Thermal comfort during temperature cycles induced by direct load control strategies of peak electricity demand management

Direct load control (DLC) is a utility-sponsored demand response program which allows a utility to cycle specific appliances on and off during peak demand periods. Direct load control of air conditioners induces temperature cycles that might potentially compromise occupants' thermal comfort. In two separate experiments, 56 subjects' thermal comfort was closely examined during 6 DLC conditions and 2 control conditions simulated in a climate chamber, representing typical DLC-induced thermal environments in university lecture theatres. Results show that half of the DLC conditions were clearly accepted by subjects. Multilevel linear modelling of thermal sensation demonstrates that operative temperature, vapour pressure and the rate of temperature change are the three most important predictors during DLC events. Multilevel logistic regression indicates that in DLC conditions with lower adapting temperatures, thermal acceptability is significantly predicted by air speed and its interaction with operative temperature whereas in DLC conditions with higher adapting temperatures, by air speed, operative temperature and the rate of temperature change. Subjects' thermal comfort zone during DLC events is wider than predicted by Fanger's PMV/PPD model in that the former is more tolerant of cooler temperatures. Results from this study suggest that ASHRAE 55-2013 is overly conservative in defining the limits for temperature cycles, ramps and drifts.