ReviewReview of passive PCM latent heat thermal energy storage systems towards buildings’ energy efficiency
Highlights
► Review of PCM passive LHTES systems to improve the energy efficiency of buildings. ► PCMs for different applications, buildings characteristics and climatic conditions. ► Survey on the potential of including PCMs into construction materials and elements. ► Survey on DSEB studies with PCMs supported by EnergyPlus, ESP-r and TRNSYS tools. ► Review on environmental and economic lifecycles assessments of the inclusion of PCMs.
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.
Section snippets
Review of research trends
The number of articles concerning the integration of PCMs in buildings to improve their energy efficiency has been increasing during the last decade. Before 2003 only 2 review articles on this subject are found in the literature. During the last years more comprehensive and particular reviews of PCM latent heat systems and their applications have been made, and more than 20 extensive review articles about the potential of integrating PCMs in buildings were published, allowing to conclude that
Building as a thermodynamic system
In a sustainable approach, buildings should be designed to ensure thermal comfort of occupants during the whole year, with a minimum auxiliary energy for heating and cooling. If the storage and insulation properties of the building envelope have a suitable role in the delay and decay of outdoor temperature fluctuation, the indoor air temperature could stay in a comfortable range without heating and/or cooling [35] and a passive ideal energy conservation building with a passive ideal energy
Types of PCMs and main criteria that govern their selection
Materials to be used for phase-change TES should have melting/freezing temperature in the practical range of application and they must have a high latent heat of fusion and a high thermal conductivity. Moreover, to be used in the design of passive LHTES systems, PCMs should have desirable thermophysical, kinetic, chemical and economic properties as suggested by many authors [13], [14], [22], [24], [26], [29], [32], [34]. PCMs should also have desirable environmental properties to decrease the
Incorporation of PCMs into building elements
Once the PCMs have been selected, based primarily on the temperature range of application and on their thermophysical properties, it is important to evaluate how they could be incorporated within passive LHTES systems (construction materials or building elements) to prevent leakage. Hawes et al. [61] considered the direct incorporation, the immersion and the encapsulation to be the 3 most promising methods of incorporating PCMs in conventional construction materials. Additionally, PCMs can also
Overview of the main PCM passive LHTES systems for building applications
In this section an overview of the main passive LHTES systems with PCMs for building applications is made. Research gaps and future outlook were also pointed out in Section 6.8.
Dynamic simulation of energy in buildings with PCMs
A good knowledge on the dynamic energy performance of buildings incorporating passive LHTES systems with PCMs is essential for building researchers and practitioners to better understand the buildings temperature response characteristics and the economic feasibility of using PCMs. This knowledge is also important to take further proper actions to fully utilize PCMs to enhance indoor thermal comfort and the overall energy efficiency of buildings. Nowadays, there are many building energy
Framework
The lifecycle of a building includes the extraction of raw materials; processing of raw materials into building materials; assembly of materials into a ready building; occupation or use; maintenance; demolition or disassembly of the building; and disposal or re-use of the materials (transport of materials is involved in several phases) [192]. Hence, buildings demand energy in their lifecycle, both directly and indirectly [193]. Gervásio et al. [194] stated that 2 main factors contribute for the
Economic impact analysis
In its “Advanced Phase Change Material Market: Global Forecast (2010–2015)” [223] MarketsandMarkets reports that the increasing demand for energy-saving and environment-friendly technology is driving the growth of the global PCM market. Furthermore, they stated that the global PCM market is expected to grow from $300.8 million in 2009 to $1488.1 million in 2015, at an estimated Compound Annual Growth Rate of 31.7% from 2010 to 2015. Another study presented the economic feasibility of using PCM
Contribution of PCM passive LHTES systems towards NZEBs
Nowadays there is a significant potential for cost-effective energy savings in the buildings sector that would lead to significant economic, social and environmental benefits. To address the buildings sector, EU regulators have published the Energy Performance of Buildings Directive (EPBD) [228], and its recast [229]. The EPBD mainly focuses on reducing the operational energy consumption of buildings, but it also establish that by 2020, every new building in the EU must be a “nearly-zero”
Conclusion
This paper provides a comprehensive review on previous studies related to the evaluation of how, and where, PCMs are used in passive LHTES systems, and how these construction solutions are related to building's energy efficiency. It was concluded that PCM passive LHTES systems can contribute to (i) increase indoor thermal comfort (air temperature peak reduction, decrease of daily temperature swing, changing in the surface temperature); (ii) improve buildings envelope performance and to increase
Acknowledgement
The first author acknowledges the financial support provided by the Portuguese Foundation for Science and Technology (FCT) under the grant SFRH/BD/51640/2011.
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