Pcm (Phase Change Material)

Topics: Thermodynamics, Heat, Heat transfer Pages: 9 (3117 words) Published: January 9, 2013
Andrew Russell and Mohammed Farid

Department of Chemical and Materials Engineering The University of Auckland, New Zealand Email: m.farid@auckland.ac.nz

ABSTRACT The use of high speed processors in modern laptops and other electronic devices leads to higher rates of heat generation that need to be dissipated efficiently. This heat generation can lead to a certain degree of discomfort when using a notebook computer on top of your lap. If heat is not dissipated efficiently it may also shorten electronic components life and cause the fan to operate for longer periods, consuming the battery charge more quickly. Phase change materials such as waxes melt at almost constant temperature by absorbing large quantities of heat as a latent heat of fusion. In this project, 10mm thick aluminium flat container was constructed as a cooling pad to contain the PCM, which has a melting point of 27oC. It was a special wax manufactured especially for the project by Shell. Although the latent heat of the wax is high, its thermal conductivity is low, which limits the rate at which it could absorb the heat. Hence it was necessary to use expanded graphite with the wax to improve its thermal conductivity. The effectiveness of heat absorption by the PCM pad was tested by placing the pad at the bottom of laptop operating at a full capacity with a heat flux sensor and a thermocouple attached at the bottom phase of the cooling pad. The discomfort condition was defined as the condition when the bottom surface reaches greater than 36oC. Results show that the use of the thin panel containing PCM impregnated in graphite could increase the comfort operation time of the laptop by more than four times. INTRODUCTION Consumer demand in the electronics industry has seen rapid advancements in laptop processor speed and capabilities, with many laptop central processing units (CPU) of more than 3GHz available. This performance increase has come at a price, however, as the total amount of heat generated increases with processor speed (Maleki and Shamsuri, 2003). A U.S. Air Force survey (Yeh, 1996) showed that more than half of all electronic failures were temperature-related. The high temperatures that modern laptops generate are starting to become too significant for common cooling techniques to handle, and hence alternative cooling techniques are necessary.

Studies have shown that the heat generated by an average laptop is 25W when idling and up to 60W under heavy load, compared with the 105W that an average human generates while sitting stationary (LapLogic Labs, 2003). Even though not all of the heat from the laptop will be conducted into the lap of the user, the extra heat is quite substantial in comparison to the heat generated by the user. The comfort temperature for humans is between 30 and 36°C, with a feeling of warmth up to 40°. It is uncomfortably hot from 40-42°C, and the onset of pain begins at 42°C (LapLogic Labs, 2003). With a large surface area of the lap in contact with the laptop, it is important to maintain the temperature of the casing below 36°C so that the laptop can be used in the comfort range of the user. Traditional laptop cooling relies on a combination of free convection and fan-forced convection. The usage of fans is a disadvantage in that they are bulky, and require extra power to run (Alawadhi and Amon, 2003). The miniaturisation of laptops does not allow the addition of more fans, or an increase in fan size, and hence becomes a constraint. These high speed fans are also known to produce noise which can be annoying to the user (Jing, 2002). In order to reduce point loads, metal heat pipes or heat sinks have been installed in modern laptops. While the installation of fans is necessary in ensuring the safe operation of the laptop, passive cooling methods have been shown to reduce the time that fans are required to operate, hence saving battery power and reducing noise...
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