Experimental Evaluation of PEMFC Water Separators Level Control System Based on Air Bubble Detection Sensor

Water separator is one of the main parts of a cascaded fuel cell stack. In separators, water droplets are separated from the wet gas. To control the water level of the separator, two lower and upper limits are considered. Improper control of the separator water level can cause the active surface of the fuel cell to become flooded which leads to fuel and oxidant starvation. As a result, the production capacity of the fuel cell is reduced. Therefore, it is very important to control the water level of the separator. In this paper, an experimental investigation is done for water level control system of a cascaded PEM fuel cell stack. The investigation is based on air bubble detection sensor.Hydrogen energy systems have been developed considering the shortcomings and pollution issues of the fossil fuels. Fuel cells are considered as the main component of the hydrogen energy system. Detecting water level in the cascaded fuel cell stacks is an important issue affecting the fuel cell proper performance.Various methods are used to measure the level of fluids such as ultrasound, magnetism and infrared, etc. Each method is developed for a specific purpose. A capacitive sensor is used to detect the presence of material in the metal tanks. Whenever the capacitance increases due to the presence of materials and exceeds a certain value, the sensor output will be activated [۱]. Surface control optical sensors are used to detect the level of liquids such as alcohol, ether, distilled water, various acids and industrial oils. These sensors are modulated based on the transmission of infrared waves and the reception of refracted waves from the prismatic tip of the sensor [۲-۳]. Another class of level detection sensors is based on pressure differences [۴]. The three mentioned methods have space limitations an can not be installed inside the cascaded stack separator, so these sensors are not suitable for detecting the water level inside the cascade fuel cell separator. Due to the importance of detecting the water level inside the cascaded fuel cell stack separators, in this paper, a method for detecting and controlling the water level inside the PEM fuel cell stack separators is presented. The method is based on air bubble detection sensor.In this method, an air bubble detection sensor is used to control the water level inside the separators. A control algorithm is developed to detect the water level. The air bubble detection sensor is mounted on a transparent tube carrying liquids and when the air bubble passes, the sensor sends a different analog signal in comparison with the signal generated during the liquid passing through the tube to the outlet. The signal will be different according to the bubble size. The bubble sensor used in the test is the product of the TT Electronics company. The sensor model is OPB۳۵۰L۲۵۰. The sensor's aperture equals to ۰.۲۵ inch tube. The air bubble detection sensor test setup is shown in figure ۱. At the beginning of the algorithm, the following inputs are taken form the user: the voltage representing air existence, the voltage representing water existence, the outlet valve closing time, the delay for water detection after opening the outlet valve, and the number of consecutive samples to determine whether water or air exists. The analog signal determines water or air existence. If water is detected, the outlet valve is opened to completely drain the existing water and then the outlet valve is closed during the time period equals to the outlet valve closing time. If air is detected, the outlet valve is closed during the time period equals to the outlet valve closing time. Due to the distance between the separator outlet and the bubble sensor, an enclosed air may already exists behind the outlet valve. Therefore, after opening the valve, the algorithm waits a constant amount of time to detect water after opening the outlet valve until the water reaches the sensor. In this case, if water is detected during this time, the separator outlet valve opens and remains open until the separator is drained and air is detected by the sensor. After detecting the air, the algorithm closes the outlet valve againduring the time period equals to the outlet valve closing time; and this cyclic process continues. If water isnot detected, while waiting for the water to be detected after the outlet valve is opened, the algorithmautomatically closes the valve during the time period equals to the outlet valve closing time. The flowchartof the algorithm is presented in figure ۲. The parameters of the flowchart are described in Table ۱. Themethod presented in this paper is experimentally investigated to control the water level of a dummyseparator. The accuracy of the sensor is confirmed by the experimental results.In this paper, a method for controlling the water level inside the separators is proposed. In the proposedmethod, using a commercial product and presenting a control algorithm, the water level inside thecascaded PEM fuel cell stack separator is controlled. The proposed method is validated by theexperimental results