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Thermal Comfort Parameters

“Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment.”\(^1\)

International standards have been established with the purpose of “specifying the combinations of indoor thermal environmental factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space.”\(^1\)

Those leading standards include:

While these standards are primarily designed to define thermal comfort conditions for spaces that are mostly consumed in sedentary positions (offices, theaters, lecture halls, etc.), the application is not limited to only spaces as such. However, it is worth noting that the empirical foundation of those standards does not include any data for spaces used for the purpose of sleeping, medical care/nursing, or heightened levels of physical activity. They also don’t take into account different requirements for the thermal comfort of children.

According to the standards listed above, occupant thermal comfort can be predicted based on air temperature, thermal radiation, humidity, air speed, as well as personal factors such as physical activity and the degree of clothing insulation.

Numerical simulation helps predict those conditions already in the early stage of conceptualization. SimScale provides thermal comfort parameter outputs in the form of Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) fields as developed by P.O. Fanger, following the static model for thermal comfort.

Thermal Comfort Parameter Study Setup

To create a thermal comfort parameter study with SimScale, first create a Convective Heat Transfer analysis. Thermal Comfort Parameter fields can then be enabled as part of Field calculations under Result controls.

menu for creating thermal comfort parameters fields within SimScale
Figure 1: Add additional output fields for PMV and PPD thermal comfort parameters as part of the Result control settings.

Those field calculations need to be defined before starting the simulation run and can’t be added retroactively for already finished runs.

In the settings panel, required additional input parameters for the field calculation need to be specified. While temperature, thermal radiation, and air speed are already indirectly defined by the boundary conditions of the simulation, additional input is required for Clothing coefficient (clo), Metabolic rate (met), and relative air humidity (%).

thermal comfort parameter settings panel
Figure 2: Settings panel for thermal comfort parameter field calculations. Specify input parameters for clothing coefficient, metabolic rate, and relative air humidity here.

Input Parameters

  • Clothing Coefficient (clo)
    • Definition: A unit used to express the thermal insulation provided by garments and clothing ensembles.\(^1\)
    • 1 clo = 0.155 \(\frac{m^2 °C}{W}\) (0.88 \(\frac{ft^2 °F}{Btu/h}\))
    • Ranges complying with standards:
      • ASHRAE-55: [0, 1.5] clo
      • ISO 7730: [0, 2] clo
    • Recommendation: Use 0.5 clo for summer clothing and 1 clo for winter clothing levels.
  • Metabolic rate (met)
    • Definition: The rate of transformation of chemical energy into heat and mechanical work by metabolic activities within an organism.\(^1\)
    • 1 met = 58.2 \(\frac{W}{m^2}\) (18.4 \(\frac{Btu/h}{ft^2})
    • Ranges complying with standards:
      • ASHRAE-55: [1, 2] met
      • ISO 7730: [0.8, 4] met
    • Recommendation: Use 1 met for passively sitting occupants (e.g. theater audience), 1.5 met for actively sitting occupants (e.g. office work)
  • Relative Humidity (%)
    • Definition: The ratio of the partial pressure (or density) of the water vapor in the air to the saturation pressure (or density) of water vapor at the same temperature and the same total pressure. \(^1\)
    • Adimensional (%)
    • Ranges complying with standards:
      • ASHRAE-55: 
        • Accepted water vapour pressure: [0 – 1910] \(Pa\), where: water_vapour_pressure = relative_humidity * 10 * exp( 16.6536 – 4030.183 / (16.8(C) + 235))
      • ISO 7730:
        • Accepted water vapour pressure: [0 – 2700] \(Pa\), where: water_vapour_pressure = relative_humidity * 10 * exp( 16.6536 – 4030.183 / (air_temperature(C) + 235))

Computed Quantities

  • Air speed
    • Ranges complying with standards:
      • ASHRAE-55: [0, 0.2] \(m/s\)
      • ISO 7730: [0, 1] \(m/s\)
    • The current implementation for computing thermal comfort parameters does not cut the actual working range for air speed according to the standard-compliant limits.
  • Air temperature
    • Ranges complying with standards:
      • ASHRAE-55: N/A
      • ISO 7730: [283.15, 303.15]
  • Mean Radiant Temperature (MRT)
    • Ranges complying with standards:
      • ASHRAE-55: N/A
      • ISO 7730: [283.15, 313.15]
    • The current implementation approximates MRT as a single uniform scalar value throughout the domain, based on a size-weighted average across all domain surface temperatures. The approximation does not take into account view factors and assumes an emissivity of 1.

Output Parameters

Thermal comfort parameters are computed based on publicly available code as detailed in ISO 7730 and ASHRAE-55 standard papers. No correction to the thermal comfort model is made for elevated air speeds above 0.2 \(m/s\).

  • Predicted Mean Vote (PMV)
    • Definition: Adimensional metric defined based on the empirical fit to the human sensation of thermal comfort.
    • Valid range: -3 (cold) to +3 (hot)
    • Comfort ranges complying with standards:
      • ASHRAE-55 recommended limit: [-0.5, 0.5]
      • ISO 7730:
        • Hard limit: [-2, +2]
        • New buildings: ]-0.5, +0.5[
        • Existing buildings: ]-0.7, +0.7[
  • Predicted Percentage of Dissatisfied (PPD)
    • Definition: Relative metric defined to predict the percentage of any population that will be dissatisfied with the environment.
    • Valid range: 5% – 100%
    • Comfort ranges complying with standards:
      • ASHRAE-55 recommended range: [0%, 20%]

Additional Notes

  • While SimScale does not prevent input parameters to exceed the standard-compliant ranges, the correctness of the computed output variables can only be guaranteed for values that comply with the limits as defined in the standards.
  • When taking radiation effects into account as part of the simulation setup, the mean radiant temperature estimation used in this model might result in higher overall temperatures than observed in reality. MRT is computed as a contributing parameter to PMV/PPD equally for simulations with and without radiation enabled. When radiation is enabled, this might lead to a small additional contribution from radiation originating from the face temperatures.


Last updated: January 2nd, 2021

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