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• Heat pump water heater with liquid-separation condensation is modelled and verified. • A 7.43 % improved COP and a 13.1 % shorter time with same condenser area is achieved. • A 22.22 % reduced condenser area and a 4.1 % shorter time with similar COP is obtained. • Optimized cases are superior at early stage of transient heating-up process. Heat pump water heater (HPWH) offers great potential to reduce energy consumption in water heating applications. But its performance enhancement and/or initial cost deduction are the main challenges to promote market penetration. In this paper, a heat transfer improvement technology, i.e., liquid-separation condensation (LSC), is implemented to HPWH and screen to achieve the maximized COP (E-HPWH) or reduced condenser area (R-HPWH). A model of HPWH with LSC is developed and validated, and its predictions are in good agreements with tested results from the baseline (B-HPWH) with an average deviation of 4.92 % at most. By using this validated model, E-HPWH and R-HPWH are obtained by adjusting the path arrangement and separation efficiencies. With the same heat transfer area, the COP of E-HPWH reaches 4.48, which is 7.43 % higher than that of B-HPWH, and the heating time is shortened by 13.1 %. With the same COP of B-HPWH being 4.17, the heat transfer area of R-HPWH is reduced by 22.2 % and heating time is shortened by 4.1 %. Then transient heating-up process of three HPWHs are comparatively investigated. Results indicate that E-HPWH has the highest compressor power as well as pressures and temperatures at the inlets of condenser and evaporator, followed by R-HPWH and B-HPWH. E-HPWH and R-HPWH overperform B-HPWH in terms of heat capacity before 4220s, due to the increased mass flowrate. Compared to B-HPWH, E-HPWH and R-HPWH have averagely 15.02 % and 7.29 % higher COP before 2790s and 1860s, respectively. Their states are less differentiated as the water temperature increases. [ABSTRACT FROM AUTHOR]

• A vapor-liquid adjustment evaporator is pioneered to heat pump water heater. • Serials experiments are conducted for performance comparisons. • Current vapor-liquid adjustment evaporator cannot ensure its superiority for each period. • A 6.35 % higher COP and 16.18 %−63.37 % lower evaporator pressure drops are found. Adjusting vapor and liquid in the evaporator could lead to heat transfer enhancement and pressure drop reduction simultaneously. Its benefits in the steady system, such as air conditioning and heat pump, have been confirmed extensively, but its effectiveness in the dynamic system is not explored yet. In this paper, a vapor-liquid adjustment evaporator is implemented to heat pump water heater (HPWH-AE) featured by a dynamic water heating process. Comprehensive comparisons to the heat pump water heater with a conventional evaporator (HPWH CE) are experimentally investigated and discussed. Firstly, the matching test reveals that the optimized HPWH-AE has a 6.35 % higher COP than HPWH CE. Secondly, the detailed comparisons indicate that HPWH-AE has less evaporator pressure drop, smaller compressor ratio, higher refrigerant mass flowrate and decreased power consumption than HPWH CE. Thirdly, the divided tests are carried out with an increment of half hour. The results suggest that HPWH-AE outperforms HPWH CE in terms of heating capacity and COP at the first one and half hours, which becomes inferior afterwards. However, HPWH-AE has always lower power consumption relative to HPWH CE. The images from FLIR camera show that the vapor-liquid adjustment evaporator is featured by effectively utilizing heat transfer area, resulting from more evenly distributed temperatures. Finally, the feasibility is examined at various operating conditions. HPWH-AE could have higher heating capacity and lower power consumption than HPWH CE simultaneously when ambient temperature is between 18 °C and 26.5 °C. HPWH-AE always has higher COP of 4.92–6.35 % than HPWH CE. [ABSTRACT FROM AUTHOR]

• First overview of the flow characteristics of CO 2 -lubricant mixtures in gas coolers. • Presents multiple factors affecting the flow characteristics of CO 2 -lubricant mixtures. • Reviews the analysis models of the flow and heat transfer of CO 2 -lubricant mixtures. • Comments on pressure drop correlations of CO 2 -lubricant mixtures. • Recommendations for future studies on the CO 2 -lubricant mixture flow characteristics. With the recent advancements in designs and optimization of gas coolers in CO 2 transcritical refrigeration and heat pump systems, the impact of the lubricant on the performance of gas coolers has been gaining increasing attention. The existing literature reviews focus on examining the lubricant effect on the heat transfer performance of gas coolers. However, the studies do not present a detailed discussion and a systematic summary of the flow characteristics. The present paper aims to offer an overall review of the lubricant impact on the flow characteristics of CO 2 in gas coolers. The authors hope it will help researchers conduct additional research about practically designing and optimizing gas coolers. This paper reveals the flow mechanism by providing critical discussions on how temperature, lubricant type, lubricant concentration, mass flux, operational pressure, tube diameter, and tube configuration influences the flow characteristics of the CO 2 -lubricant mixture. Then, the paper summarizes and evaluates the latest developments related to CO 2 -lubricant mixture in the flow and heat transfer analysis models and pressure drop correlations. Moreover, the current research gap is summarized, and the study proposes development directions. These include measuring thermo-physical properties of CO 2 -lubricant mixture, examining pressure drop characteristics under different tube configurations, developing universal pressure drop correlations, and studying flow characteristics in vertical tubes. [ABSTRACT FROM AUTHOR]

• Partial and complete condensation of R1234ze(E) and R134a was experimentally analysed. • Effects of superheating, subcooling and vapour quality were investigated. • R1234ze(E) presents heat transfer coefficients similar to those obtained with R134a. • Subcooling degree penalizes the BPHE performance at low mass flux. Condensation from superheated vapour is an essential process widely encountered in energy applications, mostly for refrigeration and heat pump technologies and for power production systems. This paper presents and analyses the experimental results obtained during partial condensation inside a brazed plate heat exchanger (BPHE) of the HydroFluoroCarbon R134a and of one of its replacements, the HydroFluoroOlefin R1234ze(E). Tests have been conducted considering different values of the outlet vapour quality, mass flux, and a saturation temperature that ranges between 34.6 °C and 42.3 °C. Partial condensation tests revealed that the heat transfer coefficients collected with R134a are 4–8% higher than those with R1234ze(E). Experimental tests of complete condensation of R1234ze(E) with different degrees of superheating and subcooling have also been performed to investigate how these parameters affect the thermal performance in real operating conditions. The experimental results, in terms of condensation heat transfer coefficient, have been compared with the predictions of some existing correlations, highlighting those that provide the best accuracy when designing brazed plate condensers. [ABSTRACT FROM AUTHOR]

• An analytical model for heat transfer inside deep borehole heat exchangers (DBHE). • Convolution theorem to address vertically increasing borehole wall temperature. • Relationships between DBHE thermal performance and various parameters. • An effective tool fast and easy enough for the design and optimization of DBHE. Deep borehole heat exchangers (DBHE) provide an effective solution for ground coupled heat pump (GCHP) systems in cold climate region where heating is dominant. Concerning the analytical heat transfer models, the simplification that borehole wall temperature is constant along the depth of ground heat exchangers (GHE) which the existing quasi-three-dimensional models have assumed in the application of shallow borehole GHE, can no longer be accepted in the application of DBHE due to the geothermal gradient in deep ground. Making this simplification cannot give the real temperature distribution of circulating fluid along the depth of DBHE. Therefore, this paper developed a new analytical model for DBHE with coaxial pipes by successfully addressing the increasing borehole wall temperature using the convolution theorem, so that the widely employed quasi-three-dimensional models for shallow borehole GHE is extended for DBHE with coaxial pipes for the first time. The new analytical model is validated by comparing with an existing numerical model. Using the newly developed analytical model, the trends revealing the relationships between thermal performance of DBHE and various parameters are firstly plotted. Because of the high accuracy and quick calculation, this new analytical model can be used as a benchmark for numerical models. More importantly, the proposed analytical model can be an effective tool for the design and optimization of DBHE, since current numerical models are always calculation-demanding, time-consuming and difficult for engineers and designers to use. Also, the method this paper proposed to address the varying borehole wall temperature can certainly be employed to improve the existing quasi-three-dimensional models for shallow borehole GHE so that they can be applicable in some other cases, for example, GHE installed in layered soils, or affected by underground seepage flow in partial depth. [ABSTRACT FROM AUTHOR]

