Re-conceiving the Built Environments of the Gulf Region
Challenging the Supremacy of Airconditioning
Simos yannas
Director, Environment & Energy Studies Programme, Architectural Association School of Architecture, London
The absolute dependence on mechanical air conditioning that characterises contemporary buildings in the UAE is a major issue that is both poorly understood and potentially intractable. While the more extreme periods of high ambient air temperatures and humidity that characterise the climates of the region may be alleviated by the use of air conditioning, there is no technical justification for the whole year to be treated the same way other than the climatic inadequacy of buildings now being built here. Nor is there any scientific or physiological evidence for the common practice of maintaining constant indoor temperatures at the kind of levels commonly provided in winter to heated buildings in cold climates. On the contrary, there is widely accepted empirical research and physiological justification for abandoning such practice so that both residents and visitors can respond to daily and seasonal variations of the outdoor climate through natural adaptive processes of the human body. Currently, the temperature difference between the airconditioned spaces inside buildings and the streets and urban spaces outside frequently rises above 20 degrees centigrade, high enough for a thermal shock when entering or exiting airconditioned buildings and motor cars. Heat discharges from airconditioning equipment, and from the power stations that produce the electricity used to drive building appliances, lead to urban warming. In Dubai this is bound to keep increasing at a fast rate owing to the intense building activity leading to additional heat discharges from airconditioning plant. Onc effect of urban warming is to drive cooling loads for buildings higher calling for larger airconditioning plant and/or more energy to operate it. Another is the deterioration of environmental conditions outdoors, undermining the usability of outdoor spaces, a serious blow to the essence of any city. At the rate at which building activity is now taking place in the UAE, a complete abandonment of the outdoor environment for a network of enclosed, airconditioned malls is probably only a matter of time. Were this to happen it could mean the return of the outdoor urban environment to a far worse desert than the one from which it was won. This could severely erode the
value of property and businesses housed here. To prevent such fate and contribute to the use and enjoyment of the city it is essential to narrow the temperature differences between indoor and outdoor spaces. This will require, first, the acceptance of adaptive standards of thermal comfort
as now commonly understood by the international scientific and engineering communities, second, a better understanding of the technical aspects of building design for these climates, third, an equally improved approach to the microclimatic design of outdoor spaces.
In view of the absence of any environmentally appropriate, contemporary built precedents, a major research effort will be required to underpin the formulation of guidelines and regulations to help redirect building design and the retrofitting of existing buildings toward climatically adaptive and environmentally sustainable models. Given that it has taken over thirty years of funded scientific research and applications in Europe and North America to make a significant, though as yet far from sufficient or satisfactory, difference in the environmental performance of buildings, the effort required will be substantial. However, the likely cost of any such research is insignificant compared to the saving in capital and running costs it can help achieve. Challenging as the technical research might be in order to bridge the present knowledge gap as quickly as possible, it cannot even start without a change in the cultural perception of the role of mechanical AC. The current dependence on mechanical AC must be challenged, its operational characteristics ought to be rethought and the environmental attributes and expectations from the buildings being built in the Region should be reconceived.
Our Masters Programme in Sustainable Environmental Design at the Architectural Association School of Architecture in London is committed to exploring architectural solutions that can achieve thermal and visual comfort at near zero carbon emission for most new buildings in most climatic regions. In response to Nader Ardalan’s call for an environmental agenda for the Gulf Region we undertook a series of studies that combined reviews of historic precedents with parametric studies using computer simulation models and fieldwork involving short-term measurements in the warmest period of the year. These are summarized in the next section of this article. The studies led to the formulation of some preliminary guidelines that were tested on a variety of
building programmes. The projects developed for these programmes are illustrated in the final section of the article.
