Steady-state Operating Temperature Research Articles

The Faster the Better? Optimal Warm-Up Strategies for a Micro Combined Heat and Power Plant

The warm-up process is a critical operation phase for micro Combined Heat and Power (mCHP) plants, directly impacting their efficiency, reliability, and lifetime. As small decentralized power generation units are increasingly expected to be operated on demand, start-ups will occur more frequently and thus the importance of the warm-up process will further increase. In this study, we address this problem by presenting a mathematical optimization framework that finds optimal actuator trajectories that significantly reduce the warm-up time and improve the thermal efficiency of an mCHP plant. The proposed optimization framework is highly flexible and adaptable to various objective functions, such as maximizing efficiency or minimizing the deviation from desired temperature references. The underlying mathematical model has been experimentally validated on a physical mCHP test rig. Selected case studies further demonstrate the effectiveness and flexibility of the framework and show that with the optimized actuator trajectories, the mCHP plant can reach its steady-state operating temperature in 40% less time. The results also indicate that the shortest warm-up time does not necessarily lead to the highest thermal efficiency. Accordingly, the methodology proposed in this paper provides a powerful tool to study higher-level operational strategies of mCHP plants and thus to maximize their overall performance, which directly translates into an improved operational cost-effectiveness, particularly in demand-driven energy landscapes.

Design optimization of a viscous clutch with an electrorheological fluid

The article describes design optimization of a viscous disc clutch with an electrorheological (ER) fluid. The design optimization is conducted based on mathematical models of ER fluid and clutch geometry. As important issues for the optimal design, the assumed values are: the torque transferred by the clutch, the dimensions of the clutch and the steady-state operating temperature of the clutch. The assumed design variables are: the angular velocity of the input shaft, the number of working gaps of the clutch and the selected dimensions of the clutch. The aim of the design optimization is to search for design variables such as the angular velocity of the input shaft, the number of working gaps of the clutch and the selected dimensions of the clutch, for which the clutch transfers the highest possible torque, while the dimensions of the clutch are small and the steady-state operating temperature is low. The design optimization is conducted searching at least two different objective functions created taking into account the volume, the transferred torque and the steady-state operating temperature of the clutch. Two numerical methods are used: the Monte Carlo method and the Genetic Algorithm method. The optimization calculations are performed for two sets of design variables, consisting of three and eight elements, and a final prototype designed via the optimization is depicted with the principal design parameters.

Gamma distribution excess minority carrier concentration model for solar cell performance modeling

Concentrated photovoltaic (PV) technology represents a growing market in the field of terrestrial solar energy production. As the demand for renewable energy technologies increases, further importance is placed on the modeling, design, and simulation of these systems. Given the cultural shift toward energy awareness and conservation, several concentrated PV systems have been installed across the world. This research presents a new model for carrier concentration within a solar cell. The goal of this innovation is to facilitate the determination of the steady-state operating temperature as a function of the concentration factor for the optical part of the concentrated PV system, to calculate the optimum concentration that maximizes power output and efficiency. This model will be shown to produce a more realistic estimate of the current through a solar cell, which will enable further research into dynamic thermal modeling.

Device-Level Multidimensional Thermal Dynamics With Implications for Current and Future Wide Bandgap Electronics

Abstract Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since the high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to submicrosecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.

The mathematical and experimental analysis on the steady-state operating temperature of bifacial photovoltaic modules

The operating temperature of bifacial photovoltaic (PV) module affects its power generation and reliability. Combined with view factor model of ground reflectivity on module backside, a thermal steady-state model is established to analyze the thermal performance of bifacial module in this paper. The module operating temperatures under three different installation conditions of roof cement ground, water surface and grassland are calculated and simulated by ANSYS software, and the experiments are designed to analyze and verify it. Based on the difference of reflectivity and heat capacity of different ground for the PV module installation, the effects of ground type and module material on bifacial module temperature and its mechanism are analyzed quantitatively. According to the simulation and experiment results, the operating temperatures of the bifacial modules installed on the roof cement ground, water surface and grassland are 44.7 °C, 41.5 °C, 43.2 °C respectively, under the setting environment condition with specific irradiance, wind speed and ambient temperature. The temperature difference of 1.7 °C–3.2 °C is caused by ground reflectivity and temperature. The operating temperature difference between glass-glass and glass-backsheet module is less than 0.4 °C under the standard condition. The conductivity and emissivity of encapsulation material does not show strong influence on module operating temperature.

