Potential of Mobile SOFC-GT Systems

Transcription

1 W. Winkler 1) Potential of Mobile SOFC-GT Systems 1. Demands on commercial fuel cells 2. Motivation and design of mobile SOFC-GT systems 3. Basic mobile SOFC-GT Inventions 4. The mobile SOFC-GT system design study 5. Future Development in the production technology 6. Conclusions 7. References Fuel Cells IBC Asia Limited Conference July 2001 Tokyo, Japan 1) Prof. Dr.-techn. Wolfgang Winkler University of Applied Sciences Faculty of Mechanical Engineering (fuel cells) Berliner Tor 21, D Hamburg Tel.: ++49 (0) 40 / Fax: ++49 (0) 40 / winkler@rzbt.fh-hamburg.de Home:

2 1. Demands on commercial fuel cells Today nearly all major automotive companies are involved in the development of PEFC propulsion systems. It is expected to improve the vehicle s environmental performance by the clearly lower emissions and the expected higher efficiency of such systems, compared with conventional propulsion systems. The SOFC systems are mainly developed for the stationary applications by the utility manufacturers. The actual development of microturbines, the excellent thermal performance of thin ceramic tubes and the developments for the use of logistic fuels are the recent motivation to discuss the mobile application of SOFC again. Generally the operation of automotive engines and power plants are following the same thermodynamic principles. But the manufacturing process, the actual load characteristics and the operational demands are completely different. stationary device technology mass production quantity of units mobile Fig. 1 Economy of scale and quantity for stationary and mobile applications distributed generation power plant The mass production of automobiles and the most mobile applications is governed by the laws of the economy of quantity, fig. 1. The economy of scale of power plants depends on the savings by the increasing size of the plant. This is one major reason of the different technical solutions and cultures of mobile and stationary applications. Only the ship engines are mobile applications that depend on the economy of scale like power plants. The use of automotive engines for stationary power applications did not really succeed yet. There is a high probability, that the stacking of fuel cells and the modularization of stacks will lead to an economy of quantity in the power industry similar to the automotive industry. The expected development to use fuel cells for a distributed generation with a high electric efficiency > 60 % will give an interesting option for an increase of the use of mass produced power generators in the future. It will be interesting to see the possible future Quality cost low pollution reliability specific volume specific weight start-up time fuel logistic efficiency utilization life time fuel cell plant technology on site erection size of plant Operational demand Power plant automobile ship Automobile W. Winkler 1999 W. Winkler 1998 interactions between the power industry and the automotive industry in that field (1). Fig. 2 Demand profile of fuel cells for stationary and mobile applications The other, probably the most important, influence on the technical development is the customer s demand, leading to the different load characteristics and the different operational demands. The demand

3 profile of the stationary application is governed by the demand on a safe energy supply of the customer. The demand profile of the mobile application is governed by the demand on mobility of the customer. Fig. 2 gives an impression of the demand profile of the both applications. The customer expects the lowest possible cost, the lowest possible pollution and the highest reliability of both applications. The power related volume, the power related weight and the startup time of the engine are very important figures for any mobile application. These figures have a strong influence on the quality of the mobility of the customer, because they influence the usable space and the performance of the vehicle, as acceleration and fuel consumption etc.. But these figures do not strongly influence the use or the performance of a stationary power generation unit. The logistic of the fuel supply is important in both cases. A power generation unit can be designed for a defined fuel that is locally available. The fuel logistic for a mobile application is more sophisticated, because it influences the customer s demand on mobility directly. The energy density of the used fuel influences the size of the fuel storage or the range of the vehicle. The possible fuel qualities that can be used influence the dependence upon a certain fuel distribution network. The efficiency, the utilisation and the lifetime of the unit are essential for any stationary or mobile commercial application. A private user may have other priorities e.g. design, luxury etc. as far as these properties don't impair the mobility. The above formulated conditions will be the measure for the later presented SOFC-GT design study. 2. Motivation and design of mobile SOFC-GT systems The use of logistic fuels as e.g. diesel in a SOFC has been already proven (2) and the efficiency potential can be estimated easily (3), (4). The high efficiency potential of a SOFC system is caused by the integration of the fuel processing and the thermodynamic value of the waste heat. The dissipated heat of any high temperature fuel cell can be used by any heat engine to generate additional power. The reference cycle of any heat engine is known as Carnot cycle and an system efficiency [% ] PEFC power consumption, losses (pumps, fans, fuel processing) SOFC cell temperature [ C] ad. heat - engine W. Winkler 1998 isothermal operation of the fuel cell as its heat source can be assumed. The simplified combined fuel cell - heat cycle consists of the pre-heaters for the fuel and the air, heated by the waste gas of the cell to be cooled and a heat engine being supplied with the dissipated heat of the fuel cell and the fuel cell itself. Fig. 3 Efficiency potentials of fuel cell systems The benefit of this reference cycle is the simple estimation of the efficiency potentials. This can be used to compare a PEFC and a SOFC system. The ratio of the efficiencies of a real heat

