Layout of SOFC-GT cycles with electric efficiencies over 80 % Wolfgang Winkler and Hagen Lorenz fuel cells and rational use of energy, faculty of mechanical engineering university of applied sciences Berliner Tor 21, D-20099 Hamburg Tel. :+49-40-428 59-3137 /-4299, Fax :+49-40-428 59-2658 e-mail: email@example.com Abstract The thermodynamic reference cycle of any combination of fuel cells and heat engines indicates an efficiency potential of 80 % for real cycles. The combination of a SOFC and a gas turbine (GT) connects the air flow with the theoretical independent heat engine supplied by the cell cooling. The waste heat extraction of a SOFC module can be done by an intermediate expansion of the waste air in gas turbines located after each SOFC sub-module of a divided SOFC module or by an external cooling of the SOFC module by the flue gas cooled down by the air and fuel heating. The combination of both principles leads to a reheat (RH) SOFC-GT cycle that can be improved by a steam turbine (ST) cycle. The first results of a study of such a RH-SOFC-GT-ST cycle indicate that a cycle design with an efficiency of more than 80 % is possible and confirm the predictions by the above mentioned theoretical thermodynamic model. The calculations show that the influence of the system pressure decreases with the increasing efficiency of the cycle by adding the ST cycle, caused by a better heat recovery of the ST cycle at lower pressures. The size of the excess air has to be sufficient for the electrochemical reaction. This indicates that the system should be operated with an excess air of about 1,5 at temperatures of about 950 °C and at a maximal pressure between about 15 to 20 bar to avoid higher pressures at higher temperatures. The additional increase of the efficiency by the higher temperature is comparable small and it seems that higher temperatures and pressures are not satisfied. The addition of the ST cycle needs a minimum capacity of the cycle of more than 10 MW to get the additional ST cycle commercial. Thus it can be expected that the market entrance with small SOFC-GT units will happen without an additional ST cycle and the RH SOFC-GT-ST cycle may become commercially interesting later. But the RH SOFC-GT system alone might be an interesting cycle for the market entrance because it allows an efficiency of more than 70 % and delivers a waste gas with a very high temperature for different industrial CHP applications without an alone power producing ST cycle. The thermodynamic principles of SOFC-GT design The efficiency potential of combinations of fuel cells and heat engines has been estimated with about 80 % by a generalised thermodynamic model already in 1993 (1). The U.S. Department of Energy has mentioned this figure for combinations of SOFC and gas turbines in its announcement for research in 1999 (2). The principles of the design of combinations can be learned by the generalised thermodynamic model : the generalised fuel cell - heat engine cycle as given in (1), fig. 1. work work work Fig. 1 The generalised heat heat engine fluid fuel cell - heat engine ∆ϑmax - cycle and the area and the fuel cell heat fuel processing tasks of the thermal engineering area of preheater waste heat thermal air The fuel cell is only a engineering fuel flue gas power delivering heat source like a "power • choice of heat engine and integration producing burner" defined • integration of fuel preparation • integration of preheaters by the ratio of delivered ⊗ restriction by ceramic cell : ∆ϑmax power and heat in the area of thermal engineering and W. Winkler 2000 the resulting engineering tasks are listed in fig. 1. The thermal integration of the fuel processing is necessary to avoid entropy losses to the environment resulting in efficiency losses in the order of 10 % as already shown in (3), (4). The generalised cycle shows that the heat recovery process for the air heating and the fuel heating is independent of the heat engine cycle. We could realise such a cycle by using e.g. a Stirling engine as the heat engine. But if we use a common gas turbine (GT) as a heat engine, we get a matching between the gas turbine cycle and the air heating because the air flow of the fuel cell becomes a part of the GT process as well. The design process of such a GT cycle is directly determined by the restrictions of the thermal stresses of the SOFC. The maximal allowable temperature difference ∆ϑmax between the inlet and outlet temperature