# Building Energy Savings by Developing Complex Smart Windows and Their Controllers
## 1 Introduction  
Korea is currently strengthening its green building roadmap. As part of this effort, we are striving to implement energy-saving measures with windows based on the \"Energy-saving Design Standards\" to minimize the energy demand for mandatory zero-energy buildings in the public sector by 2020 and the private sector by 2025 [1]. Notably, office buildings account for 60-70% of the total energy consumed by the building sector, making energy savings in offices an increasingly important issue [2, 3]. However, the development of smart envelope and Energy Management System (EMS)-linked technologies to reduce the cooling load of office buildings, which are the most thermally vulnerable parts when implementing zero-energy buildings, remains insufficient. Building energy consumption is increasing globally, and office energy consumption is experiencing significant growth. Consequently, many studies are underway to reduce building cooling loads. Some studies focused on active warning systems [4] and window systems using double skins [5]. However, these efforts concentrated solely on reducing the cooling load by blocking solar radiation from reaching the building. Consequently, lighting energy consumption in office buildings is increasing. To minimize heat lossthrough windows in conventional buildings, it is essential to produce glass that can control the transmittance of visible light radiation artificially. The need for such technological advancement has become achievable recently, thanks to the progress in the development of thin-film materials with various functionalities and research on liquid crystal materials. One of these technologies is the use of variable transmittance glass, which can artificially adjust its transmittance based on the presence or absence of light or an electric field. Such glass can be broadly categorized into polymer-dispersed liquid crystal (PDLC) technology, vacuum sputter-coated electrochromic technology, and others, depending on the type of functional material used [6,7]. These advanced materials are being pursued as next-generation, high-performance, and high-value-added products. Various foreign companies and research institutions are investing substantial budgets in their development, alongside research on control methods. Therefore, in this study, we conducted research on the reduction of lighting and cooling energy consumption in zero-energy buildings through dimming control of a developed smart window system incorporating variable transmittance glass. To achieve this, the optimal dimming control algorithm has been derived and applied in simulations to analyze the energy consumption for lighting and cooling. The developed smart skin is a composite of suspended particle device (SPD) and lighting guide glass (LGG) technologies. To address this issue, our study aimed to develop an active technology capable of reducing both cooling and lighting energy consumption by analyzing the correlation between cooling and lighting. Therefore, in this study, we developed a complex smart window by combining SPD glass [8,9] that can control light transmittance by actively responding to changes in indoor and outdoor environments, such as indoor temperature, illuminance, outdoor temperature, and solar radiation, with a LGG [10] that is capable of actively controlling solar reflectance. In this case, SPD adjusts its transparency based on the external conditions of the building to minimize the building's energy loss through heat, while LGG refracts sunlight to enhance the incoming light, thus conserving indoor lighting energy. The refractive index of LGG has been calculated using an algorithm to achieve the optimal value by adjusting its angle based on the elevation of the sun. In addition, we incorporated a dimming LED [11] to maintain the indoor illuminance level and a controller to optimally control the complex smart window [12,13]. To achieve this, an optimal control algorithm and an EMS were developed using computer simulations [14].