Numerical investigation of an ejector for anode recirculation in proton exchange membrane fuel cell system
Introduction
Proton exchange membrane fuel cell (PEMFC) has been considered the most promising candidate to replace the internal combustion engine (ICE) due to its capability of high power density, high efficiency, low emission, low operating temperature and rapid start-up [1].
In PEMFC operation, an excessive stoichiometric ratio of hydrogen is always applied to the anode in order to reduce the risk of partial fuel starvation and improve the performance of PEMFC. The unconsumed hydrogen needs to be recirculated for the sake of overall PEMFC efficiency. Generally, mechanical pumps and compressors can be used for the hydrogen recirculation [2]. However, the usage of either mechanical pumps or compressors for hydrogen recirculation consumes electrical power, generates vibration and noise [3], and occupies considerable space, in addition to regular maintenance required. These features are undesirable for mobile or even stationary applications.
An ejector can also be used for the hydrogen recirculation. The hydrogen tank pressure is much larger than the fuel cell operating pressure [4] and the large pressure difference contains sufficient pressure potential energy which can be utilized for hydrogen recirculation. Fig. 1 shows a schematic of an ejector for the hydrogen recirculation in a PEMFC system. The fresh hydrogen from the hydrogen tank with high pressure is considered as the primary flow and the anode exhaust comprised of unconsumed hydrogen and water vapor as the secondary flow. The secondary flow is sucked into the ejector by the primary flow, mixed with the primary flow, and then transported to the ejector outlet and fed to the fuel cell, accomplishing the hydrogen recirculation. The anode recirculation based on an ejector in a PEMFC system is shown in Fig. 2. Compared with the mechanical pumps and compressors, the usage of ejectors for hydrogen recirculation is more desirable in terms of its system efficiency (no parasitic power) [5], low noise, simple structure and easy maintenance (no moving parts) [6].
Many efforts have been made to develop ejectors for the recirculation in refrigeration system, as summarized in a recent review article by Besagni et al. [7]. Ejectors for the recirculation in the solid oxide fuel cell (SOFC) system have also been extensively investigated [8]. However, only a limited number of studies have been conducted on ejectors for anode hydrogen recirculation in PEMFC systems. Kim et al. [9] proposed a design methodology of an ejector used for the unconsumed hydrogen recirculation in a submarine PEMFC system, and the good performance of the ejector was verified by experiments. He et al. [10] presented a hybrid hydrogen recirculation system in a PEMFC system, in which both an ejector and a blower were involved. They also discussed the control method of the hybrid recirculation system. Bao et al. [11] reported a control model of a PEMFC system incorporating an analytical ejector model for hydrogen recirculation, and they also analyzed the dynamic characteristics of the PEMFC system under different operating conditions. Zhu et al. [12] presented a theoretical ejector model utilizing the thermodynamic and fluid dynamic principles, which applied a concave 2D velocity function in the ejector and provided more accurate ejector performance prediction than the conventional 1D ejector model. Compared with the simplified analytical ejector models, Brunner et al. [13] described the design and control approaches of ejectors for PEMFC applications using a 2D ejector model. Maghsoodi et al. [14] also established a 2D ejector model and numerically investigated the optimization of the ejector design utilizing the artificial neural network and genetic algorithm. Nikiforow et al. [15] numerically designed a hydrogen recirculation ejector for a 5 kW stationary PEMFC system using a 2D axisymmetric ejector model with experimental validation, and they found that the secondary flow inlet boundary conditions and ejector outlet pressure had significant influence on ejector performance, particularly at low primary pressures. Mazzelli et al. [16] developed both a 2D and a 3D numerical models of a ejector with a rectangular cross section, and they showed that the 3D simulation more closely matched the experimental result, since it included the friction losses caused by the front and back walls of the ejector. Although the 2D models apply well to the axisymmetric flow simulation, they may not be appropriate for the simulation of non-axisymmetric flow in an ejector having a side-branch secondary flow tube as shown in Fig. 1. The secondary flow tube is usually neglected and the secondary flow is assumed to feed to the suction chamber without any resistance from the secondary flow tube in most previous 2D ejector models. The ejector design with a side-branch secondary flow tube is the common ejector design form, due to its compactness and flexibility. For an ejector design with a side-branch secondary flow tube, the secondary flow tube may have significant impact on the flow resistance, the ejector performance and compactness, and it needs to be well considered in the ejector design. Recently, Hwang et al. [17] established a 3D ejector model with a side-branch secondary flow tube based on their previously developed vacuum ejector [18] for the hydrogen recirculation; however, the optimization of the secondary flow tube was not discussed in their studies. Besides the secondary flow tube, the suction chamber is also an important component of the ejector, which requires proper design for both the ejector performance and compactness. The other ejector geometric parameters and the operating conditions on the ejector performance also need further investigation.
In this study, the objective is to establish a 3D numerical model of an ejector for the anode recirculation in PEMFC system, and to investigate the effect of geometric parameters, especially the secondary flow tube and the suction chamber, and the operating conditions on the ejector performance. An optimization of the ejector design is also carried out.
Section snippets
Ejector model
The establishment of the 3D ejector model is described as follows.
Results and discussion
In this study, the ejector is designed based on an existing PEMFC stack whose properties and performance are given in Table 1, Table 2. The primary flow mass flow rate of the ejector depends on the stack current and is calculated by Eq. (11). The primary flow mass flow rate is 0.0014 kg s−1 for the stack current of 350 A (the stack power of 80 kW). The primary flow temperature is considered as the ambient temperature which is 298 K. The PEMFC stack works at pressurized conditions for the better
Conclusions
In this study, a 3D numerical model of an ejector for the anode recirculation in a proton exchange membrane fuel cell (PEMFC) system is established. The ejector geometric parameters are investigated and optimized using the 3D ejector model. The ejector recirculation ratio is found increasing significantly with the secondary flow tube inlet diameter, but increasing slightly with the secondary flow tube convergence and inclination angles. The ejector recirculation ratio decreases fist and then
Acknowledgements
This work is financially supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB215506) and the Program of Natural Science Foundation of Tianjin, China (Grant No. 11JCYBJC07400).
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