https://doi.org/10.6110/KJACR.2025.37.12.563

Yong-Il Kwon

To maximize the thermal performance of buffer tanks in heating and cooling systems, it is crucial to maintain effective internal thermal stratification. This study systematically examines how diffuser installation height (h) and discharge velocity (V) influence the formation and maintenance of temperature layers within buffer tanks, utilizing computational fluid dynamics (CFD) and dimensionless analysis based on the Froude number (Fr). The results indicate that the interplay between diffuser height and velocity regulates the balance between inflow momentum and thermal buoyancy. By carefully controlling these parameters within a specific range, it is possible to stabilize stratified structures and minimize exergy loss due to mixing. These findings aim to provide practical design guidelines for improving the performance of buffer tanks in HVAC and thermal energy storage systems. Future studies will focus on developing empirical correlations that integrate dimensionless parameters such as h/H, H/D, Fr, and Ri, while proposing optimal design strategies for various operating conditions.

https://doi.org/10.6110/KJACR.2025.37.12.571

Tae Seong Kim ; Jeong Taek Hong ; Chang Yong Park

This study examines the performance and economic viability of a heat pump system that utilizes discharged groundwater for heating and cooling buildings. For comparison, systems using air and river water as heat sources were also analyzed. Thermal load predictions and energy consumption assessments for an office building were performed using the TRNSYS18 simulation program. The findings revealed that the discharged groundwater system achieved the highest seasonal performance factor (SPF), with values of 2.28 for heating and 3.21 for cooling. An economic evaluation was conducted using the air-source heat pump system as a baseline, focusing on both initial investment and operating costs. The results indicated that the groundwater and river-source systems could reduce annual operating costs by 24.1% and 19.0%, respectively, compared to the air-source system. Despite the higher SPF, the estimated payback periods were approximately 9 years for the groundwater system and 12 years for the river system.

https://doi.org/10.6110/KJACR.2025.37.12.583

Young Chan Kim ; Doo Hyun Kim ; Rin Yun

As environmental concerns and demands for energy efficiency grow, zeotropic refrigerant mixtures made from low-GWP refrigerants like R-290 and R-744 are gaining attention for their thermodynamic benefits. A key characteristic of these mixtures is the temperature glide during phase change, which can either enhance or hinder the performance of heat exchangers, depending on how well it aligns with the heat source temperature profile. This study developed and validated a numerical simulation tool to analyze the heat transfer performance of a plate heat exchanger while considering the effects of temperature glide. Experimental validation indicated errors below 8% for both the condenser and evaporator. Among the tested mixtures, R-290/R-744 at an 80/20 weight percent ratio demonstrated the best thermal performance, whereas higher fractions of R-1234yf in R-744/R-1234yf resulted in excessive glide and thermal mismatching. The findings suggest that careful selection of refrigerant mixtures can enhance effectiveness and lay the groundwork for optimizing eco-friendly refrigeration systems, with potential applications in heat pump design, energy-efficient HVAC systems, and refrigerant leakage diagnostics.

https://doi.org/10.6110/KJACR.2025.37.12.594

Seong Young Jung ; Jae Sung Park ; Dong Su Kim ; Sung Ju Lee ; Song Seop Lee ; Hussain Alsalamah ; Kwang Ho Lee

This study introduces a cooling control strategy focused on thermal comfort, aimed at reducing energy consumption while maintaining or improving indoor comfort in the extremely hot and dry climates of Saudi Arabia. Experiments were conducted in a full-scale climatic chamber using three operating modes: a constant-speed air conditioner set to 24°C (Case 1), an inverter air conditioner also set to 24°C (Case 2), and an intelligent controller (Case 3) that adjusts cooling based on real-time Predicted Mean Vote (PMV) values. In Case 3, an artificial neural network (ANN) model predicted the mean radiant temperature using easily measurable inputs?indoor air temperature, outdoor air temperature, air conditioner setpoint, and time-related variables. This prediction was then used to calculate PMV and dynamically adjust the cooling setpoint. Over a 24-hour test period, energy consumption was recorded at 16.1 kWh for Case 1, 8.7 kWh for Case 2, and 8.2 kWh for Case 3. This represents a 46% reduction in energy use with the inverter system compared to the constant-speed system, along with an additional 6% saving with the PMV-based control over the inverter baseline. In terms of thermal comfort, both Case 1 and Case 2 showed a cool-side bias, with mean PMV values of ?0.9 and ?0.7, and PMV unmet rates of 100% and 56%, respectively. In contrast, Case 3 achieved a mean PMV of 0.1 and a PMV unmet rate of only 6%, indicating that approximately 94% of occupied hours fell within the ASHRAE-55 comfort range. These results highlight that PMV-targeted, machine learning-enabled setpoint modulation can correct cool bias, enhance thermal comfort, and deliver additional energy savings beyond inverter-only operation in hot and dry climates.

https://doi.org/10.6110/KJACR.2025.37.12.607

Jin Ho Kim ; Junghwan Moon

This study investigates the ventilation characteristics of multi-story underground parking facilities. The original ventilation design was inefficient due to significant non-uniformity in airflow distribution, mainly because the air supply inlet was skewed toward the exhaust outlet or predominantly directed upward. The third basement level performed particularly poorly, exhibiting high airflow velocities in the lower section and numerous stagnant zones in the upper section. An alternative design, which evenly distributed the supplied airflow between the upper and lower regions, improved airflow uniformity and reduced air age, enhancing overall ventilation performance. Although average airflow velocities remained similar under the same air supply conditions, adjusting the discharge orientation of the air supply and induced fans, along with reducing the number of airflow pathways, resulted in a more homogeneous airflow distribution and improved exhaust flow. This study also emphasizes that evaluations of multi-story underground parking facilities often focus on a single floor, neglecting the impact of lighting. Incorporating the effects of lamps is crucial, as they can alter local airflow patterns and significantly increase air age in their vicinity.

https://doi.org/10.6110/KJACR.2025.37.12.616

Sangwoo Byeon ; Baekyun Park ; Jinkyun Cho

The rapid growth of artificial intelligence (AI) and high-performance computing (HPC) has led to a sharp increase in electricity demand at large-scale data centers. In Korea, recent extensions of mandatory district heating and cooling (DHC) policies for these facilities have raised questions about the best cooling system options. This study evaluates and compares an electrical chiller system (ALT-1) and a medium-temperature absorption chiller system (ALT-2) for a 40 MW data center in Incheon. The analysis examines capital expenditure (CAPEX), annual operating expenditure (OPEX), system reliability, and compatibility with emerging cooling technologies. The results indicate that ALT-2, with a coefficient of performance (COP) of 0.736, requires significantly larger auxiliary equipment than ALT-1, which has a COP of 8.600. This difference results in a 232% higher initial investment for ALT-2. Additionally, the annual OPEX for ALT-2 is 539% greater due to the high cost of medium-temperature hot water. Its reliance on external thermal sources, the need for full backup capacity, and incompatibility with high-temperature liquid cooling further limit its applicability. Therefore, electrical chillers are the more cost-effective and reliable choice for large-scale data centers. These findings offer practical insights for future system design and policy development in regions with mandatory DHC supply.