Potential Passive Strategies

Figure 3.14 Applicability of Architectural Strategies The psychometric chart illustrated in Figure 3.12 describes the extended limits of outdoor comfort during the hot period in Sharjah when the radiation levels and air velocity are considerably higher than indoor averages. Therefore, protecting outdoor spaces from direct solar radiation through shading and channeling cool breezes can be the primary strategies to adjust outdoor comfort. This chart is based on the paper “Thermal Comfort under an Extended Range of Environmental Conditions” by Arens, Gonzalez, and Berglund (1986) and is plotted for a clo value of 0.4 (summer clothing) and 1.3 MET (sedentary activity like slow walking). It is further overlaid with hourly values of dry-bulb temperature and relative humidity as points in three different colors based on the previously described periods of Sharjah (mild, warm, and hot) while assuming that air temperature equals the mean radiant temperature.

It is evident from Figure 3.12 that shading is essential to reduce thermal stress caused by direct solar radiation above 25°C. However, strategies beyond shading are required at higher temperatures. Additional air movement can help in the evaporation of moisture from the skin and provide a cooling effect, subsequently making the higher temperatures tolerable (Auliciems and Szokolay [1997] 2007). Wind speeds up to 2m/s can effectively alleviate thermal stress in outdoor spaces at temperatures up to 40°C, as corroborated by Thapar’s fieldwork in Dubai in 2008.

3.4 Potential Passive Strategies

A deep understanding of the climate of Sharjah and the three distinct periods allowed for a more profound study of the potential climatic adaptation of architectural and adaptive environmental strategies to be employed. In a hot climate like that of Sharjah, key strategies involve controlling heat gain by providing an appropriate building shell and avoiding penetration of solar radiation while also allowing reasonable daylighting levels and views. Furthermore, increased air movement via natural ventilation is vital for enhancing comfort during the day and cooling the interior thermal mass overnight.

This section briefly segregates the architectural and adaptive passive cooling strategies and evaluates their suitability and applicability throughout the year (Figure 3.13 and 3.14). While the architectural approach, such as form and thermal mass, is more robust and enduring, the adaptive approach recognizes that people are not passive about their thermal environment, acclimate themselves to the prevailing climatic conditions, and are willing to control it to secure comfort.

Architectural Strategies

Form As solar radiation is the primary source of external heat gains in Sharjah, the geometry and material properties of the urban and built form are crucial to modify the solar radiation balance within the fabric. For example, the traditional compact courtyard form with narrow urban streets (sikkas) has the potential to provide a larger microclimatic footprint in comparison to other morphologies

in the region. Even the study by Thapar and Yannas (2008) suggested that the courtyard blocks, when compared to high-rise towers and mid-rise blocks, were the coolest, with the open courtyard itself as the coolest spot in hot climates like that of Sharjah. Furthermore, the courtyards, the sikkas, along with shaded colonnades, known as liwan, function as transitional spaces that can help achieve a gradual change between outdoor and indoor temperatures, mitigating the significant thermal shock that occupants experience while moving into and out of the buildings in Sharjah.

Chapters 4, 5, and 6 discuss in-depth the history, environmental performance, and design application of the compact courtyard form and other transitional spaces, including Liwan and Sikka, respectively.

Thermal Mass Thermal mass is a suitable strategy for hot climates with a large diurnal temperature range like that of Sharjah. It can regulate the temperature of the space by controlling and delaying the heat transfer within the building during the day, while during the night-time, when the external air temperatures are relatively lower, the building interior can be cooled by night flushing. Furthermore, the utilization of thermal inertia through the insulation on the external surface of the wall can keep the internal spaces comfortable without (or with minimal) mechanical cooling systems during the hot periods in Sharjah. Traditional materials like sun-dried mud bricks and rammed earth, along with modest insulation levels, can provide the required thermal mass and time lag in Sharjah with their density and high specific heat capacity (Gourlis and Holzer 2022).

It is also essential to consider the color and texture of materials as dark, matte, and textured surfaces absorb and re-radiate more energy than light, smooth, reflective surfaces (Gourlis and Holzer 2022).

