*4.3. System Operation Result and Analysis*

#### 4.3.1. Calculation of Operating Load

The purpose of optimization design is to improve the operational performance of RIESs. To compare the operational performance of the three systems, the annual operating load was calculated by Energy Plus based on the typical annual outdoor meteorological parameters shown in Figure 10a, as shown in Figure 10b. In Changsha, which is hot in summer and cold in winter, the cooling load of the building is much greater than the heating load. Under summer operating conditions, the system cooling load is the building cooling load, and the heating load is the domestic hot water load. However, under winter operating conditions, the system heating load includes the building heating load and domestic hot water load. In addition, since the design parameters are determined by the method of non-guarantee days, the operating load may be greater than the design load most of the time. However, to ensure the feasibility of operation optimization, this study only considers days when the operating load is less than or equal to the design load.

**Figure 10.** Outdoor parameters and building load under the operating conditions: (**a**) Annual outdoor meteorological parameters; (**b**) Building load under operating conditions.

#### 4.3.2. Operation Results

Based on the above operating loads, the carbon taxes, operating costs, and total costs of the three systems are calculated under winter and summer operating conditions, and the results are shown in Table 9. Under the summer operating conditions, the carbon tax, operating cost, and total costs of System 2 and System 3 are less than those of System 1. Compared with the carbon tax, operating cost, and total cost of System 1, the corresponding costs in System 2 are reduced by 3.2%, 1.5%, and 0.3%, respectively. Different from System 2, since the carbon tax of System 3 is larger than that of System 2, the carbon tax reduction rate of System 3 compared with System 1 is smaller, only 2.7%. However, the EES device can significantly reduce the operating cost of System 3, which results in a 6.2% reduction in the operating cost of System 3 compared to System 1. Affected by the difference in building loads between winter and summer, the carbon taxes, operating costs, and total costs of the three systems under winter operating conditions are less than those under summer operating conditions. Under winter operating conditions, the carbon tax, operating cost, and total cost reduction rates of System 2 compared with System 1 are greater, whose values are 5.5%, 7.3%, and 2.8%, respectively. Similar to the summer operating conditions, System 3 can significantly reduce operating costs compared to System 2 but can also increase its carbon tax. Therefore, under operating conditions, compared with System 1, the TES device can significantly reduce the carbon tax, operating cost, and total cost of System 2, while the EES device can reduce the operating cost of System 3 even more significantly.


System 3 512,367 1,484,274 2,660,567 294,405 688,785 1,578,434

**Table 9.** Costs of three systems under winter and summer operating conditions.

#### 4.3.3. Operation Result Analysis

According to the analysis of the design condition results, the performance difference of RIESs is mainly affected by energy consumption and the consumption of different types of energy. To analyze the energy consumption of the three systems, this paper compares the average operation efficiency of the GSHPs and CHP units under the operating conditions, as shown in Figure 11. Firstly, under winter and summer operating conditions, the average operation efficiency of the GSHP in System 1, System 2, and System 3 increases sequentially due to the influence of energy storage devices. However, there is little difference between the average operation efficiency of the GSHP in System 2 and System 3. Secondly, the average operation efficiency of the CHP unit in System 2 is greater than in System 1 and System 3, especially in winter operating conditions. This shows that the TES device can significantly improve the operation efficiency of the GSHP and the CHP unit. In addition, the average operation efficiency of the CHP unit in System 3 is greater than that in System 1 under summer operating conditions, while under winter operating conditions, the average operating efficiency of the CHP unit in System 3 is approximately the same as that in System 1. The higher the average operation efficiency of the GSHP and the CHP units, the lower the energy consumption. Therefore, under the operating conditions, the energy consumption of System 2 and System 3 is less than that of System 1.

