Generalization of second law efficiency for next-generation cooling and dehumidification systems

Abstract

Dehumidification plays a significant role in space conditioning energy use. Conventional vapor compression cooling systems employ dewpoint condensation to deal with latent loads. In contrast, separate sensible latent cooling (SSLC) and other advanced alternative dehumidification systems can significantly reduce the electricity usage for dehumidification. The second law efficiency can be used as a benchmark to evaluate thermodynamic performance of alternative dehumidification systems. However, limitations exist in previous studies that define the thermodynamic reversible limits and second law efficiency for cooling and dehumidification systems. This work presents a new physics-based definition for the reversible limit and the second law efficiencies for cooling and dehumidification systems with air recirculation. The new framework is then extended to define a novel performance metric, the seasonal second law efficiency, to form a universal benchmark for assessing various cooling and dehumidification systems. Five cooling and dehumidification systems including magnetocaloric cooling, solid desiccant dehumidification, and membrane dehumidification are evaluated using this benchmark. Steady-state thermodynamic models are constructed for each system. Second law efficiency for each system under various outdoor temperatures and indoor sensible heat ratios (SHR) are calculated. The annual electricity usage of the five systems is used to justify the seasonal second law efficiency definition. The results show that compared to conventional vapor compression systems with mechanical dehumidification, the membrane-based AMX-R cycle can reduce annual electricity use by 12.2%–22.2% and increase the seasonal second law efficiency by up to 36%.