Development of a Refrigeration and Dehumidification Cycle Using Lower Critical Solution Temperature Mixtures
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Kocher, Jordan D.
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Abstract
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: (i) investigates a new thermodynamic cycle, (ii) evaluates the extent to which it can provide both dehumidification and refrigeration, (iii) demonstrates a proof-of-concept system driven by relatively low temperature (≤ 50 °C) heat, and (iv) evaluates the cost effectiveness of the system.
The thermodynamic cycle developed 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. One phase is water-rich (WR), while the other 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 (i.e., a reduction in temperature below that of the ambient) and/or dehumidification, among other effects.
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. Furthermore, a multi-stage operation is described, which utilizes different LCST mixtures in each stage and can therefore achieve an overall chemical potential difference that is severalfold greater than what can be achieved with single-stage operation. Analysis reveals that practical LCST refrigeration and dehumidification systems would be about 60% less efficient than conventional systems, due mostly to the large sensible heating requirement of LCST mixtures with poor water uptake.
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. This could pave the way for non-aqueous LCST mixtures that do not possess negative entropies of mixing. The analytical expression for the enthalpy of separation of LCST mixtures is derived, which serves as a useful alternative to direct enthalpy measurements using differential scanning calorimetry. The properties of new LCST mixtures containing hygroscopic additives are also measured. It is revealed that even when multi-stage operation is used, the temperature lift and indoor humidity that current LCST mixtures can provide are insufficient for thermal comfort. This motivates the need to find new LCST mixtures if the LCST air conditioning cycle is to become practical.
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. A maximum temperature lift of 0.96 °C was measured for continuous, single-stage refrigeration, while a temperature lift of 2.32 °C was measured for three-stage, stepwise refrigeration.
Finally, a technoeconomic analysis is performed to understand the potential benefits and limitations of an LCST cycle air conditioner. It is revealed that LCST refrigeration is significantly hindered by the exergy associated with phase separation when aqueous LCST mixtures are used; non-aqueous LCST mixtures with higher separation temperatures could potentially mitigate this limitation. Meanwhile, LCST dehumidification would be less efficient than traditional desiccants, unless the recuperator effectiveness were very high (> 0.9). Furthermore, the cost of separation is likely to be a major barrier that must be addressed if LCST-based systems are to become cost effective.
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2024-12-19
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Dissertation