**4. Discussion**

Generally, a cooling source with a relatively higher temperature can be utilized by a radiant system, and the entire system coefficient of performance (COP) is expected to be better than a conventional CAS. The present authors investigated a practical TABS operation, and found that the supply water temperature can increase to some degree to maintain the indoor environment at an acceptable comfort level by utilizing the thermal mass and prolonging the radiant system operation time [35]. In addition, from the performances on the typical days in the transient season, the transmission loads caused by an outdoor environment are not as much as the loads on the cooling design days, and the heat transfer in the inside massive layer can be viewed as a process of cooling charging and discharging. Therefore, a potential system strategy, OPPN, could be tried in the zone with ESCS. In the strategy, the auxiliary CAS still runs continuously during the occupied period; the hydronic system starts at midnight and runs continuously with a constant flow rate until the end of the occupied period. The supply water temperature is set at a constant value according to the average outdoor temperature over 24 h and the hydronic system operation hours. Thus, the room operative temperature cannot stay constant but should be in an acceptable range.

As illustrated by Figure 13, during the period from midnight to the earliest occupied time, the inside face temperature of the external wall is relatively higher than the operative temperature in the zone (with ESCS in OPPN) on a Beijing cooling design day. As a consequence, the conduction gain on the inside face is considerably higher than the performance in strategy OPCT (Figure 14), and it is the process for cooling conservation. During the occupied period, the inside face temperature approaches to the zone operative temperature, and the conduction is as low as zero, meaning that the conserved cooling releases to compensate the heat gain on this surface. Thus, the maximum heat gain on the cooling surface in strategy OPPN is not as much as the peak value in OPCT. Considering the effect of additional thermal mass on other structures in the zone, the supply water temperature can be raised from 16 ◦C (in OPCT) to 20 ◦C (in OPPN), and the peak cooling load of radiant system can fall down 28% in turn. In addition, the strategy OPPN can be also tried in the zone where the external wall has no concrete layer. However, as the Figure 14 illustrates, little cooling can be conserved in the external wall, and the instantaneous transmission load is still close to the load in the same zone with ESCS in OPCT during occupied period. However, the risk of condensation should be avoided on the cooling surface by some measures when the single hydronic system runs in night.

**Figure 13.** Indoor temperatures in the zones with ESCS in different operation strategies on a Beijing cooling design day.

**Figure 14.** Conduction heat gains on the inside faces of external walls in north perimeter zones with ESCS on Beijing cooling design day.

Therefore, the cooling water temperature of the radiant system could be raised by taking the effect of internal thermal mass and improving the system's operation strategy. Some low-grade cooling energy sources can be directly used for free cooling, such as geothermal systems. The geothermal system has been applied in several projects where radiant heating and cooling is employed [32,36,51].

#### **5. Conclusions**

The effects of thermal mass in external walls were investigated by simulating the energy performances in a typical office building, rather than considering only the heat transfer for an individual structure. Operative temperature is employed to evaluate the thermal comfort level in both zones—those having a combined system and an equivalent CAS alone. The simulation tool of Energy-Plus is employed in the research, and it is based on heat balance method. It takes the influences of human actives and the cooling system operation strategy into account, in addition to the impacts of building physics and climatic conditions. Thus, the study results approximate real life and muse be the references for building and radiant cooling system designs, although the computation process is relatively complex.

The article introduces a new concept, relative effect (R), and takes the performance in the northern perimeter zone as an example with which to quantificationally distinguish the heat transfer process in the zone with an ESCS from the performance in the zone with an equivalent CAS, confirming that the cooling surface can decrease the heat storage capacity of the building envelope by radiation heat transfer [29]. A big portion of conduction gain on an inside face of external walls is balanced by radiation heat transfer during the occupied period on cooling design days (e.g., 66% and 88%, respectively for the zone with heavy weight and the zone with light weight in the north zone). The research shows some new findings as follows.


Based the results mentioned above, maintaining the thermal mass with a certain weight can be a key measure for a low-carbon building or green building, particularly in the zones equipped with radiant systems. In addition, the risk of condensation on cooling surfaces should be avoided, especially when the radiant system works in the situation without a dehumidification system running. In future work, some tests are going to be carried out to determine instantaneous heat fluxes on a cooling surface, and verify the interaction between a cooling surface and its surroundings.

**Author Contributions:** Conceptualization, J.N.; methodology, R.H.; software, R.H.; writing—original draft preparation, R.H.; wiring—review and editing, G.L.; funding acquisition, R.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Guangxi Natural Science Foundation, grant number 2018GXNSFBA050022; and funded by Guangxi Basic Research Ability Improvement Foundation, grant number 2019KY0220.

**Acknowledgments:** The project is supported by Guangxi Natural Science Foundation (number 2018GXNSFBA050022), and Guangxi basic research ability improvement Foundation (number 2019KY0220).

**Conflicts of Interest:** The authors declare there is no conflicts of interest regarding the publication of this paper.
