Public demonstration of PEM fuel cells
as miniature household co-generation plants in Munich

 

U. BÜNGER, E. KRAUS, 4. September 1997
Ludwig-Bölkow-Systemtechnik GmbH
Daimlerstraße 15
D-85521 Ottobrunn
e-mail: buenger@lbst.tnet.de

Th. Schmalschläger
Stadtwerke München / Energieservice
Kapellenweg 4 - 6
D-80287 München

 

 

1. Introduction

We are currently observing rapid changes in international local energy market structures. The general trend towards competitiveness can be subsummized under terms like liberalization, deregulation and privatization [1]. Starting in the U.S. with the Public Utility Regulatory Policies Act in 1978 various European countries like the UK and Norway followed. Restructuring the energy system will eventually lead to a unification of large electricity and gas suppliers, and to efficient small energy service companies installing decentral, flexible and low cost energy conversion technologies.

With a growing awareness of the costly dependency on oil imports from politically instable regions [2] and that its resource depletion may happen faster than generally believed [3] we now observe a trend to natural gas as a power generation fuel. Currently the world natural gas demand is projected to be doubled by 2030, annually increasing by 2.1 - 3.5% until 2010 [4]. This facilitates small decentralized power plants often operated by flexible independent power producers (IPP), free trade of the grid energy carriers natural gas and electricity and rising numbers of natural gas operated cogeneration systems.

Furthermore, the trend towards gaseous energy carriers will eventually lead to renewable hydrogen as the most versatile non-carbon based fuel. Its widespread application is no longer a vision, but is accepted as a reasonable means of reducing global CO2-emissions [5], although the time for its advent is still heavily debated [6].

All developments together foster the introduction of energy savings and conversion, storage and supply technologies for stationary small scale high efficiency power supply. Fuel cells (FCs) are known for their fuel flexibility. As their operation with hydrogen results in highest efficiencies they are a key component to a non-fossil era.

 

 

2. General Trends Concerning the Use of Residential FCs

In Europe an obvious trend in residential electricity and heat supply in new buildings points towards significant energy savings. For existing residential buildings new directives will further tighten reduction goals for heating demand. E.g. in Germany a new law expected for 1999 will require that so called low energy houses with 50 - 70 kWh/m² a be built. In these houses low temperature distribution systems with 40/60°C will ease the introduction of low temperature heat generating systems like PEMFC.

Other trends are the application of electric heat pumps and local district heating grids. Whereas the electric heat pump is a direct competitor for the FC technology, consuming electricity for heating instead of co-producing it locally, district heating and FC cogeneration systems could harmonize well. Furthermore, first attempts have been undertaken to evaluate the advantages of combining renewable energy technologies and FCs [7]. As the discussion is in progress general conclusions can not be drawn yet.

As only small quantities of renewable hydrogen may be available the next 10 - 20 years for economic reasons most sensitive FC fuel alternatives are natural gas and methanol. Natural gas is available from a dense distribution grid in many European countries, methanol can replace oil for remote applications. Both fuels however require efficient fuel converters, currently being developed [8, 9]. Small FCs cogenerating heat and electricity can flexibly supply valuable peak power, capable of eventually changing the electric heat pump energy supply strategy for buildings to „gas in - electricity out".

 

 

3. Technology of Natural Gas Operated PEMFC for Cogeneration

Among the five known FC technologies - mainly characterized by their operating temperature - proton exchange or polymeric electrolyte membrane FCs (PEMFC), phosphoric acid FCs (PAFC) and, for widely unmodulated operation, also solid oxide FCs (SOFC) are suitable for small scale residential heating. PEMFC typically operate at 40 - 80°C with high total efficiencies of ³ 85 % in hydrogen and 65 - 80% in natural gas operation, resulting in electrical efficiencies of 45% (31%). Thus, PEMFC are ideal for low temperature cogeneration of heat and electricity in individual single family homes or small district heating grids. They can also be used for remote power generation as e.g. in developing countries. FCs are the core of these cogeneration systems. However, the techno-economic adaptability of FC systems for individual applications is determined by the layout and performance of the tailored subsystems.