of the cathode e.g. 150 K delivers a very high air flow for the SOFC cooling only by air. If we allow this, the waste gas loss will drastically increase and the system efficiency can become lower than the cell efficiency itself. Thus any successful cooling strategy of the SOFC of a SOFC-GT system must avoid a high excess air at the outlet of the total system. Fig. 2 gives an overview over the possible strategies. One strategy is to divide the SOFC module in sub- modules and to extract the heat of the SOFC module by cooling down the waste air of the first sub-module to the inlet temperature of the cathode of the following sub-module by a gas turbine and producing additional power. This process of an intermediate expansion (INEX) can be carried on until the last GT delivers the waste gas for the air heater and the fuel heaters (HEX). The other strategy is to Intermediate expansion INEX : External cooling EXCO : cool the SOFC module by an external cooler (EXCO) ° exhaust temperature fed with the flue gas that ¬ SOFC waste heat has been cooled by the air extraction (sub-systems) and the fuel heating. Fig. 2 Cooling strategies of SOFC modules by GT cycles - pressure difference HEX walls fuel The SOFC module is the ® air inlet temperature in SOFC air heat source of the GT flue gas cycle and the air is heated reformer ¯ size of HEX surfaces W. Winkler 2000 by the flue gas as shown in the generalised model. This cycle is the result of the trial to use the cold air for the cell cooling and to use the full temperature difference between compressor and cell outlet (5). But the direct heat transfer to the cold air by the cell would lead to the damage of the cell. The integrated gas heater/cell cooler can be heated by radiation and this clearly reduces the danger of a cell damage by not acceptable temperature differences. The integrated gas heater allows an optimisation of the temperature level of the cooling flows around the cell together with an integrated air heater and this avoids unacceptable thermal stresses of the cell ceramic and disturbances of the electrochemical process. The main differences between INEX and EXCO are listed in fig. 2 and compared in fig. 3 (6),(7). The waste heat extraction ¬ is done in one pressure level in the EXCO design. The waste heat extraction is done in up to n pressure levels in the INEX design, depending on the allowable temperature difference of the cathode. The number of pressure INEX : EXCO : levels is equal to the number of the pressurised ¬ SOFC waste heat 2 - n pressure 1 pressure subsystems. extraction (sub-systems) levels (systems) level (system) - pressure difference maximal pressure only pressure Fig. 3 Comparison of the HEX walls differerence loss INEX and the EXCO design ® limit for air inlet gas turbine outlet SOFC temperature in SOFC temperature The pressure difference on temperature the heat exchanger (HEX) min. 1/2,5 of min. 1/7 of walls mainly of the air ¯ size of HEX surfaces ambient system ambient system heater - is the maximal ° exhaust temperature ~ 200 °C 500 - 600 °C pressure at the INEX design and only the pressure loss W. Winkler 2000 of the module at the EXCO design. The demands of an EXCO design on the material quality for the heat exchangers is thus comparable small. The maximal air heater outlet temperature ® is limited by the SOFC (module outlet) temperature at the EXCO design and by the lower gas turbine outlet temperature at the INEX design. The size of the HEX ¯ of an INEX design is about 2,5 times smaller than under ambient conditions caused by the pressurisation at one side, but the EXCO design has up to about 7 times smaller HEX surfaces than under ambient conditions caused by the pressurisation on both sides (6). The electric efficiency of an INEX design with two turbines is about 70 % (8) similar to the EXCO design. But the exhaust temperature ° of the INEX design is about 200 °C and of the EXCO design is about 500 to 600 °C depending on the individual parameters. The EXCO design has thus the potential for a combination with a steam turbine cycle (ST) that could be e.g. a Cheng cycle. This would lead to an electric efficiency of about 75 % (5). The layout of a reheat (RH) SOFC-GT-ST cycle The first ideas of the EXCO design included a reheat cycle (5) but with an additional heat exchanger within the SOFC module. This design didn't seem very easy to realise. But a comparison of the INEX and the EXCO design shows that the benefit of the