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Highlights • Optimal volume ratio of two-stage compressor using zeotropic refrigerant is investigated. • Optimal volume ratio firstly decrease and then increase with the increase of the R32 component. • Optimal volume ratio of 26 potential refrigerants are investigated and compared. • A modified arithmetic mean temperature method fitting for zeotropic refrigerant is proposed. Abstract Refrigerant mixtures are commonly regarded as a potential substitute in heat pumps. Two-stage compression is widely applied for improving the performance of heat pumps. The suction volume ratio is a critical design parameter for two-stage heat pumps. In this work, the optimal volume ratio of R32/R1234ze(E) mixture and 26 refrigerants named by ASHRAE Standard 34-2013 are estimated. The obtained result indicates that, for R32/R1234ze(E) mixtures, the optimal volume ratio first decreases and then increases as the R32 component increases, and the optimal volume ratio of flash tank system is lower than the intermediate heat exchanger system owing to component shifting in the flash tank. For estimated 26 refrigerants, the optimal volume ratio of the majority of zeotropic refrigerants is smaller than pure refrigerants and azeotropic refrigerants. In addition, a modified arithmetic mean temperature method is proposed to predict the optimal intermediate refrigerant states. [ABSTRACT FROM AUTHOR]

Abstract Rotary compressors encounter severe performance degradation under low-temperature ambient conditions, and the vapor refrigerant-injection technology is one of the methods to solve this problem. Traditional vapor injection technologies provide a limited improvement in performance for rotary compressors. A novel end-plate injection structure was put forward in previous research. In this paper, a rotary compressor prototype with this novel end-plate injection structure was introduced. The performance of the vapor-injected rotary compressor was tested experimentally and compared with the regular one without vapor injection. The results indicated that, compared to a single-stage rotary compressor, a rotary compressor with the novel end-plate injection structure can enhance the heating capacity and COP by 16.2–31.6% and 5.1–12.0%, respectively. [ABSTRACT FROM AUTHOR]

• An efficient CO 2 two-stage compression system is comprehensively simulated. • The main factors affecting the optimal discharge pressure are analyzed. • The correlation formula of optimal discharge pressure is proposed. • The correlation formula of optimal intermediate pressure is proposed. The CO 2 two-stage compression refrigeration system is a feasible alternative to conventional refrigeration systems for logistics cold storage cooling that provides high efficiency and environmental protection. In a previous study, we proposed a two-stage compression refrigeration cycle with an external intercooler and performed a thermodynamic analysis. Based on that, a more comprehensive simulation analysis is performed herein using the validated models. The performance, optimal discharge pressure, and intermediate pressure are investigated using a high-pressure compressor with both fixed and variable speeds. The results show that the cooling-water temperature has a significant positive linear correlation with the optimal discharge pressure, whereas the evaporator-inlet air temperature has little effect at a fixed high- pressure compressor speed. When the high-pressure speed is adjusted to achieve the optimal intermediate pressure and discharge pressure simultaneously, the coefficient of performance increases by 4.5% compared with that of the fixed-speed system, while the cooling capacity decreases by 8.4%. The correlation of optimal discharge pressure and optimal intermediate pressure is given. [ABSTRACT FROM AUTHOR]

R32/R1234ze(E) mixtures are potentially low-GWP alternative refrigerants for air conditioning and heat pumps while the rare pure refrigerants can totally meet the requirements of new international protocols on environmental conservation, thermodynamic performance, and safety. The system performance under different concentrations is important for selection of working concentration for the new R32/R1234ze(E) refrigeration or heat pump. In this paper, the thermodynamic model of an ASHP with R32/R1234ze(E) mixtures is built and used to investigate the influence of the refrigerant composition on the performance of the system. The results show that when the mass fraction of R1234ze(E) changes from 0% to 100%, the heating capacity of the ASHP decreases by 67.2%, while the COP continuously increases by 70.3%, which means the changing tendency of system COP is quite different from previous research under fixed evaporating and condensing temperature. Adjusting the refrigerant concentration will be a good system modulation method for ASHPs with R32/R1234ze(E) to meet both the heating capacity and energy efficiency requirements. Furthermore, temperature matching degree is an important factor that affects the energy efficiency of ASHPs with non-azeotropes, which can guide the circuitry optimization of evaporator and condenser in ASHPs with non-azeotropes. [ABSTRACT FROM AUTHOR]

Gas injection has been a crucial technology to avoid the serious degradation of air source heat pumps in low ambient temperature. A novel injection structure on the blade for rotary compressors has been put forward in previous research to overcome the drawback of traditional injection structures. Based on a verified numerical model, the thermodynamic performance of an air source heat pump with the new gas-injected rotary compressor is investigated. The results indicate that, compared to the air source heat pump with the regular single-stage rotary compressor, the proposed injection structure can enhance heating capacity and COP of the air source heat pump by 23.1–28.2% and 4.5–8.1%, respectively. [ABSTRACT FROM AUTHOR]

