Refrigeration with electrolytic and immiscible liquid–liquid systems

Abstract

Due to environmental concerns, new refrigerants are urgently needed to meet the refrigeration needs in the coming century. Two novel absorption refrigeration systems are considered herewith (1) the mixed-solvent electrolytes, and (2) mixtures exhibiting liquid–liquid immiscible phase separations as working fluids. When properly designed, they can avoid the global warming and ozone depletion pitfalls usually associated with the chlorofluorcarbon refrigerants. Electrolytic systems such as water–lithium bromide–ethylene glycol, and ammonia–sodium thiocyanate mixtures can serve as cooling media in an absorption refrigeration cycle with salts as absorbents and water (or ammonia) as refrigerants. The cycle efficiency (expressed in terms of the coefficient of performance—COP) plays a major role in determining their economical viability. In this paper, we study the desirable molecular characteristics that the salt systems should possess in order to yield high COP. We set out to answer the questions: how do we select and screen electrolytic mixtures as working fluids? and what molecular properties impact on design and efficiency? We shall apply an accurate theory, namely, the mean-spherical approach (MSA) to electrolytes to model the activities of the salt solutions. The results are checked with available data to ascertain its accuracy. Predictions can then be made on the performance of electrolytic working fluids in applications such as air-conditioning and refrigeration. Next, for mixtures of chemical solvents that form liquid–liquid immiscibility gaps, a Maxwell cycle can be constructed to generate refrigeration. In order for this class of solvents to work, a number of practical and thermodynamic considerations must be satisfied. We examine several functional criteria that are needed to operate the cycle: the first is concerned with the free energy-of-mixing behavior; the second, the pattern of the immiscibility gaps; and the last, most importantly, the influence of activity coefficients on absorption. This type of refrigeration, not yet commercially developed, promises to be highly economical due to hardware savings (elimination of the condenser), and energy savings (no need to supply the latent heats of evaporation).