Int. J. Environmental Technology and Management, Vol. x, No. x, xxxx 1 Performance assessment of a 5 kw SOFC cogeneration fuel cell S. Lerson, J.L. Lilien* and G. Minne University of Liège, Transmission and Distribution of Electrical Energy, B28, 4000 Liège, Belgium Fax: +32-43662998 E-mail: sebastien.lerson@ulg.ac.be E-mail: lilien@montefiore.ulg.ac.be E-mail: gael.minne@ulg.ac.be Website: www.montefiore.ulg.ac.be/services/tde *Corresponding author Abstract: The performance of a 5 kw Solid Oxide Fuel Cell (SOFC) will be studied and described in this paper. This will help to significantly reduce residential Green House Gas (GHG) emission to about 20%, when compared with the actual emissions, based on the high-performance boiler and electricity produced by high-efficiency combined cycle. The particular benefit of SOFC, when compared with other technologies of fuel cell, is the high temperature from one side (giving access to appropriate hot water for heating purposes) and the very exciting high electricity efficiency, which is over 40%, together with a global efficiency measuring over 80%. Keywords: fuel cell; cogeneration; solid oxide fuel cell; SOFC; performance; efficiency; 5 kw; residential; decentralised production. Reference to this paper should be made as follows: Lerson, S., Lilien, J.L. and Minne, G. (xxxx) Performance assessment of a 5 kw SOFC cogeneration fuel cell, Int. J. Environmental Technology and Management, Vol. x, No. x, pp.xxx xxx. Biographical notes: S. Lerson received his Degree in Mechanical Engineering (Energetic) from the University of Liège in 2001. He has worked for over four years with Professor J.L. Lilien as the Head of the demonstration project on high temperature fuel cell in residential environment. J.L. Lilien received his Degree in Electrical and Mechanical Engineering from the University of Liège, Belgium in 1976 and completed his PhD from the same University in 1984. He is presently a Professor at the University of Liège, Department of Electricity, Electronics and Computer Sciences. He is the Head of the unit Transmission and Distribution of Electrical Energy. His main area of interest lies in cable dynamics, the electric power line health monitoring, low frequency electric and magnetic field effects on human being, residential energy distributed generation. He is a member of the IEEE and CIGRE. He has published over 100 technical papers. G. Minne received his Degree in Mechanical Engineering (Energetic) from the ISIC Ht high school of Mons in 2002 and graduated as an Aeronautical Maintenance Technician in 2005 in France. He has worked as the Head of the project for PROMOCELL S.A. in complete collaboration with the industrial Copyright 200x Inderscience Enterprises Ltd.
2 S. Lerson, J.L. Lilien and G. Minne chemical unit of the University of Liège (2002 2003) on a micro-chp PEM fuel cell prototype. He has also worked as a Research Engineer with Professor J.L. Lilien on a fuel cell project at the University of Liège (2005 2006). 1 Introduction Residential cogeneration is an emerging technology that has a high potential to deliver energy services with increased efficiency and environmental benefits. The concurrent production of heat and electricity from a single fuel source can reduce primary energy consumption and the associated greenhouse gas emissions. The decentralised production of electricity and heat also has the potential to reduce electrical transmission losses and distribution congestion as well as alleviate utility peak demand problems. A number of manufacturers worldwide are developing residential-scale cogeneration devices based on fuel cells, internal combustion engines and Stirling cycles. The effective exploitation of both electrical and thermal output for space heating, space cooling, and/or heating domestic hot water is critical in realising high levels of overall energy efficiency and the associated environmental benefits. Consequently, the performance of these devices is highly dependent on how the cogeneration device is integrated to service the host s building thermal and electrical demands. To accurately assess the performance, models of cogeneration devices are therefore incorporated into whole-building simulation tool that accounts for the interactions between the building and its environment, the occupants, the thermal and electrical production, distribution systems and energy management and control systems. Voorspools (2002) underlined that mainly static and simplified methods were used in simulation tools, completely neglecting the dynamic interaction between the cogeneration system and the central power systems, and the dynamic response of the cogeneration units themselves. These methods could lead to erroneous conclusions. This signifies that users need a level of detail that is rarely available within technical specifications. The calibration of models used relies heavily upon the empirical information that can be acquired from the testing of coherent system. We obtained protocols on internal combustion engines (Voorspools, 2002), Stirling engines, turbines, Rankine cycles or Proton Exchange Membrane Fuel Cell (PEMFC) (König, 2005), from the literature results of such tests performed with others, but nothing was found on residential SOFC. In this respect, performance assessment of a 5 kw SOFC manufactured by Fuel Cell Technologies (FCT, Canada) was carried out at University of Liège (Belgium) on a dedicated test bench. The IEA Annex 42 protocol for conducting experiments on cogeneration system (www.cogen-sim.net) was followed. The performance of the system was evaluated for different operating conditions. The tests included steady-state measurement under different electrical and thermal loads, as well as an analysis of the dynamic behaviour of the system during load changes.
