Overheating mitigation in buildings: a computational exploration of the potential of phase change materials

Phase change materials (PCMs) can store and release thermal energy. The energy is stored when the material goes through a solid-toliquid phase change, and released in the reverse process. Such materials can contribute to the mitigation of overheating in buildings, if their melting and solidification temperatures are in a suitable range. The present contribution entails a computational examination of this potential as relevant to overheating mitigation in typical residential units in the Central European context of Vienna, Austria. Thereby, multiple variations of PCM application (size, thickness, location, and application thickness) under different contextual settings (fenestration and insulation, boundary conditions in terms of weather) were simulated and comparatively evaluated. Results indicate that certain PCM application configurations can significantly influence indoor thermal condition. For instance, PCM elements with larger surface areas displayed a more pronounced effect as compared to bulkier elements with smaller surface areas. Likewise, ceilingintegrated PCM application was found to be more effective that those involving other room surfaces. The results also highlight the importance of rooms ventilation regime if the PCM application potential toward overheating mitigation is to be effectively harvested.


Introduction
The present contribution focuses [1] on application potential of Phase Change Materials (PCMs) as mitigation measure against overheating in Vienna, Austria. The key research question was if a PCM incorporation in a typical Viennese building provides enough latent heat storage to increase thermal comfort and energy efficiency (by rendering active cooling unnecessary). To pursue this question, a simulation-based approach was selected. Thereby, variations of parameters of the Phase Change Materials such as application form and panel surface area were examined, as well as boundary condition parameters, such as night-time ventilation. Simulation models of two spaces in a typical Viennese Gründerzeit building stock (characterized by massive wall constructions and wooden truss slabs) were generated.
PCMs in buildings are one form of Thermal Energy Storage (TES) and can be used to enhance passive cooling potential. Thermal energy storage is achievable via sensible, latent, or thermochemical heat storage. PCMs utilize phase change enthalpy, thus they are considered to be latent heat storage. During the phase transition, thermal energy is stored in the material within a narrow temperature span. The first scientific approach onto the application of PCMs in buildings can be dated back to 1947, where PCMs based on Glauber slats were used as passive solar heating system [2]. However, there are only few further scientific publications regarding the use of PCMs in buildings until the year 2003 (Soares et al. [3] name just 2 papers that have been published before 2003). In recent years the idea of using PCMs in buildings gained momentum, also resulting in increased published research efforts by different researchers [3]. Typically, three different types of PCM-materials are distinguished: organic compounds, inorganic compounds, and eutectics [4]. Another categorization of PCMs is the distinction in PCMs with micro and macro encapsulation. Micro-encapsulation means the encapsulation of the PCM-material in very small containments and amongst other materials. Application examples of such micro encapsulations include gypsum wallboards, PCM-containing plaster products, or integrated pads in furniture surfaces. While micro encapsulation offers convenient advantages, such as easy application and integration in existing rooms, its performance is limited due to the rather small extent of material used. Macro encapsulation involves the use of large volumes of PCM materials (e.g., in panels of different form). However, their larger thickness may lead to incomplete phase-changing processes or sub-cooling phenomena.
The present contributions focuses on Macro-encapsulation PCMs. Regarding the selection of the right PCM-product for a specific task within a building, the following criteria have been suggested: PCMs used in buildings should have a large latent heat capacity and a high thermal conductivity. PCMs in buildings are required to have their phase change within regular thermal comfort ranges of indoor spaces (that is 18°C to 30°C) [5]. Khudair and Farid [6] suggest that the optimal diurnal heat storage occurs with a melting temperature of 1 to 3 Kelvin above the average room temperature. Figure 1 illustrates temperature/melting ranges of different PCM materials relevant to typical room temperatures. Building-related potential of PCMs has been explored in a number of previous research efforts (see, for example, Khuair and Farid [6], Lee et al. [7], Kenisarin et al. [8], Tyagi et al. [9], Sharma et al. [10], Skovajsa et al. [11], Menon [12]).

