Conduction cooling of large LED array systems

Large LED (light emitting diode) array lights are currently being designed and sold as replacements for lights on roadways, tunnels, parking lots and other large areas. These lights are typically 75 W to 200 W in thermal dissipation, and the lighting structure is designed to handle a predominately conductive heat path until contact with the outer air is made. At this point convection to the ambient air removes the heat from the system. To handle this internal conduction path to a suitable area for convection, most LED array lights have been developed using an aluminum heat sink, which causes a weight problem and additional costs over conventional light systems which are manufactured with sheet metal. Conventional sheet metal designs are deficient in providing an adequate thermal path for the LED thermal dissipation. A proposed solution is given in this paper with a sheet metal design enhanced for thermal conduction with flexible graphite. Eliminating the typical LED heat sink, it is shown that a sheet metal and graphite system can effectively cool the same thermal load. To prove this, an off-the-shelf LED street light was purchased, tested as a baseline, and then modified with the sheet metal and graphite design. Initial design work was performed with an analytical model, and then a prototype created from this design and shown to be of similar performance to the baseline - a system thermal resistance of 3°C/W from the PCB to the ambient, and a savings of 0.9 kg. A detailed CFD was also performed to evaluate the performance of the system and suggest possible improvements in air flow paths. Comparisons of the experimental and numerical results are given showing agreement in most areas.