**Figure 11.** The average efficiency of GSHPs and CHP units under two operating conditions.

In the RIESs, carbon tax not only depends on the energy consumption of systems but is also affected by the ratio of different energy consumptions. To this end, different power consumption ratios of the three systems under winter and summer operating conditions are counted in this paper, and the results are shown in Figure 12. Under summer and winter operating conditions, the ratio of grid power consumption in System 1 is greater than that of System 2 and System 3; hence the carbon tax of System 2 and System 3 is less than that of System 1. Although there is a slight difference in the ratio of grid power consumption between System 2 and System 3 under summer operating conditions, the average operation efficiency of the CHP unit in System 3 is lower than that in System 2. Therefore, under summer operating conditions, the carbon tax of System 3 is slightly larger than System 2. Different from the summer operating conditions, the grid power consumption ratio of System 2 is significantly smaller than for System 3 under the winter operating conditions. As a result, the carbon taxes of System 2 and System 3 are greater than that of System 1, especially System 2.

**Figure 12.** Different power consumption ratios under two operating conditions.

The system's operating cost is closely related to the TOU power price. To analyze the operating cost difference between the three systems, this paper draws the power consumption ratio of the three systems under TOU price hours, as shown in Figure 13. Under the summer operating conditions, the grid power consumption ratios of System 2 and System 3 during high and peak hours are less than that of System 1, especially for System 3, which accounts for only 22.12% during high and peak hours. Similar to the summer operating conditions, the power consumption ratios of System 2 and System 3 during high and peak hours are less than that of System 1 under winter operating conditions. However, under winter operating conditions, the difference in the grid power consumption ratio between System 1 and System 2 is greater than that between System 2 and System 3 during high and peak hours. Therefore, under winter and summer operating conditions, the operating costs of System 2 and System 3 are greater than that of System 1, and the operating cost of System 3 is the lowest, especially under winter operating conditions.


**Figure 13.** The ratio of grid power consumption under the TOU power price.

The above analysis shows that the carbon taxes and operating costs of System 2 and System 3 are less than those of System 1 under winter and summer operating conditions due to the impact of the average efficiency of the equipment, the ratio of different energy consumptions, and the grid power consumption ratio under the TOU power price. The smaller carbon tax and operating cost make the total cost of Systems 2 and System 3 less than that of System 1. However, affected by the equipment cost, the total cost reduction rate difference between System 2 and System 3 compared with System 1 is not so obvious. In addition, under winter operating conditions, the total cost of System 3 is greater than that of System 2 due to the lower operating cost difference between System 2 and System 3.

#### *4.4. Uncertainty Analysis of Energy Price*

In the actual operation process, the price of grid power and gas directly affected the operation result of the system. To explore the impact of energy price uncertainty on system operation results, this study increases or decreases the energy prices to ±30% in a 10% step, based on the energy prices in the design conditions. The three systems are optimized for operation according to different energy prices. Based on optimization results, the cost reduction rates of System 2 and System 3 compared with System 1 are calculated, as shown in Figure 14. In the case of changes in energy prices, the reduction rates of the carbon tax, operating cost, and total cost of System 2 compared with System 1 are greater than zero. Different from System 2, the operating cost reduction rate of System 3 compared with System 1 is larger, while the carbon tax and total cost reduction rates are lower. Even under partial energy prices, the carbon tax and total cost of System 3 are greater than they are for System 1. Under different energy prices, the TES device can effectively reduce the operating cost, carbon tax, and total cost of the RIES, while the EES device can significantly reduce the system operating cost.

**Figure 14.** Cost reduction rates of system 2 and system 3 compared with system 1 under changes in electricity and gas prices: (**a**) Affected by changes in electricity prices; (**b**) Affected by changes in gas prices.

#### **5. Conclusions**

The purpose of this study is to explore the impact of energy storage devices on the optimal design and operation of RIESs. For this purpose, a bi-level dynamic optimization model was first established based on the dynamic equipment model. Then, the bi-level dynamic optimization model was used to optimize the design of RIESs with different energy storage devices, and the optimization results are System 1, System 2, and System 3, respectively. According to the optimization results, the impact of energy storage devices on system performance is compared and analyzed. The main conclusions of this study include the following three points:


**Author Contributions:** Conceptualization, B.J.; methodology, B.J.; software, B.J. and Z.L.; validation, B.J. and Z.L.; formal analysis, B.J. and Z.L.; investigation, B.J.; data curation, B.J. and Z.L.; writing—original draft preparation, B.J. and Z.L.; writing—review and editing, B.J. and Y.L.; visualization, B.J.; supervision, B.J.; project administration, B.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
