Development of a Refrigeration and Dehumidification Cycle Using Lower Critical Solution Temperature Mixtures

As global temperatures continue to rise and urbanization intensifies, so grows the need for efficient, cost-effective, low global warming potential (GWP) space cooling technologies. Accordingly, there is an ongoing focus surrounding research into new space cooling cycles that are more efficient and use zero GWP refrigerants. This dissertation will: (i) investigate a new thermodynamic cycle, (ii) evaluate the extent to which it can provide both dehumidification and refrigeration, (iii) demonstrate a proof-of-concept system driven by relatively low temperature (≤ 50 °C) heat, and (iv) evaluate the cost effectiveness of the system.

The thermodynamic cycle studied in this dissertation utilizes aqueous mixtures that possess a lower critical solution temperature (LCST). These mixtures are homogenous (single-phase) and will mix with water at room temperature, but when they are heated above the LCST they separate into two phases. The first phase is water-rich (WR), while the second phase is water-scarce (WS); this difference in composition leads to a chemical potential difference when the phases are physically separated and cooled down to ambient temperature. This chemical potential difference forms the basis of the LCST cycle and can be used to produce refrigeration and/or dehumidification.

A thermodynamic analysis of this new “LCST cycle” is performed, deriving the relationship between the performance metrics (temperature lift, indoor humidity, coefficient of performance, and moisture removal efficiency) as a function of the material figure-of-merit: the chemical potential of water difference between the WR and WS phases at room temperature. The governing thermodynamic relations that are relevant to LCST mixtures are derived, which provide insight into the properties that would necessarily exist in hypothetical LCST mixtures with greater chemical potential differences than existing LCST mixtures. Furthermore, the common misconception that LCST behavior necessarily emerges from a negative entropy of mixing is dispelled. Several experimental demonstrations of this new cycle are performed, and the temperature drop, humidity drop, and coefficient of performance (COP) of the cycle are reported. Finally, a technoeconomic analysis is then performed to understand the potential benefits and limitations of an LCST cycle air conditioner.