1 Thermal Energy, Department of Mechanical Engineering, Technical University of Denmark2 Department of Mechanical Engineering, Technical University of Denmark
Reducing CO2 emissions are getting more attention because of global warming. The transport sector which is responsible for a significant amount of emissions is going to reduce them due to new and upcoming regulations. Using fuel cells may be one way to help to reduce the emissions from this sector. Battery driven lift trucks are being used more and more in different companies to reduce their emissions. However, battery driven lift trucks need long time to recharge and may be out of work for a long time. Fuel cell driven lift trucks diminish this problem and are therefore getting more attention. The most common type of fuel cell used for automotive applications is PEM fuel cell. They are known for their high efficiency, low emissions and high reliability. However, lack of a hydrogen infrastructure, cost and durability of the stack is considered the biggest obstacles to the introduction of fuel cell vehicles. The overall aim of this research is studying different fuel cell systems and find out the system with the highest efficiency and less complexity. This will be achieved by modelling and optimization of the fuel cell system followed by some experimental tests. Efficiency of the stack is about 50%. But efficiency of whole the system is less than this value, because some part of electricity produced by the stack would run the auxiliary components. The work deals with development of steady state model of necessary components in the fuel cell system (humidifier, fuel cell stack and ejector), studying different system configurations and optimizing the operating conditions in order to achieve the maximum system efficiency. A zero-dimensional component model of a PEMFC has been developed based on polynomial equations which have been drived from stack data. The component model has been implemented in a system level to study four system configurations (single and serial stack design, with/without anode recirculation loop). System design evaluations reveal that single stack with recirculation loop has the best performance in terms of electrical efficiency and simplicity. To further develop the selected system configuration, the experimental PEMFC model was replaced by a zero-dimensional model based on electrochemical reactions. The model was calibrated against available stack data and gives the possibility of running the system within the operating conditions whose experimental data is not available. This model can be used as a guideline for optimal PEMFC operation with respect to electrical efficiency and net power production. In addition to the optimal operation some recommendations have been given for water and thermal management of the system by investigation different coolants as well as operating conditions. After theoretically analyzing the system the more attempts tried to improve the anode recirculation loop basically by using an ejector instead of recirculation pump. CFD technique has been used to design and analyze a 2-D model of an ejector for anode recirculation of PEMFC system applied in a forklift truck. In order for the ejector to operate in the largest possible range of load, different approaches (with fixed nozzle and variable nozzle ejectors) have been investigated. Different geometries have been studied in order to optimize the ejector. The optimization is carried out not only by considering the best performance of ejector at maximum load with operation in the larger range as priority, but also catching the design point at maximum load even though it does not have the best efficiency at such point. Finally a hybrid drive train simulation tool called LFM was applied to optimize a virtual forklift system. This investigation examines important performance metrics such as hydrogen consumption and battery SOC as a function of fuel cell and battery size, control strategy, drive cycle, and load variation for a forklift truck system. This study can be used as a benchmark for choosing the combination of battery and fuel cell.