Stack voltage (V)

11

500

8
Cell 2 (a) 900 (a) : 55% methane (b) : 60% methane (c) : 65% methane Cell 2 (b) Cell 2 (c) Cell 7 (a) Cell 7 (b) Cell 7 (c) 900

800

800

Cell voltage (V)

700

700

600

600

500

500

400 0.00

0.10

0.20

0.30
2

400 0.40

Current density (A/cm )

Figure 4. Polarisation curves for individual cells Only part of the supplied hydrogen is utilised in the fuel cell. The hydrogen overflow ratio H is the ratio of the supplied hydrogen to that used; it is the reciprocal value to the fuel utilisation rate f. Its influence on the cell voltage is slight up to a critical limit and depends on the current density. At H < 1.25 (20 A) or H < 1.4 (40 A) the cell voltages of single cells collapse, whereas other cell voltages show only slight changes. A higher methane content in the biogas and thereby a higher hydrogen content in the reformate allows for a smaller hydrogen overflow ratio. Adjusting a constant hydrogen overflow ratio is not possible for the existing test stand, as the reformats pressure and hydrogen content fluctuate. This lowers fuel utilisation and thereby decreases electrical efficiency. 3.3 Power Balance The energetic analysis requires a model which allows the necessary values to be calculated from the measured volume fractions, volume flow rates and electric quantities (fig. 5).
& m air, Selox
& Q ref,out & Q FC,out

Fuel processor
& m CH 4 ,pro (h LHV ) CH 4 & m CH 4 ,in (h LHV ) CH 4 & m H 2O,in & m bio, in
& m bio, pro

& m CH 4 , ref (h LHV ) CH 4 & m H 2 ,ref (h LHV ) H 2 & m ref
& Q ref,in

Fuel cell
Anode chamber

Reformer

& m H 2 ,u

Cathode chamber

Pel

& m cat,out & m H 2O,out

& m CH 4 ,bu (h LHV ) CH 4
& m bio, bu

Burner

& m air,bu

& m bu,out

& & m out.an m air, cat & m CH 4 , out (h LHV ) CH 4

& m H 2 , out (h LHV ) H 2

Figure 5. Simplified mass and energy flow scheme R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

9 The conversion of volumetric flow rates into mass flow rates needed for this assumes an ideal gas as well as standard temperature and pressure.

& The hydrogen mass flow utilised is m H 2 ,u and amounts to (Winkler, 2002):
& m H2 , u = I FC z M H 2 2F

(8)

where: IFC z
M H2

= = =

Amperage Number of cells Molar mass of hydrogen

Taking into consideration the measured volume fractions the reformate hydrogen mass flow is calculated as follows:
& m H 2 , Ref = x H 2 , ref (1 - x H 2 ,an ) x H 2 , ref - x H 2 , an & m H2 , u

(9)

Where: x H

2 , ref

= =

Hydrogen fraction of reformate Hydrogen fraction of anode off-gas

x H 2 ,an

A power balance is established for different operating points to assess the performance of the test stand. The measurements show a gross electrical system efficiency Sys of 12 % at a current density i of 0.29 A/cm2 and a fuel utilisation f of 70 %. At partial load the efficiency is Sys = 11 % at f = 62 %. The reason for the low system efficiency is to be found in the inefficient fuel processor with a thermal efficiency of less than 38% as well as in the process design, as a large part of the chemically bound energy leaves the system unused as anode off-gas. A particular process design such as the one developed and tested at the Bergakademie TU Freiberg uses the anode off-gas to supply thermal energy to the reformer (Walter, 2001). For this design an estimate was made of the attainable gross system efficiency Sys for the generation of electricity from biogas with PEM-fuel cells based on the experimental results and reliable published data. Besides the efficiency rates already described, the DC/AC conversion needed to feed into the grid is taken into account assuming an efficiency I = 0.95. Calculations are made for H = 1.2 (model 2 and 4) and H = 1.4 (model 1 and 3). This corresponds to a fuel utilisation rate of 71% or 83% respectively. The fuel processor efficiency is estimated at 68% (model 1 and 2) according to manufacturer's specifications and 80% (model 3 and 4) according to published values. Operation points are determined on the basis of the polarisation curve of cell 7 with Uz = 670 mV at 0.3 A/cm2 and UZ = 720 mV at 0.2 A/cm2 (cf. figure 4). Assuming a methane conversion rate of 97%, the electrical efficiency is calculated with:
sys = (1,03 + (1 - f ) ref ) ref f UC I 1.25V

(10)

The result is a maximum gross electrical system efficiency of 39% to 42% (table 1).
R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

10 Table 1: Efficiency of an optimised PEMFC system for biogas (simulation) Model Input values i (%) ref (%) f (%) VC (mV) Results sys (%) 95 68 71 670 30 1 95 68 71 720 32 95 68 83 670 33 2 95 68 83 720 35 95 80 71 670 37 3 95 80 71 720 39 95 80 83 670 39 4 95 80 83 720 42