• The refrigerant mass distribution in an air-to-water heat pump has been studied. • Experimental tests have been performed during cooling and heating operations. • A model, for charge prediction and performance estimation, has been described and validated. • The effect of the maldistribution in the finned coil heat exchanger has been addressed. • Existing void fraction correlations have been assessed against experimental data. This paper presents the results of refrigerant mass distribution measurements carried out in an invertible air-to-water heat pump. The unit works with refrigerant R32 and produces a nominal cooling capacity equal to 60 kW. Experimental tests have been performed during both cooling and heating operations using the quick-closing valves technique to isolate the charge within the components of the system. A mathematical model, developed to predict the refrigerant charge in the heat exchangers of the heat pump and its performance, is described and validated. The model includes a physical description of the heat transfer mechanisms in the condenser and in the evaporator. The model employs void fraction correlations to calculate the refrigerant mass in the two-phase regions of the heat exchangers. In the finned coil heat exchanger, the model can account for the effects of the airflow maldistribution, which has been also experimentally measured. When the not uniform air velocity is considered, the refrigerant charge in the heat exchangers is modelled within ±10%. The results of the model showed that the airflow maldistribution can increase the total refrigerant charge in chiller mode by 12.7% and decrease the performance of the system by 11%, compared to the case of uniform air velocity. [ABSTRACT FROM AUTHOR]

The heat transfer deterioration caused by frosting is the critical factor affecting the liquefied natural gas (LNG) vaporization efficiency of the ambient air vaporizer (AAV). In this paper, an LNG vaporization system using a solar-assisted heat pump (DX-SAHPNV) was proposed for AAV defrosting and LNG auxiliary heating, and a low-temperature LNG bypass flowing through the collector/evaporator was adopted for heat pump performance improvement and LNG preheating. Moreover, a dynamic model of the DX-SAHPNV system was established. Based on the meteorological data in Beijing, the system performance throughout the year and on typical days was analyzed, as well as the main meteorological factors effecting the system performance. Results showed the annual COP and the heat collection efficiency of collector/evporator with LNG bypass were 10.77% and 12.84% higher than that without LNG bypass, respectively. Comparing with the conventional AAV, the two-phase length of AAV in the DX-SAHPNV system was reduced by 12.7% and 20.2% on the typical winter and summer day, while the outlet LNG temperature was improved by 5.33 K and 11.56 K. The proposed system may provide a new perspective for the AAV defrosting and LNG vaporization optimization. [ABSTRACT FROM AUTHOR]

• The model obtained CO 2 outlet temperature with an absolute mean error of 1.14 K. • The heat transfer rate of fluted tube-in-tube gas cooler was generally higher. • The improvement of maximum COP could be up to 29.41% by using the fluted tube. • The COP improvement could be impaired to 6.30% at water inlet temperature of 40 °C. The utilization of fluted inner tube in tube-in-tube heat exchangers is an effective way to improve the heat transfer performance. Based on the segmented model, the performance of a fluted tube-in-tube gas cooler (FTGC) in the transcritical CO 2 heat pump water heater was studied in this paper. The simulation model was validated and adopted to numerically investigate the effects of different parameters on the heat transfer performance of the gas cooler, which was compared to the smooth tube-in-tube gas cooler (STGC) with the same geometric parameters. The results showed that the FTGC had higher heat transfer capacity in most conditions. The heat transfer rate first increased and then decreased as the CO 2 inlet pressure increased. The temperature difference at the pinch point in the FTGC was lower than that in the STGC. In the FTGC, the pinch point position was mainly influenced by the CO 2 inlet pressure and water inlet and outlet temperatures. Regarding the system COP, the improvement of maximum COP could be up to 29.41% by using the FTGC compared to STGC when the water inlet temperature was 10 °C and the water outlet temperature was 80 °C. However, when the water inlet temperature was 40 °C, the improvement of COP could be impaired to 12.98%. Through changing the designs of the FTGC with the fixed total heat transfer area, when the water inlet and outlet temperatures were 10 °C and 60 °C, the highest maximum COP could be obtained with the design used in the experiments. [ABSTRACT FROM AUTHOR]

This review paper presents an overview of transpired solar collectors (TSCs) - a clean energy technology that has gained considerable attention due to its potential to minimise building energy demand and contribute to achieving net zero emissions. The review is mainly focused on scrutinising the disparities in heat and mass transfer mechanisms, specifically the impact of homogeneous and inhomogeneous suction flow distributions on the TSC, encompassing performance improvement methods, performance assessment, and optimisation strategies for TSCs. The inconsistency between the theoretical and real-world performance of TSCs was observed, attributed to the high turbulence wind with non-uniform speed distribution and suction flow interactions with the main flow in the plenum. Corrugations on the absorber improved thermal performance but increased wind heat loss on the exterior surface, which could be mitigated by adding glazing to corrugated TSCs. The review highlighted the flexibility of integrating TSCs with other technologies, including coupling with separate phase change material (PCM) thermal energy storage units, which outperformed those placing the PCM plate into the plenum. Moreover, integrating TSCs with desiccant wheel cooling systems showed some interesting results in terms of cost and payback period, while integration with heat pump systems could reduce electricity consumption, but it was not recommended in cold climates. Exergetic analysis has been frequently used to determine thermodynamic losses of TSCs and it could provide a more accurate performance assessment when integrating TSCs with power generation systems. The review identified several research gaps in related study fields and provided recommendations, encompassing dimensionless analysis of the relationships between wind effect and heat transfer of TSC absorbers, the development of convective heat transfer correlations for TSC's absorber, and the exploration of cost-effective design and optimisation strategies for hybrid TSCs. • Heat and mass transfer mechanisms of transpired solar collectors (TSCs) were analysed. • TSCs can be integrated with different energy technologies to improve flexibility. • Various designs of TSC systems and their main design parameters were discussed. • It provides some insights into developing convective heat transfer correlations. [ABSTRACT FROM AUTHOR]

• RBF-DG was a novel prediction for absorption process in falling film and membrane based absorbers. • RBF-DG method provided excellent solutions. • Comparison with analytical solutions and experiments provided good agreements. • It becomes possible to predict accurately the absorption rate of falling film and membrane. The present investigation is intended to develop an understanding of how the vapor pressure and inlet temperature of the solution affect the absorption rate, which is helpful for the design of absorption heat pump systems. Many conjugated heat and mass transfer processes in falling film and membrane during lithium bromide (LiBr) absorption have not been well addressed. This paper proposes combining the meshless Radial Basis Function (RBF) and Discontinuous Galerkin (DG) methods to solve the conjugated heat and mass transfer in falling film and membrane-based absorption problems. The derivatives in the transverse direction are discretized using the RBF method. Owing to the parabolic nature of the governing equations, the DG methods can be used to discretize the derivatives in the streamwise direction. The efficacy of the proposed method is shown by comparing its results with analytical solutions and experimental data available in the literature. Furthermore, the proposed method is used to evaluate the analytical solutions of the heat and mass transfer problem in a membrane-based absorber. The previous published works simplified the problem to obtain the analytical solutions by providing the mean temperature and concentration values as the boundary conditions at the membrane-solution interface. The comparison of the RBF-DG method and the analytical solutions reveals that there are minor changes in temperature and local concentration values along the streamwise direction, but the profiles and trends are relatively similar across the film. A comparison of mass absorption rate calculations shows that RBF-DG and analytical solutions are compatible. The advantage of the numerical method is that various types of boundary conditions can be handled easily without any simplifications. Moreover, the numerical results of the proposed method are compared with experimental data, and they present good agreement. As a results, the absorption rate of falling film and membrane becomes capable of being accurately predicted. [ABSTRACT FROM AUTHOR]

