Novel concept of composite phase change material wall system for year-round thermal energy savings

Abstract

A new type of composite wall system incorporating phase change materials (PCMs) was proposed and its potential for air conditioning/heating energy savings in continental temperate climate was evaluated. The novelty of the wall system consists of the fact that two PCM wallboards, impregnated with different PCMs are used. The structure of the new wall system is that of a three-layer sandwich-type insulating panel with outer layers consisting of PCM wallboards and middle layer conventional thermal insulation. The PCM wallboard layers have different functions: the external layer has a higher value of the PCM melting point and it is active during hot season and the internal layer with a PCM melting point near set point temperature for heating is active during cold season. A year-round simulation of a room built using the new wall system was carried out and the effect of PCM presence into the structure of the wall system was assessed. It was found that the new wall system contributes to annual energy savings and reduces the peak value of the cooling/heating loads. The melting point values for the two PCMs resulting in the highest value of the energy savings were identified.

Introduction

Heat transfer of the built environment through the building envelope has a significant weight in the overall energy balance contributing to a high extent to the cooling and heating load. Passive methods used for the thermal energy management of the built environment include the thermal mass, which can contribute to downsizing of the AC/heating equipment and reducing the AC/heating demand. In addition, increasing the thermal mass of the built environment can contribute to increasing the indoor thermal comfort.

The use of PCMs in building elements with the purpose of increasing the thermal mass was extensively studied [1], [2], [3], [4]. A review of PCMs application in the built environment can be found in [5]. PCMs can be integrated into building materials or prefabricated building elements such as concrete, gypsum wallboards, plaster, etc.

Gypsum wallboards incorporating PCMs such as fatty acids [6], mixture of lauric and myristic acids [7], eutectic mixtures of capric acid and lauric acid [8], mixture of capric acid and stearic acid [9], butyl stearate [1], etc. A key issue in developing building materials and building elements incorporating PCMs is the integration of the PCM into the structure of the matrix material. The method of incorporation depends mainly on the nature of the building material. Khudhair and Farid [10] described the most commonly used methods for integration of PCMs into the building materials and building elements.

Feldman et al. [11] used immersion of gypsum wallboards into a mixture of 93–95 wt% commercial Methyl Palmitate with 7–5 wt% commercial Methyl Stearate at 45 °C for approximately 25 s. A PCM mass absorption degree of approximately 24% was found. The thermo-physical properties of the resulting impregnated wallboard were assessed by means of DSC analysis. A phase transition temperature range of approximately 2 °C and a latent heat capacity varying between 35 and 55 kJ/kg were found. Rozanna et al. [6] used immersion of gypsum boards with the thickness values of 6 and 12.5 mm into an eutectic mixture of lauric and stearic acids for 1 h. A mass absorption degree of 27–28.5% and a maximum value of the latent heat of 56 kJ/kg were identified. The phase transition process was affected by a hysteresis-type behaviour in the sense that the melting temperature range was different from solidification temperature range. Sari et al. [9] developed a phase change wallboard by immersing gypsum wallboards into an eutectic mixture of commercial 83 wt% capric acid and 17 wt% lauric acid for approximately 1 h. A mass absorption degree of 25 wt% was found and no separation of the PCM from the matrix material was observed after 5000 melting/solidification cycles. Shilei et al. [12] used the gypsum wallboard immersion into a mixture of 82% capric acid and 18% lauric acid as a method of PCM incorporation. The immersion of the gypsum wallboards into the liquid PCM was maintained for 6–10 min achieving a PCM absorption degree of approximately 26 wt%.

Feldman and Banu [13] developed PCM wallboards by incorporating various PCMs directly into the gypsum paste prepared in a way similar to production of commercial wallboards. A PCM mass percentage of the resulting PCM wallboard was between 20 and 25 wt%. It was found that the PCM incorporation at the time of mixing resulted in a more even PCM distribution in the wallboard.

Zhang et al. [14] described a method of PCM incorporation into concrete. It was found that the thermo-physical properties of the concrete containing PCM (butyl stearate), including the latent heat and phase transition temperature range, were influenced to an important extent by the physical properties of the aggregates (especially porosity).