Performance assessment of a 5 kw SOFC cogeneration fuel cell 3 2 System description The FCT s 5 kw SOFC system (layout on Figure 1, picture of installation on Figure 2) is a cogeneration power system that operates at 900 1000 C and at a pressure close to atmospheric conditions. It converts hydrocarbon fuels (natural gas in our case) into electrical energy and heat, without the use of an intermediate combustion process. First, the natural gas was pre-reformed by steam and sent into the tubular stack where the internal reforming occurs. Then, the electrochemical reaction converts hydrogen and oxygen into water, electricity and heat. A part of the remaining mix (fuel hydrogen and steam) is recirculated in the pre-reformer to bring in the necessary quantity of water, which allows the system to work without input water. The other part of the remaining mix is post-combusted with excess of air. Exhaust gases leave the stack at 800 850 C and preheat the incoming air through a heat recuperator. The remaining heat (200 250 C) is cooled down in the heat recovery exchanger by water used for space heating or domestic hot water. An auxiliary burner is used to heat up the system until the exothermic reaction stops and the stack temperature reaches 600 C. At this temperature, the fuel is fed into the stack and the power of the burner is reduced. From this moment, the electrochemical reaction occurs and electricity is produced. The stable operation of this system (without auxiliary burner) is obtained at 970 C. A power-conditioning unit (including a DC AC converter) is used to connect the unit in parallel with the grid. Figure 1 Fuel cell s basic layout
4 S. Lerson, J.L. Lilien and G. Minne Figure 2 3 View of the test bench Data measured on the test bench The following measurements have been performed and data are integrated and stored at a 1 min interval, over the full duration of each experiment. Static measurements: total mass of cogeneration device composition of fuel (molar fractions of CH4, C2H6, C3H8, higher hydrocarbons, N2, CO2) exhaust gas composition (molar fractions of CO2, N2, Ar, O2, H2O, CH4, H2, CO, etc). Time-varying measurements: natural gas consumption rate (m3/s at standard temperature and pressure1) air supply rate to cogeneration device (kg/s) temperature of air supplied to cogeneration device ( C) flow rate of liquid water supplied to cogeneration device (kg/s) temperature of exhaust gases as they enter gas-to-water heat exchanger ( C) temperature of exhaust gases as they exit gas-to-water heat exchanger ( C) flow rate of water on plant side of gas-to-water heat exchanger (kg/s) temperature of entering water on plant side of gas-to-water heat exchanger ( C) temperature of exiting water on plant side of gas-to-water heat exchanger ( C)
Performance assessment of a 5 kw SOFC cogeneration fuel cell 5 ambient air temperature ( C) gross DC electrical production from cogeneration (W) net DC electrical output from cogeneration device prior to PCU (after parasitic losses, battery losses) (W) net AC electrical output from PCU (W) cogeneration device parasitic electrical draws (e.g., fans, controls) = Balance of Plant (BOP) (W). 4 Protocol for conducting experiments and results The IEA Annex 42 protocol for conducting experiments on cogeneration system has been used and completed to assess the performance of the unit. Thermal and electrical performances were particularly desired, as far as the fuel cell is supposed to be used as a cogeneration system. Both stationary and transient behaviours were analysed. 4.1 Stationary operation The energy performance analysis 2 showed that when in steady state, the SOFC had a net DC electrical efficiency (based on LHV natural gas) of around 40% (look at performance details on Figure 3). This definitely is a high value when compared with other cogeneration systems for such a low-power device. Losses due to BOP represent 6% of the total energy. The thermal efficiency was around 40%. Losses to the chimney were very low. The DC AC converter leads to a net AC efficiency of around 28% and needs improvements. Owing to the fact that the unit is designed for the North American market, an extra transformer had been added to connect the unit to the European grid. This leads to an extra loss, which is non-representative of the performance of a fuel cell. A energy performance review is detailed on Figure 3, as measured on our test site. Figure 3 Energy performance of SOFC
