Thermal Comfort
Thermal comfort refers to the level of satisfaction that a human has with the environmental conditions in his or her surroundings. Achieving and maintaining thermal comfort is a key goal of HVAC (Heating, Ventilation, and Air-Conditioning) system design and operation.
The thermal environment in an enclosure is affected by internal (for example, human presence) and external (for example, solar radiation) heat sources. The heat exchange between the human body and the environment occurs mainly due to radiation, convection, and evaporation. The heat that is generated by the human metabolism dissipates into the environment, thus maintaining a thermal equilibrium or, in other words, thermal neutrality. The heat transfer is proportional to the temperature difference between the skin temperature and the temperature of the surroundings. When it is hot, the body loses more heat; when it is cold, the body does not release enough heat—hence both circumstances can lead to thermal discomfort. One objective of thermal comfort simulations is to determine the time of discomfort and to ensure that the HVAC systems consume as little energy as possible.
There are several factors that influence thermal comfort: metabolic heat rate or physical activity level, air temperature, mean radiant temperature, air velocity, humidity, and clothing level.
- Equivalent Homogeneous Temperature Model
- The Equivalent Homogeneous Temperature is an environmental thermal comfort metric that is used to assess the local comfort of 17 body segments of a thermal manikin [418]. The model provides a quick and robust estimation of the EHT, depending on air temperature, manikin surface temperature, air velocity, irradiation, level of activity, and clothing. The thermal comfort level is assessed by plotting the predicted EHT in a plot that relates the prediction to the ISO 14505 standard [403].
- Fiala Thermoregulation Model
- The Fiala Thermoregulation model is composed of a thermophysiological model that accounts for metabolic heat generation, evaporation, respiration, and blood perfusion as well as global thermal comfort indices. Using a thermal network, the thermoregulation model solves for the temperature field inside and on the surface of a thermal manikin. The manikin is subdivided into 58 sectors, thus providing a high level of detail. Thermal comfort is assessed by reporting the dynamic thermal sensation (DTS) and the predicted percentage dissatisfied (PPD), which are global thermal comfort indices that apply to the entire body.
In a thermal comfort simulation, you can combine both models, so that they can complement each other.