Energy Efficiency for a Hot and Dry Climate

Nature of the climate

  • Hot-dry desert and semi-desert climates are categorised as “very hot”, with dry air and dry ground. Day-time air temperature may range between 27 and 49˚ C, but at night falls, it might reduce to as much as 29˚ C. 
  • Humidity is continuously moderate to low. 
  • The low humidity day air and minimal rainfall dissuade flora, and the ground reflects the strong sunlight producing glare. 
  • There is no cloud cover to reduce the high intensity of direct solar radiations. The local winds also carry dust and sand.

Objectives

  • Physical comfort depends on reducing the intense radiation from the sun, ground, and surrounding buildings. Heat flow study of constructions will aid the designer to select walls and roof designs which can, during the day, maintain inner surface temperature less than the outer envelope. At night, the air temperature is low enough to allow an increase in effective temperature by surface temperature less than the air temperature. 
American Peublo Homes, Source-https://rmw2wp.wordpress.com/2013/10/24/assignment-3/
  • Because of constant low humidity, surface evaporation is higher than in any other climate. 
  • A cooled dust-free breeze is advantageous for the indoors.

Planning

  • Outdoor conditions are hostile; it is hard to live in without any special provisions to mitigate its harshness. Both the buildings and the external spaces need protections. An enclosed compactly planned and inward-looking building is most suitable. 
  • Reduction of movement for the occupants will increase comfort by avoiding unnecessary effort and fatigue. 
  • The larger dimensions of the building when oriented along the north-west, these elevations receive low heat loads from the solar radiation. West is the worst orientation, as peak intensity coincides in time with the highest air temperature leading to a high heat load. Non-Habitable activity rooms like storage, toilets, if planned on the east and west which provide thermal barriers. 
  • Shading of the roof, walls and outdoor spaces is critical. Projections, verandahs, shading devices, trees, utilizing adjacent walls and buildings, are effective techniques. These shading devices should have low thermal capacity material to ensure quick heat loss after sunset.
  • The concept of “double-roof” is an effective way to shade the roof, it is a construction of a second roof over the first. Since the outer roof is gaining heat through radiation, this heat will have a slow transfer to the occupant rooms because of the air gap, increasing the thermal lag. 

External Spaces

  • The adjacent building, pavement, roads, and ground heat up quickly, causing glare and reflected heat radiation, hence it is essential to shade outdoor walls to avoid such effects.
  • Trees, plants, and water will cool the air by evaporation, help keep dust away and provide shade, visual and psychological relief.
A Moroccan narrow street shaded with adjacent buildings and pergolas
  • The best effective method in this type of climate is courtyards. This space traps a pool of cool night air, as it is heavier than the surrounding warm air.
  • Surface treatment and surface material selection will influence the thermal behaviour of the building and can benefit from reducing heat load. Light-coloured or shiny external surfaces will reflect a large part of the incident radiation; thus, much less heat will enter the building in the first place. Avoid Dark-coloured surfaces in all cases.

Ventilation

  • During the day-time, close and shade openings to the inside. Keep ventilation to the absolute minimum necessary for hygienic purposes, to reduce hot and dust external air entry.
Source-https://dss5rn.wordpress.com/2013/10/24/design-for-human-comfort-and-resilience-albuquerque-nm/
  • Internal Heat gains, the heat output of occupants, cooking and lighting cause a problem. If possible, keep such heat sources isolated and separately ventilated. 
  • Keep in mind that Ventilation does not reduce radiant heat transfer, but lowering the temperature of the inside surface, will reduce heat emission of the surface. Another way to do so is by using low emittance surfaces on the inside and a highly reflective surface on the top of the ceiling. Aluminium will benefit both in this situation. 

Vernacular observations

  • Traditional shelters have heavy walls of earth, brick, stone, and roofs of the same material, often with very less use of timber. Thick walls provide good thermal capacity. 
  • Rooms are built around a central courtyard, which provides a cool private outdoor space for family activities. 
Courtyard- the centre of all activities through the day
  • Windows and door openings are small and few. Windows are often located high on the walls, allowing little heat and dust, and reducing ground glare. Conversely, this is not very advantageous when it comes to human health. These windows provide inadequate ventilation. 

Outdoor Spaces

  • Enclosure- walls, often high, surrounding courtyards or gardens, protect from the scorching heat, hot winds, and dust; they also provide solace and privacy. 
  • Inward Looking plan- Courtyard or patio is advisable
  • Vegetation- Restricted availability of water is more feasible in enclosed spaces than outside. Evergreen trees, shrubs, cacti, creepers are advisable. Shading over the paved area, thus reducing reflected heat. Lawns are difficult to maintain. Creepers provide shade, cool surface areas, and high value. 
Source-www.nzdl.org/gsdl/collect/hdl/index/assoc/HASHb5f9.dir/p139c.png
  • Water- pond, basin, and fountains- they help cool the breeze before entering the building.
  • Shading devices- Vertical elements such as walls and the building itself provide shade in the morning and late afternoon hours. Horizontal elements, verandahs are useful, but plants provide the best solution. 
  • Protected circulation- Pedestrians and car parks need shade. Narrow alleyways over wider ones covered with awnings and pergolas and creepers. 

Conclusion

The objective is to reduce the heat load of the building due to direct or reflected solar radiations. It will help increase human comfort creating a wholesome productive environment in a hot-dry hostile climate. 



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