3.1.1. Performance on Cooling Design Days

On the cooling design days in the cities of Nanjing and Beijing, the minimum outdoor dry-bulb temperatures are higher than the room setting temperature; i.e., 26 ◦C. The inside face temperatures of external opaque walls are lower than the temperatures of the outside faces during the occupied period, for the structures with and without a concrete layer. In addition, for the external opaque surfaces which contain the same structure, the inside face temperatures appear lower in the zones with ESCSs compared to the temperatures in the zones with CASs (Figure 8a). Since the heat transfer process in the zone with an ESCS is different from the process in the zone with a CAS (Figure 1), most of instantaneous radiant heat gain can be absorbed by the cooling surface through direct or indirect radiant heat transfer (Figure 9a). This also confirms the statement by Niu et al. [29]: the cooling surface can decrease the heat storage capacity of the building envelope to the radiation heat transfer. Besides, a big portion of conduction occurring on the inside faces of the external walls is balanced by radiation instead of convection (averages of about 66% and 88%, respectively, for the zone with heavy weight and the zone with light weight). The instantaneous radiation heat fluxes on the inside faces are related to the cooling surfaces during the occupied period (Figure 9a), such that heat fluxes are conducted from the outside faces to the inside in this time, and the instantaneous conduction heat gains on the inside faces have approximate values in the most of the occupied time when these cooling systems operate to maintain the indoor environment within the given criteria (Figure 10a). However, the thermal mass in the external wall can help to maintain the inside face temperature stably, and the transmission loads through the external walls with heavy weights are lesser than the ones through the walls without concrete layers (Figure 10a). Consequently, compared to the performance in the zone with an equivalent CAS, the maximum conduction heat transfer on the inside face is slightly lower in the zone with an ESCS, and the relative effect of thermal mass (R) appears more significant on the cooling design days.



*Energies* **2020**, *13*, 1356

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**Figure 8.** Indoor temperature in the north perimeter zone. (**a**) Indoor temperature on a Beijing cooling design day. (**b**) Indoor temperature on a typical Nanjing day in transient season. (IFT—indoor face temperature, OPT—operative temperature).

**Figure 9.** Radiation heat gain in north perimeter zones. (**a**) Radiation heat gain on Beijing cooling design day, (**b**) Radiation heat gain on a typical Nanjing day in transient season. (IFEW—inside face of external wall, CS—cooling ceiling.)

**Figure 10.** Conduction heat gains on the inside faces of external walls in north perimeter zones. (**a**) Conduction heat gain on a Beijing cooling design day. (**b**) Conduction heat gain on a typical Nanjing day in transient season.

In contrast to the performances during the occupied period, more transmission loads are observed on the external walls with heavy weight structures during the unoccupied period, compared to the loads on the walls of a light weight in the zones with same cooling systems. It is due to the fact that the inside face temperatures of the external surfaces with heavy weight structures still stay higher than the corresponding room operative temperatures in this time; meanwhile, the inside face temperatures of the surfaces with lightweight structures decrease and approach the corresponding operative temperatures after the midnight. For a same structure, a wider gap exists between the inside face temperature and the corresponding operative temperature in the zone with an ESCS in comparison with the performance in that zone with a CAS. More radiation heat transfers occur on the surfaces in the zones with ESCSs, and the consequent conduction heat gains are higher in most of the unoccupied period, and the accumulated values (Figure 10a).

#### 3.1.2. Performance on Typical Days in the Transient Season

On typical days in the transient season, the outdoor dry-bulb temperature ranges surrounding the rooms set the temperatures in Beijing and Nanjing. For a surface with a lightweight structure, the inside face temperature fluctuates with outdoor temperature both in a zone with an ESCS and a zone with an equivalent CAS (Figure 8b). Both the inside face temperatures are higher than the corresponding zones' operative temperatures during occupied period. The maximum conduction gains on these inside faces are close, occurring at 19:00 (Figure 10b). However, the heat fluxes transfer from the inside faces to the outside in most of the unoccupied period, because the inside face temperatures decrease sharply as the outdoor temperatures fall down, and are even lower than the corresponding operative temperatures after midnight. In contrast, for the surfaces with heavy weight structures, the inside face temperatures have little fluctuation. The conduction heat gains on the inside faces are very minor, or even negative during the occupied period, but they increase as time goes on. The maxima occur at 5:00 when the zone operative temperatures are out of control (Figure 10b). The phase lag times are prolonged by 10 hours compared to the performances on the surfaces with lightweight structures. Thus, the 24-hour conduction heat transfers on the external surfaces can be also regarded as the process of cooling charging when the cooling systems operate, and discharging in the rest time. That is the main reason why the values of relative effect of the thermal mass (R) are enhanced on typical days in the transient season.

