This study aimed to evaluate the performance of a Photovoltaic-Thermal (PVT)-integrated Air Source Heat Pump (ASHP) system under cooling conditions during the transitional season. An AI-based optimal flow control algorithm was developed and applied to improve the cooling performance. The experimental results showed that, compared to the conventional On/Off control, the optimal control system effectively reduced the energy consumption of the circulation pump by 32.5%. Although the Heat Pump Coefficient of Performance (COP) decreased slightly due to reduced circulation water flow, the overall system COP increased by 1.66%, indicating an improvement in energy efficiency. The findings demonstrate that real-time environmental data-driven control strategies can effectively optimize energy consumption and improve system performance, paving the way for further application in building systems.

To mitigate significant energy consumption and greenhouse gas emissions from building heating and cooling, technologies leveraging subsurface geothermal energy are gaining prominence. Open-loop geothermal systems, in particular, offer high thermal efficiency by utilizing groundwater as a heat exchange medium and facilitate large-scale seasonal thermal energy storage (STES) through injection and recovery. This study classifies various open-loop geothermal system types, including Aquifer Thermal Energy Storage (ATES), Standing Column Well (SCW), Groundwater Heat Pump (GWHP), and mine-water utilization. It then critically reviews major international case studies from leading countries such as the Netherlands, Germany, and the United States. The analysis reveals a strong correlation between favorable hydrogeological characteristics (e.g., aquifer conditions, hydraulic conductivity, natural groundwater flow) and the technical viability of open-loop systems (e.g., heat recovery efficiency, groundwater level management, water quality impacts). While open-loop geothermal system adoption in South Korea has been limited compared to closed-loop ground heat exchangers, interest is rapidly growing, especially for renewable water source energy projects in groundwater-rich regions. This study proposes tailored application strategies for South Korea: implementing ATES pilot projects in areas with abundant groundwater; utilizing mine water for data center cooling in abandoned mining regions; and deploying SCW systems in greenhouse-dense zones that require significant groundwater use. These proposals aim to advance the energy efficiency and environmental benefits of open-loop geothermal systems in South Korea. Furthermore, they seek to provide foundational data to inform policy improvements, foster interdisciplinary collaboration, and drive technological development, thereby positioning open-loop geothermal energy as a competitive and vital component of the nation's sustainable energy future.

Small community public health clinics account for a meaningful and persistent share of building-sector energy use, yet retrofit guidance tailored to these facilities remains limited. This study quantified energy and operating-cost impacts of Green Remodeling (GR) packages for a 178.4 m² single-story clinic in rural Icheon, Korea. We developed a DesignBuilder (v7.3) model and simulated annual and monthly end-uses in EnergyPlus (v9.4) under a baseline and five GR cases (Case-A-Case-E) that combined envelope and airtightness upgrades, window and door replacements, HVAC and lighting efficiency measures, and energy recovery ventilator (ERV) integration. The baseline required 36.1 MWh/year (site EUI 215.6 kWh/m²·year; source EUI 592.9 kWh/m²·year) and was dominated by off-peak electricity for space heating and domestic hot water (DHW). Case-A (insulation reinforcement for walls and roof) reduced electricity use by about 10%. Case-B, adding window and door upgrades and airtightness, lowered total electricity to 17.14 MWh/year evidencing cumulative envelope and airtightness effects. Upgrading cooling COP and expanding LED lighting (Case-C) achieved a 53.2% reduction versus the baseline. Case-E (ERV addition) yielded the lowest site EUI, whereas Case-D (kerosene boiler) produced the largest source-energy reduction, reflecting fuel conversion factors. Operating cost analysis indicated that Case-E delivered the greatest savings in operation cost. These findings support envelope-first GR with ERV for small-scale public health clinics; planned post-retrofit measurement and verification (M&V) will further validate savings and refine economic guidance.

This study numerically investigates the effects of diffuser hole diameter and baffle installation on thermal stratification and charging performance in a stratified storage tank. Without baffles, the diffuser hole diameter influenced stratification through the interaction of buoyancy and momentum. A Froude number (Fr) of 1-2 resulted in relatively uniform temperature distribution and moderate thermal quality, whereas smaller or larger diameters caused increased mixing or more pronounced upper-layer heat diffusion, reducing performance. The addition of baffles structured the flow and contributed to more stable stratification while reducing sensitivity to diffuser hole diameter. In the lower portion of the tank, baffles promoted lateral spreading of inflow and limited vertical mixing, bringing temperatures closer to the 7℃ target. The final charging ratio ranged from 88.5% to 89.2%, showing a 3.3-5.6% increase compared with the no-baffle case. These results suggest that diffuser and baffle design can influence stratification and charging behavior in stratified thermal storage tanks.

This study numerically analyzed the effects of the upper and lower plate spacing and baffle geometry of a flat-plate diffuser in a square thermal storage tank on thermal stratification and charging characteristics. The upper and lower plate spacing of a flat-plate diffuser affects heat exchange characteristics and mixing behavior depending on the Froude number, a key factor determining the relative ratio of inertial and buoyancy forces in the inflow. Furthermore, the baffle geometry directly influences the stability of stratification by inducing inflow dispersion. The analysis results showed that when buoyancy was dominant, the contact area between the cold water and the hot fluid immediately after discharge increased, enhancing mixing. When inertial force was dominant, the jet flow tended to form upward, increasing mixing at the top. Conversely, when Fr=1, where inertial and buoyancy forces are balanced, the inflow maintained a stable downward flow along the bottom, resulting in the fastest charging performance. A comparison of baffle configurations revealed that a middle-plate type structure with an auxiliary plate added to the center uniformly distributed the flow and minimized mixing at the boundary between the cold and hot water layers, maintaining the most stable stratification performance.