## 6 Conclusions  
This work presents the current state of technology and aims to identify existing challenges and future trends for the utilization of ACHPs at high temperature operation. Different modifications of the ACHP cycle were presented and discussed in detail. Furthermore, a comprehensive overview of conducted experimental work was given concerning the described cycle modifications and existing challenges were identified. Recent developments and possible solutions were discussed based on current research activities and summarized in a detailed mind map. Finally, possible future trends for further research activities were defined. The following conclusions for the ACHP technology can be drawn:  
* Interest in the ACHP system has grown in recent years and has been supported by the successful implementation of first commercial units. * Many studies, both theoretical and experimental, focused on improving the achievable operating parameter and system efficiency to increase the competitiveness compared to VCHPs and conventional solutions for industrial high temperature applications. * Various configurations of the ACHP system with special characteristics have been developed and studied. The ACHP with a single-stage solution circuit was most widely investigated and is the only configuration in commercial use today. With the aim of further optimization of the ACHP system, existing challenges for the main components such as compressor, absorber/desorber and solution pump were identified based on the conducted research. * Compressors used so far have been positive displacement compressors such as reciprocating, screw and scroll. For the achievable parameters of the ACHP system, the compressor is a constraining component and associated with challenges, such as discharge temperature, lubrication, efficient and oil-free operation of the system. A variety of possible solutions to address these challenges, such as multi-stage compression with intercooling or liquid injection, were investigated. * Different types and configurations of heat exchangers have been used. For the design and operation of absorber and desorber, a clear trend towards vertically arranged PHEs has emerged. However, there are still challenges associated with the optimal design to achieve good liquid-vapor distribution and achieving high overall heat transfer coefficients. The ability to determine and predict the parameters required for the design and control of absorber and desorber more accurately is an important goal in further disseminating ACHP technology for use in industrial applications. * Various strategies to avoid cavitation in the solution pump have been successfully implemented and tested, thus this problem discussed in earlier literature was solved. * In addition to ammonia-water, a variety of alternative working fluid pairs for use in ACHP systems have been investigated. Further research and development are required to evaluate the reported potential improvements in a practical application. * Based on the conducted investigations as well as recent developments and solutions, the future trends for further research were defined for all identified challenges. * For the increased use of ACHP systems in high temperature applications, future research should focus on the development of (oil-free) liquid-injected compressors and the efficient design and operation of the absorber and desorber. Conceptualization, M.U.A., M.L., I.T., A.H., S.K., R.W. and T.M.E. ; methodology, M.U.A., M.L. and I.T. ; investigation, M.U.A., M.L. and I.T. ; resources, M.L., A.H., S.K., R.W. and T.M.E. ; writing--original draft preparation, M.U.A. ; writing--review and editing, M.U.A., M.L., I.T., A.H., S.K., R.W. and T.M.E. ; visualization, M.U.A. ; supervision, I.T., A.H., S.K., R.W. and T.M.E. ; project administration, M.U.A. ; funding acquisition, A.H., R.W. and T.M.E. All authors have read and agreed to the published version of the manuscript.