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1 Prediction of Thermal Comfort

2 Thermal Sensation Scale (Rohles & Nevins, 1974)

3 Fanger s Thermal Comfort Model (1982) Steady state model, applicable for M 3 met and a large group of people

4 Fanger s Thermal Comfort Model (1982) contd. Clothing temperature, t cl is found by iteration using the equation:

5 PMV-PPD model (Fanger, 1984) Predicted Mean Vote (PMV) is given by: PMV = (0.352 e M ) Thermal Load Thermal load = Difference between heat generation & heat loss Predicted Percent Dissatisfied (PPD) is given by:

6 Typical Results from Fanger s Thermal Comfort Model

7 PMV-PPD model requires values of the following input parameters: 1. The 4 environmental parameters (t a, p v, t r and v a ), and 2. The 2 personal parameters (M and I cl ) The valid intervals for the input parameters are (ISO 7730): Parameter Validity Range Air temperature, t a o C Vapour Pressure, p v kpa (RH: 30 70%) Mean radiant temperature, t r Air velocity, v a Activity Level, M Clothing insulation, I cl o C 0 1 m/s met 0 2 clo Fanger clearly stated that all the above input parameters have to be specified accurately after careful measurements of the environmental parameters and correct assessment of activity and clothing levels

8 Comfort charts based on PMV-PPD Model (ASHRAE Std. 55) Operative temperature (CIBSE 2006): For v a, 0.1 m/s: v a 0.2 m/s For different air speeds (ASHRAE )

9 Limitations of PMV-PPD Model PMV PPD Model assumes that the occupant is passive, i.e., does not take any action even if the environment is not comfortable Studies carried out by researchers (including Fanger s) all over the world show that: PMV-PPD model works quite well in air conditioned buildings However, it under-predicted thermal comfort in non-air conditioned and/or mixed mode buildings Fanger attempted to extend his model to non-air conditioned buildings by introducing an expectancy factor However, assigning a suitable value for expectancy factor is highly subjective, hence is not popular

10 Adaptive Thermal Comfort Models Extensive field studies have shown that: Through a suitable combination of adaptive actions, an acceptable degree of comfort is possible over a range of temperatures that is much larger than those predicted by PMV-PPD model The wider temperature ranges may be achieved by proper passive building design with little or no external energy input The adaptive thermal comfort models propose, Adaptive Comfort Equations (ACEs) that relate acceptable indoor temperature to a suitably averaged outdoor temperature Models are simple to use as they do not require details on activity, clothing, vapour pressure etc.

11 Adaptive Comfort Measures Classification of adaptive measures exercised by occupants: 1. Physiological - changes to thermoregulatory set points 2. Behavioral - change clothes, adjust posture, open a window 3. Psychological - reduced expectations, getting used to a stimulus

12 Applicability of adaptive comfort standards (ASHRAE 55(2010) The building should satisfy conditions such as: No mechanical cooling or heating system is in operation Availability of occupant operable openings to the outdoors It is permissible to use mechanical ventilation with unconditioned air Occupants are engaged in near-sedentary physical activities Occupants are free to adapt their clothing within a range at least as wide as clo The prevailing mean outdoor temperature is greater than 10 C and less than 33.5 C

13 ASHRAE 55, t comf = 0.31t o EN 15251, t comf = 0.33t o t comf, o C t o, o C ASHRAE 55 In ASHRAE 55, t o is the prevailing mean outdoor temperature (PMOT), and the acceptable indoor temperatures are within 2.5 K for 90% acceptability and within 3.5 K for 80% acceptability In EN 15251, t o is the running mean outdoor temperature (RMOT), and depending upon building class or category the acceptable indoor temperatures can vary within 2 K for Class I, within 3 K for Class II and within 4 K for Class III buildings

14 Artificial vs Adaptive Thermal Comfort Artificial air conditioning: 1. Indoor environment can be maintained at required conditions irrespective of external surroundings 2. Occupants can be passive 3. Can be energy intensive, efforts to save energy may give rise to indoor air quality issues 4. Provides freedom to the architects Adaptive thermal comfort: 1. Occupants have to take active part in ensuring comfort to themselves 2. No or very little external energy is required 3. Imposes architectural and other constraints