Performance comparison of cocurrent and countercurrent flow solid oxide fuel cells

Abstract

Solid oxide fuel cell (SOFC) is a complicated system with heat and mass transfer as well as electrochemical reactions. The flowing configuration of fuel and oxidants in the fuel cell will greatly affect the performance of the fuel cell stack. Based on the developed mathematical model of direct internal reforming SOFC, this paper established a distributed parameters simulation model for cocurrent and countercurrent types of SOFC based on the volume-resistance characteristic modeling method. The steady-state distribution characteristics and dynamic performances were compared and were analyzed for cocurrent and countercurrent types of SOFCs. The results indicate that the cocurrent configuration of SOFC is more suitable with regard to performance and safety.

Keywords

solid oxide fuel cell (SOFC) / cocurrent / countercurrent / dynamic performance

Introduction

Solid oxide fuel cell (SOFC) is an energy conversion device that converts the chemical energy of gas or gasified fuel directly into electrical and thermal power [1,2]. Fuel cells have attracted increasing attention because of their high efficiency, simple structure, and low pollution. More importantly, combining medium-high temperature fuel cells and other power cycles is easy because a hybrid power system benefits from the high-grade waste heat of medium-high temperature fuel cells [3]. For example, the gas turbine-SOFC systems that have recently emerged are suitable for distributed power plants. SOFC is expected to play a principal role in medium-sized power plants (1-10 MW) in the next 20 years [1].

Generally, planar SOFC can have three flowing configurations: cocurrent, countercurrent, and crosscurrent. The different flow types have great impact on the performance of fuel cells [4]. However, the limitation of structure and the severe operating condition of SOFC make any experimental study on the performance and the safety operation of SOFC with different flow types difficult. In this paper, based on the established simulation model of direct internal reforming planar SOFC, cocurrent and countercurrent SOFCs were modeled and simulated, and the distribution characteristic and dynamic performance of the fuel cell stack were compared and were analyzed.

SOFC simulation model

SOFC mathematical model

The models of SOFC have been extensively described [2,4-9]. However, most are computational fluid dynamics (CFD) models based on detailed parameters or external characteristic models. The CFD models were not adapted to the dynamic performance study because it is time consuming. On the other hand, too many assumptions have been made for external characteristic models. Some properly detailed models have been described after treating the model with the distributed-lumped parameter method and volume-resistance characteristic modeling [5,10,11], in which the distributed parameter characteristic of the system can be reflected, and the iterative coupling computation between pressure and flow rate in the system can be avoided. Thus, the system model can meet the real-time computation requirement. In this paper, the already established one-dimensional simulation model of direct internal reforming planar SOFC [10,11] was adopted. Figure 1 shows the principle schematic of a cocurrent planar SOFC. For the countercurrent SOFC, the air will go into the cell from the right side.

SOFC simulation conditions

Cocurrent and countercurrent SOFCs have the same operating conditions, as follows:

Fuel composition (mole fraction): CH₄ 0.1710, H₂ 0.2626, CO 0.0294, CO₂ 0.0436, H₂O 0.4934; Oxidant composition (mole fraction): O₂ 0.21, N₂ 0.79.

The air mole flow rates are 0.012 mol/s; the inlet temperature of fuel and air are 1173 K; the outlet pressure of stack is 1.0×10⁵ Pa; the fuel utilization factor is 0.75; and the cell power density is set to 2800 W/m².

Results and analysis

Steady-state results analysis

Steady-state simulation was performed based on the aforementioned conditions. Figure 2 illustrates the temperature distribution in the cocurrent and countercurrent SOFC along the cell length. The methane reforming reaction in the fuel channel absorbs a large amount of heat, resulting in decreased temperature near the inlet of the fuel channel. In the cocurrent type, the low inlet air temperature leads to a large temperature drop of approximately 50 K. In the countercurrent type, the air temperature in the outlet of the air channel is high, resulting in a small temperature drop in the fuel channel. Under normal conditions, the temperature difference at 1 cm intervals of the PEN plate should be below 10 K to prevent the cracking and deformation of the PEN plate. A large temperature gradient may cause unstable operation [12]. In the cocurrent type, the overall temperature difference of the channel length is approximately 140 K greater than that of the 100 K in the countercurrent type. However, the temperature gradient of the cocurrent type is small, whereas that of the countercurrent is very large, especially near the inlet of the fuel channel. In addition, the overall average operating temperature of the cocurrent type is low, which means the fuel cells are more stable.

Figure 3 presents the mole fractions of the fuel along the cell length for cocurrent and countercurrent SOFCs. Methane is consumed entirely through the reforming reaction near the inlet. Methane consumption at the countercurrent type is faster than that in the cocurrent type. Hydrogen concentration increases near the inlet due to the fuel reforming reaction, and then electrochemical reactions begin to play a dominant role. In the cocurrent type, the operation temperature gradually increases along the cell length and the hydrogen consumption rate increases. However, in the countercurrent type, the temperature increases at first but then decreases, resulting in a gradual decrease in the hydrogen consumption rate.

