Search Results (474)

In order to maximize the use of low-temperature heat sources for refrigeration, a Li-Br absorption chiller combined with single-effect absorption refrigeration cycle and two-stage absorption refrigeration cycle (STAC) was developed. Experimental research on STAC was conducted on a prototype with a refrigeration capacity of 500 KW. A numerical model validated by experimental data was used to study the refrigeration performance of STAC under variable operating conditions. Compared to single-effect units and two-stage units, STAC demonstrates remarkable heat source conservation capability and adaptability to a broad spectrum of heat source temperatures. This advantage renders the STAC unit more adaptable to new energy or waste heat scenarios characterized by unstable heat sources. As the inlet temperature of the hot water increases, the temperature difference between the inlet and outlet of the hot water also increases. When the inlet temperature of the hot water is 70 °C, 90 °C and 120°C, the temperature difference between the inlet and outlet of the hot water is 10 °C, 30°C and 70 °C, respectively. Both increasing the inlet temperature of hot water and decreasing the temperature of cooling water will enhance the cooling capacity and coefficient of performance (COP) of STAC. As the flow rate of chilled water increases, the refrigeration capacity of STAC will also increase, but the COP will first increase and then decreases Full article

To address the issue of performance degradation resulting from continuous thermal accumulation in the soil for conventional ground source heat pump (GSHP) systems in cooling-dominated regions, a hybrid ground source heat pump–domestic hot water system (HGSHP-DHW) is proposed, along with a corresponding mode-switching control strategy. The heat pumps for cooling, heating, and domestic hot water in the HGSHP-DHW share the same ground heat exchanger (GHE) group. To accommodate varying energy demands in different seasons, the configuration of the ground source/side loop is switched according to signals from the control strategy. The average soil temperature rise, the coefficient of performance (COP) of the heat pump units, the system performance factor (SPF), the life cycle climate performance (LCCP), and the net present value (NPV) are selected as comprehensive evaluation indicators for fifteen years of operation. A comparative analysis with traditional systems, including chiller–boiler (CB), cooling tower coupled hybrid ground source heat pump (CT-HGSHP) and GSHP, which are all equipped with an air source heat pump (ASHP) for DHW, is also conducted. By the 15th year, the average soil temperature rise in the HGSHP-DHWs is 4.94 °C, a reduction of 55.5%, effectively alleviating soil thermal accumulation. In terms of energy efficiency, the SPF is 3.79, an increase of 70.8% with 43% reduction in the accumulation of energy consumption (P_ac), achieving high-efficiency and energy-saving operation. For environmental performance, the LCCP is 2,435,587 kgCO₂, a reduction 38.8% in carbon emissions, showing a remarkable emission reduction effect. In respect of economic returns, the NPV is 644,867 CNY, which is positive and indicates favorable investment viability. Full article

This study addresses the challenge of thermal accumulation and low efficiency in conventional ground heat exchangers for building heating and cooling applications. A novel direct-expansion CO₂ borehole heat exchanger (BHE) backfilled with well water is proposed to enhance heat transfer and mitigate soil thermal imbalance. A dynamic thermal resistance-capacity model (TRCM) coupling CO₂ phase change with natural convection in well water is developed and validated against full-scale field experiments (135 m depth), with prediction errors below 5% under cooling conditions (MAPE 2.29%, RMSE 2.49%). Quantitative analysis reveals that natural convection in well water enhances overall heat transfer by 14.9% compared to soil-backfilled systems, despite intensifying thermal short-circuiting. Two practical enhancement strategies for building energy efficiency are proposed: (1) adding insulation to the rising pipe, which increases the heat transfer rate by up to 35.1%; and (2) implementing artificial well-water circulation, which achieves up to 50.5% enhancement, with an equivalent coefficient of performance (COP) reaching 52.5 under intermittent operation. The proposed system and the parametric analysis of these strategies offer effective solutions for improving the energy performance of ground-source heat pumps in buildings, contributing to reduced operational energy consumption and enhanced system reliability. Full article

