International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 62 A Study on Enhancing the Efficiency of 3-Way Valve in the Fuel Cell Thermal Management System Il Sun Hwang 1 and Young Lim Lee 2 1 Dept. of Mechanical Engineering, Graduate School, Kongju National University, South Korea 2 Corresponding Author, Dept. of Mechanical & Automotive Engineering, Kongju National University, South Korea ylee@kongju.ac.kr Abstract-- Maintaining the appropriate temperature is critical for enhancing fuel cell efficiency in fuel cell automobiles. Thus, this paper optimized the 3-way valve of the fuel cell thermal management system through normal and abnormal state analyses. The analysis results showed that when the inlet and outlet valve angles increase, the pressure difference decreases, resulting in an increase in the efficiency of the 3-way valve. Also, since the temperature and flow rate of the coolant are almost constant at the valve exit, even when the valve opening and closing speeds were increased to 1.8 rad/s, the fuel cell load has to be addressed preferably with increased valve opening and closing speeds. Index Term-- Fuel cell, Thermal management, 3-way valve, Coolant, Numerical analysis 1. INTRODUCTION The depletion of fossil fuels, which are the main source of energy today, and the issue of environmental pollution, are increasing global concerns. As a result, development of alternative energy sources which can replace fossil fuels, and environmentally friendly technologies which can resolve environmental pollution, are being accelerated. Among sources of pollution, automobiles, which are the dominant mode of transportation, are responsible for approximately 40% of total carbon dioxide emissions. In addition, automobiles consume the greatest amount of fossil fuels, underscoring the need for alternative energy and environmentally friendly automobiles to address the issues of environmental pollution and fossil fuel depletion. Representative examples of environmentally friendly and alternative energy automobiles include the electric vehicle and fuel cell vehicle. These vehicles employ engines that operate mainly on electricity, with reduced dependency on fossil fuels, and thereby producing insignificant pollutants. The fuel cell vehicle has been praised as an environmentally friendly automobile because it generates electrical power from the chemical reaction between hydrogen and oxygen. However, these vehicles still face hurdles to achieving commercialization, because of various remaining problems, such as low energy density, low efficiency, and limited battery lifespan, along with the low efficiency and high cost of the fuel cell. In the case of the environmentally friendly fuel cell car, thermal management of the fuel cell and battery is important. The temperature range for optimal fuel cell operation is approximately 10, which is not large. Moreover, when the operating temperature is high, the fuel cell becomes dry, and when the operation temperature is low, condensation occurs, reducing the efficiency and lifespan of the fuel cell. Thus, maintaining the appropriate temperature is critical to enhancing fuel cell efficiency, and various studies are being conducted in this regard. Nolan and Kolodziej [1] used a nonlinear thermal model to study the fuel cell thermal management system, and Sundaresan and Moore [2] investigated the start-up performance of the fuel cell in low temperatures. Also, Hwang [3] conducted research on optimizing the fuel cell coolant inlet temperature to 58~63, and Saygili et al. [4] used a coolant pump and radiator to develop a fuel cell thermal management model. Various other thermal management systems studies are being conducted. [5,6] The energy consumed in the thermal management system is mainly by the coolant pump and radiator operations, so reducing the capacities of the coolant pump and radiator could effectively lead to system performance enhancement. Therefore, in this paper, the 3-way valve inlet and outlet pressure difference was reduced to improve the valve s efficiency and the valve opening and closing speed, as well as controlling the deflection, to precisely control the fuel cell thermal management system temperature. To achieve this, a 3- dimensional Computational Fluid Dynamics (CFD) valve analysis was conducted to optimize the 3-way valve pressure drop, valve opening and closing speed, and deflection. 2. ANALYSIS METHOD AND CONDITIONS Fig. 1 shows a schematic diagram of the fuel cell thermal management system. Part of the coolant from the fuel cell is bypassed to enter the valve while the coolant that is not bypassed is cooled through the radiator, and then mixed with the bypassed coolant. The valve deflection is controlled in order to control the mixing ratio, which in turn controls the coolant temperature. The valve analysis was conducted using ANSYS Fluent [7]. First, a normal state analysis was performed in order to reduce the 3-way valve inlet and outlet pressure difference. The existing 90 degree model was used for the analysis by adjusting the inlet angle. The k-omega sst model was used for the turbulence model in the normal state analysis, and the inlet flow rate was assumed to be 150 LPM. Also, a model that showed the optimal inlet and outlet pressure difference, obtained through the normal state analysis, was used to perform an analysis of the outlet temperature, flow rate, and pressure difference according to the valve opening and closing speed. Here, the same k-omega sst model was used for the turbulence model as in the normal state, and the inlet pressure was set to 20 kpa, so that the pressure difference would cause
