Refrigeration Heat Pumps Research Articles

Influencing factors on flow boiling of carbon dioxide in enhanced tubes and comparison with correlations

Carbon dioxide two-phase flow characteristics are different from those of conventional refrigerants, due to the CO2 particular thermodynamic and transport properties obtained by working at high reduced pressures. Moreover, the use of peculiar heat transfer surfaces such as grooves and internal fins are often preferred to enhance the boiling heat transfer performance. This paper collects CO2 flow boiling heat transfer coefficient data from different independent databases available in scientific literature, regarding both smooth and enhanced geometries and a wide range of operative conditions, that are typical of refrigeration systems and heat pumps. The database for enhanced tubes covers internal diameters from 0.8 to 8.92 mm, saturation temperatures from -30 to +20 °C, imposed heat fluxes from 1.67 to 60 kW/m2 and mass velocities from 75 to 800 kg/m2s, collecting more than 800 points. Heat transfer data for smooth and enhanced surfaces under the same conditions are collected, in order to measure the enhancement and to correlate it to the geometry augmentation. The assessment of quoted prediction methods explicitly developed for carbon dioxide is finally carried out, with a proposal for a correction factor.

Centrifugal turbo chiller using water as refrigerant and lubricant

Water refrigerant heat pump system with a water vapour turbo compressor is developed. Water (R718) is an ideal refrigerant that is considered perfectly environment friendly. Although water refrigerant heat pump is studied extensively, the development of turbo compressors of high pressure ratio is still a technical challenge, in terms of both aerodynamics of the impellers and high-speed rotordynamics. In this study, the high-speed rotor is supported by journal bearings lubricated with water refrigerant. Additionally, other system components such as sprayed direct intercooler heat exchanger and anti-cavitation flow control valve have to be designed and developed with the new requirements as well. An experimental test rig of closed-loop heat pump is constructed which achieves the cooling capacity of 100 kW on the rated condition with COP = 5. The experimental results and the experienced challenges and their undertaken solutions are discussed in terms of the efficiency and the vibrations of the turbo compressor and the heat pump system.

Research on Effects of Vapor Injection on Twin-screw Compressor Performance

Twin-screw compressors have been widely used in refrigeration systems and heat pump systems due to their unique advantages. Vapor injection (VI) technique could effectively increase the cooling or heating capacity of a refrigeration system. But in the meantime, because the VI will increase power consumption, the efficiency of the compressor will not be equally increased with the VI pressure. The effects of VI pressure on the performance of twin-screw compressor are generally obtained by one-dimensional numerical simulations or experiments. In this paper, the working process of twin-screw compressor with VI is simulated by computational fluid dynamics (CFD) technology. A homogeneous multiphase flow model coupled with a volume-of-fluid (VOF) method for interface capturing has been applied to determine the shapes of the gas-liquid interface and to compute the turbulent flow fields in twin-screw compressor. The influences of the VI pressure and VI position on the performance of the twin-screw compressor are analyzed. Comparing the P-θ indicator diagram and the performance parameters of the compressor with the experimental results, it can be found that the results of the CFD simulation are consistent with the experimental data. At the same VI position, as the VI pressure increases, the power of the compressor will gradually increase, and the efficiency of the compressor will rise first and then drop. Therefore, an optimum VI pressure can be found to reach the peak value of compressor efficiency. Under the same VI pressure, there is also an optimal VI position to maximize the efficiency of the compressor.

A comprehensive assessment of refrigerants for cabin heating and cooling on electric vehicles

With more and more stringent environmental requirements that go into effect, the automotive industry is moving away from R134a. All major automotive manufacturers are considering next generation refrigerants, but there is still no agreement on the final choice. As for electric vehicles, the choice of refrigerant to replace R134a is even more complex, since a refrigerant’s heating capacity and efficiency should be considered with the gradual adoption of heat pump technology for cabin heating. In order to help select the best refrigerant for electric vehicle HVAC systems, in this paper a comprehensive assessment has been conducted for seven refrigerants: R134a, R152a, R1234yf, R290, R410A, R32, and R744 (CO2). TEWI method is used for evaluating the total global warming impact for each refrigerant, in which both the cabin cooling and heating are considered in the calculation of indirect emission from energy consumption. For cabin heating, heat pump is used to its maximal available capacity, whereas PTC heater is used for supplemental heating. TEWI assessment results show that R290 refrigerant generates the lowest amount of equivalent CO2 emission whereas R744 gives off the highest amount. When considering comprehensively safety, heating capacity and global warming impact, R32 is found to be a very promising heat pump refrigerant.

