Real-time temperature control for direct methanol fuel cell in off-grid renewable energy system with liquid level constraints

Direct methanol fuel cell (DMFC), with its high energy density and short refueling time, has become an essential component of off-grid renewable energy system. However, the power demand of the fuel cell system exhibits nonlinearity and uncertainty due to the influence of solar irradiance on renewable energy output and the variability of load demands. Different power levels correspond to different stack temperatures, posing challenges to stack temperature control under nonlinear and uncertain conditions. Rapid and accurate temperature control with robustness under different operating conditions is a critical technique for DMFC system to fully utilize its performance. This paper analyzes the thermal dynamics of a practical commercial DMFC system and establishes a thermodynamic model for the DMFC system. A real-time temperature control strategy, including internal model control (IMC) with liquid level constraints is proposed to control stack temperature in different operating conditions, which is characterized by its ease of implementation, rapid response, and robustness against external disturbances. In order to assess the effectiveness of the proposed strategy, comparisons were conducted between proposed strategy and the strategy without liquid level constraints under specific operating conditions. Further, the temperature control performance of the proposed IMC was compared with that of the traditional PID controller. The comprehensive results demonstrate that, compared with PID controller, IMC has better robustness against external disturbances where IMC reduced the maximum overshoot during startup by 24.8 %, decreased undershoot in the large step condition by 3.19 %, and lowered average overshoot under the off-grid renewable energy condition by 4.0098 %, and the response time improved by 7.3 %. Additionally, under long-term high-current operation, the stable working time of DMFC system increased by 14.44 % and 37.16 % for condenser fan opening degrees of 0 % and 30 %, respectively.