18./18 University of Applied Sciences Hamburg

Editors: W. Winkler, H. Lorenz :
Differences and synergies between mobile and stationary SOFC-GT designs

The combination of a SOFC and a gas turbine (GT) connects the air flow with the theoretical independent heat engine of the cell cooling. The principle strategy to reach high electric-efficiencies is to avoid a high excess air for the cell cooling and heat losses. Simple combined SOFC-GT cycles show an efficiency between 60 and 72 %. Combinations of SOFC-GT cycles and a steam injection achieve an efficiency of more than 75 %. The use of a reheat gas turbine process in a (RH) SOFC-GT cycle 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 an earlier theoretical thermodynamic model. The design principles of such SOFC-GT cycles were already shown in a design study in 1994 on the example of a 50 MW plant, Fig 1. However we expect smaller units in the future, the used design principles can be transferred to the 1 – 5 MW units.

Fig.1 Design study of a 50 MW SOFC-GT cycle

The extreme short heat-up time of a thin tubular SOFC and the market entrance of the microturbines gives the option of using these SOFC-GT designs for mobile applications. The possible use of hydrocarbons as Diesel oil is an important benefit of the SOFC. The microturbine and the SOFC stack will be matched depending on the start-up requirements of the mobile system. The minimisation of the needed volume is a key issue. The design and the efficiency of small gas turbines is lower than the efficiency of large gas turbines due to the influence of the leakage within the stages of gas turbines increasing with a decreasing size of the gas turbine. Thus the SOFC module pressure must be lower than in larger stationary SOFC-GT systems. This together leads to an electric efficiency of 45 % of a cycle used as a base for a design study. The result of the design study is that the space available in a mid class car allows the placement of such a system including space reserves, fig. 2.

Fig. 2 Design study of a mobile 75 kW SOFC-GT system

A further improvement of the system might allow an electric efficiency of about 55 %. The differences and the synergies between these two design examples for a stationary and a mobile application will be discussed.

Corresponding author :
Prof. Dr. techn. Wolfgang Winkler
Faculty of Mechanical Engineering
University of Applied Sciences Hamburg
Berliner Tor 21
Fuel cells and rational use of energy
D 20099 Hamburg
Tel : + 49-(0)40-42859-3137 (-2658 Fax)
e-mail : winkler@rzbt.fh.hamburg.de