*4.4. Radiant Floor Time Constant*

Figure 12 shows the radiant floor surface temperature change over time for different operating strategies for different outdoor meteorological conditions. It can be seen that the *τ*<sup>63</sup> of different operating strategies ranged between 4.4 and 4.7 h.

**Figure 12.** Temperature change of the radiant floor surface, *Tf* change during the (**a**) LH, (**b**) HL, and (**c**) MM operating strategies.

The differences in *τ*<sup>63</sup> between operating strategies were small, and the final floor surface temperatures all stabilized at about 23 ◦C. This indicated that all the different RFCAFC operating strategies for different outdoor meteorological conditions were effective in the high humidity environment. In particular, when high outdoor temperatures led to high initial indoor temperatures, the *τ*<sup>63</sup> value was only 4.4 h, which corresponds to a radiant floor temperature drop rate of 0.73 ◦C/h. The LH and MM operating strategies corresponded to radiant floor surface temperature drop rates of 0.19 ◦C/h and 0.21 ◦C/h. This may have been due to the high outdoor humidity levels, which required that the fan coil system be in continuous operation at all times and had a large effect on the drop in floor surface temperature. The *τ*<sup>95</sup> values among operating strategies had large differences. Comparisons showed that the higher the outdoor humidity, the greater the *τ*<sup>95</sup> value of the radiant floor, or the longer it takes the radiant floor surface to stabilize. Therefore, in cases of high outdoor humidity, we can consider activating the radiant floor system in advance to ensure the system reaches relative stability before being occupied.

#### *4.5. Thermal Comfort*

Figure 13 plots the PMV values over time for the different operation strategies. It can be seen that when the combined cooling system used the LH operating strategy, the PMV value fluctuated from −0.03 to 0.49, with the average value of 0.24. The PMV value fluctuated between −0.82 and 0.51 under the HL operating strategy, with an average of −0.08. The PMV value under the MM operating strategy fluctuated from −0.17 to 0.54, with an average of 0.14. The values of PMV under the LH and HL operating strategies were in accordance with ISO7730. However, the PMV was slightly higher than the recommended value during certain periods while using the MM operation strategy, but the comfort level was still acceptable. From the comfort point of view, the combined cooling system was able to effectively manage indoor thermal comfort.

**Figure 13.** Thermal comfort under different operating strategies.

Figure 14 shows that indoor thermal comfort was better under different meteorological conditions. Therefore, in order to optimize indoor thermal comfort and energy saving, some interior design strategies can be considered. Due to the higher amount of solar radiation reaching the room through the south window, the combined cooling system was unable to completely eliminate the indoor heat load, which finally led to a certain reduction in indoor comfort from 13:00 to 15:00. Therefore, to improve thermal comfort, it is necessary to adjust the operation strategy of the combined cooling system in real time according to the changes in outdoor meteorological conditions.

**Figure 14.** Cooling loads taken on by radiant floor and fan coil under the (**a**) LH, (**b**) HL, and (**c**) MM operating strategies.

#### *4.6. System Cooling Capacity*

Figure 14 shows the change in cooling capacity of the combined cooling system over time under different outdoor meteorological conditions. According to the figure, in the initially high humidity environment, the fan coil accounted for about 68.0% of the total cooling load under the LH operating strategy. Under the HL and MM operation strategies the fan coil made up about 73.8% and 71.7% of the total cooling capacity, respectively. In short, the fan coil of the combined system contributed more to the total cooling capacity than the radiant floor.

Comparing the proportion of fan coil cooling capacity to the total cooling capacity, we see that only during the LH operating strategy were fan coil system and floor radiation system turned on simultaneously to dehumidify and cool due to the high outdoor humidity. Under the HL and MM operating strategy, the fan coil system was turned on about 1 h before the radiant floor to compensate for the slow start and long thermal response time of the radiant floor system as well as to the eliminate indoor heat load. The cooling load taken on by the fan coil system was much higher than that of the radiant floor system because the outdoor temperature was higher under the HL and MM operating conditions.

In order to avoid thermal discomfort in the room, the fan coil was necessary for cooling when the outdoor temperature was high. Therefore, when the indoor and outdoor temperatures are high, in order to reduce indoor temperature and the risk of thermal discomfort, the fan coil system should be turned on in advance.

#### *4.7. System Operation and Energy Consumption*

Figure 15 shows the *EERh* and system energy consumption over time for the combined cooling system the operation strategies for different outdoor meteorological conditions. It can be seen that when the combined cooling system was under the LH operating strategy, the *EERh* fluctuated between 1.04 and 2.41, with an average of 1.98. During the HL and MM operating strategies, the *EERh* of the combined cooling system fluctuated between 1.69–2.96 and 1.73–2.41, with averages of 2.52 and 2.14, respectively. The *EERh* values of the combined system were slightly higher under HL outdoor meteorological conditions. This was because while using the HL operation strategy, the air source heat pump was operating at a high load when the room temperature reached a relatively stable state. In the MM operating strategy, the air source heat pump was maintained at a low load operation state. The combined cooling system consumed 13.43 kWh and 17.18 kWh when utilizing the LH and HL operating strategies, respectively, under the three different outdoor meteorological conditions. During the MM operating strategy, the daily energy consumption of the combined system was 11.36 kWh.

