Thermal modelling and statistical investigation of factors that effects thermal design of led light source based luminaires

Saving measures that can be realized in lighting installations with a significant share of about 20% in the total electricity consumed are among the issues emphasized in "energy efficiency" studies. In this context, LED (light emitting diode) technology, which can produce light by consuming less energy, has high efficacy factors (lm/W) and also has advantages such as high color rendering indexes, different color options, long life, started to be used for lighting purposes. On the other hand, the efficacy factors of LEDs are constantly increasing as a result of rapid developments in semiconductor technology. However, the announced efficacy factor values apply to the instantaneous operating conditions of the LED chips in the laboratory environment. It is known that the LED's light fluxes and lifetimes are much more affected by temperature variations than conventional light sources. When the optimum operating temperatures are exceeded, the LED's catastrophic failure ratios increase, the light intensity drops and the color characteristics deteriorate. As a result, the high efficacy factors (lm/W) announced for LED light sources are greatly reduced in the form of a luminaire. There are many luminaires in the market that use LED light sources. Due to the lack of technical information from the manufacturer, poor quality of the luminaires and and the lack of standards in this regard, the lighting installations in which the LEDs are used can not be in the superior position compared to conventional light sources as claimed. In order to be used in existing indoor/outdoor lighting installations; LEDs, which are still having problems in measurements of photometric quantities, are required to fulfill minimum lighting requirements such as illuminance, luminance, homogenity and glare limitation specified in lighting standards, in the form of a luminaire. The research on LEDs which are utilized in different areas continues for a long time. Most of the published work is about the LED's internal characteristics, production techniques and the use of different materials as semiconductors. As the LEDs gradually penetrate into lighting sector, the research focus shifted from prodction techniques to increasing performance, improving lighting characteristics, measuring thermal performance etc. Like in all lighting luminaires, optical analysis is the most important part of design for ensuring lighting quality criteria in LED luminaires. However, since the light source in LED luminaires is highly dependent on the temperature, thermal analyzes are also crucial. The thermal analyzes are carried out in two stages, the printed circuit board (PCB) on which the LED chip is mounted and the cooling elements and luminaire surroundings. In order to transfer heat in the luminaire through the cooling elements by the heat dissipated by the LED and to minimize the thermal resistance in the heat transfer inside the body, the cooling elements must be designed thoroughly. Especially for luminaires where high power LEDs are used, the efficiency is directly related to the thermal design. Depending on the type and power of the luminaire, thermal performance may greatly affect the overall luminaire performance. It is more difficult to transfer dissipated heat from the LED light sources for the luminaires with high Ingres Protection Class (IP) since the luminaires are water and dust proof whereas easier for the luminaires with low IP class. The first step in designing an LED luminaire should be a good analysis of the LED chip to be used. The optical-thermal-electrical characterizations of the LED chips must be done before the design phase and the design targets should be determined accordingly. The amount of light emitted by the LED chip depending on the temperature, the efficiency, the color change, the measurement and calculation of the optical, thermal powers and electrical power must be determined with the help of a temperature controlled measurement system. In particular, thermal power values are required for the LED chip to be thermally simulated in a computer environment. In order to determine the efficiency factor of the luminaire, it is necessary to know the electrical power and luminous flux of the LED chip for the working temperature. Usually the optical-thermal-electrical properties of the LED light sources are not given in the data sheets. Especially the luminous efficiency value is the essential part of the thermal simuation since the thermal power of the LED is calculated from this value. After characterizing the LED chip, the thermophysical properties of materials used for luminaire design must be known or measured in a laboratory environment. The accurate thermophysical properties of elements such as LED chip, body (body material), printed circuit board card (PCB) and thermal interface materials that make up an LED base luminaire will yield to get more accurate results in the transition from simulation results to application phase. The simulation software usually contains a library which the accurate thermophysical materials can be found. It is important to match the actual materials used to library materials accurately to reduce error. As a result, it is necessary to make optical analyzes according to the type of luminaire and optimize the required cooling solutions so that the luminaires with LED light sources can work with optimum performance to meet the necessary criteria for illumination. In the thesis, firstly the change of the properties of the LED light sources is investigated in the laboratory using a temperature controlled Ulbricht Sphere. For different temperatures the optical-mechanical-electrical characterization of the LEDs are shown. Then the luminaire design process is eloborated in detail and the computer aided simulation process is explained by verifying the thermal models required for the LED chips to be used in the thermal simulations. Two sample luminaires are chosen and then the compact thermal model used to characterize LED thermal models are validated using simulation results and laboratory measurements. Also photometric measurements are done and compared to predicted values using simulation results and catalog values. In order to investigate the effects of mechanical components, a statistical analysis is performed by designing a full factorial experiment on a sample road lighting luminaire prototype to understand the effects of factors such as body material, printed circuit board materials and physical properties, thermal interface elements, LED thermal power and ambient temperature and interactions between factors on a LED luminaire thermal design. A statistical software package is used to compare and calculate factor effects and interactions. After the preliminary analysis, simulations are carried out with many factor levels. Because of the increase in the number of simulations made during the research, as a second step, a multi variable linear regression model is applied to the simulation results. Factor effects and the interactions of these factors were statistically calculated by ANOVA (Analysis of Variance). Then using the multivariable regression model and the catalog values of the LEDs used in the luminaire, the photometric performance of the luminaire for different factor levels is investigated for different LED thermal power levels. Using a statistical regression model, the effects of the factors affecting the LED luminaire thermal design are explained and thermal simulations and statistical analyzes are performed on a sample LED luminaire, aiming to provide guidance about the LED luminaire thermal design process, especially to LED luminaire manufacturers and designers.