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• Installed demonstrator: high-temperature heat pump with mine thermal energy storage. • Proof of concept for a 2-stage 500 kW ammonia-butane heat pump that delivers 120°C. • Tests proved that the flooded mine is suitable for heated seasonal storage facility. • Heat pump factory tests with 70 K temperature lift showed high performance. • CO 2 emission reduction by means of a solar and geothermal integrated energy system. The research aims to prove that renewable energy sources in the heating sector can enable the energy transition towards European and German CO 2 reduction targets. To do so, at the premises of the Fraunhofer IEG Institute in Bochum a demonstrator for high-temperature heat pumps (HTHPs), coupled with seasonal high-temperature mine thermal energy storage (MTES), was developed. The goal is achieved by demonstrating the feasibility of the technology by connecting mine water present in a small flooded coal colliery (MTES) to solar parabolic trough collectors (SPTCs) and also to a HTHP system that serves the local district heating (DH) grid of Bochum south. The MTES, with an estimated water volume of approximately 20000 m3 is employed as seasonal storage for a 500 kW HTHP system that can reach a supply temperature of 120°C, as needed by the existing DH grid when the ambient temperature is lower or equal to -10°C. During the summer operation, the heat is injected into the MTES from the SPTCs to obtain a mine water temperature of 60°C; and in the winter operation, the heat is extracted through the HTHP until the mine water reaches the unheated temperature of 12°C at the end of the heating season. A literature review of the main components of the plant, the description of the installed system parts and performed tests are reported, along with the results of the successfully executed tests. [ABSTRACT FROM AUTHOR]

• Three parallel compressors more efficient than two variable speed compressors. • R452B versus R410A led to better efficiencies and compatibility at high ambient. • Reliable oil return of three parallel compressors having multiple capacity levels. • Three parallel centrifugal blowers led to improved air flow distribution. This study delineates a meticulous exploration of technologies to enhance the energy efficiency of rooftop air conditioning units, employing the DOE/ORNL heat pump design model for comprehensive engineering design and optimization. A baseline rooftop air conditioning unit, featuring a 13 ton (45.7 kW) cooling capacity and a 17.9 integrated energy efficiency ratio, served as the point of departure for substantive efficiency enhancements. Key modifications included the consolidation of two refrigerant circuits into one, integrating three parallel 2-stage (dual-speed) compressors, fan replacements with high-efficiency substitutes. Notably, a lower global warming potential refrigerant, R452B, was evaluated as a substitute for R-410A, demonstrating better performance in the lab prototype. The achieved measured integrated energy efficiency ratio of 21.4 in the lab prototype surpassed the baseline integrated energy efficiency ratio. Comparative evaluations between R410A and R452B indicated heightened efficiency with the latter, showcasing a lab-demonstrated integrated energy efficiency ratio of 22.4 at the rated capacity of 13.8 ton (48.5 kW) and 23.9 integrated energy efficiency ratio at the rated capacity of 10 ton (35.2 kW). This research underscores the successful development of a rigorous, energy efficient rooftop air conditioning unit prototype with noteworthy environmental and economic implications. [ABSTRACT FROM AUTHOR]

• The operating performances of a CASHP in LTS and HTS were tested. • Compressor speed plays opposite roles in LTS and HTS to affect the pressure ratio and COP. • Changes of the compressor speed in LTS failed to improve the COP of a CASHP unit. • Empirical models predicting COP in LTS and HTS were established. Previous studies on cascade air source heat pumps (CASHPs) mainly focused on overall performances, while impacts of the low temperature stage (LTS) and high temperature stage (HTS) on overall performances were paid less attention to. Therefore, operating performances of a CASHP in LTS and HTS were investigated in this paper, to provide a solution for the optimization of the CASHP. A CASHP unit with a variable speed compressor in LTS and a constant speed compressor in HTS was adopted, and its operating performances were investigated under 26 cases. Results showed that the pressure ratio and COP in LTS and HTS were affected differently by the compressor speed. Specifically, as the speed of the compressor in LTS increased from 30 rps to 90 rps, P rL increased by 151 % ∼ 178 %, and P rH reduced by 38 % ∼ 54 %. Meanwhile, COP in LTS reduced by 21 % ∼ 52 %, but the COP in HTS rose by 54 % ∼ 96 %. Besides, it was found that adjustments of the compressor speed in LTS could improve heating performances of the CASHP. However, when the ambient air temperature and the supply water temperature were consistent, COP changes of the CASHP were limited, only by 0.3 % ∼ 9 %. Furthermore, empirical models of the COP in LTS and HTS were established to predict the COP of the CASHP on different operating conditions. Results of this study were helpful in promoting the configurations and operation control optimization of CASHPs. [ABSTRACT FROM AUTHOR]

A novel methanol steam reforming system is proposed to enhance the utilization efficiency of waste heat in hydrogen production from methanol. The system is integrated with a monoethanolamine carbon capture system and a two-stage compression heat pump system. In this integrated system, the high-temperature products of the methanol steam reforming system exchange heat with the feedstock and undergo gas-liquid separation. The low-temperature gas produced is pre-compressed and utilized for heating the feedstock. Furthermore, a two-stage heat pump uses the low-temperature lean solution from the carbon capture system as a heat source for feedstock heating. The analysis is conducted based on a hydrogen production capacity of 200 kg/h, revealing an enhanced energy and exergy efficiency of 5.4% and 3.5%, respectively, compared to the reference system. Additionally, waste heat utilization efficiency increases from 62.1% to 92.0%. The cost of hydrogen production is reduced from 3.089 $/kg to 2.054 $/kg, which is very close to the cost of hydrogen production without carbon capture. The exergoeconomic analysis indicates that the introduction of energy levels enhances the value of high-temperature steams, and the hydrogen production unit exergy of cost rate is elevated from 3.443 $/kWh to 4.411 $/kWh. The proposed system can readily harness low-grade waste heat and exhibits high thermodynamic efficiency, presenting a promising approach for low-energy-consumption methanol-hydrogen production. • A low-energy, low-carbon methanol-based hydrogen production system is introduced. • Waste heat utilization of the integrated system is enhanced from 62.1% to 92.0%. • The overall energy and exergy efficiencies of the system reach 86.7% and 77.3%, respectively. • Hydrogen production cost is reduced from 3.089 $/kg to 2.054 $/kg by recovering waste heat. [ABSTRACT FROM AUTHOR]

• A novel parameter is introduced to refine the refrigerant leakage theory. • The refrigerant leakage process carries a higher oil content is confirmed. • The effect of leakage mass flow rate on oil content is investigated. • The oil content is highest when refrigerant leaks from the high-pressure pipe. Refrigerant leakage is a situation that often occurs in refrigeration equipment such as heat pumps. However, certain alternative refrigerants, while exhibiting superior environmental performance, possess varying levels of flammability. The leakage of flammable refrigerants into confined spaces can pose a significant risk of explosion. The amount of lubricant in the leaking refrigerant will undoubtedly affect the flammability characteristics of the refrigerant. In this paper, the content of PVE32 oil in the leaking refrigerant during the operation of a heat pump was investigated. The results showed that, under the five leakage rate scenarios set up in the experiment, the oil concentration was highest when the leakage occurred at the condenser inlet and lowest when the leakage was at the evaporator outlet. During the leakage process of the experiment, as the mass flow rate of the leakage increased, the oil content in the early stage of the leakage increased, but the oil content in the late stage of the leakage decreased. The reasons for the change in oil content were analyzed by the oil circulation rate and the degree of phase separation. The decrease of the oil circulation rate and the change of the degree of phase separation were the key factors affecting the magnitude of the oil content as the leakage proceeded. The results of the study reveal the leakage characteristics of lubricating oil with refrigerant leakage. [ABSTRACT FROM AUTHOR]

• The solubility of R1234ze(E) and R1336mzz(E) in two lubricants was tested and compared. • Experimental solubility data were correlated by the NTRL activity coefficient model. • The Henry's constants of R1234ze(E) and R1336mzz(E) in PAG and POE were obtained. Trans-1,3,3,3-tetrafluoropropene (R1234ze(E)) and trans-1,1,1,4,4,4-hexafluorobut-2-ene (R1336mzz(E)) are environmentally friendly working fluids with low global warming potential (GWP) that can be used in compression refrigeration and heat pump systems. In this study, a solubility measurement system was established to assess the solubility of R1234ze(E) and R1336mzz(E) in polyalkylene glycol ether (PAG) and polyol ester (POE) at temperatures (283.15–323.15 K). The Henry's constants for four binary systems R1234ze(E)/PAG, R1336mzz(E)/PAG, R1234ze(E)/POE, and R1336mzz(E)/POE were analyzed and compared. Additionally, The non-random two liquid (NRTL) activity coefficient model was applied to correlate the solubility data in this study. The results indicate that R1234ze(E) exhibits higher solubility in PAG lubricant, while R1336mzz(E) is more easily dissolved in POE lubricant. Moreover, the solubility of R1336mzz(E) in both PAG and POE lubricants shows greater sensitivity to pressure and temperature changes compared to R1234ze(E). The results reported in this study can serve as a valuable guide for the choice of suitable lubricants for R1234ze(E) and R1336mzz(E) in different applications. [ABSTRACT FROM AUTHOR]

