Document Type : Research Paper
Ph.D. Candidate, Art and Architecture Department, Tarbiat Modares University, Tehran
Asociate Professor Art and Architecture Department, Tarbiat Modares University, Tehran
Full Professor Art and Architecture Department, Tarbiat Modares University, Tehran
Designing an efficient skin for the building can minimize winter heat loss and summer heat gain. Double skin façade (DSF) is a good device for achieving such a goal; but its summer overheating is a problem for using it, especially in hot-dry climates such as Iran plateau. In this study, after reviewing state of the art strategies for cooling façade cavity, the effect of using some of them (such as changing size of the vents and depth of the DSF cavity) in reducing the danger of DSF overheating, in the Yazd City (Iran), is studied. First of all, the effect of using façade vents was analyzed by comparing two models: one had an unventilated DSF and the other (with the same size of DSF) had seven 200 mm2 inlet and outlet vents. The results showed that the ventilated DSF model had better performance (in reducing overheating) than the unventilated one. Then, effect of changing size of the vents in reducing overheating was analyzed. Ventilated DSFs with different vents areas including 12.5, 25, 50, 100, 150 and 200 mm2 were compared. The studies were carried out in three situations:
1- Changing the size of inlet and outlet vent simultaneously,
2- Assuming inlet vent constant (=200 mm2) and changing the outlet vent area,
3- Assuming outlet vent constant (=200 mm2) and changing the inlet vent area.
The best results were acquired by assuming 50 mm2 area for inlet vent and 200 mm2 area for outlet. Then, the effect of changing the cavity depth in reducing overheating was analyzed. Ventilated DSFs (with 50 mm2 inlet and 200 mm2 outlet vents) with 100, 200, 400, 600 and 800 mm cavity depth were compared. All models had 2400 mm cavity height. The results showed that the model with 600 mm cavity depth had the lowest temperature and the highest wind speed. At the end of this part of study, the following results were acquired:
• The ratio between Input to output vent size should be 1 to 4.
• Input and output vents should have the maximum possible.
• The ratio between cavity depth to its height should be 1 to 4.
• By using the above strategies, cavity temperature can be lower by 18.8 °C in warm months of the year.
Finally, in order to validate the results of previous parts, an improved model (equipped with the best strategies from the previous part), a model equipped with normal DSF, and a model without DSF was compared by using Design-builder simulation. The results showed that in cold months, energy consumption of the models with DSF was about 18% lower than the one without it. Also the results showed that using the strategies presented in this paper not only prevents overheating in the DSF cavity, but also can cool off the whole building. The total amount of energy consumption in the improved model was about 10% (14.4 KWh/m2) lower than the model without DSF. This showed the good potential of using double skin façades even in hot-dry climate of Iran.