Defining the energy role of buildings as flexumers: A review of definitions, technologies, and applications

Buildings have undergone a significant transformation in their energy role, from consumers to prosumers and now to flexumers—a concept that signifies the flexible nature of modern buildings in the energy domain. However, existing research lacks a comprehensive definition of the transformative features of flexumers, as well as limited knowledge about their technologies and applications. This paper aims to fill this gap by providing a comprehensive summary of the new energy roles of buildings through a literature review. By reviewing current common definitions and classification frameworks of nine popular terms related to building energy, this study proposes innovative and basic definitions applicable to flexumers that can serve as a paradigm for adaptive expansion. The flexumer combines four main energy functions: consumption, generation, storage, and flexibility. These functions are dynamically balanced and optimized throughout the building's life cycle. Furthermore, flexumers have a positive impact on the grid and the wider energy system, as they can adjust their energy status in real time to synergize and complement the energy network. The long-term goal of flexumers is to achieve positive energy using only renewable sources, while simultaneously balancing zero carbon emissions and minimizing energy costs. The key technologies that enable buildings to become flexumers are comprehensively summarized, with a primary focus on electrical system energy efficiency, on-site energy generation, energy storage systems, and interactions with energy networks. This study details specific operational strategies feasible for each technology and compares them in terms of difficulty, cost, and interrelationships. Additionally, the study summarizes four major energy applications of flexumers: positive energy districts, resilient communities/cities, virtual power plants, and synergy with transportation. Based on the review results, five feasible directions for future work are proposed to activate buildings as flexumers. This study provides the first systematic summary of the roles and technologies of buildings in the energy field and offers a valuable reference for stakeholders in addressing the challenges they face in building energy and resolving contradictions as they strive towards carbon neutrality.