Optimization of room air temperature in stratum-ventilated rooms for both thermal comfort and energy saving

Elevated room air temperature is normally accompanied by elevated room air velocity to provide thermal comfort and save energy. One problem is that an excessively high room air temperature would deteriorate the energy performance of the air conditioning system due to the increased energy consumption of the ventilation fans. Another problem is that existing thermal comfort evaluation models in the field of building energy performance may fail because most of the building simulation tools/building management systems cannot provide accurate information on the elevated room air velocity. This study proposes a room air temperature optimization method to achieve intended thermal condition and to minimize energy consumption of the air conditioning system with stratum ventilation simultaneously. Firstly, the PMV model for thermal condition evaluation is modified by representing the room air velocity in the original PMV model given in ASHRAE 55-2013 using the room air temperature and supply airflow rate. Secondly, with the modified PMV, one supply airflow rate is quantified for one room air temperature to achieve the intended thermal condition (i.e., the intended PMV value); and the energy consumptions of different room air temperatures are evaluated using building energy simulations. Objective measurements and subjective surveys in a typical classroom in Hong Kong validate the modified PMV with a mean discrepancy of 0.14 scale from the thermal sensation vote. TRNSYS simulations demonstrate the effectiveness of the proposed method that the energy consumption of the air conditioning system is reduced by 7.8% while satisfying the intended thermal comfort.