These days, the electronic components used in LED technology are packed more tightly together, and that means greater power dissipation in the housing, assembly groups and components. These factors decrease a product’s robustness and shorten its service life. The Müllheimbased Steinbeis Transfer Center for Identification Media & Identification Management, together with the companies Hollomet in Dresden and Turck-Duotec in Halver, developed a method for counteracting these effects: the hpc2, or high-performance cellular cooler.
When developing high-performance assemblies, some initial problems must be addressed: Where should the dissipated heat go? How are high environmental temperatures best tackled? How are thermal flows best conducted away from the source? How is the most homogeneity achieved in the temperature distribution across the individually placed consumer components (as in LEDs)? And is it even possible to develop a new material that possesses the right physical and chemical properties for these kinds of applications?
Since the efficiency of LEDs depends to a great extent on their energy charge levels, and, thus, on the operating temperature, it is important to attempt to keep the thermal transfer resistance between the diodes, the circuit board (where the electrical components are mounted), and the cooling elements (heat exchangers) low. At very high room temperatures of up to “measured” 80°C (e.g., environmental temperatures under the roofs of buildings like foundries, plants in galvanic industries, or gymnasiums), there is little room for passive cooling. Yet most applications don’t allow for large cooling elements due to product design and weight issues, so a medium is needed for the heat transfer, which couples the “hot” electronic components with the cooler external environment.
The service life of high-quality electronics is greatly impacted by component temperatures. To reduce these, the three project partners developed the following requirements profile for heat exchangers: highest possible heat conductivity, high permeability, coverage of the largestpossible specific surface area, and lowest-possible specific density. To achieve this, the thermal capacity generated in the assembly group was to be transferred to as large a surface area as possible, one which is exposed to airflow and a fluid coolant. Fluid materials would then be conducted to an external cooling element through piping or hoses. For the sake of service life, it is essential that the maximum temperature limit of 105°C is not exceeded, and that the thermal distribution is evenly spread in the arrays of consumer components.
All attempts on behalf of the project partners to use metal foams for an even temperature distribution proved unsuccessful, so they looked for a different technological solution – and they found one: The group was able to stabilize the temperatures in a force-cooled 200 Watt LED lamp at 35°C. In doing so, they met the objective of ensuring the viability of their development. In theory and practice, the project partners cap the diode-array capacity of 600 watts, for example, at more than 70,000 lumen. The physical limit, however, is much greater.
To achieve a high coolant flow rate in universal heat distribution across the complete cross-section of the cell, Steinbeis, Hollomet, and Turck- Duotec developed manufacturing processes for metallic, spongy cellular structures, which combine several properties in one process: lightweight construction through minimal use of materials despite high mechanical strength, implementation of defined porosity in various materials, temperature-resistance, corrosion-resistance, high allowance for through-flow and thus unimpeded coolant transfer, and freely designed geometries due to easy molding in various component sizes.
They can be varied to a great extent thanks to the geometry, layer thickness, and material selection. This provides an extremely interesting technological approach to the scalability of cooling systems: The heat exchanger connected to the component mounting element can form the greatest possible heat transfer surface between the electronics and the coolant. This allows designers to vary the cooling performance for identical assemblies through the coolant medium and its flow. This, in turn, allows for open and/or closed cooling systems.
The manufacture of the hpc2 heat exchanger is based on a powder metallurgic, patented molding process, in which an organic carrier material – similar to a bath sponge – is coated with a metal powder binding agent. The structure of the carrier material and the organic binding agent are then removed through pyrolysis, after which the metal powder particles are sintered with inert gas into the finished “metal sponge structure.” Various stainless steel materials are particularly suited to this for non-aggressive fluid coolants. By using the right materials, extremely corrosion- and oxidation-resistant structures can be created. The porosity of the hpc2 sponges can be gauged for various applications (from 60-95%) though the selection of the carrier material. This porosity can be parameterized and reproduced in the production process without any limitations.
What makes the hpc2 high-performance cellular cooler project so special is the coolant through-flow volume and its thermal coupling with the component mounting element. The coolant flow rate can be varied through the porosity of the exchange cell, and during operation it can be varied by adjusting the flow speed of the coolant. No other known manufacturing process can match this process in terms of the surface area that can be reached internally.
The manufacturing transition involved in this process resulted in a lighting program for applications with extreme demands in terms of luminosity (e.g., in manufacturing facilities with high ceilings or for exterior lighting) and temperature-resistance in extremely hot environments. A product of this kind must be 100% recyclable at the end of its service life. That’s a tall order typically, but not for the pure stainless steel used in the hpc2 project.
Armin Bäuerle
Steinbeis Transfer Center for Identification Media & Identification Management (Müllheim)
su0584@stw.de
Wolfgang Hungerbach
Hollomet GmbH
Johannes Schaefer
Turck-Duotec GmbH