2.2.3. Relative Effect (R)

In order to distinguish the effect of thermal mass in the zone with a combined system from the performance in the zone with an equivalent CAS, a parameter named relative effect is used and is defined as

$$Relative\ effect = \frac{f\_{\rm HW} - f\_{\rm LW}}{f\_{\rm LW}} \times 100\% \tag{11}$$

where *fH*(*L*)*<sup>W</sup>* is the decrement factor of a wall which is heavy (light).

#### **3. Results**

The effect of thermal mass on the room cooling load is limited in the perimeter zones. The peak room sensible cooling load and the accumulated load ( 24*h qZT*) decrease by 1% to 2% as the thickness of the concrete layer in the external wall increases from 0 to 200 mm. The data shows only a slight change as the thermal mass increases further (Figure 5). There are two main reasons. The first is that the conduction heat gain only accounts for a small portion of the total heat gain in the building at the peak time when the maximum building cooling load occurs; that is, only 3% or less in the perimeter zones (Figure 6). The second reason can be illustrated by Figure 7; only a very narrow gap in accumulated transmission load exists between the structures of light weight and of heavy weight when the same cooling system applied to maintain an identical indoor thermal environment. It also confirms some preceding research: the thermal mass has little impact on building energy consumption [15–17,27].

**Figure 5.** Effects of thermal mass on cooling loads of the combined system. (**a**) Effects of thermal mass on peak cooling loads on design days. (**b**) Effects of thermal mass on accumulated sensible cooling loads on typical days.

**Figure 6.** Composition of accumulated cooling load in perimeter zones with lightweight construction on a Nanjing cooling design day (embedded surface cooling system (ESCS)).

**Figure 7.** Accumulated conduction heat transfer rate through an external wall against degree-hours.

In addition, the peak cooling load of the zone with an ESCS is higher than the load of the zone with an equivalent CAS by 9% to 11%, and the accumulated load is also higher by 3%–4% based on same weather conditions. As Figure 7 illustrates, accumulated conduction heat gain through an external wall with a heavy thermal mass in the zone with an ESCS is still more than the amount on the wall with a lightweight structure in the zone with an equivalent CAS when the degree-hours are selfsame. These results are basically consistent with the findings from the laboratory experiment conducted by Woolley et al. [49]. The tests were carried out in a standard climatic chamber, and two separated tests

were implemented side-by-side: one with a radiant cooling ceiling, and the other with an overhead mixing air distribution system. The results showed that the peak cooling load of the radiant system was 2%–10% larger than the load of air system in the case where internal heat gain varied periodically, and the accumulated load was 2%–7% higher. The experiment also confirmed the differences existing in heat transfer processes between the zones with and without a radiant system, but it did not explain the effect of thermal mass further.

The instantaneous transmission load has obvious varietious between the constructions with different thermal masses. As Tables 2 and 3 state, the internal thermal mass helps to decrease the transmission through external structures in different orientations, especially on the typical days in the transient season. By comparing the performances in the zone with an equivalent CAS, the effect of thermal mass is more evident in the zone with an ESCS on the cooling design days, while it becomes less on the days in transient season.

In addition, the internal thermal mass prolongs the phase lag for some structures with various orientations, especially on the typical days in transient season (Tables 2 and 3).

According to the impact factors mentioned above, the climatic situation, and the thermal mass position, the specific analysis is as follows by taking the performance in the north perimeter zone as an example.
