Search Results (344)

In the practical operation of air-source heat pump heating systems coupled with phase-change energy storage tanks, wide fluctuations in outdoor temperatures often cause issues such as excessive heating, frequent unit start–stops, and low operational efficiency. Traditional start–stop control strategies struggle to balance heating quality with system energy savings. To enhance the system’s energy efficiency across all operating conditions and improve the stability of indoor temperatures, this study introduces a straightforward and easy-to-implement return water temperature zone control strategy. Using physical reference points, a three-zone control approach for return water temperature was created, which integrates outdoor temperature feedback along with combined indoor temperature adjustments. The proposed strategy’s effectiveness was confirmed through comparative experiments that split the heating season into two parts: one employing traditional control and the other using the zone control method. The results show that, compared to empirical start–stop control, the segmented control strategy increased the system’s average coefficient of performance (COP) from 3.06 to 3.11, representing a 1.63% improvement; reduced indoor temperature deviation from 1.4 °C to 1.2 °C, a 14.2% decrease; and narrowed the amplitude of extreme temperature deviations from 7.9 °C to 3.9 °C, a 50.6% reduction. Total electricity consumption for the entire heating season was approximately 4191 kWh. These findings indicate that the proposed control strategy effectively improves system energy efficiency and indoor temperature stability while meeting heating demands. It significantly suppresses excessive heating during transitional seasons and enhances heating reliability under extreme low-temperature conditions. This study involves low retrofitting costs and balances both energy-saving and comfort objectives, providing a practical, engineering-ready solution for the intelligent control of air-source heat pump heating systems. Full article

This study presents a novel investigation into the coupled effects of ambient temperature and compressor frequency on frosting behavior and thermal performance of inverter-driven air-source heat pumps (ASHPs) under low-temperature, high-humidity conditions. Unlike previous studies that focused on single environmental parameters, this work systematically explores temperature–frequency coupling. Experiments were conducted on a 3-HP DC inverter low-ambient-temperature ASHP unit using a multi-climate simulated enthalpy difference test bench. Single-factor analysis shows that frosting is most severe at 0 °C, where the frost growth rate peaks. Regarding compressor frequency, the coefficient of performance (COP) initially increases and then decreases with frequency. The maximum COP occurs near 45 Hz, representing the optimal energy efficiency balance in this experimental system. Sensitivity analysis demonstrates that relative humidity contributes less than 5% to performance degradation at the critical 10% COP reduction point. Thus, ambient temperature and compressor frequency are the core determinants of defrosting timing. A dual-factor prediction model for the critical defrosting air-to-coil temperature difference (∆T) is developed using temperature (t) and frequency (f) as independent variables. Validation confirms that the model maintains prediction error within 10% under both single-factor and multi-factor coupling conditions. Collectively, this research quantifies the coupled effects of ambient temperature and compressor frequency on frosting performance and provides a novel theoretical framework for precise defrosting control in inverter ASHPs based on performance attenuation. Full article

Traditional cryogenic air separation units are unsuitable for distributed, small-scale liquid oxygen production. Cryocooler-based liquefaction technology offers an alternative solution, featuring a large cooling capacity, high efficiency, a compact structure, and rapid start–stop capability. In this paper, an oxygen liquefaction system based on a high-capacity Stirling cryocooler was developed. Because the heat transfer performance of cryocoolers varies significantly across different temperature ranges, heat exchanger designs must be tailored to specific operating conditions. However, research on cold-end heat exchangers for large-capacity cryocoolers used in liquefaction systems remains limited. In the liquid oxygen temperature range, factors such as liquid film formation and incomplete condensation severely affect heat transfer performance and must be considered. In this paper, numerical simulations were performed to analyze the condensation behavior of oxygen, with particular attention paid to the matching between the heat exchange structure and the cooling capacity. Subsequently, a small-scale experimental system was constructed and tested. The successful operation of the experimental system validated the feasibility of the proposed heat exchanger design. Under the conditions of 300 K and an oxygen inlet gauge pressure of 0.45 MPa, the system achieved a liquefaction capacity of 7.4 L/h, corresponding to a cooling capacity of 787 W. The specific power consumption was 0.89 kW·h/kg, with a coefficient of performance (COP) of 0.116. This performance is competitive among small-scale cryocooler-based oxygen liquefaction systems. This study provides both theoretical and experimental support for further performance optimization and engineering application of such cryocoolers in liquid oxygen production. Full article