To design a high efficiency magnetocaloric heat pump for the residential sector, we focused on the improvement of the performance of the regenerator bed. In particular, placing the regenerators circumferentially on a plane, we decided to use tapered regenerators instead of the straight channel ones. Therefore, this paper investigates the effect of the tapering of the regenerators, which exhibit better air-gap utilization. Several simulations using a 1D AMR model were run to study the performance of the tapered regenerator, and the results were compared to the case of the straight regenerator bed. Moreover, the temperature span was held fixed at 25 K, and the working temperature of the regenerator was shifted to study the sensitivity to the variation of the working conditions. This paper considers a 10-layer regenerator, with Curie temperature ( T C ) spacing of 2.5 K. [ABSTRACT FROM AUTHOR]

Pile heat exchanger has been developing rapidly in recent years and is an important research direction of ground source heat pump technology. This paper, from the aspects of principle structure, heat transfer model, numerical simulation, experimental study and engineering application and so on, sums up the domestic and overseas research status of pile heat exchangers in the construction industry and describes the laws and characteristics of heat transfer, underground soil temperature field and the factors impacting energy pile heat transfer. Pile heat exchanger has a variety of pipe laying methods, in which the spiral pipe-laying method can effectively increase the heat exchange area on the premise of security and reliability and has the best heat transfer efficiency. The paper also concludes the next step of research problems and development direction of pile heat exchanger technologies. [ABSTRACT FROM AUTHOR]

In this paper, the analysis model of thermal performance and available energy of two-stage cascade cycle heat pump water heater (HPWH) was established. For the first time, the equivalent transformation thermodynamic analysis method and available energy consumption equation were employed to carry out the theoretical analyses of thermal performance and available energy consumption for two-stage cascade cycle HPWH. And the correctness of the analysis approach proposed in this paper was verified by an example. The calculation results showed a significant agreement with the results derived by the first law of thermodynamics and energy conservation law. The available energy analysis of two-stage cascade cycle HPWH clearly revealed the available energy consumption rates for different working processes, including heat transfers of heat exchangers, flow of heat-carrying fluid, working medium cycle, output of hot water, etc. This approach could be conveniently used in the analyses of thermal performance and available energy consumption for actual complex thermal system with irreversible process. This paper provided the theoretical basis for the actual optimization design and operation analysis of cascade type heat pump systems, and furthermore, the results of the calculation instance would also have a certain reference value. [ABSTRACT FROM AUTHOR]

Aimed at applications in ground-coupled heat pump and thermal energy storage systems, this paper presents a heat transfer model of a deep borehole cluster, taking into account the geothermal gradient in the subsurface and the real connection conditions of the two regions inside and outside the borehole. The superposition principle is proven valid in a mathematical sense for such models in certain conditions, and the three-dimensional heat conduction problem for a multi-borehole heat exchanger may be decomposed into a number of two-dimensional single-borehole problems. Then, a new dimension reduction algorithm is proposed for simulation of heat transfer in the deep borehole cluster. Using this algorithm, comparisons with other commercial model and simulations on example cases show that computational efficiency can be improved by orders of magnitude compared with traditional three-dimensional software. • A new heat transfer model of a deep borehole cluster was developed. • The geothermal gradient and connection conditions on boreholes were considered. • The superposition principle was proven valid on the boundary of the borehole walls. • A dimension reduction algorithm was proposed for a deep borehole cluster. • The proposed model has a high computational efficiency. [ABSTRACT FROM AUTHOR]

With the enactment of the Kigali amendment regarding the phase-down of substances with Global Warming Potential (GWP), refrigerants based on HFCs are falling into disuse and low-GWP refrigerants are proposed as replacements. R1233zd(E) is a promising new working fluid for High-temperature Heat Pumps (HTHPs) with low GWP which has yet to be further developed. A proper model needs to be imperatively constructed to predict the characteristics of R1233zd(E) applied in HTHPs. This paper proposes a mathematical model of two-stage compression and vapor injection heat pump system, and analyzes the compression ratio, injection mass flow rate, power consumption, discharge temperature, heating capacity and Coefficient of Performance (COP) under variable working conditions. When recovering 50 °C waste heat and supplying 60–130 °C hot water, the heating capacity increases from 2.09 to 2.22 MW and the COP decreases from 23 to 2.93. When recovering 80 °C waste heat and supplying 90–160 °C hot water, the heating capacity increases from 4.11 to 4.23 MW and the COP decreases from 24.4 to 4.59. The detailed model of heat exchangers can demonstrate the inner heat transfer process clearly, such as refrigerant temperature and gas content ratio, which would be instructive for component optimization. The above results indicate that the studied R1233zd(E)-based HTHP has excellent performance and broad application prospects, which plays a positive role in further development of HTHPs. [ABSTRACT FROM AUTHOR]

• Integrated desorber-analyzer-rectifier developed for compact absorption systems. • Representative ammonia-water desorber/rectifier components developed. • Tests conducted over range of representative heat pump conditions. • Measured heat transfer coefficients for desorber/rectifier geometries. An experimental study of ammonia-water desorption in compact counter-flow geometries was conducted. Two novel vapor generation units, comprising integrated desorber, analyzer, and rectifier segments that use microchannel geometries, were conceptualized. The branched-tray concept features a desorber that uses pool-boiling mechanisms, while the vertical column desorber relies on falling-film evaporation mechanisms. Both concepts rely on falling-film mechanisms in the analyzer and rectifier sections. The vapor generation unit test sections were evaluated across a range of concentrated solution mass fractions (0.400 – 0.550), desorber coupling-fluid inlet temperatures (170 – 190°C), and concentrated solution flow rates (0.70 – 1.3 g s−1). This paper, Part I, reports the experimental methodology and results while in Part II, flooding limits of these geometries and heat and mass transfer models that accurately predict component-level performance are presented. [ABSTRACT FROM AUTHOR]

• A mathematical model for the vapor-bypassed evaporator is developed. • Corresponding experiments were conducted to validate the model accuracy. • Entropy generation is applied to evaluate the evaporator overall performance. • Phase separation position and channel number of the evaporator are optimized. This paper presents a simulation study on the vapor-bypassed evaporator for heat pump applications. The vapor-bypassed evaporator, in which a phase separator is set in the middle of the refrigerant circuit to bypass vapor refrigerant, can reduce the refrigerant pressure drop for enhancing the evaporator overall performance. A mathematical model for the vapor-bypassed evaporator is developed and the experiments were conducted to validate its accuracy. Based on the entropy generation minimization theory, the optimum phase separation positions and refrigerant channel numbers of the evaporator for various operation conditions are selected out. According to the simulation results, as the required heat transfer capacity ranges 4~10 kW, the vapor-bypassed (VB) mode obtains 25.6~84.6 kPa lower refrigerant pressure drop and 3.1~25.3% lower entropy generation than the conventional operation (CO) mode. Moreover, when the ambient temperature ranges 5~15°C, the entropy generation of VB mode is decrease by 22.3~11.5% than CO mode. In general, applying the vapor-bypassed technique is a promising way for improving the evaporator performance especially under high heat transfer capacity and low ambient temperature conditions. [ABSTRACT FROM AUTHOR]

The purpose of the research was to examine the savings potential and verify how to reduce the energy consumption in new hospitals by about 50%. Our results are also very relevant in other buildings complex with surplus heating, cooling and thermal storage. Conversion and utilization of surplus heat sources represent well known technologies and are under constant development. In large building complexes there is a potential for coordinated production, storage, and distribution of energy. Heat pump, chiller and thermal storage technologies enable re-use of surplus heat/cool energy throughout the year. Energy supply and demand are typically modelled with separate tools and timescales. Through our study new methods are developed for operation of interacting simulating models. These methods provide tools to step into optimization of combinations of integrated energy systems. The main issues focus on hydraulic water flow and storage systems as a basis. The layout is important not only with regards to utilize the energy quality, i.e. the temperature. In addition, control strategies are shown to have a highly relevant impact on the possible savings. Some details of new hydraulic layout and control design will be described and discussed in the paper. Both in combination with heating, cooling and storage system. Our research indicates that it is possible to save between 20-50% of the energy consumption with optimizing the technical installations in Oslo area. Another benefit of integrated thermal modelling is to reduce the risk of permafrost in ground source heat pump installations. This risk is relevant for large buildings in northern Norway, at annual mean temperatures are below 0̊C. This paper shows how integrated simulation and design can be used to avoid permafrost. [ABSTRACT FROM AUTHOR]

Nanorefrigerants are a special type of nanofluids which are mixtures of nanoparticles and refrigerants and have a broad range of applications in diverse fields for instance refrigeration, air conditioning systems, and heat pumps. In this paper thermal–physical properties of nanoparticles suspended in refrigerant and lubricating oils of refrigerating systems were reviewed. The effects of nanolubricants on boiling and two phase flow phenomena are presented as well. Based on results available in the literatures, it has been found that nanorefrigerants have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional refrigerant. This can be considered as one of the key parameters for enhanced performance for refrigeration and air conditioning systems. Because of its superior thermal performances, latest up to date literatures on this property have been summarized and presented in this paper as well. The results indicate that HFC134a and mineral oil with TiO 2 nanoparticles work normally and safely in the refrigerator with better performance. The energy consumption of the HFC134a refrigerant using mineral oil and nanoparticles mixture as lubricant saved 26.1% energy with 0.1% mass fraction TiO 2 nanoparticles compared to the HFC134a and POE oil system. [ABSTRACT FROM AUTHOR]

This paper documents the joint performance of heat pumps that are served by a common loop buried in the ground, and which operate simultaneously: one heat pump absorbs heat from the buried loop whereas the other one rejects heat. A background flow is circulated in the underground loop even when the two heat pumps are not operating. The objective is to determine the performance and the manner in which it is affected by the way in which the two heat pumps are connected to the loop. The performance measures are the heat transfer rates into and out of the heat pumps, and the total pumping power required by the assembly. The paper documents the individual performance of the heat pumps, and their relative performance, which is the ratio of heating absorbed by one pump to the heating rejected by the other pump. [ABSTRACT FROM AUTHOR]

Ground (or geothermal) heat exchangers are attracting a great deal of attention as a way of using shallow geothermal energy. This paper provides not only a critical review but also a thorough introduction to the analysis of heat transfer by borehole and foundation pile ground heat exchangers, with an emphasis on different analytical models. The literature is reviewed in a time-scale framework because of the diversity of the time and space scales involved in the thermal processes of ground heat exchangers. We summarize, discuss, and evaluate major advances in this field, including heat-source models, short-time models, models for energy piles, in situ thermal-response tests, indoor sandbox experiments, and parameter estimation as an inverse problem. Of particular note is that the unit-step temperature response (i.e., G -function) of a ground heat exchanger with one U-shaped pipe is calculated; and six analytical models are compared: an infinite cylinder-source model, two infinite line-source models, two finite line-source models, and a composite-medium line-source model. This paper closes by identifying several unsolved problems that require solutions. [ABSTRACT FROM AUTHOR]

• Evaluated previous flow condensation heat transfer coefficient correlations. • Proposed a new correlation for condensation heat transfer in mini and macro channels. • Verified the model with 6521 data points from 40 published papers. • Discussed the effects of the input parameters on the heat transfer coefficient. A new general explicit correlation is proposed to predict the heat transfer coefficient of fluids condensing in conventional and mini channels. The expression has been developed by correlating the Nu mix number with Re mix , Pr mix , phase density ratio, P res , We GT , and Fr L using genetic programming for the two-phase flow. The model has been validated with a big dataset consisting of 6521 data samples, covering a wide range of fluids used in refrigeration and heat pump industries, cross-sectional geometries (different diameters), mass fluxes, and saturation temperatures. The new generalized correlation fits the wide range of data points used with an average relative error of 17.82 %. The same database has been used to compare predictions of eight correlations available in the literature, but they failed to give a reasonable estimation of the present experimental results. [ABSTRACT FROM AUTHOR]

• A novel microchannel heat sink with rhombus fractal-like units is proposed. • An optimization approach is developed for the design of microchannel heat sink. • Experimental validation of the optimization approach is performed. • Rhombus fractal-like units may effectively reduce pumping power of heat sink. Microchannel heat sink is commonly used as a high-efficiency heat exchanger type in electronic microchips cooling, and the parallel microchannels arranged on the heat sink not only enhance heat transfer area but also result in large pumping power. In this paper, a novel structure of microchannel heat sink with rhombus fractal-like units is proposed for improve the overall performance, and a geometry optimization approach is developed for the design of the new microchannel heat sink. The validation of the geometry optimization approach is performed, and the results show that the predicted pumping power can describe 93% of the experimental data within the deviation limit of ± 5% and the mean deviation is 1.95%. Compared to the conventional parallel microchannel heat sink, the microchannel heat sink with rhombus fractal-like units may increase the COP by 7.9%-68.7% under the same constrains of external size, specified heat flux and maximum temperature limitation, which indicates the rhombus fractal-like units may effectively improve the efficiency of microchannel heat sinks. [ABSTRACT FROM AUTHOR]

In order to solve the problem of heat build-up in subway tunnels, a subway heat pump system with capillary network as front-end heat exchanger is proposed. A one-dimensional simplified plate heat transfer calculation method for capillary heat exchanger (CHE) based on surface-heat-source is proposed. Taking into consideration of the changing heat flux during heat transfer, this paper adopts step load, superposition principle, energy conservation and iterative method to establish the heat transfer model of CHE. CHE heat transfer tests were carried out at the Zhiquan Road Station of Qingdao Metro R2 line to prove this model. The results show that the max error of the model is 8.79%. In addition, the impact of such factors as inlet water temperature of CHE, the lining thermal conductivity, the thermal conductivity of surrounding rock and thermal diffusivity of the surrounding rock to the performance of the heat exchanger has been analyzed. • A capillary heat exchanger used in subway source heat pump system is proposed. • A one-dimensional plate heat exchange model for capillary heat exchanger is proposed. • The accuracy of the heat transfer model has been verified by experiment. • The four main influence factor of the capillary heat exchanger capacity are analyzed. [ABSTRACT FROM AUTHOR]

