A study of thermal effects in a proton exchange membrane fuel cell with a two-fluid model

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Proton exchange membrane fuel cells (PEMFC) are considered a key future technology for both automotive and stationary applications. At high current densities the performance curve of a PEMFC deviates from the linear region where the cell losses are dominated by the ohmic resistance, and the cell performance deteriorates rapidly. This phenomenon has often been assigned to mass transport losses because conventional fuel cells rely on diffusion of the reactants to reach the catalyst layers. This study will investigate the role of thermal properties on expected cell performance in general and on the membrane hydration level in particular. The two key thermal properties that have been investigated in detail in this study are the thermal conductivity of the porous gas diffusion layers k, and the thermal contact resistance between the gas diffusion layer and the bipolar plates. At a high current density of 1.0 A/cm2 the difference in the average predicted membrane hydration level varies from = 8.92 for the best case to a value down to = 7.73 for the worst case. The difference in the predicted maximum temperature in the cell is more severe. The main conclusion is that it is highly recommended to use dense gas diffusion media with lower porosity but higher thermal conductivity when employing the interdigitated flow field.This project has been sponsored by EUDP, J.nr. 64012-0117 and by PSO under the ForskEL program, Project nr.2013-1-12041.am201

The effect of micro-channels in the MPL on the predicted membrane water content in a PEMFC – a modeling study

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.The micro-porous layer (MPL) in a proton exchange membrane fuel cell is frequently believed to constitute a barrier for the liquid water owing to its low hydraulic permeability compared to the porous substrate. When micro-channels are carved into the MPL on the side facing the catalyst layer, liquid water may be diverted from wet regions of the fuel cell to the drier inlet and outlet sections where it can evaporate and assist in hydrating the membrane as well as provide a pressure relieve at the interface between the catalyst layer and the MPL where delamination can occur under freezing conditions. This modeling study investigates the effect of such micro-channels on the predicted membrane hydration level for a predetermined set of operating conditions with a three-dimensional computational fluid dynamics model that utilizes the multi-fluid approach.cf201

Employing hot wire anemometry to directly measure the water balance in a proton exchange membrane fuel cell

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Water management in proton exchange membrane fuel cells (PEMFC’s) remains a critical problem for their durability, cost, and performance. Because the anode side of this fuel cell has the tendency to become dehydrated, measuring the water balance can be an important diagnosis tool during fuel cell operation. The water balance indicates how much of the product water leaves at the anode side versus the cathode side. Previous methods of determining the fuel cell water balance often relied on condensing the water in the exhaust gas streams and weighing the accumulated mass which is a time consuming process that has limited accuracy. Currently, our group is developing a novel method to accurately determine the water balance in a PEMFC in real time by employing hot-wire anemometry. The amount of heat transferred from the wire to the anode exhaust stream can be translated into a voltage signal which can be directly converted into the fuel cell water balance. In this work, experimental ex-situ results are presented and the elegance and usefulness of this method is demonstrated.This project has been sponsored by EUDP, J.nr. 64012- 0117 and by PSO under the ForskEL program, Project nr.2013- 1-12041.am201

EFFECT OF WEIGHTED JUMP WARM-UP ON VERTICAL JUMP IN FEMALE VOLLEYBALL PLAYERS

Warm-ups which elicit a post activation potentiation (PAP) effect may increase performance in subsequent activities requiring strength and power. However, finding practical methods to best manipulate and exploit PAP remains elusive. Purpose: To determine if a warm-up that included weighted jumps would elicit a PAP effect and increase subsequent vertical jump (VJ) height. Methods: Ten female NCAA Division II volleyball players participated (age = 19.8 ± 1.8 yrs; mass = 71.7 ± 9.7 kg; ht = 167.8 ±23.9 cm). Two warm-up conditions were randomly employed: 1) light jogging, high knees, carioca, shuffling, ankle pops, pogos, and tuck jumps; and 2) an identical warm-up, plus 10 maximal VJs while wearing 20% of bodyweight. At 4-minutes post, VJ was randomly assessed in two conditions: 1) 2-hand standing block VJ (SBVJ), and 2) 1-hand 3-step approach VJ (AVJ). Paired Samples T-test determined if there was a significant difference (p < 0.05) in VJ height between the two conditions. Results: Results were mixed. No significant difference (p > 0.05) occurred in the SBVJ (262.1 vs. 263.0 cm, warm-up vs. warm-up with weight vest, respectively). However, the AVJ was significantly higher by 2.7±1.5 cm (p < 0.05; 275.1±13.2 vs. 277.8±13.2 cm) with the weight vest added to their warm-up. Conclusion: This study demonstrates that a dynamic warm-up with the addition of weighted jumps may increase 3-step AVJ ability in female collegiate volleyball players. However, no effect was seen in 2-hand SBVJ ability. Coaches may consider using a warm-up that includes weighted jumps to optimize performance in sports like volleyball where vertical jumps with an approach are key components of competition.  Article visualizations

