1,179 research outputs found

    Performance and durability of thin film thermocouple array on a porous electrode

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    Management of solid oxide fuel cell (SOFC) thermal gradients is vital to limit thermal expansion mismatch and thermal stress. However, owing to harsh operation conditions of SOFCs and limited available space in stack configuration, the number of techniques available to obtain temperature distribution from the cell surface is limited. The authors previously developed and studied a thermocouple array pattern to detect surface temperature distribution on an SOFC in open circuit conditions. In this study, the performance in terms of mechanical durability and oxidation state of the thin film thermoelements of the thermocouple array on the porous SOFC cathode is investigated. A thin-film multi-junction thermocouple array was sputter deposited using a magnetron sputter coater. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterisation techniques were carried out to understand characteristics of the thin film before and after temperature (20 °C–800 °C) measurement. Temperature readings from the sensor agreed well with the closely placed commercial thermocouple during heating segments. However, a sensor failure occurred at around 350 °C during the cooling segment. The SEM and XPS tests revealed cracks on the thin film thermoelements and oxidation to the film thickness direction

    Real-time temperature distribution monitoring of SOFC cathode and contributing parameters

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    The solid oxide fuel cell (SOFC) as an energy conversion device is considered as a promising technology due to its high operating efficiency, fuel flexibility and relatively low emissions. However high operating temperature related issues such as availability and durability of the materials to operate at that high temperature and for long-time thermal cycling need to be understood and overcome. These are vital areas for research to ensure availability and durability measures are maximised subject to minimising SOFC system cost. To obtain the desired power output, especially for industrial applications, several cells are linked together to form a stack and the stacks are linked together to form a SOFC power generating system. There are two ways to overcome these issues, first; to reduce the operating temperature and second; to have better thermal management of the systems. Decreasing the operating temperature however can lead to a reduction in overall performance of the system due to an increase in the resistance in the cell as well as the electrodes becoming less active. Thus, thermal management of SOFCs is significant for minimising performance losses and maximising cell/stack lifetime. [Continues.

    Sensor technology for in situ monitoring of the surface temperature distribution of SOFC

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    Solid oxide fuel cell (SOFC) is the most efficient energy conversion technology of alltime in producing electricity from fuels. However, temperature-driven premature degradation is one of the biggest problems that impedes the widespread use of this technology. Understanding the temperature distribution of an operating SOFC is central to mitigate such degradations as well as to further enhance the performance. The published efforts on SOFC temperature sensing, except small button cells, are mainly confined to measure temperature only from the gas channels (fuel/ air) with relatively low spatial resolution. However, the electrodes’ temperature distribution measured with an adequate spatial resolution is more desirable than the gas temperature to investigate a cells’ behaviour and its correlation to a stack’s performance. The insufficiency of technology to in situ monitor the cell surface temperature distribution with an adequate spatial resolution was identified as a crucial research gap in the SOFC development cycle. Therefore, this research is aimed at developing a sensing technology to monitor in situ the cell surface temperature distribution of an operating SOFC with an adequately high spatial resolution and applying that technology to get a better insight into SOFC operation. [Continues.

    Fabrication and Thermal Characterization of Composite Cu-CNT Micropillars for Capillary-driven Phase-Change Cooling Devices

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    This paper presents the fabrication, testing, and modeling of an array of composite copper-carbon nanotubes (Cu-CNT) micropillars as a wick structure for potential application in passive phase-change cooling systems. This novel wick structure has a larger spacing at the base of the micropillars to provide a higher liquid permeability and mushroom-like structures on the top surface of the micropillars with a smaller spacing to provide a greater capillary pressure. The composite Cu-CNT micropillars were fabricated by an electrochemical deposition method on a patterned copper template. Cauliflower-like nanostructures were then grown on the top surface of the micropillars using chronoamperometry technique to improve the capillary pressure and thermal performance of the wick structure. After successful fabrication of the micropillars, a series of tests were conducted to quantify the thermal performance of the wick structures. The results demonstrate superior thermal and corrosion performances for composite Cu-CNT micropillars compared to those of copper micropillars. Additionally, a thermal resistance network analysis was conducted to model the thermal performance of the fabricated mushroom-shaped micropillar array. Model predictions were compared with the experimental results and good agreement was observed

