Maintaining equal operating conditions for all cells in a fuel cell stack using an external flow distributor
Highlights
► We present a novel fuel cell stack architecture for maximizing power output. ► External flow distributors deliver fuel and air to individual cells uniformly. ► The fuel cell stack shows an excellent performance. ► Uniform distribute of fuel/air to cells is critical to a high performance of a stack.
Introduction
Fuel cells are devices that electrochemically convert the energy from the oxidation of fuels with a relatively high efficiency [1]. Essentially, most popular types of fuel cells, such as the proton-exchange-membrane fuel cells (PEMFC), solid oxide fuel cells (SOFC), and direct methanol fuel cells (DMFC), have single cell voltages of less than 1.0 V under operational conditions [2], [3], [4], [5]. Therefore, in order to have a sufficiently high voltage for practical application, it is more common to use multiple fuel cells stacked together than to use a single cell.
In order to have high power output from fuel cells, it is important to reduce internal losses as much as possible. In a single cell, the sources that cause low power density can always be traced to the three types of polarizations—the activation polarization, concentration polarization, and ohmic polarization [6]. When these polarizations are severe, the over-potential can be too large for the fuel cell to produce a sufficient power output with reasonable energy efficiency [7], [8], [9], [10]. For a fuel cell stack, other than the losses in every single cell, the non-uniform distribution of fuel and air to the multiple fuel cells can also cause decrease of the maximum available power output from the stack [11], [12].
The multiple fuel cells in a fuel cell stack are connected serially to obtain a large voltage. As a result, the fuel cell stack must operate at a single current density. This current density is determined by the fuel cell with the lowest current density in the entire fuel cell stack, even if other fuel cells have the capability of operating at higher current densities. Therefore, the best architectural design for a fuel cell stack should ensure that each individual fuel cell in the stack receives the equal supply of fuel, and oxygen as well. Otherwise, a non-uniform fuel and air supply will cause some individual fuel cells to have higher voltage than others. In conclusion, when every fuel cell operates at the same maximized current density, the fuel cell stack will maximize power output, and also have better reliability and life span as the load/duty is even for all the cells. Researchers [13], [14], [15] have done some work to study the internal flow arrangement in a PEM fuel cell stack, and their results demonstrated that failure in a few cells can lead to the failure of the entire stack. The failure of individual fuel cells in a fuel cell stack could be due to some fuel cells always receiving less fuel and air than others. Non-uniform electrochemical reaction among individual fuel cells could also cause non-uniform heating, water flooding, and membrane drying. This increases the chances of failure of the entire fuel cell stack, which reduces the reliability and life span of the fuel cell stack [16], [17], [18].
It is clear that a uniform supply of fuel and oxygen to a number of flow channels is an important issue for the flow fields design inside any individual fuel cell. It is also a key issue for the multiple fuel cells in a cell stack in which every cell should have an equal feed of fuel and oxygen.
A significant amount of research has been done on the uniform distribution of fuel and oxygen inside a single fuel cell on the anode and cathode respectively [19], [20], [21], [22], [23]. For example, the constructal theory [24] has been applied in the design of flow distribution to gas channels of bipolar plates and endplates [25]. The current authors' group also proposed a uniform flow distribution approach by applying multiple levels of flow channel bifurcations [26], [27].
Compared to the studies of the uniform flow distribution inside a single fuel cell, flow distribution to multiple fuel cells in a fuel cell stack has not been paid sufficient attention by fuel cell researchers. Therefore, our main objective in this study is to investigate flow arrangement between cells in order to minimize failure in a fuel cell stack.
Typically, flow distribution to individual fuel cells in a fuel cell stack is based on parallel flow distribution, which is sub-divided into Z-shape and U-shape as shown in Fig. 1. Studies from literature [28], [29], [30] have shown that non-uniform flow distribution to the different channels in such flow systems is significant. The estimation by Chang et al. [15] showed that the Z-shape parallel flow distribution for a 200-cell PEM fuel cell stack operating at a current density of 0.77 A/cm2 has a flow rate difference of up to 15 percent between individual cells. This non-uniform distribution of fuel and air caused a voltage difference of up to 0.3 V between individual fuel cells in their study, which was a significant unbalanced performance amongst fuel cells. Only when the pressure loss in the flow streams is very large, the flow distribution is more uniform. However, having a large pressure drop/loss is obviously a big price to pay and is not a good strategy in the design of fuel cell stacks. Therefore, as the focus of the present study to overcome the non-uniform flow distribution to all cells in a stack, we introduce a uniform flow distribution approach by applying multiple levels of flow channel bifurcations to form an external flow distributor. Small-diameter tubes are used to connect the multiple outlets from the flow distributor to all the individual fuel cells in the fuel cell stack.
This research work will experimentally show the performance of fuel cell stack as well as that of the individual fuel cells in the stack. A four-cell fuel cell stack has been fabricated for this study. Detailed descriptions of the design of the flow distributors, the fuel cell stack architecture, as well as the test results are to be presented in the following sections.
Section snippets
Flow distribution designs
Shown in Fig. 2 is the schematic of a novel external flow distributor for the uniform distribution of fuel and airflow to all the individual fuel cells in a stack. The novel flow distribution is based on a concept of flow channel bifurcations in a cascade style, which delivers a flow to multiple flow channels [26], [31]. In this design, there are four levels of cascade flow channel bifurcations, which deliver a flow to 24 channels. The current authors have proposed several types of structures
Experimental setup and test procedures
Fig. 7 shows the layout of the experimental setup. Compressed air from the lab building was filtered and supplied to the fuel cell stack through flow distributors. Flow controllers (SERIRRA Smart Trak-2) were used to set and display the flow rates of hydrogen and air respectively. A sufficient airflow was provided with a constant flow rate of 3400 cm3/s, corresponding to a stoichiometry of 2.86 for a current density of 0.7 A/cm2 for the four-cell stack. The sufficient airflow helps remove
Results and discussion
The objective of these experimental tests is to examine the V–I performances of the individual fuel cells and the fuel cell stack, where each cell in the stack is individually provided with fuel and airflow by flow distributors. Because of the variation in the uniformity of flow distribution from different flow distributors, individual fuel cell V–I curves show disparities as well. The test results are to be presented and discussed in the following sections.
The experiments applied a constant
Concluding remarks
It is the objective of this paper to understand the effect of the flow distribution uniformity of fuel and air for individual fuel cells in a fuel cell stack, in order to achieve a high power output overall, and an equal performance of all individual fuel cells in the stack. A novel flow distributor was proposed to uniformly deliver reactants to every single cell in the stack. For comparison, a Tee-channel flow distributor was also used to deliver hydrogen and airflow to the individual fuel
Acknowledgment
The authors gratefully acknowledge the financial support from the US Office of Naval Research and the SimCenter of the University of Tennessee at Chattanooga under the contract #8500011366.
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