This paper presents the analysis, simulation and performance study of heat transfer in a single U-tube borehole heat exchanger (BHE). Unsteady heat transfer method was used to analyze the heat transfer process of both inside and outside the borehole. Implicit numerical method applied on heat transfer equations obtained from energy balance (accompanied with thermal resistance model) was used to obtain the solution. The variation of mean fluid temperature, average borehole wall, grout and nearby ground temperature as well as borehole loading with borehole depth and time were investigated. Dynamic simulation was also performed to assess the influence of important parameters. The effect of two parameters, fluid mass flowrate and thermal conductivity of grout, on mean fluid temperature, borehole wall, grout and ground temperature as well as on borehole loading and borehole thermal effectiveness were investigated. The outcome of this study can substantially reduce the time devoted for research and to quickly determine the impact of various parameters on performance of vertical single U-tube borehole heat exchanger. Furthermore, the obtained result can be utilized as a reference for the design and optimization of heating and cooling system integrated with ground coupled heat pump system. • Transient heat transfer analysis for both inside and outside the single U-tube BHE was performed. • Modeling equations that govern heat transfer process inside and outside the BHE has been developed. • Variation of borehole loading and effectiveness of BHE with depth and time was examined. • Effect of mass flow rate and grout conductivity on performance of BHE was investigated. • Impact of heat carrier fluids on performance of single U-tube BHE was assessed. [ABSTRACT FROM AUTHOR]

• A characteristic index of frosting suppression, CICO , for ASHPs was defined. • CICO was based on the configuration and operation of ASHPs. • An empirical numerical relationship for CICO was established. • CICO can be used to evaluate the frosting suppression performances of ASHPs. It has been known that for space heating ASHPs, their different configurations and operation may influence their frosting performances. However, there has been not a single index that can be used to comprehensively and realistically evaluate the frosting suppression performances for a space heating ASHP unit by considering all the influencing configurational and operational factors. Therefore, in this paper, a novel characteristic index for frosting suppression based on the configuration and operation of ASHPs has been proposed and its development is reported. Firstly, the characteristic index, CICO , was defined, a detailed analysis on the heat transfer taking place in the outdoor coil of an ASHP was carried out, and the relationship between CICO and temperature difference in the heat transfer between outdoor coil and ambient air (Δ T) was established. Secondly, the description of an experimental setup with four experimental ASHP units is given. The set was used to establish all empirical numerical relationship between CICO and Δ T. Thirdly, using the experimental results, the empirical numerical relationship between CICO and Δ T was established, which suggested that a higher CICO would lead to a smaller Δ T , thus a better frosting suppression performance. The development results suggested that the index of CICO could be used to evaluate the level of frosting suppression performance for ASHPs, and to help improve the frosting suppression performances for an ASHP during its design and operation, to ensure its best possible operation performance. [ABSTRACT FROM AUTHOR]

• A simulation algorithm integrates the newest analytical models for COP and GHEs. • The algorithm is efficient enough for hourly life-cycle simulation of GCHPs. • Simulations explain the mutual-cause coupling between load and ground temperature. This paper reports on an algorithm for hourly simulation of ground-coupled heat pump (GCHP) systems. This algorithm integrates the newest analytical models for COP of heat pumps and for heat transfer of ground heat exchangers (GHEs). These analytical models make the algorithm computationally fast and efficient. The algorithm comprises 7 independent equations for solving 7 system variables, thus including C 14 7 = 3432 simulation scenarios. The algorithm is applied to hourly 50-year simulations of a GCHP for illustrating the computation efficiency of the algorithm. The simulation cases show that the COP and seasonal COP of GCHPs depend heavily on the mutual-cause coupling between the ground-side load and the temperature field in the ground. This coupling effect renders the high-resolution simulation highly necessary. [ABSTRACT FROM AUTHOR]

• Adsorption heat exchanger based on directly crystallized flat tube-lamella design. • Created numerical model for modules with combined evaporator-condenser in Modelica. • Rigorous calibration and validation - outlet temperatures differ 1.1...1.9 K on average. • Simulated efficiency matches the experiment within uncertainty ± 0.02 (±5 %). • Evaluation of heat and mass transfer resistances reveals limitation in the evaporation. Sorption modules are the core component of thermally driven heat pumps and chillers. The efficiency of these devices strongly depends on the advantageous design of sorption modules. In this paper a calibrated and validated numerical model for an innovative sorption module with a combined evaporator-condenser is presented. The adsorption heat exchanger is based on flat tube-lamella design directly crystallized with the zeotype adsorbent silico-alumino-phosphate-34. The prediction quality of the model regarding the efficiency is within the measurement uncertainty (±0.02 kJ/kJ). Besides the good prediction quality of this integral performance indicator, the root mean square deviation of the transient outlet temperatures is in the range of 1.1...1.9 K, which is a very good agreement. Since the performance indicators efficiency and power density strongly depend on the temperature boundary conditions and half cycle times, an in-depth analysis of the experimental data using the method of heat and mass transfer resistances is suggested that overcomes this limitation. This analysis allows for a direct comparison with other sorption module designs. In a first step this analysis revealed that the evaporator-condenser component limits the sorption process during evaporation. Compared to other designs the evaporator-condenser has a 3–5 times higher volume scaled heat and mass transfer resistance (17 dm3K/kW) in the evaporation phase underlining the necessity to further optimize this component in future modules. [ABSTRACT FROM AUTHOR]

Abstract: As a natural refrigerant, carbon dioxide is safe, economic and environmentally sustainable which can be used in heat pump and refrigeration systems especially in transcritical cycles. From the early 1990s, in which the carbon dioxide transcritical cycle began, theoretical and experimental researches, as well as commercial system development, has improved to make the transcritical system performance to a level similar to that of conventional heat pump systems. This paper presents an overview of transcritical carbon dioxide heat pump and refrigeration systems. The paper introduces a summary of the history and main application of carbon dioxide's use as a refrigerant firstly. Secondly, the properties of supercritical pure carbon dioxide and that containing polyalkylene glycol (PAG) lubricants are analyzed and reviewed. In Section 3 the paper began with an analysis of some special characteristics of the basic carbon dioxide transcritical cycle such as the optimum system high pressure and so on, and then followed by a performance analysis and comparison of several novel transcritical cycles. The study finally presents a review of research on transcritical carbon dioxide heat pump systems, which covers the main components and research hotspots, such as heat transfer and expander. [Copyright &y& Elsevier]

Abstract: This paper introduced the concept, potential application and benefits relating to a novel solar photovoltaic/loop-heat-pipe (PV/LHP) heat pump system for hot water generation. On this basis, the paper reported the process and results of characterizing the performance of such a system, which was undertaken through dedicated thermo-fluid and energy balance analyses, computer model development and operation, and experimental verification and modification. The fundamental heat transfer, fluid flow and photovoltaic governing equations were applied to characterize the energy conversion and transfer processes occurring in each part and whole system layout; while the energy balance approach was utilized to enable inter-connection and resolution of the grouped equations. As a result, a dedicated computer model was developed and used to calculate the operational parameters, optimise the geometrical configurations and sizes, and recommend the appropriate operational condition relating to the system. Further, an experimental rig was constructed and utilized to acquire the relevant measurement data that thus enabled the parallel comparison between the simulation and experiment. It is concluded that the testing and modelling results are in good agreement, indicating that the model has the reasonable accuracy in predicting the system’s performance. Under the given experimental conditions, the electrical, thermal and overall efficiency of the PV/LHP module were around 10%, 40% and 50% respectively; whilst the system’s overall performance coefficient (COPPV/T ) was 8.7. Impact of the operational parameters (i.e. solar radiation, air temperature, air velocity, heat-pump’s evaporation temperature, glazing covers, and number of the absorbing heat pipes) to the performance of the system (in terms of efficiencies of the PV/LHP module and the system’s overall performance coefficient COPPV/T ) was investigated individually. The results indicated that lower solar radiation, lower air temperature, higher air velocity and smaller cover number led to enhanced electrical efficiency but reduced thermal efficiency of the module; whereas lower heat-pump’s evaporation temperature and larger number of heat absorbing pipes gave rise to both thermal and electrical efficiencies of the module. The research results would assist in developing a high efficient solar (space or hot water) heating system and thus contribute to realisation of the energy saving and associated carbon emission targets set for buildings globally. [Copyright &y& Elsevier]