• Condensation of zeotropic mixtures in horizontal tube were experimentally studied. • The processes of temperature glide and composition shift were demonstrated quantitatively. • The heat and mass transfer resistances were discussed. • Effects of vapor quality, mass flux and saturation temperature on heat transfer were presented. Zeotropic refrigerant blends have attracted wide interest due to their promises to reduce global warming potential as well as their potential ability to improve the performance of heat pumps and air conditioners. The complex phenomenon of non-isothermal phase change complicates condensation of zeotropic refrigerant mixtures and merits extensive investigation to better understand their performance profiles. The present work experimentally studied condensation heat transfer of R407C and R448A – two R134a-based zeotropic refrigerant mixtures – in a horizontal smooth tube, condensed by counter-current water cooling. Measurements for mass flux were in the range of 120 to 320 kg m −2 s −1 and saturation temperature in the range of 42 to 64 °C. Variation in refrigerant temperature and composition shift during condensation were quantitatively presented. The experimental data showed the effects of mass flux and saturation temperature on the heat transfer coefficients of R407C and R448A in different vapor quality regions. The analysis of heat and mass transfer resistances related to the flow regimes and refrigerant thermophysical properties provides a deep understanding of the condensation heat transfer of zeotropic refrigerant mixtures. The experimental data presented in this work are of value for the design of condensers. [ABSTRACT FROM AUTHOR]

• Non-invasive composition determination from six different measurements experimentally verified. • Composition determination from density data most accurate and robust. • Data from system level heat pump testing sufficient for rapid mixture model evaluation. • REFPROP mixture models for R-1234yf/1233zd(E), R-1234yf /1336mzz(Z), R32/1224yd(Z) and several ternary mixtures thereof proven useful for system level design. Refrigerant mixtures are frequently used in air-conditioners and heat pumps. However, mixtures with high glides of 30 K and more during evaporation are rare, partially because there is an increased concern about composition shifts within the system. An easy and reliable inline composition measurement could alleviate these concerns and simplify the handling of systems with high-glide mixtures. This study investigates six composition determination methods, which are all non-invasive, five of which can be conducted during system operation. All methods were applied to all available mixtures in a dataset of 380 data points. The binary and ternary mixtures tested consist of the refrigerants R-1233zd(E), R-1336mzz(Z), R-1234yf, R-1224yd(Z), and R-32. Each method was applied to all datapoints collected with a certain refrigerant mixture at varying operating conditions. The average of the calculated mass fraction was then compared to the manually charged mass fraction. For the density method, this difference of the calculated and measured mass fraction was always less than 0.03 and usually smaller than 0.01 across all tested mixtures. Most other methods performed well with deviations typically less than 0.05. Some of the methods require only low-cost sensors like thermocouples and pressure transducers. Property data of ternary mixtures is also discussed in the study but a composition determination, theoretically possible combining any two of the six methods, is not attempted in this study. [ABSTRACT FROM AUTHOR]

• COP improvements of up to 16 % experimentally shown for high-glide mixtures compared to pure refrigerants. • COP of mixtures more robust against changes of operating conditions than pure fluids. Climate change necessitates the reduction of primary energy consumption. Industrial heat pumps can contribute by replacing oil and gas fired boilers in various applications. To cater the application, heat pumps must often impose temperature differences of 25 K or more to the heat source and heat sink, causing great irreversibility if evaporation and condensation occur at constant temperatures. Many theoretical studies suggest that high-glide refrigerant mixtures designed to match the temperature differences have a superior COP for such applications. In contrast to the numerous theoretical studies, systematic experimental studies are missing. This study tests high-glide mixtures at varying compositions at constant operating conditions. COP improvements of 16 % are shown comparing the best mixture against the best pure fluid. The improvement was established despite a significant degradation in the heat transfer coefficient, which could presumably be alleviated with larger heat exchangers. Additional parametric studies are conducted on the temperature differences of heat sink and source as well as the temperature level. The results show that mixtures not only outperform pure fluids at certain operating conditions but can maintain a higher COP across varying operating conditions. The binary and ternary mixtures in this study have glides between 10 and 40 K and consist of R1336mzz(Z), R1233zd(E), R1224yd(Z), R1234yf and R32. [ABSTRACT FROM AUTHOR]

• Screening non-fluorinated refrigerants with low GWP and reduced flammability. • Performance criterion characterizing heating capacity for low-charge heat pumps. • The most attractive refrigerants were identified through decision-making techniques. • DME-CO 2 mixtures resulted in improvements in efficiency and levelized cost of heat. • Further investigations were recommended on mixtures with DME, Propylene and CO 2. The study investigated the characteristics of different natural refrigerants and their mixtures in a residential heat pump with low refrigerant charge. Three main evaluation criteria were utilized for comparing different mixtures: Coefficient of Performance (COP), Volumetric Heating Capacity (VHC), and a newly proposed indicator, the heating capacity at charge limit. Propane was used as a reference refrigerant. It was found that some mixtures significantly improved both COP and the heating capacity at charge limit while maintaining similar volumetric heating capacity and operating conditions. Alternative multi-criteria decision-making techniques were adopted to rank the best refrigerant mixtures. Mixtures rich in Dimethyl Ether (DME), such as DME-CO 2 [0.96-0.04] and DME-Propylene [0.75-0.25] were found consistently among the best options. Those were followed by Propylene-rich mixtures such as Propylene-CO 2 , Propylene-Isobutane and Propylene-Butane. The levelized cost of heat (LCOH) could be improved by up to 12 %. To accelerate the transition to natural refrigerants, without compromises on efficiency and costs, further research and certification activities on non-fluorinated refrigerants based on Dimethyl Ether (R-E170), CO 2 (R-744) and Propylene (R-1270) are recommended. [ABSTRACT FROM AUTHOR]

In cold, moisture-rich winter environments, window fogging represents a substantial safety hazard for drivers. Electric vehicles often incorporate heat pump systems to address challenges such as dehumidification and heating specific to cold weather. Therefore, it is essential to evaluate the dehumidification and heating efficiency of these systems through focused research. This study presents a dual-evaporator heat pump system designed specifically for electric vehicles, equipped with two distinct modes for dehumidification and heating. The research examines how factors such as inlet air volume and the degree of opening of the electronic expansion valve affect the system's dehumidification and heating performance. Experimental analyses were conducted to explore the system's response under various conditions of inlet air humidity and compressor speed in both modes. Results suggest that increasing inlet air volume improves dehumidification effectiveness but may reduce heating performance. Likewise, a wider opening of the electronic expansion valve enhances heating but could decrease dehumidification efficiency. Importantly, the study indicates that when the relative humidity of the inlet air exceeds 70 %, a single evaporator mode is more effective for dehumidification. However, when the relative humidity is below 70 %, the dual evaporator mode is more advantageous, showing better heating performance. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the effects of a decreasing refrigerant charge on the performance and operating conditions of an R513A water-to-water heat pump equipped with a large liquid receiver, an electronic expansion valve, and a vapor accumulator at the compressor inlet. The mass of the refrigerant inside the machine is progressively extracted to simulate refrigerant leakages occurring during the normal operation or in case of failures. The results show that the subcooling is the parameter mostly affected by a refrigerant charge variation since it rapidly collapses to 0 K with the charge reduction. Furthermore, it is possible to identify three different zones in which almost all the properties analyzed (COP, heating capacity, operating pressures and expansion valve opening) exhibit peculiar trends with the charge decrease: subcooling sensitivity zone, constant parameters zone, and compressor failure risk zone. The extension of these zones is determined by the size of the liquid receiver which is installed in the system and, for the heat pump under consideration, is between 100% and 95% of the initial charge for the first zone, between 95% and 40% for the second zone, and below 40% for the third zone. [ABSTRACT FROM AUTHOR]

