User:7alberto7/Thermal Comfort Models

Thermal Comfort Models
When discussing thermal comfort, there are two main different models that can be used: the static model (PMV/PPD) and the adaptive model.

Static Comfort Model
The static model is based on the physiological approach, according to which the comfort zone can be the same for all occupants, disregarding location and adaptation to the thermal environment. It basically states that the indoor temperature should not change as the seasons do. Rather, there should be one set temperature year-round. This is taking a more passive stand that humans do not have to adapt to different temperatures since it will always be constant.

This model is based on the PMV/PPD model, that uses the Predicted Mean Vote formula by P. O. Fanger. The PMV is the average comfort vote, using a seven-point thermal sensation scale from cold (-3) to hot (+3), predicted by a theoretical index for a large group of subjects when exposed to particular environmental conditions. Zero is the ideal value, representing thermal neutrality. This model was originally developed by collecting data from a large number of surveys on people subjected to different conditions within a climate chamber. These data were then used to derive a mathematical model of the relationship between all the environmental and physiological factors involved. The comfort zone is defined by the combinations of the six key factors for thermal comfort for which the PMV is within the recommended limits (-0.5<PMV<+0.5). The PMV model is calculated with the air temperature and mean radiant temperature in question along with the applicable metabolic rate, clothing insulation, air speed, and humidity. If the resulting PMV value generated by the model is within the recommended range, the conditions are within the comfort zone.

The Predicted Percentage of Dissatisfied (PPD) is related to the PMV as is defined as an index that establishes a quantitative prediction of the thermally dissatisfied people assuming that who votes -2, -3, +2 or +3 on the thermal sensation scale is dissatisfied. The model is also based on the simplification that PPD is symmetric around a neutral PMV.

ASHRAE Standard 55-2010 sets an acceptable range of conditions that must be complied in order to apply this method and draw the comfort zone: occupants’ metabolic rates between 1.0 and 1.3 met, clothing between 0.5 and 1.0 clo, air speeds under 0.2 m/s.

Elevated Air Speed Method
According to the standard, a different model is to be used to allow a higher air speed, in order to increase the maximum operative temperature for acceptability under certain conditions. The Elevated Air Speed Method is based on the fact that different combinations of air movement and temperatures may result in equal levels of heat loss from the skin. The model applies to a lightly clothed person who is engaged in near sedentary activity. As a matter of fact, any benefits gained by increasing air speed depend mainly on clothing and metabolic activity. Elevated air speed is more effective at increasing heat loss if the occupant is engaged in higher activities, so in this case the method would be conservative, and with lower levels of clothing. Clothing insulation higher than 0.7 clo would lead to a wrong estimation of the effects of increased air movement.

Adaptive Comfort Model
The adaptive model states that there is an optimal temperature for a given indoor environment depending on the outdoor air temperature. It takes into account that humans can adapt and tolerate different temperatures during different times of the year. The optimal temperature for a given time is determined by looking at the mean outdoor temperatures of each month of the year. Also, field studies are performed in these areas to see what the majority of people would prefer as their set-point temperature indoors at different times of the year.

This model applies especially to occupant-controlled, natural conditioned spaces, where the outdoor climate can actually affect the indoor conditions and so the comfort zone. In fact, studies by de Dear and Brager showed that occupants in naturally ventilated buildings were tolerant of a wider range of temperatures. This is due to both behavioral and physiological adjustments, since there are different types of adaptive processes. ASHRAE Standard 55-2010 states that differences in recent thermal experiences, changes in clothing, availability of control options and shifts in occupant expectations can change people thermal responses. More advanced research on thermal comfort considers the heat balance of the human body and calculates sensation and comfort for local body parts.