External Thermal Resistance Research Articles

A novel characteristic curve for thermoelectric cooler application in realistic thermal circumstances: Theory and experiment

On the materials level, the performance of thermoelectric (TE) devices can be defined by the thermoelectric figure of merit (zT). This, however, does not provide a complete picture due to the complex interplay between electronic and thermal transport properties and the uncertain thermal impedance matching between external and internal thermal resistances in different realistic thermal circumstances. Appropriate design of individual material properties and geometry parameters considering the realistic thermal circumstance can greatly enhance the device-level performance of TE devices. In this work, we develop a framework to clarify such interactions in thermoelectric coolers (TEC) with a newly proposed q-h characteristic curve, which can be used to identify how a TEC design is of effectiveness for a specific realistic thermal circumstance. The effects of zT, the individual TE material properties (i.e., thermal conductivity k, electrical conductivity σ and Seebeck coefficient S) as well as the geometry parameters (i.e., pillar height and filling factor) on the q-h characteristic curve are theoretically and experimentally investigated, respectively. TEC modules with higher zT, short pillars and high filling factor show stronger capability to handle high heat flux load, especially for the cases with high heat dissipation coefficient at hot side. The thermal property (k) and electrical properties (σ and S) are responsible for the low heat flux load (e.g., wearable TEC) and high heat flux load (e.g., on-chip TEC cooling), respectively. For wearable TEC, increasing the internal thermal resistance can prevent the back flow heat, which can be achieved with tall pillars, low filling factor and optimizing zT towards lowering thermal conductivity k. For on-chip TEC cooling, reducing the Joule heating and increasing the thermoelectric effect become dominant factors in lowering the code-side temperature of TEC modules. This can be achieved with short pillars, high filling factor and optimizing zT towards increasing the power factor S2σ. This work provides significant insights into the complex interplay between material-level properties, device-level parameters and the external thermal circumstance in determining the performance of TEC devices. The results enable researchers to design optimal TEC devices for realistic applications with different boundary conditions.

Theoretical and experimental analysis of evaporative cooling with hydrophilic screen mesh on condenser surface

In this study, we theoretically and experimentally analyzed the effect of evaporative cooling on the external thermal resistance of a condenser with a water-filled hydrophilic screen mesh. To calculate the external thermal resistance of the condenser theoretically, the external thermal resistance model was developed, considering evaporative cooling in the screen mesh. The evaporative mass flow rate in the screen mesh was calculated using the Lewis number, which can convert heat transfer to mass transfer. Especially, the maximum flow rate absorbed by the screen mesh was theoretically determined by the capillary limit. In experimental approach, hydrophilic screen meshes with mesh numbers M100, M150, and M200 were manufactured and attached to an aluminum plate simulating the condenser surface. The external thermal resistance of the condenser was measured. Based on the results, the theoretical model was well matched with experimental results. The effects of the air velocity, mesh number, and heat flux on the external thermal resistance of the condenser with evaporative cooling in the water-filled hydrophilic screen mesh were investigated. Finally, it is shown that the external thermal resistance of the condenser with evaporative cooling using the water-filled hydrophilic screen mesh can be reduced up to an average of 92 %.

Prediction models for building thermal mass of intermittently heated rooms for balancing energy consumption and indoor thermal comfort

Building thermal mass (BTM) plays an essential role in maintaining a comfortable indoor environment for intermittently heated rooms (IHR). However, how to properly determine the BTM in IHR is unclear for the comprehensive consideration of the opposite effects on the reduction of heating energy consumption (HEC) as well as the improvement of indoor thermal comfort (ITC). This study aims to investigate the comprehensive impact of BTM on the HEC and ITC of IHR in the hot summer and cold winter (HSCW) zone of China using the validated EnergyPlus model. The results indicate that BTM can significantly improve the ITC of IHR, whereas increase HEC. The degree of impact is mainly related to the internal wall heat accumulation coefficient, external wall insulation layer thermal resistance, and outdoor air mean temperature during non-heating periods. Response surface methodology is further employed to quantify the effects of BTM on HEC and ITC of IHR. Based on the predictive models, the range of parameter variations for BTM that simultaneously meet HEC and ITC requirements is obtained. The predictive models can provide a theoretical basis for balancing HEC and ITC in intermittently heated buildings in the HSCW zone of China.

