Direct absorption solar collectors: Fundamentals, modeling approaches, design and operating parameters, advances, knowledge gaps, and future prospects

Direct absorption solar collectors (DASCs) based on nanofluids offer a promising solution for achieving the dual goals of solar energy utilization: maximizing solar absorption and minimizing thermal losses. In contrast to conventional surface absorption solar collectors, which suffer from substantial heat losses, DASCs operate by replacing elevated-temperature absorption surfaces with nanofluid bulk for volumetric absorption. To bridge the gap between theoretical research and commercialization, a comprehensive understanding of DASCs is essential. This includes modeling approaches, the impact of design and operational parameters, recognizing limitations, and evaluating future prospects. This study provides a comprehensive review with a focus on resolving disagreements regarding low-flux DASC responses to specific design and operational variations that have sparked conflicting interpretations in the literature. This review, by addressing these discrepancies, serves as an invaluable resource for researchers seeking a more nuanced understanding of this evolving field, facilitating its advancement into practical applications. This review comprehensively examines the field of DASCs across eight distinct sections. Section 1 provides an overview of solar energy's potential, the evolution of solar collectors, and the rationale for the review. Section 2 focuses on theoretical modeling approaches for simulating colloidal suspensions in solar thermal systems, including optical properties, radiative transport, and heat transfer mechanisms. The strengths and limitations of these models are critically evaluated to assist researchers in selecting the most suitable one for specific colloidal systems. Additionally, a critical assessment of analytical and numerical studies in the existing literature is presented in this section. Section 3 offers a detailed view and critical assessment of experimental efforts in the field. The stability of nanofluids is discussed in section 4, while sections 5 and 6 analyze the impact of operating conditions, geometry, design parameters, and flow properties on DASC performance criteria. We address contradictions and ambiguities in the effects of some operating variables in the DASC literature, considering state-of-the-art simulation techniques. Section 7 focuses on economic and environmental analyses related to DASCs, providing insights into their feasibility and sustainability. Finally, Section 8 synthesizes conclusions from the reviewed literature, identifies research gaps, and proposes future directions based on recent advancements in DASC technology.