## 4 Results and Discussions
### The Influence of PCM Melting Temperature  
As one of the most significant factors of PCMs, which affects their thermal performance, the melting temperature, \(T_{m}\), of the PCM layer plays an essential role in the energy saving characteristics of a PCM roof. The energy saving amount, \(\Delta W\), of a PCM roof in terms of the melting temperature, \(T_{m}\), is plotted in Figure 6. It shows that the energy saving amount firstly increased with the increment of the melting temperature, \(T_{m}\), and it reached the optimum value at \(T_{m}=31\). As the melting temperature, \(T_{m}\), further increased to be higher than 31, the energy saving amount value of the PCM roof became less. When the melting temperature, \(T_{m}\), was 28, 29, 30, 31, and 32, the PCM roof energy saving amount, \(\Delta W\), during the summer cooling season was 1603 kJ/m\({}^{2}\), 1685 kJ/m\({}^{2}\), 1762 kJ/m\({}^{2}\), 1813 kJ/m\({}^{2}\), and 1748 kJ/m\({}^{2}\) respectively. Compared to the roof of \(T_{m}=28\), the energy saving amount, \(\Delta W\), for the roof with \(T_{m}=31\)  was increased by 13.10%. The above discussion indicates that using the PCMs with an appropriate melting temperature, according to climate conditions, is important for optimizing the energy consumption characteristics of PCM roof buildings. To explain the mechanism, an optimum PCM melting temperature for reducing the roof energy consumption amount exists, the transient PCM average liquid fraction \(f_{l}\) during the summer cooling season is plotted in Figure 7. At a melting temperature of \(T_{m}=28\), it was found that only 4.8% of the amount of PCM solidified during the early morning time and all of the PCM became liquid again at 12:15 p.m. When the PCM layer became fully liquid, it lost the latent heat characteristic and behaved like the SIM layer so that the internal temperature inside the PCM roof could not be controlled and started to increase at this state. Contrary to the PCM roof with a melting temperature \(T_{m}=28\), the maximum solid state PCM amount was 31.65% during the early morning for the PCM roof with \(T_{m}=31\)  under which situation the energy storage capacity of the PCM layer increased. For this reason, the time for a PCM layer of \(T_{m}=31\)  being fully melted to a liquid state was delayed to 17:00 p.m. The means that the interior surface temperature of a PCM roof could be fixed at a nearly constant value until 17:00 p.m. so that the energy consumption from using the air conditioner was highly reduced. Unfortunately, as the melting temperature \(T_{m}\) was further increased to 32, the PCM layer did not become fully liquid until 18:30 p.m. Although the PCM layer of \(T_{m}=32\)  had the capability of modulating the temperature inside the PCM roof during the whole AC working period, this PCM layer also solidified more rapidly at a relatively higher solidification temperature during  
Figure 5: The PCM roof and SIM roof temperature distribution during the summer AC cooling time. the nighttime and the early morning time from 0:00 a.m. to 9:00 a.m., as shown in Figure 7. This phenomenon makes the PCM layer have a trade-off effect on the energy saving characteristics of a PCM roof: (1) A relatively higher melting temperature, \(T_{m}\), could make the PCM layer have a longer latent heat characteristic, which delays the temperature rise in the PCM roof during the daytime so that the energy consumption of the air conditioner is reduced. (2) The PCM layer with a higher melting temperature, \(T_{m}\), solidified more rapidly during the nighttime at a nearly constant temperature. Hence, its temperature could not be further cooled down by the lower environment temperature, \(T_{e}\), under which the air conditioner energy cost during the daytime increased.