Experimental investigation of direct contact baseplate cooling for electric vehicle power electronics

An experimental setup has been built to investigate the thermo-hydraulic performance of the direct contact baseplate cooling technique for power electronics in electric vehicles, to improve the design and to validate the modelling of this technique. The setup consists of an electrical heater to emulate the heat dissipation of the power electronics and which is cooled by a 60/40% mixture by mass of water-glycol. It is equipped with a flow rate sensor, absolute and differential pressure sensors and temperature measurements at the inlet, outlet and baseplate over the channel length, to determine the performance parameters used in the comparison: thermal resistance and pumping power. Three fluid inlet temperatures, four power levels and four flow rates have been tested for three channel heights (1.5mm, 3mm and 7.6mm). Increasing the fluid temperature and/or heating power, results in a lower thermal resistance and pumping power, due to a lower viscosity of the fluid. The performance of the 1.5mm and 7.6mm channel was found to be quite similar, while the 3mm channel results on average in a 5.8% lower thermal resistance compared to the other two channel heights. The heat transfer in terms of the Nusselt number was also evaluated in function of the Reynolds number. By analyzing the hydraulic and thermal entrance lengths it could be concluded that the flow in all measurements is simultaneously developing. A comparison with two correlations from scientific literature for simultaneously developing flow did not show a good agreement, possibly due to the specific inlet and outlet effect, which is more pronounced for a bigger channel height than a smaller channel height

Quality assessment of a 2D FE based lumped parameter electric motor thermal model using 3D FE models

This paper presents an advanced thermal lumped parameter model (LPM) for a switched reluctance motor for which the quality of the results is verified with a 3D finite element (FE) simulation. An advanced lumped parameter model is proposed, which extracts a 2D LPM from a 2D FE simulation in FEMM and extends this 2D LPM into 3D based on the regular LP techniques. The assumptions and simplifications in de 3D LPM are verified with a 3D FE model by comparing the simulated average and maximum component temperatures of the models. The comparison shows a maximum deviation of 7.1% on the average temperature and 10.9% on the maximum temperature. It is concluded that the proposed advanced 3D LPM is an efficient and sufficiently accurate method against 3D FEM

Modelling and validation of a switched reluctance motor stator tooth with direct coil cooling

This paper presents the modelling and validation of an advanced thermal lumped parameter (LP) model for a stator tooth of a switched reluctance motor (SRM) with a dry lateral slot cooling method. Standard and simple lumped parameter models for electric motors can insufficiently predict the temperature distribution within the components of the motor. In standard LP models, only several nodes are used to model each component, while more accurate models are needed to predict the effect of different cooling methods on the thermal performance of the motor without the need for experiments. A fully 3D thermal finite element (FE) model could be used but this would increase effort, complexity and computing time unnecessarily. Therefore, an advanced 3D LP model including the dry lateral slot cooling method was developed and validated based on experiments on a real stator tooth cooled with the modelled cooling method. The 3D LP model is extracted from a 2D FE radial simulation of the stator tooth and extended axially in 3D to include axial heat transfer. Experiments were performed with a setup consisting of one tooth of a SRM without rotor, but including stator iron, one winding and two triangular stainless steel tubes in the slots at both sides of the winding cooled by a 60/40% mixture by mass of water-glycol. The setup is equipped with several thermocouples integrated within the components to determine the component temperatures. Three inlet temperatures (20, 35 and 50°C) and four flow rates (2, 6, 9 and 13 l/min) of the coolant were tested at three different heat losses in the winding (10, 30 and 50 W). A comparison between the simulated and measured temperatures showed generally higher temperatures in the experiment. The presence of imperfections in the manufacturing of the experimental setup was determined as the cause of this offset. These imperfections result in lower material thermal conductivities and higher contact resistances than expected from scientific literature. After fitting those thermal properties on the measurements, similar simulated temperatures could be obtained as in the experiments

1D simulations of thermally buffered prismatic batteries through the application of PCMs

