Single-stage Heat Research Articles

Performance analysis of a novel dual exhaust mixed refrigerant heat pump with large temperature lift

The substantial demand for heat energy, ranging from 70 − 100°C in industrial and commercial sectors, presents a significant challenge when utilizing traditional air source heat pumps. The conventional single-stage air source heat pumps often struggle with low system efficiency and poor operating conditions when tasked with heating with large temperature lift. To address these issues, a novel dual exhaust mixed refrigerant heat pump cycle was proposed. By incorporating an intermediate pressure compression stage in parallel, the proposed system can achieve an optimal temperature match in the recuperator, and lead to a significant enhancement in the coefficient of performance (COP). Compared to a single stage mixed refrigerant heat pump cycle, the novel system improves the COP from 5.063 to 5.756, representing an increase of 13.68 %. Concurrently, the exergy loss proportion of the recuperator decreases from 18.8 % to 13.7 %. The proposed system consistently demonstrates superior COP and exergy efficiency regardless of whether the ambient temperature is within the range of 0 °C to 20 °C or the outlet water temperatures between 80 °C to 100 °C are present. These findings provide theoretical guidance for enhancing the performance of high-temperature heat pumps with large temperature lift.

Enhanced characteristic equation method for single-stage absorption heat pumps

The method of characteristic equations aims to describe the part-load behavior of sorption heat pumps and chillers as well as heat transformers as simply as possible but yet still accurately. Based on an approach by T. Furukawa for heat transformers, the method was further developed by F. Ziegler and others for absorption heat pumps and generalized for application in multistage processes. Clever simplifications were made, to represent the cooling capacity Q˙E of absorption chillers with a characteristic temperature difference ΔΔt as a simple linear equation Q˙E=sE·(ΔΔt−ΔΔtmin,E). In ΔΔt, the average hot, cooling, and chilled water temperatures are combined, and the slope and loss parameters, sE and ΔΔtmin,E are constant. However, in practical applications of this established method, inconsistencies arise. For example, the calculated slope parameter sE does not match the slope when plotting simulated or measured cooling capacities against ΔΔt. Furthermore, the loss parameter ΔΔtmin,E is actually not constant.In this work a more precise formulation of the characteristic equation is derived which takes into account that the solution entering the absorber and desorber is generally superheated or subcooled. By means of a domain-wise heat transfer calculation, these effects can be implemented into the method and explain the above mentioned inconsistencies. The new formulation allows for explicit consideration of different heat exchanger designs and cooling water configurations. No iterations or regression analyses are required. Thus, the calculation method can be easily implemented into industrial controllers e.g. for the model predictive control of absorption chillers and heat pumps.

Numerical thermal analysis and structural optimization of cascaded PCM heat sinks for thermal management of electronics

In the present study, an innovative cascaded phase change material (PCM) heat sink using organic PCMs was proposed for electronic thermal management. The design arranges multiple PCMs with progressively lower melting points along the heat transfer path, allowing active control of the melt front. A numerical model with 5 % accuracy was developed to evaluate thermal performance. Based on the validated model, the effects of PCM stage numbers, PCM volume fractions, fin types, and fin volume fractions were analyzed. The fin volume fractions were further optimized using a support vector machine (SVM) and genetic algorithm (GA) methods. Results showed that a three-stage PCM heat sink with pin fins and increasing configurations outperformed single-stage PCM heat sinks, enhancing heat transfer by up to 36.7 %. Additionally, the innovative cascaded PCM heat sink with a uniform 20 % fin volume fraction across the three stages achieved a 38.8 % improvement compared to single-stage configurations. When the fin volume fractions were set to 24.1 %, 17.6 %, and 13.0 % for the three stages, thermal performance of the innovative cascaded PCM heat sink was enhanced by up to 39.8 % compared to single-stage configurations. The findings provide valuable insights for the design and application of PCM-based thermal management systems, potentially leading to more efficient solutions for various engineering fields.

