A critical review of cooling techniques in proton exchange membrane fuel cell stacks

Effective cooling is critical for safe and efficient operation of proton exchange membrane fuel cell (PEMFC) stacks with high power. The narrow range of operating temperature and the small temperature differences between the stack and the ambient introduce significant challenges in the design of a cooling system. To promote the development of effective cooling strategies, cooling techniques reported in technical research publications and patents are reviewed in this paper. Firstly, the characteristics of the heat generation and cooling requirements in a PEMFC stack are introduced. Then the advantages, challenges and progress of various cooling techniques, including (i) cooling with heat spreaders (using high thermal conductivity materials or heat pipes), (ii) cooling with separate air flow, (iii) cooling with liquid (water or antifreeze coolant), and (iv) cooling with phase change (evaporative cooling and cooling through boiling), are systematically reviewed. Finally, further research needs in this area are identified.     

Electron microscopy of shock deformation in alumina

Shock recovered samples of a coarse grain (10 μm), high density (>99.9% theoretical) alumina from asymmetric impact tests conducted at 6.5 GPa (e.g. 3.2 times its Hugoniot Elastic Limit) in a single stage gas gun and characterized by X-ray diffractometry, scanning and field emission scanning electron microscopy, and transmission electron microscopy showed prolific presence of reduced crystallite size, higher average microstrain, grain localized micro/nano-scale deformations, micro-cleavages, grain-boundary microcracks, micro-wing crack formation, extensive shear induced deformations and fractures localized at grains, grain boundaries and triple grain junctions, grain localized entanglement of dislocations and their pile up impeded at grain boundaries. A new qualitative model based on micro-shear and micro-twist induced deformation and fracture in single and/or multiple planes in suitably oriented grain and/or grain assembly was developed to explain the experimentally observed damage evolution process.     

Steam condenser optimization using Real-parameter Genetic Algorithm for Prototype Fast Breeder Reactor

This work explores the use of Real-parameter Genetic Algorithm and analyses its performance in the steam condenser (or Circulating Water System) optimization study of a 500 MW fast breeder nuclear reactor. Choice of optimum design parameters for condenser for a power plant from among a large number of technically viable combination is a complex task. This is primarily due to the conflicting nature of the economic implications of the different system parameters for maximizing the capitalized profit. In order to find the optimum design parameters a Real-parameter Genetic Algorithm model is developed and applied. The results obtained are validated with the reference study results.     

Physico-chemical properties of silica aerogels prepared from TMOS/MTMS mixtures

In the present work, results on the physico-chemical properties of the silica aerogels prepared by sol–gel process using mixtures of TMOS and MTMS as precursor are reported. The wide range of precursor mixture was studied with ratio of MTMS/TMOS in precursor mixtures as 0:100, 25:75, 50:50, 75:25, and 100:0 by volume. The gels with these precursor mixtures were successfully prepared using two step acid–base catalysis for gelation. Acetic acid (0.001 M) and NH₄OH (1.5 M) were used for catalysis and resulting alcogels were subsequently dried by supercritical solvent extraction method. FTIR spectroscopy revealed that the aerogels show more intense peak at 1,260 and 790 cm⁻¹ attributed to Si–CH₃ resulting in more hydrophobic nature and these results were concurrent with adsorbed water content measurements made using Karl Fischer’s titration technique. The resulted aerogels were characterized using differential thermal analysis, thermo gravimetric analysis and surface area measurements. The surface area measurements showed an interesting trend that the surface area increased from 395 to 1,037 m²/g with increase in MTMS content in the precursor mixture from 0 to 50% and then again decreased to 512 m²/g for further increase in MTMS content from 50 to 100% in the precursor mixture. It was observed from our studies that silica aerogels prepared using a starting mixture of 50% TMOS and 50% MTMS resulted in high moisture resistance (adsorbed water content of 0.721% w/w), low density of 90 kg/m³ and the highest surface area of 1,037 m²/g, which has great potential for catalysis support applications.     

Thermal management issues in a PEMFC stack – A brief review of current status

Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output and tolerates only a small deviation in temperature from its design point. The balance between the heat production and its removal determines the operating temperature of a PEMFC. These stringent thermal requirements present a significant heat transfer challenge. In this work, the fundamental heat transfer mechanisms at PEMFC component level (including polymer electrolyte, catalyst layers, gas diffusion media and bipolar plates) are briefly reviewed. The current status of PEMFC cooling technology is also reviewed and research needs are identified.     