Schossig et al. [15] investigated the incorporation of PCM microcapsules into gypsum wallboard. PCM microencapsulation prevents the interaction between the matrix material and the PCM that could change the properties of the first. Another advantage of PCM microencapsulation is leakage prevention that could occur during the lifetime of the building element. An attenuation of the indoor temperature fluctuations of approximately 2 °C was found by means of simulations compared to the case of the walls without PCM microcapsules.

The potential of PCM building materials of energy saving and improving the indoor thermal comfort was confirmed by both experimental and numerical studies.

Darkwa and Callaghan [16] carried out a simulation of a passive room with PCM drywalls modelling the phase transition process by an increase of the effective heat capacity around the PCM melting point. A Gaussian-type variation of the effective heat capacity with various values of the phase transition temperature range was considered. Two methods of PCM integration into the building materials were considered: laminated PCM boards and randomly mixed PCM boards. An attenuation of the indoor temperature profile of approximately 2 °C was found compared to an identical room built of drywalls without PCM. Ismail and Castro [4] conducted a simulation of a three-layer wall with the exterior layers consisting of conventional building materials and the middle layer consisting of PCM. It was found that the presence of the PCM in the structure of the exterior wall resulted in downsizing of the heating equipment and decreasing the energy demand for AC. Neeper [17] conducted a simulation of a wallboard with latent storage modelling the effective heat capacity of the PCM using a Gaussian function. It was found that the maximum diurnal energy storage occurs when the PCM melting point equals the average room temperature (for a sinusoidal variation of the room temperature) and for a narrow phase transition temperature range. Xu et al. [18] investigated numerically and experimentally a test room with shape-stabilized PCM floor. It was found that the suitable PCM melting point should be approximately equal to the average indoor temperature during winter sunny days. Athienitis et al. [1] developed a mathematical model for the transient heat conduction through a PCM-gypsum board. The simulation results were compared with experimental measurements obtaining satisfactory agreement, which indicated that the explicit one-dimensional non-linear finite difference model can be used successfully in simulation of the PCM-gypsum wallboard room.

Composite wall systems incorporating PCMs were investigated by Pasupathy and Velraj [19]. Chen et al. [20], Zhou et al. [21], Kuznik and Virgone [22], Ahmad et al. [23], Halford and Boehm [24], Darkwa [25], Carbonari et al. [26].

Modelling thermo-physical properties of PCMs poses some difficulties. Diaconu et al. [27] determined experimentally the apparent heat capacity of a microencapsulated PCM slurry by means of DSC. It was found that the apparent heat capacity was DSC scanning rate dependent and it followed a hysteresis-type pattern. A key conclusion was that the hysteresis of the microencapsulated PCM slurry H–t curve made impossible to predict the variation of enthalpy with temperature. Moreover, it was found that the magnitude of the hysteresis in the H–t curve was DSC scanning rate dependent. Complex and unpredictable behaviour of the H–t curves was found in cases in which temperature did not sweep completely the phase transition temperature range.

Arkar and Medved [28] used DSC to determine the thermo-physical properties of a paraffin type (RT20). It was found that the scanning rate influences to a significant extent both thermo-physical properties and the phase transition temperature range. To the authors’ knowledge, an accurate analytical model of the transient heat conduction in a PCM impregnated building material was not developed so far. Various simplifying assumptions were made in the existing models [17], [29]. The enthalpy method [16], [29], [20] is widely used for modelling the transient heat conduction in PCM wallboards.

While it is generally agreed that passive use of PCMs in buildings improves thermal comfort and reduces global energy consumption [5], results concerning the actual values of energy savings for heating and air conditioning are scarcely available in the literature. A synthesis of the results found in the literature concerning energy savings and peak load reduction resulting form application of various PCM systems in the built environment is given in Table 1.

A new type of PCM composite wall system for year-round thermal energy management in the built environment is proposed in this paper. The novelty of the concept consists of the fact that two different PCMs with different values of the thermo-physical properties are integrated into the structure of the wall system. The key thermo-physical properties that influence the efficiency of the new wall system were the PCM melting point and latent heat.