Numerical Thermal Analysis of a Module Level Power Electronics System

Transcription

1 Numerical Thermal Analysis of a Module Level Power Electronics System M. Prakash SunEdison Research Pvt. Ltd PrimRose Road Bangalore Selvam R SunEdison Research Pvt. Ltd PrimRose Road Bangalore Abstract Kavita C SunEdison Research Pvt. Ltd PrimRose Road Bangalore Solar Photovoltaics (PV) is now widely considered a reliable alternative to fossil fuel based power sources. PV systems are categorized into (a) power generation components, including the PV panels and (b) power conversion components that include inverters and related power electronic devices. The electrical and mechanical design of such systems has become critical to the performance of high efficiency PV systems. Thermal analysis, numerical and experimental, has now become a significant component of the mechanical design. The present case study describes the numerical thermal analysis of one such power conversion device. A module level power electronics system (MLPE), developed in-house, was analyzed in the study. The basic requirements of the system include good efficiency, low cost and reduced system size. In order to reduce the effective size of the system and maintain good performance, a critical thermal analysis was required. The system was analyzed using Acusolve CFD software. An initial benchmark model was created and it was compared with the results from IR camera images. A good correlation (maximum variation in temperature of about 10%) was observed and then numerical thermal analyses were carried out for different variants of the system, such as ambient temperatures, inclination of the device, etc. The system design was improved using the inputs from the thermal analyses performed in the study. Introduction The design of a MLPE (module level power electronics) system is a critical process as the unit needs to operate outdoors under various environmental conditions. The design process involves steps such as initial design, proof of concept, prototyping, testing and analysis. The thermal design and analysis of such a system is as vital as the electrical design in order to ensure that the product is efficient and reliable. The main requirement for the MLPE system from the thermal point of view was to ensure the smooth operation of the product when it is subjected to harsh operating temperatures. The rapid dissipation of heat generated by the components within the system to the external ambient was the key element of the thermal design. This ensures that the components operate within their operating temperature ranges and hence guarantee reliable performance. The thermal design was initiated during the electrical design phase of the product. A specialized potting material was selected to be used for the MLPE system. The selected material had suitable properties for the application especially thermal conductivity and durometer values. This ensured that the heat dissipation from the components to the chassis was rapid and the components were not affected by excessive stress values. The thermal analysis included the prediction of the temperatures within the system, plotting of the thermal profiles and detection of the high temperature zones within the system that could damage the components. Only steady state analyses were performed in this current study. 1

2 Modeling Procedure The system geometry was created using the SolidWorks 3D CAD. The system was surrounded by an enclosure that would simulate the ambient conditions. The air enclosure was3 times the length and width of the MLPE system and height about 10 times the heightof the system as shown in Fig. 1. The air enclosure size determined in such a way that the enclosure walls do not affect the heat transfer occurring at the system. The heat transfer from the system chassis to the ambient is mainly due to natural convection and hence in order to account for buoyancy effect the height of the enclosure was 10 times that of the MLPE system.the upper part of the air enclosure had inlet and outlet vents so as to allow air of 1ft/s to flow through in order to simulate the experimental conditions. The problem involved all the forms of heat transfer i.e. conduction, convection and radiation. The PCB had a large number of components but for this analysis only the heat dissipating components (dissipating more than 0.1W) were considered. The material properties for eachelectronic component, chassis, ambient air and potting compound were given as inputs. The Boussinesq approximation was set for air to account for the natural convection heat transfer. The PCB board was a FR4 board with copper traces; hence the thermal conductivity was designated as anisotropic conductivity. The anisotropic conductivity values were calculated based on equations suggested by Azar and Graebner [1]. The heat dissipation values for each component were known from the electrical design and hence were provided in the body force inputs as volumetric heat flux values. The ambient air was subjected to the gravity body force in the negative y direction as shown in Fig. 1. MLPE system Ambient enclosure 2

3 (a) Geometry showing the MLPE system within the ambient enclosure (b) Model of the MLPE system with the heat dissipating components Fig.1. The simulation model of the MLPE An initial bench mark study was carried out so as to compare the temperature values from the numerical simulation with that obtained from I-R camera images during experiments. During the experiment, the PCB was kept in the open ambient without the chassis and the potting compound. The system was powered at the maximum power level and made to operate for about 3 hours in order to achieve steady state. The thermal images of the top and the bottom part of the PCB were captured using the FLIR thermal camera. A similar geometry of the PCB without the chassis and the potting compound was created using SolidWorks and the model was simulated using AcuSolve. The ambient temperature for the simulation was maintained similar to that of the experiment. The temperature contours at the top and bottom part of the PCB was obtained from the AcuFieldView once the steady state was achieved. The temperature contours from the experiment and the simulation are compared as shown in Figs The temperature trends observed from the experiment and the simulations are similar and it was observed that the maximum deviation between the temperature values is about 10%. The procedure for performing the numerical analysis was validated using the bench mark study. The same procedure was followed for analyzing the actual MLPE system. The geometric model of the entire MLPE system with the potting compound was developed in SolidWorks and exported to AcuSolve. The volumetric heat source values were provided as boundary conditions for the components along with the temperature of ambient air that is external to the MLPE unit. The potting compound was defined as a solid with the values of density, thermal conductivity and specific heat capacity given as material properties. The meshing was non-uniform with the heat dissipating components having very fine mesh while regions external to the MLPE (ambient enclosure) having a relatively less fine mesh. The simulations were carried out for different ambient air temperatures so as to study the thermal profiles and possible hot spot regions within the MLPE at elevated temperatures. These would serve as inputs to modify existing design as well as developing new designs. 3

4 (a)thermal Image from I-R camera 70 C 65 C 60 C 55 C 50 C 45 C 40 C 35 C 30 C 25 C (b) Temperature contours from AcuSolve simulation Fig. 2. The temperature profiles at the top side of the PCB Results The simulations were carried out for different ambient temperatures and the thermal profiles were obtained. Fig 4 and 5 shows the thermal images of the MLPE system at ambient temperatures of 20 C and 90 C (minimum and maximum temperature limits). It was observed that both the top and the bottom part of the PCB had hot spots wherein the components would have failed if they are operated at elevated temperatures. The thermal patterns for differentambient temperatures were observed to be similar. The hot spots were observed for the same components. The affected components were evaluated on the basis of their maximum operating temperatures. Some components were replaced with parts that had higher operating temperatures in order to improve the reliability of the system. 4

5 a)thermal Image from I-R camera 70 C 65 C 60 C 55 C 50 C 45 C 40 C 35 C 30 C 25 C (b) Temperature contours from AcuSolve simulation Fig. 3. The temperature profiles at the bottom side of the PCB Conclusions The thermal analyses provided inputs for an improved design of the MLPE. The design of the MLPE was modified in order to increase the performance and reliability of the MLPE system. The improvements in the design included modifying the placement of some of the components while some components were replaced by those that could withstand higher temperatures for sustained period of time. Additional modifications were carried out on some of the accessories so as to accommodate the electronic component changes. 5

6 Top side of PCB Bottom side of PCB Fig. 4. Temperature contours from AcuSolve simulation at 20 C ambient Top side of PCB Bottom side of PCB Fig. 5. Temperature contours from AcuSolve simulation at 90 C ambient Reference [1] Azar K. and Graebner J.E., Experimental determination of thermal conductivity of printed wiring boards, Twelfth IEEE SEMI- THERM TM Symposium, 1996,