Multidimensional thermal analysis of an ultrawide bandgap AlGaN channel high electron mobility transistor

Improvements in radio frequency and power electronics can potentially be realized with ultrawide bandgap materials such as aluminum gallium nitride (AlxGa1−xN). Multidimensional thermal characterization of an Al0.30Ga0.70N channel high electron mobility transistor (HEMT) was done using Raman spectroscopy and thermoreflectance thermal imaging to experimentally determine the lateral and vertical steady-state operating temperature profiles. An electrothermal model of the Al0.30Ga0.70N channel HEMT was developed to validate the experimental results and investigate potential device-level thermal management. While the low thermal conductivity of this III-N ternary alloy system results in more device self-heating at room temperature, the temperature insensitive thermal and electrical output characteristics of AlxGa1−xN may open the door for extreme temperature applications.

High-Speed Camera Imaging of the Ignition Process in a Heaterless Hollow Cathode

The plasma discharge inside a heaterless hollow cathode (HHC) is studied during its ignition process. A partly transparent HHC device is developed for this purpose. The temporal evolution images of the plasma in the cathode are obtained by a high-speed camera imaging method. It is observed that the ignition process is started with a gas breakdown that originates between the cathode plate and the keeper plate. After that the plasma discharge spreads out of the keeper through the orifice on the keeper plate. Then, the discharge moves back into the emitter region through the orifice on the cathode plate. It is seen that an arc discharge is formed as the plasma propagates from the downstream end of the emitter to its upstream region. Because of this process, part of the emitter (that next to the cathode plate) is heated gradually to the steady-state operating temperature. The HHC achieves thermal electron emission after the ignition process.

Investigation on the effect of thermoelectric cooler on LHP operation with non-condensable gas

Non-condensable gas (NCG) is one of the main causes affecting adversely the thermal performance, operating reliability and lifespan of two-phase heat transfer devices. In this work, nitrogen was charged into two ammonia-stainless steel loop heat pipes (LHPs) with controllable amounts to simulate the NCG, and extensive experiments were conducted to investigate the effect of thermoelectric cooler (TEC) on the LHP operation with NCG. Experimental results show that the TEC can effectively improve the thermal performance of the LHP with NCG, i.e., decreasing the steady-state operating temperature, expanding the allowable heat load range, realizing a successful startup with much reduced temperature overshoot, and eliminating or inhibiting the possible temperature oscillations. However, some unexpected phenomena or operation anomalies such as startup failure and operating temperature rise were also observed during the experiments with the application of the TEC. Through analyzing these experimental results, the design considerations for the TEC assembly are proposed to better play the role of TEC in improving the thermal performance of LHPs with NCG.

Operation of 1020-MHz NMR Superconducting Magnet

We had successfully operated the 1020-MHz (24.0 T) nuclear magnetic resonance (NMR) spectrometer for 8 months, including the cooling down, shimming, and the 6-month operation for NMR measurements. The NMR superconducting magnet, which had consisted of NbTi and Nb3Sn coils, was upgraded by replacing the innermost Nb3Sn coil with a (Bi,Pb)2Sr2Ca2Cu3O10 (Bi-2223) one. The Bi-2223 innermost coil was connected in series with Nb3Sn and NbTi coils. The magnet was cooled with pressurized superfluid helium (He II). The steady-state operating temperature was 1.72 K. The magnet was power-supply driven, i.e., the transport current was always supplied by a power supply. A small normal transition and a power outage occurred during the 8-month operation; however, the magnet quench was avoided.

Steady-state modeling and analysis of a loop heat pipe under gravity-assisted operation

Loop heat pipes (LHPs) are efficient two-phase heat transfer devices that have found many space and terrestrial applications. This work addresses our insufficient understanding of LHP operation under gravity-assisted attitude, i.e. the condenser is located higher than the evaporator. A steady-state mathematical model of a LHP under gravity-assisted operation was established based on two driving modes: gravity driven mode and capillarity-gravity co-driven mode, determined by a defined transition heat load. The model was validated by the experimental results, and was employed to predict the operating characteristics of a LHP under the gravity-assisted attitude. Comparing to LHPs operating under horizontal or antigravity attitudes, some distinctive features have been identified, which include: i) the total mass flowrate in the loop shows a unique V-shape with the increase of applied heat load; ii) the steady-state operating temperature is much lower under the gravity driven mode, and is in similar values under capillarity-gravity co-driven mode and iii) the thermal conductance of the LHP increases with increasing positive elevation especially in the variable conductance zone. Such results contribute greatly to the understanding of the complicated operating principle and characteristics of LHPs especially for terrestrial applications.