Mathematical modeling and experimental analysis of the double-effect DCMD-heat pump integrated system

High energy consumption represents one of the hindrances in enabling large scale application of membrane distillation. In this work, experimental and simulation studies of a double-effect direct-contact membrane distillation integrated with a single-stage vapor-compression heat pump (DE-DCMD-HP) were carried out for concentrating tap water with the aim to evaluate the energy saving of the integrated system. It was found during our experiments that an auxiliary cooler must be added to remove the excess heat from HP to enable the integrated system to reach the steady-state condition. The temperature-heat flux plots were first utilized to analyze how HP and DE-DCMD affected each other and reveal their coupling mechanism. The simulation results showed that the increase in the first-effect feed inlet temperature, Tfi,1 and the flow rate, V caused the thermodynamic cycle line of refrigerant shift to higher temperature, which led to the increase of the input power of the compressor and the auxiliary cooler, ultimately affecting the water production mass rate, ṁd, the coefficient of performance, COP, the gain output ratio, GOR, and the specific energy consumption, SEC. The experimental and simulation results revealed that increasing Tfi,1 and V increased the permeate flux Nh and GOR and reduced the SEC. The performances for three different DCMD configurations were furthermore evaluated via experiments at constant feed temperature whereby the SEC decreased from 2168 kWh·t−1 for single-effect DCMD to 1085 kWh·t−1 for DE-DCMD to 257 kWh·t−1 for DE-DCMD-HP.

A quasi-two-stage trans-critical CO2 heat pump with in-cycle thermal storage for performance enhancement

To decarbonise the heating sector, there is a strong demand for environmentally friendly and cost-effective heating technologies. Trans-critical CO2 heat pumps offer a sustainable alternative to traditional vapor compression cycles that rely on environmentally harmful synthetic refrigerants. However, a significant challenge within trans-critical CO2 heat pumps is their high throttling loss during expansion, leading to substantial flash gas production and subsequently higher recompression power consumption. In this paper, we apply our Flexible Heat Pump cycle concept to trans-critical CO2 heat pumps to address this issue. A thermal storage unit is introduced into a traditional trans-critical CO2 heat pump cycle to recover heat from the hot super-critical CO2 leaving the gas cooler during charging mode. Once it is fully charged, the stored heat is then used as a temporary heat source for the heat pump’s operation during the discharging mode. Since the recovered heat has a higher temperature than ambient. The resultant flexible trans-critical CO2 heat pump (FlexiCO2-HP) reduces throttling losses and the compressor’s power consumption, leading to a higher average COP (Coefficient of Performance). The obtained results of the FlexiCO2-HP are compared against a single-stage trans-critical CO2 heat pump. For gas cooler temperatures ranging from 35 °C to 60 °C and across various discharge pressures, its COP is potentially 10 % to 40 % higher than the standard cycle under ideal conditions. In addition, the proposed system demonstrates a lower optimal gas cooler pressure across different gas cooler temperatures. Furthermore, this study compares the FlexiCO2-HP with other trans-critical CO2 systems, including those with expansion work recovery and two-stage configurations. The results demonstrate that the FlexiCO2-HP offers improvements in COP for most conditions, though not all. It has the potential to achieve a comparable COP when compared with ejector-based and expander-based heat pumps or two-stage systems, but it is much simpler in design without introducing an extra expander or compressor.

Conversion of high moisture biomass to hierarchical porous carbon via molten base carbonisation and activation for electrochemical double layer capacitor

Biomass-derived carbon for supercapacitors faces the challenge of achieving hierarchical porous carbon with graphitic structure and specific heteroatoms through a single-stage thermal process that minimises resource input. Herein, molten base carbonisation and activation is proposed. The process utilises the inherent moisture of Moso bamboo shoots, coupled with a low amount of KOH, to form potassium organic salts before drying. The resultant potassium salts promote in-situ activation during single-stage heating process, yielding hierarchical porous, large specific surface area, and partially graphitised carbon with heteroatoms (N, O). As an electrode material, this carbon exhibits a specific capacitance of 327F g−1 in 6 M KOH and 182F g−1 in 1 M TEABF4/AN, demonstrating excellent cycling stability over 10,000 cycles at 2 A/g. Overall, this study presents a straightforward process that avoids pre-drying of biomass, minimises base consumption, and employs single-stage heating to fabricate electrode carbon suitable for supercapacitors.

