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The low-temperature district heating system based on the air source absorption heat pump (ASAHP) was assessed to have great energy saving potential. However, this system may require smaller temperature drop leading to higher pump consumption for long-distance distribution. Therefore, the applicability of ASAHP-based district heating system is analyzed for different primary return temperatures, pipeline distances, pipeline resistances, supplied water temperatures, application regions, and working fluids. The energy saving rate (ESR) under different conditions are calculated, considering both the ASAHP efficiency and the distribution consumption. Results show that ASAHP system is more suitable for short-distance district heating, while for longer-distance heating, lower supplied hot water temperature is preferred. In addition, the advantages of NH 3 /H 2 O are inferior to those of NH 3 /LiNO 3 , and the advantages for warmer regions and lower pipeline resistance are more obvious. The primary return temperatures are optimized to obtain maximum ESRs, after which the suitable distances under different acceptable ESRs are summarized. To improve the applicability of ASAHP, the integration of cascaded heat exchanger (CHX) and compression-assisted ASAHP (CASAHP) are proposed, which can reduce the primary return temperature. The integration of CHX can effectively improve the applicability of ASAHP under higher supplied water temperatures. As for the utilization of CASAHP, higher compression ratio (CR) is better in longer distance, while lower CR is more advantageous in shorter distance. For a distance of 50 km, the maximum ESR is improved from 13.6% to 20.4–25.6% under different CRs. [ABSTRACT FROM AUTHOR]

Highlights: [•] We propose a new concept, heat adaptor, to optimize thermal system performance. [•] It is a combination of heat engine, heat pump and conventional heat exchangers. [•] The most suitable device and process can easily be identified using the concept. [•] A typical problem is used to demonstrate the advantages of applying the concept. [Copyright &y& Elsevier]

Abstract: Spaces with high heat density such as mobile phone base station (MBS) and data center increase fast recently. Huge energy is consumed for the air conditioning because of large indoor heat rejection and year round cooling in these spaces. The integrated air conditioner with thermosyphon (IACT) combines the vapor compression (VC) refrigeration and thermosyphon (TS) in one body, which can save energy and space for these spaces. After analyzing the problems in the existing IACT, a new IACT is proposed by using the novel heat exchangers and the self-operated three-way valve (STV), which works well in both VC and TS modes. The 3HP air cooled IACT is developed and tested in laboratory. The COP of TS mode reaches 9.43 when outdoor temperature is 20 °C lower than indoor, and the COP of VC mode reaches 2.94 under standard condition. This IACT has been applied in MBSs of different cities in China and the energy consumption is tested on-site and compared with the traditional air conditioners (TAC). It is shown that the IACT can save more than 30% energy compared with TAC. [Copyright &y& Elsevier]

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]

Ground-coupled heat pumps are widely applied in cold regions, but soil thermal imbalances in some projects greatly deteriorate the systems’ practical performances and heavily inhibit the systems’ reasonable applications. This paper presents a comprehensive overview of the main problems caused by soil thermal imbalance and the existing solutions targeting on them. The caused problems mainly present in the following aspects: soil temperature decrease, heating performance deterioration, heating reliability decline, and even system failure. Three kinds of existing solutions can be classified. Ground heat exchanger-modified solutions including increasing borehole space, increasing borehole length, modifying borehole layout, and improving thermal properties, can retard the thermal imbalance to some extent, suitable to projects with slight imbalance. System-modified solutions integrating auxiliary energy sources like fossil fuel, solar energy, ambient air and waste heat with ground-coupled heat pumps or using ground-coupled absorption heat pumps, can eliminate serious thermal imbalance essentially by increasing soil thermal injection and decreasing the extraction. Thus, they are dominant in the applications and attract more attention along with the utilization of renewable energies. Apart from the modifications of system design, operation-modified solutions, like seasonal and intermittent operations, are also helpful to maintain better soil thermal balance. Based on the overview, the current application situations and economic performance of different solutions are discussed. Finally, some suggestions for the applications of ground source heat pumps in cold regions are proposed to make the overview helpful for the system research and applications. [ABSTRACT FROM AUTHOR]

Two-phase thermosyphon loops (TPTLs) are extensively used in industries. Numerical simulation is always applied for their configuration design and performance rating. The conventional models are usually developed under large temperature difference, and the fully liquid-filled downcomer are taken for granted. In this study, a generalized model for TPTL is developed, in which the downcomer can be partially liquid filled or fully liquid filled under different conditions. The model is verified by experiments on an air-to-air finned-tube TPTL. Then, it is used to analyze the flow and heat transfer features of a TPTL, under different refrigerant charge and temperature difference. The results show that the downcomer is partially liquid filled in the case of small temperature difference and refrigerant charge. Additionally, the heat transfer rate first increases then decreases with the increase of refrigerant charge, and first increases linearly then increases relatively slowly with the increase of temperature difference. [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]

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]

• 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]

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]

• 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]