Experimental investigation on the hygrothermal behavior of a new multilayer building envelope integrating PCM with bio-based material
Introduction
In the context of increasingly depleted non-renewable energy sources and high CO2 emissions, the global energy system is set to be doubly challenged in the future: by the demand for more energy and less carbon [1]. However, as places where humans live, work, and play, buildings account for about 58% of energy consumption and 28% of CO2 emission [2]. Moreover, people's requirements as regards indoor comfort are increasing as society develops. Therefore, several technologies and solutions have been used to improve indoor comfort while saving energy and reducing CO2 emissions. The building envelope represents the boundary between interior and exterior, ensuring watertightness, insulation, air, and light circulation. With growing environmental concerns, building envelopes are called upon to participate in energy consumption regulation. However, building envelopes are responsible for 51% of total energy consumption [3]; therefore, huge energy saving potential can be achieved by optimizing them.
Temperature and relative humidity are two important parameters that affect indoor air quality and consequently occupants' heat and moisture comfort [4,5]. It has been proved that inappropriate temperature and humidity leads to mold growth [6,7], thereby affecting occupants' health and furniture's lifespan. Building envelope materials with excellent thermal and hygroscopic behavior are the solution to regulation of temperature and relative humidity. Bio-based materials and PCMs (phase change materials) seemed to be the answer to this question in the literature. Porous bio-based materials have moisture hygroscopic properties, while PCMs have thermal inertia properties. Also, CO2 emissions are minimal during production and utilization of such materials [[8], [9], [10]].
Hemp concrete, as a bio-based material, can be used as a building envelope material. It has been recognized by many researchers due to its sustainability and interesting hygrothermal characteristics [[11], [12], [13]]. Compared with conventional building materials such as aerated concrete, it has low density and high porosity. Therefore, it has been extensively studied as a thermal insulating [[14], [15], [16]] and hygroscopic material [[17], [18], [19]] of building, which is beneficial for building energy saving [[20], [21], [22]]. Previous studies [23,24] have proved the strong coupling between temperature and relative humidity within HLC, and both of them affect the relative humidity change within the material. Besides, the hygric property is more sensitive to temperature change than relative humidity [25,26]. Therefore, some researchers have studied the effect of temperature on hygroscopic behavior. Poyet et al. [27] presented the theoretical model based on the Clausius-Clapeyron equation and isosteric heat of sorption. Later, Colinart et al. [28,29] verified it by hemp concrete experiments, and the results highlighted the temperature dependence on the adsorption curve. Rahim et al. [30] simulated the temperature dependency of the sorption curves of the hemp concrete envelope and found that the relative humidity behavior could be predicted accurately by taking into account the influence of temperature. Chennouf et al. [31] explored the effect of temperature on hygroscopic behavior by experiment and confirmed that the sorption-desorption process and moisture buffer value were highly affected by temperature. The researches above indicate the excellent hygrothermal behavior of hemp concrete and the importance of temperature.
PCM is also an envelope technology to maintain or provide thermal comfort in new buildings [32], and it provides enough thermal inertia to utilize the cold energy at night during the hottest time of the day [[33], [34], [35]]. A great deal of thermal energy is stored or released during the phase change process [36,37]. In other applications of PCM, it behaves different when placed in different positions [38,39]. Similarly, as part of the building envelope, its thermal behavior is different when placed in different positions [40]. The thermal performance of the building envelope when PCM was placed in different positions was studied by Jin et al. [[41], [42], [43], [44]], who found that the state, heat flux, amplitude, and time delay were different according to the PCM's position. Besides, the optimal position of PCM changes with different influence factors such as thickness, heat fusion, and the melting temperature of PCM. Lee et al. [45] studied the thermal behavior of PCM when placing it in the south and west walls at different positions in the envelope, and the results indicated that the optimal position of PCM in the south wall was closer to the outdoor environment compared to the west wall. Lagou et al. [46] analyzed the melting temperature and optimal positions of PCM in different European continent climatic conditions and found that all the optimal positions were located on the interior side but varied with the melting temperature. Fateh et al. [47,48] concluded that the PCM was more active when it was placed in the middle position, which caused a 15% peak heat load reduction and obvious time delay. Similarly, the optimum position of PCM was also found to be located in the range between the middle and the external in some researches [49,50]. Therefore, the different position of PCM in the envelope produces different thermal feedback and the determination of optimal position is significant.
According to the aforementioned literature, hemp concrete has excellent hygric inertia as an envelope to dampen ambient relative humidity changes. However, it has low thermal inertia because of its light weight. Also, temperature has a significant influence on relative humidity changes within hemp concrete. On the other hand, the high PCM thermal inertia can change the temperature field and dampen the temperature amplitude. Besides, the feedback benefit in terms of heat behavior from the PCM-based envelope depends on the PCM's position.
This research focuses on the hygrothermal behavior of the new building envelope integrating HLC (hemp lime concrete, one of the bio-based materials) and PCM at experimental level. This new envelope has both high thermal inertia and high hygric inertia. First, the hygrothermal properties of PCM and HLC were presented. Then, the experimental protocol was proposed. It was carried out by connecting one side of the envelope to dynamic temperature and relative humidity changes provided by a climate chamber, which was used to imitate the outdoor environment. The other side was exposed to the laboratory ambient with relatively stable hygrothermal environment to imitate the indoor environment. Four combinations of temperature and relative humidity changes were set as boundary conditions in the climate chamber, including two identical and two opposite trends. Later, the hygrothermal behavior of the envelope using HLC only was analyzed and compared with three integrated envelopes combining HLC and PCM, which were distinguished by the different positions of PCM. The value, fluctuations, and differences between various points within the HLC were studied, temperature and relative humidity curves were quantitatively evaluated, and the impact of PCM and its position were analyzed. Finally, heat flux behavior was presented, the envelope's heating/cooling load and heat store/release capacity were calculated. The PCM's temperature distribution and specific heat capacity was investigated.
Section snippets
Materials
The materials considered in this paper are HLC and PCM (Fig. 1). The HLC sample was produced from lime binder, water, and hemp particles using the molding method. The finished HLC sample has high porosity, resulting in interesting thermal conductivity (0.12 W/(m·K)) and high water vapor permeability (2.23 × 10−11 kg/(m·s·Pa)) [30]. Therefore, it helps to weaken thermal transfer and regulate relative humidity as a building envelope material.
The PCM used in this paper is the finished panel called
Results and discussion
In this section, the hygrothermal behavior within the HLC envelope of the configuration without PCM (HLC only) is presented first as a reference. Then it is compared to the configurations with PCM. The changes in the overall trend, value, amplitude, and hygrothermal behavior differences between different positions are discussed.
Conclusion
In this paper, the hygrothermal behavior of a new multilayer building envelope integrating HLC and PCM has been studied and analyzed at experimental level. The climate chamber and laboratory ambient imitated outdoor and indoor environments respectively. Four envelope configurations were considered to study the effect of the PCM layer and its positions on the envelope's hygrothermal behavior in terms of the thermal and hygric inertia.
The results highlighted the major effect that adding a PCM
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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
We thank to the China Scholarship Council (CSC) for its financial support to the first author, No. 201808120084. CPER UL/Lorraine Region and PHC Maghreb are acknowledged. The authors would also like to thank EMPP Scientific Pole of the University of Lorraine.
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