Precedents
Courtyard Tradition in Islamic Architecture
The courtyard form has a long tradition in Islamic architecture. Study of some of the palace complexes surviving in Spain has provided insights on how the courtyard and the shaded porticos that surrounded them helped modulate indoor environmental conditions under the very intense summer conditions experienced in the south of the country. Measurements taken recently in the 14th century Palace of the Lions, Fig. 4 a-b. a residential complex of the Alhambra in Granada show the role of the courtyard and its porticos as transitional spaces mitigating the effects of the outdoor temperature and intense summer sunshine. The graph, Fig. 4c, shows outdoor, courtyard and indoor temperatures measured over four consecutive days. With the outdoor air reaching peaks of 31-35C those in the courtyard are lower at 27-30C and are further reduced indoors by thc thermal inertia of the building. The daily range of 18-25C and average of 22C achieved indoors is quite remarkable and is achieved despite the fact that the courtyard is now operating without the lavish vegetation that used to populate it and which has been recently removed to protect the building’s foundations from moisture (Jiménez Alcalá 2002).
Courtyard House and Transitional Structures in old Dubai
A series of short-term measurements of temperature and relative humidity were undertaken in outdoor and semioutdoor spaces in areas of Dubai during July 2007 in the hottest period of the year (Thapar 2007). The measurements were taken in Bastakia and Deira and in outdoor locations near the water and the city’s new developments. These were then compared with readings taken on the roof of a three-storey building of the Youth Hostel Association (YHA) that was selected as representing a reference urban temperature unaffected by built form, vegetation or water. At cach location measurements were supplemented with a thermal comfort survey among passers-by or subjects
found sitting there. The results show considerable but systematic differences between the dry” inland reference location and the wetter locations of the old town and the waterside new developments. These provide some useful insights on improving microclimatic conditions in this city.
Dating from the 1890’s the Bastakia quarters in Dubai comprise some sixty buildings that form a dense urban tissue around narrow winding streets. Measurements were taken in the courtyard of a restored house and in nearby streets and compared with readings from the YHA, Fig. 5ad.
The courtyard with a height to width ratio of 1.8:1. Fig. Sb. recorded the lowest temperature by 1-3K from the street of 2:1 height to width ratio. Fig. 5e, and by as much as 4-6K from the YHA readings. However, on the day of the measurements this advantage was counteracted by higher measured relative humidity in the area of the Bastakia owing to Northerly winds blowing from the creek.
In Deira, a densely built shopping district developed in the 1960’s, the measurements were taken in the Gold Souk, in a street parallel to the souk, and near the water along the creck. The graph. Fig. 6a, shows that although not protected from the sun the area near the water Fig. 6b registered the lowest and most stable temperatures during the hottest part of the day presumably owing to the stabilizing influence of the water mass and the effect of evaporation. The Gold Souk, Fig. 6c, which is well protected from the sun, achieves a lower temperature than the parallel street, Fig. 6d. which though narrow is exposed to the sun for part of the day. The difference reaches some 1.5K during the peak midday sunshine period falling to nil at the end of the day as both spaces cool toward the ambient air temperature after sunset. Exposure to the sun in the open street leads to surface temperature that are higher than the air temperature thus having a negative effect on pedestrian comfort. On the day of the measurements the mean radiant temperature in the open street was estimated to be higher by over 5K at 2pm. On the other hand, being protected from the sun throughout the day, the building surfaces surrounding the souk would remain close to air temperature. The importance of this was confirmed by the comfort survey which voted the souk area as comfortable whereas the open street was found to be uncomfortable. This (10 July 2007) was a hot day with a peak temperature that rose above 45C at the YHA. This makes the reduction of 7-10K achieved by the fieldwork spots in Deira particularly notable.
The dense organic forms and shaded transitional spaces of Deira and the Bastakia provide some clear microclimatic benefits that derive from the resulting solar protection and thermal inertia. Jointly these attributes lead to both lower and more stable temperatures than those resulting in less dense areas with higher exposure to solar radiation.
However, a densely built form can also prevent air flow and reduce air velocity which can be problematic at times. Near the sea the courtyard form needs to be more adaptable so as to open to cool breezes, but protect from warmer air at times.
Dubai Marina and Greens Today
Readings taken in the area of the Dubai Marina and the Greens residential area were lower than the YHA reference temperatures by up to 7K around midday, Fig. 7 a-c. These effects diminish after sunset. The Marina area is influenced by the proximity of the water and the winds blowing from the Gulf, while Greens displays the effect of vegetation and shading trees. However, again humidity levels near the sea were significantly higher. This is significant as the comfort survey identified high humidity as a major discomfort factor that undermined the effect of lower air temperatures.
