Review of passive PCM latent heat thermal energy storage systems towards buildings’ energy efficiency

Abstract

This paper aims to explore how and where phase change materials (PCMs) are used in passive latent heat thermal energy storage (LHTES) systems, and to present an overview of how these construction solutions are related to building's energy performance. A survey on research trends are firstly presented followed by the discussion of some physical and theoretical considerations about the building and the potential of integrating PCMs in construction elements. The different types of PCMs and main criteria that govern their selection are reviewed, as well as the main methods to measure PCMs’ thermal properties, and the techniques to incorporate PCMs into building elements. The numerical modeling of heat transfer with phase-change and heat transfer enhanced techniques are discussed, followed by a review of several passive LHTES systems with PCMs. Studies on dynamic simulation of energy in buildings (DSEB) incorporating PCMs are reviewed, mainly those supported by EnergyPlus, ESP-r and TRNSYS software tools. Lifecycle assessments, both environmental and economic are discussed. This review shows that passive construction solutions with PCMs provide the potential for reducing energy consumption for heating and cooling due to the load reduction/shifting, and for increasing indoor thermal comfort due to the reduced indoor temperature fluctuations.

Introduction

The energy efficiency of buildings is today a prime objective for energy policy at regional, national and international levels [1]. Buildings are one of the leading sectors in the energy consumption in the developed countries. Taking the EU as an example, the buildings sector accounts for around 40% of the total final energy consumption and produces nearly 40% of the total CO₂ emissions [2], [3]. Most of it is due to the increase in the living standard and in occupants’ comfort demands, mainly for heating and cooling. In non-sustainable approaches, buildings are increasingly dependent on active systems to ensure indoor thermal comfort. This produces an increase in the energy consumption as well as an increase in the associated greenhouse gas emissions. Consequentially, there is a surge in the operational phase cost of buildings. The reduction of the energy consumption and the improvement of the energy conservation in buildings are crucial on promoting energy efficiency and sustainability of buildings. Furthermore, the enforcement of the use of renewable energy sources is decisive concerning the reduction of buildings’ energy dependency.

Nowadays, thermal energy storage (TES) systems could be used to reduce buildings’ dependency on fossil fuels, to contribute to a more efficient environmentally energy use and to supply heat reliably. The main advantage of using thermal storage is that it can contribute to match supply and demand when they do not coincide in time [4]. The best known method of TES in buildings involves sensible heat storage by changing the temperature of a storage material. It can be used for the storage and release of thermal energy in a passive way but in comparison with latent heat storage, by changing the phase of a storage material, a much larger volume of material is required to store the same amount of energy. Hence, an effective way to reduce the buildings’ energy consumption for heating and cooling is by incorporating PCMs in passive LHTES systems of building's walls, windows, ceilings or floors. Here “passive” means that the phase-change processes occur without resorting to mechanical equipment. As suggested by Sadineni et al. [5], environmental-friendly passive building energy efficiency strategies are viable solutions to the problems of energy crisis and environmental pollution.

PCMs provide a large heat capacity over a limited temperature range and they could act like an almost isothermal reservoir of heat. As the temperature increases, PCMs change phase from solid to liquid. Since this reaction is endothermic, they absorb heat. When the temperature decreases, PCMs change phase from liquid to solid. This time they release heat, since this reaction is exothermic. The principle of PCMs use is very simple, but evaluating the effective contribution of the latent heat loads in the enhancement of the energy performance of the whole building is a challenge. The optimization of integrating PCMs within passive LHTES systems and the optimal integration of these systems within the building is complex. This is including the major design parameters, namely the PCMs phase-change temperature, its thermal mass quantity and its position within the LHTES system or, the position of the passive LHTES system within the building. Moreover, such parameters need to be specified for given indoor loads and also for specific climatic conditions. Therefore, the approach for the assessment of the potential of PCMs in buildings’ design should be different for residential buildings or service buildings or even high or low thermal inertia buildings. The approach should also be different if the inclusion of PCMs is optimized to reduce the summer cooling loads or the winter heating loads.

The main goal of this paper is to provide a comprehensive review on previous studies concerning the investigation and optimization of passive LHTES systems with PCMs, the lifecycle assessments, both environmental and economic, and the evaluation of dynamic characteristics and energy performance of buildings with those systems to present the state-of-the-art.