 

3.1. PRIMARY FUEL REFORMER

In carbon-fuel operated FCs fuel converters are responsible for producing hydrogen from a variety of fuels like natural gas, methanol, ammonia, LPG etc.. The reformer/purification unit must be customized to the individual operating requirements (load dynamics). For dynamic PEMFC especially with small fuel converters the problem is their complexity with subsystems like desulphurizer, reformer, CO shift-reactor and CO removal system (ó 10 ppm), which can be Pd-membranes or selective oxidizers. For 10 kWel-class PEMFC systems miniaturized natural gas reformer technologies are described in more detail in [10]. For large applications the specific reformer costs for natural gas operated PEMFC systems will be about 2/3 of the stack costs themselves [11], in small units they will probably approximate the stack costs.

PAFC are much more tolerant to CO than PEMFC, decisively lowering the degree of fuel converter complexity. SOFC may not require a fuel converter at all as it can be integrated into the high-temperature (» 600°C) stack [12]. For an optimization towards uninterrupted FC operation it may be necessary from case to case to consider a hydrogen buffer storage system.

 

3.2. GAS AND HEAT FLOW SUBSYSTEMS

PEMFC can be characterized by their operating pressure level. The lowest secondary energy demand is necessary for unpressurized systems. The highest energy densities and material intensity can be reached with pressurized systems. The pressure is typically below 3 bar, resulting in relatively simple hydrogen/air blowers. The exhaust gases have to be removed from the system confinement, however no over-the-roof ventilation stack will generally be required in residential systems. The heat generated will be taken out by a circulating flow of water, which can also be stored in conventional warm water storage equipment, thus lowering system complexity.

 

3.3. DC / AC CONVERTER AND SYSTEM CONTROLS

In grid connected residential FC systems the electricity has to be transformed to the grid quality and safety specifications. The development of FC integrated inverters will eventually require least costs, which can only be reached by a highly integrated single-chip-design of power and control logics within the system controller. Specific inverter investment costs, which can be as low as 400 DM/kWel [13] today, may well reach 50 DM/kWel when mass produced in the future [14].

 

3.4. SAFETY COMPONENTS

For small residential FC cogeneration systems, generally installed in the basement of single family residences, special attention must be paid to safety. Preliminary results of a German inspection authority require a forced ventilation of about 5 h-1 either by natural or forced convection. An alternative is the installation of cheap customized sensors at locations of highest possible hydrogen concentrations, requiring customized low maintenance sensors as under development in Europe [15] and the U.S [16].

 

 

4. Operating Modes of Small 10 kWel Class Household PEMFC

In a follow-up project to previous hydrogen related studies of the Stadtwerke München (municipal utility owned by the city of Munich) and Ludwig-Bölkow-Systemtechnik [17] an analysis of three operating modes (see table 1) for small natural gas operated PEMFC was carried out in comparison to a conventional calorific gas burner [18]. A flow diagram of the system is sketched in fig. 1.

 

 

FIGURE 1. Flow diagram of a small natural gas residential co-generation PEMFC (Siemens)

 

The specific operating conditions (local climate, electricity and gas tariffs, etc.) of Munich were considered. The techno-economic PEMFC development goals of the industry involved were anticipated. The system was expected to be operated priorizing heat production in winter and electricity production in summer. During summer the decision about the time and period of operation lies within the responsibility of the local utility. As the period for maximum electricity demand in the grid can coincide with the warm water demand of the individual household by means of a warm water storage system (e.g. 150 l) the utility can now flexibly levelize its electricity production capacities within the grid, hence improving overall system economy.

The economic evaluation of the system was based on a complex simulation model (see table 2) and a consecutive sensitivity analysis.

 

 

FIGURE 2. Functional entities of the PEMFC simulation tool [18]

 

The conclusions can be summarized as follows.

Although the techno-ecological determinants are highly favorable the commercial chances for the proposed system are limited to specific operating conditions (table 1).

TABLE 1. Commercialisation chances for different small PEMFC operating modes

Owner / Operator Evaluation
Private

not competitive if connected to the electricity grid

small systems possibly competitive if grid-independent
Energy service company

currently not competitive
Utility

competitive by replacing peak power (typically at noon)

 

The utility option currently seems to be most promising for small natural gas and electricity supplying communities, because only then the operational flexibility of small decentralized heat and electricity generation systems can be utilized.

For larger 200 kWel-class FCs (PEMFC, PAFC) a recent study revealed that when reaching their investment cost development goals both technologies may not only be competitive with natural gas fired boilers, but also with gas engine cogeneration plants [7]. The next years of intensive development efforts and pilot projects such as the first 250 kWel PEMFC for Berlin [19] will unveil verifyable development potentials.

