## 4 Identification of Recent Developments and Possible Solutions
### Alternative Working Fluid Pairs
Except for CO2/Acetone and NH3/NaSCN, the applicable temperature range is always 200 \({}^{\circ}\)C or higher. However, the minimum temperature depends strongly on the refrigerant selected and is generally lower for inorganic refrigerants. A further comparison of the cycle performance among the working fluid pairs is not appropriate due to the large differences in vapor pressure. [PERSON] and [PERSON] (1990) [84] as well as [PERSON] and [PERSON] (1992) [85], added LiBr as absorbent to the ammonia-water mixture and studied the suitability of the three-component mixture for the ACHP cycle. They stated inconsistent results regarding the improvement potential and recommended further investigations. [PERSON] and [PERSON] (1999) [86] and [PERSON] (2015) [87] investigated NH3 paired with salts to increase the amount of refrigerant in the gas phase, eliminating the separation that may be required at increased source temperatures to reduce the water content in the vapor phase. Conducted experiments demonstrated the general functionality of this approach, although further research is required to establish prototypes and applications. The investigations on H2O/LiBr in a temperature range between 2 \({}^{\circ}\)C and 35 \({}^{\circ}\)C, done by [PERSON] et al. (2018) [89], and in double-effect cycles, done by [PERSON] et al. (1991) [88], are both far from practical application. [PERSON] et al. (2017) [90] paired NH3 with CO2 and H2O in a wet compression cycle. The simulations of these cycles predict advantages in efficiency by increasing the amount of CO2. However, there are many uncertainties like the formation of solid phases, ternary property data and the compressor model used so that the authors suggest experimental validation as a necessary next step. CO2/Acetone is an interesting working fluid pair for safety aspects, but with standard components only feasible below 80 \({}^{\circ}\)C. Experiments by [PERSON] and [PERSON] (1992) [91] and [PERSON] (1994) [100] revealed poor heat transfer properties with the coaxial heat exchangers used and little further experience with this working fluid pair in the literature. In recent years, the thermodynamic properties have been increasingly studied experimentally, and [PERSON] et al. (2019) [92] simulated the usability for ACHP cycles in automotive air conditioning based on the available composition data. [PERSON] (2020) [93] conducted thermodynamic modelling and simulation of the cycle and theoretical-experimental investigations of the desorption process in a PHE. DME/MeOH was theoretically investigated by [PERSON] et al. (2007) [94] in the temperature range between 7 \({}^{\circ}\)C and 60 \({}^{\circ}\)C for the single stage cycle and the DAHX cycle. The differences in efficiency between pure DME and the mixture in both cycles were small. They suggest to further proof their findings in experiments. TEGDME or PEGDME as solvent use flammable and toxic TFE or MeOH as the refrigerant. These mixtures are suitable for temperatures above 65 \({}^{\circ}\)C and PEGDME for cases where the compressor may be lubricated by the lean solution [98]. [PERSON] et al. (1991) [95] stated that TFE/TEGDME shows good thermal stability and no corrosion on copper or carbon steel up to 180 \({}^{\circ}\)C. [PERSON] et al. (1997) [96] investigated the performance for TFE/TEGDME and MeOH/TEGDME for use in the single-stage and DAHX cycles. They reported interesting features for use in high temperature applications with better results for TFE/TEGDME and suggested further investigations. [PERSON] et al. (2000) [97] conducted further investigations for a three-component mixture with additional water and reported further benefits such as an improvement in performance. [PERSON] et al. (2003) [98] investigated the working pairs TFE/PEGDME and MeOH/PEGDME to develop a 15 kW pilot plant. Here, MeOH was selected as the refrigerant for further investigations due to the lower pressure ratio required. In addition, [PERSON] (2005) [99] investigated the working pair HFIP/PEGDME, which was not tested experimentally.