Heat supply systems based on absorption heat pump were assessed to have great potentials on energy savings and emissions reduction. A prototype of NH 3 -H 2 O water source absorption heat pump (WSAHP) designed for low temperature heating was experimentally investigated under different working conditions. The effects of driving source, hot water and source fluid temperature on the heating performance were studied. As driving source increases from 110 °C to 140 °C with 15 °C evaporator inlet and 45 °C hot water, COP increases from 1.429 to 1.552 and then decreases to 1.495, while the heating capacity increases from 32.23 kW to 88.35 kW. As hot water increases from 30 °C to 45 °C with 130 °C generator inlet and 15 °C evaporator inlet, COP decreases from 1.653 to 1.449, while the heating capacity drops from 94.55 kW to 60.37 kW. As the source fluid increases from −10 °C to 30 °C with 130 °C generator inlet and 45 °C hot water, COP increases from 1.203 to 1.609, while the heating capacity increases from 39.51 kW to 79.72 kW. Comparisons with former work indicate that the developed prototype can operate under evaporator inlet temperatures as low as −18 °C, which is significant to improve heating applicability in colder conditions. [ABSTRACT FROM AUTHOR]

The cooling performance of exhaust air heat pumps is limited by the high condensing temperatures owing to insufficient exhaust air volume. In this study, a high-efficiency solution for an exhaust air heat pump was developed and numerically studied. In this novel system, the exhaust air is stepped energy recovered by an indirect evaporative cooler and a heat pump with an evaporative condenser. It is considered that the combination of these two heat transfer components can provide a system with a significantly high cooling efficiency. To explore the application potential of this system further, a detailed mathematical model was established. Meanwhile, a prototype based on this design was developed, and its performance in four operating modes was tested for model validation. When the test results are compared with those of a conventional system of the same size, they show that the cooling capacity and energy efficiency ratio of the novel system increased by 53% and 135%, respectively. Subsequently, to clarify the principles underlying the design of this system, parameter sensitivity analysis and system matching optimization were conducted numerically. • IEC and ECHP is combined into a novel ventilation treatment method. • The improvements of the novel system in cooling capacity and EER are 53% and 135%. • Key design parameters of the novel IEC–ECHP was investigated. • The novel system is especially recommended for hot and humid areas. [ABSTRACT FROM AUTHOR]

• A high fidelity scroll compressor model with numerically efficient equations is developed for performance prediction. • The governing equations are generic for both single-phase compression and two-phase compression. • A detailed scroll wrap temperature distribution calculation method is proposed. • Validation shows predicted cooling capacity error within 3.5% and predicted power error within 2.8%. • Simulation speed is 7.9–23.7 s per case and convergence rate is 100% for various conditions. A general scroll compressor model with high accuracy, fast simulation speed and good numerical robustness is proposed in this study. The governing ordinary differential equations derived from mass and energy conservation use pressure and specific enthalpy as state variables, which can be applied to both single-phase compression and two-phase compression. Both the governing equations for inner compression and the nonlinear equations for internal leakages are explicit in pressure and specific enthalpy to avoid severe numerical stiff problems. In addition, the detailed scroll wrap temperature distribution is calculated based on the proposed second order differential equation considering heat conduction and periodic heat convection. Validation against experimental data shows that cooling capacity errors are less than 3.5% and power consumption errors are less than 2.8% in non-injection and liquid injection conditions. Evaluation of model speed and robustness shows that the model has a convergence rate of 100% and an average calculation speed of 12.4 s per case in various conditions. [ABSTRACT FROM AUTHOR]

Heat pump heating is one of the most promising solutions for transitioning residential heating in China's cold regions, as well as hot summer and cold winter regions from centralized, continuous heating to decentralized, intermittent heating, promoting low-carbon transformation. However, the fundamental issue of the actual capacity requirements for heat pumps in intermittent heating has not yet been addressed. In view of this, this study developed a numerical model that considers the dynamic coupling characteristics between the heat pump and the building environment. Using the model, the worst startup conditions for heat pumps in intermittent heating, actual heat pump capacity demands, and the differences in capacity compared to continuous heating mode for residential buildings in 230 Chinese cities are analyzed. The findings revealed an exponential relationship between intermittent heating capacity demands and time constraints, as well as a linear relationship with heating temperature. Specifically, the impact of the set temperature on capacity demands in cold regions and hot summer and cold winter regions is 22.8 W/(m2·°C) and 32 W/(m2·°C), respectively. These findings provide insights into the planning and design of future intermittent heating renovation projects in China. [ABSTRACT FROM AUTHOR]