Effect of non-condensable gas on steady-state operation of a loop thermosyphon

Non-condensable gas (NCG) generated inside two-phase heat transfer devices can adversely affect the thermal performance and limit the lifetime of such devices. In this work, extensive experimental investigation of the effect of NCG on the steady-state operation of an ammonia-stainless steel loop thermosyphon was conducted. In the experiments, nitrogen was injected into the loop thermosyphon as NCG, and the thermal performance of the loop thermosyphon was tested at different NCG inventories, heat loads applied to the evaporator and condenser cooling conditions, i.e. natural air cooling or circulating ethanol cooling. Experimental results reveal that NCG elevates the steady-state operating temperature of the evaporator, especially when the loop thermosyphon is operating in the low temperature range; meanwhile, the more NCG exists in the loop thermosyphon, the higher the operating temperature of the evaporator, and the lower the reservoir temperature. In addition, the existence of NCG results in the decrease of the overall thermal conductance of the loop thermosyphon, and the overall thermal conductance under the ethanol cooling condition may be even lower than that under the air cooling condition when the heat load is smaller than a certain value. Finally, the experimental results are theoretically analysed and explained.

Thermal Management of a 15 kV/100 kVA Intelligent Universal Transformer

The thermal management of power electronics presents a significant challenge to thermal engineers due to high power loads coupled with small footprints. Inadequate thermal dissipation of these loads can lead to excessively high equipment temperatures and subsequent system failure. In this study, a unique power electronics-based transformer, called the intelligent universal transformer (IUT), is thermally analyzed using the computational fluid dynamics software ICEPAK. The objective of this work is to examine the use of a finned heat pipe array for the power electronics in the IUT. A design sensitivity study was performed to determine the effect of the number of fins attached to the heat pipe array, the number of heat pipes in the heat pipe array, and the fin material on the steady-state operating temperature of the power electronics. It was determined that a set of 33 copper fins attached to an array of 36 heat pipes on each side of the containment unit is sufficient for continuous operation of the power electronics. This analysis and thermal management solution will be applicable not only to this situation but also to other high density power electronics applications.

Effect of Creep of Ferritic Interconnect on Long‐Term Performance of Solid Oxide Fuel Cell Stacks

AbstractHigh‐temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and coefficient of thermal expansion (CTE) compatibility with other components for most of the solid oxide fuel cells (SOFCs). However, creep deformation becomes relevant for a material when the operating temperature exceeds or even is less than half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the SOFCs under development are around 1,073 K. With around 1,800 K of the melting temperature for most stainless steel (SS), possible creep deformation of ferritic IC under the typical cell operating temperature should not be neglected. In this paper, the effects of IC creep behaviour on stack geometry change and the stress redistribution of different cell components are predicted and summarised. The goal of the study is to investigate the performance of the fuel cell stack by obtaining the changes in fuel‐ and air‐channel geometry due to creep of the ferritic SS IC, therefore indicating possible changes in SOFC performance under long‐term operations. The ferritic IC creep model was incorporated into software SOFC‐MP and Mentat‐FC, and finite element analyses (FEAs) were performed to quantify the deformed configuration of the SOFC stack under the long‐term steady‐state operating temperature. It was found that the creep behaviour of the ferritic SS IC contributes to narrowing of both the fuel‐ and the air‐flow channels. In addition, stress re‐distribution of the cell components suggests the need for a compliant sealing material that also relaxes at operating temperature.

Operational characteristics of the miniature loop heat pipe with non-condensable gases

The present paper experimentally investigates the effect of non-condensable gases (NCGs) on the thermal performance of the miniature loop heat pipe (mLHP). Copper mLHP with the flat disk shaped evaporator, 30 mm diameter and 10 mm thick, and fin-and-tube type condenser, 50 mm length and 10 mm height, located at a distance of 150 mm was used in the study. The device which was designed for the thermal control of computer microprocessor was capable of transferring maximum heat load of 70 W while maintaining evaporator temperature below 100 °C limit for electronic equipments. Water was used as the heat transfer fluid inside the mLHP. All the tests were conducted with the evaporator and condenser at the same horizontal level. Simple methods were devised to detect and purge the generated NCG out of the loop heat pipe without disassembling the system. Experiments conducted to classify the trends in the NCG production and storage revealed that majority of the gas is generated in the first few thermal runs and is accumulated in the compensation chamber. Sensitivity tests show that overall effect of the NCG is to elevate the steady-state operating temperature of the loop and increase the start-up time required by the evaporator to achieve stable conditions for the given heat load. As an outcomes of the research work, it can be concluded that mLHPs are more tolerable to the NCGs than conventional heat pipes due to the presence of compensation chamber that can accumulate most of the released gas without major performance degradation.

