In order to enhance the understanding of occupants’ thermal comfort, methods have been developed to evaluate thermal comfort levels of occupants. The Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfaction (PPD) are the most common thermal comfort models used; these were developed by Fanger (1970) and stated in ISO7730 (2005). This standard uses 22 °C as the neutral temperature (Tcomf) in winter and 24°C in summer
(Taleghani, Tenpierik, & Dobbelsteen, 2012). PMV uses a seven point thermal sensation scale where +3 is hot and -3 is cold. Therefore, when the PMV is 0 it is in a neutral state, where thermal comfort is achieved for the majority of occupants (Abdullah, 2007; R. Li, 2007). The PMV is calculated using six basic variables, which are humidity, mean radiant temperature, air velocity, air temperature, clothing, and activity. The results agree with a number of tests involving a large population exposed to a wide range of given environments.
Moreover, PPD is another commonly used term, it is dependent on the PMV value and relates to the six main factors. PPD predicts the dissatisfaction of occupants that can be expected under certain conditions, or at each PMV. A total of 90% and 80% of people thermal satisfaction are calculated using the PPD model. The relationship between the PMV and PPD is illustrated in Figure 2. 2, which shows that, when the PMV is 0, the minimum value of PPD is no less than 5%. This means that, even if the environment is classified as thermally comfortable, there is never 100% satisfaction for all occupants and there are always some individuals who are dissatisfied with the comfort level that most other people seem to be fine with. This is because comfort evaluation differs from person to person. It is believed that the range of comfort is -0.5 to +0.5 in the PMV graph for 90% thermal satisfaction (Figure 2. 2) (Kim, Min, & Kim, 2013).
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Figure 2. 2: Predicted Percentage of Dissatisfied (PPD) in relation to Predicted Mean Vote (PMV) (Fanger, 1970)
Kim et al. (2013) and Hussain and Oosthuizen (2013) state that the PMV indices are only applicable if used in HVAC or air conditioned buildings and are not adequate for naturally ventilated (NV) buildings. Furthermore, De Dear and Brager (2002) present examples that compare observed and predicted indoor comfort temperatures in an HVAC building as well as a naturally ventilated building, and confirm that the PMV index is more applicable to buildings with HVAC (Figure 2. 3 and Figure 2. 4).
Figure 2. 3: Observed and predicted indoor comfort temperature for HVAC buildings (De Dear & Brager, 2002)
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Figure 2. 4: Observed and predicted indoor comfort temperatures for naturally ventilated buildings (De Dear & Brager, 2002)
A comparison of Figure 2. 3 and Figure 2. 4 suggest that, in HVAC buildings, the PMV is successful in predicting the comfort temperature of occupants whereas in naturally ventilated buildings, it does not always predict the comfort of occupants correctly. Fanger and Toftum (2002) also agree that in warm climates, the occupants of a naturally ventilated building may observe the warmth as being less severe than the PMV prediction. Furthermore, De Dear and Brager (2002, p. 552) state that the occupants of an HVAC building, ”become more finely adapted to the narrow, constant conditions typically provided by mechanical conditioning, while occupants of NV buildings prefer a wider range of conditions that more closely reflect outdoor climate patterns”.
By comparing Figure 2. 3 and Figure 2. 4, it becomes obvious that there should be another comfort index, which includes naturally ventilated buildings. Therefore in the new, revised ASHRAE Standard 55 (2013), another thermal comfort model was provided, named the ‘Adaptive Comfort Standard’ or ACS (Figure 2. 5).
The Adaptive Comfort Standard is developed from a global database where the comfortable neutral temperature in winter is 22°C, whilst in summer it is achieved via a formula (Taleghani et al., 2012). The average comfort range formula presented in the ASHRAE project is dependent on the outdoor dry bulb temperature (De Dear and Brager, 2002. This
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formula is acceptable when the average outdoor temperature is in the range of 10°C to 33°C in warm parts of the world (De Dear & Brager, 2002):
Tcomf =0.31Ta,out + 17.8 Tcomf is the monthly average thermal comfort temperature
Ta,out is the average outside monthly temperature
A total of 90% and 80% satisfaction amongst occupants are assumed at Tcomf ±2.5°C and
±3.5°C respectively, respectively; this is illustrated in Figure 2. 5. The line in the middle is the neutral operative temperature, or the average comfort range. The 80% and 90% acceptable limit line on either side of the dotted middle line of the neutral temperature are the ±2.5°C and 3.5°C respectively.
Figure 2. 5: Adaptive Comfort Standard graph (ASHRAE Standard 55, 2004)
According to previous formula, and as presented in Figure 2. 5, when the outdoor temperature is 10°C the neutral temperature given by the formula is 20.9. Moreover, according to ASHRAE, ±3.5°C as the maximum and minimum acceptable range from the neutral temperature are also in the comfort range for 80% of occupants. Thus, 20.9 minus 3.5 equals 17.4, which is the minimum comfortable temperature. However, when the outdoor temperature is 33°C, the neutral temperature given by the formula is 28.03, which, when added to 3.5, is 31.53°C; this gives the maximum comfort temperature for 80% of the people in the building. So for 80% comfort, the range is almost 17.5°C to 31.5 °C, depending on the outside temperature.
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Nevertheless, Fanger and Toftum (2002) believe that the adaptive comfort standard model has limitations as it is only applicable to mean monthly temperatures from 10°C to 33°C, and for large spaces.
“[It] does not include [a variety of] human clothing or [a variety of] activity... [However,] the adaptive model predicts the thermal sensation quite well for non-air- conditioned buildings…located in warm parts of the world.” (Fanger & Toftum, 2002, p. 533).
Since this research will be investigating passive ways of providing thermal comfort with the help of natural ventilation in large, open plan office buildings within a semi-arid climate, it is thus important to consider the ACS model in the analysis.