If you’ve ever kept a laptop computer on your lap for any extended length of time, you know how hot they can get. Cramped computing spaces, such as microTCA, PC104 and custom small form factors can have much the same effect where the chassis is small and close to the components. In the case of laptops, a laptop heat sink with heat pipes abutting a metal chassis can be a solution to transfer the heat from the CPU, chipset and other components. In the case of cramped (or more politely, small form factor computing) heat sinks, larger fans and ducted airflow are some default choices. However, leading edge thermal management developments can provide some unique approaches to cooling these use cases. Our two part article here will cover three in particular.
As integrated circuits (ICs) have to provide increased functionality and computational power through a greater number of transistors in smaller and smaller packages, the removal of the heat dissipated by these electronic chips has become a serious challenge in the design of portable and other space-limited electronics devices. The cooling of these electronic chips in notebook computers is especially challenging due to the notebooks small footprint. Currently, heat pipes, as shown in figure 1 [1], are used to transport the heat from the high power components to a remote heat exchanger. The heat is then dissipated to the air passing through the remote exchanger. However, the heat dissipation using a heat pipe is approaching an asymptotic limit for the size restrictions of a notebook-shape form.
Figure 1 – Heat Pipe Cooling in Existing Notebook Computer [1]
Alternative cooling approaches have been investigated to achieve the required dissipation rates, while satisfying the required reliability and cost considerations. These methods are thermoelectrics and refrigeration. Given the small cooling capacity and low efficiency of thermoelectrics, a refrigeration system is the only viable method to further increase the heat dissipation of high power components in notebook computers. Refrigeration cooling allows high heat flux dissipation at low junction temperatures, which will increase microprocessor performance at lower operating temperatures and increase chip reliability. However, refrigeration cooling also increases the size, complexity and cost of the cooling system. The complexity would increase the uncertainties in the system reliability.
Vapor Compression Refrigeration System
A basic vapor-compression refrigeration system consists of four major components: an evaporator, a compressor, a condenser and a throttling device. Figure 2 [2] shows a schematic of a vapor-compression refrigeration system. The main heat transfer mode of the vapor-compression refrigeration cycle is evaporation/condensation of the refrigerant.
Figure 2 – Vapor-Compression Refrigeration System Schematic [2]
When the refrigerant enters the evaporator, it evaporates due to the low pressure and absorbs heat from the evaporator at a constant temperature. Then, the vapor refrigerant travels through the compressor, which increases the pressure of the refrigerant. After the compressor, the vapor refrigerant condenses in the condenser due to the high pressure and rejects heat to the condenser at constant temperature. The refrigerant then travels through a throttling device, which reduces the liquid refrigerant pressure. The low pressure liquid refrigerant re-enters the evaporator and restarts the cycle. Figure 3 [2] shows the thermodynamic state point diagram of a vapor compression cycle.
![Figure 3 - Vapor Compression Cycle Thermodynamic State Point Diagram [2]](http://qats.com/cms/wp-content/uploads/2011/07/Leading_Edge_Strategies_to_Cool_Laptop_Computers_figure_31.png "Leading_Edge_Strategies_to_Cool_Laptop_Computers_figure_3")
Figure 3 – Vapor Compression Cycle Thermodynamic State Point Diagram [2]
For electronic cooling, the evaporator would be directly attached to the high power electronic chip, absorbing the chips heat dissipation. The heat dissipation would be rejected to the ambient environment through the air-cooled condenser. Figure 4 [3] shows the schematic of the vapor-compression cycle within a computer.
Figure 4 – Schematic of a Miniature Refrigeration System [3]
Recently, there have been a lot of studies that aim to further investigate the feasibility of the refrigeration system for electronics cooling. Studies of vapor compression systems and system simulation were directed at electronics cooling in laptop computers. One such study was conducted by Mongia, Masahiro, and DiStefano. The small-scale refrigeration system, within the study, included a compressor, cold plate, condenser and throttling device. These components were specially designed, such that the entire cooling system can be incorporated within a notebook form factor. Figure 5 [2] shows the schematic of the entire system with temperature and pressure measurement points.
Figure 5 – Small Scale Refrigeration System Schematic [2]
Iso-butane was chosen as the working fluid. The cold plate and condenser contain microchannels to efficiently transfer heat to and from the refrigerant. Prototypes of each of the components were built and tested in order to assess their individual performance. Figure 6 [2] shows a complete form factor loop that was also built and tested to determine the system feasibility and overall performance. The test results, as shown in Figure 7 [2], show that this system can achieve a coefficient of performance (COP) > 2.25 at a moderate temperature rise. The thermal resistance of this system ranges from 0.28 – 0.7 °C/W. Figure 8 [2] shows the cooling loop within a notebook.
 
Figure 6 – Complete Small Scale Refrigeration Form Factor Loop[2]
Figure 7 – Results from Small Scale Refrigeration [2]
In part two we’ll cover Miniature Scale Diaphragm Compressors and Micro-Channel Heat Sinks.
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.
