Experimental study of the thermal characteristics of phase change slurries for active cooling W. Lu ⇑ , S.A. Tassou School of Engineering and Design, Brunel University, Uxbridge UB8 3PH, UK article info Article history: Received 2 August 2011 Received in revised form 3 October 2011 Accepted 4 October 2011 Available online 5 November 2011 Keywords: Phase change material Slurry Paraffin–water emulsion Thermophysical property Heat transport abstract Phase change materials (PCMs) are increasingly being used for thermal energy storage in buildings and industry to produce energy savings and reduce carbon dioxide emissions. PCM slurries are also being investigated for active thermal energy storage or as alternatives to conventional single phase fluids because they are pumpable and have advanced heat transport performance with phase change. The pres- ent study investigates several types of phase change materials for the preparation of PCM slurries which have potential for cooling applications. The thermophysical properties of paraffin in water emulsions, such as latent heat of fusion, melting and freezing temperature ranges, viscosity and the effect of surfac- tants, have been tested using appropriate experimental techniques. It has been identified that the use of small quantities of higher melting temperature paraffin and surfactants in the emulsion can reduce the effect of supercooling and increase the useful heat of fusion. However there are negative impacts on vis- cosity which should be considered in heat transport applications. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction With rising concerns over global warming, there is increasing interest in the application of innovative technologies to improve the energy efficiency of commercial buildings. Phase change mate- rials (PCMs) are increasingly being used to achieve a better balance between heating and cooling supply and demand. They can also in- crease comfort and the energy efficiency of buildings, particularly during transient heating or cooling loads, due to their high appar- ent specific heat arising from the heat of fusion [1]. The most com- monly used PCMs are salts for high temperatures, up to 1000 °C, and ice/water, organics or salt hydrates for lower temperatures, up to 150 °C. The last three PCMs have been widely studied and used for thermal storage and comfort cooling applications. However, the phase change process, especially solidification, of static PCMs is very slow because heat is transferred mainly by con- duction and the PCMs normally have low thermal conductivity. To increase the heat transfer rate, researchers have used materials with high thermal conductivities, such as metal foams, aluminium powder, carbon fibre or expanded graphite, to improve the thermal conductivity of PCM [1–10]. Agyenim et al. [11] summarised PCMs and commonly used heat transfer enhancement methods. Martin et al. [12] studied thermal storage using direct contact between the PCM and the HTF (heat transfer fluid). This required that the PCM is insoluble in the HTF, and that the difference in density is high enough to ensure a phase separation between the PCM and HTF. They investigated Paraffin/water storage and identified the flow rate, temperature difference, and paraffin drop size as key parameters to achieve high heat transfer rates for charging and dis- charging. However, they also reported that PCM–water bed expan- sion and trapped water and liquid PCM in the storage vessel reduces the storage capability of the system. Packaging of the PCM in smaller containers, for example PCM encapsulated in polymer material in the shape of balls, contained in a larger storage container is another approach. The heat transfer fluid is circulated in the void between the stacked containers to melt and solidify the PCM but packaging can be expensive and introduces a heat transfer resistance between the heat transfer fluid and the PCM. Another approach is to store the PCM in a con- tainer that includes pipes through which the heat transfer fluid is circulated to transfer heat to and from the PCM [13]. PCM slurries which have PCM in micro-size may be more suit- able for energy storage and heat transport in rapid cooling and heating applications, as they are fluid and pumpable during phase transition [14], which means the convection heat transfer is dom- inated instead of conduction heat transfer during solidification process. They also possess high thermal energy densities and high- er heat transfer capacities than single phase fluids due to the phase change process. Different PCM slurries have been considered for comfort cooling. Ice slurries have attracted significant attention because of the high latent heat of ice (334 kJ/kg) and low cost [15]. The 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.10.004 ⇑ Corresponding author. Tel.: +44 (0) 1895 267791; fax: +44 (0) 1895 269782. E-mail addresses: wei.lu@brunel.ac.uk (W. Lu), Savvas.Tassou@brunel.ac.uk (S.A. Tassou). Applied Energy 91 (2012) 366–374 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy