Investigation of Thermal Effect of Plants in Urban Spaces by PMV Model (Case Study of Ozgol neighborhood of Tehran)

Throughout the past century, cities have lost their vital connection with the nature due to the fundamental changes in their functions and physical forms. The development of urbanization and attachment of villages to metropolises in recent years have led to destruction of vegetation in these regions. The importance of identifying the effects of cities on their climates can be indicated only through accurate research and quantitative assessment. The overall increase in air temperature as a result of urbanization is 1-3 degrees Celsius. In stable climatic conditions, however, the air temperature difference between urban and suburban areas can be as high as 10 degrees Celsius. Increasing the amount of vegetation, planting trees, and developing ecological networks are some of the effective passive methods available for improvement of microclimates. Similar methods have been used in the ancient cities of Iran, where the temperature inside the cities has always been lower than that of the surrounding areas due to the utilization of a special type of urban planning which has been capable of providing appropriate climatic conditions at micro scale by controlling climatic elements. Ozgol is one of the northern neighborhoods of Tehran, where green spaces have diminished as a result of the development process and space quality at the centralized focal points has decreased. This study attempted to identify the relationship between green areas and temperature decrease on the hottest day of the year to help improve the quality of the public spaces and increase thermal comfort at the focal points of the neighborhood. This research used a hybrid methodology based on logical reasoning and software-assisted assessment to study a site in the Ozgol neighborhood under two conditions: current status and optimal design. The data on the current situation of the neighborhood were obtained by field measurements, and those on the latter situation were obtained via computer simulation and addition of plants to the area. Then, the PMV model was used for assessment of the comfort conditions and specification of the exact effect of plants on microclimate elements. The two conditions were compared, and the impact of urban design with plants on environment quality was evaluated. The evaluation criteria included temperature and relative humidity. The measurements were made at a height of 1.65 meters to prioritize pedestrian comfort. The findings suggested that allocation of a low percentage of city space to greenery can greatly improve microclimatic conditions. The computations indicated that every square meter of green space added to the area under study would expand the comfort zone by 3.71 times. The results also demonstrated that retention of the amount of relative humidity can reduce air temperature by 1.5-2 degrees Celsius. The findings of this study highlighted the significance of preserving the current green spaces over the neighborhood and the necessity of avoiding any change in the land use of the remaining gardens (about 6,000 m2 in the 4-hectare area of the site). The computations also showed that addition of the existing green spaces over the area by 2.69% can expand the area within the range of thermal comfort to twice as wide as the pre-intervention area. One the other hand, the issue of global warming, the 1-degree rise in the world’s temperature during the past century, highlights the value and significance of this 1.5-degree temperature decrease. Thus, it is possible to make significant changes through extension of the results of this study to a larger scale to involve the entire city.