• Developed energy consumption prediction using transient thermal model. • Validated the accuracy of the model through field test. • Investigated the impact of two control strategies on energy consumption. • Cooling during valley electricity price period can save energy and reduce costs. The fast development of global cold chains has led to a rapid increase in cold storage facilities capacity, in which consumes substantial amounts of energy. However, limited research has been conducted on the energy consumed by cold storage facilities, and the time and economic costs of obtaining long-term measurements of annual energy consumption of cold storage facilities are high. Therefore, in this study, short-term field tests and simulations were combined to establish a transient thermal network model for cold storage, using a specific cold storage case study for validation. The model considers the structure of the hall, which strongly impacts the cooling load. The refrigeration system performance was analyzed on the measured data. Literature and field research have shown temperature range control to be the baseline control strategy used in cold storage refrigeration systems, which ensures that the cold storage temperature remains within the set temperature range at all times. In addition, some researchers have proposed a control strategy based on time-of-use tariff that provides cooling almost throughout the valley period. These two control strategies were applied to frozen and chilled rooms, separately; the results indicated that for both frozen and chilled rooms, the latter control strategy consumes slightly less energy than the former strategy. However, the electricity bill for the latter control strategy are 0.35 and 0.5 times lower than those for the former strategy for frozen rooms and chilled rooms, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reviewed the development and research status of logistics cold storage. • Natural refrigerants such as R717 and CO 2 have great potential in the future. • More field tests are needed to obtain the actual energy consumption of cold storage. • The huge heat capacity of cold storage can provide a flexible demand-side response. The rapid development of the cold chain addresses food waste, ensures the quality of expensive products, and supports the pre-cooked dish industry development. However, carbon emissions from logistics cold storage are significant. Reducing these emissions is critical for the sustainable development of the cold chain industry in the context of carbon neutrality. This review examines the development, research, and policy status of logistics cold storage. It first outlines the current state of cold storage in China and globally. On this basis, it summarizes research findings on logistics cold storage in terms of building and refrigeration system. Policies from major economies and organizations regarding cold storage are also reviewed. Future research directions and development trends in logistics cold storage are discussed. The results indicate a shift towards environmentally friendly refrigerants like R717 and CO 2 , with significant potential for pure CO 2 systems. More field tests are needed to assess energy consumption accurately and inform regulatory policies. Additionally, the logistics cold storage with large heat capacities holds untapped potential for energy storage applications, which could enhance grid stability and promote clean energy utilization. [ABSTRACT FROM AUTHOR]

• Efficient operating range of the original district heating substation is below 1℃. • Proposed system formed by original components operates efficiently in whole winter. • Control strategies are based on the lowest primary return water temperature. • The proposed system has advantages in energy and economic performance. With the rapid development of heating technology, heating terminals are developing toward low temperatures and diversification, resulting in the simultaneous presence of different temperature demands in district heating systems. To reduce the primary return water temperature and satisfy the temperatures of different terminals, a dual-temperature district heating substation supplying dual temperatures of hot water with two absorption heat pumps and two heat exchangers has been proposed based on the design conditions. However, whether the dual-temperature district heating substation can operate and how to control this system during the entire heating period have not been explored. In this paper, the models for the substation with two absorption heat pumps and two heat exchangers and its subsystems were established to analyze and compare the operational conditions for the substation and its subsystems at different outdoor temperatures. By comparing the primary return water temperatures of all subsystems, control strategies and a combined system for efficient operation during the heating season were determined. Then a case study and analysis of the combined system were conducted under the proposed control strategies and compared with two other systems – two heat exchangers and two absorption heat exchangers – used in dual-temperature heating stations. Results show that 1) The original system with two absorption heat pumps and two heat exchangers could only operate when the outdoor temperature was −8–1℃. 2) The subsystem with a high-temperature absorption heat pump and two heat exchangers and the subsystem with two heat exchangers which could achieve efficient operation under outdoor temperature ranges of 1–7 °C and 7–11 °C respectively were considered in the combined system to broaden the operating range of the original system. 3) The combined system had the lowest return water temperature (23.8℃) and the highest coefficient of performance (6.60) during the entire heating season. The payback period of the combined system was 1.54 years. This study provides a reference for the efficient operation of dual-temperature district heating substations at different ambient temperatures during the heating season. [ABSTRACT FROM AUTHOR]

Indirect evaporative cooler (IEC) applied as an exhaust air heat recovery component for fresh air pre-cooling in air-conditioned zone, which successfully extends its application range to hot and humid areas. To explore more effective application method of the heat recovery IEC, an energy and exergy transfer model for IEC operating as a heat recovery devices was proposed in this study. The thermal performance and exergy transfer characteristics of three basic types of IECs: cross-flow, counter-flow, and parallel-flow IECs were numerically investigated. The results show that the irreversible heat transfer in the primary air channels and the irreversible mass transfer in the secondary air channels are the main causes of exergy loss. These two factors account for more than 90% of the total exergy destruction in all the three types of IECs. To alleviate this situation and improve the energy efficiency of the IEC, two two-stage modified heat recovery systems which combine IEC with sensible heat exchanger and energy recovery exchanger were proposed and numerical analyzed. The simulation results show that, for the modified systems, the exergy transfer efficiency is increased by 104.6% and 131.7%, while the heat transfer capacity is increased by 47.6% and 48.5%, respectively. • The performance improvement of IEC by changing the airflow direction is limited. • The irreversible loss mainly occurs at the entrance of heat recovery IEC. • Cooling capacity of two modified systems increases by 47.6% and 48.5%. • Multi-level processing for the heat recovery IEC systems is the way forward. [ABSTRACT FROM AUTHOR]

