Generation classification of solar thermal technologies

Solar thermal technologies (STTs) have undergone significant evolution over the past five decades. Yet the literature still lacks a unified framework for systematically categorizing these technologies based on their functional, thermal, and application characteristics. This study introduces a comprehensive four-generation classification framework (STT-G1 to STT-G4) that maps the technological evolution of solar thermal systems using operational temperature ranges, design complexity, and end-use suitability as key differentiators. The proposed framework bridges fragmented descriptions across existing studies and enables clearer benchmarking of system capabilities, performance boundaries, and application domains spanning domestic heating, industrial process heat, and high-temperature concentrating solar power (CSP) operations. By consolidating performance ranges, deployment maturity, and emerging trends, the classification provides a structured foundation for comparing technologies, identifying research gaps, and supporting future policy and market-driven decision-making. The framework also highlights opportunities for next-generation development, including hybrid thermal configurations, high-flux receivers, sCO2-based cycles, and solar-driven thermochemical pathways. Overall, this work offers a unified perspective that strengthens understanding of STT evolution and serves as a strategic guide for researchers, system designers, and energy planners in advancing solar thermal innovation.