**Figure 15.** *Cont*.

**Figure 15.** Energy consumption under the (**a**) LH, (**b**) HL, and (**c**) MM operating strategies.

Calculations showed that the energy consumption of the LH operating strategy was about 21.8% lower than that of the HL operating strategy. The energy consumption of the MM operating strategy was about 15.4% lower than that of the LH operating strategy. The energy consumption of the HL was much larger than those of the other two operating strategies. This showed that energy consumption was mainly influenced by the outdoor temperature. Thus, the higher the outdoor temperature, the more energy is consumed by the combined system. Furthermore, the *EERh* was smaller during the initial operating period each day compared to other time periods. This was because the air source heat pump was operated at low load during the initial time period each day. In addition, the *EERh* tended to increase as the cooling load taken by the radiant floor system increased. Therefore, from the perspective of reducing energy consumption and achieving efficient operation of the system, the radiant floor system should take on as much cooling capacity as possible and be continuously operated, while the fan coil can be operated intermittently.

#### **5. Discussion**

This experiment is one of the few that integrate an RFCAFC system into a standalone outdoor building in China. Previous experimental studies on the RFCAFC system have been based in office or residential buildings wherein not all walls are exterior walls. This experiment assessed the automatic operation of the RFCAFC system based on outdoor meteorological conditions, which freed the system from defects due to manual control present in previous studies. The subsequent results have created a strong theoretical basis for the promotion and application of combined cooling systems in the construction of intelligent buildings. They also provide new ideas for the operation of RFCAFC systems in actual building projects. The effectiveness of the operation strategies for different outdoor meteorological conditions can be used to guide future system operations and help to improve the reliability of the combined cooling system, especially in spaces that are frequently characterized by high humidity.

However, there were five limitations in this study that need to be further explored. First, this experimental study used only three different outdoor meteorological conditions (HL, LH, and MM) and did not consider others, such as high temperature and high humidity/low temperature and low humidity. Therefore, the operation strategies that were effective here may be different from those required for a combined cooling system in an actual project. In-depth studies will need to be conducted to consider more types of outdoor meteorological conditions in the future.

Second, the effect of indoor moisture dissipation on the combined cooling system was not considered in the study. Moreover, the study only assessed control strategies and did not study the control principles and the implementation process of the controls, which introduces certain limitations. A more in-depth study is needed to ensure the combined cooling system is widely applicable to different outdoor meteorological conditions.

Third, the experimental test was only conducted for six days, which is relatively short considering the length of the cooling season. The outdoor meteorological conditions were limited and cannot be considered representative of all outdoor meteorological conditions in Jinan, China during the cooling season. Therefore, when promoting the application of the combined cooling system, the influence of different outdoor meteorological conditions on the combined cooling system should be fully considered.

Fourth, this experiment was conducted in Jinan, China, which is in a hot summer and cold winter climate zone. Therefore, appropriate validation studies should be carried out before promoting the application of this combined cooling system in other climate zones to ensure that the RFCAFC system is the most effective and energy-saving option. The next steps will gradually address each of the above deficiencies to provide a stronger theoretical basis for the application of this type of the RFCAFC system in high humidity environments.

Fifth, this study will need to be augmented by developing numerical models and conducting simulation tests for validation. These methods will allow us to obtain results from different, more general case studies and under a wider variety of conditions.

#### **6. Conclusions**

In this study, a RFCAFC system for a single building was established and tested. The effects of different operation strategies on indoor temperature, vertical temperature difference, thermal comfort, cooling capacity, *EERh*, and system energy consumption under different outdoor meteorological conditions were analyzed. The conclusions of the study are as follows.

(1) The level of radiation floor surface uniformity coefficient was low, mostly within 0.7– 1.0. The range of *τ*<sup>63</sup> of different operation strategies for different outdoor meteorological conditions was 4.4–4.7 h. The differences among operation strategies were small. The final floor surface temperature was stable at about 23 ◦C. In order for the radiant floor to reach its relatively stable state earlier, the radiant floor system could be turned on in advance.

(2) For all different operating strategies, the indoor thermal comfort was good and the temperature distribution was uniform in the indoor high humidity environment. Therefore, interior design approaches can be considered to further reduce the temperature and ensure indoor thermal comfort.

(3) The fan coil of the combined system took up a large proportion of the cooling load, accounting for about 68.0–73.8% of the total cooling capacity. The fan coil system plays a significant role in compensating for the long thermal response time of the radiant floor system.

(4) The energy consumption during the HL operating conditions was much larger than those of the other two outdoor meteorological conditions. The energy consumed by the LH operating strategy was about 21.8% lower than that of the HL operation strategy. The energy consumption of the MM operating strategy was about 15.4% lower than that of the LH operation strategy. From the energy saving point of view, use of the radiant floor

system for cooling should be prioritized and it should be operated continuously, while the fan coil can be operated intermittently.

**Author Contributions:** Conceptualization, X.Z. (Xuwei Zhu) and J.L.; methodology, J.L.; formal analysis, X.Z. (Xuwei Zhu), X.W. and J.L.; investigation, X.Z. (Xuwei Zhu), J.L., Y.D. and X.Z. (Xiangyuan Zhu); writing—original draft preparation, X.Z. (Xuwei Zhu); writing—review and editing, J.L., X.Z. (Xiangyuan Zhu), J.M., X.W. and X.Z. (Xuwei Zhu); supervision, J.L.; project administration, J.L.; funding acquisition, J.L. and J.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by Natural Science Foundation of Shandong Province (ZR2021ME199) and Key Research and Development Project in Shandong Province (2018GSF121003).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work acknowledges the support of the Plan of Introduction and Cultivation for Young Innovative Talents in Colleges and Universities of Shandong Province.

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

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#### **References**

