Experimental and theoretical study of a vertical tube in shell storage unit with biodegradable PCM for low temperature thermal energy storage applications

Abstract

This article presents the experimental investigations of the coconut oil-based TES module for HVAC applications in the ambient and-sub ambient temperature range. To properly study this problem modular experimental module and test loop were developed. Special attention has been paid to study the physical mechanism of the melting/solidification process for natural substance (coconut oil) which has perspectives to be used in thermal energy storage systems. It has been a highlight that the melting/solidification process is much more complex for non-eutectic mixtures and there are limited analytical models witch could be used to design TES working with such substances. Due to the limit of literature data for thermophysical properties of coconut oil own experimental data has been conducted.The article presents both visual and thermal characteristics of the melting and solidification process. It has been confirmed the strong influence of natural convection phenomena at the melting process. What is more, it has been observed that in the case of coconut oil it is better to talk about appearance melting temperature than melting temperature. The experimental analysis confirmed that appearance melting temperature is a function of radial distance from the heat source. In the case of solidification, the influence of natural convection is reduced quickly due to the increasing domination of the conduction heat transfer mechanism. For the solidification temperature profiles are characteristic of the occurrence of subcooling temperature. The level of temperature drop in sub cooling is a function of radial distance from the heat source. It has been also confirmed that in case of melting as well as solidification process the shape of the phase-change profile seems to form a truncated cone (in case of melting inverted).To validate the engineering approach to vast discrepancies in the literature data own experiment has been evaluated with a simple mathematical model to verify the applicability of the literature data in system behavior prediction. Based on the best-fit approach the parameters thermal resistance of the module R = 1.3 W/K was found. Additionally the latent heat of the coconut oil of 178 kJ/kg with Gaussian distribution (σ=1.1) for melting in temperature of 25 °C and solidification at 21 °C. The proposed correlation can be easily used to optimize latent thermal energy storage cooperated with the HVAC system.