A Comprehensive Set of Cooling Measures for the Overall Control and Reduction of High Temperature-Induced Thermal Damage in Oversize Deep Mines: A Case Study

The mining process in deep mines occurs at elevated temperatures and thus is significantly jeopardized by the thermal damage. In this study, the main factors causing high-temperatures under particular mining geological and prevailing conditions of coal mine production, namely for the Longgu Coal Mine (LCM) in Shandong Province of China, were specified and analyzed in detail. This included exothermic heat from the surrounding rock of an underground roadway, inflow of high-temperature water, seasonal temperature rise, mechanical and electrical equipment operation, and airflow compression in the mine. The integrated artificial cooling mode was implemented on the basis of the original normal ventilation and cooling facilities of the LCM, which involved cooling by mobile refrigeration units, water source heat pump refrigeration units, and a ground centralized ice-cooling radiation system, as well as the underground centralized cooling system provided by Wärme-Austausch-Technik (WAT) GmbH. Eventually, a comprehensive set of measures for the overall control and reduction of high-temperature-induced damage was realized, which ensured more effective cooling of the LCM. Thus, the average temperature of the main operation sites was reduced by 8 K, while that of the underground working faces was maintained at 299.15 K. These measures also resulted in excellent technical and economic benefits: the total three-year increase in revenue and savings reached 76.3 million USD, hence relevant findings of the study are expected to provide technical guidance on the treatment of high-temperature-induced damage in deep mines.

Analysis of the effect of thermohydraulic irreversibility of processes in the cycle of a refrigeration machine with a non-azeotropic mixture of refrigerants

The object of research is the effect of thermohydraulic irreversibility of energy processes in the cycle of a refrigeration machine using an ozone-safe non-azeotropic mixture as a refrigerant on its energy efficiency. One of the most problematic places during the development and design of such refrigeration machines is that, due to the peculiarities of the thermodynamic properties associated with the different chemical composition of the components, non-azeotropic mixtures are characterized by differences in the equilibrium concentrations of the components in the liquid and vapor phases. This property of non-azeotropic mixtures presents a certain difficulty for their effective application in refrigeration machines and heat pumps. During the study, modern methods of analysis and synthesis of thermodynamic systems are used, based on the application of the theoretical apparatus of technical thermodynamics, thermoeconomics, the theory of heat and mass transfer, as well as elements of the theory of systems engineering. The issue of assessing the energy efficiency of the cycles of refrigeration units operating on a non-azeotropic mixture of refrigerants is considered, taking into account the variability of the composition of the components of the mixture. A method has been developed for the formation of the composition of a multicomponent mixture, taking into account the influence of the non-isobarity of processes in the hydraulic circuit of the refrigerant circulation on the energy efficiency of the refrigeration machine. Based on a numerical experiment, the influence of changes in the concentrations of the components of the mixture R32, R125, R134a on the non-isothermal phase transition in the evaporator and condenser, as well as on pressure losses in the hydraulic circuit elements of an autonomous air conditioner, is established. The effect of friction at a temperature level below ambient temperature on the exergy efficiency of the refrigeration machine is analyzed. An advanced exergy analysis of the refrigeration cycle with a non-azeotropic mixture is carried out, as a result of which the avoidable and unavoidable, as well as the endogenous and exogenous components of the destruction of exergy in the elements are determined. The proposed method, due to its visibility, can significantly simplify the finding of the thermodynamic parameters of the refrigerant at the nodal points of the refrigeration machine cycle during numerical simulation.

