Evaluation of Thermal Comfort Indices in Outdoor Space

The goal of this research was to clarify the relationship between outdoor thermal comfort indices and perceived user satisfaction. The paper presents findings from a research survey in a selected Tehran urban park، Iran. Theoretical framework: One of the most affected environmental qualities is human thermal comfort in urban outdoor space. Thermal comfort in outdoor settings is a topic that، until recently، has received little research attention. The factors that have an effect on thermal comfort are objective and subjective parameters. There are four environmental variables: air temperature، mean radiant temperature، air humidity and air speed، and additionally، two personal variables influence thermal comfort: clothing insulation and the level of activity. Thermal comfort indices (empirical or rational) try to combine variables that affect thermal comfort into a single expression. Thermal comfort indices were calculated to assess the relationship between meteorological، environmental and personal variables، and also study how this relationship affects human health and activities. The main question in this study was: “Is the current Physiological Equivalent Temperature ranges valid for Tehran residents”? Three commonly used indices in a field of thermal comfort are Physiological Equivalent Temperature (PET)، Predicted Mean Vote (PMV) and Standard Effective Temperatures (SET*). Fanger (1972) developed PMV for indoor climate with the aim of offering an index for rating thermal comfort at various states of activity and clothing insulation. Gagge (1986) developed ET* and suggested the new Standard Effective Temperature which is used regularly both as an indoor and outdoor thermal comfort index. Hoppe (1999) developed PET as an index which takes into account all basic thermoregulatory processes and is based on a thermo-physiological heat balance model. Mean Radiant Temperature is the most important factor influencing PET in summer. This study included outdoor thermal comfort surveys and simultaneous measurements of outdoor environmental variables such as air temperature، globe temperature، relative humidity، surface temperature، wind velocity and sky view factor. The Mean Radiant Temperature، Physiological Equivalent Temperature، Predicted Mean Vote and Standard Effective Temperatures were calculated based on the measured parameters. The study was conducted in five park areas (Melat، Saei، Laleh، Shahr and Besat) in Tehran، Iran. Field surveys were performed from October 11 to 28، 2010 and have taken place between 10 a. m. and 5 p. m.، on weekdays and weekends. The range of Physiologically Equivalent Temperature and a model for calculating Physiological Equivalent Temperature based on metrological and environmental data in Tehran were calculated. AMOS 17. 0 analysis software was used to establish Structural Equation Modeling (SEM)، and maximum likelihood estimation perform a confirmatory factor analysis. Conclusion and Suggestions: The results confirmed that Physiological Equivalent Temperature is a more reliable index for predicting thermal comfort in comparison with Predicted Mean Vote and Standard Effective Temperatures indices in outdoor space. The range of Physiological Equivalent Temperature was higher than the standard ranges in European countries. Thermal comfort was defined as Physiological Equivalent Temperature values between 24. 8°C and 28. 4°C for a person wearing typical outdoor clothing in summer. Calculating Physiological Equivalent Temperature in urban open space is a complicated task that should be done by specialists، since it requires meteorological data from field survey and complex computer modeling. One of the results of this research was determining a simple model that can calculate outdoor thermal comfort، using meteorological data from a city station and urban design elements such as sky view factor، albedo and density of green space. The below formula was designed for open space in Tehran based on the meteorological and urban design criteria: PETTehran = 21. 7 + 0. 4*Tamet - 0. 2*RHmet - 2. 1*Va - 6. 1*a + 1. 8*LC + 2. 6*SVFWhere، “Tamet” is urban air temperature (°C)، “RHmet” is urban relative humidity (%)، “Va” is local wind speed (m/s)، “a” is albedo، “LC” is the density of green space (%) and “SVF” is sky view factor (0-1).