Designing building envelope with PCM wallboards: Design tool development

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Abstract

While space conditioning load contributes largely in grid critical peak, shifting a part or full to the off-peak period could have significant economic effects on both energy supply and demand sides. This shifting technique is accomplished by storing energy during off-peak periods to be utilized during peak periods. The wallboard enhanced with PCM can provide latent heat thermal energy storage (TES) distributed in the whole surface area of the building envelope and evade the enhanced thermal mass in light weight buildings. Identifying the best design parameters of the PCM wallboard is the main key to apply this latent heat TES efficiently.

The effective dimensionless numbers on the thermal dynamics of a PCM wallboard were identified. Moreover, the impact of the change of those numbers on the time required for the wallboard to become fully charged was evaluated. This parametric study provided a tool to characterize the required thickness of a PCM wallboard which needs to be charged during the off-peak. The tool presents the Fourier number as a correlation of Biot number and Stefan number. Moreover, the impact of melting range on the charging time of a PCM wallboard was investigated.

Introduction

Building sector contributes largely in total energy consumption, particularly for its space conditioning. According to The Natural Resources CANADA [26], while more than 30% of the total secondary energy was used by residential and commercial/institutional buildings, space heating accounts up to 56% of the total energy used in non-industrial buildings. In Quebec, Canada, 70% of residential buildings use electrical space heating system which accounts for 29% of the grid critical peak [9]. Its total usage varies during a day due to the activities of industrial, commercial, and residential sectors together. This results in peak period (mostly in early morning and evening) and off-peak periods. According to HydroQuebec [10], during the peak period in winter the electricity cost to the utility company is 10 $/kW to generate and could increase up to 40 $/kW in 2015. Taking this information into account, shifting a significant portion or the whole required energy for space heating to off-peak periods would have significant economic effects on both energy supply and demand. The shifting can be accomplished by storing energy during off-peak periods to be utilized during peak periods. Building envelope and central thermal storage have been used as thermal energy storage (TES). Recently, TES has attracted increasing attention due to the potential benefits it can offer in energy efficiency, in shifting load from peak to off-peak, in emergency heating/cooling load, in economics and in environmental impact [2], [5], [18], [24]. Moreover, TES is known as an essential mean in designing net zero energy buildings (NZEB) [25]. TES has a significant role in remitting the mismatch between the energy demand time and renewable energy production time. Advanced design tools and technical improvements are required in TES technologies and systems. Indeed the design of the building and TES are often not coordinated. A building integrated with distributed thermal storage materials could shift most of peak load to off-peak time period.