Distributed multienergy and low-carbon heating technology for rural areas in northern China

In the context of China's “double carbon” policy and “rural revitalization” strategy, clean and low-carbon heating in rural areas of the northern China has become a key problem that needs to be urgently addressed. Practice has proved that “coal-to-gas” and “coal-to-electricity” projects have various limitations, such as high operating cost, high carbon emission, and high cost of distribution network upgrade, making them unsuitable for nationwide promotion. To address this issue, this research proposes a distributed multienergy and low-carbon heating (DMLH) system, which integrates the photovoltaic power generation, combined heat and power (CHP) system, heat storage, and newly developed multisource efficient heat pump (MEHP). The MEHP operates by simultaneously absorbing heat from air and waste heat sources of the CHP generation, providing a stable and efficient heat output. The multienergy complementary operating strategy of the DMLH system has been presented, considering variations of heating demand and environmental temperature. Comprehensive case studies were performed under typical and extremely cold winter scenarios to examine the effectiveness of the DMLH system. Simulation results revealed that 66.7 % of the operating cost can be reduced by the proposed system in typical scenario and the heat supply stability of MEHP has been considerably enhanced compared to a single-stage heat pump system. Furthermore, the DMLH system has low-level requirement of distribution network capability, which is conducive to promotion in the rural areas of northern China.

System Optimization and Operating Strategy of Single-Stage Air Source Heat Pump with Thermal Storage to Reduce Wind Power Curtailment

Wind power generation has increased in China to achieve the target of decreasing CO2 emissions by 2050, but there are high levels of wind curtailment due to the mismatch between electricity supply and demand. This paper proposes a single-stage air source heat pump coupled with thermal storage for building heating purposes. The main objective is to find the proper system designs and operating strategy, which can help to avoid peak demand periods while obtaining minimized running costs and reduced wind energy curtailment. Dynamic simulations were performed using TRNSYS to investigate its use in a typical office building based on an actual electricity tariff, wind power, and meteorological data. The proper system designs, including the tank size and thermal storage temperature, were determined to maximize the system’s performance. It was found that a proper combination of the two parameters exists for a specific application. Further, results showed that the use of auxiliary electric heating is necessary for single-stage air source heat pumps to participate in a wind curtailment reduction. The operating strategy of the system was also studied. Results indicate that by implementing a proper operating strategy, non-renewable power consumption can be reduced by 11% for the studied building, with a total wind power utilization of 3348 kWh during the heating season while still satisfying the heating demands of users. These findings can contribute to the green and low-carbon development of the building industry and further enhance the grid’s accommodation capacity for renewable energy sources.

Performance analysis of double-stage heat pump and refrigeration cycles

Multistage refrigeration systems are investigated to/enhance the performance of vapor compression refrigeration cycles based on our previous work on multistage low-grade thermal energy conversion. A double-stage heat pump and refrigeration cycles (D-S) is expected to reduce the irreversible loss in the heat exchange process. However, certain characteristics of independently configured systems, namely, compressor power and coefficients of performance (COPs), have not been sufficiently clarified. This study aims to elucidate highly advanced heat pump and refrigeration systems by comparing D-S, a basic single-stage heat pump and refrigeration cycles, and a two-stage compression cycle. D-S shows higher COPs for both the refrigeration and heat pump cycles compared with the other cycles, especially at high thermal conductances. The reduction of the irreversible loss in the heat exchange process is confirmed via the entropy generation rate; in particular, the COP increases with a decrease in the entropy generation rate. The evaporation and condensation temperatures of the refrigerant approach the temperature of the heat source when the thermal conductance exceeds 150 kW/K for the refrigeration cycle. The heat exchangers exergy destruction of D-S is 169 kW, S-S is 189 kW, the two-stage cycle is 184 kW/K when the thermal conductance is 100 kW/K.

