عوامل موثر بر تهويه غيرفعال دود‌كش خورشيدي (نمونه موردي: شهر اصفهان)

Influential Parameters in a Solar Chimney’s Passive Ventilation (Case Study: Isfahan)

تهويه غيرفعال روشي كم‌مصرف از لحاظ انرژي است كه مي‌تواند هواي نامطلوب داخل را از ساختمان خارج كند. يكي از روش‌هاي تهويه غير‌فعال استفاده از دودكش خورشيدي است كه در يك كانال هوا، با استفاده از انرژي خورشيد و با كمك پديده اثر دود‌كشي تهويه فضاهاي مجاور، تامين مي‌شود. در اين پژوهش ابتدا عوامل موثر بر كارآيي دود‌كش خورشيدي از جمله مصالح، عرض ديوار جاذب، مساحت خروجي هوا از دود‌كش بررسي ‌شد. سپس تاثير تعداد طبقات بر نرخ تهويه مورد مطالعه قرار گرفت و در نهايت در يك جدول مساحت فضاهايي كه در طبقات مختلف دود‌كش خورشيدي قادر به تامين نياز تهويه آن است، ارايه شد. همچنين عملكرد دود‌كش خورشيدي در فصول سرد سال بررسي شد. به‌دليل رايج نبودن ساخت دود‌كش‌هاي خورشيدي، امكان دسترسي به نمونه‌هاي اجرا شده وجود ندارد، از اين‌رو شبيه‌سازي كامپيوتري مدل‌ها، در نرم‌افزار انرژي پلاس به‌عنوان روش جايگزين براي مطالعات ميداني انتخاب شد. مدل‌هاي دود‌كش خورشيدي متصل به يك ساختمان اداري هفت‌طبقه در اقليم گرم و خشك شهر اصفهان در روز 31 تيرماه به‌عنوان نماينده يك روز گرم و ماه‌هاي گرم به‌صورت ميانگين مورد بررسي قرار گرفت. نتايج نشان داد كه افزايش ارتفاع دود‌كش خورشيدي، قدرت تهويه آن را بالا مي‌برد. همچنين مناسب‌ترين مصالح و ابعاد بهينه ديوار جاذب، ورودي و خروجي هوا نيز مشخص شد.

Passive ventilation is an energy conservation method by which sufficient quantity of air is directed into the building in order to maintain desired air change and proper fresh air needed for inhabitants inside. There are many passive methods to achieve required interior ventilation. Solar chimney is a passive ventilation system that uses solar radiation to induce stack effect and to maintain ventilation of the adjacent spaces in the building. Solar chimneys usually consist of a glazed south wall, a duct and a mass wall for absorbing solar energy. Solar incidence transmitted through the glazing is absorbed by the wall and heats the air inside the chimney. Warm air flows upward due to buoyancy effect and as a result the interior air of the adjacent rooms will be drawn into the chimney and directed to the outside. Thus the ventilation rate of the interior spaces will increase and the desired fresh air is maintained. In the buildings of hot- arid climate, to prevent from direct solar radiation and entering dust, windows are usually closed during the day, and then ventilation of the interior spaces may be maintained by solar chimneys. By integrating a solar chimney into the south wall of a building thermal resistance of the wall will also improve. Advantages of a solar chimney in comparison to natural ventilation in buildings are: 1. preventing inside being viewed from outside, 2. maintaining inside ventilation with a low air velocity, 3. preventing dust, pollution and outside noise from entering inside, 4. providing thermal comfort during summer nights when outdoor air velocity is low and air temperature is within the comfort range. This paper investigates the specifications of the main parameters affecting the solar chimneyʹs efficiency such as the material of the absorber wall, the width of the wall and the area of the chimneyʹs inlet and outlet. The effect of the number of floors on ventilation rates is also studied, and finally the area of the interior spaces which may be ventilated by the solar chimney is introduced. Moreover solar chimneyʹs performance during cold periods is examined and discussed. Since the application of solar chimneys to buildings is not common and constructed models are not available for experimental studies, their performance is usually studied numerically or by simulation as substitute methods. Here the models under consideration were simulated in EnergyPlus software. We studied a seven-storey office building in hot-arid climate of Isfahan with an attached solar chimney on its south wall. The results revealed that by increasing the height of the chimney, the ventilation rate enhances. Furthermore, the most proper material for the absorber wall, the appropriate dimensions of the wall as well as the inlet and outlet air was identified. It was found out that the thermal mass of the absorber wall will affect the night ventilation and increases it. A double glazed south wall is effective for increasing the air flow rate inside the chimney. Overall, a solar chimney with optimum dimensions will result in 18.52% reduction of the buildingʹs heating load during the year.