The Relevance of Surface Resistances on the Conductive Thermal Resistance of Lightweight Steel-Framed Walls: A Numerical Simulation Study

The accurate evaluation of the thermal performance of building envelope components (e.g., facade walls) is crucial for the reliable evaluation of their energy efficiency. There are several methods available to quantify their thermal resistance, such as analytical formulations (e.g., ISO 6946 simplified calculation method), numerical simulations (e.g., using finite element method), experimental measurements under lab-controlled conditions or in situ. Regarding measurements, when using the heat flow meter (HFM) method, very often, the measured value is based on surface conditions (e.g., temperature and heat flux), achieving in this way the so-called surface-to-surface or conductive thermal resistance (Rcond). When the building components are made of homogeneous layers, their Rcond values are constant, regardless of their internal and external surface boundary conditions. However, whenever this element is composed of inhomogeneous layers, such as in lightweight steel-framed (LSF) walls, their Rcond values are no longer constant, depending on their thermal surface resistance. In the literature, such systematic research into how these Rcond values vary is not available. In this study, the values of four LSF walls were computed, with different levels of thermal conductivity inhomogeneity, making use of four finite elements’ numerical simulation tools. Six external thermal surface resistances (Rse) were modelled, ranging from 0.00 up to 0.20 m2·K/W. The average temperature of the partition LSF walls is 15 °C, while for the facade LSF walls it is 10 °C. It was found that the accuracy values of all evaluated numerical software are very high and similar, the Rcond values being nearly constant for walls with homogeneous layers, as expected. However, the variation in the Rcond value depends on the level of inhomogeneity in the LSF wall layers, increasing up to 8%, i.e., +0.123 m2·K/W, for the evaluated Rse values.

Heat transfer enhancement with tube inserts: How can we define the best benefit?

This study recommends the use of two criteria FG-1a and FG-1b for evaluating the benefits that could be obtained when different heat transfer enhancement techniques are implemented in the heat exchanger design, instead of the most commonly used criterion PEC, based on the constraint of fixed pumping power. When the thermal performances of two heat transfer channels are compared, they must be put under equal conditions, such as fixed heat transfer area, mass flow, and initial temperature. It is also important whether an external thermal resistance of the channel is available or not. The first case is typical for experiments in shell-and-tubes heat exchangers where the objective is an increase in the heat flow, whereas the second case is encountered in experiments with electrical heating of the tube wall, where the objective is a decrease in the driving temperature difference with fixed heat flow. An additional constraint in both cases is the augmentation entropy generation number Nsa ? 1. Through manifold examples, using different twisted tape inserts, we demonstrate how these two criteria have to be implemented for assessing the thermal benefits. The use of the criterion PEC is connected with many erroneous results and misunderstood conclusions that have been revealed in this study.

Thermal analysis of thermoelectric active cooling including external thermal resistances

Thermoelectric active cooling uses nontraditional thermoelectric materials with high thermal conductivity, high thermoelectric power factor, and relatively low figure of merit (ZT) to transfer large heat flows from a hot object to a cold heat sink. However, prior studies have not considered the influence of external thermal resistances associated with the heat sinks or contacts, making it difficult to design active cooling thermal systems or compare the use of low-ZT and high-ZT materials. Here, we perform a non-dimensionalized analysis of thermoelectric active cooling under forced heat flow boundary conditions, including arbitrary external thermal resistances. We identify the optimal electrical currents to minimize the heat source temperature and find the crossover heat flows at which low-ZT active cooling leads to lower source temperatures than high-ZT and even ZT→+∞ thermoelectric refrigeration. These optimal parameters are insensitive to the thermal resistance between the heat source and thermoelectric materials, but depend strongly on the heat sink thermal resistance. Finally, we map the boundaries where active cooling yields lower source temperatures than thermoelectric refrigeration. For currently considered active cooling materials, active cooling with ZT < 0.1 is advantageous compared to ZT→+∞ refrigeration for dimensionless heat sink thermal conductances larger than 15 and dimensionless source powers between 1 and 100. Thus, our results motivate further investigation of system-level thermoelectric active cooling for applications in electronics thermal management.

General Figures of Merit ZQ for Thermoelectric Generators Under Constant Heat-In Flux Boundary.

