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Thermal and Ventilation Performance of Stratified Air Distribution Systems Coupled with Thermal Beams
Maintaining good air quality and providing thermal comfort in indoor environments is not only critical for occupants’ health, but also important for their productivity at work. Among the various heating, ventilation and air conditioning (HVAC) systems, displacement ventilation (DV) was reported to be able to provide good air quality to indoor spaces while saving energy. However, since DV supplies fresh air to occupied zone directly, its supply-air temperature must not be too low. This feature limits the cooling capability, and hence applicability, of DV. Furthermore, researches showed that a DV system, when used to remove a cooling load larger than 40W/m2, could create a large vertical temperature gradient in the occupied zone, which causes thermal discomfort. On the other hand, passive chilled beams (PCBs) have a high heat-removal capability and are a good candidate for coupling with DV to address the issue of low cooling ability. However, PCB produces a downward jet which could significantly change the local airflow velocity and temperature. It can also recirculate airborne contaminants near the ceiling downwards to the occupied zone, which could be counterproductive to the contaminant stratification produced by DV. Therefore, a systematic study of the coupled air-water system, DV-PCB, is needed, for evaluating its thermal and ventilation performance.
To evaluate the performance of a DV-PCB system, this study first conducted experimental measurements in a full-scale environmental chamber with this coupled system to obtain the distributions of air velocity, temperature and contaminant concentration. A computational fluid dynamics (CFD) model was developed to simulate air distribution in an enclosed environment with the DV-PCB system, which was then validated by the measured data. The validated CFD model was employed to analyze thermal comfort and indoor air quality in the enclosed environment with the DV-PCB coupled system using four indices: vertical temperature gradient, draft rate, normalized contaminant concentration and age of air. PCBs were found to effectively reduce the temperature gradient, especially when the PCBs were used to remove 40% or more of the cooling load. Nevertheless, they also significantly increased the draft rate in the regions right beneath PCBs. Besides, PCBs also to some extent disrupted the contaminant stratification created by DV systems and increased the mean age of air in the room.
Furthermore, this research used the CFD model to establish a database of 70 indoor environment cases that include four common room layouts: office, classrooms, workshops, and restaurants. With the database, the study developed simple mathematical models for predicting the thermal and ventilation performances of DV-PCB based on various design parameters. Using these mathematical models, this investigation then proposed a step-by-step procedure for designing a DV-PCB system to create a thermally comfortable and healthy indoor environment without causing condensation on the chilled beams. Moreover, a user-friendly design interface was developed such that engineers can use it to make design decisions in a convenient way. In addition, this research studied the energy performance of the coupled DV-PCB system by evaluating the year-round energy consumption when it is used in different US climate zones. The evaluation used different air handling processes when outdoor climate was location in different sections of psychrometric chart. Compared with all air system, the coupled DV-PCB system was shown to be most energy-efficient when it was used in dry and hot climate regions.
However, although the coupled DV-PCB system could enhance the capability of traditional DV system in removing high cooling load, and with the developed design guide, the indoor air quality could be achieved, it is not applicable in heating seasons. Hence, this research further investigated the performance of a novel displacement induction ventilation (DIV) system that comprises multiple DIV units, which could expand the applicability of coupled DV-PCB. In cooling seasons, all of the units could be used for providing cool air. And in heating seasons, some units can be used for heating while others used for ventilation by supplying relatively cool air into the room. Experimental and CFD simulation results showed that DIV system was able to accomplish stratified air distribution as well as satisfactory thermal comfort in both cooling and heating seasons. Hence, it effectively remedies the heating limitations of DV and coupled DV-PCB system with one system.
Moreover, this study developed occupant-oriented control schemes for advanced air-water systems to further optimize their thermal and energy performances. In a large indoor space, the proposed control method was able to achieve multi-zone temperature control. In addition, with a bio-signal based control module that was implemented in an air-water system, it was able to accurately condition the indoor space based on occupants’ real-time thermal sensations. These control and operation strategies not only improved occupants’ thermal comfort, but also accomplished high energy efficiency by preventing overcooling or overheating of the indoor space.