6 S. Lerson, J.L. Lilien and G. Minne 4.2 Transient behaviour (start stop) Figure 4 show a start-up phase of the system. These tests show that the SOFC transient heating takes around 48 h for a cold start. The burner operates to increase the stack temperature. When the stack temperature reaches 600 C, the fuel is sent into the stack zone and the thermal and electrical power increases rapidly. This electrical start-up occurs after 12 h and the nominal electrical power is reached after 150 h. These delays are highly dependent on the stack temperature. Figure 4 Start-up period Table 1 presents the main energy balances during start-up phases. Table 1 Energy balance for start-ups Stack s fuel consumption Burner fuel consumption Electricity production Heat production 157 kwh 214 kwh 85 kwh 165 kwh
Performance assessment of a 5 kw SOFC cogeneration fuel cell 7 5 Conclusion The tests have shown that the steady-state performances of this unit were fairly high when compared with the other cogeneration technologies (45% DC electric, 38% thermal). We have showed that there is a need for a few auxiliary improvements (a non-minor aspect is the liability), but the heart of the technology itself (SOFC cells) has a remarkable performance behaviour. The start-up phase (cold start) still takes a long time for widespread use in residential areas. Also, the use of new material (low SOFC temperature) or the continuous operation of the system would cancel this bottleneck. Finally, tests under non-nominal state showed that the heart of the system as well as electricity production was not influenced by the inlet water temperature and flow. The thermal performance of the system is highly dependant on the (lost) exhaust gas temperature, i.e., by the water inlet temperature. These experiments allow us to postulate that the development of SOFC technology is on a good track. It still needs vital research efforts to become widespread but has showed remarkable performance paving a new horizon for the study on SOFC. Acknowledgements This work has been supported by the government of Walloon Region, Belgium, under the Green Family Project. The authors gratefully acknowledge the government, and particularly the Energy Research Division (DGTRE). This work was also undertaken as part of the International Energy Agency s Energy Conservation in Building and Community Systems Programme s Annex 42, the Simulation of Building-Integrated Fuel Cell and Other Cogeneration Systems (www.cogen-sim.net). This work has produced a general report referred as Knight et al. (2005). References König, P., Weber, A., Lewald, N., Aicher, T., Jörissen, J., Ivers-Tiffee, E., Szolak, R., Brendel, M. and Kaczerowski, J. (2005) Testing and model-aided analysis of a 2kWel PEMFC CHPsystem, Journal of Power Sources, Vol. 145, pp.327 335. Voorspools, K. and D haeseleer, W. (2002) The evaluation of small cogeneration for residential heating, Int. J. Energy Research, Vol. 26, pp.1175 1190. Notes 1 The natural gas pressure is constantly 145 mbar. We assumed the temperature to be 10 C. 2 Reference temperature = 25 C. Website www.cogen-sim.net
8 S. Lerson, J.L. Lilien and G. Minne Bibliography Beausoleil-Morrison, I., Cuthbert, D., Deuchars, G. and McAlary, G. (2002) The simulation of fuel cell cogeneration within residential buildings, Proc. E-Sim, Bi-annual Conf. of IBPSA-Canada, Montreal, pp.40 47. Daoud, I. and Pierreux, N. (2001) Analyse de systèmes de micro-cogénération, Travail de fin d étude, Université catholique de Louvain, Louvain, Belgium. Dorer, V. and Weber, V. (2003) Integrated Cogeneration with Fuel Cell and Thermal Solar System for the Home of the 2000 W Society, to be published. Knight, I., Ugursal, I. and Beausoleil-Morrison, I. (2005) Residential cogeneration systems: a review of the current technologies: a Report of Subtask A of FC+COGEN-SIM: the simulation of building-integrated fuel cell and other cogeneration systems, Annex 42 of the International Energy Agency, Energy Conservation in Buildings and Community Systems Programme, 92 pages. Lerson, S. (2004) Piles à combustible résidentielles: projet d installation, DEA (Diplôme d étude approfondie), Université de Liège, Belgium. Lilien, J.L, Pochet, N. and Lerson, S. (2003) Fuel Cell for Residential and Inherent Thermal Management, Groove Fuel Cell Symposium. Singhal, S. and Kendall, K. (2003) High Temperature Solid Oxide Fuel Cell, Fundamentals, Design and Applications, Elsevier, Oxford, UK. Vetter, M. and Wittwer, C. (2002) Model-based development of control strategies for domestic fuel cell cogeneration plants, France-Deutschland Fuel Cell conference, Franhofer Institute for Solar Energy Systems ISE. Symbols and abbreviations DHW NG LHV SOFC PEMFC PCU BOP Domestic Hot Water Natural Gas Low Heating Value Solid Oxide Fuel Cell Proton Exchange Membrane Fuel Cell Power Conditioning Unit Balance of Plant