Building energy flexibility (BEF) is getting increasing attention as a key factor for building energy saving target besides building energy intensity and energy efficiency. BEF is very rich in content but rare in solid progress. The battery energy storage system (BESS) is making substantial contributions in BEF. This review study presents a comprehensive analysis on the BEF with BESS, in terms of the current study statues, connection to building energy management, integration with renewable energy sources and electric vehicles. In addition, electric vehicle (EV) is found to be flexible by itself for BEF. This study delivers some new insights and findings that the synchronized aging of building and battery and complex optimization with multiple indexes, is still challenging. Additionally, the opportunities behind the challenges are also highlighted that the higher level of BEF can be achieved by tackling the higher level of system complexity. Further, the suggestions about distributed energy control for distributed energy system, as well as coupling building energy pattern with users’ behavior pattern, are also proposed. The review study may further facilitate the BEF with BESS and push forward the synergistic improvement of building energy efficiency, intensity, and flexibility.

Air-permeable building envelopes (APBEs) utilize the infiltrated or exfiltrated airflow within porous materials to directly change their temperature distribution to reduce heat loss/gain. APBEs effectively integrate building ventilation and heat recovery to achieve excellent thermal insulation while improving indoor air quality. This paper presents a comprehensive review of the fundamentals and classifications, historical evolution over time, opportunities and benefits, and future views on APBEs. It can be treated as a responsive building envelope that enables building envelopes to dynamically change the U-values by varying the infiltrated or exfiltrated airflow rate within a porous material. Previous studies have indicated that the U-value of 0.1 W/(m²·K) can be realized by employing APBEs. Moreover, some research demonstrates that APBEs could act as high-performance air filters that reduce over 90% of particulate matter within fresh, ventilated air. Some factors, such as airflow rate, thickness, and thermal conductivity of porous materials, have a significant influence on the effectiveness of APBEs. For practical applications, integrating the APBE with passive building ventilation can help reduce the initial cost and facilitate decarbonization in buildings. Moreover, advanced control strategies could collaboratively optimize the operation of ABPEs and build energy systems to maximize their energy-saving potential.