Next generation of working pairs in absorption heat pumps
The history and latest developments in working pairs for absorption heat pumps were discussed in a recent review article.
Most process industries generate a large quantity of waste heat which is a by-product of industrial processes. A significant amount of this heat is low-temperature and therefore cannot be reused in other processes of these industries, which often require a high-temperature heat supply. In an Informatory Note, the IIR points out that globally, an estimated 42% of industrial waste heat is available at temperatures below 100°C. 
In this context, absorption heat pumps are an energy-efficient solution because of their ability to exploit low-grade waste heat. Studies have shown that for a heat source at 45–60°C, an absorption heat pump can produce about 2.5 to 11 times more energy than other waste heat recovery systems.  According to the IIR, there is a strong untapped potential for absorption heat pump technologies to be integrated in process industries with high heat demand at temperatures above 160°C. 
Absorption refrigeration and heat pump systems use refrigerant–absorbent (or solvent) pairs, called working fluids. In a review article published in Energy Reports, the authors summarised the progress of largely developed working pairs, which play an essential role in the performance of absorption heat pumps.  Absorption heat pumps have the advantages of using natural working fluids, for example ammonia/water and lithium bromide/water mixtures. However, these mixtures have disadvantages that limit the heat pump operating ranges. 
Over the past decade, working pairs based on ionic liquids (ILs) have attracted a lot of attention. This unique class of solvents offer several advantageous properties over traditional solvents. When used as a refrigerant or absorbent, their beneficial properties include negligible vapour pressure, high thermal and chemical stability, excellent miscibility with polar and nonpolar solvents, and recyclability.  The authors of the review article noted a limited number of studies detailing the thermodynamic properties of ionic liquids. Furthermore, Ji et al. concluded that achieving new working pairs remains a challenge, as their COPs are still lower than those of NH3+H2O and LiBr+H2O. However, by adjusting the operating conditions, the performance of the newly developed working pairs can be improved and the LI + H2O ([MSME][DMP] + H2O) pair seems to be the most promising at present.
For the most part, working pairs based on ionic liquids are still at the experimental research stage, and far from practical engineering applications.
Did you know? The IIR has published an Informatory Note on high-temperature heat pumps (HTHPs) for industrial applications prepared by Dereje S. Ayou, José Miguel Corberan and Alberto Coronas.
 Ayou D. S., Corberán J. M., Coronas A. High-temperature heat pumps for industrial applications, 45th Informatory Note on Refrigeration Technologies. https://iifiir.org/en/fridoc/high-temperature-heat-pumps-for-industrial-applications-amp-nbsp-144137
 Ji, L., Shukla, S. K., Zuo, Z., Lu, X., Ji, X., & Wang, C. (2023). An overview of the progress of new working pairs in absorption heat pumps. Energy Reports, 9, 703-729. https://doi.org/10.1016/j.egyr.2022.11.143
Credits image: Oak Ridge National Laboratory via Flickr