Optimal operation and design of ejectors are the subject of recent concerns, especially for the enhancement of refrigeration and heat pump cycles based on natural refrigerants like carbon dioxide. In this study, a thermodynamic analysis of an ejector-based refrigeration cycle is performed to determine both what operating pressures lead to the highest physically possible performance depending on the ambient conditions, and what are the main dimensions of the ejector leading to the best performance at a given ambient temperature. A state-of-the-art thermodynamic model for the prediction of the ejector performance is for the first time utilized to generate reliable operation and performance maps of the ejector cycle. Most notably, it is found that the optimal coefficient of performance is not necessarily found when the ejector operates at critical conditions but mostly when the device is under off-design regime, depending on the ejector internal efficiency and the hot side temperature. In addition, the analysis reveals that the performance of the cycle is not highly sensitive to the throat area ratio of the ejector given that the latter lies within an acceptable range. Those findings contribute to getting a better understanding of how the cycle benefits from the ejector and define design and control strategies for the cycle. [ABSTRACT FROM AUTHOR]

• Conducted a numerical study on the leakage of R1234yf and R290 into the passenger compartment of electric vehicles. • Evaluated and compared the volume concentrations of R1234yf and R290 with experimental data. • Discussed the volume concentration distributions of R1234yf and R290 within the passenger compartment in detail. • Analyzed and compared the highest volume concentrations of R1234yf and R290 against their respective LFL. Electric vehicles that utilize a heat pump system have a refrigerant charge increase of at least 400 g compared to traditional fuel vehicle air conditioning systems. If combustible refrigerants are used, the risk of combustion increases when the refrigerant leaks and spreads to the passenger compartment. This paper dynamically monitored the volume concentrations of combustible refrigerants R1234yf and R290 as they leaked and entered the passenger compartment accompanied by air supply by numerical simulation. The results indicated that refrigerants are more prone to accumulate in the rear row than in the front. After a leak, the average volume concentration of R1234yf at the four air outlets was 1.58 %, and 3.36 % for R290. at the breathing points of four passengers, the average volume concentration was 0.99 % for R1234yf and 2.39 % for R290. Near the feet of the passengers, the average volume concentration was 0.95 % for R1234yf and 2.27 % for R290. The highest volume concentration of R1234yf in the passenger compartment was below its LFL, whereas all monitoring points for R290 exceeded its LFL. Compared to experimental data, the difference in maximum refrigerant volume concentration was approximately 1.2 %. [ABSTRACT FROM AUTHOR]

Vapor injection technology represents a highly promising avenue for enhancing the efficiency of heat pump systems within electric vehicles, especially in challenging cold ambient conditions. Although a simplified isentropic process is commonly employed to assess the thermodynamic functioning of scroll compressors with vapor injection (SCVI), it diverges significantly from actual operational dynamics. This study introduces a sophisticated 1D mathematical model that incorporates key factors such as internal leakage and thermal losses, thereby providing a more accurate representation of SCVI's operational realities. The research includes comprehensive performance evaluations of a short wrap profile SCVI, with a specific focus on low-temperature ambient conditions, supported by rigorous experimental validation. Comparative analyses against non-injection scenarios reveal notable enhancements, including a maximum 17.2 % increase in mass flow, a 10.5 % rise in heating capacity, and a 2.15 % improvement in heating COP. Both the simplified isentropic process calculation model and the enhanced 1D mathematical model are utilized to analyze compressor operations. The integration of internal leakage and heat loss considerations significantly narrows the gap between calculated and experimental results for heating capacity and discharge temperature, reducing discrepancies from nearly 20 % to a mere 4 %. This refined mathematical model demonstrates a high level of alignment with experimental data, achieving an accuracy within 5 % when assessing the compressor's real-world operational dynamics. [ABSTRACT FROM AUTHOR]

• Low dew-point desiccant wheel systems are summarized and discussed in detail. • Single-stage and two-stage low dew-point desiccant wheel systems are examined. • Cold and heat sources are key factors affecting energy efficiency. • Optimization measures to improve dehumidification capacity are summarized. Humidity control is very important in production and life, especially in low dew-point industries where the dew-point temperature is less than 0 °C. A low dew-point desiccant wheel system (LDDWS) can meet the ambient humidity ratio required for production well. The mechanisms, configuration, and optimization of the LDDWS are comprehensively reviewed. The air heat-moisture handling processes of the LDDWS are initially introduced. The influence of the cold and heat source configurations on the dehumidification capacity and energy consumption is illustrated. Free cold sources, renewable heat sources, and heat pump technology can effectively increase energy utilization, thereby reducing refrigeration consumption and promoting energy savings. The optimization measures of system performance, including efficient desiccant materials, desiccant wheel partitioning, heat recovery technology, and low dew-point return air recycling, are presented. These optimization measures can effectively improve the dehumidification capacity and reduce the consumption of primary energy. In addition, to enhance the further development of LDDWS, this paper presents prospects for future developments in terms of industrial demand, dehumidification capacity, thermodynamic cycle and energy utilization. [ABSTRACT FROM AUTHOR]

• Impact of refrigerant leakage on R513A when replacing R134a. • R513A has better stability in response to refrigerant leaks. • Refrigerant leakage affects both direct and indirect carbon emissions. • A 20 % leakage could increase indirect carbon emissions by 6.3 %. The escalating threat of global warming has highlighted the imperative to address the greenhouse effect. R513A is recognized as a viable substitute for R134a, providing a lower global warming potential (GWP) while preserving similar thermodynamic properties. However, refrigerant leakage is one of the common faults in heat pump equipment. For industrial heat pumps, refrigerant leakage can make the system less stable and affect normal industrial production, while long-term leakage can also affect the carbon emissions of the industry. When substituting refrigerants, it is crucial to consider not only their distinct properties under typical operating conditions but also the stability and environmental impact of the alternative refrigerant in cases of leakage. This paper focuses on experimentally evaluating the impact of using R513A to replace R134a on the performance of refrigeration systems under the condition of rapid refrigerant leakage. Then the life cycle climate performance evaluation (LCCP) theory is used to assist experimental results in evaluating the carbon footprints of R513A and R134a systems at different annual leakage rates. The results show that R513A has better stability than R134a when responding to rapid refrigerant leakage. This paper determines the changes in annual electricity consumption and indirect emissions under several annual leakage rates and finds that the impact of leakage on indirect emissions is also not negligible. During the utilization phase of the equipment, when leakage was taken into account, the carbon emissions of the R134a system were higher. [ABSTRACT FROM AUTHOR]

In response to the ongoing optimization of energy systems and the promotion of clean energy, this paper introduces a new high-efficiency cogeneration system based on proton exchange membrane fuel cells (PEMFC). Named the Transcritical Combined Cooling Heating and Power with Subcooling Coupled ORC (CPTSO) system, it undergoes energy, exergy, economic, and environmental (4E) assessments to evaluate its performance in both summer and winter modes. This paper also analyzes the effects of key factors on the new system's performance. The results indicate that the new system performs more effectively overall and achieves faster cost recovery during winter operations. With varying degrees of subcooling (ΔT subcooling), the system's average energy efficiency, fuel energy saving ratio (FESR), exergy efficiency, and pollutant reduction in winter are 60.97 %, 4.51 %, 17.16 %, and 33.35 % higher, respectively, than in summer. Changes in the main evaporation temperature (T e) result in winter improvements of 74.37 % in energy efficiency, 16.86 % in FESR, 17.6 % in exergy efficiency, and 45.5 % in pollutant reduction compared to summer. Similarly, adjustments to the outlet temperature of the gas cooler (T g) lead to winter increases of 56.75 % in energy efficiency, 0.49 % in FESR, 17.1 % in exergy efficiency, and 29.3 % in pollutant reduction over summer values. [ABSTRACT FROM AUTHOR]

This paper presents a conventional air source heat pump cycle (CHPC) and two dual-temperature air source heat pump cycles (DTHPC1 and DTHPC2). DTHPC1 employs two condensers, while DTHPC2 adds an extra compressor and combines the ejectors on top of DTHPC1. Compared with the conventional heat pump cycle, the new dual-temperature heat pump DTHPC2 can provide hot water at one additional temperature and still have high performance. In this paper, the refrigerants R1234yf and R1234ze(E) were selected as suitable for the cycle conditions. The cycle performance under different conditions was simulated and compared based on energy analysis methods and exergy analysis methods. The main performance parameters included COP h , η ex , etc. The results demonstrate that the COP h and η ex of DTHPC2 and DTHPC1 are greater than those of CHPC under identical conditions. Specifically, at an ambient temperature of approximately -10 °C, high-temperature hot water of approximately 65 °C, and low-temperature hot water of approximately 35 °C, the COP h of DTHPC2 and DTHPC1 increased by 45% and 32.7%, respectively, in comparison to CHPC. Similarly, the η ex of DTHPC2 and DTHPC1 increased by 27.1% and 28.9%, respectively. [ABSTRACT FROM AUTHOR]