CONTINUOUS WEIGHTED JUMPING: EFFECTS ON VERTICAL JUMP HEIGHT

High intensity, near maximal exercise is a conditioning activity which can cause neural hyper-stimulation and lead to acute enhanced power production known as post-activation potentiation (PAP). Investigations need to be conducted to better understand the duration and intensity of the conditioning activity and subsequent effects on the fatigue-potentiation relationship. Purpose: To investigate the effect of 30 seconds of continuous vertical jumping while wearing a vest loaded with 30% of body weight on power output as measured with a maximal vertical jump (VJ). Methods: 14 volunteers (8 weight trained males [23.0+2.9 yrs, 79.8+13.8 kg, 179.9+8.6 cm] and 6 weight trained females [23.0+2.9 yrs, 69.9+13.3 kg, 171.6+7.0 cm]) participated in the study. Pre-testing consisted of each participant performing 3 VJs. The highest VJ was recorded as baseline. A weighted vest was then loaded equaling 30% of the individual's body weight; while wearing the weighted vest, participants performed 30 seconds of continuous VJs. Immediately after jumping participants were seated in a chair for 3 minutes; at 3 minutes, they performed a maximal VJ without the weighted vest in the same manner as done during pre-testing. Two additional VJs were repeated at 4 and 5 minutes post weighted jumping. A mixed design ANOVA with repeated measures was performed. Results: Results showed main effects for each of the independent variables tested, gender and jump. The posttest values at all 3 intervals for both males and females were significantly different from the pretest scores (F [3, 36] = 21.74, p < 0.05). Post hoc analysis indicated that the pretest VJ scores (M = 280.1±18.6 cm) decreased significantly at the 3 minute interval (M = 278.1±18.3 cm), followed by a significant increase in height at the 4 minute interval (M = 281.1±18.7 cm), and another increase at the 5 minute interval (M = 283.0±19.2 cm). Males jumped significantly higher than the females across all trials (p<0.05), however there was no significant difference between male and female VJ gain scores between pre-PAP VJ and 3, 4 and 5 minute post weighted exercise VJ. Conclusion: Results suggest that 30 seconds of weighted VJs causes fatigue which decreases VJ at 3 minutes post exercise. However, a PAP effect was seen at 4 and 5 minutes post weighted exercise significantly increasing VJ above pre testing. From a practical perspective, coaches must be aware of the complex nature of the fatigue-potentiation relationship when attempting to elicit a PAP effect in the individual athlete.   Article visualizations

Functional and multiscale 3D structural investigation of brain tissue through correlative in vivo physiology, synchrotron microtomography and volume electron microscopy

Understanding the function of biological tissues requires a coordinated study of physiology and structure, exploring volumes that contain complete functional units at a detail that resolves the relevant features. Here, we introduce an approach to address this challenge: Mouse brain tissue sections containing a region where function was recorded using in vivo 2-photon calcium imaging were stained, dehydrated, resin-embedded and imaged with synchrotron X-ray computed tomography with propagation-based phase contrast (SXRT). SXRT provided context at subcellular detail, and could be followed by targeted acquisition of multiple volumes using serial block-face electron microscopy (SBEM). In the olfactory bulb, combining SXRT and SBEM enabled disambiguation of in vivo-assigned regions of interest. In the hippocampus, we found that superficial pyramidal neurons in CA1a displayed a larger density of spine apparati than deeper ones. Altogether, this approach can enable a functional and structural investigation of subcellular features in the context of cells and tissues