Fig. 8 shows the integrating value of air-conditioning load on the hottest day in Tokyo when p*=0.1~0.9 in different insulated states of the rooftop slab. Fig. 9 shows the convective heat flux from the rooftop during the daytime under the same conditions. When the rooftop is insulated, the air-conditioning load decreases (Fig. 8). The thermal resistance level of the rooftop slab was particularly conspicuous in its drop rate between no insulation and the next-generation energy-saving level of Japan (2.29m2K/W). Even in the building without insulation, by implementing a rooftop cooling of p*=0.9, the effect of air-conditioning load reduction corresponding to that in the next-generation energy-saving level could be obtained. By comparison, as the thermal resistance value of the rooftop rises, the convective heat flux from the rooftop during the daytime becomes greater (Fig. 9). However, that increase rate was merely 2~3%, a small figure in comparison to the air-conditioning load.

Fig. 10 shows the relationship between the daytime and nighttime rooftop surface temperature and the heat flux to the indoor side and the outdoor side air on the hottest day in Tokyo. In the daytime, there was a correlation between the p* value and the rooftop surface temperature; as the p* value increased 0.2, the surface temperature decreased 5.3 ~5.5°C (Fig.10(a)). The rooftop surface temperature greatly changed depending on the p* value rather than the insulated state of the rooftop slab. When p*=0.1, the rooftop surface temperature was 21.0°C higher than the outdoor temperature. As the surface temperature increased, so did the heat flux to the indoor side and the atmosphere. Yet, by strengthening insulation, the heat transfer could be controlled to a low level. Similarly, at nighttime, as the p* value increased, the rooftop surface temperature decreased (Fig.10(b)). It is believed that the increase in the p* value helped the effect of inhibiting the daytime thermal storage occur.

Thermal resistance of roof slab [m2K/W]

Fig.9 Convective heat flux to the atmosphere during the daytime (Tokyo, the hottest day, 10:00-15:00).

Surface temperature [°С]

Heat flux to the atmosphere Heat flux to indoor

Ф, (dotted line): Decreasing rate of air-conditioning load of the day due to the increase of the thermal resistance of roof slab, comparing to the condition of no thermal insulation

30 40 50 60

Fig.10 Average rooftop temperature during the daytime and the nighttime, and heat flux to indoor and the atmosphere (Tokyo, the hottest day, daytime: 10:00-15:00, nighttime: 22:00-following 3:00).

Just as in the daytime, as the surface temperature increased, so did the heat flux to the indoor side and the atmosphere. Yet, when the surface temperature went below the outdoor air temperature, the heat flux value to the atmosphere side became negative. Thus, the heat flux to the atmosphere was negative when the p* value was 0.7 or greater regardless of the insulated state of the rooftop slab. The above findings clarified that the rooftop surface temperature was closely related to both the heat load to the atmosphere and the indoor heat load. This suggests that observed values of the rooftop surface temperature in Fig. 4 had a significant role in measuring the thermal performance of the rooftop slab.

Fig. 11 shows the daily integrating value of air-condition load on the hottest day when w and p changed to 0.1~0.9 respectively. It became clear that even when the rooftop slab was not insulated, an energy-saving performance similar to that in the next-generation insulation standard of Japan (thermal resistance=2.29m2K/W) could be obtained from p: 0.5

or greater (w=0.0) and w: 0.1 or greater (p=0.1). It also became clear that, with p: 0.5, an effect of air-conditioning load reduction similar to that with w: 0.1 could be obtained. Also, regardless of the insulated state of the rooftop slab, it was confirmed that with an increase in p, w would contribute to the reduction in the indoor air-conditioning load.

Up to this point, we have examined the cooling effect of the rooftop surface in summertime. In winter, the reverse effect such as a heating load increase is believed to occur. Therefore, we made an annual load calculation targeting the regions having different climate conditions, conducting a yearlong evaluation. Fig. 12 shows the integrating value of the region-by-region annual air-conditioning load on the non-insulated rooftop slab when p*:

0. 1, 0.5, and 0.9. The greater p* was, the cooling load became smaller and the heating load increased. This problem of the heating load increase can be resolved by passive actions such as watering only in summertime using water-retentive materials on the rooftop. The details, though, will be a future task. In Tokyo, the cooling load which had been reduced due to the rise in p* was offset by the increase in the heating load and showed no change as the annual air-conditioning load. However, the problem of the peak electric power in summer was one of the major tasks in terms of energy issues. Thus, we believe that, in regions south of Tokyo, one should strive to decrease the surface temperature on the rooftop surface.

Conclusion

In this study, we conducted an outdoor experiment using test pieces with a focus on the passive cooling on the rooftop surface in summertime to clarify the cooling effect of various kinds. We also clarified the influence of the rooftop cooling upon the indoor heat load and heat load on the atmospheric side. The findings are as follows:

1) The results of the outdoor experiment verified that one needs to actively utilize such methods as the latent heat of evaporation and the reflection of solar radiation on the rooftop surface in order to reduce the rooftop surface temperature in summer and improve the cooling effect on the atmosphere. To obtain the cooling effect on the indoor side, it is effective to use finishing materials high in heat resistance and heat capacity while at the same time blocking off as much incoming radiation heat as possible on the rooftop surface.

2) To reduce the cooling load in summertime, it is necessary to insulate the rooftop slab and cool the rooftop surface. Also, to reduce the heat load on the atmosphere, it is effective to employ passive cooling methods by which the surface temperature on the building rooftop surface can be maintained as low as possible. After showing the correlation between the summertime rooftop surface temperature and the heat flux to the indoor side and outdoor

side, we also pointed out that the rooftop surface temperature was an important index in evaluating the thermal performance of the rooftop slab.

3) We learned that, even when the insulation work has not been done on the rooftop slab, an energy-saving performance similar to that in the next-generation insulation standards could be achieved with solar reflectance of 0.5 or greater and an evaporation rate of 0.1 or greater.

4) From the calculated findings of region-by-region annual air-conditioning load, we noted that regions where the cooling of the rooftop surface would be effective throughout the year are the regions south of Tokyo.

Symbols

subscript a: atmosphere subscript s: surface

subscript t: physical quantity at thickness t Qr: radiant heat transfer [W/m2]

QS: heat transfer by short wave radiation [W/m2]

Ql: heat transfer by long wave radiation [W/m2]

QV: sensible heat transfer [W/m2]

Qe: latent heat transfer [W/m2]

Qa: conductive heat transfer [W/m2] ac: convective heat transfer coefficient [W/(m2K)] ar: radiant heat transfer coefficient [W/(m2K]] aw: moisture transfer coefficient [W/(m2h(kg’/kg))] p: reflectance [-] e: emittance [-]

a: Stefan-Boltzmann constant [W/(m2K4)]

Br: ratio of emission [-]

Js: solar radiation [W/m2]

T: absolute temperature [K]

9: temperature [°C]

K: height coefficient of clouds [=0.62]

CC: amount of clouds [-]

a, b: coefficient of one-dimensional approximate equation of saturation vapour pressure [-] w: evaporation efficiency [-] f: vapour pressure [kg’/kg] l: latent heat of evaporation of water [=2512kJ/kg]

X: thermal conductivity [W/(mK)]