Urban Canyons and Solar Heat Gains

buildings and communities more sustainably in the UAE) (Figure 5.1). In the case of the mild and warm period (Figure. 5.1), all the processes of heat flow through the unit’s envelope are sources of heat gain, except the building envelope in the mild period. The most significant parameter is solar radiation from both glazed and opaque surfaces. It has the highest impact on the heat gain within the unit.

In the hot period (Figure. 5.1), the most significant external heat gain contributor is conductive heat gains through the building envelope. They are a result of the large difference between the preferred indoor and external dry bulb temperatures.

The following section investigates the microclimatic strategy of urban canyons, also a transitional space that could be adopted to reduce heat gains caused by solar radiation. At the same time, chapter 6 will explore strategies to reduce heat gains through conduction (Figure 5.2).

5.2 Urban Canyons and Solar Heat Gains

As solar radiation is the primary source of external heat gains for the mild and warm period in Sharjah, the two main urban design factors that can modify the solar radiation balance within an urban context are explored in this section: aspect ratio and the orientation of urban canyons. They can affect the thermal environment of the adjacent indoor spaces in two ways: one is by either providing shade or sun in those urban canyons, which eventually affects their thermal behavior and that of the adjacent indoor spaces; the second is that the urban canyons can specify the direct solar exposure of the adjacent buildings (Yannas 2000).

Urban canyons are geometrically defined by two parameters: The orientation of their major and minor axis and their height-to-width ratio (H/W). Figure. 5.3 represents a comparative analysis between urban canyons for major and minor axis for different orientations and H/W ratios. For the major axis, N-S and NW-SE orientation are analyzed for H/W ratios 1 and 2. For the minor axis, perpendicular to the major axis, E-W and NE-SW orientation are analyzed for H/W ratios 2 and 4. These orientations and H/W ratios are based on recent

Figure 5.2 Illustration of the heat exchange processes in the street canyon

Figure 5.3 Sun Path Diagrams of Major and Minor Urban Canyons

Figure 5.4 Monthly Average Solar Radiation received by the ground of Major and Minor Urban Canyons

Figure 5.5 Monthly Average Solar Radiation received by the wall of Major and Minor Urban Canyons

studies of urban morphology in the UAE by Elkhazindar, Kharrufa, Sahar N, and Arar (2022). They suggest that the major axis should be aligned to N-S and NW-SE to benefit from increased wind flow (the predominant direction of the wind in Sharjah is North and North-West). In all cases, the reflectance of the surfaces of the canyon is 0.6, which represents a light-colored finish (Szokolay 2017). The comparison was made by calculating the amount of average monthly incidents and reflected solar radiation on the ground and walls of the canyons.

Orientation and Height-to-Width Ratio

As illustrated by Figure 5.4, for the major axis, NS urban canyon with H/W ratio=2 represents the case when maximum ground protection is achieved in the hotter period (May-September). For the minor axis, although EW urban canyon H/W ratio=4 receives slightly higher solar radiation on the ground than the NE-SW canyon during the hot period, on average, it is better performing throughout the year.

When comparing the amount of average monthly solar radiation (incident and reflected) on the walls of the canyons (Figure 5.5); for the major axis, H/W ratio=1, walls that are facing East-West (meaning NS canyon) will receive less amount of direct solar radiation in the hotter period (May-September) in comparison to walls of NW-SE canyon. However, for H/W ratio=2, walls of both NS and NW-SE canyons receive the same amount of solar radiation. This is because when the space between buildings is narrower, orientation becomes less important when comparing the amount of average monthly solar radiation received by the grounds and walls of the canyons.

For the minor axis, the walls of the EW canyon for both H/W ratios 2 and 4 receive less amount of solar radiation during the hot period in comparison to the NE-SW canyon. The walls of EW canyon, H/W ratio=4, represent the case where the least amount of radiation is received throughout the year.

Conclusions

This comparative analysis has illustrated that for a major axis urban canyon, NS orientation with a H/W ratio=2 will provide the maximum direct solar radiation protection for the ground and wall surfaces of the canyon throughout the year. For the minor axis, EW orientation with H/W ratio=4 are better performing. Figure 5.6 illustrates the total solar radiation received on the ground for all the cases discussed previously.

Figure 5.6 Total solar radiation received by the ground of urban canyons in various orientations and H/W ratios