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 63 a flow. The valve opening and closing speed was increased sliding mesh method. Approximately 50,000 elements were from 0.45-1.8 rad/s to investigate the changes caused by the used for the analysis. increase in valve opening and closing speed. Fig. 3 shows the mesh system of the analysis. Tetrahedron mesh was employed and the valve opening and closing was reproduced using the Fig. 1. Schematic of fuel cell thermal management system Fig. 2. Schematic of 3-way valve Fig. 3. Mesh system
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 64 3. RESULT AND DISCUSSION valve angle reduces the valve flow resistance, increasing the 3.1 Pressure Difference Change According to the Normal valve efficiency and making it more advantageous for State Valve Inlet and Outlet Angle obtaining higher flow rates. Analysis was conducted by varying the valve inlet and outlet Fig. 5 shows the flow path line according to the valve angle. angle in order to reduce the pressure difference. Fig. 4 shows When the valve angle is 90, the flow passes the valve mixing the pressure distribution according to the valve angle. When chamber causing a large pressure loss, making discharge from the valve angle was 90, the inlet and outlet pressure the valve outlet less smooth. When the valve angle becomes difference was approximately 17 kpa and the maximum 130, the flow was observed to pass through the mixing pressure was around 20 kpa. When the valve angle was chamber relatively smoothly in comparison to when the valve increased to 130, the inlet and outlet pressure difference was angle was 90. Also, when the valve angle was 145, the approximately 14 kpa, which is 3 kpa less than when the valve pressure loss was minimized in the mixing chamber, angle was 90. When the valve angle was 145, the pressure increasing the velocity distribution. difference was 11 kpa, around 6 kpa less than the initial model. The inlet and outlet pressure difference was found to decrease nonlinearly according to the valve angle. Thus, using a larger Fig. 4. Pressure contours with valve angle
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 65 Fig. 5. Variation of flow path line with valve angle 3.2 Pressure, Temperature, Mass Flow Rate Changes According to the Valve Opening and Closing Speeds Analysis results of the normal state showed that when the valve angle was 145, the pressure resistance was approximately 11 kpa, which was the lowest. So, this model was used to conduct the abnormal state flow analysis for varying valve opening and closing speeds. Fig. 6 shows the pressure difference in the valve inlet according to time. When the valve opening and closing speed was 0.45 rad/s, the pressure at the inlet varied from a minimum of around 7.5 kpa to a maximum of 20 kpa. The minimum pressure occurred when the valve was completely open and the maximum pressure occurred when the valve was completely closed. Meanwhile, when the opening and closing speed increases, the maximum and minimum pressures stay virtually constant and the pressure variation is decreased. When the pressure variation decreases, the flow is more stable, therefore, having a fast valve opening and closing speed is more beneficial to flow stability. Fig. 7 shows the outlet temperature variation according to the valve opening and closing speed. When the valve opening and closing speed is 0.45 rad/s, the temperature at the outlet varies from a minimum temperature of approximately 27 to a maximum of 65. The minimum temperature at the outlet occurs when the bypass side of the valve is closed, the radiator side of the valve is open, and only the coolant that passed through the radiator is flowed in. Afterwards, the bypass side opens and the radiator side closes, resulting in an increase in the temperature and the maximum temperature of around 65 occurs when the bypass side is completely opened. Additionally, even when the valve opening and closing speed is increased to 1.8 rad/s, the coolant temperature at the valve outlet is virtually constant, so the fuel cell load has to be addressed with preferably fast valve opening and closing speeds. Fig. 8 shows the coolant mass flow rate variation according to the valve opening and closing speed. The maximum coolant mass flow rate was approximately 2.7 kg/s when one side of the valve was completely open and the minimum coolant mass flow rate of around 2.1 kg/s occurred when both sides of the valve were open 50% each. The difference between the maximum and minimum flow rate was around 0.6 kg/s, which indicates that the degree of valve opening and closing resulted in significant differences. However, the effect of the valve opening and closing speed on the maximum and minimum difference in mass flow rate was insignificant, so it is more advantageous to use faster valve opening and closing speeds. In addition, there is a section in the maximum mass flow rate occurrence interval where the mass flow rate rapidly varies and this is due to the rapid change in pressure. An increase in the valve opening and closing speed relieves the phenomenon. Therefore, using high valve opening and closing speeds can help maintain the fuel cell stack temperature in a stable manner.