Effects of carbon nanofibers on cell morphology, thermal conductivity and crush strength of carbon foam

The effects of low volume fractions of carbon nanofibers on the structure, thermal conductivity and crush strength of carbon foam were examined. Bulk density of the foam increased linearly with the fiber fraction reflecting the morphological changes in the cells. Thermal conductivity increased at low fiber fractions, but dropped at higher fiber fractions. Crush strength increased linearly with fiber fraction for short length fibers, but decreased for the longer length fibers. Scanning electron microscopy, petrography, and X-ray diffraction illustrated the complex effects of the carbon nanofibers on the foam. Available models for thermal conductivity and crush strength have been extended to accommodate these effects incorporating cell structure and morphology (macroeffect), presence of fibers (microeffect), and graphite crystal d-spacing (nanoeffect). This research has shown that the nanofibers have a complex role in the macro, micro, and nanoproperties of the composite foam.     

Response of Materials During Sliding on Various Surface Textures

In the present investigation, soft materials, such as Al-4Mg alloy, high-purity Al and pure Mg pins were slid against hard steel plates of various surface textures to study the response of materials during sliding. The experiments were conducted using an inclined pin-on-plate sliding apparatus under both dry and lubricated conditions in an ambient environment. Two kinds of frictional response, namely steady-state and stick-slip, were observed during sliding. In general, the response was dependent on material pair, normal load, lubrication, and surface texture of the harder material. More specifically, for the case of Al-4Mg alloy, the stick-slip response was absent under both dry and lubricated conditions. For Al, stick-slip was observed only under lubricated conditions. For the case of Mg, the stick-slip response was seen under both dry and lubricated conditions. Further, it was observed that the amplitude of stick-slip motion primarily depends on the plowing component of friction. The plowing component of friction was the highest for the surfaces that promoted plane strain conditions and was the lowest for the surfaces that promoted plane stress conditions near the surface.     

Effects of ventilation rate per person and per floor area on perceived air quality,sick building syndrome symptoms,and decision‐making

Ventilation rates (VRs) in buildings must adequately control indoor levels of pollutants; however, VRs are constrained by the energy costs. Experiments in a simulated office assessed the effects of VR per occupant on perceived air quality (PAQ), Sick Building Syndrome (SBS) symptoms, and decision‐making performance. A parallel set of experiments assessed the effects of VR per unit floor area on the same outcomes. Sixteen blinded healthy young adult subjects participated in each study. Each exposure lasted four hours and each subject experienced two conditions in a within‐subject study design. The order of presentation of test conditions, day of testing, and gender were balanced. Temperature, relative humidity, VRs, and concentrations of pollutants were monitored. Online surveys assessed PAQ and SBS symptoms and a validated computer‐based tool measured decision‐making performance. Neither changing the VR per person nor changing the VR per floor area, had consistent statistically significant effects on PAQ or SBS symptoms. However, reductions in either occupant‐based VR or floor‐area‐based VR had a significant and independent negative impact on most decision‐making measures. These results indicate that the changes in VR employed in the study influence performance of healthy young adults even when PAQ and SBS symptoms are unaffected.     

Artificial neural networks for recognition of 3-d partial dischargepatterns

Partial discharge (PD) measurements have been carried out over the years to assess insulation systems in power apparatus for their integrity and design deficiencies. Digital PD recording, processing and its presentation as 3-d patterns are recent trends in both industry and testing laboratories. Interpretation of these patterns can lead to evaluation of the cause of PD. A need arises to look for methods in the domain of pattern recognition for automating this process. In this context, this paper presents results to demonstrate the possibility of using pattern recognition capabilities offered by a multilayer neural network to recognize 3-d PD patterns     

The use of fractal features for recognition of 3-D dischargepatterns

This work presents further results on the use of fractal features for recognition of 3-D partial discharge patterns. Two fractal features, the fractal dimension and lacunarity were calculated from 3-D discharge patterns and their power to discriminate among various discharge patterns was analyzed. The results indicate that fractal features possess fairly reasonable discriminating abilities