Thermo-Physical and Performance Analysis of Bio-Based Phase Change Material for Medium Temperature Latent Heat Thermal Energy Storage Applications

Abstract

It is well known that present and immediate future policies are focused on reducing fossil fuel consumption and thus promoting renewable energy sources for heating, cooling, and domestic hot water production. Thermal energy storage (TES) is essential to match production and demand and, therefore, provide heat or cold to consumers when required independently of when it was obtained. High energy density TES systems employ phase change materials (PCM) as storage mediums, where thermal energy is mainly stored in the form of latent heat, the so-called Latent heat thermal energy storage (LHTES) system. The most used phase change material today is paraffin. However, paraffin is highly flammable, which limits its applications. Paraffin is also a by-product of fossil fuel which implies that paraffin is a non-eco-friendly material. Only a few existing studies have investigated the thermophysical properties of Bio-PCMs (BPCM) in this detailed manner. The present research on the thermo-physical properties and performance of environmentally friendly and less flammable phase change material for medium temperature latent heat thermal energy storage applications. Studying the effect of aspect ratio on the PCM characterization in the T-history method (THM). Conventional characterization of BPCM thermo-physical properties and compared the differential scanning calorimetry (DSC) results with the non-conventional THM. Based on the thermo-physical properties, developing the experimental and numerical model of shell and tube LHTES system. The second fold of this study includes the performance of BPCM in the LHTES system by varying the mass flow rate and inlet temperature of heat transfer fluid (HTF). Adopting the cascaded heat transfer enhancement technique and numerically comparing the proposed tapered geometry performance with the existing cylindrical unit. Studying the effect of aspect ratio on characterizing the PCMs in the T-history method (THM) concludes that the application of high aspect ratio tubes minimizes the error. Characterizing the BPCM (OM55 and Shellac wax) thermo-physical properties using the conventional and non-conventional methods, concluding the OM55 and Shellac wax as potential BPCM for medium temperature LHTES applications. Experimental analysis was carried out to study the performance of the shellac wax in a vertical shell-tube LHTES system by varying the mass flow rate and inlet temperature of heat transfer fluid (HTF). The optimum flow rate and inlet temperature were obtained as 4 LPM and 120 °C, respectively, yielding a charging efficiency of 73.4%, discharging efficiency of 62.6%, and maximum reduction in the melting rate of 43.6%. It is evident from the experimental study that shellac wax is a potential bio-PCM for the medium temperature range (60-90 °C) applications. Further, the numerical comparison of tapered type shell and tube cascaded latent heat storage (CLHS) with that of the existing cylindrical CLHS model. The mean power for the tapered model during the charging cycle is 17.6% higher than the cylindrical, attributed to convection heat transfer's dominance during the melting process. The tapered CLHS model utilizes convection heat transfer effectively by enhancing the PCM's melting rate without additional structural configurations such as fins. Hence, higher energy storage for the same volume is economically justifiable compared to conventional cylindrical CLHS units.