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 66 Fig. 6 Variatio.n of pressure at valve inlet with time Fig. 7. Variation of coolant temperature at valve exit with time
International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 67 Fig. 8. Variation of mass flow rate at valve exit with time 4. CONCLUSION In this study, 3-way valve optimization was performed to improve fuel cell thermal management system efficiency. For this, a 3-dimensional nonlinear flow analysis was performed on the effect of the valve angle, valve opening and closing speed, and inlet and outlet pressure difference. The following conclusions were obtained in this study. 1) Normal state analysis revealed that when the valve angle increased, the inlet and outlet pressure difference decreased nonlinearly. Also, when the valve angle increased, pressure loss was minimized, resulting in a higher velocity distribution. Similarly, minimizing the pressure resistance was very effective for system efficiency enhancement, as even a small capacity pump can provide sufficient flow rates. 2) Abnormal state analysis revealed flow variation depending on the valve opening and closing speed. As the valve opening and closing speed increased, the rapid pressure variation interval decreased, stabilizing the internal flow. Also, even when the opening and closing speed increased to 1.8 rad/s, the coolant temperature and flow rate at the valve outlet was essentially constant, so the fuel cell load has to be addressed preferably with fast opening and closing speeds. As future work, the cooling effect of the fuel cell needs to be investigated through numerical analysis based on these 3- way valve analysis results, along with experimentally validating research. Advancement of Technology(KIAT) through the Project for Expansion of Reliability-related Technology. REFERENCES [1] Nolan, J., Kolodziej, J., 2010, "Modeling of an automotive fuel cell thermal system", Journal of Power Sources, Vol. 195, No. 15, pp. 4743~4752. [2] Sundaresan, M., Moore, R. M., 2005, Polymer electrolyte fuel cell stack thermal model to evaluate sub-freezing startup, Journal of Power Sources, Vol. 145, No. 2, pp. 534~545. [3] Hwang, J. J., 2013, Thermal control and performance assessment of a proton exchanger membrane fuel cell generator, Applied Energy, Vol. 108, pp. 148~193. [4] Saygili, Y., Eroglu, I., Kincal, S., 2015, " Model based temperature controller development for water cooled PEM fuel cell systems", International Journal of Hydrogen Energy, Vol. 40, No. 1, pp. 615~622 [5] Yu, X., Zhou, B., 2005, Water and thermal management for Ballard PEM fuel cell stack, Journal of Power Sources, Vol. 147, No. 1, pp. 184~195. [6] O Keefe, D., EI-Sharkh, M. Y., Telotte, J. C., Palanki, S., 2014, Temperature dynamics and control of a water-cooled fuel cell stack, Journal of Power Sources, Vol. 256, pp. 470~478. [7] Ansys Fluent version 14, 2013, Ansys inc., User's Manual version 14. 5. ACKNOWLEDGEMENTS This research was financially supported by Korea Institute for