Although almost all radiation heat gain in the zone with an ESCS can be extracted by a cooling surface through direct or indirect radiation heat transfer during the occupied period, the portion of conduction heat gain on the inside face of the external surface balanced by radiation is not as much as the one on a cooling design day (Figure 9b), average 30% on a surface with lightweight structure and less than 10% on a heavy wall. This is due to the fact that the inside face temperatures of the external surfaces decrease as the outdoor weather becomes cooler. Even so, the inside face temperatures in the zones with ESCSs are relatively lower than the values on the surfaces with the same structures in the zones with CASs (Figure 8b). The difference in heat transfer process between the zones with the different cooling systems leads to an interesting phenomenon on typical days in the transient season: the operative temperatures increase significantly when the CASs are switched off, and stay above the 26 ◦C during un-occupied period, whereas in the zones with ESCSs the operative temperatures rise slightly and then fall down after the internal heat gains completely disappear. Thus, similarly to the statement in [5,50], the application of night ventilation to cool down a surface with an interior massive layer could be feasible in a zone with a CAS. It may result in a considerable reduction in conduction gains, but it is not necessarily for a zone with a cooling surface. In addition, since a bigger difference exists between the inside face temperature and zone operative temperature in the zone with an ESCS, more conduction heat gain is observed by comparing it with the gain in the zone with an equivalent CAS. The values of relative effect of thermal mass (R) in the perimeter zones with ESCSs become less than the ones in the zones with CASs on the typical days in the transient season.

#### *3.2. The E*ff*ect of Thermal Mass Position on System Performance*

Preceding research [13–17,20] indicated that the insulation layer should be placed inside when the cooling system runs intermittently. Thus, the external wall structure can be rearranged (i.e., outside concrete + inside insulation). The corresponding thermal mass performances are discussed based on identical thermal environments in the same office building equipped with the combined system. The peak sensible cooling load decreases by approximately 2% to 3% as the thickness of the concrete layer extends from 0 to 200 mm, whereas the corresponding accumulated cooling loads for 24 h change minimally as the thermal mass increases.

Taking the performances in the north perimeter zones with ESCSs as examples, the inside face temperature on the surface with inside insulation approximates to the temperature on the surface with a massive layer inside during the occupied period on the Beijing cooling design day (Figure 11). The conduction gains at these inside faces are also close (Figure 12). Since the conduction heat gain only accounts for a small portion of the total gain in the thermal zone, the peak room cooling load reduces slightly as the thickness of outside concrete layer increases on the cooling design day. However, the accumulated conduction heat gain for the wall with inside insulation is higher than that for the surface with a massive layer inside, because the inside face temperature rises more significantly as the cooling system is turned off.

**Figure 11.** Inside face temperatures of external opaque walls with different structures in north perimeter zones with ESCS.

**Figure 12.** Conduction heat gains at the inside faces of external opaque walls in north perimeter zones with ESCS.

On a typical Nanjing day in June, more conduction gain exists on the external wall with inside insulation during the occupied period compared to the gain of the wall with outside insulation (Figure 12), because little internal heat gain is absorbed by the inside insulation layer, and the portion distributed to the external wall immediately becomes the cooling load through convection and radiation heat transfers. In a typical office building, internal heat gain from occupants, lighting, and electrical equipment accounts for 60% or more of the total gain. Correspondingly, the inside face temperature on a wall with inside insulation is higher than that of the wall with a massive layer inside, most times, on the typical day in June. Although the temperature falls down after midnight, and the corresponding

conduction gain decreases and becomes less than the gain on the surface with a massive inside layer, it cannot compensate the excess gain from 09:00 to 23:00 during the day. Therefore, the structure (outside massive layer + inside insulation) has little positive impact on the saving of heat transmission. The accumulated conduction gain on the inside face is close or even higher than the amount on the surface without concrete layer when the degree-hours are absolutely the same.