Thermal management of Li-ion batteries is critical for its performance and lifetime. Furthermore, when batteries are submitted to excessive temperatures by a bad thermal management system, thermal runaway can occur which can destroy the afflicted cell and the adjacent cells in a battery pack. Batteries are subject to cyclic behavior, charging and discharging, which is accompanied by a non-steady-state heat dissipation. Through thermal buffering, heat can be stored temporarily, which allows the heat transfer to the environment to be more evenly and thus reducing the maximal cooling load. Phase change materials or PCMs for thermal buffering are studied in this paper. By melting and solidifying, these substances take up and release a large amount of heat in a small volume and mass. To be able to design a thermal buffering system with PCMs, a one-dimensional transient model is developed to identify which influence design parameters have on the battery temperature. Simulations are performed for pure PCMs and for PCMs enhanced with three types of thermally conducting structures: metal foam, expanded graphite and carbon fibers. The results show that the effectiveness of thermal buffering is highly dependent on the cycle duration. For long cycles in the order of one day or more, thermal buffering can reduce peak temperature by around 4°C. For medium duration cycles in the order of several hours, peak temperatures can be reduced by around 13°C. For shorter cycles, heat buffering in the simulated cases was only slightly beneficial for the battery temperature. Furthermore, the simulations show that thermal buffering for battery packs requires a relatively small amount of PCM which results in short heat paths through the PCM. Enhancing the thermal conductivity by using thermally conductive structures slightly improves the thermal buffering performance, but might not be advisable due to the added complexity and cost

Experimental study of a switched reluctance motor stator tooth with slot and end winding cooling

This paper presents an experimental study of direct coil cooling applied to a stator tooth of a switched reluctance motor where a direct contact is realized between the winding and fluid. Experiments were performed with a setup consisting of one tooth of a SRM without rotor, but including stator iron and one preformed winding. Three configurations of the cooling method were investigated: slot cooling, end winding cooling and a combination of the two by pumping an Automatic Transmission Fluid (ATF) over the designated sides of the winding. The setup is equipped with 17 thermocouples integrated within the components to determine the temperatures. Three inlet temperatures (21, 33 and 44°C) and four flow rates (1.5, 2, 3.5 and 5 l/min) of the coolant were tested at four different heat losses in the winding (10, 30, 50 and 70W). The results show that the maximum temperature is always located in the centre of the winding and is the lowest for the combined cooling (73.0°C), followed by the slot cooling (79.7°C) and then by the end winding cooling (91.6°C) for the lowest ATF inlet temperature and the highest heat losses and flow rate. With a determined current density in the range 13.8A/mm² to 19.5 A/mm², all three direct coil cooling methods show a great potential in increasing the power density of electric motors

Combined conduction and natural convection cooling of offshore power cables in porous sea soil

The power that can be carried by offshore power cables is often restricted by the temperature limit of the materials inside the cable. It is therefore essential to predict the heat transfer behavior of the dissipated power from the cable to the environment. Offshore cables are buried in the seabed, which is a porous structure of sea soil saturated with water. Both conduction of heat through the soil, as well as natural convection due to the flow of water through the porous soil, are possible ways of heat transfer. Most cases are best described as a combination of these heat transfer effects. In this paper, a numerical model is made to predict the heat transfer from the cable to the environment by modeling the surrounding soil as a porous medium. The influence of soil parameters such as conductivity, heat capacity and permeability, as well as geometrical parameters, such as burial depth and cable diameter, are tested. An analytical expression, which can estimate the heat transfer rate for conduction dominated heat flows, is used. For convection dominated heat flows, a correlation in function of the Darcy-modified Rayleigh number is used. For heat flows which are a combination of conduction and convection effects, an algebraic summation of the thermal conductance due to convection and conduction is found not to give adequate agreement with the simulations. It is shown that an asymptotic expansion of the limiting equations for conductive and convective heat transfer rate can be used to determine the total heat flow effectively. Several soil samples in the North Sea are analyzed, and the thermal properties are used as inputs for the model. These calculations show that conduction is the main heat transfer effect and that convection has a limited effect on the heat transfer

Advanced lumped parameter model for switched reluctance motors with high performance cooling