Figure 4 displays the distribution of various voltage losses and current densities in the cocurrent and countercurrent types of SOFCs along the cell length. In Fig. 4, U_ocp represents the open circuit operating voltage; the difference between the dotted line and U_ocp represents ohmic losses; the difference between the dotted line and the triangle line represents the anodic polarization losses; and the difference between the triangle line and U_op represents cathodic polarization losses. In high-temperature fuel cells, the ohmic resistance of the material only accounts for a small part of the losses, while the main losses are those from anodic and cathodic polarization. Influenced by temperature, the voltage losses of the cocurrent type are larger than those of the countercurrent type are. In the cocurrent type, the current density decreases slightly at the inlet due to the large temperature drop of the PEN plate, and then increases gradually with the increase in location. The current density drop may also exist near the outlet. However, in the countercurrent type, the current density first increases dramatically and then decreases dramatically, whereas the large current density gradient affects the normal operation of the cell. Current density is a complicated physical quantity that is mainly concerned with the temperature distribution of the PEN plate. It has much to do with fuel partial pressure and fuel utilization [1,2]. Under the operating conditions described in this paper, the operating voltage of the cocurrent type is approximately 0.61 V, and the average current density is about 4616 A/m². The operating voltage of the countercurrent type reaches 0.71 V, and the average current density approximates 3952 A/m².

The distributions of pressure, velocity, and gas physical property parameters (density, dynamic viscosity, thermal conductivity, and specific heat capacity) in the gas channel of the cocurrent and countercurrent types of SOFCs along the cell length, as shown in Figs. 5 and 6, are consistent with that of correlation attributes in Ref. [5]. The pressure drop in the gas channel of the cocurrent and countercurrent types are almost the same, at about 3% in the air channel, and only 0.1% in the fuel channel. Gas velocity is not only related to mass flow rate, but also to gas density and temperature. In the air channel, the gas velocity of the cocurrent type decreases first and then increases, while that of the countercurrent type does the reverse. The gas velocity increases gradually in the fuel channel. Gas density is mainly affected by the pressure of the gas channel, the gas temperature, and the gas components. The specific heat capacity, thermal conductivity, and dynamic viscosity of the gas are mainly related to gas temperature and gas components. For these gas properties, the cocurrent and countercurrent types have the opposite tendency in the air channel, and they have the same tendency in the fuel channel.

In the working condition of the fuel flow rate of the cocurrent type, the fuel cell is approximately 1.31 mol/s and the power generation efficiency is about 41%. The fuel flow rate of the countercurrent type fuel cell is approximately 1.12 mol/s, and the power generation efficiency is about 47.9%. Based on the comparison and analysis under the same load power requirements, the operating voltage and the power generation efficiency of the cocurrent type are low, but the countercurrent type has large temperature gradient and current density gradient, which will seriously affect the life of the cell system and the safety and stability of the operation. The cocurrent type of fuel cell, which operates more stably, has more advantages than the countercurrent type. The conclusion agrees completely with Refs. [2] and [4].

Dynamic results analysis

Based on the steady-state analysis, while keeping the fuel utilization constant using the PID controller, the dynamic response of various parameters of the cocurrent and countercurrent types of fuel cell systems are analyzed after a sudden 10% increase in fuel cell load power density at 200 s.

The dynamic response trends of the parameters of the cocurrent and countercurrent types of fuel cell systems are the same. Figure 7 demonstrates the power density demand and the dynamic responses of the cell power density in these two cases. In both cases, the stability of the system is about 500 s; in comparison, the response speed of the countercurrent type is faster. When the load power density demand increases, the inlet fuel increases, and the max temperature of the fuel gradually increases (Fig. 8) when fuel utilization is unchanged (Fig. 9(a)). Due to the unchanging air flow rate, air consumption increases gradually when fuel increases; therefore, air utilization increases gradually (Fig. 9(a)). Figure 10 gives the dynamic responses of the average current density and operating voltage for both cases. The average current density increases gradually, while the operating voltage decreases gradually.

Figures 7-10 show that the inertia delay of system parameters is about 500 s, mainly due to the large thermal inertia of the fuel cell, which will be of referential significance for the design of the control system.

Conclusions

Based on the already established simulation model of direct internal reforming planar SOFC, steady-state and dynamic simulations were conducted on cocurrent and countercurrent fuel cells.

The steady-state simulation indicates that the countercurrent type of fuel cell has higher power generation efficiency under the same operating condition. However, it has a large temperature gradient and current density gradient, which greatly affects the life and stability of the cell. Therefore, the cocurrent type of SOFC is more suitable in operating the cell system.

The dynamic simulation shows that the cocurrent and countercurrent types of SOFC systems have large thermal inertia, resulting in an inertia delay time of about 500 s for the system. However, the inertia delay time of the countercurrent type is relatively small.

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