    NASA welding assessment program

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    A long duration test has been conducted for comparing various methods of attaching electrical interconnects to solar cells for near Earth orbit spacecraft. Representative solar array modules have been thermally cycled for 36,000 cycles between -80 and +80 C on this JPL and NASA Lewis Research Center sponsored work. This test simulates the environmental stress of more than 6 years on a near Earth spacecraft as it cycles in and out of the Earth's shadow. Evaluations of the integrity of these modules were made by visual and by electrical examinations before starting the cycling and then at periodic intervals during the cycling tests. Modules included examples of parallel gap and of ultrasonic welding, as well as soldering. The materials and fabrication processes are state of the art, suitable for forming large solar arrays of spacecraft quality. The modules survived his extensive cycling without detectable degradation in their ability to generate power under sunlight illumination

    Development of Highly Functional, Surface Tunable, and Efficient Composite Coatings for Pool Boiling Heat Transfer Enhancement

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    Rapid growth and advancements in high-power electronic devices, IC chips, electric vehicles, and lithium-ion batteries have compelled the development of efficient and novel thermal management solutions. Currently used air and liquid cooling systems are unable to remove the heat efficiently due to significant pressure drops, temperature differences, and limited heat-carrying capacities. In contrast, phase-change cooling techniques can remove the larger amount of heat with higher efficiency while maintaining safer operational temperature ranges. Pool boiling heat transfer is a type of phase-change cooling technique in which vapor bubbles generated on the boiling surface carry away the heat. This pool boiling performance is limited by the maximum heat dissipation capacity, quantified by the Critical Heat Flux (CHF), and efficiency of the boiling surface, quantified by the Heat Transfer Coefficient (HTC). This work emphasizes on improving both CHF and HTC by developing highly surface functional and tunable microporous coatings using sintering and electrodeposition techniques. Initially, graphene nanoplatelets/copper (GNP/Cu)-based composite coatings were developed using a multi-step electrodeposition technique. And 2% GNP/Cu coating rendered the highest reported CHF of 286 W/cm² and HTC of 204 kW/m²-°C with increased bond strength. To further enhance the cohesive and adhesive bond strength of the electrodeposited coatings, a novel multi-step electrodeposition technique was developed and tested on copper-based coatings. This technique dramatically improved the overall functionality, pool boiling performance, and durability of the coatings. Later, a sintering technique was used to develop the coatings using GNP and copper particles. Uniform spreading of GNP over the coatings was obtained via ball milling technique. This technique yielded a CHF of 239 W/cm² and the HTC of 285 kW/m²-°C (~91% and ~438% higher than a plain copper surface, respectively). A novel approach of salt-templated sintering was developed in the final part to attain a better control on porosity and wicking properties of the sintered coatings. This generated interconnected porous networks with a higher nucleating activity, and attained record-breaking CHF of 289 W/cm² and the HTC of 1,314 kW/m²-°C

    Development of nanowire structures on 2d and 3d substrates for pool boiling heat transfer enhancement

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    Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation, refrigeration and many other systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF). Increasing the CHF and the HTC has significant impacts on system-level energy efficiency, safety and cost. As the surface modification at nano-scale has proven to be an effective approach to improve pool boiling heat transfer, the enhancement due to combination of nanomaterials with micro-scale structures on boiling heat transfer is an area of current interest. In this study, metallic- and semiconductor- material based nanowire structures were fabricated and studied for boiling enhancement. A new technique is developed to directly grow Cu nanowire (CuNW) on Si substrate with electro-chemical deposition, and to produce height-controlled hydrophilic nanowired surfaces. Using a two-step electroless etching process, silicon nanowire (SiNW) have been selectively fabricated on top, bottom, and sidewall surfaces of silicon microchannels. An array of the SiNW coated microchannels functioned as a heat sink and was investigated for its pool boiling performance with water. This microchannel heat sink yielded superior boiling performance compared to a sample substrate with only microchannels and a plain substrate with nanowires. The enhancement was associated with the area covered by SiNWs. The sidewalls with SiNWs greatly affected bubble dynamics, resulting in a significant performance enhancement. The maximum heat flux of the microchannel with SiNW on all surfaces was improved by 150% over the microchannel-only heat sink and by more than 400% over a plain silicon substrate. These results provide a viable solution to meet the demands for dissipating a high heat transfer rate in a compact space, with additional insight gained into the boiling mechanism for the microchannel heat sinks with nanostructures