Within the realm of sustainable heating technologies, this study examines the performance of a Stirling heat pump employing chemically reactive working fluids in contrast to conventional inert counterparts. Reactive working fluids are energy vectors that enable the conversion of not only thermal but also chemical energy within the heat pump. The investigation spans a wide range of theoretical reactive gaseous mixtures, leveraging the ideal gas mixture thermodynamic model. Each fluid is characterized by an equilibrated chemical reaction, denoted as A 2 (g) ⇄ 2 A (g) , and distinguished by a set of reaction coordinates: the standard entropy change of reaction and standard enthalpy change of reaction. The chemical reaction evolution and thermodynamic properties are observed in each transformation, and the overall coefficient of performance (COP) of the system is evaluated and benchmarked against that of comparable inert working fluids. It is observed that the exothermic reaction during isothermal compression significantly increases the thermal energy supplied to the heat sink, as well as the thermal energy density per unit maximum volume, by up to 269 %, compared to an inert gas system. However, for the majority of reactive fluids studied, chemical reactions introduce irreversibility in the internal regenerator due to heat transfer across a finite temperature difference, contrary to the case of inert working fluids, penalizing the COP. Consequently, a reduction of up to 28 % in the COP is observed. Nevertheless, there exists a range of reactive fluids, characterized by reversible heat exchange in the internal regenerator, offering increased thermal energy transfer to the heat sink without compromising the COP. [ABSTRACT FROM AUTHOR]

• Residential heat pumps in cold climates were analyzed through an in-house model. • CO 2 -based mixtures are used as refrigerant of a combined demand heat pump. • The refrigerant mixture of 90% of R32 and 10% of CO 2 obtained the best performance. • Refrigerant mixtures rich in CO 2 reduced the heat pump pressure ratio. • CO 2 -rich refrigerant mixtures were able to return more compact heat pumps. Extremely cold climates subject residential heat pumps to significant temperature differences between the heat source and the ambient being heated, which may lead to system failure and reduced compressor performance. The present study considers the possibility of improving the performance of heat pump systems that are simultaneously used for residential space and domestic water heating while subjected to climates varying from -25 °C to 5 °C by exploring the use of zeotropic mixtures. CO 2 -based binary mixtures composed of low-GWP (global warming potential) refrigerants are considered – R32, R1234yf, and R290 – aiming to benefit from environmentally friendly and flame suppressants characteristics of CO 2 , as well as the improved thermal efficiency granted by the addition of a secondary refrigerant. A thermodynamic model was developed for a standard vapor-compression heat pump cycle and used to maximize the coefficient of performance limited by the minimum pinch point in the heat exchangers. Our analysis explores the effect of several parameters, such as, the mixture components, mass fractions, space and water heating demands, and heat source temperature on the heat pump's performance. For cold climates, the mixture of R32 (90%)/CO 2 (10%) yields the highest COP and CO 2 -rich mixtures exhibit the lowest. However, by increasing the mass fraction of CO 2 within the zeotropic mixture, the pressure ratio of the heat pump was improved. When considering combined performance criteria, such as the volumetric heating effect and the total heat delivered related to heat pump size, CO 2 -rich mixtures tend to allow more compact systems, especially in colder climates. [ABSTRACT FROM AUTHOR]

• High pressure density and viscosity of R515B were measured using vibrating-wire viscosimeter in the temperature of 253 K to 363 K when pressure changes from 1 MPa to 12 MPa. • By incorporating the parameterization method of thermodynamic equation of state and the modified residual entropy scaling model from our previous work, a predictive viscosity model for azeotropic refrigerants, considered as pseudo-pure fluids, was proposed. • The variation rule of the viscosity property can be described accurately and reasonably.Please check, thank you in advance. One essential aspect of the studies on the refrigeration and heat pump technology is to search for new alternative working fluids. Meanwhile, the azeotropic mixtures of hydrofluorocarbons (HFCs)/hydrofluoroolefins (HFOs) have attracted researchers not only in fundamentals research field but also in the industry fields due to its good performance and applicability. Therefore, to promote application research, this study is focused on the characteristics of viscosity for azeotrope R515B, which is widely recognized and studied from the thermodynamic aspect. Hence, the high-pressure density and viscosity in liquid phase of R515B were measured with a vibrating-wire viscosimeter within the temperature range in 253 K to 363 K when pressure changes from 1 MPa to 12 MPa. The combined expended uncertainties with a confidence level of 0.95 (k = 2) of density and viscosity are 0.2 % and 2 %, respectively. In addition, a modified viscosity model is proposed with combining the parameterization method of thermodynamic equation of state (EoS) in previous work and the modified entropy variable as well as reduced viscosity reference term of residual entropy scaling (RES) theory. Furthermore, the systematical comparison results among this model and three benchmark viscosity models available illustrate that the RES model proposed in this research is robust and precise in a wide range of operation condition. [ABSTRACT FROM AUTHOR]

Performance of available air-source heat pumps (ASHP) for buildings is extremely influenced by the outdoor air temperature (OAT). When OAT falls below –20 °C, especially in subarctic climate, most commercially available ASHPs lose their defined functionality. To address this issue, a novel cascade heat pump (CHP) is proposed that utilizes two natural (with zero global warming index) refrigerants: ammonia, aka R717, as the low stage (LS) refrigerant, and water, aka R718, as the high stage (HS) working fluid. The proposed system is equipped with two rotary vane compressors for LS and HS. To desuperheat the compressor discharge temperatures and increase the HS heating capacity, liquid refrigerant is injected into the chamber of both LS and HS compressors. The wet compression is thermodynamically modeled and validated against experimental data available in the literature. Optimum design parameters such as the maximum allowable temperatures during compression and the minimum vapor quality after injection are obtained where the COP is maximized. Total exergy destruction, number/location of injectors and the injected amounts are also investigated. The proposed CHP can deliver 6.35 kW heating load at 50 °C with a COP of 1.985 when OAT is –40 °C. From a broader perspective, the proposed CHP not only proves its efficacy in extremely cold subarctic climates but also emerges as a pivotal player in addressing the global warming issue. This is accomplished through a dual approach to decarbonization—firstly, by electrifying heating processes and secondly, by substituting phased-out refrigerants with environmentally friendly natural alternatives. [ABSTRACT FROM AUTHOR]

• A review dedicated to the state on compression high-temperature heat pump steam systems. • The refrigerant and compressor that can be used in compression high-temperature heat pump steam systems are reviewed. • Research trend and the main technical problems of compression high-temperature heat pump steam systems to be solved are proposed. • The progress in developing, manufacturing, and optimizing high-temperature, high-pressure ratio compressors is summarized. • The primary technique for lowering discharge temperature in water vapor compressors is indicated. Compression high-temperature heat pump steam system (HTHPSS) has higher energy efficiency than electric, coal-fired and gas boilers for steam production. The system efficiently recovers waste heat from the factory and the CO 2 emissions can be reduced depending on the driving electricity mix, which is in line with the carbon neutral energy saving and environmental protection development strategy. However, there are few reviews published in the existing literature that specifically address the status of research on HTHPSS. This work not only reviews the status of research on compression HTHPSS in two directions: increasing the efficiency of high-temperature heat pump (HTHP) and increasing the steam temperature, respectively, but also indicates the future development trend of HTHPSS. The key to the research of HTHP is the development of high-temperature refrigerant and high compression ratio compressors. In this work, the refrigerants that can be used in HTHP are reviewed. Natural refrigerants R718 show promise as HTHP refrigerants because they have zero ozone depletion potential (ODP), zero global warming potential (GWP) and a high critical temperature. In addition, the research status of high-temperature and high-pressure compressor with a focus on their design and strategies for reducing the discharge temperature of scroll, piston, and screw compressors. The future focus of research is to develop a high compression ratio for the compressor. Moreover, the three main water vapor compressors, centrifugal, Roots and screw compressors, are introduced respectively. Finally, according to the operational requirements of the compression HTHPSS, research trend and the technical problems to be solved are proposed. [ABSTRACT FROM AUTHOR]

• Two comparable high-temperature heat pumps at different scales are investigated. • Both heat pumps show similar COP trend lines depending on the temperature lift. • Differences in heat losses and heat transfer are related to volume-to-surface ratios. • Experimental data for a water-cooled compressor show promising results. • Analysis of over 200 data points shows the possibility of upscaling laboratory results. High-temperature heat pumps (HTHP) are crucial for decarbonizing district heating and industry. Hence, a reliable performance estimation considering effects of scaling is of interest for cost-efficient system integration. This study compares two laboratory HTHPs with the same refrigerant R1233zd(E). These systems have a similar plant design but a capacity scale factor of 3.2. The aim is to evaluate scale effects and make general valid statements about the challenges of upscaling laboratory test results. The analyses show that it is essential to distinguish between design effects like temperature differences at the pinch point or different heat exchanger surfaces and scale effects like relative heat losses and efficiencies of components and the system. At the component level, large surface reserves in the heat exchangers lead to 3–5 K smaller approach temperature differences in the lower capacity HTHP. At the system level, the relative heat losses are about 15 % higher than in the system with a larger capacity. The coefficient of performance of both HTHPs shows a similar trend as a function of the temperature lift from the heat source to the sink. Cross-referencing these performance results with over 200 data points from the literature, validated the possibility of upscaling. [ABSTRACT FROM AUTHOR]