EXCO design to reduce the excess air in one process step at one pressure level with small HEXs can be combined with the benefit of the INEX design to allow a simple cascading of gas turbine cycles as needed for a reheat GT- cycle. This led to the following proposal of the reheat SOFC-GT cycle with a bottomed steam turbine (ST) cycle - the RH SOFC-GT-ST cycle - as shown in fig. 4. compressor LP-SOFC module Fig. 4 The layout of the RH SOFC-GT-ST cycle HEX flue gas The EXCO design is the HP-SOFC module first stage of the RH turbine fuel gas waste air SOFC-GT-ST cycle. The air turbine first GT, after the high st waste air 1 stage pressure section (HP- flue gas steam cycle steam /water SOFC module) as the first stage, is named as "waste Target : ηel ≥ 80 % air turbine" to show that W. Winkler 2000 the waste air is used as the combustion air of the second stage. The second stage, the low pressure (LP) section, doesn't need any external gas cooler because the comparable small LP- SOFC module is cooled by a comparable high waste air flow coming from the HP-SOFC module. The waste gas boiler of the ST cycle is supplied with the flue gas of the last GT - the "flue gas turbine" - to use the waste heat of the cell in a most efficient way. This cycle has been calculated by a PC based SOFC-GT model with methane as the fuel and a cell efficiency of 55 %. Some results are presented in the following figures to give a first impression of the performance of this design. The examinations of the RH SOFC-GT-ST cycle will be continued. Fig. 5 shows the electric efficiency of the system (produced power related on LHV) of the RH SOFC-GT cycle and of the RH SOFC-GT-ST cycle as well. 0,85 SOFC module Fig. 5 The electric efficiency of the temperature in °C RH SOFC-GT cycle and of the RH SOFC-GT-ST cycle electric efficiency 0,8 950 The efficiency of the RH SOFC-GT RH SOFC-GT-ST cycle 1000 cycle depends more on the HP- 0,75 1050 SOFC module pressure than the 950 efficiency of the RH SOFC-GT-ST 1000 0,7 cycle. The results of the RH SOFC- 1050 GT cycle are similar to the results RH SOFC-GT cycle of a RH GT cycle cooling the SOFC 0,65 module as published in (5) in a 5 10 15 20 25 30 combination with a Cheng cycle. But the new design shows a more HP-SOFC module pressure in stable performance if the system bar pressure is changed. It seems to be a benefit to separate the RH- ζsteam = 0,8 H.Lorenz, W. Winkler 2000 SOFC-GT cycle and the ST cycle. The effect of the addition of the ST cycle is that the optimal HP-SOFC module pressure is reduced and the differences between the efficiencies at different HP- SOFC module pressures at one certain temperature level are reduced as well. However the influence of the SOFC module temperature is comparable small. Thus near 80 % seems to be a border, as predicted. The reason why the ST cycle reduces the differences in the system efficiency can be easily understood 1000 by fig. 6. Fig. 6 shows the outlet temperature of flue SOFC module influences on the outlet gas turbine in °C 800 temperature in °C 600 950 temperature of the flue gas 1000 turbine that is the outlet 400 1050 temperature of the RH 200 SOFC-GT system as well. 0 5 10 15 20 25 30 Fig. 6 The outlet tempera- HP-SOFC module pressure in bar ture of the flue gas turbine depending on the HP- SOFC module pressure H.Lorenz, W. Winkler 2000 The outlet temperature of the RH SOFC-GT system allows a higher heat recovery of the ST cycle for lower HP-SOFC module pressure levels than for higher pressure levels at all temperature levels. The importance of the ST cycle increases with a decreasing HP-SOFC module pressure. The above discussed analysis of 2,5 the cycles is purely directed on the demands of heat cycles. But 2 SOFC module it is still necessary to discuss the temperature in °C consequences of the optimisation Excess air 1,5 950 of the process flows on the 1000 oxygen supply of the SOFC. Fig. 1 1050 7 shows the excess air as a function of the HP-SOFC module pressure and the SOFC module 0,5 impossible area temperature. 