Difference analysis on optimal high pressure of transcritical CO2 cycle in different applications

Optimal high pressure has a key role in transcritical CO2 system performance. In this work, we conducted a quantitative difference analysis on optimal high pressure between CO2 refrigeration system and heat pump water heater (HPWH) using response surface methodology. It revealed that temperature of CO2 leaving gas cooler, with 96.38% contribution, is the predominant factor to optimal high pressure in CO2 refrigeration system. However in HPWH, multiple factors affect optimal high pressure. Water outlet temperature, evaporating temperature, pinch point temperature difference and water inlet temperature account for 55.42%, 25.65%, 5.67%, 3.72% contributions, respectively. Furthermore, new quadratic correlations for HPWH with different number of variables were developed. As a result, when the number of variables increased from 1 to 4, mean error of COP almost linearly decreased from 5.28% to 0.22%.

Fluid Retrofit for Existing Vapor Compression Refrigeration Systems and Heat Pumps: Evaluation of Different Models

The global warming potential of many working fluids used nowadays for vapor compression refrigeration systems and heat pumps is very high. Many of such fluids, which are used in currently operating refrigerators and heat pumps, will have to be replaced. In order to avoid a redesign of the system, it would be very helpful if efficient and ecological alternative working fluids for a given plant could be found. With modern process simulation tools such a selection procedure seems possible. However, it remains unclear how detailed such a model of a concrete plant design has to be to obtain a reliable working fluid ranking. A vapor compression heat pump test-rig is used as an example and simulated by thermodynamic models with different levels of complexity to investigate this question. Experimental results for numerous working fluids are compared with models of different complexity. Simple cycle calculations, as often used in the literature, lead to incorrect results regarding the efficiency and are not recommended to find replacement fluids for existing plants. Adding a compressor model improves the simulations significantly and leads to reliable fluid rankings but this is not sufficient to judge the adequacy of the heat exchanger sizes and whether a given cooling or heating task can be fulfilled with a certain fluid. With a model of highest complexity, including an extensive model for the heat exchangers, this question can also be answered.

Thermodynamic analysis of the scheme­cycle design of a heating­coolingmachinefor an individual house

The direction of the research is the thermodynamic analysis of the scheme-cycle designof a heating and cooling machine for an individual house powered by autonomous solar photovoltaic system of providing private consumers with the year-roundheating and cooling in conditions of dry tropical climate. For the analysis, a single-stage compressor refrigerating machine was used, which operates in two modes: refrigeration forair conditioning and heat pump for heating, covering all rooms in the house. Change of the modes is made seasonally or during the day depending on the ambient temperature. The energy efficiency of the refrigerationcycle(“energy” problem) associated with the properties of the working fluid, and cycle size(“transport”problem) associated with the scheme-cycle design, equipment mass and investment costs,were determined. The study used the working fluids R404a, R134a, R410, R290, R600a, R32, which are neither prohibited nor expired. Calculations were performed separately for each mode. The results showed that the R290, R600a working fluids have a high efficiency in both modes, R404A, R410, R32 have the same energy efficiency, differing by no more than 10%, R134a is incompetitivein the heating mode. Among the cyclesizes, R32, R410 have the advantagewith the valueshalf the size of R290, R404A, R600a and R134a are not included in the alternative group. Based on the thermodynamic analysis and monitoring of the market of working fluids, only R32 can be recommended for real projects. A separate thermodynamic analysis of thescheme-cycle designs for CO 2 – the real prospect of refrigeration equipmentwas carried out.

Demand side management analysis of a supermarket integrated HVAC, refrigeration and water loop heat pump system

Supermarkets are intensive energy consumers because of a high electricity demand, mainly due to refrigeration utilities. Thus, in this work a supermarket integrated HVAC, refrigeration and water loop heat pump (WLHP) system was analyzed according to a demand side management approach, adopting a demand response strategy coupled with real-time pricing predictive rule based controls. The system was modeled with TRNSYS and several DR strategies were applied to both the space heating/cooling and the WLHP to determine the plant configuration with the most effective electricity cost saving. It was found that two setups guarantee the highest economic savings. The first consists of a predictive rule based control applied to the space heating/cooling only, which is basically inexpensive and allows an annual cost saving of 4.06% respect to the baseline configuration. The second, instead, combines predictive rule based controls applied to both the space heating/cooling and the WLHP auxiliary heater, and shows the best performance with the adoption of a 200 m3 water-based thermal energy storage. Respect to the baseline, this configuration provides an annual cost saving of 4.67%.

