## 3 Studies of PCMs Incorporated in Passive Systems
### Experimental Studies
#### 3.1.1 At the Composite Scale
The results of chemical and thermal analysis showed good thermochemical stability of the composite material after more than 2000 cycles. However, the PCM impregnation reduces the mechanical properties of the composite material by a factor of two, which remain acceptable for non-building applications. The addition of recycled graphite increased the thermal conductivity by 415% (from 0.2 to 0.83 W m\({}^{-1}\) K\({}^{-1}\)). [PERSON] et al. [33] developed a new thermal energy storage concrete consisting of lightweight aggregates impregnated with vegetable oil PCMs, of melting temperature between 23 and 26 \({}^{\circ}\)C, mixed in a cement mortar. Three types of mineral aggregates were used (bentonite, sepiolite and silica gel), as well as two impregnation methods, namely direct incorporation and vacuum impregnation. The best impregnation rates were obtained with silica gel (56%), but sepiolite (32%) should be preferred for regulatory reasons according to the mechanical tests. In fact, the regulations for masonry elements stipulate that the value of compressive strength should not be less than 15 MPa [53]. Despite a loss of mechanical strength, the authors demonstrated that composite mortars with a compressive strength greater than 7-10 MPa are feasible. In addition, the results showed an improvement of 24.4% and more than 13.5% for energy storage capacity and thermal conductivity, respectively. [PERSON] et al. [54] studied the effect of incorporating micro-encapsulated PCMs (MICRONAL(r) DS 5040X) with temperature between 22 and 23 \({}^{\circ}\)C with a heat of fusion of 95 J/g into gypsum board. The proportion of PCM is chosen lower than 30 wt% in order to guarantee the mechanical properties of the composites. Thus, four different compositions of gypsum boards are manufactured: reference gypsum board (0 wt% of PCM), gypsum with 5 wt% of PCM, gypsum with 10 wt% of PCM, and gypsum board with 15% of PCM. The incorporation of PCMs leads to a reduction in compressive strength but the value is still permissible given the recommendation for gypsum board in the building structure, which suggests a compressive strength greater than or equal to 2 MPa [55]. In addition, the thermal conductivity decreases with the addition of PCM that increases the insulation properties of the gypsum boards. [PERSON] et al. [56] conducted an experimental study on the thermal performance of wall-boards containing a micro-encapsulated PCM (MPCM for micro-encapsulated PCM), PH-31, using two methods: DSC and a home-made experimental device. Three different techniques were used to obtain the enthalpy-temperature (H-T) curve: theoretical calculation from DSC data, a dynamic method, and the stepwise method. The sample analyzed is composed of gypsum with 20 wt% microencapsulated kerosene having a phase change temperature of 31 \({}^{\circ}\)C. The addition of MPCM decreases the thermal conductivity of the gypsum board due to the lower density and thermal conductivity of PCM, but increases the apparent heat capacity up to 2.71 times for a temperature range of 26-32 \({}^{\circ}\)C.  
Renewable materials, such as natural fibers, are considered as promising carbon sequestration resources with beneficial effects on the planet's ecosystems, living environment and energy efficiency [57; 58; 59]. [PERSON] et al. [60] investigated the specific heat capacity and thermal conductivity of a wallboard made from hemp shives mixed with a commercial organic PCM, S50 at 5 wt%. The addition of nano-capsules increased the heat capacity by 62% to 2.369 J g\({}^{-1}\) K\({}^{-1}\). The thermal conductivity of the samples is in the range of 64-74 mW m\({}^{-1}\) K\({}^{-1}\), which is consistent with the class of commercially available hemp insulation products. To improve the thermal conductivity of building envelopes incorporating PCMs, additives, such as expanded graphite (EG for \"expanded graphite\"), montmorillonite, pen taerythnitol, and melamine polyphosphate can be used [61].