There is a critical situation we frequently encounter in our thermal comfort analyses:
Even though the room is conditioned in accordance with design conditions by the HVAC system, an increase over time is observed in the perceived temperature of occupants sitting near the window.
Especially in time-dependent (transient) analyses, as the sun moves throughout the day, energy enters the indoor space through radiation via the windows.
This effect is clearly observed in our studies:
▬ In the first video, it can be clearly seen that the indoor air temperature decreases over time due to the effect of the HVAC system.
▬ However, in the second video during the same time period, the perceived temperature (operative temperature) near the windows is observed to increase gradually as direct sunlight strikes these areas.
The main reason for this is that shortwave solar radiation passes through the glass surfaces and is directly absorbed by the human body and interior surfaces. Even if the air temperature is under control:
▬ Mean Radiant Temperature (MRT) increases
▬ PMV/PPD values shift in a negative direction
▬ Users experience the perception of “the air conditioning is on, but it still feels hot”
Therefore, in comfort evaluations near windows, considering only air temperature and air velocity is not sufficient.
Solar gains, solar position, glazing properties (g-value / SHGC), shading elements, and occupant position must be included in the analyses on a time-dependent basis.
