Composite adsorbents and working pairs for sustainable adsorption refrigeration: a thermodynamic and materials review

The rapidly increasing global demand for cooling has intensified the need to transition from conventional vapor compression systems to sustainable and carbon–neutral alternatives. Adsorption Refrigeration Systems (ARS) have gained significant attention due to their ability to utilize low-grade waste heat or renewable energy sources while employing environmentally benign refrigerants. However, their large-scale deployment remains limited by the relatively low performance of conventional adsorbent–refrigerant working pairs. This review provides a critical assessment of recent advancements in composite adsorbents and advanced working pairs aimed at improving ARS efficiency and feasibility. Traditional systems, particularly silica gel water pairs, exhibit moderate performance with coefficients of performance (COP) ranging from 0.3 to 0.5 and specific cooling power (SCP) between 100 and 350 W kg−1. Substantial improvements have been achieved through the development of advanced materials such as CaCl2/silica gel composites, which demonstrate enhanced adsorption capacities (0.75 kg kg−1) and COP values approaching 0.8. Similarly, expanded graphite-based adsorbents significantly improve heat transfer properties, resulting in SCP values exceeding 1000 W kg−1. Metal–Organic Frameworks (MOFs) further enhance system performance, offering 10–20% higher thermodynamic efficiency and achieving COP values of 0.85–0.90 due to their high surface area and tunable pore structures. Additionally, graphene-enhanced composites improve thermal conductivity up to 1.55 W m K. Despite these advancements, challenges such as high material costs, scalability issues, and long payback periods persist. Addressing these barriers through cost-effective and scalable solutions is essential for widespread ARS adoption.