Experimental apparatus and methodology to test and quantify thermal performance of micro and macro-encapsulated phase change materials in building envelope applications
Introduction
In United States, buildings use about 41% of primary energy. This energy consumption contributes to the electric peak energy demand and increases the need for more fossil fuel based peaking power plants [1]. About 15% of this electricity is used for building cooling. Furthermore, during the summer the non-coincident peak demand of electricity for cooling can be as high as 50%. There are different strategies to reduce peak electricity demand of buildings and overall reduction of building space conditioning energy such as: on site generation [2], smart controls [3] and distributed energy storage [4]. One way to increase building thermal energy storage is to use phase change materials (PCMs). Sun et al. [5] finds that simple load shifting control strategies with PCM can reduce peak load from 10% to 57%, overall cost savings (from 11% to 96.6%) and improved thermal comfort.
PCMs can be either organic, inorganic, or eutectic compounds [6,7]. Organic PCMs are primarily categorized as Paraffins and non-Paraffins [8,9]. Organic PCMs crystallize with little or no subcooling [10]. Non-paraffinic PCMs consist of fatty acids, fatty alcohols, esters, emulsifiers, and thickening agents [11,12]. Inorganic PCMs are mostly made of hydrate-salts or metallics [13]. Each type has its pros and cons based on cost, supercooling, and thermal stability [14], [15], [16]. In particular stability is important and many commercial PCMs must maintain its properties after 10,000-20,000 cycles to be applicable for buildings, which can last 50-100 years [17]. Both organic and inorganic PCMs are used in building applications [12,[18], [19], [20], [21], [22], [23], [24]]. PCMs are typically microencapsulated, macroencapsulated, or shape-stabilized to contain PCM in place [9,[25], [26], [27]]. Although this study focuses on PCM applications on the building envelope, its use is broad even in building applications, such as PCMs used in heat exchangers for cooling or heating applications [28] and water heaters [29,30]. On one side, microencapsulated PCMs can be mixed with other building materials such as concrete or gypsum drywall but microencapsulation tend to decrease PCM concentration and increase cost [31]. On the other side, macroencapsulated PCMs avoid additional costs related to microencapsulation and there are several building products already on the market. Macroencapsulated PCM implementation in the wall are passive or active [32], [33], [34], [35], [36]. Most popular PCM application is to use macroencapsulated PCM in pouches behind drywall or above false ceiling [[22], [23], [24],[37], [38], [39], [40], [41]]. PCMs generally have low thermal conductivity [42]. The container/pouch material for macroencapsulation can be selected to increase the thermal conductivity [17,43].
Macroencapsulated PCMs experimental studies are done at material scale [44], system scale [45], and whole building scale [46]. Previous studies with macroencapsulated bio-based PCM (BioPCM) at system scale level investigate the impact of the PCMs on the temperature oscillations in the interior of the building [20,21], and the impact PCMs have on the operative temperature and cooling power demand of an office cubicle in a full-scale test room [12]. These studies conclude that macroencapsulated PCMs have greater ability to reduce indoor air temperature oscillations compared with a non-PCM envelope. Another study did a partial validation of EnergyPlus using indoor air temperature data from a room with macroencapsulated PCMs on the ceiling [47]. An EnergyPlus model has also been used and partially validated for another macroencapsulated PCM in a passive house [48]. There has been significant numerical and experimental work conducted to validate PCM models for microencapsulated and shape-stabilized PCM [49], [50], [51], [52]. However, no study has validated PCM models used in building energy programs for cases when macroencapsulated PCMs are used in the building envelope. Previous studies used indoor air temperature to validate, which does not fully validate a model and/or use only one PCM. Another study proposes neuronal networks but in simplified geometries [53]. Validation is an important task, since encapsulation affects PCM heat transfer and therefore influences the charge and discharge rates of energy stored. Lack of validated models is partly due the lack of well documented data, with well-defined boundary conditions done in full-scale wall assemblies. With the increased use of macroencapsulated PCMs in the building envelope, it is important to have laboratory data to validate PCM models.
This paper proposes a simplified wall to test multiple PCMs without the complexity of realistic walls. The simplified wall construction allows to focus on the PCM heat transfer modeling, avoiding three-dimensional heat transfer that takes place in stud walls. This study also provides laboratory data for three different PCM types and two different encapsulations types that can be used to validate PCM models used in the building energy simulation programs. Using the data provided in the current study to validate PCM models with different PCM types increases the confidence of energy modeling for PCMs.
Section snippets
Methodology
All laboratory tests are done in a full-scale, state-of-the-art environmental chamber using simplified 1D PCM walls that has no studs and a large surface area compared to the wall thickness so that the heat transfer perpendicular to the wall surfaces are dominant. Full cycle tests are conducted simultaneously for all four panels to generate data to validate PCM modeling algorithms in different building energy programs.
Results and discussion
Fig. 7 shows radiant supply water temperature (actual and setpoint) and average chamber interior air temperature. 0h in the horizontal axis corresponds to 2 hours prior to changing the setpoint for the first melting cycle and the 100h corresponds to the two hours after the end of 3rd cooling cycle. The variations in the chamber air temperature is caused by the temperatures being allowed to float but both internal and external temperatures are above and below the analyzed PCMs melting
Conclusions
This study tests three different PCMs used in building envelopes inside a full-scale testing apparatus using a simplified 1D wall to develop data for PCM model validation. Four wall panels are studied with (i) regular drywall, (ii) PCM enhanced drywall, (iii) BioPCM pouches, and (iv) PCM hydrate-salts pouches. Experimental data indicates that the apparatus can conduct full cycle tests of wall panels with different PCMs. The data also indicates that temperature profiles obtained at different
Author Statement
Sajith Wijesuriya: Data collecting, curation, writing- original draft preparation.
Paulo Cesar Tabares-Velasco: Supervision, writing- reviewing and editing.
Data Availability. Data is attached as part of this manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge National Gypsum, Phase Change Energy Solution, and Insolcorp for the donation of their panels and their technical support. We will also thank Drs. Greg Jackson and Brian Thomas for their critical feedback.
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