Design of a Battery System for a Fuel Cell Powered UPS ApplicationWith Extreme Temperature Conditions

In a currently running research project, a UPS system for peripheral components in rural applications in Siberia is being developed. As main components, the UPS contains a battery system, a high temperature proton exchange membrane fuel cell (HT-PEM FC) with an auto thermal reformer (ATR), as well as two DC/DC converters to adapt the voltage levels between the individual components and the load. This work describes design aspects of the battery system, which is needed for a successful operation of the complete system. In case of a power failure, the fuel cell is not able to deliver power immediately, as it has to be heated up first. In addition, the ATR, which converts natural gas to a gas with a larger share of hydrogen, also needs about 20 to 30 minutes to heat up. Thus, for about 30 minutes the load and the heating process itself have to be supplied completely from the battery. In order to operate the fuel cell at an efficient and sustaining operating point, the battery is still needed after the heat up phase and is recharged by the fuel cell. The needed energy content as well as the peak and mean power have been calculated by a full system simulation. Implemented in Matlab Simulink, this simulation models the behavior of each system component during system operation. The main challenge for all components is the extreme temperature range in Siberia. For the desired location of the UPS built in the project described here, an environmental temperature range of -40 ºC up to +50 ºC is specified. From the battery point of view this means, that both, heating and cooling could be necessary. Thermal simulations were carried out in COMSOL Multiphysics, using the preliminary battery case design, to confirm these necessities. For the simulations worst case scenarios for winter and summer temperature profiles were used. The summertime simulations included influence from the resistive battery losses, as well as heat input from fuel cell and reformer as an addition to the environmental heat. In order to meet the performance requirements at -40 ºC, all battery types in question for this application have to be heated. In this regard different heating methods are considered. Apart from traditional electrical heating (e.g. using resistors), approaches for heating the battery with a superposed AC current are examined. Furthermore the battery cell selection, cell tests, passive components selection, the Battery Management System (BMS) and thermally optimized operation of the battery module are addressed in this work.