Abstract: The article described the dilatancy technology of district heating system with high-temperature heat pump (HTHP). For the exploitation of low-temperature return network sources, a high-temperature heat pump with a heat exchanger was planned. As the core technology of the dilatancy district heating system, HTHP units and heat exchanging systems had been described detailedly through theoretical and experimental analysis. The model was proved to be in good agreement with the experiment data. It could be used for engineering design. In this paper the district heating system in Datong was taken as a case study. The main characteristics of the dilatancy district heating system had been presented in the form of diagrams. The principal advantage of dilatancy technology in the district heating system lies in increasing the temperature difference, reducing the diameters and the initial investment of primary side network. This paper shows clearly the analysis of the dilatancy technology for characterising and improving the performance of district heating systems. [Copyright &y& Elsevier]

Highlights • Typical 15 factors influencing the heat pump performance were investigated. • Yearly heat pump performance was analyzed in the different scenarios. • The Borehole Heat Exchanger installed behaved 8% in sand better than in clay. • The investigated factors affected between 0 and 45.3% the heat pump performance. • The meteorological condition is the most crucial factor. Abstract Borehole Heat Exchanger is one of the main elements of Ground-Coupled Heat Pump and its performance is directly related to the heat pump Coefficient of Performance. In this paper, a comprehensive numerical simulation model was built to evaluate the annual performance of the shallow Borehole Heat Exchanger installed in different soils subjected to land surface seasonal hydrothermal variations. It was found that the shallow Borehole Heat Exchanger installed in sand had a better performance of 8% than in clay. The same performance difference between sand and clay was observed in most of the investigated scenarios. The annual average heat pump performance presented a variation more than 27.2% with the meteorological condition, the grout thermal conductivity and the heat load level. The multi-pipe solution and the carrying fluid velocity also influenced the heat pump performance, ranging between 12.2 and 16.2%. Other factors such as the hydraulic condition, the groundwater flow, the grout volumetric heat capacity, the grout diameter, the shank spacing, the pipe inner diameter, the pipe thickness, the pipe thermal conductivity, the carrying fluid material and the heat load mode had less impact on the heat pump Coefficient of Performance, varying between 0 and 11.6%. These findings could be used as a guideline for the future studies of Borehole Heat Exchanger, especially for the applications installed in unsaturated soils. [ABSTRACT FROM AUTHOR]

Highlights • A novel heat pump water heater with a freezing latent heat evaporator was proposed. • The energy recovery of the novel system was 4.4 times that of the conventional one. • The novel system could save 60% of energy compared with the electrical water heater. • The heat transfer coefficient increased significantly at the moment of freezing. Abstract The temperature of domestic drain water is relatively high, but its flux is sometimes insufficient as a heating source. Thus, the conventional wastewater source heat pump water heater is incapable of meeting user heating demand given the limited sensible heat temperature difference in many situations. To address this problem, a novel in–house heat pump water heater with a freezing latent heat evaporator and assisted by domestic drain water was proposed. A prototype of the system was first constructed to determine the performance of the system. An experiment was then conducted, in which a parallel helical coil tube evaporator was designed and developed to extract freezing latent heat from domestic drain water. Experimental data was obtained in different experimental conditions. Results of the investigation show that the heat pump water heater with a freezing latent evaporator recovers 3.4 times more energy than the conventional wastewater source heat pump water heater, and saves 60% energy compared with the traditional electric water heater. Furthermore, an interesting result is that the heat transfer coefficient increased significantly at the moment the wastewater was just frozen. This paper provides a reference for the wastewater source heat pump water heater to extract freezing latent heat from domestic drain water. [ABSTRACT FROM AUTHOR]

Highlights • The practical energy performance of MD-GHPs applied for space heating is introduced. • The MD-GHPs reduce the space occupation and eliminate thermal imbalance of the ground. • The average temperature of the outlet water from the ground can reach 33.0°C. • The COP of the heat pump reaches 5.43 and the COP hs of heat source reaches 4.58. Abstract This paper introduces the practical energy performance of medium-depth geothermal heat pump systems (MD-GHPs), which use vertical concentric tube as ground heat exchangers (GHEs) to extract heat from the medium-depth geothermal energy for space heating in buildings. Hereby the medium-depth geothermal energy refers to heat embodied in rocks nearly 2–3 km underground with temperature around 70–90°C. The operation performance and energy efficiency of MD-GHPs is tested in five residential buildings in Xi'an, North China. According to the short-term and annual field test results, the energy performance of MD-GHPs as well as the heat transfer performance of GHEs are introduced and analyzed. Results show that the average temperature of the outlet water from the ground can reach 33.0 °C, promoting the performance of whole systems greatly, with the COP (Coefficient of Performance) of the heat pump reaching 5.43 and the COP hs of heat source reaching 4.58. Therefore, the energy saving rate (ESR) of the MD-GHPs can reach 41.0%, 29.3% and 47.5% compared with air source heat pump systems (ASHPs), conventional ground source heat pump systems (SD-GHPs) and gas boilers. [ABSTRACT FROM AUTHOR]

Highlights • Experimental results at high condensation temperatures. • R245fa and its HFO replacements R1233zd are tested. • Different working fluids show diverse heat transfer mechanisms. • New heat transfer and pressure drop correlations are developed. Abstract The fundamental understanding of the thermal–hydraulic performance of working fluids during condensation is important for the optimal design of the condenser in various thermodynamic cycles. This paper is aimed at obtaining flow condensation heat transfer and pressure drop characteristics in a plate heat exchanger during the working conditions of the condenser of either organic Rankine cycle power systems or heat pump units. The selected working fluids are two hydrofluorocarbons, R134a and R245fa, as well as their hydrofluoroolefin replacements, R1234ze(E) and R1233zd(E). Measurements of heat transfer coefficients and pressure drops were carried out with varying saturation temperature, mass flux, and liquid Reynolds number, ranging from 30 °C to 70 °C, 16 kg/m2s to 90 kg/m2s and 65 to 877, respectively. Based on commonly used existing correlations, new heat transfer and pressure drop correlations were developed, including the effect of the surface tension. The experimental data indicate that different heat transfer mechanisms occur at low liquid Reynolds number with the different working fluids. The results suggest higher heat transfer coefficients and pressure drops for R1234ze(E) and R1233zd(E) than for R134a and R245fa at the same working conditions. The new correlations enable significantly better prediction accuracies for the experimental results in this study than existing correlations, indicating that the surface tension is a suitable parameter to consider in mini and micro-scale condensation heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: Based on the problem of underground vertical U-tube heat exchangers in ground source heat pump systems in Shenchi station of Shuohuang railway, the transient heat transfer physical and mathematical model of the soil and borehole temperature field is established. Finite element numerical simulation is made by virtue of MATLAB software. Through the method of numerical simulation, the paper analyzes the soil temperature, obtains the distribution of isotherms around the pipes and the scope of the heat transfer, and understands the heat transfer performance around the pipe intuitively. Based on the model and numerical solution, the paper analyzes the changes of temperature field about the soil around ground heat exchangers in different ways which involve geological conditions operating mode, layout, pipe depth, and pipe coupling interval situation, and then the dates are obtained to help the engineering design of ground heat exchangers. [Copyright &y& Elsevier]

Abstract: Low and moderate geothermal resource can be found in most areas of the world and heat pump system as an efficient device to utilize the geothermal is prevalent. It is important to know temperature field around the heat-exchange pipelines for efficient conversion and use of geothermal energy. The temperature distribution around the heat pipe underground was studied by the heat source method in the paper. Based on the method, every heat pipe could be supposed as a heat source that would produce a corresponding temperature field, and the total temperature underground could be the superposition of every temperature- field. The temperature fields around heat pipes were given in the paper in different containing water rate of the soil, gap rate of the soil and different disposal of heat pipes. It would be beneficial for design and better operation of the heat pipe system underground to use geothermal resource. [Copyright &y& Elsevier]

