In this paper, the optimal scheduling results of electric heating/cooling microgrid and CCHP microgrid with heavy floor VESSs are analyzed respectively, and comparisons are made between them and the optimal scheduling results with light floor VESSs.
5.2.1. Optimal Scheduling Results of Electric Heating/Cooling Microgrid
(1) Optimal scheduling results without heavy floor VESS
The operative temperature is kept at the optimum operative temperature (
Tzopt is 22 °C for winter and is 25 °C for summer) when the electric heating/cooling Microgrid is optimized without VESS. The optimal scheduling results for typical day in winter as well as typical day in summer are shown repectively in
Figure 8a,b, including the power exchange with external grid, the power consumed by EHs/ECs, the charging/discharging power of the BESS, the operative temperature and the radiant floor surface temperature. In order to facilitate the representation, the power selling to external grid and charging power of the BESS are taken negative in
Figure 8. Meanwhile, in order to illustrate the pure effect of the VESS later, the corresponding optimal scheduling results of electric heating/cooling Microgrid without BESS are shown in
Figure 9.
It is known that the charging/discharging behavior of the BESS is mainly influenced by electricity price, charging behavior usually happens at lower electricity price periods, while discharging behavior usually happens at higher electricity price periods. The power exchange with external grid and the power consumed by EHs/ECs in
Figure 8 are quite similar with that in
Figure 9. Power consumed by EHs/ECs is mainly determined by solar radiation intensity and outdoor temperature. Because both outdoor temperature and solar radiation intensity in the daytime are higher than that in the nighttime, for typical day in winter, the power consumed by EHs in the daytime is obviously lower than that in the nighttime; while for typical day in summer, the power consumed by ECs in the daytime is obviously higher than that in the nighttime. There is few electric power selling to the external grid for typical day in winter, while there is some electric power selling to the external grid for typical day in summer. The operating costs of the electric heating/cooling Microgrid for typical day in winter and typical day in summer are 4224.71 CNY and 596.93 CNY respectively, while without BESS, the corresponding operating costs are 4432.78 CNY and 786.08 CNY respectively.
(2) Optimal scheduling results with heavy floor VESSs
The operative temperature is kept around the optimum operative temperature (
Tzopt is 22 °C for winter and is 25 °C for summer) with adjustable range of ±2.5 °C when the electric heating/cooling microgrid is optimized with VESSs. The optimal scheduling results (thermal comfort sensitive coefficient
γ = 0.1) for typical day in winter and typical day in summer are shown respectively in
Figure 10a,b. Meanwhile, the corresponding optimal scheduling results of electric heating/cooling microgrid without BESSs are shown in
Figure 11.
It is known that for BESS, charging behavior usually happens at lower electricity price periods, while discharging behavior usually happens at higher electricity price periods, which is quite similar with the scheduling results without VESSs. Power consumed by EHs/ECs and the operative temperature are quite different with the scheduling results without VESSs. At lower electricity price periods, more power is consumed by EHs/ECs, thus the operative temperature rises slowly on a typical day in winter while it falls slowly on a typical day in summer; at higher electricity price periods, less power is consumed by EHs/ECs, thus the operative temperature falls slowly in typical day in winter while rises slowly in typical day in summer. For a typical day in winter, it appears that some electric power is sold to the external grid at higher electricity price periods; for a typical day in summer, more electric power is sold to the external grid at higher electricity price periods. The operating costs of the electric heating/cooling microgrid with VESSs for a typical day in winter and typical day in summer are 3183.60 CNY and 388.16 CNY respectively, which decrease 24.64% and 34.97% respectively compared with the scheduling results without VESSs; while without BESSs, the corresponding operating costs are 3334.83 CNY and 616.82 CNY respectively, which decrease 24.77% and 21.53% respectively compared with the scheduling results without VESSs.
(3) Charging/discharging characteristics of heavy floor VESSs
For electric heating/cooling microgrids, the power consumed by EHs/ECs is represented as the heating/cooling demand of residential buildings, then heating/cooling demand curves with/without VESSs are shown in
Figure 12. Meanwhile, the corresponding heating/cooling demand curves of electric heating/cooling microgrids without BESSs are shown in
Figure 13.
Take the heating/cooling demand curve without VESSs as reference curve, it could be understood that the heating/cooling demand curve with VESSs fluctuates around the reference curve. The part above the reference curve means heat/cool energy is stored in VESSs, which can be regarded as ‘charging’; while the part below the reference curve means heat/cool energy is released from the VESS, which can be regarded as ‘discharging’. The heating/cooling demand difference between the two cases is the charging/discharging power of a VESS. It is known that charging/discharging power of VESSs in
Figure 12 are quite close to that in
Figure 13, which indicates that BESSs have little impact on the charging/discharging characteristics of VESSs.
Comparing the charging/discharging power of VESSs in
Figure 12 with the optimal scheduling results of BESSs in
Figure 8 and
Figure 10, it can be seen that both VESSs and BESSs could enact reasonable charging/discharging responses to the variation of the electricity price. In addition, similar to the BESSs, the heavy floor VESSs could continuously work in ‘charging’ or ‘discharging’ states over multiple periods owing to the considerable thermal storage capacity of the radiant floor and envelope structure.
(4) Effect of thermal comfort sensitive coefficient on the scheduling results
Considering users’ different requirements for indoor thermal comfort level, take typical day in winter as example, the thermal comfort sensitive coefficient
γ is adjusted, and the corresponding scheduling results of VESSs are shown in
Figure 14. It can be found that the charging/discharging behaviors of the VESSs for different
γ are basically consistent, that is, the charging process mainly happens at the lower electricity price periods, and the discharge process mainly happens at the higher electricity price periods. The larger the
γ is, the less obvious the charging/discharging behavior of VESSs is due to the more serious punishment to the decline of thermal comfort level in the optimization model.