A novel CO 2 air-source heat pump system (ASHPs) with a secondary throttling based on a single-stage compression was proposed to further improve the heating energy efficiency of electric vehicles (EVs) for heating in cold climates. The heating characteristics of the secondary throttling system (STS) and conventional throttling system (CTS) at different indoor air supply temperatures were compared. The STS performance characteristics were investigated under varying ambient temperature, outdoor air velocity, and exergy analysis was performed on the system and its components. A fitting correlation equation for optimal discharge pressure was obtained based on experimental data. The results demonstrate that implementing STS significantly reduces CO 2 ASHPs energy consumption (W com), enhances the coefficient of performance (COP), and reduces exergy destruction. When the indoor air supply temperature reaches 45 °C, using STS can result in an 8.1 % reduction in W com , a 9.4 % increase in COP, and a 6.1 % improvement in the exergy efficiency (Ex η) compared to that of CTS. Furthermore, the advantages of STS become more pronounced with higher indoor target air supply temperatures. As the ambient temperature decreases, the exergy efficiency of the STS reaching the target air supply temperature decreases. The velocity of outdoor air significantly affects the STS performance, and increasing outdoor air velocity can effectively enhance the Ex η of STS. The optimal discharge pressure fitting equation for the STS was obtained through multiple linear regression analysis. These findings can provide ideas for architectural optimization and design of CO 2 ASHPs for EVs, which are important for further improving system performance. [ABSTRACT FROM AUTHOR]

The performance of a magnetocaloric heat pump (MCHP) consisting of active magnetocaloric regenerators (AMR) of 12 layers of MnFePSi magnetocaloric materials (MCM) with a linear distribution of Curie temperatures was investigated using a 1D numerical model. The model predicted the heating power and coefficient of performance (COP) of the AMR for a fixed temperature span of 27 K, between 281 K and 308 K, and variable flow rate and AMR cycle frequency. A maximum applied magnetic field strength of 1.4 T was used. A well-insulated house with a maximum heating power demand of 3 kW (under quasi steady state conditions) was considered. Ambient temperature in The Netherlands was taken as a reference for the estimation of the seasonal heating power demand. Without optimizing the design of the AMR, the model predicts a maximum single-AMR heating power equal to 43.5 W when the AMR operates at 3 Hz and 3 L min-1, and a maximum COP equal to 5.8 when it operates at 1.5 Hz and 1 L min-1 Considering the maximum heating power of a single AMR, approximately 69 AMRs are needed to provide the design heating power demand of the house. It was found that it is possible to achieve an AMR seasonal COP of 5.6 by continuously adjusting the flow rate and frequency of operation of the MCHP along with the ON/OFF switching of some groups of AMRs in order to adjust the heating power of the MCHP to the heating power demand of the house. • MnFePSi is used for the setup of a 12-layer active magnetocaloric regenerator (AMR). • AMR heating power and COP are mapped against flow rate and frequency using a 1D model. • A part-load operating strategy is proposed for residential magnetocaloric heat pumps. • ON/OFF switching of AMR groups is used along with flow rate and frequency modulation. • A heating AMR seasonal COP of 5.6 is obtained under Dutch climate conditions. [ABSTRACT FROM AUTHOR]

• The article introduces a novel semi-empirical model for a TEV operating with R290. • No prior experimental work was found involving TEV operating with R290 in DX-SAHP. • The article emphasizes a careful data collection process encompassing 150 experiments. • The multiple linear regression resulted in a R² of 0.9648 with predictions not exceeding more than 15 % deviation. • The model was applied and compared with 44 experiments from the literature, resulting in a deviation of less than 15 %. The expansion device plays a crucial role in the operation of heat pumps and refrigeration systems, making its study integral to advancements in the fields of refrigeration and heating. Among various expansion devices, the Thermostatic Expansion Valve (TEV) stands out due to its rapid response time and mechanical nature, which grants it high control capacity over refrigerant superheating. In this study, was developed a semiempirical mathematical model for a TEV incorporated in a DX-SAHP utilizing R290. The research was carried out in an experimental way, conducted using a fully instrumented DX-SAHP. A series of experiments tested different operational modes for water heating, subject to varying solar radiation intensities. A uniform methodology collected operational parameters, resulting in 150 experiments. The semiempirical model creation involved combining basic equations from the literature describing TEV operation and formulating an equation using multiple linear regression based on experimentally evaluated parameters. These parameters included solar radiation incidence, inlet and outlet pressures, and superheating degree. The regression model achieved an adjustment higher than 96 %. Combining this regression with equations suggested by previous researchers, resulted in a model with an adjustment exceeding 94 %. Furthermore, this study culminated in the derivation of a generalized model primed for broad application across various heat pump configurations for water heating purposes. Notably, the model underwent rigorous validation across a separate set of experiments, reaffirming its reliability and versatility. While excelling in steady-state operations, the model's transient data analysis falls short, as intended. [ABSTRACT FROM AUTHOR]

• Focuses on thermophysical properties of propylene glycol solutions at low temperatures. • Experimental thermal conductivity results are higher than ASHREA data. • Experimental viscosity results are lower than ASHREA data. • Experimental specific heat capacity curves do not have the linear trend as most known databases. • Solutions with concentrations below 25 wt-% have higher specific heat capacity than water. More than 1.7 million ground source heat pumps are currently in operation in the European Union and a large majority of systems is using propylene glycol as secondary fluid. In this study the thermophysical properties of propylene glycol solutions with specific freezing points between −5 and −50 °C are investigated at low temperatures of interest for refrigeration and heat pump applications. The experimentally determined data are compared to a large number of data from the literature. It has been found that the freezing point results for all concentrations were in very good agreement with both Melinder (2010) and ASHRAE (2021) data. Moreover, the literature data and present study for concentrations above 40 wt-% have different density curve slopes compared to ASHRAE data. The viscosity results for concentrations below 40 wt-% were up to 21.5 % higher than ASHRAE data and the viscosity results for solutions with concentrations above 40 wt-% were lower by up to 9.7 %. The thermal conductivity results for concentrations above 40 wt-% were 10 % higher than ASHRAE (2021) data. The experimental specific heat capacity curves and other studies do not have the same linear trend as Melinder (2010) and ASHRAE (2021) data. This difference indicates that currently available ASHRAE data have been indirectly computed from thermal conductivity data or that they have not been fully experimentally validated. Finally, more studies are required to investigate concentrations below 25 wt-% to identify solutions with higher specific heat capacity than water that could be of interest for specific applications. [ABSTRACT FROM AUTHOR]

• Heat transfer coefficient of R32 is 50 % higher than that of R410A in 1 mm chevron depth. • Heat transfer coefficient of R410A increases monotonically with mass flux, while that of R32 doesn't. • At low mass flux pressure drop of R32 exceeds that of R410A; the opposite applies at high mass flux. • Some correlations work well with R32 and R410A condensation in small gap plate heat exchangers. Compact brazed plate heat exchangers (BPHEs) with low chevron depths are widely used in new heat pumps and air-conditioners for reducing inventory and improving efficiency. This study analyses the complete condensation of R410A and R32 inside BPHEs with a chevron depth of 1 mm. The effects of the mass flux and chevron patterns (V-shaped and W-shaped) on the heat transfer coefficient (HTC) and frictional pressure drop are investigated. The HTC of R32 is 30–50 % higher than that of R410A, which has a 10–20 % higher friction factor at high mass fluxes and vice versa at low mass fluxes. The HTC of R410A monotonically increases with the mass flux, whereas that of R32 does not. The W-shaped pattern results in 30 % and 15 % higher pressure drops compared with the V-shaped pattern for R410A and R32, respectively. The friction factor of R32 decreases more sharply than that of R410A as the mass flux increases, particularly at low mass fluxes. The correlations developed by Longo et al. and Zhang et al. show the best agreement with the experimental data for the HTCs of R32 and R410A, respectively, with mean absolute percentage deviations (MAPDs) of 15 %. The friction factor correlation of Zhang et al. accurately predicts the pressure drop of R32 for the W-shaped and V-shaped patterns. In addition, it accurately predicts the pressure drop of R410A for the W-shaped pattern, with an MAPD of less than 10 %, but over-estimates it by 37 % for the V-shaped pattern. [ABSTRACT FROM AUTHOR]

