REVIEW ON ENHANCING TECHNOLOGIES OF HEAT TRANSFER THROUGH PHASE CHANGE MATERIALS

This review concentrates deeply on the recent studies performed on the phase change materials (PCMs) in order to enhance their performance. Most of these enhancing techniques fall in the following areas of interest (i) Development of  new PCMs with promising thermo-physical properties. The researches recommend some promising substances to be used as a PCM such as pure inorganic salts (low cost chlorides, nitrates and carbonates), salt eutectics (KNO3–LiNO3 and KNO3–NaNO3–LiNO3), liquid metals (Gallium), esters, nanofluids (Al2O3/water, CuO/water and alumina-water), and polyols (sugar alcohol like polyethelene glycol, erythritol and xylitol).  (ii) Introducing of composite mixtures to exaggerate the benefits of used materials. Composites could be obtained by mixing PCMs like fatty acid esters with building materials like concrete pavements, cement
or gypsum or with isolating materials like perlite to keep satisfying room temperature for long time. PCMs could be mixed or be used in cascaded configuration layered and ordered according to their melting temperature. Good conducting materials like graphite and aluminum were introduced to be mixed with PCMs to enhance the thermal conductivity of PCM. Stable materials like Diatomite and silica were introduced to enhance the stability of the PCM to retain its properties for a lot of melting/freezing cycles. Also, nano particles of TiO2, ZnO, CuO, and Silver-Titania, are added to generally enhance the PCM thermo-physical properties. (iii) Configuration of PCM container to achieve higher heat transfer rates by changing the aspect ratio or by adding fins. The studies focused on adding internal fins, external fins, and increasing the aspect ratio of the PCM vertical container to induce natural convection. Experiments advise to focus on increasing the number and height of fins more than focusing of fins thickness. (iv) Encapsulation technology of PCM to provide the largest heat transfer surface and avoid leakage when in liquid state as well as allowing larger surface area for heat transfer and protection in handling hazardous materials. It was observed that the core-to-coating ratio plays an important role in deciding the thermal and structural stability of the encapsulated PCM. An increased core-to-coating ratio results in a weak encapsulation, whereas, the amount of PCM and hence the heat storage capacity decreases with a decreased core-to-coating ratio. Among all the microencapsulation methods, the most common methods described in the literature for the production of microencapsulated phase change materials (MEPCMs) are interfacial polymerization, suspension polymerization, coacervation, emulsion polymerization, and spray drying.