Windows are one of the significant indicators of the energy efficiency of a building and have undergone extensive research since the last decades. This paper reviews the performance of various window technologies covering the physical and optical properties of traditional windows and advanced window technologies. In window technologies, one of the most critical parameters is its thermal transmittance value or also known as U-value. In this paper, we discuss the relationship between the physical and optical parameters of the different types of windows and its U-value. Additionally, this paper will also provide interested readers with a wide range of information, including the research gaps in window technologies. Among the main conclusions, we found that, although several advancements have been achieved in this field in the last decade, further research is needed to develop window technologies that not only have high insulating properties but also can generate power.

No abstract

Introduction Phase change materials (PCMs) absorb, store, and passively release available thermal energy via latent heat transfer during phase change, thereby reducing peak demand and improving thermal comfort (Salunkhe and Shembekar, 2012; Kalnaes and Jelle, 2015; Wang et al., 2020). The thermal performance of PCMs is based on their melting point, thermal conductivity, and energy storage density. For this reason, when applied as energy storage, they require an instant melting and solidification point (Ji et al., 2014; Ma, Lin and Sohel, 2016). Paraffins, salt hydrates, and fatty acids are the most commonly used PCMs, having a melting temperature within human thermal comfort, making them suitable for building applications. However, such materials have major drawbacks, including low thermal conductivity, especially for organic PCMs. As a result, performance enhancements of PCMs are eagerly researched, to develop improved techniques (Fan and Khodadadi, 2011). Such methods require the addition of highly conductive materials, which can be done by modification of the encapsulation material, the shape of the container, using heat pipes, heat exchangers, micro-and macro-encapsulation, or the addition of highly conductive nanoparticles in the base fluid, creating nano-enhanced PCM (Babaei, Keblinski and Khodadadi, 2013; Ma, Lin and Sohel, 2016). Further techniques proposed the integration of metallic fins, foam wools, and graphite (Ji et al., 2014; Fan et al., 2013). The literature views of PCM enhancement materials have identified graphite, aluminium, and carbon as the most frequently applied materials for organic PCM enhancement. There are two methods to integrate PCM in building elements. The first method, "shape-stabilized", considers the direct addition of the PCM into a building element, such as a gypsum wall (Silva, Vicente and Rodrigues, 2016). The second method requires the PCMs to be encapsulated for technical use, as otherwise the material would disperse from the location (Cabeza et al., 2011). For this reason, the encapsulation method is the most commonly used form of integration and has become a topic of analysis in recent years. The geometry of the encapsulation can take any shape, but the most popular forms are tubes, pouches, spheres, and panels. Encapsulation geometry could potentially be harnessed as a heat enhancement method, improving the thermal conductivity of the PCMs (Amin, Bruno and Belusko, 2014). Additional benefits of encapsulation include the capacity to counteract phase segregation, which is a regular phenomenon particularly prevalent with salt hydrates, in which the high storage density of the material disperse in layers, leading to the decline in the storage efficiency.

No abstract