Abstract

Phase change materials (PCMs) are materials that can store large amounts of heat during their phase transition from solid to liquid without a significant increase in temperature. While going from liquid to solid this heat is again released. As such, these materials can play an important role in future energy-efficient buildings. If applied in facades as part of a thermal buffer strategy, e.g., capturing and temporarily storing solar energy in so-called Trombe walls, the PCMs are exposed to high solar radiation intensities, which may easily lead to issues of overheating. This paper therefore investigates the melting process of PCM and arrives at potential solutions for countering this overheating phenomenon. This study uses the simulation program Comsol to investigate the heat transfer through, melting of and fluid flow inside a block of PCM (3 × 20 cm2) with a melting temperature of around 25 °C. The density, specific heat and dynamic viscosity of the PCM are modeled as a temperature dependent variable. The latent heat of the PCM is modeled as part of the specific heat. One side of the block of PCM is exposed to a heat flux of 300 W/m2. The simulations show that once part of the PCM has melted convection arises transporting heat from the bottom of the block to its top. As a result, the top heats up faster than the bottom speeding up the melting process there. Furthermore, in high columns of PCM a large temperature gradient may arise due to this phenomenon. Segmenting a large volume of PCM into smaller volumes in height limits this convection thereby reducing the temperature gradient along the height of the block. Moreover, using PCMs with different melting temperature along the height of a block of PCM allows for controlling the speed with which a certain part of the PCM block starts melting. Segmenting the block of PCM using PCMs with different melting temperature along its height was found to give the most promising results for minimizing this overheating effect. Selecting the optimal phase change temperatures however is critical in that case.

Highlights

  • In order to mitigate climate change and move towards a society based on renewable energy sources, the primary energy consumption of societies needs to be significantly reduced

  • Buildings are responsible for a large share of this primary energy consumption, which in the European Union is in the order of 40% [1]

  • Space heating and space cooling are the dominant contributors to this building-related energy consumption

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Summary

Introduction

In order to mitigate climate change and move towards a society based on renewable energy sources, the primary (fossil) energy consumption of societies needs to be significantly reduced. Buildings are responsible for a large share of this primary energy consumption, which in the European Union is in the order of 40% [1]. Like phase change materials, may play a pivotal role in such future high performance buildings. Phase change materials are materials that can store large quantities of heat in the transition from solid to liquid without significant change in temperature. When they cycle from liquid to solid this heat will again be released. Paraffins are Energies 2019, 12, 3286; doi:10.3390/en12173286 www.mdpi.com/journal/energies

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