Highlights • A spray tower with upward spraying and downward gas flow (ST-UD) is proposed. • ST-UD has no restriction on the minimum droplet diameter and no drift eliminator. • A mathematical model was developed for the quick performance evaluation of ST-UD. • ST-UD has high heat and mass transfer performance and particle scavenging performance. • The impacts of operating parameters on thermal performance of ST-UD was evaluated. Abstract This paper presents the design and evaluation of a novel spray tower which called the reversibly used cooling tower with upward spraying and downward gas flow (RUCT-UD). Unlike conventional spray towers, RUCT-UD has no high-resistance drift eliminator and no restriction on the minimum droplet diameter, which gives it the potential to achieve better performance of heat and mass transfer and particle scavenging. A mathematical model was developed and validated by field experiments. This model is highly efficient in calculation due to the simultaneous consideration of rising and falling droplets. Using the model, the heat and mass transfer characteristics were investigated by a parametric study, which provides a theoretical basis for the tower design. After that, the performance of RUCT-UD was compared with that of other spray-type RUCTs. Results show that RUCT-UD is 45% shorter than RUCT-UU (the reversibly used cooling tower with upward spraying and upward gas flow) and RUCT-DU (the reversibly used cooling tower with downward spraying and upward gas flow), meanwhile it could realize a high thermal performance like RUCT-UU. The particle collection efficiency of PM 2.5 for RUCT-UD is 75.9%, which is higher than that of RUCT-UU (48.9%) and that of RUCT-DU (61.1%). The configuration of the proposed RUCT-UD might be applied to other spray towers for the performance improvement. [ABSTRACT FROM AUTHOR]

Highlights • A novel graded-grooves wick design was put forward for flat heat pipe. • An accurate mathematical model is established for mini-grooves flat heat pipe. • Effective thermal conductivity of proposed heat pipe is increased by 12.4%. • Local wick size & vapor chamber volume have great effects on thermal performance. • Grooves wick in slope type is suggested for practical application. Abstract Thermal management of micro electronic devices is one of the major issues for electronics application. Mini-grooves flat heat pipe is suitable to be used for small space electronics rapid cooling due to its high thermal conductivity, simple structure and good temperature uniformity. In this paper, an axial fluid flow and heat transfer mathematical model of mini-grooves flat heat pipe in steady state is built to study the axial distributions of key parameters such as pressure, velocity and wall temperature. As the wick structure plays an important role in the mini-grooves flat heat pipe heat transfer enhancement, the influence of mini-grooves size and wall with grooves along the axial direction on the heat transfer coefficient of mini-grooves flat heat pipe is studied. It is found that aiming at designing the optimum mini-grooves flat heat pipe, the local wick needs to be close to in saturated state, and the vapor chamber needs to be large. In this way, a novel graded-grooves wick design is put forward for flat heat pipe, which can increase the effective thermal conductivity coefficient by 12.4% with heat transfer rate 5.0 W. And the grooves wick in slope type with simple fabrication accords with practical application. [ABSTRACT FROM AUTHOR]

Abstract Liquid desiccant dehumidification system is regarded as a promising and energy-saving solution for dehumidification. A heat pump driven internally cooled desiccant dehumidification system is investigated in this paper. Dehumidifier and regenerator are both internally cooled/heated types compared to previous adiabatic ones, and heating and cooling capacities of a heat pump cycle are utilized for regeneration and dehumidification respectively. System model is established and validated to investigate system performance and effects of different influencing parameters. It's indicated that two main issues, which are restricting the system performance, are encountered, i.e. mismatching property among the three fluids inside the dehumidifier/regenerator and heat offset due to solutions circulating between dehumidifier and regenerator. Heat transfer mismatching ratio (x t) and mass transfer mismatching ratio (x ω) are defined to indicate the degree of mismatching property. A lower solution flow rate and a solution-solution heat exchanger with a better recovery efficiency are highly recommended to obtain a better system performance. Further, an improved system with external pre-cooler and pre-heater was proposed to improve the matching property among the three fluids, and COP sys is found to be increased from 5.96 to 6.68 with an increasing ratio of 12%. x t is found to be reduced from 0.357 to 0.168, and x ω from 0.282 to 0.127. Highlights • Performance of a novel heat pump driven, internally cooled dehumidification system is analyzed. • Key issues restricting performance of the internally cooled system are identified. • Effects of operational parameters on system performance are investigated. • Mismatching ratio are defined to indicate the degree of mismatching property. • An improved system with pre-cooler leads to an efficiency increase of 12%. [ABSTRACT FROM AUTHOR]

An absorption-compression heat pump is a promising way to recover low-temperature waste heat efficiently in industrial applications. In this paper, an advanced ammonia-water absorption-compression heat pump is proposed to recover the sensible heat of flue gas below 150 °C to generate saturated steam at 0.5 MPa (151.8 °C). The sensible heat is cascade utilized in the hybrid heat pump system. The high-temperature waste heat is recovered to generate pure ammonia vapor in the rectifier, and the low-temperature heat is used to evaporate the ammonia liquid. In the ammonia vapor compression process, the gas compression process is combined with a liquid compression process, leading to the clear decrease in power consumption. The simulation results indicate that the coefficient of performance and exergy efficiency of the proposed system reaches 5.49 and 27.62%, which is almost two times and 4.69% higher than that of the reference system, respectively. Subsequently, a sensitivity analysis is conducted to optimizing the key parameters, and the optimums values are obtained. Finally, an economic analysis is adopted to evaluate the economic performance of the proposed system. The payback period of the proposed system is 6.26 years compared to the reference system. This study may provide a new way to produce saturated steam by efficiently using the low-temperature waste heat. [ABSTRACT FROM AUTHOR]

The industrial exhaust flue gas contains tremendous waste heat that cannot be efficiently recovered by conventional condensing heat exchanger. This paper proposes a novel full-open absorption heat pump for the total heat recovery of flue gas. The recovered heat is used for district heating. By applying direct-contact heat and mass transfer without solid transfer interfaces, the novel system can save considerable metallic tube or plate materials and therefore the initial investment can be reduced substantially. A prototype full-open absorption heat pump is developed and tested, achieving the COP (coefficient of performance) of 1.621 and the exit flue gas dew point of 36.2   °C at most. A detailed theoretical model in Eulerian–Lagrangian formulation is established, involving in conjugate heat and mass transfer and particle dynamics. The numerical simulation agrees well with the experiment results. On the basis of the validated model, three critical parameters, including excess air coefficient of the burner, moisture content of the objective flue gas and return water temperature of the district heating network, are selected to study their effects on system performances. As a controlled variable, especially, the excess air coefficient plays an important role by influencing the regeneration of the liquid absorbent in the generator. The full-open absorption heat pump outperforms the condensing heat exchanger by 19.7–178.1% in heat recovery capacity as the return water temperature increases from 40 to 55   °C. [ABSTRACT FROM AUTHOR]

Closed loop ground-source heat pump (GSHP) systems use ground heat exchangers (GHEs) to transfer heat to and from the ground and efficiently provide clean and renewable energy for heating and cooling purposes. Vertical GHEs contain pipes with circulating fluid (loops), which transfer thermal energy between the ground and the fluid. One very common assumption made in designing GSHP systems is that, when installed, these loops remain evenly separated along the length of the GHE, something that due to the nature of construction is rarely true. This can result in thermal interference not accounted for in the design, leading to a potential negative impact on the performance of the system. This paper investigates the effect of this interference, using detailed numerical simulations to compare different geometries, modelling fixed and variable pipe separations. A comprehensive parametric analysis is conducted to identify some of the most influential design parameters and the potential consequences on running and capital costs. Amongst the key findings of this study is the importance of the borehole filling material, as a highly thermally conductive material can minimise these negative effects from the thermal interference by up to 60%. Moreover, potential increases in drilling (capital) costs of up to 24% are shown, while the potential increases in running costs due to the reduced efficiency were found to be relatively minor. [ABSTRACT FROM AUTHOR]