Adaptive Strategies

Comfort Ventilation and De-coupling Comfort ventilation is one of the most common passive design solutions and is used to move fresh outdoor air through the interior spaces in order to remove heat and enhance human comfort. This strategy results in direct physiological cooling for the inhabitant. However, it is only applicable when the outdoor air temperature can provide the desired indoor temperature and is strictly according to the inhabitant’s preference. Givoni (1994) claims that the strategy can still apply when the air outside is warm, around 30°C but not when the outdoor average maximum temperature exceeds 32°C.

Therefore, comfort ventilation and the cool¬ing effects of elevated air speed could be applied during the warm (mornings and evenings) and the mild period from November to April, representing the period when the outdoor average maximum temperature does not exceed 32°C. Beyond this temperature, during the hot period, the indoors need to be de-coupled from the outdoors and selectively ventilated through mechanical fans for cooling. During the warm period, de-coupling is primarily required only during the afternoon (the warmest time of the day), while at other times, the coupling for increased ventilation is encouraged.

Night Cooling This strategy can be a sensible approach to cool down any effective building structure with high thermal mass. Night flushing is deployed to get rid of the

Figure 3.14 Applicability of Adaptive Passive Strategies throughout the year. heat stored in the mass during the daytime and to maintain indoor temperatures within a comfortable range for the next day (Givoni 1994).

In Sharjah, the mean temperatures vary from one period to another and differ from day to night. Figure 3.4 illustrates that the diurnal variation can exceed 8-10K in the region. The potential of night cooling as a passive strategy is apparent in the mild and warm periods and parts of hot periods (September to May) when the night dry bulb temperatures drop below the indoor operative temperature. The mean temperature drops to a minimum at night-time and goes below 20°C, especially during the mild period. However, this procedure might have bleak potential during July and August when the night-time minimum temperatures exceed 30°C.

Evaporative Cooling Both direct and indirect evaporative cooling can be used as a potential cooling technique in Sharjah due to the yearly wet bulb depression that ranges from

8-14 K (Figure 3.2). This strategy has high potential from October to May due to the high wet bulb depression. However, during the hot period from June to September, when the wet bulb temperatures exceed 24ºC, it is impractical to use either direct or indirect evaporative cooling (Givoni 1994), as discussed in Section 3.1. Historically, evaporative cooling has been a prominent part of the vernacular architecture of the UAE, including the wind towers or the malqaf that placed water at the air intake to create a comfortable interior climate. Even the mashrabiyas or the perforated screens operated on evaporative cooling. They were traditionally watered either manually or by the constantly seeping water from a porous water jar placed on the sill (Gourlis and Holzer 2022). Chapter 4, Section 4.1 further explores the various vernacular strategies employing evaporative cooling and their environmental performance and application.

Adaptive Shading Controlling heat gain and avoiding penetration of solar radiation while allowing reasonable daylighting levels and views is critical in a hot climate like that of Sharjah. Shading devices can be beneficial in such cases, including but not limited to loggias, balconies, roof canopies (permanent and adaptive), and window shading screens. The most effective forms of protection are operable adaptive shading devices designed to suit the weather and the occupant’s need. Perforated screens, often known as mashrabiya, are a common adaptive shading element of vernacular architecture in the UAE, offering adequate protection against intense sunlight when required (Gourlis and Holzer 2022). Adaptive tensile structures are also a predominant feature of the vernacular architecture in the UAE. They can shade streets, courtyards, and roofs during the hot and warm period in Sharjah, especially in the afternoons when the high radiation rates reach daily peaks of almost 750Wh/m2. They can be withdrawn during the evenings and early mornings when the outdoors is within comfort range.

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“[...] deliberately permanent rather than temporary, which is traditional rather than academic in its inspiration, which provides for the simple activities of ordinary people, their farms, and their simple industrial enterprises, which is strongly related to place and the climate, through the use of local building materials, but which represents design and building with thought and feeling rather than in a base or strictly utilitarian manner.”

- Henry Glassie (2000)