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There are three main alternatives for the improvement of heat transfer in phase change materials (PCM) such as the use of additives, support matrices and customized designs for heat exchangers. The first two alternatives imply incurring additional material costs, as well as a decrease in the specific heat storage capacity by using components that remain solid during the process. The present work therefore sought to study the effect of the use of a coil shell heat exchanger with a square cross section on natural convection during the melting process of hydrogenated palm stearin. Three different sections of the profile called square profile, U-profile, and L-profile and two types of material, copper, and stainless steel, were studied. A test bench was built with a capacity of 5 kg of this PCM, implementing a hot water recirculation system with flow and temperature measurement and including 9 temperature sensors inside the PCM. The melting process was conducted by recirculating water with a temperature of 75° C and a flow rate of 2 L/min. The results show a marked effect of natural convection on the melting process, presenting characteristic profiles for this phenomenon. It was found that the highest temperatures are recorded in the upper part of the PCM and that the heat transfer in the solid material is appreciably low. The results allow us to identify for each profile, the zones in which heat transfer is relatively high because of natural convection and those zones that require the use of additives or support matrices. It is therefore expected to contribute to the design of more efficient and lower commercial cost configurations. [ABSTRACT FROM AUTHOR]

The search for new phase change materials (PCM) of renewable origin and low cost, as an alternative to the use of paraffin wax, contributes to the environmental resilience of solar thermal energy systems. An important source of PCMs of renewable origin are vegetable oils and fats, especially those with established agro-industrial chains. Globally, palm oil has positioned as an indispensable product in many industrial sectors. One of the main by-products of palm oil refining is palm stearin, which can be hydrogenated to improve its thermal properties. This paper presents the results of the performance of a Solar thermal energy storage system (STESS) located in a Colombian paramo area with an altitude of 3,200 m.a.s.l. The system store heat in 550 kg of Hydrogenated Palm Stearin (HPS), contained in a rectangular vessel, which allow to supply hot water and space heating for a country house. The average ambient temperature is 12°C during the day and 7°C at night. This experimental station implements a robust measurement and control system including 39 PT-100 type temperature sensors, and different actuators connected to a control system based on Arduino and Raspberry Pi devices. This configuration permit also to implement a remote monitoring system. The constructed STESS employs 140 collector tubes, running at a maximum temperature of 95°C. According to the energy load tests during a period of 7 hours of operation, approximately 40.0 MJ were stored as heat in the hydrogenated palm stearin. For the same period, the heat transfer fluid received approximately 170 MJ of energy. In subsequent discharge tests, the heat stored in the HPS allowed the ambient temperature inside the room to be maintained 8°C above the external ambient temperature for up to 8 hours during the night, demonstrating the technical feasibility of using this new PCM from renewable sources. [ABSTRACT FROM AUTHOR]