Traditional inter-station crude oil heating processes rely heavily on fossil fuels, leading to high energy consumption and environmental pollution. To address this issue, this paper develops a dynamic thermal simulation model for a novel pipeline heating system that couples geothermal and solar energy. The model synergistically utilizes abundant solar energy and abandoned geothermal well resources in the Jilin region, and is applied to analyze the thermal performance of the Xinmiao Station on the Qingtie Fourth Line pipeline. The results show that the system achieves approximate thermal stabilization during long-term operation: the produced water temperature stabilizes at approximately 30.85 °C, and the average coefficient of performance (COP) of the heat pump remains above 4.79, demonstrating good stability. Solar energy contributes about 23.5% of the total annual heat supply (7.0 × 10⁶ kWh) over 1600 effective hours, significantly reducing the annual electricity consumption of the heat pump and water pumps. The integration of solar energy effectively mitigates the decline in the average soil temperature; after 25 years, the soil temperature remains at approximately 54.43 °C. Through optimized configuration, the system reduces its life-cycle cost and levelized cost of heat (annual cost reduced by about 4.35%), showing excellent economic performance. Comprehensive analysis indicates that the coupled system exhibits outstanding energy efficiency and sustainability, providing technical support for the optimized design and engineering application of clean heating systems for crude oil pipelines. This paper contributes four novelties: first application of a coupled geothermal–solar system to a crude oil pipeline (Xinmiao Station, Qingtie Fourth Line); reuse of abandoned deep oil wells as geothermal boreholes to cut drilling costs; a 25-year dynamic simulation quantifying long-term soil temperature evolution and proving sustainability gains over a standalone geothermal system; and multi-scenario economic optimization identifying the optimal collector area under site land constraints. Based on these, a dynamic thermal simulation model is developed and its synergistic operation strategy is investigated, aiming to provide theoretical and technical support for clean-energy-driven crude oil heating. Full article

This study investigates integrating an air-to-water heat pump (HP) into an existing office building served by a district heating (DH) system to improve energy performance and reduce environmental impact. The system was modelled using Polysun software, considering two operating scenarios: a conventional configuration based solely on DH and a hybrid configuration combining DH with a HP. The analysis was performed using hourly simulations over a typical meteorological year, allowing a detailed evaluation of system behaviour under varying climatic conditions. The results indicate that the hybrid system reduces total energy consumption by approximately 24%, while natural gas consumption decreases by about 36%. Although electricity consumption increases due to HP operation, the overall energy performance is significantly improved. The HP operates efficiently within the analysed temperature range, with COP values ranging from 1.8 to 3.0 and a seasonal performance coefficient of approximately 3.6. The system ensures full coverage of the heating demand, with a negligible deficit, confirming appropriate sizing and control strategy. From an environmental perspective, the hybrid configuration results in approximately 29 t CO₂ per year less than the conventional system. These results demonstrate that integrating HPs into existing DH systems can represent a viable solution for similar buildings under comparable operating conditions. Beyond the quantified energy and environmental benefits, the novelty of the study lies in evaluating a hybrid solution under real operating conditions affected by DH instability. The results highlight practical implications for system resilience, operational flexibility, and the applicability of this retrofit strategy to existing buildings connected to conventional DH networks. Full article

Decarbonizing HVAC in hot–arid regions is challenging for natatoriums because year-round cooling must be delivered alongside stringent dehumidification and occasional heating under high ambient temperatures. In this paper, a fully renewable system has been developed and evaluated for an indoor swimming pool located in Abu Dhabi with a 679 m² swimming pool hall designed to accommodate 200 pool users. The hybrid system includes a high-temperature linear Fresnel reflector (LFR) solar field, stratified thermal energy storage (TES), a single-effect LiBr–H₂O absorption chiller for cooling, a water-to-water heat pump as a backup system for the stability of cooling and heating rates, and a photovoltaic (PV) system to offset the ancillary equipment power input of the hybrid system. The system performance was simulated and validated by using hourly data from Abu Dhabi. Optimization of design/operation parameters was carried out by a multi-objective genetic algorithm to achieve the maximum coefficient of performance (COP) and the minimum levelized cost of cooling (LCOE). The initial COP and LCOE were 0.701 and 0.037 $/kWh, respectively. They were optimized to 0.825 and 0.0254 $/kWh, respectively. The annual energy balance revealed a synergistic operation of the solar field, TES, and heat pump. The lifecycle assessment was utilized to compare the proposed hybrid system with the conventional vapor-compression systems in terms of energy, cost, and CO₂ emissions, in which the proposed system proved superior over conventional systems with a positive net present value (NPV) and net zero carbon emissions. Full article

Rapid growth in fossil-fuel consumption has amplified the severity of global climate issues, making the deployment of renewable energy solutions increasingly imperative. Among candidate approaches, adsorption-based technologies are attractive; however, the limited adsorption capacity and kinetic performance of traditional adsorbents constrain composite-cycle efficiency and hinder large-scale implementation. In this work, we develop and evaluate a new class of composite adsorbents prepared by impregnating metal–organic frameworks (MOFs) with hygroscopic chloride salt solutions (LiCl, CaCl₂, and MgCl₂). Owing to their enhanced sorption characteristics, the resulting materials support an integrated adsorption cycle in which one device can simultaneously realize refrigeration, space heating, seawater desalination, and power generation. Under standard operating conditions, experiments demonstrate that MOF–vermiculite composites deliver a cooling coefficient of performance (COP) of 0.71, a heating COP (COP_h) of 1.30, a specific power-generation output of 27.2 kJ/kg, and a desalination yield of 0.71 g/g. Collectively, these metrics outperform the majority of previously published results, indicating that composite adsorbents can substantially improve the efficiency and practicality of renewable energy conversion systems. Full article

