Measuring moisture sorption and diffusion kinetics on proton exchange membranes using a gravimetric vapor sorption apparatus

The understanding of water sorption and diffusion properties of proton exchange membranes is crucial to the fuel cell's ultimate performance. In this study, a dynamic gravimetric vapor sorption (DVS) instrument was used to measure the water vapor sorption properties of three Nafion^® based fuel cell membranes: N-117 (extruded film, 183 μm thick); N-112 (extruded film, 51 μm thick); and NR-112 (dispersion cast film, 51 μm thick). Water sorption characteristics were studied between 0 and 95% relative humidity (RH) at 30, 40, 50, 70, and 80 °C. The thicker dispersion cast, N-117, film had a lower water vapor sorption capacity (based on percentage weight gain) than the thinner, N-112 sample. The dispersion cast, NR-112, film had a lower percentage water uptake than the extruded, N-112, film. Below 80% RH, the water sorption capacity increases with temperature for all three samples. Above 80% RH, the moisture sorption capacity increases from 30 to 50 °C, but decreases at 70 and 80 °C compared to the lower temperature data. Moisture diffusion coefficients were also calculated over the humidity and temperature range studied. In general, maximum diffusion coefficients were measured at intermediate humidities. Water heat of sorption calculations at low coverages yielded higher values for the extruded (N-112) film compared to the dispersion cast (NR-112) film indicating a higher affinity for water.     

Subcooled flow boiling CHF enhancement with porous surface coatings

The effect of micro/nanoporous inside surface coated vertical tubes on CHF was determined during water flow boiling at atmospheric pressure. CHF was measured for smooth and three different coated tubes, at mass fluxes (100–300 kg/m² s) and two inlet subcooling temperatures (50 °C and 75 °C). Greater CHF enhancement was found with microporous coatings than with nanoporous coatings. Al₂O₃ microporous coatings with particle size <10 μm and coatings thickness of 50 μm showed the best CHF enhancement. Maximum increase in CHF was about 25% for microporous Al₂O₃. A wettability test was performed to study an increase of CHF with microporous coated surfaces.     

A thin film silicon anode for Li-ion batteries having a very large specific capacity and long cycle life

A thin film of Si was vacuum-deposited onto a 30 μm thick Ni foil from a source of n-type of Si, the film thickness examined being 200–1500 Å. Li insertion/extraction evaluation was performed mainly with cyclic voltammetry (CV) and constant current charge/discharge cycling in propylene carbonate (PC) containing 1 M LiClO₄ at ambient temperature. The cycleability and the Li accommodation capacity were found to depend on the film thickness. Thinner films gave larger accommodation capacity. A 500 Å thick Si film gave a charge capacity over 3500 mAh g⁻¹ being maintained during 200 cycles under 2 C charge/discharge rate, while a 1500 Å film revealed around 2200 mAh g⁻¹ during 200 cycles under 1 C rate. The initial charge loss could not be ignored but it could be reduced by controlling the deposition conditions.     

Thin yttria-stabilized zirconia electrolyte and transition layers fabricated by particle suspension spray

In order to develop high performance intermediate temperature (<800 °C) solid oxide fuel cells (SOFCs) with a lower fabrication cost, a pressurized spray process of ceramic suspensions has been established to prepare both dense yttria-stabilized zirconia (YSZ) electrolyte membranes and transition anode layers on NiO + YSZ anode supports. A single cell with 10 μm thick YSZ electrolyte on a porous anode support and ∼20 μm thick cathode layer showed peak power densities of only 212 mW cm⁻² at 700 °C and 407 mW cm⁻² for 800 °C. While a cell with 10 μm thick YSZ electrolyte and a transition layer on the porous anode support using a ultra-fine NiO + YSZ powder showed peak power densities of 346 and 837 mW cm⁻² at 700 and 800 °C, respectively. The dramatic improvement of cell performance was attributed to the much improved anode microstructure that was confirmed by both scanning electron microscopes (SEM) observation and impedance spectroscopy. The results have demonstrated that a pressurized spray coating is a suitable technique to fabricate high performance SOFCs and at lower cost.     

