Higher volumetric heat generation is anticipated when miniaturization of electronics is done in typical engineering applications. Recent days, this value of heat flux released from electronics is as much as 1 MW/m^2 (100 W/cm^2).
For reliable functioning of the electronics, the generated heat has to be dissipated quickly and efficiently such that the electronics are maintained below a set point temperature (SPT, usually the junction temperature of the transistors, ~ 90 to 120 C) during their time of operation.
In this regard, heat sinks using Phase Change Materials (PCM) such as high carbon paraffin find extensive applications in the thermal management of portable electronic devices, owing to the PCM property of absorbing a large quantum of heat at a constant temperature while melting.
However, the low conductivity of the PCM is a hindrance by the creation of hot spots, which is overcome by a Composite Heat Sink (CHS) comprising a judicious distribution of high thermal conductivity Base Material (BM) along with the PCM.
The thermal design of such a CHS, constructed using a vertical array of �fins� (or Elemental CHS, ECHS) of similar shape and size, made of PCM and BM, for its performance improvement has also been studied in detail by many researchers in the recent decade [Prof. Suresh Garimella at Purdue Uty., Prof. Cristina Amon at Carnegie Mellon Uty., Prof. Pradip Dutta and Prof. Srinivasan at the IISc]. One such simple design is shown in Figure 1 below.
Figure 1
For a CHS constructed with the design of BM fins placed vertically inside a PCM bath (Fig. 1a), operating under a given SPT (i.e. keeping the electronics that need to be cooled), under-usage of the latent heat of the PCM because of incomplete melting is a more critical CHS issue than the complete melting of PCM before the CHS reaches the SPT. The former results in a poorly performing CHS while the latter merely reduces the CHS potency for heat transfer enhancement (from PCM latent heat to sensible heat) in time.Of course, this maximization of the operation of CHS can be resolved, as was done in [Akhilesh et al., 2005] – I am one of the co-authors. For a chosen PCM quantity, a “critical upper-bound dimension” (d_c and B_c) for the ECHS, as shown in Figure 2b, can be determined by analysis. Constructing a CHS (shown in Figure 2a) with a finite number of ECHS (shown in Figure 2b) made of the critical dimensions (d = d_c and B = B_c) ensure complete melting of all of the PCM exactly when the SPT is reached. This design gives an increased time of operation for the CHS.
In other words, in a CHS design of Figure 1, exhausting all of the PCM latent heat storage before SPT is reached, should yield maximum time of operation of the CHS.
Figure 2
By the way, the above CHS idea is not far-fetched. See for instance [ ESLI Products: PCM Heat sinks ]. This product, shown in the right here, is a typical PCM based CHS that uses light weight brush like metal fibers as interconnecting media pervading the PCM matrix, different from the simple parallel ECHS arrangement shown in Figure 1.
According to the ESLI website,
ESLI’s paraffin based heat sinks can be designed to operate at or near 5, 18, 28, 37, 44, 55, or 61 °C. Non-paraffin based designs are also available for cryogenic or high temperature applications. ESLI’s PCM composite heat sinks have been space flight tested on the Space Shuttle (STS-95) aboard the CRYOTSU experiment module, and will fly aboard NASA’s Vegetation Canopy LIDAR experiment in 2000.
Another product developed by NASA to cool on-board space batteries in rockets, utilizes carbon fibers as the metal BM, a variant from the above CHS design of Figure 1 and 2.
For an illustration of the applicability of the CHS explained in Fig. 1 and 2, consider a cooling application that requires a thermal dissipation of 62.5 W and the electronics to be maintained below T_SET = 90 C through a typical CHS area of 5cm x 5cm. For this case then the heat flux would be q” = 25 kW/m^2 and by choosing a CHS with D = 5 cm (left to right length, in Figure 1) and height A = 5 cm (see Fig. 2a), the number of critical dimensioned ECHS (Fig. 2b) made of 50 percent of PCM that should ‘fill’ the CHS (Fig. 2a) can be calculated from analysis. This turns out to be d_c = B_c = 0.004 m (see Fig. 2b) and the number of such ECHS that would fill the CHS is equal to 4. For such a design the CHS would operate successfully for about 453 seconds. After this, the CHS has to be ‘cooled’ to release all of the stored energy in the PCM, before charging it again.