Internal thermal load fluctuations and variations in occupant density affect the performance of Hybrid Geothermal Heat Pump (HGHP) systems. Traditional control strategies cannot provide the rapid adjustments needed to operate efficiently in real time and can be inefficient, leading to increased energy consumption and reduced thermal comfort. A data-driven fuzzy logic control framework is developed in this paper to dynamically adjust the performance of an HGHP system in real time as a function of occupancy and environmental conditions (e.g., temperature and humidity differences). The controller analyzes input data related to real-time outdoor ambient conditions like temperature, humidity and occupied spaces; a real-time flow sensor attached to the occupants of the building (a count of the number of occupants currently in each occupied space); and the coefficient of performance (COP) of the HGHP system, and uses the analysis to generate a “smart” control decision for the following device types: variable speed drive (VSD), fan number, operating modes, system control and valve positions. The controller also controls the overall system. The model was developed and simulated in MATLAB Simulink^®, with realistic system parameters, and validated and calibrated using operational data from an HGHP system at a university, based on operating conditions. The simulation results indicate that our fuzzy controller achieves higher energy efficiency for thermal comfort than traditional thermostat-based controls, with COP improvements ranging from 7.36% to 11.76% and power consumption reductions between 4.13% and 8.55% across various occupancy scenarios. The improved COP also demonstrates the device’s responsiveness and effectiveness, even under frequent changes in occupancy patterns (dynamic occupancy), making it suitable for use in automated climate control systems in modern buildings. Full article

The motionless wind turbine with opposing paired airfoils offers a compact and noiseless alternative to conventional wind energy systems, but its performance remains well below the Betz limit, limiting urban deployment potential. To address this gap, this study conducts a dual-parameter optimization of angle of attack (0–16°) and inter-foil spacing (0.4c–1.0c) for S1210 airfoils, focusing on maximizing suction while minimizing flow asymmetry/separation a critical trade-off unexplored in the prior literature. This study optimizes the aerodynamic efficiency of an S1210 airfoil pair through an integrated approach that combines numerical with experimental analysis. The numerical results show that a reduced spacing of 0.4c amplifies suction but causes premature flow separation and instability, whereas larger spacings of 1.0c produce more stable flow. The optimal configuration is found at an angle of attack of 12° with a spacing of 1.0c, which attains the highest average suction pressure with minimal flow disturbances. Experimental validation with a prototype confirms computational fluid dynamics (CFDs) predictions: a 12° angle of attack yields the highest duct velocity, corresponding to a peak coefficient of performance (COP) of 0.31. The study also identifies that the key design balance to achieve stronger suction requires closer spacing or higher angles, but this comes at the cost of increased flow instability and separation. Conversely, wider spacing improves stability but reduces peak suction. The system’s improved efficiency stems from enhanced venturi effects and controlled flow asymmetry, making the design suitable for scalable urban deployment. Full article

This study presents the design, thermodynamic modelling, and numerical optimisation of a medium-scale (100 kW) transcritical CO₂ air-source heat pump water heater (ASHP-WH) intended to deliver 90 °C domestic hot water under sub-zero ambient conditions. A detailed component-sizing methodology was established and implemented in AMESim 2404 using REFPROP-based property calculations, with model convergence confirmed by the mass and energy balance closure. Parametric investigations covering the discharge pressure, refrigerant charge, ambient air temperature, and water outlet temperature were conducted through 140 steady-state simulations. The results show that the system achieved a heating capacity of 100–121 kW with a coefficient of performance (COP) of 2.7–3.3 across −15 °C to +10 °C ambient conditions. The optimal discharge pressure (≈11.2 MPa) and charge inventory (10 ± 2 kg) define a broad operating window that ensures COP stability (±2%) and avoids liquid carry-over. The exergetic efficiency remained above 0.75 throughout the tested climate range. Compared with published laboratory prototypes, the proposed 100 kW module demonstrates a superior performance at harsher sub-zero boundaries, highlighting its potential for commercial hot water and industrial applications. The findings provide actionable guidelines for component sizing, charge management, and adaptive pressure control, and establish a pathway from a numerical prototype to scalable field deployment of medium-scale transcritical CO₂ systems. Full article