Comparison study of liquid replenishing impacts on critical heat flux and heat transfer coefficient of nucleate pool boiling on multiscale modulated porous structures

The critical heat flux (CHF) and heat transfer coefficient of de-ionized (DI) water pool boiling have been experimentally studied on a plain surface, one uniform thick porous structure, two modulated porous structures and two hybrid modulated porous structures. The modulated porous structure design has a porous base of 0.55 mm thick with four 3 mm diameter porous pillars of 3.6 mm high on the top of the base. The microparticle size combinations of porous base and porous pillars are uniform 250 μm, uniform 400 μm, 250 μm for base and 400 μm for pillars, and 400 μm for base and 250 μm for pillars. Both the CHF and heat transfer coefficient are significantly improved by the modulated porous. The boiling curves for different kinds of porous structures and a plain surface are compared and analyzed. Hydrodynamic instability for the two-phase change heat transfer has been delayed by the porous pillars which dramatically enhances the CHF. The highest pool boiling heat flux occurring on the modulated porous structures has a value of 450 W/cm², over three times of the CHF on a plain surface. Additionally, the highest heat transfer coefficient also reaches a value of 20 W/cm² K, three times of that on a plain copper surface. The study also demonstrates that the horizontal liquid replenishing is equally important as the vertical liquid replenishing for the enhancement of heat transfer coefficient and CHF improvement in nucleate pool boiling.     

Solid/vapour sorption heat transformer: Design and performance

A high temperature high lift solid sorption based heat transformer has been successfully designed and tested. The sorption reactor concept is based on a tube-fin heat exchanger where the heat exchanging fluids can flow through the hollow fins. The plates were brazed together with porous metal foam that was impregnated with either of the sorbents, LiCl and MgCl₂. The adsorbate is ammonia. The batch system was tested as to the power delivered at high temperatures, 150–200 °C. Peak power at 200 °C was about 0.8 kW, the average power about 0.4 kW. The thermal efficiency, COP, was calculated from the experimental results to be 0.11. This is only 40% of the expected theoretical value and can largely be attributed to the thermal mass of the reactor.     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

Preparation and characterization of thin electrodes for lead–acid batteries

Thin-film electrodes were prepared by spraying aqueous suspensions of soaked leady oxide over both sides of lead sheets previously heated at 150 °C. Uniform coatings of controlled thickness were obtained under optimum deposition conditions. The coatings, which retained their homogeneity after the electrochemical formation, were found to consist of small grains of β-PbO₂ that tended to coalesce into micron-size units. The best electrochemical response was obtained from coatings of reduced thickness (20–40 μm). Cells were found to retain a discharge capacity in excess of 100 Ah/kg after extensive cycling. Also, electrodes exhibited excellent electrochemical performance at different discharge rates. However, if the coating was too thick, the paste was easily sulfated and the cell capacity faded after the first few cycles.     

Development of a multiple layer vacuum insulation chip

Advent of micro thermal devices such as lab-on-a-chip and micro heat pump necessitates development of highly effective insulation chips or layers. This paper reports the development of a vacuum insulation chip (VIC) having very low effective thermal conductivity and very small thickness. Fifty nanometer thickness metal coating on both sides of an LCD glass chip and 5 μm vacuum gap are stacked in a series to decrease the heat transfer by radiation. An array of support legs is necessary to maintain the structure under the atmospheric pressure. Design of VIC involves trade-offs between the heat conduction through the multi-layer structure and the mechanical strength. A model to determine the actual design values is proposed. The results are in reasonable agreement with the more refined results using commercial numerical codes. Based on these results, a VIC of 32 × 32 × 1.88 mm³ is manufactured, and the effective thermal conductivity is measured by guarded hot plate method. The chip shows effective thermal conductivities of 0.0015 and 0.001 W/m K at vacuum levels of 1.33 and 0.24 Pa (N/m²), respectively.     

High heat flux flow boiling in silicon multi-microchannels – Part I: Heat transfer characteristics of refrigerant R236fa

This article is the first in a three part study on flow boiling of refrigerants R236fa and R245fa in a silicon multi-microchannel heat sink. The heat sink was composed of 67 parallel channels, which are 223 μm wide, 680 μm high and 20 mm long with 80 μm thick fins separating the channels. The base heat flux was varied from 3.6 to 221 W/cm², the mass velocity from 281 to 1501 kg/m² s and the exit vapour quality from 2% to 75%. The working pressure and saturation temperature were set nominally at 273 kPa and 25 °C, respectively. The present database includes 1217 local heat transfer coefficient measurements, for which three different heat transfer trends were identified, but in most cases the heat transfer coefficient increased with heat flux and was almost independent of vapour quality and mass velocity. Importantly, it was found for apparently the first time that the heat transfer coefficient as a function of vapour quality reaches a maximum at very high heat fluxes and then decreases with further increase of heat flux.     