 

6. Conclusions and Recommendations for Further Work-

Further techno-economic studies are necessary to study the economic chances for PEMFC and SOFC in the 10 kWel-class, and for 200 kWel-class PAFC or PEMFC district heating systems. Among the specific topics to be examined are

In the current development stage practical experience from pilot and demonstration projects is necessary. A first small PEMFC co-generation project is planned for the city of Munich during the next years, others will hopefully follow in the not too far future.

 

 

7. References

1. Schmitt, D.: Europäische Elektrizitätswirtschaft und EU-Binnenmarkt. 10 Thesen. Proc. Energieinnovation als Wirtschaftsfaktor, Ed. Kurt Friedrich and Wolfgang Wallner, Vienna, 1996, pp. 37 - 42.

2. Riva, J.P.: World oil production after year 2000: business as usual or crises? U.S. Congress report 35-925 SPR, 18 August 1995.

3. McKenzie, J.J.: Oil as a finite resource: When is global production likely to peak? Paper by World Resources Institute, Washington, March 1996.

4. Study of the International Gas Union, Presented at 20th World Gas Conf., Kopenhagen, 8 - 13 June 1997.

Decarbonisation of fossil fuels. Report PH2/2 by Foster Wheeler for Statoil and IEA-GHG, March 1996.

6. Nitsch, J.; Dienhart, H.; Langniß, O.: Entwicklungsstrategien für solare Energiesysteme - Die Rolle von Wasserstoff in Deutschland, Energiewirtschaftliche Tagesfragen, 47(1997), No. 4, pp. 223 - 229.

7. Blandow, V.; Bünger, U.; Eckstein, U.; Loerbroks, A.; Maier, S.; Niebauer, P.: Die Bedeutung von Energiespeichern in zukünftigen Niedrigenergiehäusern. Report Ludwig-Bölkow-Systemtechnik, April 1997.

8. Jones, R.; von Waveren, T.: Novel compact steam reformer for fuel cells with heat generation by catalytic combustion augmented by induction heating. To be published in Catalysis Today, 1997.

9. Product brochure of the Fraunhofer Institute for Solar Energy Systems, Freiburg, April 1997.

10. Weindorf, W.; Bünger, U.: Verfahren zur Reinigung von Wasserstoff aus der Erdgasdampfreformierung für den Einsatz in Brennstoffzellen. To be published in Brennstoff-Wärme-Kraft, 7/8 1997.

11. Gavalas, G.R., Voecks, G.E.; Moore, N.R.; Ferrall, J.F.; Prokopius, P.R.: Fuel cell locomotive development and demonstration program - phase I: systems definition. Report to SCAQMD, California, 1995, p. 4-12.

Barp, B.; Dienhart, R.: Solid oxide fuel cells for small scale cogeneration. Conf. Proc. Eurogas Trondheim, 3 - 5 June 1996.

13. Rasch, H.: Untersuchung zum Einsatz von Frequenzumrichtern für Photovoltaikanwendungen. Thesis FH München, FB Elektrotechnik, Oktober 1996.

14. Schwab, M.; Reismayr, D.; Fechner, U.: Concept and cost degression potential of PV-inverters at very high mass production. Conf. Proc. EuroSun ’96, pp. 876 - 878.

15. Sensorsystem mit minimalem Energieverbrauch basierend auf gasempfindlichem Feldeffekttransistor zur Wasserstoffdetektion. Project proposal for the Hydrogen Initiative Bavaria (WIBA), 1996.

16. Haberman, D.: Advances in sensor technology may expand hydrogen applications. NHA Advocate, newsletter of National Hydrogen Association, Vol. 2, No. 1, 1997, pp. 1 - 2.

17. Bünger, U.; Zittel, W.; Schmalschläger, T.: Hydrogen in the public gas grid - a feasibility study about its applicability and limitations for the admixture within a demonstration project for the city of Munich. Xth World Hydrogen Energy Conf., Cocoa Beach 1994, pp. 173 - 183.

18. Kraus, E.: Einsatz und Betreibermodelle kleiner stationärer Brennstoffzellen-Blockheizkraftwerke im Versorgungsgebiet der Stadtwerke München. Thesis FH München, FB Versorgungstechnik, April 1997.

19. Das Brennstoffzellen-Blockheizkraftwerk im Bewag-Heizwerk Berlin-Treptow. Unpublished description of the first 250 kWel PEMFC in Berlin, June 1996.