Thermal Safety Analysis of Direct Methanol Micro-Fuel Cell
LOU Wenzhong & QI Bin
(School of Mechatronic Engineering, Beijing Institute of Technology, Beijing 100081, China)
Abstract: A simulation of thermal safety management in micro direct methanol fuel cell(DMFC) is developed. Finite element analysis (FEA) software, ANSYS,
is employed to explore the heat transfer issue of micro fuel cell, in order to protect the micro fuel cell itself. The thermal performance of DMFC is characterized.
25 microchannels are built inside the micro fuel cell, which decrease the temperature gradient between the two sides of the bipolar plate, and enhance the
working reliability of DMFC.
Keywords: DMFC; thermal safety management; microchannel; Micro-Fuel cell
Safety and clear energy is the most important problem today. Fuel cells are attractive for portable power systems because of
their very high energy density, high efficiency, clean and quiet operation. As another innovative invention, micro direct methanol
fuel cell(DMFC) is one of the most outstanding forms of portable power supply. However there are still many questions unanswered
about the scale up of high temperature fuel cell, especially for DMFC. Among them are questions concerning the heat transport inside
the fuel cell, battery temperature uniformity, and maintaining the fuel cell at operating temperature range. The performance, energy
efficiency, safety and reliability of the fuel cell depend on answers to these questions. The examination of the thermal behavior and
design of a proper thermal management system are important topics in the design of fuel cell.
In order to generate electricity, DMFC must be supplied with methanol, which at the same time, producing extra thermal energy
(heat). Because chemical reaction in the micro fuel cell operates best within a certain temperature range, the cooling system must
provide an adequate means of heat removal. As a result, engineering issues concerning thermal management need to be resolved to
robustly meet the requirements for commercial micro DMFC.
Traditional methods, such as natural-cooling and forced air-cooling, have become inadequate to remove the heat fluxes and
incompatible with the compact, micro-size design. Compared with the traditional methods, the structure of the microchannel
enhances heat transfer coefficient, which is an alternative cooling method for micro DMFC systems.
2 Figure of Fuel Cell’s Design
As shown in fig 2, the whole fuel cell is in cylinder shape. Bipolar plate is also in cylinder shape. Both top and underside are
riveted by insulated fixed part. Exterior is made of metal shell with many small holes. The bottle of deposit liquid is load with
methanol aqueous solution. Mini pump runs when fuel cell require to work. The methanol aqueous solution firstly comes into anode
plate from the bottom of the fuel cell. Then it passes through the flow channel from the bottom to the top. And then it runs back to
the bottle of deposit liquid through top pipeline. In this way, electricity is generated by circulation. In order to keep the thermal
equilibrium, extra produced heat is taken away by water flowing through the internal channels in the bipolar plate.
3 Design of Cooling Channel in Bipolar Plate
The flow direction of fuel and oxidant in the fuel cell is different from the flow direction of the cooling water in channel, which
result in two kinds of thermal boundary conditions in the heat transfer between the coolant and the surface. One is the boundary
condition of invariable heat flux density; the other is the boundary condition of invariable surface temperature.
Fig.1 Figure of Micro-Fuel cell
3.1 Invariable Heat Flux Density
Fig.2 is a quarter of the cross section A-A of the fuel
cell (Fig.1). As mentioned in Section 2, it is supposed that
the fuel and oxidant flow from the bottom to the top. Any
longitudinal section of the bipolar plate has a temperature
gradient from the bottom to the top. The temperature
gradient degree means the temperature changes relative to
change in the distance, which therefore is determined by
the fuel velocity. Hence, a suitable velocity is of
importance to make the fuel cell work smoothly. The
coolant in the channel also runs from the bottom to the top.
And it also has a temperature gradient in the flow direction.
The thermal energy transferred between the coolant and the Fig.2 Quarter of the cross section A-A of the Micro-Fuel cell
channel surface takes place in the boundary condition of
invariable heat flux density. Because the coolant carries the same amount of thermal energy from every part of the channel surface,
which makes the temperature gradient of the cross section keep invariable, the fuel cell is able to work stably.