Comparative analysis of thermodynamic performance of high-temperature absorption heat transformers using various ionic liquids as working pairs

The study investigated six sets of ionic liquid (IL) working pairs based on high-temperature vapor-liquid equilibrium data. These sets were modeled using the NRTL activity coefficient model and integrated into a single-stage absorption heat transformer (AHT) for system cycle simulation analysis. A comparison was made with traditional H2O + LiBr working pairs commonly used in AHTs. The study aimed to assess the feasibility of using IL working pairs in high-temperature AHTs to achieve higher output temperatures when dealing with heat sources exceeding 120 °C. Compared to the traditional H2O + LiBr working pair, which has strong COP and ECOP but a limited temperature range, IL pairs offer advantages under various conditions. For instance, H2O + [HMIM][Cl] and H2O + [BMIM][Br] can operate at higher condensation temperatures, providing broader temperature ranges. H2O + [HMIM][Cl] has a wider operational temperature range, suitable for unstable waste heat sources. It also has the highest optimal ECOP value of 0.64 at 197 °C absorption temperature, and shows good cyclic performance, achieving temperature rises of about 78 °C. ILs can maintain stable COPs and circulation ratios over a wide range of absorption temperatures, thus achieving higher temperature rises or meeting the need for higher absorption temperatures. Notably, although the H2O + IL combination exhibits slightly higher exergy loss than the traditional H2O + LiBr pair when comparing exergy losses in the AHT system among different working pairs, its low corrosiveness, lack of crystallization, and wide operating temperature range make these drawbacks insignificant.

Experimental study on performance and compressor characteristics of transcritical CO2 heat pump system

The accelerated advancement of the economy has led to a heightened focus on the issue of energy conservation and reduced consumption within heat pump systems. An experimental investigation was conducted to analyze the impact of discharge pressure and outlet water temperature on the performance of the transcritical CO2 single-stage heat pump system and vapor injection heat pump system. The experimental results show that the heating capacity first increases and then tends to be constant or even slightly decreases with the increase of discharge pressure in single-stage heat pump system. When subjected to the same operating conditions, the coefficient of performance of vapor injection heat pump system exceeds that of single-stage heat pump system. Compared with single-stage heat pump system, the performance coefficient of vapor injection heat pump system has increased by an average of 6.21% and the heating capacity has increased by an average of 9.54%. Additionally, the volumetric efficiency of the compressor substantially decreases when producing high-temperature hot water. Semi-empirical models for evaluating the compressor performance in both single-stage heat pump system and vapor injection pump system are established through the parameterization of the volumetric efficiency with respect to the pressure ratio and compressor speed. These models effectively forecast compressor performance.

Increase of the Efficiency of Heat Supply Units of the CHP Plants Due to the Choice of Rational Heat Release Modes

The possibilities of increasing the efficiency of heat supply turbines of CHPPs due to the choice of rational modes of operation of network water heaters are analyzed. With the help of a software and computing complex developed at the Institute of Mechanical Engineering Problems of the National Academy of Sciences of Ukraine and adapted by the authors to the operating conditions of CHPP generating equipment with one or two network heaters, a set of calculation studies of various ways of connecting them depending on the outdoor air temperature is conducted in the paper. Areas of positive effect associated with increase in the electric power of the power plant have been established. The calculation study was carried out at typical power plant water consumption from 1000 t/h to 4500 t/h, in the range of changes in the outdoor air temperature from -11 ºС to 10 ºС (heating season) and more than 10 ºС (hot water supply). The load change of the power unit was carried out due to the consumption of fresh steam at a constant pressure and temperature at the turbine inlet. As shown by the results of the operation of the T-100/120-130 heat supply turbine in operating conditions with one or two heat supply steam selections, in the area of positive outdoor air temperatures above 2 ºС, it is advisable to use one lower selection (with the upper one turned off) for all network water consumption. At the same time, additional electric power in the area of outdoor air temperatures above 6ºС can be from 0.25 MW to 2.15 MW. However, when the outdoor air temperature is less than 2 ºС, work with one lower heating selection becomes irrational. From the point of view of choosing rational modes of operation of turbine plants, the most important are the results of determining the optimal distribution of heat load between network heaters. The gain in electrical power of the turbine can be up to 2.46 MW in the nominal mode of operation with two heaters, and up to 7.84 MW in comparison with the use of single-stage heating. The nature of the influence of the distribution of the heat load indicates that during deprivation from the instructional uniform distribution of the heat load between network heaters, it is possible to obtain additional electricity.