    In-situ monitoring of temperature distribution in operating solid oxide fuel cell cathode using proprietary sensory techniques versus commercial thermocouples

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    Real time surface temperature distribution monitoring of Solid Oxide Fuel Cell (SOFC) systems is important to identify temperature related degradation and understand cell performance. This type of monitoring is limited due to the harsh operating environment of SOFC. Therefore, the temperature variation of an operating SOFC is generally predicted by applying modelling tools which take into account the conventional I-V (current (I)-voltage (V)) curve. However, experimentally obtained temperature data is vital for management of high temperature related degradation and for more reliable modelling of the SOFC. In this study, the temperature distribution of the SOFC is in-situ monitored along the entire cell cathode simultaneously, using commercial TCs on the gas flow channel (the present conventional method) alongside the in-house-developed sensor sensing points (SSPs) directly from the cell cathode surface under both open circuit voltage (OCV) and loading conditions. A considerable difference is observed, especially under the loading condition, between the temperature obtained from the TCs and SSPs even from the same locations. Furthermore, the contribution(s) of different parameters on the temperature variation are investigated, including fuel/air amount under OCV, gas cooling effect, contact area effect and flow direction effect under the loading condition for the given SOFC. There is a fivefold increase in spatial resolution, alongside higher temporal resolution, being observed with the implemented sensor compared to the resolution obtained from the conventional TCs, which yields promise for further development and investigation into test cells and stacks

    Parameters and their impacts on the temperature distribution and thermal gradient of solid oxide fuel cell

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    The commercialisation potential of Solid Oxide Fuel Cell is hindered due to certain technical issues. One of these is the thermal gradient across the cell structure during its operational period that can deteriorate the system’s performance. In this study, a newly developed multipoint thermal sensor is deployed across the cathode to understand the impact of various factors including cell’s operating temperature, fuel flow rate and drawing current density on temperature distribution and its stability. Here we report that direct oxidation of hydrogen due to fuel crossover has been the most impactful contributor for the cell’s average temperature increment during both open circuit voltage and loading conditions, while electrochemical oxidation of hydrogen is the most impactful contributor for cell temperature gradient during loading. A relationship has been established between the temperature profile of the cell surface and the source of the temperature variation which allows identification of the responsible parameter

    Visible light driven photoanodes for water oxidation based on novel r-GO/\u3b2-Cu2V2O7/TiO2 nanorods composites

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    This paper describes the preparation and the photoelectrochemical performances of visible light driven photoanodes based on novel r-GO/-Cu2V2O7/TiO2 nanorods/composites. -Cu2V2O7 was deposited on both fluorine doped tin oxide (FTO) and TiO2 nanorods (NRs)/FTO by a fast and convenient Aerosol Assisted Spray Pyrolysis (AASP) procedure. Ethylenediamine (EN), ammonia and citric acid (CA) were tested as ligands for Cu2+ ions in the aerosol precursors solution. The best-performing deposits, in terms of photocurrent density, were obtained when NH3 was used as ligand. When -Cu2V2O7 was deposited on the TiO2 NRs a good improvement in the durability of the photoanode was obtained, compared with pure -Cu2V2O7 on FTO. A further remarkable improvement in durability and photocurrent density was obtained upon addition, by electrophoretic deposition, of reduced graphene oxide (r-GO) flakes on the -Cu2V2O7/TiO2 composite material. The samples were characterized by X-ray Photoelectron Spectroscopy (XPS), Raman, High Resolution Transmission Electron Microscopy (HR-TEM), Scanning Electron Microscopy (SEM), Wide Angle X-ray Diffraction (WAXD) and UV\u2013Vis spectroscopies. The photoelectrochemical (PEC) performances of -Cu2V2O7 on FTO, -Cu2V2O7/TiO2 and r-GO/-Cu2V2O7/TiO2 were tested in visible light by linear voltammetry and Electrochemical Impedance Spectroscopy (EIS) measurements
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