تعيين تحليلي تاخير زماني و عامل كاهيدگي براي ديوارها در شرايط اقليمي تهران

Analytical Determination of Time Lag and Decrement Factors for Walls in Tehran’s Climate

در تعيين ظرفيت ذخيره‌سازي انرژي مواد، «تاخير زماني» و «عامل كاهيدگي» پارامترهاي بسيار مهمي هستند. در اين تحقيق، تاخير زماني و عامل كاهيدگي براي ديوارها در شرايط اقليمي شهر تهران، به طور تحليلي مطالعه شده است. برخلاف تحقيقات قبلي كه اساساً بر روش هاي عددي متمركز بوده، در تحقيق حاضر روشي تحليلي با كاربرد تابع گرين براي حل معادله‌ي گرماي گذرا با شرايط مرزي جابه‌جايي (نوع سوم) وابسته به زمان مربوط به تعيين عامل كاهيدگي و تاخير زماني، ارايه شده است. اثر پارامترهاي مختلف ــ نظير ضخامت و ضريب انتقال گرماي محيط داخل و خارج ــ و نيز اثر لايه‌ي عايق گرمايي در ديوار ه‌هاي ساندويچي معمول بر تاخير زماني و عامل كاهيدگي مورد بررسي قرار گرفته است. نتايج اين تحقيق را مي‌توان در طراحي كاراتر ساختمان هاي غيرفعال خورشيدي و طراحي بهينه ي ديوارها و ساير زمينه هاي مرتبط در كشور، كه سبب كاهش مصرف انرژي و همچنين كاهش آلودگي محيط زيست مي‌شود، به كار برد.

Although the direct heat gain method is simple and inexpensive, it suffers from large temperature swings, besides strong directional daylight. In addition, the direct heat gain method can be affected very quickly by outside temperature fluctuations, which results in an indoor bed comfort level. For thermal storage buildings, walls and floors are used as heat storage elements, and stored energy in the walls and floors during the day period can be used for heating during the night. Time lag and decrement factors are very important characteristics for determining the heat storage capabilities of materials. Evaluation of the time lag and decrement factor provides a measure of the developed indoor thermal comfort conditions and, from an energy point of view, the possibility of reducing energy load demands. In the present study, time lags and decrement factors for different building materials utilized in Iran have been investigated analytically. Unlike previous studies, which mainly focused on numerical methods, in this research, the transient heat conduction equation is solved analytically using a Green function under time-dependent convection boundary conditions. According to the climate of Tehran, periodic boundary conditions are applied to the outer surface of the wall for modeling outdoor temperature variations over 24 hours. The effects of different parameters, such as wall thickness and both inner and outer heat transfer coefficients, as well as the effect of thermal insulation layers in sandwich walls, on the time lag and decrement factor, are investigated. The results indicate that the thickness and type of material have a significant effect on the time lag and decrement factor. The results of the present study are applicable for designing more effective passive solar buildings, optimizing the design of walls and other related areas, resulting in a reduction of energy consumption and environmental pollution through diminishing pollutants such as CO2.