• Energy Research

Abstract This review quantitatively examines and compares the heat transfer characteristics of several cooling technologies with potential application in the server electronics industry. Strategies that have been examined include traditional air cooling, single and two-phase indirect liquid cooling, heat pipes, pool boiling, spray cooling, and jet impingement. The characteristics that have been examined include heat flux values, coolant temperatures, and coolant flowrates; which serve as indicators of the heat transfer limitations and power requirements of each cooling solution. A direct comparison against anticipated server heat loads has shown that some form of liquid cooling is necessary in high performance computing applications; where individual processor heat loads are expected to reach 300 W by the year 2020. While in the case of general purpose computing, where individual processor heat loads are expected to reach 190 W, air cooling remains a viable option; although other factors such as operating costs, chip reliability, and waste heat recovery may still encourage the use of liquid cooling.

Abstract Tablet computers (PCs) are continuously moving towards light, thin, and small shape factors to achieve an eye appealing design while providing extreme mobility. The increased integration of electronic components with high power density, smaller size, and more compact layout is leading to exceptionally high operating temperatures for tablet PCs. This research focuses on development of an inexpensive and practical solution for thermal management for tablet PCs using phase change materials (PCMs) encapsulated in thin aluminized laminated film. In this paper, the performance of the tablet PC with this PCM thermal energy storage (TES) unit under continuous operation was investigated. PT-37, a commercial PCM from PureTemp, and n-eicosane were used as PCMs, as they have been shown to be safe and relatively inexpensive, and have melting temperatures in the range suitable for thermal management of portable electronics such as tablet PCs. It was found that the insertion of the thin PCM-based TES system within that tablet PC resulted in a reduction of the rate of temperature increase, for both the electronics and the tablet cover, during the transient start-up phase of operation; leading ultimately, to lower operating temperature of the entire tablet. Findings from this research confirm that the implementation of PCM–based TES unit is a viable option for thermal management of tablet PCs.

This paper details a numerical study of solid-liquid phase change heat transfer in a rectangular enclosure with the aim of optimizing the number, dimension, and positioning of fins in the enclosure. The enclosure is exposed on one side to a constant heat flux of 1000 W/m2 for 3 h and both the total fin mass and the PCM mass are kept constant in all simulated cases. The heat diffusion equation in the PCM uses the equivalent heat capacity method while natural convection driven PCM motion in the enclosure is modeled through the buoyancy force, using a modified viscosity and an additional volume force term in the Navier-Stokes momentum conservation equation. Three efficiency assessment parameters are used: (i) the height-averaged temperature of the front hot plate which should remain as low as possible for the longer possible time, (ii) the energy stored inside the PCM as a function of time, and (iii) the heat transfer rate and heat flux between the front plate and the PCM. For each simulated case, complementary data such as the standard deviation of temperature along the heated plate and the ratio of sensible heat to latent accumulation are also calculated. Results show that increasing the number of fins diminishes both the stabilization temperature and the stabilization time of the front plate during phase change, and accelerates the sensible and latent storage of energy in the PCM. Using thinner but longer fins provides the same impact except that the stabilization time also increases as the fin length is increased. Besides, at constant fin mass, varying fin spacing have marginal impact on the latent heat storage performance and on the hot plate temperature stabilization. The study also shows that the surface area plays the largest role in the increase of the heat transfer rate between the front plate and the PCM, while configurations which can promote stronger natural convection lead to higher heat flux. Finally, it was observed that systems allowing easier heat transfer to the back plate during melting provide a higher ratio of sensible heat to latent accumulation, while the standard deviation of the front plate temperature decreases as the number of fins or the spacing between fins increases.

