Layout of SOFC-GT Cycles with Electric Efficiencies over 80

摘要

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.