For industrial lighting fixtures, in particular the UFO-style high bays in which circuitry and LEDs are housed in an enclosed housing, an efficient thermal design is critical to bring down the operating temperature of such optoelectronic device while improving performance and reliability. The thermal design is usually focused on the heat sink which is typically an integrated luminaire housing when it comes to high bay designs. A heat sink is designed to pull heat away from the junctions on each LED and from the driver housing. Heat sinks typically comprise a heat conducting material such as a metal, and include fins or channels to increase the heat sink's surface to provide for greater convection heat exchange with the ambient air. The housing may contain a built-in thermal venting chamber cast into the housing. Thermal conductivity of a high bay housing is determined by material composition and environmental conditions. The removal of waste heat by thermal conduction is also structured on the geometries of system elements. Heat sinks can be constructed from any high thermal conductivity material including but not limited to copper, aluminum, or metal alloys. Although copper can have a thermal conductivity as high as 400 W/m-K or more. Aluminum is the most preferred metal for heat sinks because of its relatively high thermal conductivity and ease of manufacturing. To improve heat dissipation and resistance corrosion, an acrylic powder coat finishes can be applied to both the inside and outside surfaces of the aluminum housing.
The aluminum heat sink can be manufactured in different processes with varied cost and performances. Stamped heat sinks are the lowest cost thermal solution but less efficient than extruded heat sinks and die cast heat sinks. The extrusion process is advantageous at manufacturing complex fin profiles which allow greater heat dissipation through increased surface area. Forged heat sinks have a very high aluminum purity and accordingly have an excellent thermal conductivity - usually 20% higher than the extruded and die cast heat sinks. High purity aluminum can have a thermal conductivity at room temperature of approximately 210 W/m-K. Extruded and die cast manufacturing often involves alloying elements for easier processing, but these impurities are negative to the thermal properties. An extruded or die cast aluminum heat sink has a thermal conductivity of approximately 160-200 W/m-K. As the cost/performance ratio is often the key consideration in system design, forged heat sinks are used less frequently than other types of heat sinks. Furthermore, die cast high bay light housings offer one-piece construction and eliminate secondary operations such as machining and assembly and can be molded with many features such as fins, chambers, dedicated vents or openings, or specific shapes for maximum heat dissipation. Modern UFO high bay fixtures are increasingly designed with streamlined form factors for aesthetic considerations as well as better thermal management. Properly designed luminaire housings, by way of example, can avoid dust accumulation in the long run and the system thermal conductivity will not deteriorate.
Better thermal management allows the high power LEDs of a high bay luminaire to be driven at higher current levels while mitigating the negative effects on life and light output typically connected with high ambient temperatures. Designers have a couple of ways to keep the high power LEDs cool through the use of other passive thermal management technologies, such as heat pipe-based assemblies. A heat pipe system utilizes two-phase heat transfer through the evaporation and condensation of a working fluid. Other thermal management strategies have been developed that make use of active cooling devices, such as fans, to radiate heat from the LEDs. Forced air convection generated by a fan can increase heat transfer to the ambient.