The diameter and the length of the channel are D and L, respectively. When L / D 60 , the flow is turbulent current. The
average of the convection coefficient is not influenced by the inlet length. The inlet length is decided by L / D 0.05Re P r 
when it is of laminar flow.
Respecting the average temperature of the coolant as the reference temperature, we can calculate the Reynolds number ( Ref )
Re f ud / f . (1)
Where u is the coolant velocity; d is hydraulic diameter; f is kinematical viscosity;
If Ref 10 4
, it is turbulent current, the forced convection coefficient formula is shown :
Nu f 0.023Re0.8 Prf0.4
Where Nu f is Nusselt number in the reference temperature; Pr f is the Prandtl number in the reference temperature;
If Re f 104 , it is laminar flow, and the Nusselt number is independent of Re, so the forced convection coefficient formula
Nu f hd / f 4.366  (3)
Where h is the heat transfer coefficient; d is the length along the channel; f is the thermal conductivity of the coolant in the
3.2 Invariable Surface Temperature
It is supposed that the fuel and oxidant flow from the bottom to the top. Any longitudinal section of the bipolar plate has a
temperature gradient from the bottom to the top, while the coolant runs into the channel from the top to the bottom. Since the coolant
temperature is much lower than the working temperature of fuel cell, it produces a temperature gradient from the top to the bottom in
the flow direction. If the parameters can be suitably designed to eliminate the two temperature gradients, it can be considered that the
heat transfer in the channels carries out in the boundary condition of invariable surface temperature.
The analysis process is the same as 3.1, but the difference is:
If Re f 104 , Nu f hd / f 3.658  (4)
3.3 Comparison of Two Kinds of Boundary Conditions
From the two kinds of boundary conditions, it indicates that the Nuf in the boundary condition of invariable surface
temperature is smaller than it in the boundary condition of invariable heat flux density. It shows that the heat transfer effect, which in
the invariable heat flux density condition, is better than it in the other one. But, according to the practical working state, to ensure a
better performance of the cooling system, the temperature gradient should be as low as possible, which prevents certain part of the
channel from being too hot. So the boundary condition of invariable surface temperature is more advantageous to the work of the fuel
4 Simulation of the Cooling Channel
Here the coolant we select is water that has a good property for heat transfer. The whole cooling system is shown in Fig.3.
What we analyze in this paper is the fluid mechanics and thermal performance in the channels.
According to the dimensions of the anode and negative plates, the length and width of the coolant channel we designed are 2 mm
and 1mm, and the number of the channels is 25. The distance between the coolant channel and the plate edge is 1 mm. The arrays of
the coolant, fuel and oxidant channels are illustrated in Fig.2.
The flux of CH3OH and H2O is 52 mg/min, and the ratio of CH3OH and H2O is one to one; the diffusivities of CH3OH and H2O
are both 0.8, and the percolation ratio of CH3OH through the MEA is 0.05; The inlet and outlet temperatures of fuel are 20 ℃ and
Fig.3 Cooling system of Micro-Fuel cell
In order to guarantee working stability of the fuel cell, we select invariable surface temperature as the boundary condition.
Supposed that heat dissipation is ignored in the two sides of the plates and heat transfer of every channel is the same, we
modeled and simulated the flow and heat transfer process in the channel using ANSYS .
We simulated the surface without the coolant channel, and acquired the temperature distribution data(Fig.4) which shows 40 ℃
temperature difference. While there is only a 13 ℃ temperature difference (Fig.5) between the two sides with coolant channel, and
the maximal velocity of flow is 5.73 cm/s in the middle of the channel(Fig.6).
Fig.4 40 ℃ temperature difference Fig.5 15 ℃ temperature difference Fig.6 The maximal velocity
In this paper, we have shown the significance and advantage of a microchannel cooling approach to the thermal management in
direct methanol micro-fuel cell. The thermal performance of the micro fuel cell with and without the cooling channels is analyzed
and simulated. The result indicates that, with microchannel, the surface temperature of micro DMFC can be maintained around
30 ℃ with a temperature difference of 13 ℃ between the two sides of bipolar plate. This temperature range is suitable for
chemical reaction in the micro DMFC, and ensures the best performance of micro DMFC.
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