• Principles for determining the standard rating conditions of heat pumps are proposed. • Standard rating conditions for different ASHP systems in China are established. • Technical characters, energy savings and economic benefits of ASHPs are analyzed. Air source heat pumps (ASHPs) are widely used in various types of buildings in different regions of China. Considering the significant differences in the characteristics in heat source side and user side, providing different products for different zones is an effective strategy to promote the systematic development of the ASHP technology. To realize the goal of energy conservation, standard rating conditions are extremely important. The principles for determining the standard rating conditions based on a two-side (heat source/user side) multi-grade (different conditions) method were proposed along with the development of related standards. Therefore, the establishment of standard rating condition is introduced and summarized systematically. For heat source side, the standard rating conditions for heating were classified into four types according to the lowest ambient temperature (−25, −12, −2, 7 °C), while for cooling they were constant (35 °C). For user side, the conditions for supply air (20/27 °C for heating/ cooling), floor heating (35 °C), radiator (50 °C), fan coil unit (41/7 °C for heating/cooling), domestic hot water (55 °C), and industrial use were also determined. This study introduced the implications on ASHP by different standard rating conditions, especially the technical requirements, which indicated the necessity of providing different system cycles and compressors. The energy savings and economic benefits were also evaluated. The primary energy saving rate reached 15.3% and 41.6% compared with coal heating and conventional ASHP respectively by using low ambient temperature ASHP. The results showed that these standards have been satisfactorily developed. [ABSTRACT FROM AUTHOR]

As heating requirements continue to evolve and heating technology advances, radiators and radiant floor heating devices, which require different supply-water temperatures, have gradually become the two main heating terminals in district heating systems. However, the primary return-water temperature of current solutions based on heat exchangers and heat pumps is relatively high and fails to recover waste heat from the combined heat and power plants, owing to improper use of heat sources. To address this issue, a heat exchanger network optimisation method—the thermal energy discretisation and matching method—was used to design a dual-temperature water district heating system consisting of two absorption heat pumps and two heat exchangers. The primary supply water flows through the two generators, two heat exchangers, and two evaporators of the absorption heat pumps. The radiator and floor heating return water was divided into two parts and heated by the heat exchanger and the absorber and condenser of the absorption heat pump, respectively. The performance of the designed heating station was evaluated using the coefficient of performance and the primary return-water temperature and compared with that of other district heating systems. The changes in the district heating system's configuration when the boundary conditions changed were investigated. Results showed that (1) the designed system based on thermal energy discretisation and matching method -had the lowest primary return-water temperature of 18.8 °C and the highest coefficient of performance of 5.62. (2) When the low-temperature load ratio changed from 0% to 100% while maintaining a constant total load, the system's form changed regularly. (3) When the primary supply-water temperature increased, the heating station's form remained unchanged and the capacity of the heat exchange equipment changed regularly. (4) When the secondary supply and return-water temperatures increased, the evaporation temperature of the absorption heat pump increased, resulting in a change in the system's form. This study provides valuable insight into the design of district heating systems that supply dual-temperature water under various design conditions. • A dual-temperature DH system including two AHPs and HXs is designed by TEDM method. • Thermal energy of primary hot water can be used more reasonable in designed system. • Energy performance of designed system is better than that of other DH heating system. • The changes in DH system's form with different conditions are investigated. [ABSTRACT FROM AUTHOR]

The pipe-embedded double-skin façade (PDSF) is a superior approach to decrease building cooling and heating load by using natural energy. With pipes embedded in the shading, the overheating problem in the conventional double-skin façade (DSF) can be eliminated. Unlike the traditional DSF or glass windows, the thermal performance of PDSF depends on the ambient environment and water temperature in the embedded pipes. Although the complicated heat transfer process of PDSF can be numerically solved, it is difficult for engineers to obtain the whole picture of the PDSF at first glance. To easily understand the thermal property of the PDSF, the overall heat transfer coefficient (U value), solar heat gain coefficient (SHGC) and equivalent ambient temperature of the PDSF are derived based on the thermal network. The heat transfer characteristic of the PDSF at different cooling water temperatures is compared with that of the conventional DSF. The results show that the three proposed indices can clearly reflect the thermal property of the PDSF: U value and SHGC reflects the influence of insulation and the transparency respectively, and the equivalent temperature reflects the integrated influence of water temperature and outdoor temperature. The typical PDSF has a higher U value than the DSF in the same structure. However, the SHGC of the PDSF is only 60% of that of the DSF. Although the equivalent ambient temperature depends on the outdoor temperature and water temperature, the weighting of the water temperature is 0.8, which indicates that the equivalent ambient temperature for the PDSF can reach a better level with the appropriate water temperature. Moreover, relatively warm water is suggested on sunny days to reduce the solar heat gain, since the solar heat gain is dominant in the total heat gain. Correspondingly, cool water is recommended for cloudy days to decrease the conductive heat gain. [ABSTRACT FROM AUTHOR]