0 Fig. 7 The excess air depending 5 10 15 20 25 30 on HP-SOFC module pressure HP-SOFC module and SOFC module temperature pressure in bar The results showing an excess H.Lorenz, W. Winkler 2000 air < 1 are impossible. However the Nernst voltage is influenced by the excess air too and thus the excess air in real plants must be clearly higher than 1 (7). We see that the SOFC module temperature of 950 °C is a reasonable figure for values of the excess air > 1,5. If we consider that the mentioned excess air is after the waste gas burner and the excess air in the last SOFC is thus higher we could assume that a SOFC module temperature of about 1000 °C might be possible. This will depend on the mass transfer within the SOFC too. percentage of produced power 60 53,01 The technical possibility of a design finally depends 50 on the size of the unit. Fig. 40 8 shows the percentage of the produced power by the in % 30 different power generating 20 14,45 14,22 components of the system. 12,8 10 5,52 Fig. 8 The percentage of 0 power generation by the HP-SOFC LP-SOFC waste air flue gas steam system components module module turbine turbine cycle ζsteam = 0,8 The SOFC modules H.Lorenz, W. Winkler 2000 deliver more than two third of the produced power in the case of the RH SOFC- GT-ST system and about more than three quarters of the RH SOFC-GT system. This shows that the optimisation of the steam cycle is an important part of the system design to reach an efficiency target of more than 80 %. But if we need a ST cycle we need a minimal size of capacity of the total system > 10 MW. Only the demand of a very high efficiency > 73 % leads to an additional ST system including water treatment and boiler operation etc. Thus it can be expected that the market entrance with small SOFC-GT units will happen without an additional ST cycle and the RH SOFC-GT-ST cycle may become commercially interesting later. But the RH SOFC-GT cycle can be built for smaller capacities, depending on the available gas turbines, with an efficiency over 70 % and the benefits of delivering a waste gas with a very high temperature for different process applications in CHP units for industrial applications and operating at a comparable low SOFC module temperature. Conclusion The results of a study of a RH-SOFC-GT-ST cycle indicate that a cycle design with an efficiency of more than 80 % is possible and confirm earlier predictions by theoretical thermodynamic models. The calculations show that an additional optimised ST cycle is necessary to boost the RH SOFC-GT technology. This technology seems too costly for the market entrance with SOFC-GT systems with a comparable small capacity as needed for a distributed generation. The RH SOFC-GT-ST technology can be used later for units with a capacity > about 10 MW for a more centralised generation. But the RH SOFC-GT system alone allows an efficiency of more than 70 % and delivers a waste gas with a very high temperature for different industrial CHP applications at a comparable low SOFC module temperature but without an only power producing ST cycle and it may be interesting for a market entrance as well. Acknowledgement This work was done within the project : "Einsatz von Hochtemperaturbrenn- stoffzellen (SOFC) in der Energietechnik " (aFuE - FKZ 1701998) funded by the German Bundesministerium für Bildung und Forschung (BMBF). The authors want to thank for this funding. References (1) Winkler, W. : Analyse des Systemverhaltens von Kraftwerksprozessen mit Brennstoffzellen. Brennstoff - Wärme - Kraft 45 (1993) Heft 6. p. 302 - 307. (2) U.S. Department of Energy : Broad Agency Announcement (BAA) NO. DE-BA26-99FT40274 for research entitled : "Multi-Layer Ceramic Fuel Cell Research". 7th June1999. (3) Winkler, W. : SOFC-Integrated Power Plants for Natural Gas. Proceedings First EUROPEAN SOLID OXIDE FUEL CELL FORUM. 3 - 7 October 1994. Ulf Bossel. Lucerne. 1994. p. 821 - 848 (4) Winkler, W. : Lay out principles of the integration of fuel preparation in fuel cell systems. Proceedings 2nd IFCC (International Fuel Cell Conference). NEDO, Kobe, Japan. 1996. p. 397 - 400. (5) Winkler, W. : Möglichkeiten der Auslegung von Kombikraftwerken mit Hochtemperaturbrennstoffzellen. Brennstoff-Wärme-Kraft 44 (1992) Heft 12. p. 533 - 538. (6) Winkler, W. : Thermodynamic influences on the cost efficient design of combined SOFC cycles. Proceedings 3rd EUROPEAN SOLID OXIDE FUEL CELL FORUM in Nantes. 1998. Ed. Philippe Stevens. Oral Presentations. p. 525 - 534. (7) Winkler, W. : Cost effective design of SOFC-GT. Proceedings 6th International Symposium on Solid Oxide Fuel Cells. American Electrochemical Society in Honolulu, USA. 1999. p. 1150 - 1159. (8) George, R.A. : SOFC Combined Cycle Systems for Distributed Generation. American Power Conference. Chicago, Illinois, April 1, 1997. Paper No. DOE/MC/28055-97/C0837.
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