Overview of Ionic Liquids Used as Working Fluids in Absorption Cycles

Abstract

The cycle performance of refrigeration cycles depends not only on their configuration, but also on thermodynamic properties of working pairs regularly composed of refrigerant and absorbent. The commonly used working pairs in absorption cycles are aqueous solutions of either lithium bromide water or ammonia water. However, corrosion, crystallization, high working pressure, and toxicity are their major disadvantages in industrial applications. Therefore, seeking more advantageous working pairs with good thermal stability, with minimum corrosion, and without crystallization has become the research focus in the past two decades. Ionic liquids (ILs) are room-temperature melting salts that can remain in the liquid state at near or below room temperature. ILs have attracted considerable attention due to their unique properties, such as negligible vapor pressure, nonflammability, thermal stability, good solubility, low melting points, and staying in the liquid state over a wide temperature range from room temperature to about 300 ∘ C. The previously mentioned highly favorable properties of ILs motivated us for carrying out the present research and reviewing the available ILs found in the literature as the working fluids of absorption cycles. Absorption cycles contain absorption heat pumps, absorption chillers, and absorption transformers.

Are Ionic Liquids Suitable as New Components in Working Mixtures for Absorption Heat Transformers?

“…They can be divided into three classes: absorption heat pump (AHP), absorption chiller (AC), and absorption heat transformer (AHT). Absorption cycles become of great interest since electrical energy is replaced with low grade or waste heat allowing both primary energy savings and energetic efficiency improvements [2]. Consequently, absorption cycles enhance the atmospheric conditions by reducing the emissions of greenhouse gases.…”

“…Nowadays, the most used binary systems in the absorption heat cycle are {water + lithium bromide (H 2 O + LiBr)} and {ammonia + water (NH 3 +H 2 O)}. The aqueous solution of LiBr is the most successful working mixture in absorption cycles and widely spread all over the world [2,3]. Nevertheless, LiBr aqueous solution has some main drawbacks as follows:…”

“…Crystallization of {H 2 O + LiBr} at high concentrations is a common problem. High vacuum conditions should be preserved in the system for suitable operation of the {H 2 O + LiBr} system; otherwise, the performance of the absorption cycle would be greatly reduced [5].…”

“…High vacuum conditions should be preserved in the system for suitable operation of the {H 2 O + LiBr} system; otherwise, the performance of the absorption cycle would be greatly reduced [5]. {H 2 O + LiBr} is corrosive to metals [2][3][4][5][6].…”

“…Performance development and enhancement of absorption cycles were evaluated. They concluded that double-stage absorption refrigeration cycle based on {H 2 O + LiBr} has the highest coefficient of performance (COP) if compared to other systems in the market. In addition, they stated that multistage absorption cycles have a promising future.…”