Highlights • A novel heat recovery solution regeneration system is proposed for heating tower. • Regeneration performance of internally heated regenerator is experimentally studied. • Correlations of heat and mass transfer coefficients are developed based on results. • A low-grade heat source can be used for heating tower solution regeneration. Abstract Owing to their high cooling efficiency in summer and frost-free characteristics in winter, heating tower heat pumps are becoming increasingly popular. However, the solution will be diluted during heating due to absorbing moisture from the air, and should be properly regenerated for continuous operation. In this study, a novel heat recovery solution regeneration system using an internally heated regenerator is proposed for antifreeze solution regeneration. A test bench is fabricated to investigate the regeneration performance of the internally heated regenerator. Correlations of the heat and mass transfer coefficients are proposed, and agree well with the experimental data. The influences of inlet parameters on the moisture removal rate, latent heat ratio, and humidity effectiveness are evaluated and compared to those for internally heated liquid desiccant regeneration. The results indicate that: the regeneration performance of the internally heated regenerator is affected mainly by the air flow rate, heating water flow rate, and inlet temperature of heating water. The moisture removal rate increases remarkably with increasing the heating water flow rate and inlet temperature of heating water. The latent heat ratio decreases significantly with increasing the inlet temperature of heating water. The humidity effectiveness decreases remarkably with increasing the air flow rate. The influences of the above inlet parameters on performance indices in the present study exhibit the same trends as those reported for internally heated liquid desiccant regeneration. However, compared with internally heated liquid desiccant regeneration, the regeneration temperature for internally heated antifreeze solution regeneration is much lower. This suggests that a low-grade heat source can be used to meet the antifreeze solution regeneration demands. [ABSTRACT FROM AUTHOR]

Abstract To reduce the energy consumption for fresh air handling, a novel fresh air pre-handling system that fully exploits the shallow energy to precool and preheat fresh air is proposed in this study. The model of a typical all-air system with a pre-handling system is built on the TRNSYS platform. The energy saving potential of this fresh air pre-handling system is simulated based on the diverse climate of China. The results show that the ratios of the cumulated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load range from 35% to 45% in the temperate zone and the subtropical zone, such as Shenyang, Beijing, and Shanghai, while the ratio in the tropical zone is approximately 10%. The energy saving rates are approximately 30% in most climate zones, but less than 5% in Guangzhou, the tropical climate zone. Moreover, the payback periods are within 4 years in the temperate and subtropical zones. However, the system is not economical in the tropical zone. All these results indicate that the fresh air pre-handling system with shallow geothermal energy is quite promising in most climate zones except in the hot summer and warm winter region with a tropical climate. [ABSTRACT FROM AUTHOR]

• To reflect the impact of indoor units on system performance, a novel index and model were proposed. • Simulation and experimental results validated the proposed performance model. • Proposed index was applied in a field test in an office and a residential building. In the context of a large market share, enhancing the actual performance of variable refrigerant flow (VRF) air-conditioning systems is of considerable importance. The operating states of indoor units (IUs) significantly influence the energy efficiency of a VRF system with multiple indoor terminals. Therefore, it is challenging to address accurate and concise representation of the operational characteristics of IUs for analyzing their impact on system performance. This study explored a novel performance index called capacity utilization rate (CUR) to illustrate the effect of IUs on system performance and developed a performance model based on this index. Simulation and experimental results demonstrated that energy efficiency and evaporating temperature decrease with a reduction in CUR under specific ambient temperatures and part-load rates. The proposed performance model was applied to real buildings in field tests to examine the operational characteristics of the VRF system. This research offers valuable insights for system design, performance simulation, and performance evaluation of VRF systems. [ABSTRACT FROM AUTHOR]

• Circulating air is treated with dual-temperature chilled water in two stages. • Multiple natural energies are used to produce dual-temperature chilled water. • Longer free cooling duration and lower energy consumption are realized. • Dual-temperature chilled water system has very short payback period. Indoor high-temperature circulating air is only treated with one type of low-temperature chilled water in a conventional waterside free cooling system, resulting in a short free cooling duration and high energy consumption in a data center. This study develops an air conditioning system using dual-temperature chilled water for a two-stage treatment of air. Mathematical models are developed to calculate the system performance throughout the year. A data center located in Hohhot, China is taken as a case, and the energy consumption, energy efficiency, and economics of the grading treatment system are analyzed and compared with those of two conventional systems (i.e., waterside economizer and air-cooled heat exchanger systems). The results show that the grading treatment system achieves increased free cooling duration and energy efficiency in the data center compared with the conventional systems. Compared with the conventional waterside economizer and air-cooled heat exchanger systems, the grading treatment system realizes annual energy-saving rates of 27.4% and 25.4%, energy efficiency increments of 37.8% and 34.0%, and payback periods of 2.55 and 2.59 years, respectively. [ABSTRACT FROM AUTHOR]

Heat pump technologies play significant roles in building energy saving and emission reduction. Combing the opposite characteristics of the electrical and absorption heat pumps, a novel internally hybrid absorption-compression heat pump was proposed for performance improvement. The thermodynamic models for the hybrid heat pump using two cycles (single-effect and generator-absorber-heat-exchange) were built for component design and performance simulation with various refrigerant distribution ratios. Results show that a higher absorption-side refrigerant ratio generally makes the primary energy efficiency less sensitive and the capacity more sensitive to working condition. In the heating mode, a higher absorption-side refrigerant ratio brings about a higher efficiency when the evaporator inlet temperature is below 16 °C and 20 °C for the two cycles. In the cooling mode, a lower absorption-side refrigerant ratio contributes to a higher efficiency. The hybrid absorption-compression heat pumps can realize flexible heating/cooling capacity ratios to accommodate the design heating/cooling loads as necessary. Generally, a higher absorption-side refrigerant ratio is preferred in colder conditions owing to the higher heating efficiency and the higher heating/cooling capacity ratio, well matching the heating-dominant building loads. This study focuses on the principle and performance of the novel hybrid heat pump, the applications in actual buildings will be conducted through year-round simulations in future studies. [ABSTRACT FROM AUTHOR]

