Preparation and characterization of phase change materials for energy efficiency in white good applications

Nowadays, there is considerable demand for phase change materials (PCMs) within a wide temperature range to accomplish the energy efficiency in many areas. PCMs offer tremendous potential to fulfill the growing energy needs for cooling and heating applications across various industries including construction, commercial refrigeration, textiles, transportation packaging for temperature-sensitive products, several solar energy based systems, electronics and biomedical materials. The use of different PCM technologies is expected to improve the global efforts to conserve energy in the wake of fast depleting fossil fuels. However, effective integration of PCMs in end-products remains a challenging task. The increasing demand for energy-saving and ecologically friendly systems is driving the growth of the global PCM market. The paraffin-based PCM market commands the largest share of the overall PCM market in terms of value, while salt hydrate-based PCMs lead the market in terms of volume. PCMs that are used for the storage of thermal energy as sensible and latent heats are an important class of modern materials which substantially contribute to the efficient use and conservation of waste heat and solar energy. The storage of latent heat enables them to have considerably higher thermal energy storage densities with a smaller temperature difference between storing and releasing heat compared to sensible heat storage materials. The basic operation principles of solid liquid PCMs is based on that PCMs absorb heat with increasing in temperature until phase change temperature, heat absorption at phase change from solid to liquid occurs at almost constant temperature and the same process repeats with release of heat during the cooling process and liquid completely converts into solid. In this research, studies have been conducted in four different groups of which three are related with the development of suitable PCM formulations by using fatty acids, polyethylene glycols (PEGs) and salt hydrates. Another one is conserned with increasing the thermal performance of phase change materials by the way of adding some high thermal conductivity materials. According to the theoritical knowledge, organic phase change materials have more remarkable properties in terms of working life and performance. And inorganic phase change materials also have disadvantages such as supercooling and phase separation. But these disadvantages're dissolved by the adding some agents in the salt hydrate formulations. In the light of this notion, the choice of raw materials was evaluated taking account of the temperature of developed phase change materal which is desired between 25°C to 40°C. 44 different fatty acid, polyethylene glycol and salt hydrate based formulations were prepared and their thermal characterization measurements're done. Formulations for using at prototype're chosen according to application set-up temperature, chemicals' melting temperature, latent heats and thermal conductivity values. Then, these chosen formulations're used for performans measurements at both premise and advanced prototype. Besides, the effects of adding high thermal conductivity materials such as graphite on the thermal performance at advanced prototype have been observed by two selected formulations. The findings from this study are introduced below: • Short-life problems of salt hydrates dissolved by the way of adding thickening agent and and nucleating agent. Also, 1650th cycle by Glaubert C salt-based formulation, 1000th cycle by Glaubert D salt-based formulation reached. • Capric acid – palmitic acid formulations have shown wide range melting and crystallization. Moreover, lactic acid – palmitic acid formulations have shown two phase mixing by not dissolving in each other. Because of these problems, these formulations cannot be used. • >40oC melting temperature formulations such as YA_15, YA_6 – YA_11 cannot be considered as suitable. They should be used for high temperature applications, dryer and bakery. • PEG400–PEG1500 formulations have shown two step melting and crystallization because of their phase change characteristic. Most of PEG300–PEG1500 formulations demonstrate one step solid liquid phase change except PEG_4 and PEG_9. They also have shown wide range peaks. Due to the fact that, for dishwasher applications require more regular melting behavior, these formulations cannot be considered as suitable for white good - home appliances. • The melting and crystallization temperatures of caprylic acid-stearic acid eutectic mixtures from YA_18 to YA_22 are approximately 10oC. Although, because of this situation, they cannot be used direct usage in refrigerator properly, especially when they are frost utterly in freezer, they can be used in fresh food cabinet if power outage is existed. This group can be extended the decay time of foods and beverages. • Within the context of this study, it observed that tetradecane – pentadecane mixtures become prominent for home appliances with low operating temperature. • Mixtures of PEG, fatty acid and salt hydrate based phase change materials with relatively high phase change temperature (between 25 - 40°C) can be utilized for white good applications such as in heating rinse water of dishwasher. • Selected formulations which have melting-crystallization phase change temperature (between 25-40°C) are YA_17, YA_23 and YA_25 as fatty acid mixtures, PEG_1 and PEG_6 as polyethylene glycol mixtures, TH_6 and TH_7 as salt hydrate mixtures. • As a result of comparison of premise and advanced prototype, minimum 50 percent of performance raise due to usage of advanced prototype. Performance of premise prototype is increased by minimizing of heat losses and improving the heat exchange rates. • Fatty acids removed the adhesives of prototype after a few repetitive experiments. On account of the fact that fatty acids and salt hydrates are corrosive, the non-corrosive, low-cost PEGs should be used for dishwashing applications. • When we consider the time of transferring of the stored energy from main wash water on phase change material, Croda_21 and Croda_29 coded commercial samples have shown better performance than other benchmark samples. These Croda samples are fatty acid and their esters' structured. But while considering without time of transferring the stored energy, Croda_29 and capric acid commercial PCMs, which have phase transition around 30oC, provide higher energy saving values. • Although graphite reinforced PEG formulations (PEG_1, PEG_6) have shown smaller heat of fusion and melting temperature values, stored energy quantities of them increase. The thickening agent which is added for graphite reinforced PEG formulations works as a crosslinked medium when graphite material additives are added. Thickening agents and graphite addings decrease both melting enthalpies and melting temperatures of formulations due to that mass fractions of latent heat absorbing parts decrease in the overall PCM formulation. • The overall thermal performance of the prepared reinforced-PCM was further enhanced with graphites with mass fraction of 1%. With improving of the heat transfer coefficient depanding of adding graphites, duration of energy storage diminishs and the losses of energy minimize. This results are supplied from kinetic effects. • While the highest energy saving value can be achieved with YA_23 formulation, the lowest savings can be handled with PEG_6. These energy values are calculated independent of the duration of the experiment. • Between the samples of PEG_1/GF, PEG_6/GF and TH_7 changing phase at the same temperature, TH_7 which has the highest heat of fusion value supply the highest amount of energy storage. • Fatty acids and salt hydrates cause permanent damages to the advanced prototype. For example, they remove adhesives and corrode copper plate especially at points where silicones are after several repetitive experiments. Because of these damages, using of non-corrosive and cheap PEGs has a primary priority. • Fatty acidic formulations like as YA_23 and YA_25 with the highest energy values may be preferred as a result of performance measurements. However, when their negative and corrosive effects on prototype are considered, graphite reinforced PEG_1/GF formulation preferred with regard to both in terms of test process and amount of stored energy. At the same time, these PEG formulations have cost advantages compared to fatty acids. • In the cycle of a dishwasher, 923 Wh energy is consumed. After experiments in large volumes approximately 2,5% energy saving will be possible with selected PEG_1/GF formulation as a result of preliminary studies if the tap water received in the system reaches the same temperature.