Performance analysis of a two-stage evaporation heat pump drying system for graded cooling/dehumidification

Single-stage evaporation heat pumps are widely used in drying fields, but they suffer from relatively low efficiency. To enhance the performance of conventional heat pump drying systems with single-stage evaporating, a two-stage evaporation heat pump drying (TSE-HPD) system is proposed in this paper for graded cooling/dehumidification. In this innovative system, the quasi dual-stage compression, matching a double temperature level evaporation process, achieves graded cooling and dehumidification of air. Its performance is investigated using energy and exergy analysis methods and compared with that of a single-stage compression heat pump drying (SSC-PHD) system. The results indicate that the TSE-HPD system exhibits 15.5 %–19.3 % improvement in the coefficient of performance (COP) and 16.3 %–18.9 % enhancement in the specific moisture extraction rate (SMER) compared to those of the SSC-PHD system when the condensing temperature varies between 50 °C and 70 °C. The parametric analysis reveals that the highest COP, SMER, and exergy efficiency (ηex) occur when the optimum first-stage evaporating temperatures are 16 °C, 25 °C, and 34 °C, respectively, under three standard operating conditions. At this time, the first-stage evaporator handles 36 %–40 % of the cooling/dehumidification tasks, the refrigerant mass flow ratio at the intermediate-pressure suction port of the compressor ranges from 47 %–58 %, and the optimum first-stage and second-stage compression ratios are 1.49–1.52 and 2.45–2.61, respectively. The research results provide guidance and reference for the determination of matching design parameters and efficient operation control parameters of drying systems.

A simulation of Al-Si coating growth under various hot stamping austenitization parameters: An artificial neural network model

22MnB5 hot stamping steels are pre-coated with an Al-Si layer to prevent oxidation and decarburization during the austenitization process, which includes heating and soaking within a furnace. Above ∼577 °C, the Al-Si coating transforms into Fe-Al and Fe-Al-Si intermetallic phases because of diffusion and solidification. In the present work, a deep neural network (DNN) model was developed to predict coating intermetallic growth under various heating rates and soaking conditions. The DNN model was trained using experimental data (energy dispersive x-ray spectroscopy chemical scans) collected from our coating growth characterization work. The heating rate (r), heating time (th), soaking time (ts), and the distance of each measured point away from the coating surface (d) are defined as input parameters, and the outputs are the weight percentage of Al, Fe, and Si. The model successfully predicted the chemical composition of the training dataset, which consisted of a single-stage heating process, followed by a soaking stage at a constant temperature. To validate the model, complex two-stage heating tests were conducted, and the intermetallic coating growth was quantified. The DNN model successfully predicted the two-stage heating test coating profiles, despite a minimal error that was due to the variation in as-received coating thickness. The DNN model predicts the overall experimental trends that show the heating rate has little effect on the transformed intermetallic species, but it affects the transformation rate of the intermetallic phases. Moreover, the higher heating rate test (after 750℃) resulted in the formation of the island shaped τ1 and/or τ2 phase, while the heating rate before 750℃ does not affect the morphology of the τ1 and/or τ2 phase.

Thermal performance study of a solar-coupled phase changes thermal energy storage system for ORC power generation