The heat and mass transfer simulation model of a ground heat exchanger (GHE) directly affects the design and operation performance of a ground-coupled heat pump system. The GHE models based on the response function (like the Green function and g-function) can achieve a fast calculation speed. However, the heat sources in these models are limited to points or whole boreholes, leading to low calculation accuracy in heat transfer during a short time period and limitation to a certain GHE. A general distributed parameter model for a ground heat exchanger (RF model) is proposed based on the principle of response factors in this paper. A sandbox experimental platform is then built to test the temperatures of typical points in the double-layered soil and to validate the RF model. After that, the calculation of the RF model is simplified by determining suitable positions for the soil boundaries and the numbers of sub pipes and sub soil boundaries. Finally, the RF model is applied in different scenarios to demonstrate its characteristics. The results show that: (1) the RF model is suitable for different kinds of GHEs with arbitrary shape and type of heat sources releasing heat in arbitrary time steps; (2) the RF model has only 0.01 °C and 0.23 °C temperature response errors compared to those from numerical solutions and experiments, respectively; (3) the general RF model has similar accuracy to the numerical solution in calculating the distributed temperatures of the borehole and pipes, heat transfer in the short term, and heat transfer of borehole groups and the energy pile. [ABSTRACT FROM AUTHOR]

The gas injection technology had been developed to enhance the performance of the single cylinder rotary compressor at high compression ratio. However, the improvement of current designs was unsatisfactory because of the limited effective mass injected. A novel end-plate injection structure with check valve for rotary compressor was proposed and its advantages were confirmed in a previous study. In order to enlarge the injection port, increase the gas injected, and eventually increase performance enhancement, the general expressions of the envelopes and areas of the injection regions are developed based on an analytical geometry method in this paper. Based on them, the effects of the design parameters, such as the eccentricity and the thickness of the rolling piston, on the injection region area are investigated. Finally, an optimal design method for the rotary compressor oriented to proposed end-plate gas injection with check valve is raised. As a conclusion, for an arbitrary rotary compressor with designated cylinder radius, the area of the injection region reaches the maximum when the thickness of the rolling piston is equal to two times of the eccentricity. [ABSTRACT FROM AUTHOR]

The heat compensation unit with thermosyphon has been developed to eliminate the annual soil thermal imbalance of ground-coupled heat pump in heating-dominant buildings. But the issues on heating capacity deficiency at peak heating loads and on the high borehole investment are still unsolved in this non-coupled system. In this paper, a coupled operation of the heat compensation unit and ground-coupled heat pump system is proposed. That is, the heat compensation unit reheats the borehole outlet fluid and improves the temperature of the fluid entering the evaporator of heat pump during the heating season. By doing so, it can enhance the heating capacity at peak loads and reduce the number of boreholes required. The heat compensation required by the heat compensation unit in the non-heating season is also reduced, increasing the system efficiency. To demonstrate the effectiveness of the coupled system against the non-coupled one, the system models are built in TRNSYS to analyze the system reliability, efficiency and economy. Results show that, while the number of boreholes is reduced to 40%, the coupled system can maintain the soil thermal balance and meet the indoor heating demand. For the coupled system with different numbers of boreholes (40–100%), the seasonal average heating COPs of the ground-coupled heat pump and heat compensation unit are 4.39–4.70 and 3.67–3.80, respectively. The hourly heating capacity of the ground-coupled heat pump is also increased by 19–65%. The annual system COP of the coupled system is about 2.48–2.61, which is higher than that of 1.82–2.45 for non-coupled one. Compared to the conventional Boiler assisted ground-coupled heat pump system, the payback period of the coupled system is only 1 year. [ABSTRACT FROM AUTHOR]

• A method for constructing refrigerant cycle and heat exchange network is developed. • The method can obtain global optimal cycle structure and heat exchanger layout. • The method is verified by constructing reverse Carnot and two-stage cooling cycles. • Combined cooling and heating systems developed by the method improve COP by 38.6%. An integrated system of vapor compression cycle and heat exchanger network can comprehensively use heat transfer and improve the temperature grade of thermal energy to achieve efficient heating and cooling. Constructing an integrated system is necessary for saving energy and reducing CO 2 emissions. Existing construction methods of integrated systems cannot cover a global optimal cycle structure and are unsuitable for integrated optimization with external heat exchanger networks. This study proposes an innovative integrated construction method, named the GraPHsep method, to realize one-step optimization of integrated structures of vapor compression cycles and external heat exchanger networks. The proposed method is verified by constructing an ideal cycle and efficient two-stage compression cycle under two heat reservoirs. Furthermore, the proposed method is applied to construct a combined cooling and heating system with waste-heat recovery. The results show that the three novel systems constructed using the GraPHsep method exhibit 38.6%–40.3% higher coefficient of performance (COP) than the existing energy-efficient system. [ABSTRACT FROM AUTHOR]

