Improving the energy efficiency of existing building external windows: Insights from integrated renovation technologies

The construction industry consumes a significant amount of energy, and the resulting carbon dioxide emissions adversely affect the human living environment. As one of the least energy-efficient components in building envelope systems, building external windows are characterized by photothermal performance indicators, including heat transfer coefficient (U-value), visible light transmittance (Tvis), and solar heat gain coefficient (SHGC), that directly affect building energy consumption. In recent years, although numerous studies have focused on energy-saving technologies for building external windows, few have addressed window energy performance issues in a systematic manner, especially in terms of energy-saving renovation. First, the current development status of energy-saving technologies for building external windows was reviewed using a bibliometric approach. Second, the principles and characteristics of energy-saving technologies for building external windows, such as window-to-wall ratio, shading devices, window frames, glazing, and airtightness, were summarized. Finally, using CFD simulation for a specific case under the climatic conditions of hot-summer and cold-winter regions, the thermal performance of an intelligent adjustable airflow photovoltaic integrated window was investigated. The results show that, compared with commonly used double-glazed windows, through intelligent dynamic control of the window vents, this integrated window not only reduces heat gain by 53.04% during the daytime of a typical summer day but also increases heat input into the room by 51.12% during the daytime of a typical winter day. Even during the daytime of cloudy winter days, the window can reduce heat loss by 21.12%, thereby maximizing annual building energy savings and demonstrating excellent climatic adaptability. Despite existing challenges, integrated external-window technologies can help overcome the limitations of single technologies and remains one of the key approaches for future sustainable building design.