## 8 Conclusions  
An optimal scheduling strategy of BIPV microgrid considering VES is proposed in this paper. Firstly, the ACVES model and EWHVES model are built by exploiting the heat/cold storage capability of the building and the electric water heater, respectively. The ACVES and EWHVES are integrated into BIPVmicrogrid as flexible and controllable units for optimal scheduling. Then the optimal scheduling model containing the system operation constraints and the objective function is established. At last, the CPLEX solver is used to solve the proposed day-ahead optimal scheduling model, and the following main conclusions can be derived with the simulation analysis.  
* The integration of both ACVES and EWHVES into the optimal scheduling model of the BIPV microgrid can effectively reduce the operating cost of the system by managing the charging/discharging of the VES in different seasons. When two kinds of VES are integrated together into the system, the cost saved is not the linear superposition of the effect when they participate in the system separately.
* Compared with ACVES, EWHVES shows more stable VES utility. The VES utility of EWHVES is primarily affected by the capacity of the water heater's tank. The VES utility of ACVES is affected by a number of factors such as outdoor temperature, solar radiation, indoor heat sources, and seasonal variations. However, the ACVES shows great potential to provide VES utility in the summer.
* Through proper control of VES, the investment cost of the energy storage system can be significantly reduced. At the same time, the VES can replace the energy storage system and perform a similar function when its capacity is curtailed or the energy storage system fails.  
The main contribution of this paper includes:  
* An optimal scheduling strategy of BIPV microgrid considering VES is proposed, which provides new approaches to further improve its operating economy.
* The VES characteristics of ACVES and EWHVES are compared and analyzed to provide a reference for designing a better strategy in future work.
* This paper validates the effectiveness of VES in day-ahead optimization scheduling in different seasons, which provides a basis for its participation in real-time optimization scheduling.  
* During the summer months, the operating costs of the system are significantly reduced due to the sufficient amount of PV power generation, and VES is also effective in reducing the operating costs of the system in different seasons, which can effectively promote government departments to formulate a series of policies to promote the development of BIPV and VES.  
Conceptualization, Z.C. and X.L.; methodology, X.L. and Z.L.; software, Z.C., X.L. and Z.L.; validation, Z.C. and X.L. and S.X.; formal analysis, Z.C. and X.L.; investigation, X.L.; resources, Z.L.; data curation, S.X. and R.N.; writing--original draft preparation, X.L.; writing--review and editing, Z.C. and X.L.; visualization, Z.C., X.L. and R.N.; supervision, Z.L., J.S. and S.X.; project administration, Z.L.; and funding acquisition, Z.C., Z.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (61803343, 61876169), Key R&D and Promotion Project of Henan Province (202102210096, 202102210296), and Key Projects of Higher Education of Henan Province (19A120012).  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} BIPV & Building integrated photovoltaic \\ VES & virtual energy storage \\ AC & air conditioner \\ EWH & electric water heater \\ ACVES & air conditioner based VES \\ EWHVES & electric water heater based VES \\ PV & Photovoltaic \\ MPPT & maximum power point tracking \\ DR & demand response \\ ETP & equivalent thermal parameter \\ \end{tabular}