Po-Ya Abel Chuang

The Nature of Flooding and Drying in Polymer Electrolyte Fuel Cells

Two different 50 cm2 fuel cells operated at high current density (1.3A/cm2 –1.5A/cm2 ) were visualized using neutron imaging, and the liquid water content in the flow channels and diffusion media under the lands and channels was calculated and compared. At high current density with fully humidified inlet flow, a direct comparison between flooded and non-flooded conditions was achieved by increasing the fuel cell temperature over a small range, until voltage loss from flooding was alleviated. Results indicate that a surprisingly small mass of liquid water is responsible for a significant voltage loss. The deleterious effects of flooding are therefore more easily explained with a locally segregated flooded pore model, rather than a homogeneously flooded pore and blockage phenomenon. Anode dryout was similarly observed and quantified, and results indicate that an exceedingly small mass of water is responsible for significant voltage loss, which is consistent with expectations. The results presented help to form a more complete vision of the flooding loss and anode dryout phenomena in PEFCs.Copyright

The Property and Performance Differences Between Catalyst Coated Membrane and Catalyst Coated Diffusion Media

Two fuel cell architectures, differing only by the surfaces onto which the electrodes wereapplied, have been analyzed to determine the root causes of dissimilarities in perfor-mance. The basic proton exchange membrane fuel cell is comprised of the proton trans-porting membrane, platinum-containing anode and cathode electrodes, porous carbonfiber gas diffusion media (GDM), and flow fields that deliver the reactant hydrogen andair flows. As no optimal cell design currently exists, there is a degree of latitude regard-ing component assembly and structure. Catalyst coated diffusion media (CCDM) refers toa cell architecture option where the electrode layers are coated on the GDM layers andthen hot pressed to the membrane. Catalyst coated membrane (CCM) refers to an archi-tecture where the electrodes are transferred directly onto the membrane. A cell withCCDM architecture has tightly bonded interfaces throughout the assembly, which canresult in lower thermal and electrical contact resistances. Considering the fuel cell as a1D thermal system, the through-plane thermal resistance was observed to decrease by5–10% when comparing CCDM to CCM architectures. This suggests that the thermalcontact resistance at the electrode interfaces was significantly reduced in the hot-pressprocess. In addition, the electrical contact resistances between the electrode and GDMwere observed to be significantly reduced with a CCDM architecture. This study showsthat these effects, which have a potential to increase performance, can be attributed tothe hot-press lamination process and use of CCDM architecture.

Comparison of Experiments and 1-D Steady-State Model of a Loop Heat Pipe

A modern, effective, two-phase heat transfer device, a loop heat pipe (LHP), was studied analytically and experimentally. A 1-D steady-state model was developed based on energy balance equations. The mathematical modeling procedures of each component are explained in detail, including a model of the secondary wick in the evaporator. Other models neglect the existence of the secondary wick because the detailed designs of the secondary wick are often proprietary. Three sets of experiments were performed at different elevations. Results of experimental data are compared with 1-D steady-state model predictions. The comparisons show that the model predictions of steady state operating temperatures for both zero elevation and adverse elevation are within 2 percent. It has been clearly demonstrated that the 1-D steady-state model is a useful tool for future LHP study.Copyright

Optimal Oxygen Stoichiometry for Maximum Net Power Output of Proton Exchange Membrane Fuel Cell Systems

This paper investigates the issue of maximum net electrical power output for proton exchange membrane (PEM) fuel cell systems. Under certain operating points characterized by output current or equivalently load resistance, the electrical power output of the PEM fuel cell is proportional to oxygen stoichiometry. However, the extra electrical power needed for the air blower is proportional to the supplied oxygen. In this paper, the optimal oxygen stoichiometry is derived as functions of the stack output current and the blower energy efficiency. Therefore, to obtain a maximum net power output for a PEM fuel cell, the implementation of oxygen stoichiometry can be tuned on-line according to the specification of blower efficiency and the instantaneous value of the output current. The analytical derivation is based on third-order nonlinear PEM fuel cell dynamics with the system parameter values of a Ballard 5 kW PEM fuel cell system. The proposed approach is verified through the quantities of net power output as obtained from both equilibrium operation conditions and time response simulation based on the formulated fuel cell nonlinear dynamics

The Effect of Catalyst Coated Diffusion Media on PEM Fuel Cell Performance

Two fuel cell architectures, differing only by the surfaces onto which the electrodes were applied, have been analyzed to determine the root causes of dissimilarities in performance. The basic proton exchange membrane fuel cell (PEMFC) is comprised of the proton transporting membrane, platinum-containing anode and cathode electrodes, porous carbon fiber gas diffusion media (GDM), and flow fields which deliver the reactant hydrogen and air flows. As no optimal cell design currently exists, there is a degree of latitude regarding component assembly and structure. Catalyst coated diffusion media (CCDM) refers to a cell architecture option where the electrode layers are coated on the GDM layers and then hot-pressed to the membrane. Catalyst coated membrane (CCM) refers to an architecture where the electrodes are transferred directly onto the membrane. A cell with CCDM architecture has tightly bonded interfaces throughout the assembly which can result in lower thermal and electrical contact resistances. Considering the fuel cell as a 1-D thermal system, the through-plane thermal resistance was observed to decrease by 5–10% when comparing CCDM to CCM architectures. This suggests the thermal contact resistance at the electrode interfaces was significantly reduced in the hot-press process. In addition, the electrical contact resistances between the electrode and GDM were observed to be significantly reduced with a CCDM architecture. This study shows that these effects, which have a potential to increase performance, can be attributed to the hot-press lamination process and use of CCDM architecture.Copyright

Analytical Modeling of a Loop Heat Pipe at Positive Elevation

A two-phase heat transfer device, a loop heat pipe (LHP), is studied analytically. It is noted that a LHP behaves differently when it is operated against gravity (adverse elevation) or at gravity assisted (positive elevation) conditions. Steady-state modeling of LHP operating characteristics at adverse or zero elevation was broadly studied in the past. This paper presents a steady-state model of a LHP when it is operated at positive elevation based on experimental results. The effects of elevation on the trend of steady-state operating temperature (SSOT) are then studied using the newly developed steady-state model. Experimental results agree with the model predictions at adverse (88.9mm), zero, and positive (88.9mm) elevations. This steady-state model is the only model known to have the capability to predict the operating characteristics at positive elevation. The model will help to design the LHPs utilized in terrestrial applications.Copyright

Theoretical and Experimental Study of a Loop Heat Pipe at Positive Elevation

A loop heat pipe (LHP), which is a two-phase heat transfer device, was studied experimentally and theoretically. The steady-state operating characteristics of a LHP when it is operated at adverse (the condenser is below the evaporator) and zero elevations (the evaporator and the condenser are at the same level) had been studied intensively in the past. However, study of a LHP when it is operated at positive elevation (the condenser is above the evaporator) is still lacking. This paper presents detailed theoretical analysis of the steady-state behavior of a LHP operated at positive elevation. The present analysis agrees with experimental results, and is confirmed by flow visualization images. Testing was performed for a wide range of heat loads (20 W-600 W) at three positive elevations: 25.4mm, 76.2mm, and 127mm. Flow visualization images were taken by neutron radiography when the LHP was operated at 102mm positive elevation.Copyright