## 2 PCMs: Basic Characterization
### PCM Applications  
The use of PCMs covers several applications, with a higher amount of studies focused on solar energy and Heating, Ventilation and Air Conditioning (HVAC) Systems [16]. In addition, many other areas can benefit from using PCM systems, including the construction field, space crafts and satellites, waste heat recovery, photovoltaic-thermal solar collectors, electronic equipment, personal comfort, and medical systems, among others. Thermal energy storage always plays an essential role in concentrating solar power plants, as it increases the availability of the plant to periods without sunlight. In such systems, part of the solar energy collected during the day is redirected to a reservoir with PCM that keeps the electricity generated during the night [8]. An improvement in the performance of the thermal energy storage and power plant is the use of a cascaded storage system [9; 10]. Cascaded PCM systems are composed of several PCMs with different phase change temperatures and are configured with the phase change temperature in ascending order. Thus, the direction of the HTF flow is reversed between charge and discharge. In photovoltaic solar collectors, the usage of PCMs results in a reduction in the peak temperature. Thus, the losses due to overheating are reduced in the collector [14]. Another important application of PCMs is in photovoltaic-thermal solar collectors, where PCMs have been observed to improve the overall efficiency of the collector [12; 35; 36]. The same occurs in the utilization of PCMs in solar distillation processes in which the operational period after sunset is extended. As a result, both efficiency and productivity are increased [37; 38]. Another essential building application of PCMs is the thermal storage in solar heating systems for domestic water, as pointed out in works [15]. Commonly, such systems use water as sensible heat storage due to the low cost of installation. The use of PCMs as latent heat storage in solar heating systems entails the main advantages of the reduction in reservoir size and improvement of the overall system efficiency [39]. However, the cost of the PCM may represent an obstacle to large-scale employment in domestic hot water systems [16]. In air HVAC systems, for example, the utilization of PCMs with phase change temperature lower than ambient temperature can bring about several advantages in order to reduce the peak energy load and slightly increase the efficiency of the equipment [40]. In the construction field, PCMs are used for passive storage daily, in the form of microcapsules contained in thermal insulation boards. The use of these boards provides temperature peak attenuation inside the building, lowering the maximum temperature during the day and increasing the minimum temperature during the night. Therefore, the thermal load is reduced, and consequently, the load of heating or air conditioning is also reduced [2]. Space crafts and satellites orbiting the Earth operate in a cyclical thermal condition that varies between extremes. When the spacecraft is exposed to the sun, the temperature reaches its maximum. Oppositely, when the spacecraft is in the shadow of the Earth, the temperature reaches its minimum. Thus, the use of PCM attenuates the high- and low-temperature peaks and represents great potential to reduce energy consumption [41]. Even waste heat recovery in industries using PCMs represents excellent potential for energy saving [18]. A representative example is the steelmaking industry, in which vast amounts of heat are rejected to cool the alloys produced [42]. According to [PERSON] and [PERSON] [43], after having been stored in the PCM, the waste heat can be directed to a supply center for public use. In vehicles with internal combustion engines and hydraulic machines, waste heat can be directly stored in the PCM and used to preheat the engine, as a cold start represents higher fuel consumption and more significant wear of the moving parts [16].