This paper presents a derivation of design guidelines for plate heat exchangers used for evaporation of zeotropic mixtures in heat pumps. A mapping of combined heat exchanger and cycle calculations for different combinations of geometrical parameters and working fluids allowed estimating the trade-off between heat transfer area and pressure drops on the thermodynamic and economic performance indicators of the cycle. Compressor running costs constituted the largest cost share, and increased due to a steep decrease of the heat pump coefficient of performance at high refrigerant pressure drops. It was found that the pressure drop limit leading to infeasible designs was dependent on the working fluid, thereby making it impossible to define a guideline based on maximum allowable pressure drops. It was found that economically feasible designs could be obtained by correlating the vapour Reynolds number and the Bond number at the evaporator inlet as Re V − 0.42 Bd 0.26 ≈ 0.040 . The use of the proposed guideline was illustrated for the mixture Propane/Iso-Pentane (0.5/0.5), leading to evaporator designs with net present values deviating maximum −4.4% from the best value found in the mapping. The presented methodology can be applied in different scenarios to develop similar guidelines, thereby decreasing the cost of combined cycle and component optimizations. [ABSTRACT FROM AUTHOR]

Highlights • Suitable working fluids for high temperature heat pumping up to 125 °C are evaluated. • Technological, safety and environmental limitations constrain working fluid selection. • The operating parameters and performance of each fluid are compared. • R600 was found to have the highest performance and flexibility for the considered conditions. Abstract Industrial processes with applications such as pasteurization, drying and distillation have heat demand at a temperature between 100–125 °C. Industries with large amounts of waste heat and cooling demand will benefit from the development of high temperature heat pumps (HTHPs) which can replace conventional heating and waste heat disposal systems. This potentially saves both investment and operational cost of a boiler and a cooling tower with a single more efficient and less complex energy system. At the same time, the environmental footprint is reduced. The working fluids for these high temperature heat pumps using the vapour compression technology are often constrained by their high compressor suction and discharge temperature as well as the safety and environmental impact of the fluids. Within these limits, this paper conducts a theoretical analysis to evaluate available and potential fluids with thermodynamic properties that are applicable for HTHPs. The evaluations show that certain hydrocarbons and halocarbons are the most promising fluid candidates for waste heat upgrade from low temperatures to high temperatures, up to 125 °C. [ABSTRACT FROM AUTHOR]

Abstract: A large number of ground-coupled heat pump (GCHP) systems have been used in residential and commercial buildings throughout the world due to the attractive advantages of high efficiency and environmental friendliness. This paper gives a detailed literature review of the research and developments of the vertical-borehole GCHP technology for applications in air-conditioning. A general introduction on the ground source heat pump system and its development is briefly presented first. Then, the most typical simulation models of the vertical ground heat exchangers currently available are summarized in detail including the heat transfer processes outside and inside the boreholes. The various design/simulation programs for vertical GCHP systems primarily based on the typical simulation models are also reviewed in this paper. Finally, the various hybrid GCHP systems for cooling or heating-dominated buildings are well described. It is found that the GCHP technology can be used both in cold and hot weather areas and the energy saving potential is significant. [Copyright &y& Elsevier]

The paper focuses on the operation of simple refrigeration cycles. With equipment given, there are, from a control and operational point of view, five steady-state degrees of freedom; the compressor power, the heat transfer in the condenser, the heat transfer in the evaporator, the choke valve opening and the active charge in the cycle. Different designs for affecting the active charge, including the location of the liquid receiver, are discussed. With a given load (e.g. given cooling duty) the compressor power is set. Furthermore, it is usually optimal to maximize the heat transfer. The two remaining degrees of freedom (choke valve and active charge) may be used to set the degree of super-heating and sub-cooling. It is found that super-heating should be minimized whereas some sub-cooling is optimal. For a simple ammonia cycle, sub-cooling gives savings in compressor power of about 2%. In this paper, refrigeration (cooling) cycles are considered, but the same principles apply to heat pumps. [Copyright &y& Elsevier]

The purpose of this paper is to discuss the role of heat pumps in total site heat integration. It will be shown how heat pumps can help de-bottleneck heat transfer so that maximum energy savings are attained. Non-linear targeting models to get maximum cost savings and optimum heat pumping temperature levels are presented. Finally, the paper describes optimal solutions with heat pumps that do not cross the pinch, contradicting an old result from pinch technology. [Copyright &y& Elsevier]

• ILS-based k gr estimations when BHE penetrates a single or multiple ground layers. • q ratio and the g 0 -function needed for performing the FFT spectral method are linked. • If q ratio <1 the g 0,j (τ) function affects the T f,j (τ) profile for each fluid node. • Geothermal gradient affects ILS-based k gr estimations if q ratio far from q ratio >>1. • q ratio indicates if the k gr estimations depart from the weighted-thickness average. The present paper introduces new concepts related to the modeling of vertical Borehole Heat Exchangers for Ground Coupled Heat Pump applications. A sensitivity analysis on how specific parameters affect the ground thermal conductivity k gr estimation when the Infinite Line Source model is used to interpret a Thermal Response Test has been performed. The study has been conducted considering shallow and deep BHEs, with and without geothermal gradient, and for homogeneous and stratified ground thermal conductivities. The q ratio parameter scales the external heat rate to a natural heat rate associated with the geothermal gradient. The effect of q ratio on the TRT analysis has been related to a specific dimensionless g -transfer function called g 0, which incorporates the geothermal gradient. Three in-house built Fortran90 codes implementing the finite-difference models related to coaxial, single and double U-BHE geometries are exploited to evaluate the dimensionless g -transfer functions related to each fluid volume. A spectral method aimed to reconstruct the fluid temperature profiles by superposing two separated convolutions in the time domain exploiting the Fast Fourier Transform leads to considering q ratio as the dominant parameter when the ILS model is used to estimate k gr. In the case of a single-layered subsurface, q ratio >>1 guarantees the correct ILS-based k gr estimation for any BHE geometry. In the coaxial center-pipe inlet case with a single-layered subsurface and q ratio <1, the ILS-based k gr estimation when the g 0 -function is taken into account can differ by -14% from the correct ILS-based k gr estimation without taking into account the g 0 -function. In the case of a multilayered subsurface, the q ratio parameter indicates when the effective k gr estimated by the ILS model departs from the weighted-thickness average. A departure of 10% occurs for q ratio between 2 and 2.5 for the coaxial center-pipe inlet cases considered and the departure increases with decreasing q ratio. [ABSTRACT FROM AUTHOR]

The use of heat pumps in district heating is one of the most promising technologies for enhancing the efficiency of systems and for meeting the 2030 and 2050 European energy and climate targets. It is essential to assess the available data, the practice and experience of planners and engineers to determine heat pump placement, connection and operational modes in district heating networks, as the systems ought to be capable of covering residential heat requirements for all year around operation. The main selection criteria for the appropriate heat pump system are the heat source, the heat pump technology and the heat requirements. These parameters form a technical triangle, which should be used as a comprehensive instrument to enable heat pump integration into residential district heating. The triangle allows the definition of the bidirectional interdependences and the design of a high efficiency, sustainable heat pump based system. This paper presents the solution to scenarios of how to integrate heat pumps into district heating, in terms of heat pump placement, connection and operational modes based on available heat sources and heat requirement profiles and their environmental impact. [ABSTRACT FROM AUTHOR]