Enhancing the Performance of Night Ventilation Systems in Office Buildings Using Phase Change Materials, Case Study in Yazd

Night ventilation is a well-known passive method for reducing cooling load of buildings with HVAC systems and providing thermal comfort by storing coldness of night in buildings’ fabric. This method is related to the properties of the thermal mass which are used in the building elements. While the use of Phase Change Materials (PCMs) in night ventilation is a powerful method for reducing cooling load in buildings, in this article, the effect of PCMs as a lightweight thermal mass for reducing buildings’ cooling load is discussed. This research is carried out to compare the results of this experimental study with those obtained from simulation and logical analysis arguments. The studied building is an office building located in Yazd with hot-dry climate. In order to define warm period of the year with cooling load, a simple model with HVAC system is simulated while providing cooling during the day and free night ventilation during the night. At first, the main parameters that are related to night ventilation are analyzed and optimum conditions for starting night ventilation and fans ventilation with different ventilation rates are defined. Then, the effect of PCMs melting points are calculated, to select the optimal PCM melting point while cooling loads of the alternative models are compared with those of a model using common construction materials. Similarly, the amount of reduced cooling load for PCMs coupled with night ventilation is compared with common conditions in this system. PCM materials are those that melt and solidify at a certain temperature, and due to this phase change property, they can store a great amount of energy. The heat stored in these materials, related to their phase change characteristics, is more than that stored through temperature change (sensible heat). Then PCMs are more ideal as thermal mass in new constructions due to their weight compared with traditional heavy weight masonry thermal storage elements. In this study EnergyPlus 1 software was used as the simulation tool. To validate the software, the results of an experimental model in Phoenix, Arizona were compared with those obtained by simulation. The coefficient of determination between the results of the experimental model and simulated model was 0,93. Therefore, regarding to the weather data of these two locations, Yazd and Phoenix are very similar, then this model can be used for validation. According to the published researches, the best time schedule for night ventilation in Yazd office buildings is 24 pm to 7 am. In other words night ventilation is utilized, when outdoor temperature is lower than the set point temperature. In this regard the entrance of warm air into the interior spaces is prohibited and its negative effect on thermal comfort is avoided. Here we studied the effect of two PCMs with 27 and 29 degrees melting point temperatures and 10, 20 and 30 ventilation air change rates per hour during the working hours (8 am to 18 pm) on annual cooling load of the building. We found that when the set point temperature of the fans was adjusted above 30 degrees centigrade, the cooling load would be increased, therefore the results showed that 30 oC is the most appropriate set point temperature for the fans, which is the highest outdoor temperature for starting night ventilation. In the next step simulation was carried out using various PCMs with 19 to 29 degrees melting point temperatures. It was observed that PCM 27 has the best thermal performance in combination with night ventilation in Yazd office buildings. The phase change materials used in this research are Organic Paraffins. In the next step, the effects of various air change rates were analyzed on the cooling load. We examined the effects of 0 to 30 ventilation air change rates in 5 steps intervals. We found that the increase in ventilation rate will decrease the energy consumed for cooling during the working hours. The reason is the cooling down of the thermal mass in the model under study with regards to increase in night ventilation rate. The cooling load of the model including the energy consumed by fans shows that with the increase of the ventilation rate, the energy consumed by fans also increases, and resultantly more than 15 ach per hour increase of ventilation rate does not have a significant effect on reducing energy consumption which will cause draught inside the office building. The results show that through using appropriate PCM with desired night ventilation rate, we can reduce up to 47 percent of energy consumption for cooling load in the office buildings in dry-hot climate.