# Energy Saving Potential of PCMs in Buildings under Future Climate Conditions
## 1 Introduction  
Reducing the cooling and heating requirements of buildings as the climate changes is one of the main challenges in buildings design worldwide [1; 2]. The building sector consumes 20-40% of all energy, especially in developed countries [1; 3], and nearly 50% of the energy used in buildings is consumed by Heating Ventilation and Air Conditioning (HVAC) [3; 4]. In addition, 30-40% of greenhouse gas emissions are caused by energy consumption in buildings [2; 5]. Therefore, in the absence of energy improvements in buildings, global energy demand will increase about 50% by 2050 [6].  
Improving energy performance in buildings can be achieved by both passive means (development of the building envelope) [6] and active means (smart HVAC equipment) [7]. Investment in the building envelope is an ideal solution because a passive design can achieve long-term energy efficiency and carbon removal with the help of the sun. In addition, it will enhance thermal comfort, which increases energy savings by stabilizing the indoor air temperature [3; 4]. Building envelopes are distinguished by their ability to regulate the heat exchange between the interior and exterior environments, which affectsthe cooling and heating requirements, occupant comfort, and the ability to integrate new building materials and systems [8].  
Many technologies have been developed to reduce building energy consumption and improve thermal comfort, for example insulation materials [6], double-glazed reversible window systems [9], heat insulating solar glass [10], hybrid walls integrated with heat collectors, and solar thermal power generators [11]. Reinforcing building envelopes with thermal insulation is an effective strategy that has been widely used to reduce heat dissipation from the interior environment to the exterior environment, especially in lightweight buildings [12]. Thermal mass application in building envelopes has enhanced energy efficiency and reduced greenhouse gas emissions by allowing buildings to store heat energy as latent heat, which affects the indoor temperature, energy requirements, and thermal comfort of the occupants [12].  
Systems of thermal energy storage can create a balance in the energy demand between the nocturnal and diurnal periods by storing latent heat [8]. Several studies [3; 4; 13; 14; 15; 16] have been published about using phase change materials (PCMs) in buildings. PCMs are distinguished by their ability to store high amounts of energy inside short temperature ranges through their high melting temperatures [13]. They are a unique alternative for increasing energy efficiency and improving thermal comfort in buildings [14; 15]. They can be included in the envelopes of buildings in different ways to achieve passive heating and cooling. In this context, a passive PCM system is a sustainable way to improve occupant comfort and energy performance, especially in the extreme weather conditions of winter and summer. Therefore, this study evaluates the effect of future climate change on the energy saving potential of applying various PCMs to lightweight building envelopes in different climatic zones (Seoul, Tokyo, and Hong Kong), analyzing the buildings' heating and cooling loads and peak energy demands according to the phase change temperatures of the PCMs. An additional investigation considers the buildings' thermal performance in the interior environment.