## 3 Studies on Using Organic PCM in Concrete for Thermal Energy Storage  
There are two main categories of organic PCMs: paraffin and non-paraffin. Paraffin works well as a passive cooling and energy storage PCM. In contrast, organic PCM's poor thermal conductivity results in a slower rate of heat release/storage capacity. In the next section, a detailed discussion of the relevant related research will be presented. The list of references [49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66] presents the research that is relevant to this review. For structural-thermal multifunctional applications in high-energy-efficiency building envelopes, [PERSON] et al. (2018) [49] reported the results of multiphysics thermomechanical research into novel concretes that include paraffin-based PCM. In the new composite preparation, shown in Figure 1, both traditional Microencapsulated Phase Change Materials (MPCMs) and the more innovative, ground-breaking Macroencapsulated PCMs with an 18 \({}^{\circ}\)C phase transition temperature were utilised. The results corroborated PCM's thermal advantages and showed that incorporating PCM into concrete has decreased mass density by almost twice PCM's weight. While adding PCM generally reduces the average compressive strength, it has less effect on the coefficient of variation, which is a good sign for the structural dependability of the material. Figure 2 shows that the coefficients of variation (or the standard deviation intervals) of the compressive strengths were reduced, and the characteristic compressive strength was even increased if MPCMs and Macroencapsulated PCMs were incorporated with 1% of the total weight. To fabricate environmentally friendly concrete with an elevated capability of TES, [PERSON] et al. (2018) [50] produced Geo-polymer Concrete (GPC) including Microencapsulated Phase Change Materials (MPCMs). GPC's microstructure and thermal characteristics were studied using many MPCM types to determine the impact of hygroscopicity, latent heat, and microcapsule size. The capacity of the GPC to store energy was observed to increase as MPCM concentration and latent heat did. An energy balance and heat capacity approach were built from the model's basis using the technique of Implicit Finite Differences. A novel equation was effectively used to fit the specific heat capacity of GPC, including MPCM, as a function of temperature, which allowed for the improvement of the model. The thermal performance of the GPC was measured experimentally to validate the computational model. With more MPCM and thicker concrete walls, it takes less energy to keep a room at a comfortable 23 \({}^{\circ}\)C. Using a wall of concrete with MPCM (5.2 wt.%) and 75 mm thickness led to a decrease in electricity consumption of about 35%. To use the GPC as a TES concrete for passive structure uses, [PERSON] et al. (2018) [51] incorporated MPCM into the GPC. The microstructure, thermal characteristics, and com  
Figure 1: MPCMs and Macroencapsulated PCMs in concrete sample preparation [49]. Figure 2: (**a**) The average compressive strength with the standard deviation intervals; and (**b**) The characteristic compressive strength of studied concretes without PCM, with MPCMs, as well as Macroencapsulated PCMs [49]. pressive strength of GPC made with three different MPCMs were investigated to look at the influence of the MPCM shell's hygroscopic nature, the dimension of the MPCM, and the ratio of PCM core to polymer shell. Improved interface bonding between the matrix of the GPC and the microcapsules led to an increase in the GPC's energy storage capability. The excellent MPCM dispersion in the matrix of GPC was also observed when a polymer shell's hygroscopic nature was combined with an elevated ratio of core to shell and a small dimension of MPCM.