| dc.description.abstract | Heating, Ventilation and Air-Conditioning (HVAC) systems consume large amounts of energy to maintain acceptable thermal environments for occupants; therefore, it becomes important to develop energy efficient systems to support this primary HVAC system goal. The Fixed Bed Regenerator (FBR) is a type of energy recovery system that offers preconditioning of fresh supply air without compromising on Indoor Air Quality (IAQ). This device consists of a cylindrical core made of ceramic for thermal energy storage and laced with numerous hollow channels for air flow so the device can act as a regenerative heat exchanger. The FBR also contains a reversible fan that facilitates air movement between the outdoors and indoors by alternating channel airflow in two directions. The ceramic core absorbs energy from the hot outside air flowing through channels during the supply cycle, which spans 50 seconds for this particular unit, thus cooling fresh outdoor air before it discharges to the indoor space. Next, the core transfers energy to the indoor space air flowing back through the channels during the exhaust cycle, which also spans 50 seconds. Thus, a complete cycle, which consists of a combination of the supply cycle and the exhaust cycle, lasts 100 seconds. The objective of this project is to evaluate the thermal behavior, the core charging and discharging phenomenon, and the effectiveness of the FBR for a range of indoor-to-outdoor temperature differences.
Before taking data for analysis, it was necessary to develop a methodology that ensures the onset of pseudo steady state after an initial FBR startup by monitoring how long it takes the core temperatures to stabilize; it was determined that pseudo steady state was reached after about 16 minutes for all cases. As a first step in the FBR analysis, the temperature change of air flowing through the core provided an indication of the FBR effectiveness. For example, at an outdoor-to-indoor temperature difference of ΔTmax =25℉, the inlet-to-outlet air temperature difference during the supply cycle decreased from 22.9℉ to 11.8℉ from the cycle beginning to the end. For the exhaust cycle, this inlet-to-outlet temperature difference decreased from 15.4℉ at the cycle beginning to 8.6℉ at the end. These decreasing temperature differences from the start to the end of both cycles is an indicator that the core has a decreasing ability to exchange thermal energy with the air or, in terms of this FBR study, a decreasing effectiveness as time passes during a cycle. For example, in the ΔTmax=25℉ case, the effectiveness, which is calculated from temperature data, decreased from 0.90 at the start to 0.47 at the end of the supply cycle, and similarly from 0.80 to 0.39 for the exhaust cycle. The effect that ΔTmax has on mean and peak effectiveness was also evaluated. An analysis of the complete cycle, which is a combined supply and exhaust cycle, showed that the mean effectiveness increased slightly from 0.50 at ΔTmax=20℉ to 0.53 at ΔTmax=30℉. The peak effectiveness showed a similar trend of a slight increase from 0.85 at ΔTmax=20℉ to 0.94 at ΔTmax=30℉. Thus, it was determined that, among the three ΔTmax samples analyzed, the optimal operating conditions for the Fixed Bed Regenerator were at ΔTmax=30℉. | |
