## 5 Conclusions  
In this paper, the results of an economical optimization of the MGT MTT Enertwin were presented. The study aimed at determining whether an operation at partial-load could be economically more convenient than at full-load. Germany was selected as a case study, being Central Europe's main market of this CHP. As constraints, the system had to fulfill two different heat demand profiles and comply with the electrical demand. The analysis spanned over 15 climate zones, investigated single-family and multi-family buildings, and included environmental and external elements such as weather, season, and day of the week.  
This paper showed that an operation at full-load is, in the vast majority of cases, more convenient than at partial-load. The price difference is usually between 10% and 20%. This fact indicates that firing the system at full-load, but less frequently allows burning, in total, less fuel than firing more frequently at partial-load. The inefficiency of transient maneuvers and intermediate SP is the major cause for this result. The partial-load inefficiencies being a common characteristic of MGTs, this conclusion can be transferred to other MGT-based CHP systems as well.  
A sensitivity study on the potential of batteries showed that their use is highly beneficial to reduce the operating costs. Depending on the building size, batteries between 1.5 and 3.0 kWh are recommended. The battery type can have a large impact on the installation costs and should be decided carefully. Installation costs are generally paid back within two or three years, depending on the electricity prices (sell/buy), hence on the agreements with the power provider.  
A qualitative parametric study on the thermal storage size was also conducted, showing that larger water tanks allow for more flexibility and work like a thermal buffer. The advantages of having more water ready for use outweigh the higher thermal losses given by a larger tank, provided that this is properly thermally insulated. An upper limit for the tank size is not definable; however, a minimum of 1200 L is recommended in order to have enough operational flexibility and to possibly obtain, in some cases, incentives for the installation.  
Finally, in the case of full-load mode, the system operating profile generally shows a lower number of starts and stops compared to partial-load. Since the transient loads are thought to be responsible for thermo-mechanical fatigue, a full-load operation could also be beneficial in terms of components' life. Future studies from the same authors will investigate this topic in a future work.  
Figure 16: Comparison of the optimal operating profile for two example scenarios: climate zone 15, typical day UWB (**a,b**), and climate zone 13, typical day WSB (**c,d**). On the left, the full-load mode, on the right, the partial-load mode, for a 2-living-unit building.  
**Author Contributions:** D.C.: conceptualization, methodology, software, formal analysis, data curation, writing--original draft. F.G.: writing--review and editing, supervision. P.K.: project administration. M.A.: funding acquisition. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to thank [PERSON] and her colleagues from Lisbon University for setting the framework for this analysis and [PERSON] and [PERSON] (DLR Stuttgart) for the experimental data.  
**Conflicts of Interest:** The authors declare no conflict of interest.