The design method of a general heat exchanger network with heat pumps (GHEN) is significant owing to its wide application in different industrial and building processes. However, the thermal energy grade of the energy pocket cannot be fully used in the pinch method, and the thermodynamic meaning of the optimal solution cannot be clearly explained by the mathematical programming method. In this paper, a design method with as little high-grade energy use as possible is proposed for GHENs based on thermal energy discretization and matching (TEDM). This method obeys thermodynamic laws and designs GHENs through programming. Two types of advanced systems, an absorption heat exchanger and a composite district heating substation, were designed using the TEDM method to demonstrate its correctness and practicability. The energy and economic performance of a GHEN in an actual scenario were analyzed and compared with a system designed using the pinch method. The results indicated that the advanced systems presented in the literature can be easily constructed using the TEDM method. Compared with the system designed using the pinch method, the exergy consumption and operational cost of the system designed using the TEDM method decreased by 14.9% and 30.9%, respectively. The TEDM method provides a more promising alternative to the pinch method for the design of GHENs. • Method of thermal energy discretization for heat sources and sinks is developed. • Design method of general heat exchanger network with heat pump is established. • High-grade thermal energy can be used more reasonably than that without energy discretization. • Advanced systems can be easily constructed based on the proposed method. • Energy performance of optimized system is better than that with pinch method. [ABSTRACT FROM AUTHOR]

• Relay-evaporation cycles for better temperature matching are proposed. • Relay-evaporation cycles can realize 6.8–16.0% higher COP than single-stage cycle. • Proposed cycles can get 6.6% higher COP than two-stage compression cycle. Refrigeration and air conditioning systems consume 20% of the overall global electricity. Developing refrigeration systems using zeotropic refrigerants is an effective approach to save energy in air conditioning systems, because the temperature glide of zeotropic mixtures can improve the temperature matching between the refrigerant and airflow. However, in air conditioning systems, the temperature variation of the indoor airflow is considerably larger than that of the outdoor airflow. Thus, a simple zeotropic vapor compression system cannot simultaneously match the temperatures in the evaporator and condenser. This significantly compromises the benefits of adopting zeotropic refrigerants. In this study, four new relay-evaporation cycles are proposed. They can simultaneously improve the temperature matching in the condenser and evaporator. Thermodynamic models of the proposed cycles using R32/R1234ze(E) are established, and their performances are compared to that of the single-stage and two-stage compression cycles in terms of the coefficient of performance (COP), exergy destruction, and temperature match indicators. Under the basic condition, the relay-evaporation cycles can improve the COP by 6.8%–16.0% compared to the single-stage compression cycle. In particular, the relay-evaporation cycle with a recuperator and an economizer exhibits 3.4%–6.6% higher COPs than that of the two-stage compression cycle. [ABSTRACT FROM AUTHOR]

Multi-split variable refrigerant flow (VRF) systems have wide applications in offices and commercial buildings. In the multi-split VRF system, a subcooler is usually adopted to avoid flash vapor generation in the liquid pipeline. At present, the effects of the refrigerant pipeline length of the multi-split VRF system with a subcooler have not yet been investigated, nor the optimization of the subcooler. In this paper, the model of a multi-split VRF system with a subcooler has been developed and validated. In nine cases with different lengths of the main pipelines and the subcooling heat exchanger (SCHX), the cooling performance of a multi-split VRF system has been studied by simulation. The results indicate that the subcooler has little benefit in a system with a short pipeline, while it can help improve the coefficient of performance ( COP ) of a system with a long pipeline by controlling the bypass mass flow rate ratio in a proper range, and a higher maximum COP can be obtained by increasing the length of the SCHX in the subcooler. Furthermore, it is recommended to control the outlet superheated degree of the bypass stream in the SCHX in order to maximize the COP of a multi-split VRF system. [ABSTRACT FROM AUTHOR]

The coefficient of performance (COP) and heating capacity of the absorption heat pump (AHP) decreased obviously as the evaporator inlet temperature dropped. Compression-assisted AHP (CAHP) could operate efficiently in colder conditions, and a prototype was constructed for experimental investigation. At a generator inlet of 130 °C, as the evaporator inlet decreases from −5 °C to −25 °C, the COP drops from 1.513 to 1.372, while the heating capacity deteriorates from 77.26 kW to 47.11 kW. Comparisons between CAHP and normal AHP indicated that CAHP can extend the lower limit of evaporator inlet temperature from −10 °C to −25 °C. Besides, CAHP can enhance the heating capacity by approximately 55.5–85.0% even when AHP can operate normally. Moreover, the improvement contributed by CAHP is greater under much colder conditions. The heating COP and capacity of CAHP are improved in all the conditions, while the primary energy efficiency is advantageous under lower evaporator inlet temperatures. [ABSTRACT FROM AUTHOR]

