Thermal Comfort Evaluation of a Heat Pump System using Induced-air Supply Unit

Traditional heat pump systems supply conditioned air to space at certain temperatures such as, in summer, about 16°C. When the supply-air temperature drops too low, most occupants tend to feel uncomfortable. On the other hand, a certain amount of dehumidification has to be carried out and sometimes, the velocity of supply-air has to be high which in turn creates a draught. This paper introduces a new air supply method to reduce fan power consumption as well as to improve thermal comfort of occupants. Using this method, colder fresh air at 13°C is generated by air handling units and pumped to the ceiling supplies. As supply air temperature is lower, less air flow rate is required to meet the latent load. The reduced air flow rate decreases the fan power consumption. The supply air units mounted on the ceiling are specially designed so that space air flows upwards to inside due to negative pressure created by the supply unit and mixed with the 13°C cold air. The mixed air consists of roughly 60% return air and 40% fresh air at 13°C. Due to the mixing process, the supply air temperature is around 19°C and supply air velocity is around 0.7 m/s. Both the supply temperature and velocity are more thermally-favored compared to those from traditional units. A commercially-available computational fluid dynamics (CFD) tool was used to evaluate the operative temperature as well as air velocity distribution inside an office-setting equipped with induced-air supply unit for both heating and cooling cases. Since the air velocity may change due to various latent loads, Proper Orthogonal Decomposition (POD) technique was applied to study the thermal comfort conditions when air velocity changes between 0.1 m/s to 1 m/s. The POD method uses a sophisticated interpolation method based on a set of CFD snapshots which are generated once beforehand. The method can reduce the computation time from hours for CFD to seconds when predicting temperature and velocity fields at different supply air velocities. The model outputs are compared with field measured data for model validation. The simulation results show that the induced-air supply unit creates a good thermal comfort conditions for the occupants in terms of predicted mean vote (PMV). The average PMV of a 45-person office shows a value of - 0.2 in the summer and - 0.6 in winter.