Human body exergy analysis and the assessment of thermal comfort conditions

Exergy analysis is applied to assess the quality of the energy conversion processes that take place in the human body, aiming at developing indicators of thermal comfort based on the concepts of destroyed exergy rate, exergy transfer rate to the environment and exergy efficiency. In literature only destroyed exergy has been used to evaluate thermal sensation. To perform the exergy balance it is necessary to calculate the exergy variation of the body over time which is a composition of metabolic exergy and the exergy variation due to transient environmental conditions. The exergy transfer to the environment is calculated as the sum of the terms associated with radiation, convection, evaporation and respiration. The thermal behavior of the human body is simulated by a model composed of 15 cylinders, naked and dressed for winter seasons, as a function of the air temperature, mean radiant temperature and relative humidity. The energy equation is solved to obtain transitory response of the body due to a variation in environmental conditions and the energy transfer to the environment. For relative humidities between 40% and 60%, results indicate that the destroyed exergy is minimal for thermal comfort conditions. Nevertheless, for low relative humidities and high temperatures the destroyed exergy is also minimal, indicating the necessity of another physical quantity to evaluate thermal comfort conditions. At this point the exergy transfer to environment is high, showing that the body may not be at thermal comfort condition. This article proposes is to use two terms of the exergy analysis to evaluate the thermal comfort condition: destroyed exergy and exergy transfer to environment.