Control Issues of SOFC-GT Hybrid System

It should be underlined that, in the case of a system which contains both a pressurized SOFC-M and a gas turbine, varying the amount of fuel injected is not the only way of changing the system power output. Fuel cell voltage and current are quite dependent on the variable rotational speed of the compressor-turbine unit. This is accompanied by varying system efficiencies. Hence there is a need to formulate an appropriate control concept (control strategy logic) and its approach for technical realization. 

Off-design (part-load) analysis is an important issue for any type of system including an SOFC-GT and should be taken into account during designing and defining operational characteristic. Results of the system behavior analysis under part-load conditions should aid in defining the system structure and its nominal parameters, as well as the constructional solution and characteristics of a given subsystem. 

Off-design operation of a solid oxide fuel cell hybrid system was analyzed previously by many authors and a few notes on chosen works are set out below. 

Bessette et al. [23] investigated the prediction of on-design and off-design performance for a solid oxide fuel cell power module. In this study, the SOFC module was investigated separately from turbo-machinery. The SOFC module consisted of tubular cells and worked at 1000°C. They found that the linear scaling of single cell results does not give an accurate assessment of the whole stack s performance. 

Costamagna et al. [24] investigated design and part-load performance of a hybrid system based on a solid oxide fuel cell reactor and micro gas turbine. This study contains models of a centrifugal compressor and inflow expander. There is no ejector model analyzed in this paper. The hybrid system operated at 0.38 MPa and 800-1000°C, and generated about 290 kW at 60% efficiency. 

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