Overall the comfort survey confirmed people’s high adaptive potential with temperatures close to 40C reported as comfortable by subjects in the shade and exposed to air flow. Wind velocities of 2.0m/s were reported as desirable at these temperatures. As in all hot climates urban activity in Dubai avoids the hottest part of the day. Markets in Deira open at 6am and are deserted at midday. Evenings are the most popular time to enjoy outdoor activity.
Parametric Studies
Climate Analysis for Sustainable
Environmental Design
Weather data for Abu Dhabi City (24.28°N 54.25 E) and Dubai (25.14 N 55.17°E) were obtained using the Metronorm global meteorological database (Meteotest 2004). The data files representing ten-year average data for the two cities are almost identical. A summary of mean daily values of the main parameters is given in Table 1.
The monthly variations of the outdoor dry-bulb temperature show that the annual cycle can be divided into three distinct periods, Figs 1-2: a four-month period of mild weather (December to March inclusive) characterized by daily mean temperatures of 20-23°C: a warm period (November and April) with mean temperatures of 2526″C, and a hot period (May-October inclusive) with mean temperatures of 29-34°C..
The diurnal temperature range of 10-12K involves night-time ambient air temperatures that are low enough for convective cooling of building structures for most of the year. However, the useful cooling potential available from this source is being eroded by the urban warming caused resulting from the heat discharges from airconditioning appliances on buildings and motor cars.
- Winds average 4.0-4.5 m/s throughout the year the strongest coming from the direction of the Gulf on most months except for the hottest months (July-September inclusive) when the predominant direction is recorded as South
- .Thermal comfort criteria as defined by the international standard ISO 7730 and the ASHRAE Standard 55-92 can be satisfied at air and mean radiant temperatures in the range of 19-30°C (Yannas 2007). Temperatures above 30°C are also commonly tolerated in hot climates when air movement is available. Air velocities of up to 1.0m/s are generally acceptable indoors whereas outdoors in the city air velocities of up to 2.0m/s will help extend the comfort range further provided subjects are protected from direct solar radiation. Such wider range is consistent with fieldwork undertaken to assess adaptive practices in hot climates (Auliciems and Szokolay 1997: Humphreys, Nicol and Raja 2007) and a comfort survey undertaken last summer (Thapar 2007). With these considerations thermal comfort can be achieved by natural means in this climate for much of the year.
- Sunshine is strong throughout the year with an annual average of 8 hours of bright sunshine per day, rising to some 10 hours per day in the hot period. Clearly, solar protection of occupied spaces is essential outdoors as well as indoors throughout the year. The incident solar radiation is high all year varying in the range of 3.77.0 kWh/m on unobstructed horizontal surfaces. Roofs. streets, pavements and other exposed manmade surfaces will get extremely hot affecting outdoor comfort as well as building cooling loads unless specially treated.
For solar energy applications the prospects are extremely good for all types of applications both thermal and electric, sun-tracking appliances can intercept as much as 6.5-8.5 kWh per m collector area daily throughout the year.
- Relative humidity mean daily values of 50-65% conceal fairly high levels of absolute humidity that rise to 15-25 g/kg during the hot period. However, with the exception of three months when the wet-bulb temperature is too high, the temperature difference between dry-bulb and wet-bulb (wet-bulb depression) reaches regular peaks of 10-15K during daytime indicating useful potential for evaporative cooling if needed.
- Calculated hourly sky temperature depressions which provide a measure of radiative cooling to the night sky are in the range of 10-12K in the hot period indicating a useful potential at night time.
The sky luminance is high throughout the year in the range 15,000-70,000 lx during work hours in the mild period to 50,000-100,000 lx in the hot period about half is diffuse illuminance from the sky vault. Under these conditions 1-2% of the outdoor illuminance is sufficient to meet required illumination levels for any indoor activities.