4 engine and a Carnot cycle is roughly 0,7 to 0,8. The total efficiency of a combined fuel cell - heat cycle can be estimated easily therefore and the efficiency potential of a combined SOFC system can be roughly estimated between 75 and 80 % as shown in fig. 3. The high temperature of the dissipated heat of a SOFC makes a combination with a gas turbine (SOFC-GT) attractive and the system efficiency increases, compared with a stand alone fuel cell system. The low value of the dissipated heat of a low temperature fuel cell as a PEFC does not allow an additional power production by a heat engine. Therefore the necessary power to operate the system must be supplied by the fuel cell itself and the system efficiency decreases compared with the efficiency of the fuel cell. The necessary external heat supply for the fuel processing is another source of thermodynamic losses of the low temperature fuel cell systems (3), (4), (5). There are two possibilities to approach the reference cycle of an isothermal operated fuel cell with a direct heat extraction by a connected heat cycle. The SOFC stack can be cooled by an integrated cooling loop and the coolant is the operating fluid of the connected heat cycle. Alternatively the SOFC stack can be divided in sub-stacks and the oxygen rich flue gas of any sub-stack is cooled by a connected heat engine and is fed to the cathode of the next sub-stack again. The adiabatic SOFC operation is not an adequate solution because the excess air and thereby the waste heat loss increases to too high values (6). A SOFC-GT cycle with an external cooling as used for the following study is shown in fig. 4 (6). The incoming air and the incoming fuel are heated by the flue gas of the SOFC. The connected heat cycle is a GT cycle. The cooled flue gas is reheated by the integrated coolers of the SOFC and thereafter it is expanded in the gas turbine. The integrated reformer is used as an additional cell cooling. The depleted anode gas flow can be recycled as a steam source for the reforming. air compressor fuel gas fuel gas air flue gas HEX mixer balance border integrated SOFC stack gas heater cathode anode reformer mixer throttle burner gas turbine anode gasrecycling jet pump W. Winkler 1998 Fig. 4 The SOFC - GT cycle with an external cooling The pressure difference on both sides of the heat exchanger walls is low - only the pressure loss of heat exchangers (HEX) and stack. Thus the SOFC-GT cycle with an external cooling has comparable low restrictions referring to the choice of the high temperature material. The much shorter lifetime of automotive components facilitates such developments additionally compared with power plant applications. Additionally the SOFC-GT cycle with an external cooling has the benefits of using only one pressure level for the total SOFC process, of using total pressurised HEX surfaces with a high heat transfer coefficient and of operating at low pressure differences between the HEX surfaces. This gives a good option to design a very compact, cost effective HEX system as needed for mobile applications.

5 3. Basic mobile SOFC-GT Inventions One technical key issue for the further discussion of a mobile SOFC is thus the design of a SOFC-GT cycle, including the availability of a small gas turbine and the influence of the design of the stack and of the system on the power related volume and the power related weight. The other one is the potential to reach the required short start-up time of the mobile SOFC system. The interesting developments of the last years that made a mobile SOFC-GT application to a realistic option are collected in fig. 5. SOFC temperature [ C] SOFC cycles, source : K. Kendall et al. tubular SOFC 2 mm diameter time t [ s ] thin tubular SOFC short start - up time recuperator compressor combustor turbine microturbine small SOFC-GT cycles realistic chance for mobile SOFC-GT development W. Winkler 2000 Fig. 5 The basic innovations for a mobile SOFC-GT system The temperature gradients that can be realised by a thin tubular SOFC, as shown by K. Kendall et. al. (7) for a tube with 2 mm diameter in 1997, gives the impression that thin tubes could solve the start-up problem of the SOFC in mobile applications. Here the temperature gradient is 200 K/min in stead of the usual 200 K/h. The development and the market introduction of the microturbines shows the availability of small sized turbines down to 25 kw capacity (8). The second benefit of thin tubes in general is the high power density > 1 kw/l. A smaller SOFC diameter reduces the cost of the BOP (balance of plant) by smaller pressure vessels and insulation as well (9), (10). Thus the thinner SOFC tubes are very interesting options here and in other applications as well. The influence of the tube diameter on the power related volume of the stack can be seen in fig. 6. A SOFC diameter of 20 mm leads to a power related volume of 5 l/kw but a 2 mm thin allows 0,5 l/kw. Fig. 6 The influences on the power related volume of SOFC stacks The high efficiency of SOFC-GT cycles, the comparable simple use of Diesel oil and other hydrocarbons and the long lifetime of the SOFC were very motivating to start investigations of such designs with students (11). The main purpose of the reported work was to find out, what G

