# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Phase-Change Materials for Passive Air Conditioning  
Almost 25% of the energy produced worldwide is used to heat and cool homes and commercial buildings [86]. Therefore, passive cooling techniques are a highly promising family of solutions to help reduce the energy consumption of homes and buildings. Passive cooling techniques usually use large heat capacity materials (heat sinks), such as building materials or water, to mitigate temperature rises due to heat sources, such as hot ambient air, direct solar heat gain and internal heat gain [87]. Another way to massively expand the heat capacity of a building (in a pre-defined temperature range) is to incorporate phase-change materials (PCM) into building materials [88]. PCMs can store or release especially large amounts of thermal energy during the solid-liquid or liquid-solid phase transition. In Figure 4, we present the operating principle of an ideal PCM material (the curves do not correspond to real PCM materials) [89]. During phase transition, PCMs also absorb or release a predictable amount of thermal energy in a predictable amount of time [90]. When room temperature rises, excess heat gets absorbed (and stored) in the PCM, thanks to the endothermic solid-liquid transition. When room temperature falls again, the PCM releases the stored heat back into the room, thanks to the exothermic liquid-solid transition. This mechanism helps to (passively) stabilize the room temperature around the material's transition temperature. The ideal PCM for passive air conditioning applications must have a small volume change, be non-toxic and non-corrosive, have high thermal conductivity and specific heat capacity, and not supercool or decompose. Only PCMs that have a phase transition  
Figure 4: Stored heat around Tm (typical curves). Red: PCM with a solid–liquid transition temperature Tm; Blue: standard material, with no phase change, in the considered temperature range. close to human comfort temperature can be used. This temperature is located between 18 and 24 \({}^{\circ}\)C [91]. Of the many available hydrated salts and organic PCMs, potassium fluoride tetrahydrate, calcium chloride hexahydrate, butylstearate, dodecanol, octadecane, propyl palmit and capric-lauric acid are therefore possible candidates for passive air conditioning applications in buildings [92]. In order for PCM material to perform its thermal mitigation function during the day when it is hot, it needs to be regenerated (i.e., to return to the solid phase) at night. There are various options for regenerating the PCM: natural ventilation (by opening the windows), automatic ventilation using an air circulation system or cooling using water-cooled panels in direct contact with the PCM panels (surface activation). PCMs are particularly well suited to passive air-conditioning applications in hot countries, such as sub-Saharan Africa, where the days are hot and the nights cold. However, they can also find applications in other parts of the world. In the following section, we use a numerical study to illustrate the stabilizing effect of PCM panels in a typical residential room in Villach, southern Austria (see Figure 5). Real climatic data were used for this study. The software tool PCMexpress 1.0.3 was used for the computations (Valentin Software GmbH, www.valentin-software.com, accessed on 10 August 2023). In the simulation model, PCM panels cover the entire ceiling. In some of the simulations, they also cover the entire wall surface. Alba Balance 25 solid gypsum boards (ABG) were selected as PCM. The main technical characteristics of these PCM panels are shown in Table 1. The construction of the walls plays a major role, as it obviously influences the thermal capacity and conductivity of the walls, and therefore the heating/cooling demand. In our case, a simple construction (i.e., elevated/suspended reinforced concrete with 25 mm ABG) was selected.