The thermoelectric figure of merit ZT bridges the efficiency and material parameters for a thermoelectric device operating under constant temperature of the hot- and cold-source thermal boundary (Type-I TB). However, many application scenarios fall under the constant heat-in flux (qh ) and constant cold-source temperature (Tc ) thermal boundary (Type-II TB), for which a figure of merit is absent for more than half a century. This study aims to fill this gap and propose a figure of merit ZQD for the thermoelectric devices under the Type-II TB condition, defined as , where Z, h, and κ are the traditional figure of merit, leg height, and thermal conductivity, respectively. The effectiveness of ZQD is verified through both numerical calculations and experiments, which are more accurate and practical than ZT. Furthermore, a system-level figure of merit ZQS is suggested after considering the external thermal resistance. Finally, optimization strategies for thermoelectric systems based on ZQS are proposed, showing a 30% enhancement in the efficiency. ZQD and ZQS are expected to be widely used in the thermoelectric field.

Bushfire resistance of external light steel wall systems lined with fibre cement boards

Bushfires/wildfires have become a widespread and frequent occurrence in many parts of the world. With the adverse effects of climate change and the increasing population in the Bushland Urban Interface (BUI)/Wildland Urban Interface (WUI), ensuring adequate bushfire resistance of buildings in bushfire-prone areas has become very important. Improving the safety and integrity of the external building envelope is identified as a solution to minimise the bushfire related building damage and losses. This paper presents the details of bushfire resistance experiments conducted on external Light gauge Steel Framed (LSF) wall systems and the results. These LSF wall systems were externally lined with fibre cement boards and exposed to both bushfire radiant heat and flame zone fire conditions under indoor and outdoor test conditions. The results have shown that the rapid increment of the heat flux led to sudden explosive failure of the wall system whereas the gradual increment of the heat flux resulted in a progressive failure. This study has highlighted the importance of external cladding integrity and thermal shock resistance of building elements to enhance the bushfire resistance of buildings.

Soil Dryout in the Vicinity of Cables With Cyclic Load Installed in a Backfill

The IEC Standard 60287-1-1 provides the procedure for modelling of soil dryout in the vicinity of current currying power cables. Two important assumptions limit the application of the formula to the cases with uniform burial conditions and a uniform current with 100% load factor. Another standard in this series, 60287-2-1, contains the formula for the calculations of the external thermal resistance of cables laid in thermal backfills. It is based on the pioneering work of Neher and McGrath published in 1957. The Neher/McGrath paper provides also a modification of the external thermal resistance of buried power cables with a non-unity load factor with an assumption of the uniform burial conditions. This paper develops new formulae for the calculation of cable current ratings taking into account all three factors; that is, consideration of the soil dryout for cables laid in a thermal backfill and loaded with a variable current with known load factor. The results obtained with the proposed approach are compared with detailed finite element studies.

Human body heat-driven thermoelectric generators as a sustainable power supply for wearable electronic devices: Recent advances, challenges, and future perspectives

Thermoelectric generators are devices that directly convert heat flux or the temperature difference between two hot and cold surfaces into electricity. With the advancement of the Internet of Things (IoT) technology and the development of wearable and portable devices, the issue of providing a sustainable power source is one of the main challenges in the development path of these tools. Creating electric power by harvesting the waste heat from the human body is one of the effective solutions in this way. For this reason, the development and improvement of the technology of wearable thermoelectric generators have received much attention recently. Due to the low-temperature difference between the two sides of wearable thermoelectric generators and the high thermal resistance between the skin and the heated surface of these modules, the performance of these systems is highly dependent on their structural parameters and environmental factors. In this paper, it has tried to review all the previous studies regarding the impact of structural factors (such as the matching of internal and external thermal resistances, geometrical parameters of the module, design of heat source and sink, and flexibility of thermoelectric module) and environmental parameters (including the effect of ambient air temperature and humidity, skin temperature, and the interaction of power consumers with thermoelectric modules). Based on the studies, it seems that in optimizing the performance of wearable thermoelectric generators (WTEGs), it is necessary to consider the effect of the human body's thermoregulatory responses, such as skin temperature and sweating rate. The change in skin temperature directly affects the performance of WTEGs, and the change in sweating rate can also affect the thermal resistance between the skin and the hot plate and overshadow the matching of thermal resistances during operation.

