Phase Change Material Melting Process in a Thermal Energy Storage System for Applications in Buildings

Túlio Nascimento Porto,Balbina Brito Correia,João M P Q Delgado,Hortência Luma Fernandes Magalhães,Antonio Gilson Barbosa De Lima,Ana Sofia Guimarães,Gicelia Moreira,Tony Freire De Andrade

doi:10.3390/en13123254

Túlio Nascimento Porto, Balbina Brito Correia + Show 6 more

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https://doi.org/10.3390/en13123254

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Abstract

The development of thermal energy storage systems is a possible solution in the search for reductions in the difference between the global energy supply and demand. In this context, the ability of some materials, the so-called phase change materials (PCMs), to absorb and release large amounts of energy under specific periods and operating conditions has been verified. The applications of these materials are limited due to their low thermal conductivity, and thus, it is necessary to associate them with high-conductivity materials, such as metals, to make the control of energy absorption and release times possible. Bearing this in mind, this paper presents a numerical analysis of the melting process of a PCM into a triplex tube heat exchanger (TTHX) with finned copper tubes, which allowed for the heat transfer between a heating fluid (water) and the phase change material to power a liquid-desiccant air conditioning system. Through the analysis of the temperature fields, liquid fractions, and velocities, as well as the phase transition, it was possible to describe the material charging process; then, the results were compared with experimental data, which are available in the specialized literature, and presented mean errors of less than 10%. The total required time to completely melt the PCM was about 105.5 min with the water being injected into the TTHX at a flow rate of 8.3 L/min and a temperature of 90 °C. It was observed that the latent energy that accumulated during the melting process was 1330 kJ, while the accumulated sensitive energy was 835 kJ. The average heat flux at the internal surface of the inner tube was about 3 times higher than the average heat flux at the outer surface of the TTHX intermediate tube due to the velocity gradients that developed in the internal part of the heat exchanger, and was about 10 times more intense than those observed in the external region of the equipment.

Highlights

The development of a country is intrinsically connected with the energy demand; the scarcity of energy sources justifies the improvement of studies that enable better use of the sources that are still available to humanity
The results revealed that the geometry of the triplex tube heat exchanger (TTHX) with eight fins achieved a 34.7% reduction in the time required for the complete phase change materials (PCMs) fusion compared with that of the triplex tube without fins, that the complete melting time required for the case with 42 mm length in the fins with a configuration of eight fins produced a reduction of 37.5% relative to the case with 10 mm fins, and that the use of copper instead steel reduced the complete melting time by 42.8%
The proposed physical study was the analysis of the PCM (RT82) fusion process, which exchanges heat with the water through a TTHX made of copper tubes with eight longitudinal fins with a 1 mm thickness and mm length with the Energies 2020, 13,42 x FOR

Summary

Introduction

The development of a country is intrinsically connected with the energy demand; the scarcity of energy sources justifies the improvement of studies that enable better use of the sources that are still available to humanity. The scarcity and intermittency of renewable energy sources, such as wind and solar energy, and the pollution produced by the use of fuels, such as oil, which is the fundamental base of the global energy matrix, motivates the search for ways to optimize energy processes. In this sense, the development of a thermal energy storage system provides an important contribution in the search for reductions in the difference between the global energy supply and demand, as well as in the search for greater thermal efficiency in thermodynamic processes. This storage can occur through the heat from reversible chemical reactions; sensitive heat, which allows liquid or solid media to increase its internal energy by raising its temperature; and latent heat, which is a characteristic of some materials such that when subjected to thermal changes, the state of the matter that composes them is modified, increasing their internal energy and keeping their temperature almost constant

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