Positive thin electrodes obtained from hydrothermally synthesized 4BS for lead-acid batteries

Tetrabasic lead sulfate, 4 PbO·PbSO₄ (4BS), was prepared from an aqueous suspension of leady oxide by using a simple hydrothermal method. Digesting the paste at a moderate temperature (125 °C) and heating for a short time (30 min) ensured the obtainment of particles of small, uniform size. The material was deposited on a lead alloy substrate 0.2 mm thick by spraying from aqueous suspensions. The deposits were highly uniform and homogeneous, with a coating thickness of 100 μm. A multi-step charge algorithm involving no preliminary soaking provided the best 4BS → PbO₂ conversion. The resulting electrodes delivered a capacity of 115 Ah kg⁻¹ with excellent capacity retention over more than 500 cycles at 100% depth of discharge (DOD).     

Study on a re-heat two-stage adsorption chiller – The influence of thermal capacitance ratio,overall thermal conductance ratio and adsorbent mass on system performance

Silica gel/water based adsorption cycles have a distinct advantage in their ability to be driven by heat of near-ambient temperature so that waste heat below 100 °C can be recovered. One interesting feature of refrigeration cycles driven by waste heat is that they do not use primary energy as driving source. From this context, some researchers investigated the performance of multi-stage adsorption refrigeration cycles those can be operated by heat source of temperature 60 °C or lower which are usually purged to the environment, with a heat sink of temperature at 30 °C. However, the performances of multi-stage systems are low. To improve system performance, an analytic investigation on a re-heat two-stage chiller is performed to clarify the effect of thermal capacitance ratio of the adsorbent and inert material of sorption element, overall thermal conductance ratio of sorption element and evaporator along with silica gel mass on the chiller performance. Results show that cycle performance is strongly influenced by the sorption elements overall thermal conductance values due to their severe sensible heating and cooling requirements resulting from batched cycle operation. The effect of thermal capacitance ratio (C_s/C_m) becomes significant with relatively higher mass of silica gel. It is also found that the chiller performance increases significantly in the range of silica gel mass from 4 to 20 kg.     

Analysis for heat transfer enhancement of helical and electrical heating tube heat exchangers in vacuum freeze-drying plant

This paper investigates the heat transfer rate of the combined cooling-and-heating heat exchanger by using computational fluid dynamics (CFD) method. Several factors, such as additional baffles and heat transfer areas, are also discussed in order to improve the efficiency of heat exchanger in the vacuum freeze-drying system. The simulated result indicated that, for addition electrical heating tube, the heat transfer rate of the heat exchanger increased with the increasing length of the electrical heating tube. The increasing rates of secondary and primary drying stages were 2.774 and 2.986 W/mm, respectively. For additional vertical baffle, the variation of the heat transfer rate with respect to vertical baffle length was in the U-shape format. The minimum heat transfer rates of secondary drying, primary drying and freezing stages were 716.79 W and − 195.17 W and − 670.71 W, respectively. For additional W-shape vertical baffles, the heat transfer rate of this heat exchanger was maximum among these four designs. For the three stages of heat exchangers with these four designs, the shell side Nusselt number had the inverse linear relationship with the Reynolds number.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Multifunctional heat exchangers derived from three-dimensional micro-lattice structures

A micro-scale cross-flow heat exchanger is constructed from a hollow nickel micro-lattice structure, which is fabricated by conformally electroplating nickel onto a sacrificial polymer micro-lattice formed from self-propagating photopolymer waveguides. The periodic unit cell of the hollow nickel micro-lattice structure tested here includes lattice members with a diameter <1 mm and a nominal pore size <9 mm. The heat transfer performance of the micro-lattice-based heat exchanger is analyzed in terms of thermal conductance per unit volume, which is equal to the value of overall heat transfer coefficient multiplied by surface area to volume ratio. Calculated values range from 0.84 to 1.58 W/cm³K for Reynolds number ranges of between 3400 ± 200 and 6500 ± 500 for hot water flow inside the hollow lattice members and 85 ± 6 and 240 ± 20 for cold water flow around the lattice members. Based on a developed correlation, the experimental heat transfer data is used to predict the thermal performance of larger and smaller micro-lattice-based heat exchangers, as well as various micro-lattice feature dimensions that are tunable with the fabrication process (node-to-node spacing, inner diameter, etc.). The micro-lattice heat exchanger was tested under quasi-static compression and the results illustrate the multifunctional capability for load bearing and energy absorption applications. This work demonstrates a multifunctional heat exchanger with a fully-scalable fabrication process which is useful for size and weight constrained heat transfer applications, including those in the automotive and aerospace industries.     

