Temperature and Humidity Control Using Lower Critical Solution Temperature (LCST) Mixtures

Traditional air conditioning systems contribute significant greenhouse gas (GHG) emissions throughout their lifetimes, with roughly one third coming from the refrigerant, one third coming from energy consumption for temperature control, and one third from energy consumption for humidity control. There exists a need to create air conditioning systems that consume less energy and do not use high global warming potential (GWP) refrigerants. Aqueous mixtures that possess lower critical solutions temperatures (LCSTs) will mix with water when below the LCST and separate into a two-phase mixture above the LCST. This allows for the creation of a new air conditioning cycle. The first process in this cycle uses low-grade heat to cause the LCST mixture to separate into a phase that is water-rich and a phase that is water-scarce. Those two phases are then physically separated and cooled back down to ambient temperature. Due to its high chemical potential, the water-rich phase can desorb water into the outdoor air, which produces indirect evaporative cooling that can be provided to a building. Meanwhile, the low chemical potential of the water-scarce phase allows it to absorb moisture from building air, thereby providing dehumidification. After this, the two phases are reheated to once again induce phase separation and repeat the cycle. We have performed a step-wise demonstration of this new cycle with several different LCST mixtures and regeneration temperatures less than 100 C. We rank the LCST mixtures based on their effectiveness in this particular application. However, all of the mixtures fall short of reaching the conditions required for building thermal comfort; namely, the water-scarce phase is unable to dehumidify air to sufficiently low humidities. This demonstrates the need to discover new aqueous LCST mixtures that can provide superior performance when used in this cycle. In particular, we show that more effective LCST mixtures would have more negative enthalpies and entropies of mixing. The discovery of these new mixtures would enable an air conditioning cycle that is driven by inexpensive, low-grade (i.e., solar) heat, consumes no water and very little electricity, and uses no high GWP refrigerants.