Operating Temperature Window for SiC Ceramics and Composites for Fusion Energy Applications

Limitations in operating conditions, primarily the steady-state operating temperature, of silicon carbide-based ceramics and composites for applications to structural and functional components in fusion blanket systems were critically examined based on the latest experimental results. Irradiation-induced high temperature swelling and irradiation creep were identified to be the likely factors limiting the upper temperature bound for structural applications, whereas irradiation-induced thermal conductivity degradation was identified to be the primary factor to limit the lower temperature bound when substantial heat flux is anticipated. For the application to flow channel inserts in liquid metal blankets, insulating properties will likely limit the upper temperature bound, whereas the lower temperature bound may be limited by swelling-induced secondary stress. Additionally, key scientific issues which need to be addressed for the better definition of design limitations were identified.

Thermal simulations of three-dimensional integrated multichip module with GaN power amplifier and Si modulator

A finite-element simulation was used to quantitatively estimate the heat transfer in a three-dimensional multichip module (MCM) consisting of a GaN power amplifier with solder-bump-bonded Si modulator and integrated antenna on a high-resistivity SiC substrate under various conditions of power density and substrate and epi thicknesses via wire thickness and effective heat transfer coefficient. The maximum temperature in the integrated-antenna approach occurred in the center of the MCM. At a GaN power amplifier power level of 3W∕mm, a steady-state temperature of ∼125°C was reached in ∼20s. Bulk GaN substrates were also found to provide good thermal transfer characteristics, while sapphire produced an increase in temperature almost a factor of 3 higher than for SiC. At a power density of 10W∕mm, the steady-state operating temperature was ∼400°C even with SiC substrates.

Magnetoresistive stripe temperature calculation

A method of calculating the steady-state operating temperature of a shielded magnetoresistive element in three dimensions is described. The method of images is used to account for any number of layers of different thermal conductivities, which may include, for example, gaps, shields, undercoat, overcoat, and substrate. The temperature may be calculated at any point, allowing the average temperature of the stripe to be calculated. The effect of the thermal coefficient of resistance is included. Results of calculations using the model compare favorably to measurements.

Installation effects on fan acoustic and aerodynamic performance

Sound power measurements were performed on an upright vacuum cleaner in order to identify predominant noise generating mechanisms and to recommend corrective noise control modifications that would cause no increase in production costs. These modifications were then implemented and their effectiveness quantified. In general, the noise radiated by the vacuum cleaner is dominated by aerodynamic sources. Aerodynamic interactions occur between the electric motor cooling fan and nearby motor mounting fixtures resulting in intense tones at blade rate and harmonic frequencies. These tones are reduced by as much as 25 dB in level, to the broadband sound power level of the unit, by eliminating certain flow obstructions, and mounting the motor in an alternative fashion. Motor cooling performance is also dramatically improved by the new mounting arrangement resulting in a 26 °C reduction in steady-state operating temperature. © 1999 Institute of Noise Control Engineering. †S0736-2501„99...00101-0‡

Stack-Up Forces for Normal and Abnormal Operating Conditions in a Railroad Roller Bearing Assembly

Stack-up force distributions are obtained for normal and abnormal operating conditions in a class F (6 1/2 × 12) railroad tapered roller bearing-axle assembly for various friction coefficients, press-fits and cap screw preloads. Three conditions that affect the stack-up force distribution are considered: a steady-state normal-operating temperature distribution, a steady-state temperature distribution resulting from a stuck brake or a jammed outboard roller, and a transient temperature distribution due to multiple jammed outboard rollers. The steady-state normal or abnormal operating temperature have insignificant effects for nominal values of interference, friction and cap screw preload. However, the effect of the transient temperature due to multiple jammed outboard rollers on the stack-up system is detrimental due to localized high temperature gradients near the outboard cone area and a distortion of the original surface geometry of the cone.

A Study of Macrosegregation in Low-Density Polyethylene Autoclave Reactors by Computational Fluid Dynamic Modeling

Flow and reaction in a typical commercial scale autoclave LDPE reactor were modeled by a three-dimensional computational fluid dynamic (CFD) k-e model in order to shed light on the macrosegregation effects that can occur in these reactors. It is shown that the CFD model predicts significant differences from CSTR behavior. Results are discussed in terms of the effects of macro- and microscale inhomogeneities of concentration and temperature on free radical polymerization kinetics. The observed nonidealities in terms of minima in the initiator consumption curves and multiple steady states are explained on the basis of competing turbulent transport and chemical kinetics. Microsegregation effects are shown to be negligible in comparison to macrosegregation effects. Given the fact that the CFD model is based on reactive scalar and energy balances without adjusted parameters in the three-dimensional flow field of the entire reactor, it is tentatively concluded that commercial-scale LDPE vessel reactors can have significant macrosegregation effects beyond a certain steady-state adiabatic operating temperature that is specific to the initiator being used.