Optimization of finned-tube heat exchangers for diesel exhaust waste heat recovery using CFD and CCD techniques

In this paper, response surface methodology (RSM) based on central composite design (CCD) is applied to obtain an optimization design of finned type heat exchangers (HEX) to recover waste heat from the exhaust of a diesel engine. The design is performed for a single point operation (1600 rpm and 60 N m) of an OM314 diesel engine obtained from experimental measurements. Based on the CCD principle, fifteen HEX cases with different fins height, thickness and number are modeled numerically and the optimization is done to have the maximum heat recovery amount and minimum of pressure drop along the heat exchanger.     

Experimental investigation of fluid flow and heat transfer in a single-phase liquid flow micro-heat exchanger

This work presents an experimental analysis of the hydrodynamic and thermal performance of micro-heat exchangers. Two micro-heat exchangers, characterized by microchannels of 100 × 100 and 200 × 200 μm square cross-sections, were designed for that purpose. The fluid used was deionized water and there was no phase change along the fluid circuit. The fluid pressure drop along the heat exchanger and the heat transfer were measured and corrections were made to isolate the contribution of the microchannels. The results were compared with the predictions of the classical viscous flow and heat transfer theory. The main conclusions show that the experimental results fit well with these theories. No effects of heat transfer enhancement or pressure drop increase were observed as a consequence of the small scale of the microchannels.     

Energy-saving transparent heat mirrors based on tungsten oxide–gold WO3/Au/WO3 multilayer structures

Transparent heat mirrors are multilayer structures that transmit visible light while reflecting infrared heat. Heat mirrors based on tungsten oxide and gold multilayers WO₃/Au/WO₃ were fabricated by thermal evaporation, and their performance was investigated as a function of the thickness of the gold layer. First, the properties of individual layers were investigated. Atomic force microscopy revealed that all layers possessed smooth surfaces that were suitable for optical applications. The transmittance of the gold layers was found to decrease as the thickness is increased, with an opposite trend followed by infrared reflectance. In the multilayers, the thickness of the WO₃ was fixed at 34 nm, whereas the thickness of the gold layers was varied in the range 20–44 nm. X-ray photoelectron spectroscopy was used to investigate the elemental diffusion among the various layers, and it revealed the presence of inter-diffusion of elements throughout the layers. The performance of the heat mirrors was evaluated on the basis of their optical behavior. The optimum thickness of the gold layer was found to be 36 nm, with a peak spectral transmittance of 84%.     

Micro thermoelectric cooler: Planar multistage

A suspended, planar multistage micro thermoelectric (TE) cooler is designed using thermal network model to cool MEMS devices. Though the planar (two-dimensional) design is compatible with MEMS fabrication, its cooling performance is reduced compared to that of a pyramid (three-dimensional) design, due to a mechanically indispensable thin dielectric substrate (SiO₂) and technical limit on TE film thickness. We optimize the planar, six-stage TE cooler for maximum cooling, and predict ΔT_max = 51 K with power consumption of 68 mW using undoped, patterned 4–10 μm thick co-evaporated Bi₂Te₃ and Sb₂Te₃ films. Improvement steps of the planar design for achieving cooling performance of the ideal pyramid design are discussed. The predicted performance of a fabricated prototype is compared with experimental results with good agreements.     

Experimental study of the effect of winglet location on heat transfer enhancement and pressure drop in fin-tube heat exchangers

Local heat transfer coefficients were measured on fin-tube heat exchanger with winglets using a single heater of 2 inch diameter and five different positions of winglet type vortex generators. The measurements were made at Reynolds number about 2250. Flow losses were determined by measuring the static pressure drop in the system. Results showed a substantial increase in the heat transfer with winglet type vortex generators. It has been observed that average Nusselt number increases by about 46% while the local heat transfer coefficient improves by several times as compared to plain fin-tube heat exchanger. The maximum improvement is observed in the re-circulation zone. The best location of the winglets was with ΔX = 0.5D and ΔY = 0.5D. The increase in pressure drop for the existing situation was of the order of 18%.