## 4 Identification of Recent Developments and Possible Solutions
### Alternative Working Fluid Pairs  
In general, all working fluid pairs suggested for absorption heat pumps may also be used in ACHP cycles [79]. The unique feature of a working fluid pair is a greater temperature glide during complete evaporation and condensation compared to the glide of heat source and sink, causing the partial phase change that makes a solution circuit necessary. For a comprehensive review and possible identification of interesting alternatives, Table 2 lists an overview of literature sources investigating alternative working fluid pairs besides ammonia-water for use in ACHPs. There are mainly theoretical studies with few experimental investigations on a laboratory scale (below 30 kW heating capacity). Following the Montreal Protocol and Kyoto Protocol addressing the ozone depletion potential (ODP), the main issues in recent years have been the EU Regulation No. 517/2014 (2014) [80], also known as F-Gas Regulation, and the European Directive 2006/40/EC (2006) [81], establishing very strict limits on the GWP values of applicable refrigerants. Due to this legislation, many sources dealing with substances having an ODP \(>\) 0 or a GWP \(>\) 1000 are not listed and will not be discussed further. The Globally Harmonized System of Classification and Labelling of Chemicals (GHS) classification was carried out using the \"GESTIS-Stoffdatenbank\" [82]. Maximum workplace concentration (MAK) values are taken from the \"DFG MAK- und BAT-Werte-Liste 2020\" [83]. The applicable temperature range is defined so that saturation pressure is between 1 and 25 bar. If 200 \({}^{\circ}\)C is still possible at the sink with a vapor pressure below 25 bar, 200 \({}^{\circ}\)C is assumed to be the upper limit. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Author** & **Epc.** & **Epc.** & **Epc.** & **Epc.** & **Epc.** & **Epc.** & **Epc.** & **Epc.** \\
**(Sveden)** & 1990 & & & NH\({}_{3}\) (\({}^{1}\)) & & H\({}_{2}\)O (\({}^{2}\))/Libr (\({}^{3}\)) & & 0/200 & [84] \\
**Chatzidakis, Regdakis** & 1992 & & & NH\({}_{3}\) (\({}^{1}\)) & & H\({}_{2}\)O (\({}^{2}\))/Libr (\({}^{3}\)) & & 0/200 & [85] \\
**(Germany)** & 1992 & & & NH\({}_{3}\) (\({}^{1}\)) & & NaSCN (\({}^{4}\)) & & \(-\)35/160 & [86] \\
**(India)** & 1999 & & & NH\({}_{3}\) (\({}^{1}\)) & & LN\({}_{3}\) (\({}^{1}\)) & & \(-\)40/200 & [87] \\
**Hanni** & 2015 & X & NH\({}_{3}\) (\({}^{1}\)) & & LNO\({}_{3}\) (\({}^{1}\)) & & \(-\)40/200 & [88] \\
**(Ausria)** & 2015 & X & & & & & & \\
**(Ausria)** & 2018 & & H\({}_{2}\)O (\({}^{2}\)) & - & Libr (\({}^{3}\)) & & 0/200 & [88] \\
**(India)** & 2017 & & & NH\({}_{3}\) (\({}^{1}\))/CO\({}_{2}\) (\({}^{3}\)) & & & & \\
**(Netherlands)** & 1992 & X & CO\({}_{2}\) (\({}^{6}\)) & & Acetone (\({}^{7}\)) & & \(-\)40/80 & [91] \\
**(Germany)** & 2019 & & CO\({}_{2}\) (\({}^{6}\)) & & Acetone (\({}^{7}\)) & & \(-\)40/80 & [92] \\
**(Spain)** & & & CO\({}_{2}\) (\({}^{6}\)) & & Acetone (\({}^{7}\)) & & \(-\)40/80 & [93] \\ \hline \end{tabular}
\end{table} 
Table 2: Overview of alternative working fluid pairs proposed in the literature for use in ACHP cycles. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & & & & & & \\ \hline [PERSON] et al. & & & & & & & \\ (Japan) & 2007 & & & DME ([8]) & & MoOH ([9]) & \(-\)30/200 & [94] \\ [PERSON] et al. & & & & & & & \\ (Japan) & 1991 & X & & TE ([10]) & & TEGDME ([11]) & & 75/200 & [95] \\ [PERSON] et al. & & & & & & & \\ (Spain) & 1997 & & & TE ([10]), MoOH ([9]) & & TEGDME ([11]) & & 65/200 & [96] \\ [PERSON] et al. & & & & & & & \\ (Spain) & 2000 & & & TE ([10]), /H\({}_{2}\)O ([11]) & & TEGDME ([11]) & & 75/200 & [97] \\
**[PERSON] et al.** & & & & & & & \\ (Spain) & 2003 & X & TE ([10]), MoOH ([9]), & & & & & \\ (Spain) & 2005 & HFIP ([13]) & & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._There were 13 alternative working fluid pairs investigated in the literature, divided into inorganic and organic refrigerants with TFE and HFIP as the only non-natural refrigerants.