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In response to the increasing need for carbon neutrality in the building sector, this study evaluates the performance of photovoltaic (PV), solar thermal (ST), and photovoltaic-thermal (PVT) systems as renewable strategies for residential Zero Energy Buildings (ZEBs). While PV and ST systems are widely applied, their independent installation is often constrained by limited roof and facade areas. PVT systems, which simultaneously generate electricity and heat from a single surface, offer a promising alternative; however, quantitative assessments under diverse climatic and installation conditions remain limited. In this study, EnergyPlus simulations were conducted to compare the load coverage performance of PV, ST, and PVT systems under identical installation areas, six tilt angles (reference 21.3[degrees], 0[degrees], 30[degrees], 45[degrees], 60[degrees], 90[degrees]), and four representative Korean climate regions (Chuncheon, Daejeon, Busan, Jeju). Coverage rate was defined as the reduction in auxiliary heating energy consumption relative to a baseline model without solar systems. Results showed that PVT systems consistently achieved the highest performance, reaching a maximum coverage rate of 94.2% in Jeju at 30[degrees], confirming their suitability as high-density renewable solutions for ZEBs. PV systems exhibited limited coverage capability, particularly in high-latitude regions, while ST systems produced relatively high thermal output but faced structural limitations from surplus summer generation. Tilt angle was found to strongly influence system performance, with 30[degrees] optimal in most regions, but 45[degrees] more effective in Chuncheon due to lower solar altitude and concentrated winter demand. Monthly auxiliary consumption analysis further revealed that winter heating performance was the dominant factor determining coverage. These findings highlight that ZEB design should move beyond simple energy yield maximization and adopt strategic system selection based on climate, seasonal load distribution, and installation constraints, with PVT systems offering significant advantages for enhancing residential energy self-sufficiency.

The global expansion of data centers, driven by rapid digitalization and emerging technologies such as artificial intelligence (AI), connected energy systems, and autonomous operations, has led to a significant rise in energy consumption and carbon emissions. This presents a critical challenge to sustainable development, particularly in managing the high energy intensity of data center cooling systems. While conventional direct expansion (DX) cooling systems often utilize free cooling with water/glycol loops, they suffer from limitations such as high fluid viscosity, reduced thermal conductivity, and increased pumping energy. To address these issues, this study proposes and evaluates a refrigerant-based free cooling strategy integrated with an air-cooled chiller system for data center applications. Unlike glycol-based systems, the refrigerant-based design eliminates the need for antifreeze agents and reduces associated pumping losses, enhancing both energy efficiency and system reliability. A new refrigerant cycle model was developed using Simulink to replicate free cooling operations, and its performance was analyzed based on data center's thermal load profiles obtained from EnergyPlus simulations. The case study involves a data center modeled as a single thermal zone with 3 MW of air-cooled information technology equipment (ITE). The proposed configuration includes an air-cooled chiller with an evaporatively cooled condenser, a fluid cooler, and a fluid-to-fluid heat exchanger, allowing the refrigerant loop to be used directly for heat rejection under favorable ambient conditions. Simulation results show that the refrigerant-based free cooling system achieves up to 35% annual cooling energy savings, depending on the climate condition. The number of available free cooling days is also increased due to the favorable thermal properties of the refrigerant compared to the case without free-cooling operation. Overall, this study demonstrates the potential of refrigerant-cycle-based free cooling as an effective strategy to reduce cooling energy use and improve the sustainability of data center operations. The findings provide valuable insights for designing energy-efficient cooling systems tailored to climate conditions and operational requirements in large-scale digital infrastructure.

Heat recovery technologies of variable refrigerant flow (VRF) systems have good potential to reduce the building energy consumption and maintain good thermal comfort in many commercial building types by allowing individual indoor units to heat or cool as required while the compressor load benefits from the internal heat recovery. This study investigates an integrated heat recovery system design of a VRF system with a domestic hot water (DHW) loop using a hydraulic heat exchanger. With the integrated heat recovery VRF configuration, the condensing heat from vapor compression cycles can be recovered and used for space heating and/or DHW production when space cooling is simultaneously required. This strategy could potentially improve the energy savings of the existing heat recovery VRF technology and minimize waste heat through VRF outdoor units. The hydraulic heat exchanger linked to the VRF refrigerant circuit box in this VRF system is connected to the water tank and is able to increase the heat recovery potential on the domestic hot water side. To evaluate energy savings potential of the proposed VRF system, a case study of a nine-story hotel building with 395 guest rooms is carried out in this study using DOE-2.2 in 6 U.S. climate zones. Based on simulated hourly building loads, a calculation tool is also developed to estimate heating/cooling and DHW energy usages. The results from the study show that the annual energy savings of the proposed VRF system can be around 22% to 24% and around 0.3% to 1.4% in cooling and heating main mode, respectively. In terms of annual DHW energy savings, the proposed VRF system can achieve around 54% to 91% of annual DHW energy savings when compared to the baseline VRF in 6 U.S. climate zones.

