https://doi.org/10.6110/KJACR.2024.36.9.429

Beomjun Kim ; Dongchan Lee

This study experimentally compared and evaluated the heat transfer coefficient and frictional pressure drop characteristics of R-134a and its low-GWP alternative refrigerants (R-1234yf, R-1234ze(E)) in a plate heat exchanger. The experiments were conducted at various vapor qualities, different mass fluxes of 32, 45, and 58 kg m-2 s-1, heat transfer rates of 0.51, 0.65, and 0.7 kW, and saturation temperatures of 5, 10, and 15℃. Vapor quality and mass flux had significant effects on the heat transfer coefficient and pressure drop, whereas the heat transfer rate and saturation temperature had little impact. However, saturation temperature had a noticeable effect on pressure drop. In terms of heat transfer performance in a plate heat exchanger, R-1234yf showed the best characteristics among the tested refrigerants.

https://doi.org/10.6110/KJACR.2024.36.9.442

Jong Hyeok Ryu ; Je Hyeon Lee ; Seok Kwon Jeong ; Ho Won Byun ; IHl wan Choi ; Dong Kwon Kim

This study dealt with the auto-tuning of a PID controller designed to precisely control the indoor temperature to a setpoint during heating operations in an electric-heat pump (EHP)-based VAV air conditioning system. The proposed method tunes PID gains online through an on/off operation near the setpoint and automatically switches to PID control once tuning is complete. The performance of the PID gain auto-tuning and feedback control with the tuned controller was evaluated under various conditions using a calorimeter. The experiments demonstrated that the PID controller gains were properly tuned under different thermal load conditions and provided a detailed analysis of the tuned controller’s control performance. The auto-tuned controller was shown to precisely control the indoor temperature to the setpoint, even under varying thermal load conditions that differed from the ones used for tuning. Notably, the tuned PID controller maintained the indoor temperature within a 0.2℃ error of the set point (22℃) despite disturbances such as the introduction of low-temperature outdoor air and defrost operations, which are inevitable during heating operations. Finally, the stability and reproducibility of the auto-tuned PID controller were confirmed through the dominant pole analysis of the closed-loop system in the complex plane.

https://doi.org/10.6110/KJACR.2024.36.9.454

San Jin ; Hong-Ik Yoon ; Sung Lok Do

A dry bulb temperature-based HVAC control method has been traditionally used; however, the method is limited to considering the variables (e.g., humidity, air flow, mean radiant temperature, and others) that affect indoor thermal comfort. This feature may result in an uncomfortable indoor thermal environment and increase energy consumption. Thus, a thermal comfort control method based on effective draft temperature (EDT) was developed to improve these shortcomings. This study aimed to evaluate the impact of the EDT-based thermal comfort control method on indoor thermal comfort and energy consumption by developing a whole-building energy simulation model to implement the thermal comfort control method. The simulated results of indoor thermal comfort and energy consumption were analyzed by comparing the EDT-based control method to a dry bulb temperature-based one. The comparison showed that the indoor thermal environment operated with the EDT-based control method maintained a better comfort level (increased from 35.0% to 97.2% for cooling and from 87.6% to 95.5% for heating). In addition, the energy consumption decreased by 10.7% for cooling and 21.2% for heating compared to the dry bulb temperature-based control method. Therefore, operating an HVAC system using the EDT-based control method is expected to improve the indoor thermal comfort level and save energy consumption.

https://doi.org/10.6110/KJACR.2024.36.9.466

Byeong Guk Kim ; Hyun Jung Kim

Various methods exist for joining fire pipes. This study investigated the pressure loss caused by grooves in the pipe using the grooved joint bonding method. The groove size was very small, and the equivalent length presented by each design workbook was different, making it difficult to design. Particularly, fire extinguishing facilities in systems with many grooves compared to other facilities, such as tunnel fire extinguishing facilities, are expected to vary greatly. As the uneven part is fine, the actual measurement experiment is difficult, so it was analyzed using numerical analysis. A sudden change in pressure caused by the expansion and reduction of the groove and pressure loss caused by a recirculation vortex in the expansion part occurred. The grooved joint pressure loss equivalent length at 100A was 0.225 m, 125A was 0.333 m, 150A was 0.305 m, 200A was 0.285 m, 250A was 0.276 m, and 300A was 0.330 m, for an average equivalent length of 0.292 m.

https://doi.org/10.6110/KJACR.2024.36.9.474

Jinhwan Oh ; Sangmu Bae ; Hobyung Chae ; Yujin Nam

Thermal energy accounts for more than 40% of the total energy consumption in buildings. Thus, production methods for thermal energy must be secured to achieve zero-energy buildings. The Building Integrated Photovoltaic?Thermal (BIPVT) system is acknowledged as a promising technology for achieving zero-energy buildings by generating both electrical and thermal energy. In this study, the energy performance of a BIPVT module was analyzed by applying it to an actual building. During the experimental period, the maximum daily electricity and thermal generation were calculated to be 5.16 kWh and 2.57 kWh, respectively