Comparison of Transcritical CO2 and Conventional Refrigerant Heat Pump Water Heaters for Domestic Applications

Although CO 2 as refrigerant is well known for having the lowest global warming potential (GWP), and commercial domestic heat pump water heater systems exist, its long expected wide spread use has not fully unfolded. Indeed, CO 2 poses some technological difficulties with respect to conventional refrigerants, but currently, these difficulties have been largely overcome. Numerous studies show that CO 2 heat pump water heaters can improve the coefficient of performance (COP) of conventional ones in the given conditions. In this study, the performances of transcritical CO 2 and R410A heat pump water heaters were compared for an integrated nearly zero-energy building (NZEB) application. The thermodynamic cycle of two commercial systems were modelled integrating experimental data, and these models were then used to analyse both heat pumps receiving and producing hot water at equal temperatures, operating at the same ambient temperature. Within the range of operation of the system, it is unclear which would achieve the better COP, as it depends critically on the conditions of operation, which in turn depend on the ambient conditions and especially on the actual use of the water. Technology changes on each side of the line of equal performance conditions of operation (EPOC), a useful design tool developed in the study. The transcritical CO 2 is more sensitive to operating conditions, and thus offers greater flexibility to the designer, as it allows improving performance by optimising the global system design.

Experimental study of thermodynamic efficiency of the absorption refrigeration machine and heat pump complex

Experimental study of thermodynamic efficiency of the absorption refrigeration machine and heat pump complex

Molecular simulation and experimental investigation for thermodynamic properties of new refrigerant NBY-1 for high temperature heat pump

This paper presents the differences in thermodynamic properties of refrigerants in different spatial conformation, showing the superiority of molecular simulation in predicting thermodynamic properties of fluids. And molecular simulation was modified to improve prediction accuracy of thermodynamic properties of fluids at high temperature and pressure. Meanwhile, a new high temperature heat pump refrigerant mixture named NBY-1 was proposed, and its thermodynamic properties were estimated with the modified molecular simulation. Then the experiment investigation was carried out and showed the good cycle performance of NBY-1 as the refrigerant. The experimental results indicated that high temperature heat pump using NBY-1 could supply high temperature water of 130 °C with the highest coefficient of performance of 2.74 and the condensing pressure of 2.55 MPa, when the temperature lift was 50 °C. Moreover, the experiment also demonstrated the high accuracy with molecular simulation in predicting thermodynamic properties of NBY-1 in the two phase region, and the average relative deviation of condensing pressure between predicted results and experimental results was 2.59% with the highest value of 3.89%.

THE EFFECT OF SYSTEM PARAMETERS ON THE CONDENSATION PERFORMANCE OF HEAT PUMP SYSTEM USING R290

Since global warming has reached critical levels, limitations have been placed on the use of certain fluoride-containing refrigerants by F-Gas regulations. The EU F-​​Gas Regulation has introduced quotas for the use of refrigerants with a global warming potential(GWP) greater than 150. Hydrofluorocarbons(HFCs) from restricted refrigerants are widely used in heat pump systems. Considering the environmental impact of these refrigerants, it is important to look for long-term alternatives to comply with F-gas regulations. Hydrocarbon(HC) refrigerants are shown as suitable alternatives for heat pump applications. R290 as HC refrigerant is a potential refrigerant suitable for existing HFCs systems due to zero ODP and low GWP. In heat pump systems, there are many system components or parameters that are effective in condensing the air passing through the evaporator. It is very important to know how these elements affect the condensation performance in different design situations. In this study, the effect of different parameters such as capillary length, charge amount and evaporator tube volume on the condensation performance of a R290 hydrocarbon refrigerant heat pump was investigated by the experimental design approach. The experimental results obtained was compared with the theoretical model. It has been determined that the most effective parameter on the condensation performance is the capillary tube length with the effect of 35%.

