As shown in Table 2, solar gain through windows accounts for approximately 40 percent of the total cooling energy load for a base case unit in Shenzhen. Over a year, the energy from direct sunlight per area incident on all building surfaces (north, south, east, and west) totaled nearly 2.53 MWh/2. Northern windows received 3 percent of the total direct annual solar energy per area, while southern windows received 33 percent, and western and eastern facing windows each received 32 percent (64 percent combined). As a result, it was desirable that building designs minimized western and eastern exposures in order to maximize energy-efficient designs. Recommendations for window design in Shenzhen follow in the next several paragraphs.
Northern windows should have small overhangs and vertical fins, or architectural projections, for the purpose of shading. Although shading the northern windows does not have a large impact on energy savings (less than one percent annually), it will greatly improve comfort during the summer months when the sun is to the north. Overhangs should be sized with a depth equal to ten percent of the window height and fins with depths equal to ten percent of the window width. This shading plan will block 100 percent of direct sunlight from August to April, 70 percent in May and September, and 50 percent in June.
Southern windows should have overhangs and fins. Shading the southern window has a large impact on energy savings. The recommended overhang has a depth equal to 20 percent of the window height and fins with depths equal to 10 percent of the window width. This shading plan will block 100 percent of direct sunlight from April through August, 60 percent in March and
September, 40 percent in February and October, and 30 percent from November through January.
Western and eastern windows must be "self-shaded" to block late-afternoon and early-morning sunlight. Overhangs and fins are ineffective because the sun angles are so low in the morning and afternoon. As shown in Figure 15, the only overhang and fin configuration that reduces energy levels more than the solar heat gain coefficient described in Case #2 is overhangs and fins at 100 percent of the window width and height, or Case #6. This is impractical, so we suggest reducing the solar heat gain coefficient (SHGC) of the window to less than 0.4. This may be achieved by using double-pane windows with low-e or reflective coatings or mounting an exterior screen (similar to an insect screen) on a standard single-pane window. Similar to the Shanghai case study, a shading worksheet was prepared for designers to test various combinations of window shading techniques. Figure 16a shows the difference between a window facing south in June and September. The June window receives no solar radiation. Figure 16b shows the difference between overhangs and fins sized at 100 percent and changing the SHGC to 0.4 on solar radiation through a window in August.
Walls with western exposure should have R= 1.5 Km2/W or greater (e.g., block wall with six centimeters of glass fiber insulation inside the weather barrier). This will help keep the west-facing units cooler late in the day during the hot summertime months. This upgrade will increase the comfort of occupants late in the day during the warmest months, but does not save much energy over the entire year (less than one percent).
The color of the exterior walls was found to have negligible impact (less than one percent) on the annual energy use (comparing red brick with white paint). Therefore, the aesthetics of the color scheme was more important than the potential energy savings impact.
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