The CHS design in Figure 1 and 2, or something similar, is a required one for cooling the on-board electronics in the space vehicles launched by ISRO, the Indian Space Research Organization. Typically, this means, the CHS has to operate successfully (taking maximum energy and not allowing the electronics to reach SPT) for about 20 to 30 minutes (one way).
10 responses so far ↓
TechBizMedia » Composite Heat Sink for Cooling Electronics // August 20, 2006 at 9:46 pm |
[...] [cross-posted also at Nonoscience, my science blog] [...]
drkatte // August 21, 2006 at 9:25 am |
“the CHS has to be cooled to release all of the stored energy in the PCM, before charging it again…. is a required one for cooling the on-board electronics in the space vehicles launched by ISRO…..”
Of course an electronics cooling system is required for all the spacecraft and launch vehicles. However, it is challenging to transport the heat from the electronic equipment and dissipate the same (to space) in the absence of convection. Space radiators (a true radiator unlike an automobile” radiator”) are the only way to achieve this (to throw away the heat in order to cool CHS or whatever). Prof. S.P. Venkateshan and Dr. C. Balaji of IIT Madras have done pioneering research in this area. I also have done a bit of research in this area and I am planning a few posts on space radiators.
A nice technical post, but I want other (and more) people (not me again!) to read this post.
Venkat // August 21, 2006 at 9:58 am |
The PCM based Heat sink paper was quite an interesting idea and that led/inspired us (me and Subramanya) to come up with the design for the PCM based cooling vest. It seems worthwhile for a company to take this up and come up with a whole array of products with the PCM idea. Also, I’d be interested in looking at some experimental papers for the ECHS. Am sure others also, would be interested in looking at experiments on this, so that they’d be more convinced with the viability of taking the idea to a product.
On a personal note, I am in the process of trying to demonstrate a prototype for the PCM-based cooling vest and Lucas-TVS seems to be interested in looking at it.
Arunn // August 21, 2006 at 10:02 am |
Thanks for your comments Dr. Katte.
The CHS discussed here is to be used as an ‘one way’ use-and-throw device. Of course you cannot ‘throw’ it in space, but its pourpose is served if it successfully works for the period the launch vehicle goes into space from Earth and puts the satellite in its orbit. This time period, as you would agree, is about 30 minutes (if you leave out the ‘parking time’ on the launch pad), during which period is what the CHS discussed above should be designed to be used.
And, just for clarification, the CHS discussed here doesn’t opearte using convection or radiation, but melting/diffusion – convection effects in the molten liquid pool is not considered for its obvious (to us) low impact on the time of operation…
Yes, space radiators are another option for cooling. Would love to see your posts on this topic.
Audience will come, if our contents are original and reasonably well informed and reliable. Spread the word on this post to your blog audience, if you like it, and want more of us to read it (I started “panta rei” at my blog, for doing this for other bloggers). Thanks.
Arunn // August 21, 2006 at 10:16 am |
Venkat: Thanks for your comments. Yes, PCM based cooling vests are viable (I was wondering what happened to our ‘discussions’ – past and supposed ‘future’ ones…). Let me know of how it went with Lucas TVS.
Experiments are underway with ECHS at our lab. Some work should be reported soon. As I said in the post, experimental work has also been done by more researchers like Prof. Suresh Garimella (Purdue), Prof. Pradip Dutta and Prof. Srinivasan (at IISc).
Yes, A company could take up and explore more ideas of using PCMs. But such ‘start-ups’ or SBIRs are not ‘popular’ in India at present. Perhaps due to the inherent high risk nature of the SBIR business model, which may not thrive in India.
Arunn // August 28, 2006 at 12:38 am |
This comment has been moved here as a separate post.
Panta Rei - 2nd Edition: What’s the Difference at Kyun.org // August 29, 2006 at 10:50 am |
[...] Moving on from the macro to something that affects all you folks reading this – cooling in computers. Bad things can happen when your electronics are not cooled efficiently. Arunn tells us how important it is to dissipate heat quickly. He presents two conceptual designs and is looking for folks to fulfill the challenge of taking these to market. [...]
Nonoscience / Comments on My Composite Heat Sinks for Cooling Electronics Post // September 1, 2006 at 9:42 pm |
[...] This post is a reply to a set of questions that I received by email for the idea I discussed in my earlier post titled Composite Heat Sinks for Cooling Electronics…[...]
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My Laptop Heat Sink « Unruled Notebook // March 18, 2009 at 4:52 pm |
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