The objective of this work was to experimentally determine the feasibility of using a phase change material (PCM)-based temperature control module, in conjunction with a heat spreader and thermal interface material, to improve the thermal management of a tablet computer. An experimental apparatus was designed to be representative of a tablet computer. This mock tablet was used to perform a series of transient heating and cooling experiments to compare the impact of the PCM module on the thermal response of the system. The PCM module consisted of n-eicosane encapsulated with heat-sealable laminated film forming a 2 mm thick sheet of encapsulated PCM. A full comparison, including the use of a heat spreader and a thermal interface material (TIM), was conducted at heat generation rates of 4.5 and 7 W. The temperature control module was able to reduce the mean and peak temperatures of the internal components and at a heat generation rate of 7 W it extended its operating time by 30% before it reached a critical threshold temperature.

This paper presents an experimental and numerical investigation of the thermal behavior of thin phase change material (PCM) packages as a solution to thermal management in portable electronic devices. The thin packages are made of encapsulated PCM in aluminized laminated film. The experimental setup is designed to include the most fundamental aspects of a portable electronic system while also being simple enough to be easily simulated using the finite element method; it is rectangular in nature. Two different types of PCM are used for the experimental work; the commercially available PT-37 and n-eicosane. It was determined that the use of a thin PCM thermal energy storage package significantly improved the temperature behavior of the experimental setup, by reducing the rate of temperature increase at the heater and the back cover. The time needed to reach a critical cover temperature of 45°C was increased by 12 to 18% while the time for the heater temperature to reach 70°C was increased by up to 66%. Numerical simulations of the system were in good agreement with the experimental data.

This study employs thermo-economic analysis to establish a homeowner-centric valuation of renewable energy technologies. The evaluation methodology is illustrated using several classes of renewable energy technology, including, ground-source heat pumps, wind turbines, photovoltaic panels, and solar thermal water heaters. Energy systems employing each of these technologies are examined when servicing the typical domestic energy loads of a single-detached home. Through the analysis, it is learned that single-home photovoltaic systems produce energy at an annual cost approximately four times that of grid electricity, while wind energy systems deliver this energy at approximately twice the cost of grid electricity. Further, single-detached homeowners currently heating with fuel oil and/or grid electricity may expect to save between $1000 and $1800 per year when switching to a ground-source heat pump system; whereas those considering the implementation of a solar thermal water heater can expect to spend an additional $245 per year for hot water. Investigation of larger scale renewable systems examined the performance of wind turbines and ground-source heat pumps when serving groups of single-detached homes; wind turbines were found to exhibit significant economies of scale.

Abstract This paper demonstrates that liquidus line (T-x) data can be obtained from calorimetric determinations of phase transition enthalpy profiles (H-T) for incongruent-melting phase change materials (PCMs) more efficiently than using traditional cooling curves. An accurate and reliable equilibrium mixture enthalpy model bridges the H-T and T-x gap to provide a full suite of high density H-T-x data to assist latent heat energy storage researchers to evaluate composition-dependent two-phase equilibrium processes. The proposed method is validated for T-history method H-T determinations of 1:1 diluted sodium acetate trihydrate in water, and can also be used with other laboratory calorimetric techniques used to determine the phase transition enthalpy profiles of incongruent-melting compounds.

The two-region fin model captures the heat spreading behaviour in multilayered composite bodies (i.e., laminates), heated only over a small part of their domains (finite heat source), where there is an inner layer that has a substantial capacity for heat conduction parallel to the heat exchange surface (convection cooling). This resulting heat conduction behaviour improves the overall heat transfer process when compared to heat conduction in homogeneous bodies. Long-term heat storage using supercooling salt hydrate phase change materials, stovetop cookware, and electronics cooling applications could all benefit from this kind of heat-spreading in laminates. Experiments using laminate films reclaimed from post-consumer Tetra Brik cartons were conducted with thin rectangular and circular heaters to confirm the laminate body, steady-state, heat conduction behaviour predicted by the two-region fin model. Medium to high accuracy experimental validation of the two-region fin model was achieved in Cartesian and cylindrical coordinates for forced external convection and natural convection, the latter for Cartesian only. These were conducted using constant heat flux finite heat source temperature profiles that were measured by infrared thermography. This validation is also deemed valid for constant temperature heat sources.