## 5 Conclusions  
As an illustrative example, a typical residential building located in Yangzhong, China, has been examined as a case study. In this paper, the economic performances of the five systems are compared from NPV, payback, and CSR by formulating the evaluation  
Figure 8: Comparison of NPV for CHP in different control strategies and domestic gas-fired heater.  
Figure 9: Annual energy cost and saving for CHP with different strategies.  
model for different DHW systems' configuration, including GB, ASHP, domestic gas-fired heater, domestic electric heater, and CHP systems. First, the economic performance of five configurations is compared from the aspects of initial investment, efficiency and annual energy costs. Then, the optimal two configurations of DHW system and DHW system are selected respectively, and the economic performance is further compared with that of traditional DHW system, including payback period, net present value (NPV) and cost-saving ratio (CSR). The results show that the DHW system based on domestic gas-fired heater is the most economical option in the AHUE is 31.28%. The main reason for this type of system is that once the hot water leaves the water storage tank and flows through the system piping, the water temperature drops due to the traveling distance and ambient air temperature, which leads to large amounts of energy being wasted and increases the energy cost. To eliminate the impact of heat use rate on economy, the investment payback period of CHP and domestic gas-fired heaters under different AHUE is analyzed. The results show that the DHW system based on CHP is the best when the ratio is more than 55.35%. Finally, three different operation modes of micro-CHP are considered, including minimum cost operation and minimum energy consumption. The analysis results show that the \(S_{chp,2}\) consists of an internal combustion engine, and thermal energy system (TES) is recognized to be a better choice for the examined residential building under investigation from the perspective of economic operation and payback.  
The micro-CHP system produces excess electricity that can be sold, the electricity price has a significant impact on the marketization of the micro-CHP system based on internal combustion engines. Meanwhile, specific subsidies for energy savings are adopted by governments. The use of internal combustion engines increases the profitability of CHP when subsidy is available. As a limitation, with the increasing penetration of renewable energy, the price of electricity may further decrease in the future power grid, which weakens the advantages of micro-CHP system, because its economic advantages are realized by saving electricity costs.  
Future work will establish a complete evaluation model on AHUE, which can forecast the heat demand of the DHW system at the beginning of system construction. Moreover, it will combine game theory to conduct real-time economic analysis of micro-CHP units to optimize local operating strategies and scheduling strategies.  
Conceptualization, P.Z. and P.L.; methodology, P.Z. and P.L.; software, P.Z.; validation, Y.Z. and P.L.; formal analysis, P.Z.; investigation, P.Z.; resources, P.L.; data curation, P.L.; writing--original draft preparation, P.Z.; writing--review and editing, P.Z.; visualization, P.Z.; supervision, P.L. and Y.Z.; project administration, P.L.; funding acquisition, P.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Key R&D Program of China [grant numbers: No.2017 YFC0804800]; the Key R&D plan of Jiangsu Province [grant numbers: BE2018111] and the Special Funds for Fundamental Scientific Research Expenses of Central Universities [grant numbers: 2020 MS057].  
Not applicable.  
Data Availability Statement: Data is contained within this article.  
The authors declare no conflict of interest.