The restrictions set by the F-gas Regulation and the Kigali Amendment to the Montreal Protocol have pushed an extensive research into alternatives for fluorinated greenhouse gases in air conditioning and refrigeration. Hydrofluoroolefins (HFOs) are emerging as promising substitutes for hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) in HVAC and refrigeration. Amongst these, R1243zf raised interests as low-GWP substitutes for R134a. The pursuit of low GWP refrigerants has also led to the examination of hydrocarbons (HCs), like propane (R290) and isobutane (R600a), due to their efficiency, low charge, and affordability, despite their flammability. The mixtures of R1243zf and isobutane could be of interest for medium temperature heat pump applications. Moreover, blending HFOs with HCs usually leads to azeotropic mixtures. While HCs benefit of a wide range of data and reliable Equations of State (EoS), HFOs, and in particular their mixtures, lack comprehensive information. This study reports vapor–liquid equilibrium (VLE) measurements for the 3,3,3-trifluoropropene (R1243zf) + isobutane (R600a) binary system in the temperature range between 283.15 K and 323.15 K. The measurements have been carried out by means of a vapor-recirculation apparatus combined with gas-chromatographic analysis. This work reports 53 experimental data with an expanded uncertainty (k = 2) of 0.003 mol mol−1. The reported data, along with the data available in the literature, have been used to developed two new mixture models based on the Helmholtz free energy EoS, both leading to a significant improvement in the VLE prediction, with RMSE values lower than 0.005 mol mol−1 for both the liquid and the vapor phase composition. [ABSTRACT FROM AUTHOR]

• 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]

Degradation of performance over lifetime is a challenge for interstitial metal-hydride hydrogen storage systems, especially those employed in thermodynamic machines such as compressors and heat pumps, since these must execute many thousands of cycles of absorption and desorption with minimal loss of throughput. Degradation manifests typically as diminished reversible hydrogen capacity, sometimes accompanied by undesirable changes in the shape of the absorption/desorption isotherm or slower kinetics. Understanding the origin and evolution of degradation during absorption/desorption cycling is crucial to designing for long service life with minimal maintenance and replacement costs. This review examines the degradation mechanisms observed in interstitial metal hydrides, focussing on those relevant to thermodynamic machines. Based on the reviewed literature, identifying standard measures for evaluating the degradation of metal hydrides in thermodynamic machines proves challenging. This challenge stems from the variety of reported alloy compositions, and from the widely differing operational configurations employed in published research studies. Furthermore, the degradation mechanisms that manifest during extended absorption/desorption cycling (decrepitation, sintering, hydrogen trapping, loss of crystallinity, disproportionation) are intertwined, sometimes acting sequentially and sometimes concurrently. Time spent in the hydride phase at elevated temperature is a key controlling factor. This review offers insights to aid the selection of alloys for service in hydrogen storage generally and thermodynamic machines in particular. Additionally, an in-depth exploration of the degradation of LaNi 5 , by far the most studied hydrogen storage alloy, is presented, highlighting the intertwined nature of the acting degradation mechanisms. [Display omitted] • In-depth review presented of atomistic mechanisms that degrade metal hydrides during cycling. • Focus on thermodynamic machines, distinguishing compressors and heat pumps. • Decrepitation, sintering, hydrogen trapping, loss of crystallinity and disproportionation examined. • Drivers of degradation – elevated temperature, impure hydrogen, prolonged cycling – considered. • Detailed case study on LaNi 5 presented. [ABSTRACT FROM AUTHOR]

International regulations, such as the European Regulation EU No 517/2014, are pushing the refrigeration, heat pumps and air conditioning sectors to find suitable low-GWP working fluids as alternatives to the currently widespread high-GWP refrigerants, such as R134a and R410A. Halogenated olefins (HFOs and HCFOs) have been considered promising and, among these, trans-1,3,3,3-tetrafluoropropene (R1234ze(E)) has been identified as a potential alternative to R134a in medium temperature applications, including air-cooled and water-cooled chillers, heat pumps, refrigerators and combined with CO 2 in cascade systems. The thermodynamic properties of R1234ze(E) have been widely studied in the last decade. Instead, a comprehensive experimental knowledge on the transport properties of R1234ze(E) is still missing. In order to increase the available experimental transport property data, the present study reports thermal conductivity measurements carried out with a new transient hot-wire (THW) apparatus on R1234ze(E) in the liquid phase at temperatures from 243 K to 343 K and pressure up to 8 MPa. The reported uncertainty is estimated to be lower than 2% and the measurements resulted in agreement with the available literature data. [ABSTRACT FROM AUTHOR]

• A high-temperature heat pump steam system using single-screw compressor that recovers industrial waste heat to produce steam at 170 °C is proposed. • The feasibility and reliability of replacing a coal-fired boiler with a high-temperature heat pump steam system was evaluated using a 4E analysis. • A more advanced economic analysis model is proposed in the 4E analysis, which considers the recovery value of waste heat and CO 2 trading prices. Industrial steam is widely recognized as a valuable form of heat currency, possessing considerable social and economic significance. As the industry shifts to low carbon emissions, there is an urgent need to reduce CO 2 emissions from industrial steam generation. The analysis of a high-temperature heat pump steam system (HTHPSS), which recovers industrial waste heat to produce high-temperature steam at 170 °C, includes evaluations of its energy, exergy, economic, and environmental (4E) to determine whether it can replace a coal-fired boiler. The economic analysis considered the recovery value of waste heat and the effect of CO 2 trading prices. Optimization calculations were performed to investigate changes in the waste heat temperature and condenser outlet temperature. The study also offers recommendations for operational conditions and optimization strategies for the HTHPSS. The results indicate a positive correlation between the system coefficient of performance (COP) and the waste heat utilization rate with waste heat temperature and condenser outlet temperature. The maximum COP value is 2.73. The total exergy efficiency initially increases and then decreases as waste heat temperature increases. It also decreases as the condenser outlet temperature increases, reaching a peak value of 43.26 %. The waste heat temperature needs to be at least 59 °C to generate a net profit, with a corresponding payback period (PBP) of 4.40 years. The minimum PBP is 0.03 years, independent of waste heat temperature. The annual emission reduction increases as waste heat temperature increases and decreases as condenser outlet temperature increases. These results indicate that a HTHPSS could replace a coal-fired boiler and offer guidance for design optimization. [ABSTRACT FROM AUTHOR]

• Examined the feasibility of CO 2 heat pump combined with rotor drying of banana slices. • Drying kinetics characteristics and drying quality of banana slices were investigated. • Low-humidity drying can shorten the drying time of low-temperature drying. • Low-temperature and low-humidity drying improves drying quality of banana slices. A CO 2 closed heat pump drying system combined with rotary dehumidification (HPRD) was investigated using both experimental and theoretical approaches. The advantages of HPRD over traditional CO 2 closed heat pump drying systems (THPSs) for drying banana slices were analyzed. After drying, the drying kinetics and quality of banana slices were investigated. The results demonstrated that HPRD reduced the banana slice drying time during low-temperature processing while retaining more color and vitamin C content than THPS. In particular, HPRD increased the drying rate of banana slices dried at low temperatures by up to 41 % compared to the rate obtained by THPS. In addition, it increased the vitamin C content by an average of 12 % and reduced the total color difference by an average of 16 %. And the specific moisture extraction rate of HPRD increases by 26 % compared to THPS. The highest-quality banana slices, which were 3 mm thick and dried by HPRD at 43 °C, maintained a high drying rate and high drying quality. The optimal model of Henderson & Pabis predicted the drying behavior within the experimental range. The results indicated that the drying rate and quality of banana slices dried at low temperatures using HPRD were superior to those dried using THPS. [ABSTRACT FROM AUTHOR]

In light of the global industrial sector's steadfast pursuit of sustainable solutions to mitigate carbon emissions and efficiently minimize energy inefficiencies, the significance of novel approaches to energy optimization becomes increasingly evident. This research presents a novel digital twin concept that is designed to enhance the efficiency and effectiveness of high-temperature heat pumps in industrial renewable energy systems. Through the utilization of real-time data and complex computer modeling techniques, our proposed digital twin model seamlessly presents a comprehensive perspective on energy flows. This approach identifies inefficiencies and delivers practical insights to mitigate waste. The incorporation of this approach into pre-existing eco-conscious renewable energy systems has the potential to greatly enhance the effectiveness, predictability, and long-term viability of industrial processes. The empirical findings, obtained from multiple case studies conducted in industrial settings, provide evidence of the potential benefits in terms of energy waste reduction, and maximize the durability of systems. The results of our initial studies provide a foundation for the utilization of digital twin technologies in the field of industrial renewable energy systems optimization, representing a significant advancement towards a more environmentally sustainable industrial landscape. [ABSTRACT FROM AUTHOR]

