Micro Heat Exchanger

Silicon electronics, and solar cells under concentration, need to be cooled. Heat fluxes in concentration at 300 suns can be of the order or 200 W/cm².

How do we remove this heat?

Standard approach resorts to a soft thermal interface material (e.g. Glue, thermal pad or so) and a metal heat exchanger. The soft material is needed to establish thermal contact between the surfaces, that are not atomically flat, and to accommodate the different thermal expansion coefficient of silicon and the metal (aluminum or copper).

What if we could develop an heat exchanger in silicon directly bonded to the heat generating element? Or possibly even integrated into it? We discussed this concept in the Solar Cells Cooling chapter in the book “Concentrator Photovoltaics” published by Springer (1).

Together with my research group at that time (2003) we designed a simple single mask wet etching process (where trenches are obtained in silicon by a SiO₂ masking, patterning and Tetramethylammonium hydroxide etching) to create a set of microchannels (80 microns width) into a silicon wafer, we, then, bonded it by wet silicon bonding process to a silicon cap with inlets, and run water through it testing its heat exchange properties.

Contrarily to most heat exchangers, where the liquid flux is turbolent to overcome the low thermal conductivity of water in laminar flow and disrupt the limit layer at the water-pipe interface, the use of microchannels allows to remain in the laminar flow region with significant advantages in terms of hydraulic pressure drops (that is greatly diminished with respect to the turbulent regime) and, thus, pumping power.

The device was succesfully realized and tested exhibiting 1 order of magnitude improvement in thermal exchange coefficient with similar area conventional heat exchangers. The results were presented at an ISES conference in Gothenburg in 2003 (2) and the poster is available here.

Unfortunately the project was then abandoned because the student doing the work found a position in an Alluminum Smoldering plant (!) and we were unable to find financing to continue with it….

1) (2007). Solar Cell Cooling. Concentrator photovoltaics. A. L. Luque and A. Viacheslav, Springer Berlin Heidelberg. 130: 133-149. ISBN 978-3-540-68798-6. http://www.springer.com/engineering/energy+technology/book/978-3-540-68796-2

2) D. Vincenzi, F. Bizzi, M. Stefancich, C. Malagu, GL. Morini, A. Antonini, G. Martinelli (2003). “Micromachined silicon heat exchanger for water cooling of concentrator solar cells.” ISES, Gothenburg, Sweden.

Figure 1: The poster presented at ISES conference in Goteborg in 2003