6 restrictions could occur by integrating such a system in a car. Thus a principle design study was necessary. The general thermodynamic theory is the same as for the stationary system. But the process design is strongly influenced by the demand on a very short start-up time. 4. The mobile SOFC-GT system design study The basic innovations thin SOFC tube and microturbine and results of stationary system studies were the base that led to the conceptual system design as shown in fig. 7. The power system of the car is organised like an electric grid. The pressurised SOFC-GT module is the mid load power plant. battery The battery system is the peak load fuel tank power plant and the power electronic is the load distribution of the grid (12), (13). SOFC - GT air supply power electronic el. wheel motors W. Winkler 2000 Fig. 7 The integration of the SOFC- GT system in the power train One important task of this system design is to deliver sufficient power for all load situations including a short start-up time. The total energy system on board of a vehicle driven by a SOFC-GT system consists of three sources of electric power : the SOFC, the gas turbine and the battery. Two of them, the battery and the gas turbine will be immediately available for the propulsion. There are two principle extreme possibilities to assure the power supply of the car during the heating-up time of the SOFC module. The capacity of the batteries can be extended to deliver the required power during the start-up time or the capacity of the microturbine can be increased to deliver the needed power during the heat-up phase of the SOFC. If the batteries are extended the dead load of the car increases and this will reduce the total efficiency of the automotive system. The electric efficiency of the SOFC-GT system decreases with a decreasing SOFC capacity share too because an increasing part of the used fuel is burnt in the burner of the microturbine and not in the SOFC. Both influences should be deeper analysed and optimised. The base of such studies are principal design studies that allow to analyse the consequences of the variations. The necessary decisions to start the presented design study was thus very simple. The minimum available microturbine of today has a capacity of 25 kw. The size of the microturbine was thus fixed to 25 kw for the design study. This is one third of the total system capacity of 75 kw. Fig. 8 shows the principles of the chosen start-up procedure. The capacity of the microturbine of 25 kw is big enough to move the car in residential areas. This low power operation can be supported by battery power if necessary. After 2-5 minutes the total system is operating.

7 available power low power operation battery power for peak load target : < 2-5 min. only GT operation SOFC - GT operation normal operation 2/3 SOFC power 1/3 GT power W. Winkler 1998 Fig. 8 Start up and design targets The difference between a stationary and a mobile SOFC-GT lead to the following restrictions for a mobile system design. The required space of the system is a key issue. Thus the higher pressure losses must be accepted to decrease the required volume of the SOFC module including the heat exchangers. The influence of the leakage within the stages of gas turbines increases with a decreasing size of the gas turbine. Thus the compressor and the turbine of a microturbine have a smaller isentropic efficiency as a gas turbine in a power plant and the pressure ratio is smaller than in a large gas turbine. A cycle model was used to calculate the basic design data. The calculations of the mobile SOFC-GT system showed the connections between the electric efficiency and the above mentioned restrictions of the mobile system. Fig. 9 shows the results. electric efficiency 0,9 0,8 0,7 Fig. 9 The influences on the efficiency of mobile SOFC-GT systems at 900 C SOFC module temperature 0,6 0,5 0,4 0,3 0,2 The decrease of the isentropic efficiency of the compressor and the turbine is illustrated by an efficiency curve for an isentropic efficiency of 88 % of the compressor and an isentropic efficiency of 92 % of the turbine as known from big power plant SOFC module pressure in bar gas turbines and the reduced values 80 % and 85 % of a microturbine. The relation of the sizes of the SOFC and the gas turbine is the ideal relation for a maximum efficiency at any pressure level. The influence of the isentropic efficiency increases with the SOFC module pressure. The fixed combination of the capacities, 1/3 gas turbine and 2/3 SOFC, as chosen for the study delivers always a lower efficiency than the ideal relation. But the influence on the efficiency decreases with an increasing SOFC module pressure. Finally the demand of reducing the volume leads to an increase of the pressure loss of the heat exchangers to increase the heat transfer coefficient. It is important to choose a microturbine with a SOFC module pressure of 4 bar or higher to reduce the negative influences of the pressure loss. It is necessary to consider the fixed capacity relation of microturbine and SOFC module in combination with the battery as mentioned above in a later stage and an efficiency of about 55 % ore even more may be obtained. time