The current solar organic Rankine cycle power generation (ORC) system cannot run smoothly under the design conditions due to the shortcomings of solar fluctuations, and thermal energy storage (TES) can effectively buffer the fluctuations of solar energy. Cascaded heat storage (CLTES) has been shown to be more suitable for solar heat storage than single-stage heat storage (LTES). The main objective of this study is to analyze the thermal storage characteristics of thermal storage systems under real-time solar energy fluctuations, and to improve the thermal storage efficiency and total thermal storage capacity of solar phase change thermal storage systems in distributed scenarios. The current research on solar CLTES system needs to focus on the heat storage characteristics under the actual solar fluctuations to provide guidance for the design and practical operation of CLTES system. Analyzing the effect of solar fluctuations on thermal storage performance is the focus of solar thermal storage research, which can provide guidance for the design and actual operation of CLTES systems. However, the current research on solar CLTES system lacks the analysis of thermal storage characteristics under typical solar energy fluctuations. Therefore, a simplified two-dimensional enthalpy-dynamic model of heat transfer for shell-and-tube latent heat storage system is developed in this paper to simulate the heat storage characteristics of single-stage and cascade heat accumulators containing different phase change materials (PCMs) at a fixed temperature of the heat source, and the heat storage performance of the system is simulated based on the solar fluctuations of typical four seasons. Finally, based on the shortcomings of large-capacity thermal storage, a dual-tank recirculation thermal storage model is proposed and the thermal storage characteristics of the improved recirculation model are analyzed. The results show that the optimized cycle heat storage mode can bring about a minimum of 10 % and a maximum of 17.2 % thermal efficiency improvement. On a typical summer day with the most abundant solar energy resources, four times of complete phase change heat storage and one incomplete phase change heat storage were completed (melting fraction = 81.83 %), and on a typical winter day with the least solar energy resources, two times of complete phase change heat storage and one incomplete phase change heat storage were completed (melting fraction = 75.81 %). The maximum difference of heat storage between them is 45.38 %.

Влияние предварительной подготовки исходного материала и состава газовой среды на характеристики активированных углей из хлопкового пуха при двухэтапном сверхвысокочастотном нагреве

Influence of starting material preliminary preparation and gaseous medium composition on adsorption and microstructural characteristics of activated carbons (AC) obtained from cotton fluff by two-stage microwave heating was investigated. Cotton fluff samples were impregnated with a 5 % solution of H3PO4, molded in the form of cylinders without incisions (type I), in the form of cylinders with transverse incisions (type II), and in the form of cylinders made of crushed “fluff–crumbs” (type III). During two–stage microwave heating (the first stage is carbonation in Ar, CO2 or N2 for 60 s, the second is activation in air for 50 s), fluff samples are completely carbonized and activated. It was found that the two-stage process allows to achieve higher rates of adsorption capacity of the finished AC according to the methylene blue indicator compared with single-stage microwave heating for 120 s. It is shown that by incising or grinding the processed samples, the adsorption capacity can be further increased. The reasons for the higher adsorption capacity of carbonized type III samples are higher density uniformity due to the grinding of the starting material, which makes it possible to achieve higher carbonation temperatures in a steady state (up to 1100 °C in a CO2 environment). Grinding also leads to additional surface development both due to the dissection of the fiber into several fragments, and due to the stratification of the fiber wall during the carbonization process.

Optimization of Operating Modes of CHPPs as a Way to Reduce Greenhouse Gas Emissions

The aim of the work is to analyze and evaluate the operating efficiency of CHPPs to develop an optimal operating strategy based on minimizing fuel consumption and greenhouse gas emissions. To achieve this goal, the following tasks were solved: a quantitative assessment of the most effective methods of operating units in typical operating modes was given; dependences of changes in the main energy and environmental parameters based on mathematical models of turbine units using real energy characteristics of turbine stages and compartments were constructed; long-term optimization method to exploitation operation of CHPРs to the point of view of reducing greenhouse gas emissions was shown on specific examples. The most significant results are the following: a comprehensive method-ology for applying mathematical models for solving the most typical problems for CHPP was used, both from the point of view of saving primary fuel and reducing carbon emissions; the dependences of the influence of operating parameters on the carbon footprint were calculated; it has been established that the specific consumption of equivalent fuel and the carbon footprint are reduced when the loads are redistributed between turbine units during the heating period; the advantages of two-stage heating of delivery water over single-stage heating and sequential loading of turbines of the same type over parallel loading have been determined. The significance of the results obtained lies in their applicability in solving problems of optimizing the operating modes of CHPPs, taking into account the determination of the values of reducing greenhouse gas emissions.

