In part 1 of our 2 part series on leading edge strategies to cool laptop computers and cramped computing spaces, we primarily covered Vapor Compression Refrigeration Systems. In part 2 we’ll finish up our two part series by examining Miniature Scale Diaphragm Compressors and Micro-Channel Heat Sinks.
Miniature Scale Diaphragm Compressors
The researchers at Purdue have developed tiny compressors that pump refrigerants using penny-sized diaphragms, mainly of two contoured conductive planes that serve as electrodes. These planes are separated by dielectric insulation layers and a gas/refrigerant gap. As a voltage potential is applied between the electrodes, the electrostatic force deforms the diaphragm and pulls the diaphragm towards the electrode on the chamber wall. The contour of the compression chamber causes a progressive and continuous zipping action of the diaphragm until the membrane mates with the entire chamber wall. At the end of the compression strokes, the compression volume has almost zero dead space and the flexible diaphragm provides perfect rectification. Thus, the pressure of the refrigerant inside the chamber rises. The refrigerant flow in and out of the compressor chamber is controlled by suction and discharge flapper mini-valves. Target operational parameters for the miniature compressor are a heat removal of 200 W, pressure head of 750 kPa, pressure ratio of 2 and flow rate of 3000 ml/min. The targeted dimensions of the diaphragm compressor are 80 mm in diameter and 20 mm in height.
Although the new technology seems promising, there are still several challenges. One complication is that many diaphragms must operate in parallel in order to pump a large enough volume of refrigerant for the cooling system. One possible solution is to stack the diaphragms within the system small enough to fit inside a laptop. The design can be optimized using computational methods, which enables the engineers to determine how many diaphragms to use and how to stack them, either in parallel to each other or in series. By stacking in one direction, the pressure might increase. While stacking in the other direction, the necessary volume would be able to be pumped. Another major challenge is to manufacture the compressors at a low cost.
Figure 9 – Miniature Diaphragm Compressor Schematics [4]
Micro-Channel Heat Sinks
Another research project at Purdue is focusing on heat transfer in microchannel heat sinks, which circulate coolant through numerous channels about three times the width of a human hair. The micro-channel heat sink is a copper plate containing numerous grooves 231 microns wide or about three times as wide as a human hair and 713 microns deep. Figure 10 [5] shows Purdue researchers testing their microchannel heat sinks.
Figure 10 – Purdue Researchers Testing Micro-Channel Heat Sink [5]
Currently the researchers at Purdue are seeking to characterize and predict the enhancement due to boiling heat transfer provided by randomly roughen the surfaces in micro-channel heat sinks. Some results indicate that increasing the surface roughness by a factor of 3 yields a 30% enhancement in the amount of heat that can be removed while keeping the heat sink temperature constant. Further increases in surface roughness appear to be of little additional benefit.
There has been a lot of research in the feasibility and operational performance of small scale vapor-compression system. One promising area is the microchannel heat exchanger, which circulates coolant through numerous channels about three times the width of a human hair. Another promising area is a micro-diaphragm compressor. As the challenge of heat removal from more powerful electronic chips in smaller form shape, small scale vapor-compression system might be a promising solution.
Reference:
1. Review Core Duo vs. Core 2 Duo, NotebookCheck website http://www.notebookcheck.net/Review-Core-Duo-vs-Core-2-Duo.2404.0.html
2. Mongia R. et al., Small Scale Refrigeration System for Electronics Cooling within a Notebook Computer, Proceedings SEMITHERM XXII, March 2006, San Clara, pp.751-758
3. Miniature Refrigeration System for Electronics Cooling, Minicool website http://www.minicool.co.uk/project.html
4. CTRC Breakthroughs, CTRC website https://engineering.purdue.edu/CTRC/research/breakthroughs.php
5. Purdue Miniature Cooling Device Will Have Military, Computer Uses, ScienceDaily website http://www.sciencedaily.com/releases/2005/04/050414173948.htm
6. Miniature Cooling Device, California Science & Technology News website http://www.ccnmag.com/article/miniature_cooling_device
7. Trutassanawin S. et al., Experimental Investigation of a Miniature-Scale Refrigeration System for Electronics Cooling, CTRC Research Publications, Sept. 2006, pp.678-687.
8. Chiriac V., Chiriac F., Optimized Refrigeration Vapor Compression System for Power Microelectronics Cooling, Proceedings of Clima 2007 WellBeing Indoors.