Siemens is currently testing tubular fuel cells with the SFC-200. Future prototypes will feature planar cells. 

Siemens first launched its activities in this field with tubular fuel cells — a development taken over from the U.S. company Westinghouse, which Siemens partially acquired in 1998. Rather than being flat, as in a PEM stack, the individual cells in this type of system are composed of long tubes. The anode, which is on the outer surface, is separated — by an electrolyte — from the cathode, on the inside of the tube.

This type of system comprises a whole bundle of such tubes installed in a casing. During operation, air flows through the tubes and hydrogen along the outside of them. Oxygen ions diffusing through the electrolyte react with the hydrogen to form steam. In the process, the cells — which are made of ceramic materials — reach temperatures of around 1,000 °Celsius. Using this heat, the natural gas used to power the fuel cell can be reformed directly in the stack itself. Beforehand, the gas is simply desulfurized, passed over a catalyst and enriched with steam to facilitate reformation.

Siemens has already commissioned a whole range of test facilities featuring this type of tubular design. The latest addition is the SFC-200 Power System, a stationary plant with an output of 125 kilowatts of electrical power and 100 kilowatts of thermal power. With such a rating, this kind of system would be ideal for providing apartment blocks or small industrial plants with electricity and heat, whereby the thermal power could be used for both heating and air conditioning.

The prototype plant already boasts an electrical efficiency of 44 percent and an overall efficiency of 80 percent. Moreover, Siemens engineers are confident that improvements in surface geometry will further enhance efficiency. Development activities are currently focusing on cells with a rectangular profile, which can be packed closely together. Such a design yields a power density some two or three times higher and therefore also a greater efficiency. Siemens anticipates having the first systems ready for a market launch by the end of the decade.