Photovoltaic Thermal Heat Pump Assessment for Power and Domestic Hot Water Generation

The efficient utilization of solar energy significantly contributes to energy efficiency in buildings. Solar photovoltaic thermal (PVT) heat pumps, a hybrid of photovoltaic and solar-assisted heat pumps, have demonstrated a significant development trend due to their multi-generational capacity for heating, power, and cooling with reliable operational performance. This research work presents and investigates a single-stage compression PVT heat pump system, along with the operation principle of the system’s heating and power co-generation throughout the winter and transitional season. The construction of the testing facility, data reduction, error analysis, and performance evaluation indices of the system are all explained theoretically. A continuous experiment research project focusing on system heating and power performance was carried out in Dalian during the transition season (November in this study) and winter season (December in this study) as part of our investigation into the potential uses for space heating, residential hot water, and power supply in northern China. The findings of the experimental research demonstrate that the proposed system can generate electricity and heat at high efficiency during the winter and transitional seasons, with long-term stable performance. The system’s average heating COPt is 5 during the transitional season and 4.4 during the winter season. Meanwhile, the average photovoltaic power efficiency under both weather conditions is 11.9% and 10.2%, with a peak value of 15.7% and 12.0%, respectively. Additionally, the system compression ratio’s variation range is 2 to 3.88, which is lower than the standard heat pump system. As a result, the entire system heating operating process remains constant.

Experimental and theoretical assessments on the systematic performance of a single-stage air-source heat pump using ternary mixture in cold regions

The performance of air-source heat pump is strongly influenced by ambient temperature variation. Improving its stability and efficiency during wintertime is important for promoting its applications, while existing studies paid few attentions to flexible parameter regulation of heat pump using mixture. In this paper, a single-stage compound air-source heat pump using ternary mixture was theoretically simulated and experimentally tested, in which operating parameters including pressure and concentration could be regulated with the cooperation of multiple valves and reservoirs. Using CO2/R134a/R600, operating pressure and mixture concentration were successfully regulated at ambient temperature from −30 ℃ to 0 ℃, achieving a coefficient of performance from 1.834 to 2.546 when producing hot water at 75 ℃. Whether R134a inclusion was favorable to energy performance depended on its influence on the temperature matching in recuperative process, especially the heat transfer process in recuperator. The addition of R134a significantly alleviated the increase of operating pressure and thus enlarged the operation range of this heat pump. The flammability range of CO2/R134a/R600 was calculated, indicating the insignificant influence of R134a on flammability retardance in the studied range of this paper. The significance of this study is further revealed with the potential advances of heat transfer technique and new-generation refrigerant. The findings in this paper contribute to: 1) the modification of air-source heat pump for operation at low temperature; 2) the regulation method for mixed-refrigerant system; 3) the exploration of new mixed-refrigerant in thermodynamic cycle.

Performances of an absorption heat transformer cycle with water/deep eutectic solvents working fluids: Modelling and screening using a multiscale approach

In this work, a tool was developed to evaluate the performances of working fluids composed of deep eutectic solvents as absorbents and water as refrigerant in a single-stage absorption heat transformer. The thermophysical properties required for the simulation were obtained using a multiscale approach involving the COSMO–SAC(+OH–ClBr) model for the vapour-liquid equilibria and group contribution models for the densities and heat capacities of the DESs. The predictive power of the model was assessed by comparing the results with the ones obtained with a conventional approach based on the non-random two-liquid model and empirical correlations derived from experimental data. The performances of the absorption heat transformer were evaluated using several criteria including the coefficient of performance, the circulation ratio and the heat input. The results are in agreement with the ones obtained with the classical approach. The performances of 32 deep eutectic solvents as well as the influence of the different temperatures of the process on the results are investigated. Results show that the best performances are obtained with deep eutectic solvents composed of ethylene glycol, glycerol, tetramethylammonium chloride, choline chloride and betaine, with coefficient of performance values comprised between 0.438 and 0.441, circulation ratio between 14.75 and 9.66 and heat input between 1982.11 and 2006.77 kJ.kg−1. In particular, the best performances were achieved with the pair water/{ethylene glycol:[tetramethylammonium][chloride]}, ratio [2:1]. Overall, the deep eutectic solvents with a higher ratio of hydrogen bond acceptor and lower molar masses showed better performances as absorbents in absorption heat transformers.