Aiming to improve cooling and dehumidification performance in air conditioning systems and to meet the trend toward environmentally friendly refrigerants, this study proposes a coupled system that combines a CO₂ transcritical refrigeration cycle (CTRC) with a liquid desiccant dehumidification cycle. The system takes advantage of high-grade waste heat from the exothermic side of the CTRC to drive the regenerating process of the liquid desiccant dehumidification. A cooling evaporator is adopted to cool indoor air, while another evaporator (i.e., Evaporator II) is utilized to cool the concentrated solution, improving dehumidification capacity and enabling independent control of sensible and latent heat loads. Through thermodynamic modeling and the exergy analysis model, a mathematical model of the system is developed to examine how key parameters (such discharge pressure and the CO₂ mass flow rate ratio in Evaporator II (λ)) affect performance and to analyze exergy loss features. Results show that the system’s coefficient of performance (COP) and dehumidification coefficient of performance (COP_deh) initially rise and then fall with increasing CTRC discharge pressure, achieving an optimal pressure of around 10,500 kPa (COP up to 4.32) under a specific working condition, surpassing those of standalone CTRC systems. Properly increasing λ enhances dehumidification capacity and energy efficiency, with a low specific dehumidification energy (SDE) of 0.2033 kWh/kg, indicating high economic efficiency. Most exergy losses occur in the CO₂-solution heat exchanger and dehumidifier (over 60% of total losses). The system’s maximum exergy efficiency reaches 12.4%, leaving room for further improvements. This coupled system offers an efficient, eco-friendly way for air conditioning in high-humidity environments, combining cooling and dehumidification with the potential for energy recovery. Full article

To realize high heat transfer capacity with low energy consumption in machine tool thermal control systems under high-flow-rate conditions, a vortex-inducing–microchannel composite enhanced thermal control plate is proposed. Numerical simulations combined with experimental validation are conducted to investigate the effects of vortex-inducing geometry and microchannel configuration under unified boundary conditions. Heat transfer capacity, pressure drop, coefficient of performance (COP), and performance evaluation criterion (PEC) are employed for comprehensive assessment. The results show that vortex induction enhances fluid mixing and boundary layer renewal, while microchannels effectively suppress pressure loss and energy consumption. Their synergistic coupling enables a balanced optimization between heat transfer enhancement and flow resistance control. Compared with a conventional thermal control plate, the proposed composite structure achieves over 20% improvement in heat transfer capacity and more than 50% increase in COP within the tested operating range. Among the investigated configurations, circular and square vortex-inducing structures combined with microchannels exhibit superior overall performance, with the circular configuration reaching a maximum COP enhancement of 72% at a flow rate of 7 L/min. This study provides practical guidance for structural selection and parameter optimization of composite thermal control plates for machine tools. Full article

This study proposes a CO₂ re-liquefaction system utilizing the cold energy of LNG and liquid hydrogen (LH₂) to efficiently manage boil-off gas in alternative fuel-based CO₂ carriers. Process simulations using Aspen HYSYS V11 under 100% and 70% propulsion loads evaluated the Specific Energy Consumption (SEC), Coefficient of Performance (COP), UA of heat exchangers, and Specific Life Cycle Cost (SLCC). The results demonstrate that under both 100% and 70% propulsion load conditions, the utilization of cold energy decreases the SEC by 24.5% and improves the COP by approximately 34% compared to the reference model without cold energy utilization. Sensitivity analysis on the minimum temperature approach indicates limited impact on performance. The UA of the heat exchangers decreased by up to 83% (LNG) and 87% (LH₂), offering significant downsizing advantages. Economically, SLCC was reduced by up to 14.8% and 15.9% for the LNG and H₂ models, respectively, due to lower Capital Expenditure (CAPEX) and Operating Expenditure (OPEX). Consequently, this study demonstrates that exploiting the cold energy of alternative fuels significantly improves both the thermodynamic performance and economic feasibility of CO₂ re-liquefaction systems, providing foundational data for future optimization. Full article