The solar collector–evaporator is a pivotal component in a direct-expansion solar-assisted heat pump (DX-SAHP) system, and its structural design directly affects the heat-transfer performance and overall coefficient of performance (COP). To investigate the effects of collector–evaporator geometric parameters, a system-level DX-SAHP simulation model was developed in MATLAB (R2016b). Specifically, a roll-bond plate solar collector–evaporator was represented using a distributed-parameter model. The effects of three key geometric parameters—namely, the channel-number allocation ratio between the two flow passes, channel pitch, and equivalent channel diameter—on the system COP were examined. The results show that each parameter exhibits an optimal value that maximizes COP. Furthermore, a coupled parametric analysis was conducted to account for their interactive effects, revealing that the DX-SAHP system achieves a global optimum COP when the channel-number ratio is 1:1, the channel diameter is 2.11 mm, and the channel pitch is 43 mm. Using the climatic conditions of Shanghai as a case study, the performance improvement achieved with the optimized collector–evaporator design was evaluated. The results demonstrate that, under representative meteorological conditions in spring, summer, autumn, and winter, the optimized collector–evaporator increases the system COP by approximately 4.13%, 4.72%, 4.50%, and 3.04%, respectively. These findings provide practical guidance for the structural design and optimization of collector–evaporators in DX-SAHP systems. Full article

Irreversible loss caused by viscosity-dominated viscous dissipation is an important factor affecting high-temperature heat pump (HTHP) performance. To quantify the effect of viscosity on the coefficient of performance (COP) of HTHP systems, this study developed a contribution analysis model based on data samples from multiple working conditions, working fluids, and device types. Factor analysis and Varimax orthogonal rotation were employed to achieve multi-factor dimensionality reduction and mapping, quantitatively analyze viscosity factors, and compare the weight contribution distributions of other influencing factors with and without viscosity parameters. Results show that, in the global sample, viscosity corresponding to condensation temperature ranks among the top three negatively correlated factors, with a contribution of 7.40%. The sum of the absolute contributions of viscosity corresponding to condensation temperature and evaporation temperature reaches 9.86%, second only to temperature lift (16.10%). In the three local temperature ranges, the contributions of viscosity corresponding to condensation temperature are 6.31%, 6.75%, and 7.11%, respectively. The total contribution of irreversible loss parameters increases from 46.21% to 49.39%, and the increase reaches 12.02% in the high-temperature range. These results provide a theoretical basis for HTHP system design, working fluid selection, and performance improvement under high-temperature operating conditions. Full article

In vehicle-mounted thermoelectric refrigerators, limited installation space and fluctuating ambient conditions make it difficult to achieve both sufficient cooling capacity and low power consumption. However, most previous studies have focused on thermoelectric materials or standalone devices rather than system-level optimization under realistic vehicle constraints. To address this issue, a three-dimensional multiphysics-coupled finite element model combined with a parametric optimization approach was developed for a vehicle-mounted thermoelectric refrigerator used in one of Dongfeng Motor’s new energy vehicle models. Based on this model, the effects of key geometric parameters, including thermoelectric leg height (l), leg width (w), and leg number (p_d), as well as operating conditions, namely input voltage (U) and ambient temperature (T_a), on the overall performance of the refrigerator, including cooling capacity (Q_c), coefficient of performance (COP), and interior center temperature (T), were systematically investigated. The results show that under nominal operating conditions (U = 13.5 V, T_a = 25 °C), increasing p_d from low to moderate values significantly improves cooling capacity, reduces the interior temperature, and decreases power consumption. However, further increases in p_d lead to diminishing improvements in cooling performance because of the heat dissipation limitation on the hot side. By comprehensively evaluating cooling performance and energy consumption, the optimal design was determined to have 322 legs, a leg width of 1.4 mm, and a leg height of 1.8 mm. Under these conditions, the refrigerator achieved a cooling capacity of 13.95 W, a power consumption of 38.4 W, a COP of 0.36, and a compartment center temperature of 10.71 °C. Compared with the conventional 254-leg module (w = 1.4 mm, l = 1.6 mm), the optimized design improved the COP by more than 45.1% and reduced power consumption by 28.8%. In addition, the results indicate that under high ambient temperature conditions, the overall system performance is mainly limited by the hot-side heat rejection capacity. Overall, this study provides an effective structural optimization approach for improving the energy efficiency of compact thermoelectric refrigerators in confined spaces and offers a useful reference for the low-power design of vehicle-mounted cooling devices. Full article

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

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