The operative temperature curves for different
γ are shown in
Figure 15. It can be seen that the larger the
γ is, the smaller the fluctuation of the operative temperature is due to the more serious punishment of the decline of thermal comfort level in the optimization model. The operating cost of the microgrid and average deviation of the operative temperature to the optimum operative temperature are calculated for different
γs, as shown in
Table 11. It is known that the larger the
γ is, the smaller the average deviation of the operative temperature is but the higher the operating cost of the microgrid is.
5.2.2. Optimal Scheduling Results of CCHP Microgrids
(1) Optimal scheduling results without heavy floor VESSs
The operative temperature is also kept at the optimum operative temperature (
Tzopt is 22 °C for winter and is 25 °C for summer) when the CCHP Microgrid is optimized without VESSs. The optimal scheduling results for a typical day in winter and a typical day in summer are shown respectively in
Figure 16a,b, including the power exchange with external grid, the output electric power of MTs, the charging/discharging power of the BESSs, the operative temperature and the radiant floor surface temperature.
It can be seen that the charging/discharging behaviors of BESS are mainly influenced by electricity price, which is quite similar to that of the electric heating/cooling microgrid. As the CCHP unit operates with the strategy of FTL, the output electric power of MTs depends on the heating/cooling demand determined by the outdoor temperature and the solar radiation intensity. For a typical day in winter, the output electric power of MTs in the daytime is obviously lower than that in the nighttime. While for typical day in summer, the output electric power of MTs in the daytime is obviously higher than that in the nighttime. There is almost no electric power purchasing from the external grid for both a typical day in winter and a typical day in summer. The operating costs of the CCHP microgrid for a typical day in winter and a typical day in summer are 1127.86 CNY and 303.27 CNY respectively.
(2) Optimal scheduling results with heavy floor VESSs
The operative temperature is also kept around the optimum operative temperature (
Tzopt is 22 °C for winter and is 25 °C for summer) with adjustable range of ±2.5 °C when the CCHP microgrid is optimized with a VESS. The optimal scheduling results (thermal comfort sensitive coefficient
γ = 1) for a typical day in winter and a typical day in summer are shown respectively in
Figure 17a,b.
It can be seen that the charging/discharging behaviors of BESSs are mainly influenced by electricity price, which is quite similar with the scheduling results without VESS. The output electric power of MTs is quite different with the scheduling results without VESS. At lower electricity price periods, less electric power as well as thermal power are generated by MTs, thus the operative temperature falls slowly on a typical day in winter while it rises slowly on a typical day in summer; at higher electricity price periods, more electric power as well as thermal power are generated by MTs, thus the operative temperature rises slowly on a typical day in winter while it falls slowly on a typical day in summer. For a typical day in winter and a typical day in summer, more electric power is sold to the external grid at higher electricity price periods. The operating costs of the CCHP microgrid for a typical day in winter and a typical day in summer are 657.01 CNY and 209.77 CNY respectively, which decrease 41.75% and 30.83% respectively compared with the scheduling results without VESSs.
(3) Charging/discharging characteristics of VESSs
For CCHP microgrids, the output thermal power of MTs could represent the heating/cooling demand of the residential building, then heating/cooling demand curves with/without VESSs are shown in
Figure 18.
Take the heating/cooling demand curve without VESSs as a reference curve, it also could be understood that the heating/cooling demand curve with VESS fluctuates around the reference curve corresponding to the charging/discharging of the VESSs. Contrary to the VESSs in electric heating/cooling microgrids, the VESSs in CCHP microgrids work in a charging state at higher electricity price periods while in a discharging state at lower electricity price periods. This is because the MTs need to generate more thermal power to increase the electric power sales to the grid at higher electricity price periods, which corresponds to the charging behavior of VESSs, and the MTs need to generate less thermal power to decrease the electric power selling to the grid at lower electricity price periods, which corresponds to the discharging behavior of VESSs.
5.2.3. Contrast of Scheduling Results for R-Microgrids with Light/ Heavy Floor VESSs
Take a typical day in winter as an example, the scheduling results of operative temperature and operating cost for r-microgrids with a light floor VESS and a heavy floor VESS are compared.
(1) Operative temperature contrast
For electric heating/cooling microgrids and CCHP microgrids with light floor VESSs, the corresponding scheduling result of operative temperature with different
γs are shown in
Figure 19a,b respectively.
Compared with the operative temperature with heavy floor VESSs in
Figure 15, it is known that the operative temperature with light floor VESSs fluctuates more frequently due to the relative weak thermal storage capacity of the light floor. In addition, when smaller
γ is given in the optimization model, the operative temperature equals the lower limit for most scheduling periods, and only changes at the scheduling periods before or after the electricity price varies, which indicates that the continuity of the charging/discharging behavior of light floor VESS is relatively poor.
(2) Operating cost contrast
For electric heating/cooling Microgrid and CCHP Microgrid, the corresponding operating costs with heavy floor VESS and light floor VESS are shown respectively in
Table 12 and
Table 13. It is known that, for all cases, the smaller the
γ is, the lower the operating cost is. Compared with the scheduling result without VESS, for both electric heating/cooling microgrids and CCHP microgrids, more decrease of operating costs could be achieved with heavy floor VESSs than with light floor VESSs when a smaller
γ is given in the optimization model. Take
γ = 0.1 as an example, for electric heating/cooling microgrids, the operating cost can be decreased by 24.64% with heavy floor VESSs and only 10.37% with light floor VESSs; for the CCHP Microgrid, the operating cost can be decreased by 42.53% with heavy floor VESSs and only 8.87% with light floor VESSs.