In this paper an advanced thermal lumped parameter model for a switched reluctance electric motor (SRM) is constructed, based on a 2D thermal finite element simulation of a radial cross section of the motor. When applying and combining advanced cooling methods such as direct coil cooling, end winding cooling (radial stretched) and spray cooling on an SRM, the conventional lumped parameter models can no longer be used due to the 3D and complex temperature gradients in the motor. In standard LP models, mostly one simple cooling method is implemented by which the thermal gradients are also quite simple (1D or 2D). When combining different cooling methods, the gradients become highly 3D and these LPM are no longer valid. To improve the accuracy of this problem, a fully 3D thermal finite element simulation could be performed, but this would unnecessarily increase effort, complexity and computational time. To avoid this an advanced lumped parameter model is constructed in this paper, such that the high thermal gradients are modeled in more detail. The results from one 2D finite element simulation of a radial cross section of half of a stator tooth are reduced to a simpler lumped parameter model with more nodes in the most crucial parts, i.e., where the highest thermal gradients are expected. The 2D thermal model is then expanded to a 3D lumped parameter model, including the gradients in axial direction. Using this model, various cooling configurations and geometry parameters can be varied easily such that the design of an SRM with advanced cooling can be optimized efficiently

Assessment of nucleate pool boiling heat transfer and critical heat flux for power electronics cooling with a low-GWP refrigerant

To combat climate change, greenhouse gas emissions have to decrease across all sectors. In the transport sector, several options to reduce the emission of carbon dioxide are considered, such as hydrogen and bio-fuel powered engines. For vehicles, the most adopted strategy is to convert to battery electric vehicles, which can be charged using renewable electrical energy. Currently, electric vehicles have several drawbacks compared to conventional internal combustion engine vehicles, such as lower driving range and higher cost. These drawbacks can be combated by making the electric drivetrain (motor, power electronics and battery) more compact and therefore more power-dense. Further increasing the power-density requires more effective cooling systems. The power modules in the power electronics of the drivetrain feature the highest heat fluxes and are therefore one of the most challenging components to cool. State-of-the-art thermal management systems of electric drivetrains utilize liquid cooling with water-glycol mixtures. Two-phase cooling, with a coolant that starts to boil as it comes into contact with heat generating components, has the potential of acquiring higher heat transfer rates and thus more effective cooling. Two-phase cooling does come with a major risk. If the heat flux from a drivetrain component to the coolant increases above a maximal value, the critical heat flux, a continuous vapour film will be formed on that surface. This layer acts as a thermal insulator and thereby reduces the heat transfer rates dramatically, which results in overheating and failure of the component. In this study, the fluid chosen for the two-phase cooling system is FK-649, a fluoroketone. It has a low global warming potential equal to 1, which means that sporadic leaks of the fluid will not contribute significantly to global warming. Boiling heat transfer is a complex phenomenon with a strong interaction of heat and mass transfer. Although research on boiling heat transfer has been conducted for almost a century, there is still no consensus on which heat transfer mechanisms are dominant. This lack of fundamental understanding renders it difficult to predict heat transfer rates. Several heat transfer correlations have been proposed by different authors, taking into account to various degrees the effects of heat flux, fluid properties and boiling surface properties. Several correlations have also been proposed for the critical heat flux and these show a larger agreement with each other than the heat transfer correlations. Limited data is available for boiling heat transfer of FK-649. It is also unclear which correlations are best suited to predict heat transfer rates and critical heat flux for FK-649. To determine the heat transfer rates associated with pool boiling of FK-649, an experimental setup is designed and constructed. The setup consists of a sealed reservoir containing FK-649. Boiling occurs on a horizontal boiling surface at the bottom of the reservoir, while the generated vapour is condensed by a spiral condenser at the top. Polycarbonate windows are added for visual access to the boiling phenomenon. The type of boiling surface can be altered and the saturation temperature of FK-649 can be controlled to the desired value. The saturation temperature during the experiments was varied in three levels: 36 °C, 41 °C and 46 °C. In the power module under test, IGBTs are the highest heat dissipating components. These components are placed on a baseplate which has a larger area than those of the components. This allows to spread the heat generated in the components with the goal of having a larger heat transfer area. As the heat is not generated uniformly over the entire internal baseplate area, this may result in a non-uniform heat flux at the boiling surface. To analyse this effect, measurements of pool boiling on the power module are compared to measurements with a boiling