• A cascade high-temperature heat pump to produce 1 MW steam at 160 °C is proposed. • District heat is introduced as available heat source for the proposed HTHP system. • Use of alternative low GWP hydrocarbons in the low-temperature loop are examined. • Water injected compressor delivers steam by just 0.9 K superheat directly to sink. • The proposed HTHP shows a better business case compared to the conventional boilers. This work aims to assess and optimize the performance of cascade high-temperature heat pump (HTHP) integrated with district heating (DH) to produce 1 MW steam at 160 °C for the industrial processes. The heat available in the primary loop of the DH network at 80 °C is considered as the heat source; which is cooled down 70 °C through the HTHP evaporator, before supplying the DH secondary loop. The use of alternative hydrocarbons in the low-temperature loop are examined; and considering the gas compressor limitation, the HTHP performance of using each refrigerant is optimized and compared to each other. The optimization results reveal that pentane- hydrocarbon with the highest critical temperature- is the most promising refrigerant to be paired with steam in the high-temperature loop, reaching the highest COP of 2.66. However, concerning safety and compressor sizing issues, butane is an excellent candidate; with volumetric heating capacity (VHC) of about two times more than that of pentane, in the expense of just about 4 % reduction in the HTHP COP. In addition, water injection theoretically controls the steam compressor discharge temperature successfully, with just 0.9 K superheating at the compressor outlet; and reduces its power consumption and the HTHP COP up to 4.3 % and 1.7 %, respectively. Moreover, techno-economic analysis demonstrates that the HTHP technology shows a better business case compared to the conventional natural gas and electric boilers. [ABSTRACT FROM AUTHOR]

• A 2 kW-class FPSHP prototype was designed, built and tested. • The analysis of the distribution of acoustic field and exergy loss were carried out. • The influence of key parameters was studied by simulation and experiment. • The prototype can provide 2253 W of heating with a COP h of 2.76. Compared with conventional vapour-compression heat pump technology, the free-piston Stirling heat pump technology attracts wide attention for its high efficiency, reliability, and environmental friendliness. In this work, a free-piston Stirling heat pump prototype with a two-kilowatt capacity, which can operate with an outside ambient temperature ranging from -20 to 10 °C, was designed, built and tested. First, numerical simulations were carried out to explore the distributions of the acoustic field and exergy loss. Second, the effects of several operating parameters on the heating performance were numerically investigated in detail. Finally, to validate the accuracy of the simulation results, verification experiments were conducted on the experimental platform of a free-piston Stirling heat pump driven by a dual-opposed linear compressor. Experimental data show that with a hot-end temperature of 45 °C and outside ambient temperature of 0 °C, the prototype achieved a heating capacity of 2253 W with a corresponding COP h of 2.76 (based on PV power input) and a relative Carnot efficiency of 39.1 %; in particular, the prototype was able to achieve a COP h of 2.5 when pumping heat from -20 °C to 45 °C for 3.25 MPa mean pressure. The simulation and experimental results demonstrated that the free-piston Stirling heat pump system has good potential to achieve high efficiency as a heating supply in a cold region. [ABSTRACT FROM AUTHOR]

• Reproducibility assessment for dynamic performance rating tests. • Reproducible dynamic emulation of testing conditions within different facilities. • Characterization of dynamic and on-off cycling operation. • Results coherence with COP deviation within 3 % for all examined conditions. • Consistent cycling time intervals and compressor speed modulations for all tests. Current testing procedures for the performance rating of air conditioners (ACs) and heat pumps are conducted while maintaining the valve opening and compressor speed at fixed values, thus providing measurements that are often not representative of the actual field operation of these systems. The proposed emulator-type load-based testing methodology makes it possible to reproducibly test different units under various environmental and load conditions while maintaining the native control of the system active. This study presents and discusses the results of round-robin tests at four different facilities to assess the reproducibility of the proposed testing methodology. A 10-kW R32 ceiling-type system is tested, and promising coherence of the results is found with coefficient of performance (COP) deviation within 3 % for all the examined conditions, including consistent cycling intervals and compressor speed modulation behaviors. [ABSTRACT FROM AUTHOR]

The interstage subcooling vapor-injection technique has been introduced to transcritical CO 2 heat pumps to alleviate performance degradation under conditions of low ambient temperature and high water-outlet temperature. This type of solution holds considerable appeal and finds utility across a wide range of applications. However, the efficacy and efficiency of such systems are contingent upon their management, which frequently requires the implementation of suitable control systems. The primary objective is to enhance the overall system operational performance, resulting in tangible advantages in terms of economic viability, energy conservation, and environmental sustainability. To achieve these objectives, one can utilize both explicit and implicit optimization methods ranging from Model Predictive Control (MPC) to Extremum Seeking Control (ESC), each with its strengths and weaknesses. This paper compares the performance of an ad hoc integrated MPC strategy and a multi-variable ESC one. Based on the results of the system operation employing the latter strategy, a significant linear correlation was discovered between the optimal medium pressure and the optimal discharge and suction pressure. Consequently, an intrinsic self-optimization strategy for the medium pressure was developed. Subsequently, a dynamic system identification was performed on the new system, incorporating this proposed strategy. To optimize the system Coefficient of Performance (COP) while maintaining the water outlet temperature, ensuring the thermal load constraint, the MPC strategy was employed. The main focus of the MPC was to determine the optimal Electronic Expansion Valve (EEV) opening degree and water pump settings. The integrated MPC strategy was evaluated and compared to the multi-variable ESC strategy under various conditions, including fixed design conditions, changing ambient conditions, and step-change water outlet temperature. The results clearly demonstrated the effectiveness and superiority of the integrated MPC. [ABSTRACT FROM AUTHOR]

The Chinese government has considered the transcritical CO 2 heat pump (TCHP) as a promising solution for space heating with the goal of "carbon peaking" and "carbon neutrality." Considering the convenient implementation and the performance improvement, the vapor injection technique should be taken into consideration. This paper presents the experimental and simulation investigation of the SCVI TCHP prototype with energetic and economic analysis. Under the design condition, the SCVI technique could improve the peak COP by 5.02 % with a 10.11 % improvement in heating capacity. Compared to the direct electric heater solution, the annual operation cost was reduced by 11966CNY, 29.03 % higher than the Base TCHP. The payback period was decreased from 8 to 6.9 years due to the application of vapor injection. The effect of ambient temperature on the system performances was evaluated based on the experimental data. As the simulation progressed, a control strategy optimizing both the discharge and the medium pressure was developed, and the influence of inlet and outlet water temperature was studied. [ABSTRACT FROM AUTHOR]

Due to its excellent thermodynamic properties, low GWP refrigerant R290 is found to be promising alternative refrigerant. However, due to its special physical properties, high oil discharge rate and throttle valve blockage may occur for the R290 heat pump system under low temperature start-up conditions. In this paper, the transient characteristics, including the temperature, heat exchange rate, phase change rate, oil discharge rate and the liquid level of the oil sump, of a R290 rotary compressor used in the heat pump system under low temperature conditions are theoretically and experimentally studied. A transient heat and mass transfer model in the compressor is proposed, which considers the phase change of refrigerant in the shell. The experiment was carried out on the start-up characteristics of the compressor and the mathematical model was verified. The results show that high rotation speed and low temperature are the main reasons for the condensation of R290 in the shell. The excessive liquid refrigerant in the shell leads to a high oil discharge rate of up to 22 %. This paper will provide guidance for the use of R290 refrigerant under low temperature conditions. [ABSTRACT FROM AUTHOR]

