## 1 Introduction  
Phase Change Materials (PCMs) are a solution widely used for decreasing a building's energy consumption [1] by storing and releasing heat within a certain temperature range, raising the buildings inertia, and stabilizing indoor climates. However, because PCMs cyclically change their phase from solid to liquid during the charging and discharging of the heat storage process, there is a necessity for retaining the PCMs in containers or matrices. The methodologies by which PCM are confined depend on the thermal energy storage (TES) application. Among the available methodologies, the encapsulation of PCM in a container as a supporting material that maintains the shape of the Microencapsulated Phase Change Material (MPCM) when transitions occur has been explored, this being this one of the most efficient ways of confining PCM [2]. There are several advantages for MPCM over PCM, such as providing a large heat transfer area, the reduction of the PCM reactivity towards the outside environment, and the ability to control the variations in volume of the storage materials as phase change occurs [3], among others. In building applications, for example, the organic PCM is encapsulated not only for preventing leakage but for preventing the evaporation or release of some of their components [4]. Although it is important to distinguish between different MPCM usages in passive and active solar systems, the chemical, physical, thermal, and mechanical properties have to be evaluated in the same way. A passive solar system does not involve mechanical devices or conventional energy sources beyond those needed to regulate dampers and other controls. Examples of basic passive solar structures are greenhouses, sunrooms, and solariums. In terms of design, the success of the passive solar system will depend on the orientation and the thermal mass of the structure's exterior walls, as these factors determine their ability to store and redistribute heat. On the other hand, active solar energy systems use the same basic principles as passive systems, except that they use a heat transfer fluid (for example water) to absorb heat. Active solar systems use external sources of energy to pumps and other types of equipment to collect, store and convert solar energy. Around fifty different polymers, both natural and synthetic, can be employed as shell materials in the microencapsulation process for MPCMs [5]. The polymeric shell has to be capable of creating a thin film, cohesive and stable, coating the PCM. Hence, the polymeric shell has to be chemically compatible and non-reactive with the core material, as well as able to polymerize in an aqueous medium. Furthermore, the strength, flexibility, impermeability, and stability have to be adequate for the whole system. The thickness of the shell can vary depending on the encapsulation methodology. A good option for a polymeric shell is poly (methyl methacrylate) (PMMA), because it has suitable mechanical properties, is an adaptable material, and offers decent protection against the environment [6; 7; 8]. It has also been reported elsewhere [9; 10; 11; 12; 13; 14] that polystyrene (PS) is also a good option for encapsulating PCM. Regarding PCM, the material that is to be coated will depend on its final application [15; 16; 17; 18]. Its composition can be a single solid substance or a combination of active constituents [19; 20; 21]. PCM can be organic, inorganic, or eutectic, but a high thermal energy storage capacity is required, and is the most important property to take into account. Although there are a lot of studies considering commercial samples such as Micron(r) DS 5001 [22; 23; 24; 25; 26; 27; 28; 29; 30; 31], there have also been MPCMs produced in research laboratories with good results [9; 32; 33; 34], some of which have been characterized for evaluating their possible usage at industrial scale.