This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

For reducing the energy consumption in cities, LED street light systems are frequently used. A long life span of these systems is closely related to the maximum temperature present in the system. The so-called junction temperature, which is the highest operating temperature of the LEDs, influences the life span dramatically. An increase of the temperature above the junction temperature leads to a rapid decrease of life span. Additional cooling systems like fans increase the energy consumption of the whole system and are not the method of choice. Therefore, new approaches are required.



In this work, a new cooling system based on natural convection will be presented. The heat generated from the LEDs lead to temperature gradients in the lamp which induce a volume flow due to natural convection. The design of the cooling channels should allow a maximum of heat transfer, i.e. large areas of heat exchange, as well as a low pressure drop leading to higher volume flows in the system. This produces a contradicting effect of small channels with large areas but high pressure drops and small volume flows. As the system is strongly dependent on these geometrical features a numerical simulation model was developed in order to identify the relevant parameters and optimize them. The model is based on a conjugate-heat transfer model which was solved using the Open Source CFD-code OpenFOAM®. The heat sources of the LEDs are modelled as source terms in the energy equation. To reduce the computational effort several model reductions like symmetric conditions are applied. The simulation results show that the main source of improving the temperature level is the shape of the channel and the design of the outlet to the surrounding. Experiments with different prototypes show the same tendencies observed in the model. By applying this new approach with cooling channels, the maximum temperature could be reduced in the experiment by approx. 12 K compared to the system with blocked cooling channels. The results of the simulation model are in good agreement with experimental data (difference between 2-6 K).