Heat recovery technologies of variable refrigerant flow (VRF) systems have good potential to reduce the building energy consumption and maintain good thermal comfort in many commercial building types by allowing individual indoor units to heat or cool as required while the compressor load benefits from the internal heat recovery. This study investigates an integrated heat recovery system design of a VRF system with a domestic hot water (DHW) loop using a hydraulic heat exchanger. With the integrated heat recovery VRF configuration, the condensing heat from vapor compression cycles can be recovered and used for space heating and/or DHW production when space cooling is simultaneously required. This strategy could potentially improve the energy savings of the existing heat recovery VRF technology and minimize waste heat through VRF outdoor units. The hydraulic heat exchanger linked to the VRF refrigerant circuit box in this VRF system is connected to the water tank and is able to increase the heat recovery potential on the domestic hot water side. To evaluate energy savings potential of the proposed VRF system, a case study of a nine-story hotel building with 395 guest rooms is carried out in this study using DOE-2.2 in 6 U.S. climate zones. Based on simulated hourly building loads, a calculation tool is also developed to estimate heating/cooling and DHW energy usages. The results from the study show that the annual energy savings of the proposed VRF system can be around 22% to 24% and around 0.3% to 1.4% in cooling and heating main mode, respectively. In terms of annual DHW energy savings, the proposed VRF system can achieve around 54% to 91% of annual DHW energy savings when compared to the baseline VRF in 6 U.S. climate zones.

Concrete structures in marine environments are prone to deterioration and damage due to chloride ion penetration, freezing and thawing, and chemical erosion. Ultra-high-performance concrete (UHPC) mixed with steel fibers has been proposed as a solution to enhance the durability and mechanical properties of concrete in marine environments. Although several studies have been conducted in this regard, they have yet to focus on addressing errors that may be caused during the construction of offshore piers. Therefore, this study proposes a modular system to control horizontal and vertical errors during construction using a new connecting core type. UHPC with a fiber content of 0.75% was considered the optimum mix proportion because this met the tensile and compressive strength requirements and the chloride attack resistibility requirements of marine structures. The structural performance of a specimen constructed using modular technology was evaluated. The results of the lateral load resistance experiments showed minimal deformation in the girder and pier. Additionally, both the precast and cast-in-place types met the criterion of load resistance. This study contributes to the advancement of construction technology in marine environments by considering both material performance and construction conditions.

This paper presents a developed manipulator and control method for automatic lapping process applied to a large work surface. A lapping machine consists of a newly designed parallelogram-type 5-axis manipulator and a conventional 3-axis gantry machine. The gantry machine is utilized to precision positioning over a large work area, and the manipulator is controlled by a separate controller for lapping operation in the local surface area which is provided by the CNC controller of gantry machine. The lapping process is, as a first step, needed to remove milling tool marks without injuring original shape, and then to correct the shape error. Wheel compliance becomes different in accordance with the special purpose such as tool mark removal and shape correction. Both processes require high stiffness of the manipulator. In this study, newly designed joints are adopted to secure the stiffness of the joints in both normal and lateral directions. Deadweight of manipulator including wheel motor as well as feed-forward torque control are adopted to suppress the vibration caused by abruptly dynamic cutting force. It was confirmed that the lapping operation was successful in wiping the curved contour without the structural vibration, and that the feed-forward torque control produces twice higher machining efficiency as well as 30% higher surface quality in comparison with the existing compliance control method. The experiment investigated the effects of the abrasive wheel compliance and the dynamic cutting force. In results, the smooth surface was achieved within a satisfied quality level of less than Rz 0.5 μm. It is considered that the proposed manipulator has great potential to be applied in unmanned lapping systems for a large work surface.