Utilization of Latent Heat Thermal Storage in CO2 Refrigerant Heat Pump in Vending Machine

Utilization of Latent Heat Thermal Storage in CO₂ Refrigerant Heat Pump in Vending Machine

Thermodynamic model for reciprocating compressors with the focus on fluid dependent efficiencies

Fluids with high global warming potential, which are used in existing refrigeration cycles and heat pumps, will have to be replaced soon by less harmful fluids, but the fluid selection is difficult especially due to the unknown compressor performance. In this work a differential compressor model for reciprocating compressors is introduced which predicts volumetric and isentropic efficiencies quickly and can be easily fitted with measured data at only one operation point of an existing compressor. In order to characterise the influence of different fluids two semi-physical correlations for the valve flows are fitted here, and a procedure of transferring them to different compressors is shown. The model is validated on, in total, 63 measured points based on numerous fluids from one semi-hermetic reciprocating compressor which is part of a heat pump cycle. The calculations lead to mean prediction errors of 3.0% for the isentropic and 2.3% for the volumetric efficiency.

Nanorefrigerants for energy efficient refrigeration systems

Nanorefrigerants, which are a new class of nanofluids, are mixtures of nanoparticles and refrigerants. They have several potential applications in various fields, such as for refrigeration and air-conditioning systems and heat pumps, among others. Accordingly, adding nanoparticles to conventional refrigerants results in anomalous improvements in the thermophysical properties and heat transfer characteristics of refrigerants, thereby further improving the performance of refrigeration systems. In the present study, R134a-based nanorefrigerant from R134a + CuO/Polyalkylene glycol (PAG) oil nanolubricant was prepared by dispersing copper oxide nanoparticles in the PAG lubricant. The effect of adding nanoparticles was investigated in relation to the thermal conductivity of lubricants and tribological characteristics of nanolubricants. The overall performance of a vapor compression refrigeration system with nanorefrigerants was evaluated. Enhanced heat transfer rate, coefficient of performance, freezing capacity, and reduced power consumption were observed for the refrigeration system. The thermal conductivity of the nanolubricant was better than that of the pure lubricant. Experimental results also showed that the low concentrations of CuO nanoparticles suspended in synthetic oil enhanced the tribological properties of the base oil.

Research on Environmental Friendly Refrigerant Heat Pump Water Heater Based on Enhanced Vapor Injection

This paper studied the substitutability of R417A in enhanced vapor injection air source heat pump, mainly analyzed the feasibility of R417A instead of R22 under the low temperature conditions by experimental performance test. By adjusting the openness of the electronic expansion valve to regulate refrigerant volume, and found out the optimal openness of R22 and R417A system under winter conditions(7℃,6℃) and low temperature conditions(-7℃,-8℃) . The results show that for the direct-heated heating system, the lower ambient temperature, the more feasibility of R417A instead of R22.

二相流ノズル出口に発生する衝撃波による圧力変動に関する研究

Recently, the refrigeration air conditioner and heat pump system using the two-phase flow ejector is beginning to appear, as the efficiency of the ejector cycle is improved largely in comparison with the conventional system. Shock and expansion waves can occur in the internal flow fields of the ejector, and the effect of the pressure fluctuation by those waves must be considered on the designs of the ejector. In this study, the pressure fluctuation of the shock wave appeared at the outlet of the two-phase flow nozzle are measured in the HFO1234yf refrigeration cycle. It was clarified by experiment that the amplification frequency components of pressure fluctuations are increased with the strength of the shock waves and the attenuation frequency components are increased with increase of liquid void fraction of downstream shock waves.

Подбор смесевых хладагентов для парокомпрессионных холодильных машин и тепловых насосов

Подбор смесевых хладагентов для парокомпрессионных холодильных машин и тепловых насосов