Figure 3 shows the hourly patterns of the dry-bulb, wetbulb and sky temperatures on a typical day at the beginning of the hot period in May. The sky temperature depression (dry-bulb minus sky temperature) is of 10-13K. This gives a measure of the cooling potential by longwave radiation to the sky. Although this appears to be as high during daytime as at night, during the day the outgoing longwave radiation is overtaken by the incoming longwave and shortwave radiation from the sun and sky. At night-time, however, the net outgoing longwave radiation is sufficiently high to lower temperatures of surfaces exposed to the sky below that of the ambient air temperature. This is shown on the graph by a reference surface temperature (yellow line) calculated for a horizontal surface with a solar reflectance typical of urban surfaces and with unobstructed view of the Sun and sky. In this case this can be seen to be of the order of 2K relative to the air temperature. During daytime exposure to solar radiation raises the temperature of such surface well above that of the ambient air making it contribute to the urban heat island effect. The temperature elevation would be much higher on darker surfaces such as asphalt The graph also indicates the potential for direct evaporative cooling which with a wet-bulb depression varying in the range of 5-15K is quite substantial during daytime at this time. This is progressively reduced in the summer becoming unavailable in the form of direct evaporative cooling toward the middle of the hot period.
These fractions can be achieved in buildings with very modest areas of glazing. Highly glazed facades risk serious problems of glare as well as excessive cooling loads and overheating.
Urban Form
A number of courtyard variants based on the forms encountered in old Dubai were modelled using the ENVImet three-dimensional microclimate model for simulating microclimatic interactions in an urban environment. Coutyard height-to-width ratios H/W of between 1.5:1 and 2:1 were considered as providing reasonable shading as well as potential for holding cooler air during daytime, Figs 8 and 9.
The simulation studies investigated the effect of openings in the courtyard blocks to improve ventilation conditions. Openings perpendicular to the main streets helped increase air flow. Moreover:
- Broader N-S streets (ie in the direction of the predominant wind) and narrower E-W streets provide deeper penetration of wind as well as reduced incident solar radiation
- Wind speeds are highest where the wind enters the urban blocks; these areas are good for public functions that require good airflow.
- Chamfering of the edges of blocks helps improve ventilation and wind speeds especially in streets perpendicular to the main wind direction.
- Staggering of blocks leads to improved air flow Removing parts of the lower two floors of the courtyard blocks improves wind penetration inside the fabric as well as creating covered double height urban spaces at road intersections; these provide transitional spaces that have better wind access as well as being shaded.
Building Design
Hourly cooling loads and indoor resultant temperatures were calculated for a number of building variants using climatic data for Abu Dhabi and Dubai with the Tas dynamic thermal simulation model (EDSL 2006). The building specification assumed for these simulation studies was of compact square plan with good access to daylight. This was tested for both office and residential occupancy with an average rate of internal heat gain of 15 Watts per square metre floor area. The following building parameters were varied as part of parametric studies:
Window Areas were varied from a minimum required for daylighting to fully glazed elevations.
Window Orientations: windows were assumed to be equally distributed between two orientations, either NorthSouth or East-West
- Glazing Type: clear single glazing, clear double glazing, coated double glazing
- Solar Control: none, maximum (no direct radiation at any time)
- Opening area for convective cooling was assumed to vary in the range 10-50% of occupied floor and to
- be activated when the outdoor air temperature provided potential for free coolingThermal Transmittances of opaque elements were considered within the range 0.25-1.0 W/mK for external walls and roofs
- Cooling setpoints: 22 °C and 29-C
For a base case with a cooling setpoint of 22 °C and unprotected windows of a surface area equivalent to 50% of the building’s floor area, the cooling energy requirement was calculated at 230 kWh/m-building floor area. When the main building parameters were optimized this dropped to 96 kWh/m for the same cooling setpoint and window to floor ratio, and to 70 kWh/m’ when window areas were reduced to a lower window-to-floor ratio of 10-25%. Finally, adopting a cooling sctpoint of 29 C as suggested by the adaptive comfort algorithms eliminated the need for airconditioning for a total of at least six months in the year leading to a total cooling energy demand of only 25 kWh/m’, an overall saving of some 90 percent compared to the base case, Fig. 10.