surface that is uniformly heated by an electrical resistive heater. The measured heat transfer rates matched within 10%. Also visually, no evidence of non-uniform heat fluxes were perceived. Bubble nucleation occurred uniformly across the boiling surface and was not correlated with the location of the IGBTs. It is concluded that the baseplate effectively spreads the heat and that a quasi-uniform heat flux is achieved at the boiling surface. These results indicate that the correlations developed for surfaces with uniform heat fluxes can be directly applied to power module baseplate cooling. Thirteen correlations that predict nucleate pool boiling heat transfer are assessed. In order of the date of publication, these are the correlations of Kruzhilin, Rohsenow, Forster-Zuber, Kutateladze-Borishanskii, Borishanskii-Mostinksi, Shekriladze-Ratiani, Labuntsov, Gorenflo, Stephan-Abdelsalam, Cooper, Kutateladze, Leiner and Pioro. Of these correlations, the Rohsenow and Pioro correlations require fitting one or more constants to experimental data for each specific surface-fluid combination. The predicted thermal performance is compared to the experimental results. As expected, the correlations with fitting constant perform best. The Rohsenow correlation is the preferred correlation, it has an average deviation of 7.5% and predicts all measurement points within 25.6%. Of the non-fitted correlations, the Labuntsov correlation is best at predicting heat transfer rates with FK-649. The average deviation of the prediction from the measurements is equal to 7.5% and all data is predicted within 29.1%. Remarkably, the average error of the Labuntsov correlation is equal to that of the fitted Rohsenow correlation. Both correlations take into account accurately the effect of saturation temperature on heat transfer. The Labuntsov correlation is thus preferred for predicting heat transfer rates without available measurement data for fitting. Caution should however be exercised if the correlation is used for non-metallic surfaces or surfaces with a non-standard surface roughness, as the Labuntsov correlation does not take into account influences of the boiling surface. The experimental results showed that the effect of the heat flux on the nucleate boiling heat transfer rates can be divided into three separate regimes. These regimes were also perceived visually. From low to high heat flux, the three regimes are the partial nucleate boiling, fully developed nucleate boiling and partial dryout boiling regimes. These regimes are also reported by several authors in literature. However, none of the heat transfer correlations take into account the different nucleate boiling regimes and they therefore do not predict adequately the variation of the heat transfer rates with heat flux. The measurements in this study were correlated using a specific power-law relation for each of the three regimes. For more accurate predictions, future heat transfer correlations should take into account the differences between the three regimes. Four critical heat flux correlations are compared to the experimental data: those of Kutateladze, Zuber, Lienhard-Dhir and Mudawar et al. Both the Zuber as the Lienhard-Dhir correlations predict all data within 10%. It is concluded that the critical heat flux correlations perform much better than the heat transfer correlations. To assess the feasibility of two-phase power electronics cooling with FK-649, the measured thermal performance is compared to that of water-glycol cooling. The maximal heat transfer coefficient measured in this study was 4965 W/m²K and the highest critical heat flux was 146 kW/m². The heat transfer coefficients are similar to those of water-glycol cooling of a flat baseplate. However, the maximal attainable heat fluxes with water-glycol are more than two times higher than the critical heat flux of FK-649 boiling. For two-phase cooling of power modules with FK-649 to outperform water-glycol cooling, strategies for increasing the critical heat flux such as increasing the saturation temperature, using subcooled boiling or using flow boiling should be investigated

Thermal analysis of absorption heat pump implementation in an industrial dryer

Industrial dryers are very energy-intensive, contributing to a large share of the thermal energy demand in industry. Most of this energy is discharged as moist air to the environment. The calorific content of the exhaust air is high due to the large amount of water vapour in this waste stream. Heat pump systems can be used to recover heat from the exhaust air, hereby raising the energy efficiency of the dryer. Simulations have been performed using data from an existing drying installation with conventional heating. The increase in energy efficiency is analyzed for the implementation of several types of absorption heat pumps in the drying cycles. The simulation results show the influence of different working fluid pairs and different configurations: type I, type II and double lift cycle. The highest amount of energy savings is achieved with type I absorption heat pumps using water–lithium bromide as the working fluid pair. With optimized temperature levels in the different components, the thermal energy use of the complete dryer can be reduced with 20%. The performance of absorption heat pumps in drying systems is however still bounded by the temperature limit of the waterlithium bromide working fluid pair. Searching for alternative working fluid pairs for higher temperature applications is therefore still essential and can increase energy savings even more.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016