Zhang’s thermal comfort model predicts that the local comfort of feet, hands, and face predominate in determining a person’s overall comfort in warm and cool conditions. We took advantage of this by designing a task-ambient conditioning (TAC) system that heats only the feet and hands, and cools only the hands and face, to provide comfort in a wide range of ambient environments. Per workstation, the TAC system uses less than 42W for cooling and 60W for heating. By reducing the amount of control normally needed in the overall building, it could be possible to save much larger amounts of energy in the building HVAC system.
We tested the TAC system on 18 subjects in our environmental chamber, at temperatures representing a wide range of practical winter and summer conditions (18-30ºC, or 65-86 ºF). A total of 90 tests were done. We measured subjects’ skin and core temperatures, obtained their subjective responses about thermal comfort, perceived air quality, and air movement preference. The subjects performed three different types of tasks to evaluate their productivity at white-collar-types of work during the testing.
The TAC system was able to maintain positive comfort levels across the entire temperature range tested. TAC did not significantly affect the task performance of the occupants compared to a neutral ambient condition. Whenever air motion was provided, perceived air quality was significantly improved, even if the air movement was re-circulated room air. There was no dry-eye discomfort with the head ventilation device as designed, even at 1 m/s in the breathing zone. The acceptable thermal sensation levels were from –2.2 to 2. In our tests, subjects found thermal environments acceptable even if they were judged slightly uncomfortable (-0.5).
Simulated annual energy savings with the TAC system in Fresno, Oakland, and Minneapolis were each about 40% with intensive use of TAC (allowing 18-30ºC ambient interior temperature), and 30% with a moderate use (in 20-28ºC ambient temperature).