Power Cycling Method for Power Modules With Low Thermal Resistance

In this article, an advanced power cycling method for power modules with low thermal resistance is proposed. Thermal resistance of power module is getting smaller and smaller, and for power cycling test (PCT), it is more and more difficult to obtain the desired maximum junction temperature <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\mathrm {vjmax}}$ </tex-math></inline-formula> and junction temperature swing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta T_{\mathrm {vj}}$ </tex-math></inline-formula> under rated current. By analyzing the influencing factors of parameters in the PCT, this article proposes a new method to affect the parameters of the power cycle by changing the external thermal resistance so that the low thermal resistance module can perform the PCT at the rated current. Then, FZ750R65KE3 modules are selected to analyze the influence of different external thermal resistances on PCT parameters. Finally, PCTs are carried out using discrete devices and power modules to analyze the influence of changing the external thermal resistance on the typical failure modes bond wire liftoff and solder layer degradation. The new method enables power modules with low thermal resistance to perform PCT at rated current without changing lifetime and failure modes.

Research on Methods of Increasing Current Carrying Capacity of 110 kV Single Core Cable

Power cables have been used more frequently in planning and designing the urban power grid. With the rapid development of power grid engineering, the power grid’s capacity is increasing, and the transmission capacity of the cable is also growing. Therefore, it is of great significance to improve the cable carrying capacity. This paper puts forward some methods to enhance the current-carrying capacity of the cable, taking a 110 kV cable as an example, including adopting a reasonable cable arrangement, adopting proper grounding mode and segment length, and reducing the external thermal resistance of the cable and adopting reasonable laying mode. The above methods are calculated and analyzed, and the influence degree of the above methods on the current carrying capacity of the cable is counted, which provides a reference and basis for the design and selection of power cables in the future. Combined with the theoretical calculation and engineering practice of the current carrying capacity of the cable, the cable diameter is reasonably selected, the engineering investment is reduced, and the urban power grid is well constructed to provide a stable power guarantee for urban construction.

A novel lumped thermal characteristic modeling strategy for the online adaptive temperature and parameter co-estimation of vehicle lithium-ion batteries

Accurate modeling of thermal characteristics is critical to the safe use and reliable management of lithium-ion batteries. However, limitations in sensors and testing methods make online real-time acquisition of internal temperatures extremely difficult. This paper uses the similarity of dynamic system modeling to construct a lumped thermal characteristic model of the battery. By analyzing the heat conduction mechanism inside the battery, the optimized heat path model is combined with the classical Bernardi equation to realize the state description of the battery thermal characteristic system. In addition, the forgetting factor recursive least squares algorithm is used to realize the online identification of the parameters of the lumped thermal characteristics model. Meanwhile, the identification of the external thermal resistance is coupled with the estimation of the internal temperature, and a novel online adaptive co-estimation strategy based on the forgetting factor recursive least squares — joint Kalman filter is proposed, which solves the problem that the external thermal resistance cannot be accurately identified adaptively in a complex environment. The experimental results show that the maximum root-mean-square error of the model under different experiments is 0.53 °C, which verifies the high-accuracy of the lumped thermal characteristics modeling strategy.

Experimental and computational thermal analysis of partial-body cryotherapy

During the partial body cryotherapy, human surface is cooled with gas for several minutes. For this, special heat and mass exchanger is used – a cryocabin. From a thermal engineering viewpoint, to improve this empirical treatment, detailed data are needed on the temperature and velocity of the cooling gas, as well as on the biological tissues cooling. For this, current study proposes an improved human, partial body cryotherapy simulation models and study of the partial body cryotherapy heat transfer problem in a single system “equipment-gas-biotissue” using experiment and computer simulation. For the first time the cryoexposure duration up to 420 s was analyzed. The data on the actual cooling gas temperature and velocity distributions, skin and internal biotissue temperature, bioheat flux for the considered typical gas supply mode is obtained. The impact of body mass index, external thermal resistance (body hair) and internal structure of a body parts on cooling dynamics is analyzed. This information is expected to be useful for understanding the effect-exposure correlation of the cryotherapy and in the design of the cryotherapy equipment.