The indoor environment and energy performances of convective terminals have been widely studied under the strong heating demands in cold regions. As different operative conditions affect the performances of heating devices, investigating these factors under different conditions can help improve the performances. In this study, the thermal performances of fan coil units (FCUs) and room air conditioners (RACs) were studied using comparative experiments, based on overall indoor environmental factors, local discomfort indices, energy performances, terminal characteristics, and integrated analysis. The results for the indoor environment were classified into different types, with the aim of analyzing approaches to obtaining a comfortable environment. The proper operating conditions reduce the percentages of dissatisfied caused by vertical temperature, floor temperature, and radiant asymmetry by 30.0%, 9.7%, and 28.8% for FCUs, respectively, and by 45.8%, 22.2%, and 0.2% for RACs, respectively. Long-term operation, high airspeed, downward air supply, and low set-point temperature were recommended to form a comfortable environment. The energy efficiency is improved by 9.6% and 2.7% with a low set-point temperature and downward supply air. The factors influencing thermal comfort and energy consumption are systematically analyzed, and the differences between continuous and intermittent convective terminals are compared. •Indoor environment and energy performance were studied with different terminals. •Conditions that improving indoor environment and energy performance are determined. •Dynamic process for start-up and steady stage of FCU and RAC were analyzed. [ABSTRACT FROM AUTHOR]

• A heat-source tower heat pump using subcooling heat for regeneration is proposed. • Three heating modes for heat-source tower heat pump are compared and discussed. • The novel system shows a nearly unreduced heating capacity with a high efficiency. The heat-source tower heat pump, a frost-free heat pump with superior energy efficiency, has been used for heating in cold regions. Because the solution in the heat-source tower absorbs moisture continuously from the outdoor air and becomes diluted during heating, solution regeneration is necessary. However, existing regeneration technologies have various disadvantages, such as a reduced heating capacity, decreased system efficiency, or the need for additional equipment. Hence, it is urgent to develop a regeneration technology with low cost, high energy efficiency and little impact on heating capacity. In this study, a heat-source tower heat pump with a self-regenerator is proposed and experimentally investigated. To regenerate the weak solution in the tower, the system uses the subcooling heat of the refrigerant at the condenser outlet as a heat source. The operating characteristics of the system in three heating modes are investigated experimentally. The results indicate that in the heating mode with self-regeneration, the heating capacity of the system is similar to that in the heating mode without regeneration under the same operating conditions. The system realizes efficient regeneration and can operate continuously, although the system efficiency is 2.5–5.1% lower than that of a system without regeneration. In addition, the regeneration efficiency of the proposed system reaches 3.13–11.98 under the condition where the regeneration temperature ranges from 18 to 45 °C. These results demonstrate that the proposed system can be used in cold regions. [ABSTRACT FROM AUTHOR]

Traditional single source heat pump systems, e.g., air source heat pump (ASHP) and ground source heat pump (GSHP), cannot work efficiently in the whole year. To develop an energy-efficient heat pump system for year-round operation, temperature grades of common natural energies are compared with each other. Based on the comparison, the combination principle of hybrid sources with complementary characteristics is discussed firstly. Then three typical types of heat pumps with hybrid sources are selected as representatives to show how to take full advantages of different heat sources or sinks for energy-efficient operation and how to extend functions of hybrid source heat pumps for new solutions to the disadvantages of conventional heat pumps. Finally, the annual performances of these three selected types of hybrid source heat pump systems for three corresponding specific cases are simulated by TRNSYS. The results show that hybrid source heat pump systems can maintain nearly the same heating capacity as they do without frost during the defrosting period; maintain energy-efficient year-round cooling with no risk of freezing during winter; keep the thermal balance of soil and maintain reliable cooling and heating over long periods of operation; fully utilize solar radiation of different intensities; and achieve year-round energy-efficient performance. In contrast to a conventional ASHP and GSHP system, the energy saving rate of a typical hybrid source heat pump system is approximately 15%, and the payback period is approximately five years. Hybrid source heat pumps provide a way to match multiple heat sinks/sources for different levels of building demand in different climates. Image 1030 • The HSHP can fully combine the advantages of different kinds of natural energies. • The HSHP achieves almost no decline in heating capacity during defrosting period. • The HSHP achieves the thermal balance of soil over long periods of operation. • The HSHP fully utilizes solar radiation of different intensities. • The HSHP operates efficiently year-round, and the payback period is about five years. [ABSTRACT FROM AUTHOR]

Here we show that it is possible to control magnon-magnon entanglement in a hybrid magnon-atom-cavity system based on atomic coherence. In a four-level V-type atomic system, two strong fields are applied to drive two dipole-allowed transitions and two microwave cavity modes are coupled with two dipole forbidden transitions as well as two magnon modes simultaneously. It is found that the stable magnon-magnon entanglement, one-way steering and two-way EPR steering can be generated and controlled by atomic coherence according to the following two points: (i) the coherent coupling between magnon and atoms is established via exchange of virtual photons; (ii) the dissipation of magnon mode is dominant over amplification since one of the atomic states mediated one-channel interaction always keeps empty. The coherent control of magnon-magnon correlations provides an effective approach to modify macroscopic quantum effects using the laser-driven atomic systems.