Previous studies investigated the possibility of the peak-load shifting by storing the sensible heat in building materials [3], [15], [16]. Moreover, the application of phase change materials (PCMs) as a latent heat thermal storage draws interests as a TES with higher capacity. The application of PCM as a TES in buildings was reviewed in details by previous researchers [4], [8], [12], [14], [17], [18], [19], [22], [27], [29], [30]. One promising way to improve the thermal inertia of a building is to integrate PCM layers in its envelope. Due to considerably large area of the envelope of a building, integrating PCM in the envelope can provide a TES with a large capacity. Regular building wallboard can be a mean to accommodate PCMs. The wallboard enhanced with PCM can provide latent heat TES distributed in the whole surface area of the building shell and evade the enhanced thermal mass in light weight buildings. The design parameters of a PCM wallboard consists of its thickness and thermo-physical properties, directly impact the thermal dynamics of the TES and consequently the temperature profile and the space condition enclosed by the wallboards. Identifying the best design parameters of the PCM wallboard is the main key to apply this latent heat TES efficiently. The design parameters of the wallboard depend on its application and require the knowledge of the influence of those parameters on the thermal dynamics of the PCM wallboard. Reviewing previous studies presents some achievements regarding the best design of a PCM wallboard. Neeper [23] investigated the optimum melting temperature and melting range of a gypsum wallboard impregnated with fatty acid and paraffin waxes in a passive design. He, also, conducted the study for two different magnitudes of the convective heat transfer coefficient between a wallboard to the room. He concluded that the optimum melting temperature depends on the outdoor seasonal conditions and the average room temperature. He, also, suggested a melting temperature equal to the room average temperature with a narrow phase change temperature range (around 2°). Kuznik et al. [13] investigated the effect of insulation thickness, indoor temperature and outdoor temperature swing on the optimum thickness of an encapsulated PCM wallboard. The objective of the optimization was to maximize the energy storage capacity with the smallest PCM thickness. Their results showed that the variation of the outdoor temperature swing and insulation thickness does not affect the optimum design thickness of the PCM layer. However, increasing the indoor temperature swing resulted in an increase in the optimum design thickness of the PCM wallboard. Ahmad et al. [1] reported that the PCM thermo-physical properties impact the thermal dynamics of the PCM layer and consequently the building thermal performance. Lin et al. [20] studied the effect of the thickness, conductivity, melting temperature and the latent heat of a shape-stabilized PCM on the room air temperature profile. Xu et al. [28] concluded that there is minimum thickness of the PCM layer, and suggested that the conductivity and latent heat of the PCM should be more than 0.5 [W m−1 K−1] and 120 [kJ kg−1], respectively. Overall, the outcomes of the abovementioned studies emphasize that the best design parameters depends on the design objectives, and the environmental conditions. Although those works brought some general suggestions for the application of a PCM in building envelope, the results are inherently case specific and cannot be generalized for the design of an integrated building envelope with PCM. A procedure to generalize the results, specifically for numerical studies, is to conduct the simulation and parametric studies using dimensionless numbers. Ettouney et al. [6], [7] characterizing the heat transfer process in PCM applied in double pipe and spherical storage. They provided some correlations for the melting and solidification: Fourier number (Fo) as a function of the Stefan number (Ste) and Biot number (Bi) of the PCM. Those dimensionless numbers content the thickness, heat transfer coefficients and the thermo-physical properties of the materials. Therefore, any possible change of the characteristics of the materials, which affects the thermal storage/release process inside the PCM, can be evaluated deploying the correlation available between those dimensionless numbers. Yet, the lack of similar correlations for PCM wallboards requires extended studies on the application of these materials in building envelope. The aim of this study is to investigating the application of PCM wallboard inside the building envelope to be able to shift the peak load for space conditioning to the off-peak period. Indeed, the interest is to store energy during the off-peak period for space conditioning during the peak period, to evade the application of spacing heating/cooling systems at those peak hours. For this purpose, the PCM wallboards need to be evaluated based on their characteristics in a way to determine the required time to be charged. Therefore, the main objective of this study is to develop a procedure to design a PCM wallboard for a given application.

Section snippets

Methodology and problem formulation

To obtain the best design parameters of a PCM wallboard, characterizing the impact of the design parameters on its thermal performance is required. Generally, the influential parameters are the ones which affect the heat transfer rate through the wallboard and the storage capacity. Therefore, the thermo-physical properties of the PCM wallboard, its thickness, and convective heat transfer coefficient characterize its thermal performance. To investigate the impact of these parameters, recognizing

Simulation scenario

To simulate the thermal performance of a PCM wallboard, it is assumed that the layer is mounted as the interior layer of a wall section. Thus, the PCM has room temperature as its boundary condition in one side, and adiabatic boundary condition on the other side (prefect insulation or similar room air condition on the other side). This assumption is valid for internal walls which are utilized as partition or separating two zones with the same thermal conditions, and the external walls of well

Conclusions

PCM wallboard can be applied to shift the peak load to the off-peak period. Either the thickness or the thermo-physical properties of the wallboard can be used as design parameter in order to properly find the charging time. In order to take advantage of the PCM's latent heat, it needs to be liquefied in a sufficient time to be utilized during peak period. Therefore, to select an appropriate PCM wallboard, a design tool needs to relate the PCM thickness and its thermo-physical properties to the

Acknowledgements

The authors would like to express their gratitude to the Public Works and Government Services Canada for their support.

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