Parametric analysis of the mechanism of creating indoor thermal environment in traditional houses in Lhasa

Traditional Tibetan houses exhibit a response to the unique local climate, reflecting a green wisdom in terms of architectural design. This paper therefore aims to investigate the mechanism of creating indoor thermal environment of traditional houses in Lhasa. Field measurements indicate passive design methods can improve the thermal environment in winter. Simulation was used to analyze the winter comfort time ratio (WCTR) and indoor average temperature on the coldest days of two typical traditional houses. Then parametric analyses were deployed to clarify and rank the effects of individual passive design methods related to architectural form design and thermal design of envelope. Based on the criteria from current regional standard, the WCTR of was 32.1% and 10.7% for the L-shaped house and the oblong-shaped respectively. For the two houses studied, the importance of individual passive design method is different. Specifically, as to the four architectural form design methods, orientation and depth-width ratio (DWR) are two most important methods for the two house types. Additionally, the semi-open space is also considered as a significant method. Regarding the five thermal design methods, the window-wall ratio (WWR) and the external wall thermal resistance are the two most important methods in the two houses. These results contribute to a deepened understanding of the thermal mechanism in traditional houses and of select passive design methods and offer implications for architecture design practice.

Fast calculation method of directly buried cables ampacity

Accurate calculation of the ampacity of underground cables is an important prerequisite for ensuring the safe and economical operation of cables, the realization of this goal mainly depends on the accuracy of the calculation of the external thermal resistance of the cable. The accuracy of numerical method is high but slow calculation speed, the analytical algorithm (IEC standard supply) is fast, but the calculation error is large when dealing with the non-uniform soil problems with poor heat conduction performance backfill soil. In this paper, the external thermal resistance of cable is calculated by finite volume method and equivalent thermal circuit method, substitutes the calculation result into the analytical calculation formula, which can quickly and accurately calculate the ampacity of the cable group. This method is faster than the pure numerical method, and which is more accurate than the analytical algorithm. The superiority of this method is proved by comparing with analytical algorithm, coordinate combination method and field-circuit combination method in uniform and non-uniform soils.

Ampacity Estimation for Submarine Power Cables Installed in Saturated Seabed—Experimental Studies

The maximum current carrying capacity (ampacity) of power cables depends on the heat transferability of its surrounding medium. Submarine power cable ampacity is calculated conventionally following the international standards defined for underground cables. In reality, the thermal behavior of submarine environments differs significantly from its underground counterpart as the porous sediments are constantly water saturated. In this article, finite element method (FEM) results were used to determine the factors affecting the ampacity in the submarine environment. A method to calculate the ampacity using external thermal resistance considering two series resistances—conductive resistance (of the solid phase sediment) and convective resistance (due to the natural convection in the pore fluid, water) is proposed. An experimental setup was developed to validate the FEM simulations and the numerical method. The results indicated that the inclusion of convective heat transfer in the proposed method obtained a more precise ampacity estimate compared to that of the standard methods.

Cables in Backfills and Duct Banks – Neher/McGrath Revisited

The Neher/McGrath paper and the IEC Standard 60287-2-1 contain the formulae for the calculation of the external thermal resistance of cables installed in backfills or duct banks. These models are based on several assumptions, which often are not valid for practical installations. This paper examines the applicability of the Neher/McGrath model and proposes a new solution for the calculation of the external thermal resistance for such installations. Numerical examples illustrate the difference between the two methods.

Experimental characterization of a novel soft polymer heat exchanger for wastewater heat recovery

Wastewater released from showers, sinks, and washers contains a considerable amount of waste heat that can be recovered by using a heat exchanger. Conventional metal heat exchangers for wastewater heat recovery have common problems of corrosion, fouling and clogging, which makes it necessary to develop a new type of heat exchanger for such low-grade thermal energy recovery applications. This study deals with a novel patented polymer heat exchanger (WO2020049233A1) made of soft polyurethane tubes that are capable of oscillation once subjected to external forces. Laboratory tests coupled with theoretical analyses show a stable global heat transfer coefficient of 100-110 W/m2·K, achieving 67-92% of the performance of titanium-, aluminum-, and copper-made heat exchangers with the same configuration. It further reveals that the performance of the soft heat exchanger can be enhanced by 30% when it is under oscillation. In addition, the external convective thermal resistance appears to be the dominant one instead of heat conduction through the wall material. The special operating condition of heat recovery from a sewer pipeline makes the polymer heat exchanger particularly adapted with its equivalent thermal performance but advantages of high flexibility, modularity, and low cost.

INFLUENCE OF THERMAL RESISTANCE OF AIR CONDENSER TUBES ON STEAM COOLING EFFICIENCY

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