Within the Fraunhofer innovation cluster “Regional Eco Mobility 2030” (REM2030) concept developments to improve the energy efficiency of regional eco mobility of the future are investigated. An AUDI A1 Sportback is used as a technology demonstrator with an entirely electric powertrain, completed to a serial hybrid by a fuel cell range xtender. A methanol reformer provides hydrogen for the high temperature fuel cell. The main focus of this paper is the thermal management system of the car, which has to deal with different temperature levels and must be designed for zero emissions and energy efficiency. The model-based development of such a system using Modelica is described, comprising a conception, simulation and testing phase.