Renewable energy and 20th-century architecture: mixed-methods research through documentary investigation, case-based inquiry, and expert consultation

The integration of Renewable Energy Sources (RES) into 20th-century Architecture presents a complex challenge at the intersection of energy transition, technological innovation, and heritage preservation. Modern design paradigms (rationalism, spatial fluidity, modularity, biophilic intent, natural lighting and ventilation) often resulted in lower energy performance compared to pre-industrial buildings. Trust in industrialization and technological advancement led to early RES experiments but also contributed to the progressive degradation of materials and systems, exacerbated by limited knowledge of mass-customized material and techniques. These distinctive characteristics require tailored RES integration criteria, as current guidelines still equate them to traditional heritage buildings. This study addresses this critical gap through mixed-methods research that integrates documentary investigation, case-based inquiry, and structured expert consultation to comprehensively examine both technical constraints and cultural values. The methodology reconstructs the energy challenges and transition of 20th-century Architecture, maps 133 international RES interventions, and incorporates the perspectives of 26 experts through interviews, working tables, and focus groups. This framework enables the identification of recurrent barriers, context-sensitive strategies, and differences with historical examples. The findings culminate in a set of multidisciplinary criteria for RES integration, considering cultural significance, aesthetic compatibility, structural and hygrothermal performance, electrical safety, environmental impact, and system reversibility. It also defines three strategic approaches (visual concealment, mimetic adaptation, tailored design) to guide context-sensitive implementations. Beyond offering a replicable approach, the study reframes RES integration as an opportunity to reinterpret the legacy of 20th-century Architecture through the lens of sustainability, resilience, and cultural continuity.