To address the coupled challenge of heat-transfer enhancement and energy consumption in machine-tool temperature control plates under high-flow-rate conditions, a comprehensive performance evaluation method based on an equivalent thermal resistance network is developed. By introducing heat-transfer power, equivalent total thermal resistance, and a coefficient of performance (COP), the thermal performance and energy cost are quantitatively characterized. Building upon established thermal resistance modeling approaches, the method provides a systematic framework for performance evaluation. The effects of inlet flow rate and heat-source temperature are investigated using CFD under consistent conditions, and experimental validation is conducted. The results show that increasing the flow rate enhances heat transfer but exhibits diminishing returns, while the rapidly increasing pressure drop reduces energy efficiency. Increasing the heat-source temperature mainly improves heat-transfer power by strengthening the temperature difference, with a limited impact on thermal resistance. Good agreement among theoretical, numerical, and experimental results confirms the validity and engineering applicability of the proposed method. Full article

This study investigates the operation of an air source heat pump (ASHP) working with combined radiant floor (RF) and fan coil unit (FCU) heating systems in hot summer and cold winter (HSCW) regions. Intermittent heating demands and ASHP sensitivity to supply water temperature in these regions lead to insufficient steady-state assumptions, while experimental evidence on transient heating behavior, thermal comfort development, and operational limits remains limited. In this study, experiments were conducted to analyze six supply water temperatures (ranging from 35 °C to 45 °C) with respect to the system’s dynamic thermal response, vertical air temperature difference, floor surface temperature, power consumption, and coefficient of performance (COP). The results show that start-up heating is dominated by FCU convection, causing pronounced vertical temperature stratification, while radiant heat becomes dominant as the system approaches steady operation. A good vertical air temperature difference with respect to breathing zones and ankle-level temperature differences below 2 °C was achieved after sufficient operating time. Increasing the supply water temperature accelerated the heating response, where the time required for the average indoor temperature to reach 18 °C decreased from 5.5 h at 35 °C to 2.2 h at 45 °C. However, this improvement was accompanied by reduced energy efficiency, with the mean ASHP unit COP declining from 2.5 to 2.3. Excessively high supply temperatures further induced premature indoor overheating and the frequent start–stop cycling of the heat pump, thereby limiting thermal benefits and increasing power demand. These findings provide experimentally grounded insight into the operation and performance limits of ASHP RF–FCU heating systems. Full article

This paper presents an experimental investigation of a low-temperature ejector-based air-conditioning system designed to utilize waste heat from Combined Heat and Power (CHP) plants. The system operates with isobutane (R600a) as the working fluid and is driven by low-grade heat sources in the temperature range of 80–120 °C. A prototype experimental rig was developed to evaluate the influence of key operating parameters, including motive steam pressure and evaporator temperature, on the system’s Coefficient of Performance (COP) and entrainment ratio. The results demonstrate that the system can maintain stable operation even at ultra-low heat source temperatures, achieving a maximum COP of 0.35 under optimal conditions. The findings confirm the feasibility of using R600a in ejector-based systems for sustainable cooling applications. Furthermore, the study highlights the potential for integrating such systems into existing district heating networks to enhance overall energy efficiency. Overall, the presented results provide valuable experimental data supporting the development of sustainable, thermally driven cooling technologies that reduce reliance on grid electricity and high-GWP refrigerants. Full article

