From Fundamental Understanding to Fuel Cell System Design

Abstract

Electrochemical energy conversion and storage are key technologies for a friendly and environmentally friendly energy supply and for securing future individual mobility. However, to realize the environmental goals it is of paramount importance to use renewable energies sources for the generation of electrical power and fuels (e.g. hydrogen). Advanced fuel cell systems for electric mobility are predominantly hybrid configurations with batteries. The advantage of high power density and dynamic range of batteries in combination with the high energy density of fuel cells improves reliability, durability and operation range of the systems in the relevant applications. The advantages of hybridized fuel cell power trains in comparison to pure battery cars will be discussed taking into account recent public studies. The group at DLR believes that in order to optimize systems with fuel cells a detailed fundamental understanding of fuel cell technology (as well as battery technology) is required. For that purpose ex-situ and in-situ diagnostic methods are used and further developed. For example, segmented bipolar plates have been developed for low-temperature polymer electrolyte fuel cell as well as high-temperature polymer fuel cells to provide a detailed view into the distribution of current densities and temperatures. The research also comprises the characterization of elementary processes like the conductivity of fuel cell membranes on the nanometer scale. Different membranes have been successfully investigated and their understanding is being used for improvements. Fundamental investigations provide information or relevant degradation mechanisms under specific operation conditions which can be used for optimization of system control and design. The system configuration is described based on DLR demonstrations for aircraft applications which have been developed in the last years. A flying testing platform realized in 2009 is the motor glider Antares DLR-H2 the first piloted aircraft capable of starting with only fuel cell power. The Antares DLR-H2 25 kW propulsion system (efficiency ca. 44%) is based on high-temperature polymer fuel cell due to the implications of high-altitude operation and is being momentarily designed as an integrated battery-hybrid. The next step will be the Antares H3 with wider range and improved operational stability. Another investigation concerns the development of an electrical drive for the nose wheel of an A320 aircraft. DLR is currently investigating a so-called “direct hybrid” system design which promises high efficiencies and cost effectiveness. It consists of reducing power electronic devices to a minimum with simple, inexpensive electronic components. Results so far are promising and are being validated by modelling.