## 1 Introduction
### Phase Change Material  
Materials to be used for phase-change TES should have phase-change temperature in the practical range of application and they must have a high latent heat of fusion and a high thermal conductivity. PCMs should also have desirable environmental properties to decrease the environmental impact of the systems during their lifecycle [9]. Different materials considered as potential PCMs include hydrated salts, paraffin waxes, fatty acids, the eutectics of organic and non-organic compounds, etc. PCMs can be divided into three main groups--based on the temperature ranges over which the TES phase-change occurs: low temperature (phase-change temperatures below 15 \({}^{\circ}\)C), mid temperature(the most popular--from 15 to 90 \({}^{\circ}\)C) and high temperature PCMs (above 90 \({}^{\circ}\)C) [10; 11]. In our case, PCMs are classified as organic, inorganic and eutectic. Organic PCMs include a wide selection of organic materials such as fatty acids, esters, alcohols and glycols. The principal advantages are their chemical and thermal stability, and high latent heat of fusion. They are usually recyclable, non-corrosive, and have little or no subcooling [1; 12]. Unfortunately, they have some disadvantages: flammable, non-compatible with plastic containers, relative large volume change, more expensive, low thermal conductivity, and lower phase-change enthalpy [1; 13; 14]. Inorganic PCMs are described as hydrates salts and metals. They cover a wide range of application. The main advantages are higher enthalpy per volume, higher thermal conductivity, non-flammable, and lower volume change. However, they have the disadvantages of phase segregation, corrosion, lack of thermal stability and subcooling [9; 12]. The eutectic PCMs are a combination of chemical compounds or elements that have a single chemical composition and that solidify at a lower temperature than any other composition obtained from the same components [15]. Due to their higher density and stability in their liquid state, they have been used widely as ionic liquids in high temperature sensible thermal storage systems (thermonuclear energy, concentrated solar thermal power) [16]. Theoretically, most materials can be considered as PCMs. On the other hand, only some of them can be used for effective and predictable energy storage. Figure 2 shows some of the many materials suitable for TES. Depending on the application, the PCMs should be selected based on their phase-change temperature, should have a large latent heat, should melt congruently with minimum subcooling, and should be chemically stable, inexpensive, nontoxic, and noncorrosive [15]. The most commonly used PCMs in buildings are paraffin and salt hydrates [17; 18]. As previously mentioned PCMs can be incorporated into building structures. It contribute to lower energy demand by storing thermal energy in the form of heat or cold [20]. Several configurations  
Figure 1: Latent and sensible heat, temperature range of the phase change [7]. Figure 2: Classes of the phase change materials and their common parameters [19]. have been proposed in the last two decades in order to incorporate PCM into construction materials. Traditionally, as one of the best option is taken application of the PCM in the envelope of the room because of its large area [21]. Some of the recent research show some improvements and specific use of the PCM in the buildings and also their advantages and disadvantages [22; 23]. The efficiency is not dependent only on the location of the PCM, but also on their specific parameters, such as melting temperature range. For example, one of the research shows that the highest value of energy exiting from the indoor environment is ensured by PCM with a melting temperature equal to 20 \({}^{\circ}\)C [24]. Integration of the PCMs into the building's energy system can increase efficiency of the systems. These active systems use of the air flow as forced convection to increase amount of energy obtained from storage materials [25].