Dual-source heat pump systems combining photovoltaic-thermal (PVT) and air-source technologies have attracted considerable research interest due to their energy complementarity. Based on the climatic characteristics of the Dalian region, this study conducted field measurements and data analysis on a developed dual-source heat pump system incorporating three adaptive operational modes: (1) PVT mode, (2) PVT/air dual-source mode, and (3) photovoltaic (PV)/air-source mode. Compared to Mode (3), Mode (1) achieves a 5.76% higher heating capacity and an 11.56% greater electrical efficiency. Meanwhile, Mode (2) demonstrates a 12.23% increase in heating capacity, and a 9.14% improvement in electrical efficiency relative to Mode (3). A data-driven methodology is provided to quantify the system’s evaporation temperature, the thermal efficiency of PVT mode, and the coefficient of performance (COP) of the PVT heat pump. The economic assessment demonstrates that the proposed dual-source heat pump system achieves a heating cost as low as RMB 0.1125/kWh and a payback period of 6.4 years, indicating favorable economic benefits. This study provides fundamental data and computational methods for the optimized operation of the PVT/air dual-source heat pump. Full article

In ground-source heat-pump (GSHP) systems equipped with a single U-tube borehole heat exchanger (BHE), the heat-carrier fluid in the return leg may release heat to the surrounding ground in the shallow part of the borehole. From a fluid energy balance perspective, this is an exothermic process; however, it is detrimental during heating operation: It lowers the effective source temperature available to the heat pump and therefore degrades the overall coefficient of performance (COP). This study proposes a measurement-driven procedure to determine the exothermic transition depth $z_{\uparrow }^{*}$ from temperature profiles recorded at multiple depths along the ascending (return) pipe. The borehole is discretized into axial segments and, assuming a constant mass flow rate, the linear heat-exchange rate is estimated from the segment-wise enthalpy change. Time integration yields the segment-wise net energy exchange $Q_{\uparrow ,i}$, which is then classified as exothermic or endothermic using an uncertainty-based threshold derived from the standard uncertainty of the temperature sensors. The exothermic transition depth $z_{\uparrow }^{*}$ is defined as the first statistically stable sign change in the integrated segment energy (from exothermic to endothermic) and is obtained by linear interpolation between adjacent segment centres. By summing the exothermic energy exchange and the corresponding average loss power, an equivalent change in source-side outlet temperature $∆T_{out}$ is estimated and interpreted in terms of COP impact using a Carnot-scaled surrogate model. For two representative operating conditions, $z_{\uparrow }^{*}$ was found at $31.17 \mathrm{m}$ and $24.01 \mathrm{m}$, respectively, while the average exothermic loss power remained approximately 0.48 kW. The estimated $∆T_{out}$ ranged from $0.52$ to $0.75 \mathrm{K}$, corresponding to a diagnostic COP improvement if this parasitic exothermic exchange could be mitigated. The present results should therefore be interpreted as a case study-based demonstration of the method on one instrumented borehole rather than as a universal quantitative prediction for other sites or borehole fields. Full article

This study investigates the applicability of alternative low-global warming potential (GWP) refrigerant blends in water-source heat pump systems. Binary and ternary refrigerant mixtures were generated using REFPROP 10 to identify suitable candidates. Among 379 novel blends, 18 mixtures with glide temperatures below 10 °C, high critical temperatures, and GWP values lower than 750 were selected for analysis. Thermodynamic analyses were conducted for the selected refrigerants at target water outlet temperatures ranging from 35 to 75 °C, with a heat source temperature of 15 °C and an evaporation temperature of 5 °C. In addition, compressor discharge temperature, volumetric heating capacity, and coefficient of performance (COP) were evaluated. Among the refrigerants, MX1 was recommended for condenser temperatures of 40–80 °C in large-scale heat pump and district heating applications. For refrigerants with GWP values below 150, MX7 exhibited the highest COP and second-law efficiency (η_II) and is therefore suitable for small-capacity systems. In the GWP range of 150–750, MX16 demonstrated the highest COP and η_II values over the entire temperature range. Overall, MX7 achieved the highest COP and η_II among all refrigerants considered, while MX4 emerged as the most favorable mixture in terms of low GWP (below 150) and thermophysical performance. Full article