An APU (auxiliary power unit) is a power generator, which covers a vehicle's demand for energy during a standing phase. The APU is especially set up for the application in a commercial vehicle, as during the dead time the engine may not idle for power generation - a consequence of stringent anti-idling-laws like in the USA. Still during stand-still, energy is required for e.g. independent cab air conditioning, cab electrification, pre-heating for cab and engine etc. Current solutions like the ICE-APU have significant disadvantages like noise, and pollutant emissions like diesel particles.

Fuel cells generate electrical energy out of gaseous fuel such as hydrogen, without making a detour to mechanical energy. Hence these electrochemical energy converters can especially apply in a vehicle, where electricity is needed. Development of a fuel cell-APU (Auxiliary Power Unit) for vehicle application is based on the SOFC-technology for vehicle application (Solid Oxide Fuel Cell). Compared to competing fuel cell-technologies, the SOFC has the advantage that for electricity generating every kind of fuel can be used, which is available today and in the future. Thereby the market entry does not depend on the duly availability of an area-wide infrastructure of a new fuel, but on the successful technical implementation.

The fuel cell-APU concentrates Diesel with air in a so called reformer mixture, which then flows over a catalyst - similar to a normal exhaust gas system. In the catalyst arises a gas which contains hydrogen and carbon monoxide. This gas is now forwarded into a solid oxide fuel cell to its anode. In parallel, ambient air flows to the cathode of the fuel cell. Oxygen-ions dissolve from the air and move to the anode. Thereby a power flow arises, which can be used for car air conditioning of similar electrical loads. This electrochemical process is very efficient and offers a theoretical efficiency factor of more than 35 %. Moreover it proceeds at temperatures of more than 800°C. This leads to the option of using the arising rejected heat also for the function of a vehicle heater in the future, for example. In the first test rigs the rejected heat is given to the ambient air (also in summer unproblematic), without being used.