Case study rooms
The two case study rooms used in this study are situated in a typical Viennese Gründerzeit building. Both rooms (denoted as TR1 and TR2) have their cardinal orientation towards southeast and feature two windows. TR1 is situated in the third floor of the building and features two regular windows, while TR2 is situated in the attic space and has two skylights. Thermal properties of the constituting building elements are based on the OIB Guideline 6 [13]. Table 1 includes key information on both rooms.

Phase Change Material and Integration in test rooms
The chosen PCM is a (macro-encapsulation) product of the company Entropy Solutions LLC (Plymouth, Minnesota), namely PureTemp 25 [14] (see Table 2 for key technical information and Figure 4 for melting/solidification characteristics). It is of organic origin, has been extensively tested, and is suggested to accommodate 10.000 diurnal cycles without significant change of performance or characteristics. This corresponds to a lifecycle of 27 years (daily usage scenario) or 50 years (summer time usage scenario). Table 2 denotes the characteristics of the material. The main applications scenario in the present study involves mounting of panels to the ceilings of the test rooms. Nonetheless, the effect of placing the panels on other surfaces (especially walls) was examined as well. Three layer thicknesses of the PCM panels were evaluated (3, 5, and 7 cm).

Simulation tool, simulation scenarios, settings & boundary conditions
The effect of the applied PCM-panels was studied using EnergyPlus [15], which has been suggested as a suitable tool for the topic at hand [16] [17]. Climate data was used based on measurement results obtained from our Department's weather station in Vienna. Regarding internal conditions, applicable Austrian Standards data was used [18]. Several application scenarios were simulated for both test rooms. Thereby, the application of PCM panels and the ventilation regimes were varied. Moreover, as base-line scenarios, both rooms were simulated without PCM application but under different ventilation scenarios. Toward this end, both tilted and fully open window positions were considered, implying air change rates between 0.5 and 4.0 h -1 . Moreover, a differentiation between Day time ventilation and Night time ventilation was considered. Table 3 illustrates some the scenarios evaluated. The key to the scenario labels in this  3 Results Figure 5 illustrates the resulting TR1 temperatures for different scenarios for TR1 as cumulative graphs. The information entailed in this Figure reveals both the PCM application influence and the importance of ventilation rates. PCM application as such cannot replace proper ventilation, especially during night-time. The performance of those scenarios with PCM, where ventilation is drastically reduced, can be worse than properlyventilated cases without PCM. Table 4 encapsulates the main results for both TR1 and TR2.   These results suggest that the application of PCMs can significantly lower both indoor temperature peaks (maximum temperatures during summer seasons) and average indoor air temperatures. Furthermore, the number of overheating hours (hours with an indoor air temperature of higher than 27°C) can be reduced during summer season, if PCM deployment is combined with proper ventilation regimes. Increasing the thickness of the PCMs can further reduce overheating hours. However, the effect involves a diminishing return, as it weakens the thicker the panel becomes. Table  5 shows the impact of 3, 5, and 7 cm of ceiling-mounted PCMs (based on scenario TR1 _C5_DV0.5_NV2).

Conclusion, limitations of the study, and future research
The present contribution utilized a simulation-based assessment of the impact of PCM application in typical rooms in the building stock in Vienna, Austria. Thereby, the application of PCMs was shown to be effectual in view of reducing peak and average temperatures in the examined rooms, if combined with appropriate ventilation regimes. As such, deployment of PCM can enhance, but not replace, the utilization of smart ventilationbased cooling strategies. Details of the entire set of conducted simulations [1] could not be covered within the framework of the present paper. Nonetheless, it would be appropriated to briefly mention at least two additional findings of the study: i) The application of PCMs on the ceiling appears to represent most efficient positioning option; ii) Large-area panels appear to be more effective than thicker ones.
Needless to say, a purely simulation-based study has shortcomings, including -most importantly -the absence of measurement-based validation. Thus, empirical studies must be performed in the course of future research efforts. Likewise, a larger set of case study buildings must be considered, addressing different building types, construction methods, use patterns, urban context, and microclimatic boundary conditions.