In the ever-evolving landscape of technology and industry, there's a growing awareness of the need for environmentally friendly solutions. This survey marks a pioneering investigation into utilizing eco-environmentally friendly organic mixtures within indirectly integrated heat pump-driven humidification-dehumidification desalination systems. Its primary objective is to determine the most effective approach for enhancing the performance of the integrated cycles: employing organic blends or implementing structural modifications. The proposed layouts for distilled water production are structured around two distinct scenarios: the first scenario involves a basic heat pump, and the second incorporates a vapor injection heat pump. To scrutinize the devised systems' performance, an exergoeconomic analysis is performed, taking into account a meticulous heat exchanger modeling approach. In addition, extensive sensitivity analysis and NSGA.II optimization are conducted based on the most efficient mixture in terms of electricity consumption at a consistent feed-water flow rate. The findings highlight a notable enhancement in the efficiency of the two developed systems when organic blends are employed instead of pure working fluids, resulting in a substantial reduction in electricity consumption. Specifically, replacing R134a with R22/R142b increased the Gain-Output-Ratio (GOR) by 42.26% for the first scenario and 29.06% for the second scenario. As a result of the cost assessment, distilled water unit costs have decreased by 12.87% and 14.32%, respectively, for the first and second scenarios. Notably, using a ternary blend slightly enhances the performance of the proposed systems compared to when a binary blend is employed. Specifically, the highest Gain-Output-Ratio (GOR) of 12.28 was achieved for the second scenario when using R142b/R22/R236fa, representing a modest 3.2% increase compared to the case where R22/R142b was utilized. Furthermore, optimizing the first and second scenarios and utilizing R22/R142 leads to an improvement of 7.31% and 12.55% in exergetic efficiency and 7.36% and 16.21% in UCDW, respectively. Ultimately, it is evident that incorporating eco-environmentally friendly organic blends in the simple heat pump, alongside their minimal ecological footprint and their role in promoting sustainability and adhering to environmental mandates, stands out as a remarkably efficient choice. In fact, the GOR of the first scenario is approximately 27.66% higher when using R142b/R22/R236fa compared to the second scenario employing a pure working fluid. Also, PP of the second scenario increases from 7.76 years to 9.66 years. [ABSTRACT FROM AUTHOR]

• A modified heat recovery ejector heat pump cycle is proposed. • Comparisons between HREHP, CEHP and ASHP is performed based on energy and exergy methods. • The HREHP achieves approximately 30.0% and 27.0% improvements in COP and PEE over the CEHP. • The PEE of the HREHP is 9.08%-35.56% higher than that of the ASHP. • R365mfc is a promising candidate from the viewpoint of thermodynamic. The paper aims to analyze the performance and identify suitable working fluids for a new heat recovery ejector heat pump (HREHP) system. This system introduces a flue gas waste exergy recover circuit combined with the traditional ejector heat pump. Considering the factors of performance, environmental impacts, flammability, and fluid type, four working fluids are selected (i.e., R245fa, R365mfc, RE245fa2, and R1233zd(E)). Comparative study initially conducted to demonstrate the advantages of the proposed heat recovery ejector heat pump facility. Subsequently, the effects of the operational parameters on the novel system performance are investigated to derive design and operation criteria through energy and exergy analysis. Finally, sensitive analysis of generation temperature (T g), flue gas temperature (T f g , o u t), and supply water temperature (T w) on the proposed system is carried out. Compared with traditional air source heat pump, the application of the HREHP shows higher energy efficiency and lower operating costs at lower ambient temperatures. The results indicate that the primary energy efficiency of the proposed system is 27.0 % and 35.56 % higher than that of conventional ejector heat pump cycle and air source heat pump cycle, respectively, at an ambient temperature of −10 °C. Besides, T g has a stronger effect on the entrainment ratio and heating coefficient of performance; The primary energy efficiency is more sensitive to T w variation. Lastly, the R365mfc and R1233zd(E) show good potential as the alternative for the R245fa. The design methods, evaluation methods, and operational characteristics obtained in this study can provide reference and guidance for the application of heat recovery ejector heat pump technology in practical engineering. [ABSTRACT FROM AUTHOR]

Heat pumps can be switched between cooling and heating mode, requiring accurate charge modeling capabilities to enable the design of heat exchangers for near optimum efficiency in both operating modes. The charge modeling tools also require high-fidelity experimental validation data to tune their predictions. However, little experimental charge validation data is available in the open literature. This study addresses this need by providing complete, high-fidelity, experimental charge data of Round-Tube-Plate-Fin Heat Exchangers (RTPF) with ±2.2% of relative charge-measurement uncertainty and 0.8% of charge-measurement repeatability. In addition to R410A, charge data on two low-Global Warming Potential (GWP) refrigerants, R1234yf and R468C, is additionally collected. Furthermore, an accurate data-driven charge model is developed, tuned by the high-fidelity experimental charge data, with 12.3%, 12.8%, and 12.9% of the Mean Absolute Percentage Error (MAPE) based on the Taitel–Barnea, the Zivi, and the Baroczy void-fraction model, respectively. This experimentally validated charge model can contribute to enabling effective designs of heat pumps with high-accuracy charge predictions. [Display omitted] • High-fidelity experimental charge validation data with ±2.2% uncertainty. • Low-GWP refrigerants, R468C and R1234yf, as well as the baseline R410A. • The accuracies of popular void-fraction correlations are experimentally evaluated. • Tuning methods for a charge model are presented. • The model predicts the refrigerant charge in a heat exchanger with a MAPE of 12.3%. [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]

This study investigates the long-term thermodynamic performance of a direct-expansion solar-assisted (DX-SAHP) system for hot water production in an average Canadian household. Performance is evaluated using realistic time-varying demand profiles and DX-SAHP with and without glazing under cold climatic conditions. A mathematical model for determining the system's long-term thermodynamic performance based on the first law of thermodynamics and heat transfer fundamentals is developed and thoroughly validated. The solution is implemented in MATLAB®. The CoolProp® database is coupled with MATLAB® and used to determine the working fluid's thermal physical properties. Besides, the thermodynamic performance of the DX-SAHP throughout the year, operating with low global warming potential (GWP) and class A safety refrigerants, was evaluated. Moreover, the influence of solar irradiation and ambient air temperature on the DX-SAHP's thermodynamic performance is studied. Results indicate that the monthly average coefficient of performance (COP) varies between 2.5 and 3.8, while the monthly average energy consumption of the system varies between 170 and 275 kWh. The obtained figures for COP and power consumption demonstrate the high-efficiency potential of the DX-SAHP system, making it an attractive alternative to reduce energy costs and CO 2 emissions in residential water heating. In addition, results show that R1233zd gives the highest COPs, followed by R152a, R1234ze, R134a, R1234yf and R32. Besides, the long-term simulation results indicate that the DX-SAHP water heater without glazing has superior thermal performance compared to one with glazing. Also, results show that the COP rises as the ambient temperature or solar radiation level increases. [ABSTRACT FROM AUTHOR]

• Experimental investigation of a natural refrigerant blend in a household heat pump dryer. • Successful drop-in with minor adjustments to the system. • Increased drying performance that result is a shorter drying time. Due to their high global warming potential (GWP), fluorinated refrigerants are being replaced in many heat pump and refrigeration systems. Especially in small systems such as heat pump tumble dryers (HPTD), the replacement of the globally used working fluid in this application, R134a, appears to be manageable. One established alternative is the use of hydrocarbon refrigerants. Within this research project, new zeotropic refrigerant blends of hydrocarbons are theoretically evaluated as retrofit substitutes for R134a in heat pump tumble dryers. The aim is to use the temperature glide inherent to zeotropic mixtures during phase change to increase the heat transfer efficiency and thus, the COP of the system. A mixture of R290 (propane) and RE170 (dimethyl ether) was selected and experimentally investigated in a commercially available HPTD. To assess the new mixture, parameters relevant to the cycle such as pressure, temperature, compressor power, and the dryer moisture extraction rate are compared to those values achieved using the refrigerant R134a. Retaining the heat exchanger and compressor from the R134a system, the system charge with the new mixture was 40 % lower, while the thermal capacity was equal to or higher than that of the R134a system. The increased temperature level supplied by the mixture reduced the drying time and the energy consumption of the HPTD. The investigation showed that the heat exchanger design needed to be adapted to the temperature glide properties of the mixture in order to realize meaningful heat transfer efficiency gains. [ABSTRACT FROM AUTHOR]

This study evaluates a potential coupling between AEC and SOEC through a CO 2 heat pump unit that works in super-critical conditions. In such a way, thermal power requested by SOEC can be provided by recovering heat loss from AEC via the CO 2 heat pump. The coupled configuration is characterized by renewable electric input. Hence, the power is provided for electrolysis without more external thermal resources. The energy, economic and environmental impacts of such intervention have been determined and compared with SOEC in solo mode. The layout is created and simulated in a MATLAB SIMULINK environment. The simulation results show a potential improvement in the total energy efficiency of the whole system without an external heat resource, up to 68%, which is higher than 65% of SOEC itself, thanks to the heat recovery section that is correlated with sCO 2 HP contribution. LCOH varies widely depending on SOEC size from around 2.59 €/kg H2 up to 9.27€/kg H2. • Green hydrogen production. • Coupling layouts between low and high-temp water electrolysis. • Featuring a cutting-edge technology of supercritical CO 2 HP for heat recovery. • Improving total energy efficiency through coupling layout compared. • Lower LCOH in coupling configuration compared with SOEC. [ABSTRACT FROM AUTHOR]