8 fuel tank power electronic Fig. 10 Design study of a mobile 75 kw SOFC-GT power train system batteries fuel preheater el. motor generator microturbine SOFC module H. Lorenz 2000 These results led to the conceptual design as shown in fig. 10. The necessary information about the dimensions of the car was taken from an actual compact midclass car. The pressurised SOFC module including the air and the fuel heaters is under the hood of the car. The microturbine is flanged directly at the SOFC module. The batteries, the power electronic and the fuel tank are positioned in the mid and at the rear of the car. One interesting result of the design study is that the placement of the components left a still available space. Thus the failure tolerance of the system design is comparable high because there is still a reserve of space available that could be used for possible corrections if the further developments should show that some assumptions had been too optimistic. 5. Future Development in the production technology An important step to realise such designs is the development of stacks allowing the demanded short start-up times within a few minutes and the necessary power density. A key component will be the thin SOFC tube. The options to produce this key component are increasing. The extrusion process as reported in e.g. (7) with the results shown in fig. 5 and a tube production by the electrophoretic deposition technology (14) are already documented in the literature. Fig. 11 Micro-tube for a mobile SOFC application. Source: K. Rennebeck (15) An other interesting emerging technology is the use of the spinning process to produce the needed thin tubes at comparable low cost. The simultaneous spinning of two layers is a further interesting option of this production process (15). Fig. 11 shows a sample of the micro-tubes produced by a spinning process.

9 The increase of the power density with the decrease of the tube diameter shows the importance of matching the system design and the stack design as well to achieve the lowest possible unit cost. The connection between tube diameter and the insulation volume and the pressure vessel material volume and thus the cost has been already shown in (9). The high power density of small tubes leads to a smaller power related surface and thus to a smaller power related consumption of surrounding material - as mentioned before. 6. Conclusions The small tubular SOFC will become a key component of the further development. The main benefits are the short start-up time and the high power density that allows automotive applications with a high number of produced units and that reduces the cost of the surrounding system at stationary applications too. The efficiency of the mobile and the stationary SOFC-GT application is clearly higher than any competing technology. The actual fuel logistic and any regenerative fuel as well can be used by such systems. But the measure for the cost are the competing technologies however there are a lot of benefits of SOFC-GT systems that are important for an economic and sustainable development. The results of the studies encourage a further and deeper consideration with the mobile SOFC-GT technology. 7. References (1) Winkler W. : Benefits and risks of mobile SOFC applications. European Fuel Cell News, Vol. 6, No 2, July 1999, S (2) Veyo S.E. : Tubular Solid Oxide Fuel Cell demonstration activities. 2nd IFCC February 5-8, 1996, Kobe. p (3) Winkler W. : Der Einfluß der Prozeßkonfiguration auf das Arbeitsvermögen von Verbrennungskraftprozessen. Brennstoff Wärme Kraft 46 (1994) Nr.7/8. p (4) Winkler W. : SOFC-Integrated Power Plants for Natural Gas. 1st European Solid Oxide Fuel Cell Forum. Ulf Bossel. Luzern p (5) Bossel U. : Solid Oxide Fuel Cells for Transportation. Proceedings 3rd EUROPEAN SOLID OXIDE FUEL CELL FORUM in Nantes, Juni Ed. Philippe Stevens. Oral Presentations. S (6) Winkler W. : Thermodynamic influences on the cost efficient design of combined SOFC cycles. Proceedings 3rd EUROPEAN SOLID OXIDE FUEL CELL FORUM in Nantes Ed. Philippe Stevens. Oral Presentations. p (7) Alston T., Kendall K., Palin M., Prica M., Windibank P. : A 1000-cell SOFC reactor for domestic cogeneration. Journal of Power Sources 71 (1998). S

10 (8) de Biasi V. : Low cost and high efficiency make 30 to 80 kw microturbines attractive, Gas Turbine World, Jan.-Febr p (9) Winkler W. : Cost effective design of SOFC-GT. Proceedings zu 6 th International Symposium on Solid Oxide Fuel Cells. American Electrochemical Society in Honolulu, USA. p (10) Winkler W., Krüger J. : Design and manufacturing of a tubular SOFC combustion system. Fifth Grove Fuel Cell Symposium Commonwealth Institute. September 1997 in London. Journal of Power Sources 71 (1998). p (11) Collection of intermediate reports on the seminar : Mobile SOFC-GT systems. University of applied sciences Hamburg.1999/2000. Not published. (12) Winkler W., Lorenz H. : The design of stationary and mobile SOFC-GT systems. (Proceedings 7 th UECT Ulm Electrochemical Talks in Neu Ulm). (13) Winkler W., Lorenz H. : Differences and synergies between mobile and stationary SOFC- GT designs. To be published in : Proceedings zu 7 th International Symposium on Solid Oxide Fuel Cells. American Electrochemical Society, in EPOCHAL, Tsukuba, Japan (14) Negishi H. et al. ; Fabrication of small tubular SOFCs by electrophoretic deposition technology. Solid Oxide Fuel Cells (SOFC VI) Proceedings of the Sixth International Symposium, Editors S.C. Singhal, M, Dokiya, Electrochemical Society Proceedings Volume ISBN Pennington p (15)Rennebeck K. : personal communication.