4. CONCLUSIONS

The suitability of PEMFC technology for generating electricity from biogas has been experimentally confirmed for the first time. Measurements taken at a 650 Wel test stand exhibit a cell efficiency of 58% at a current density of 0.2 A/cm2. However, this value is only achieved by individual cells. Cell voltages vary considerably. Differences increase with increasing power output and lower excess hydrogen overflow. The reasons for this are assumed to be the irregular gas flow and cell humidification and can thereby be regarded as a constructive problem of the fuel cell stack. The fuel processor proves to be a particularly problematic component with a thermal efficiency of approximately 38% and a methane conversion rate of 75% to 90%, resulting in a low hydrogen yield. The influence of methane content on the energetic efficiency of the fuel cell system is small at the low power densities, which are necessary to obtain a high electrical efficiency. A calculation on the basis of the strongest single cell results in a maximum gross electrical system efficiency of 40% at a power density of 0.14 W/cm2 in an optimised system using the anode off-gas to supply thermal energy to the reformer. In summary, the assessment can be made that the system is not fit for the market. The use of biogas requires an efficient and reliable biogas fuel processor and a basic system optimisation. Once this is achieved, biogas cogeneration on the basis of PEMFCs has very good energetic prospects. Studies are continuing in cooperation with fuel cell and fuel processor developers. The aims are system optimisation, determining the durability and degradation of components and identifying the harmful components in the biogas in order to ascertain the appropriate tolerance limits of the fuel cell system.
5. ACKNOWLEDGEMENT

Particular thanks to Dr.-Ing. J�rgen Arnold and Dipl.-Ing. Frank Beckmann from Schalt- and Regeltechnik GmbH Berlin for building the test stand and providing technical support. We would like to thank PD Dr. agr. Dipl.-Ing. Bernd Linke and Dipl.-Ing. Jan Mumme from ATB Potsdam for their kind collaboration and the provision of biogas.

R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

11
6. REFERENCES

Beckmann, F. 2003. Einsatz von Biogas in Polymer-Elektroly-Membran-Brennstoffzellen. In Proc. 2th Potsdamer Biogaskonferenz, 72-84. Potsdam, Germany, 17-18 November. Krause, H., Grosser, K. and Gerber, J. 2001 Kompaktreformer f�r die Aufbereitung von Erdgas � Stand und Anforderungen. In Proc. Innovationsforum Brennstoffzellen in dezentralen Energieversorgungsanlagen, 22-25. Riesa, Germany, 29-30 November Koschowitz, M. 2003. Felderfahrung mit Vaillant Brennstoffzellenheizger�ten. In Station�re Brennstoffzellen, 15-24. VDI-Berichte 1752. D�sseldorf, Germany: VDI Linke, B., Schelle, H. and Mumme, J. 2001. Solid state anaerobic digestion of farmyard manure and plant residues. In Proc. Renewable Energy Sources on the Verge of the XXI Century, 332-335. Warsaw, Poland, 10-11 December Ott, M. and Tamm, D. 2003. Anforderungen beim Einsatz von Biogas in Brennstoffzellen. In Proc. Aufbereitung von Biogas, 60-70, G�lzow, Germany: FNR Pokojski, M.2001. Die erste 250 kW PEM Brennstoffzelle in Europa 2001. In Station�re Brennstoffzellen, 73-85. VDI-Berichte 1596. D�sseldorf, Germany:VDI Schmersahl, R. 2004. Energetische Nutzung von Biogas in Polymer-Elektolyt-MembranBrennstoffzellen, Dipl.-Ing. Thesis. Berlin, Germany: Fachhochschule f�r Technik und Wirtschaft Berlin Schmersahl, R. and Scholz, V. 2004. Biogas in Brennstoffzellen � Stand und Perspektiven. In Proc. of 1st Mitteldeutscher und 10th. Th�ringer Bioenergietag, 76-87. Jena ,Germany, 10 November Schmitz, S. 2002. Thermodynamische Analyse eines Brennstoffzellen-Systems zur dezentralen Energieversorgung. VDI Fortschritt-Berichte 475, D�sseldorf, Germany: VDI Schuler, A. 2001. Sulzer Hexis SOFC System. In Proc. Biogas-Brennstoffzellen Systeme, 4158, Steyr, Austria, 15 January U.S. Department of Energy. 2000. Fuel Cell Handbook (Fifth Edition). Winkler, W. 2002. Brennstoffzellenanlagen. Berlin: Springer-Verlag Walter, G. 2002. Innovationspotenzial der Brennstoffzellentechnologie in dezentralen Energieanlagen. In Proc. Innovationsforum Brennstoffzellen in dezentralen Energieversorgungsanlagen, 11-13. Riesa, Germany, 29-30 November
NOMENCLATURE Symbols

AC F (hLHV)i i IFC
& mi

Membrane surface area Faraday constant (= 96487 As/mol) Specific enthalpy (ref. to lower heating value) Current density Amperage Mass flow of component i

cm2 kJ/mol A/cm2 A g/h

R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

12 Mi
& Q

Pel xi u VC Uc,rev z H BZ C I ref sys f

Molar mass of component i Heat flow Electrical power Volume fraction of component i Methane conversion rate Cell voltage Reversible cell voltage Number of cells Hydrogen overflow ratio Electrical stack efficiency Electrical cell efficiency Inverter efficiency Thermal fuel processor efficiency Gross electrical system efficiency Fuel utilisation rate

g/mol kJ/h W % % mV V % % % % %

ABBREVIATIONS

CO-Shift CHP LHV MCFC MEA MFC PEMFC Selox SOFC PLC ppm

Carbon monoxide shift reactor Combined Heat and power production Lower heating value Molten carbonate fuel cell Membrane electrode assembly Mass flow controller Proton exchange membrane fuel cell Selective oxidation Solid oxide fuel cell Programmable logic controller Parts per million

R. Schmersahl and V. Scholz. "Testing a PEM Fuel Cell System with Biogas Fuel". Agricultural Engineering International: the CIGR Ejournal. Vol. VII. Manuscript EE 05 002. September, 2005.

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