Two-phase heat exchangers provide passive cooling for high-power LED light engines (MAGAZINE)

Passive cooling using a finned heat-exchange system provides a lightweight, adaptable and noise-free method to maintain a lower LED operating temperature, writes ABOUDÉ HADDAD.

Content Dam Leds En Articles 2012 05 Two Phase Heat Exchangers Provide Passive Cooling For High Power Led Light Engines Magazine Leftcolumn Article Thumbnailimage File
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This article was published in the April/May 2012 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete April/May 2012 issue, or view the E-zine version in your browser.

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Efficient removal of heat from LED light sources is a crucial factor in ensuring that an LED lamp or luminaire exhibits the expected levels of performance and lifetime. Conventional light sources release heat to the ambient through the emission of infrared radiation, and convection is also an important mechanism in the case of fluorescent lamps. These modes of heat transfer typically do not require advanced cooling methods to facilitate the process. However, LED sources lose heat through conduction, which requires the additional knowledge of how to efficiently carry waste heat out of the LED in order to prevent performance degradation and system failure.

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FIG. 1.
Various techniques can be used to remove heat from LED sources in LED-based lighting systems, including both passive and active approaches. Many LED luminaires and lamps use passive cooling based on finned heat-sinks made from cast or extruded metal. These tend to be heavy and bulky, and their operation is orientation-dependent. However, they do not suffer from performance degradation over time, and do not create any noise.

Active cooling approaches include conventional fans and diaphragm-based forced air cooling. While enabling higher power dissipation, these generally produce noise and may have issues of reliability and performance degradation over time. They are not orientation-dependent, but add slightly to the operating cost since they consume electricity. In general, there is a split between the advantages and disadvantages of active and passive approaches.

Two-phase heat exchangers

Another passive cooling approach is the two-phase heat exchanger, which works on the same principle as a heat pipe. One example, the HPK-Fin from FrigoDynamics, is a two-phase heat exchanger that aims to combine the advantages of both active and passive systems.

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FIG. 2.
Two-phase heat-transfer devices do not consume electricity, but only require the disposable heat in order to initiate their powerful heat-transfer cycle. In this case, two-phase means the use of evaporation and condensation in order to dramatically increase the heat-transfer characteristics of the device. The process happens at nearly the speed of sound, and takes place inside a hermetically-sealed tube that is filled with a minute quantity of liquid. Fig. 1 outlines the heat-exchange mechanism. Such devices have been field-tested by the military and in various commercial industries for decades, proving their reliability in electronic systems.

The effective thermal conductivity across a two-phase heat-transfer device can be up to several hundred times greater than an equivalent aluminum or copper rod. The process can even work independently of the device’s orientation. The HPK-Fin coolers have a series of fins around the tubes to dissipate heat (see Fig. 2).

Thermal tests

The effectiveness of the passive HPK-Fin two-phase heat exchangers was tested using an SC HPK-Fin 150 (fin diameter of 100 mm with 150-mm overall length and weight of 220g) in combination with a Xicato XSM 2000-lm, 36W LED module. A variety of orientations were tested.

The key measurement is the case temperature (Tc) of the LED itself. This is the temperature on the LED surface that makes contact with the HPK-Fin two-phase heat exchanger. For this particular LED module, the maximum allowable Tc is 90°C, in order to guarantee the specified lumen output and 50k hours of operation.

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FIG. 3.
At an ambient temperature of 40°C, the Tc point was measured at around 70°C, giving a margin of 20°C on the maximum allowable temperature. Furthermore, very little effect on performance was seen at different orientations. With such a performance margin, this means in practice that it would be possible either to extend the lifetime of the LED significantly beyond 50k hours, or to use a module with an even-higher lumen output.

Passive cooling relies heavily on surface area for free convection to occur, in contrast to active cooling which places more emphasis on the forced air-flow rate. Therefore, it is crucial that the heat can be spread effectively across the available area. To illustrate this point, a longer SC HPK-Fin 230 (230 mm in length) was analyzed with an infrared (IR) thermal-imaging camera. Fig. 3 demonstrates that the heat is distributed very evenly, at a temperature difference of less than 2°C along the whole length and surface of the two-phase heat exchanger.

This is particularly notable when considering that a large amount of heat is coming from a relatively small footprint at the base of the two-phase heat exchanger. Using only solid extrusions or castings would lead to considerable thermal gradients when subjected to a concentrated heat load, resulting in less-effective heat dissipation at the areas far away from the heat source.

Adaptable design

The configuration and layout of the two-phase heat exchangers and fins can be adapted and tailored to meet specific product-design embodiments. Two-phase heat exchangers are typically larger than active cooling devices, but are still low in weight. Given their performance, they are compact relative to other passive solutions.

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FIG. 4.
All air-cooled solutions, whether active or passive, operate better when in an open-air environment. However, many applications require that a housing is built around the light engine for aesthetic and safety reasons. Products from FrigoDynamics with specially-designed fins can have enough thermal margin to enable proper operation and cooling inside a housing. However, without airflow, passive cooling solutions are rendered useless, thus the designer must ensure that correctly-situated air inlet and outlet ventilation holes are available.

One recent example can be seen in Fig. 4, where the SC110 FrigoDynamics HPK-Fin two-phase heat exchanger is integrated into the AlphaLED Metropole light fixture from Projection Lighting Ltd, enabling high performance in an entirely passive fashion and with a lightweight design.

In summary, the passive two-phase heat exchanger approach combines the advantages of other passive and active cooling systems, including high reliability, no sensitivity to orientation, and zero operating cost.

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