Refrigeration Heat Pumps Research Articles

Selection of refrigerant based on multi-objective decision analysis for different waste heat recovery schemes

Waste heat generation, upgrading, and refrigeration are the fundamental ways to recover and utilize waste heat. The rationalizing the use of refrigerants also contribute in creating energy savings and minimizing carbon emissions. This study evaluated the thermodynamics, economics, and environmental of different types of refrigerants in three kinds of waste heat recovery schemes: generate electricity, heat pump, and refrigeration. Based on this, the entropy weight and Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) were combined to assess the overall performance of the refrigerants. A thorough analysis revealed that R1234ze(E) can replace R245fa and R123 in the organic rankine cycle (ORC). The best refrigerant for vapor compression refrigeration (VCR) and high temperature heat pump (HTHP) systems is R1243zf. In addition, the multi-objective decision analysis shows that the performance difference among the nine selected refrigerants is the total cost, followed by the environmental impact. The approach is successful in recognizing the variations between different refrigerants in the same waste heat recovery scheme and gives a thorough evaluation. It sets instructions for the future use of eco-friendly refrigerants and their application waste heat recovery schemes.

Low-GWP refrigerants in heat pumps: An experimental investigation of the influence of an internal heat exchanger

In heat pumps, the use of refrigerants with a low global warming potential (GWP) gains importance due to increasingly strict regulations regarding their ecological impact. In this regard, hydrocarbons (HCs), hydrofluoroolefins (HFOs), and their mixtures are the most promising options due to their thermodynamic properties. Besides the impact of the refrigerant, the cycle configuration (e.g., basic cycle and internal heat exchanger cycle) strongly influences the efficiency of the heat pump. While the potential of low-GWP refrigerants in internal heat exchanger (IHX) cycle configurations is widely investigated in numerical studies, there is a lack of experimental validation. Therefore, this work experimentally evaluates four HCs (R290, R600a, R436A, R1270) and the HFO R1234yf in comparison to R134a in a brine-water heat pump test bench. The test bench design allows to switch between the basic and IHX cycle, thus, enabling the simultaneous impact evaluation of the cycle configuration and the refrigerant on the performance. In the experiments, R1270 shows the highest efficiency for all operating points followed by R290 in the basic cycle. The IHX cycle improves the efficiency for all refrigerants in comparison to the basic cycle. The zeotropic mixture R436A achieves the highest efficiency improvements of up to 27.5% compared to the basic cycle, whereas the efficiency of the single-component refrigerants increases by 10% on average. Despite the significantly higher improvements of R436A due to the IHX, R1270 still leads to the highest coefficient of performance (COP) of up to 6.6 (B12/W35) in the IHX cycle configuration.

Analysis of the oil temperature in a scroll compressor with oil carter at discharge pressure using propane as refrigerant: Performance analysis for abnormally low oil volume

Scroll compressors are widely utilized in refrigeration systems and heat pumps, operating through the interaction of two spiral-shaped elements, one stationary and the other orbital. These elements fit together and continuously oscillate, compressing the fluid into diminishing volumes. They offer significant advantages such as high volumetric and compressor efficiencies, low vibrations, and reduced leaks. In direct suction scroll compressors, the oil sump is at discharge pressure, resulting in a wide pressure difference between the housing and the backpressure chamber; also, the oil temperature is higher than in the most common suction-side oil crankcase design. Knowing the oil temperature is crucial in determining the properties of the oil/refrigerant mixture that affect the system's performance. It also allows us to estimate the amount of refrigerant dissolved in the oil. In the current study, a series of tests are conducted under various operating conditions to measure the oil sump temperature in a direct suction scroll compressor. This experimental campaign was conducted using a compressor with an unusually low oil level, and an analysis of the system's performance was performed until a failure occurred.

Matching Characteristics of Refrigerant and Operating Parameters in Large Temperature Variation Heat Pump

Characterizing the optimal operating parameters for a heat pump with a specific refrigerant is paramount, as it provides valuable guidance for refrigerant selection. The temperature mismatch between cold and hot fluids in the evaporator and condenser can lead to degraded thermal performance in heat pumps with large temperature variations. To address these two key issues, we selected several pure refrigerants with varying critical temperature levels for use in a large temperature variation heat pump configuration. The corresponding thermal performance was then investigated using the Ebsilon code under fixed temperature lift conditions as the operating temperature varied. It indicates that the maximum coefficient of performance (COP) is typically achieved when the deviation factors of temperature and pressure from their critical parameters fall within the ranges of 0.62~0.71 and 0.36~0.5, respectively. Our research recommends the binary refrigerant mixture of R152a/R1336mzz(z) (COP = 3.54) for the current operating conditions, as it significantly improves thermal performance compared to pure R1336mzz (z) (COP = 2.87) and R152a (COP = 3.01). Through research on the impact of the compositional ratio of R152a/R1336mzz(z) on the thermal performance of the heat pump, we found that that the optimal ratio of R1336mzz(z) component to R152a component is 0.5/0.5. This study offers valuable guidance for selecting the most suitable refrigerants for heat pumps in practical engineering design scenarios.

Experimental and chemical kinetics study on the flammability limit of CO2/C3H6 mixture working fluid at variable initial temperature

CO2/C3H6 mixture has a wide range of applications in refrigeration systems, heat pumps and Organic Rankine Cycle (ORC) systems, but its flammability safety needs to be considered. An experimental and computational investigation has been conducted on the determination of the flammability limits of CO2/C3H6 and on the understanding of associated limit phenomena in general. We tested the flammable zone of CO2/C3H6 at variable initial temperature. The accuracy of the prediction model of calculation adiabatic flame temperature were verified based on the experimental data. Meanwhile, a high-speed camera was used to record the process of CO2 suppression of flame propagation near flammability limit. Furthermore, based on kinetic numerical calculation, the effect of CO2 chemical and thermal on the flammability limit of C3H6 were analyzed. The results show that the thermal effect of CO2 is greater than that of chemical effect near the flammability limit. The theory of chemical reactivity inhibition reduces the yield and molar ratio of key radicals. The addition of CO2 can inhibit the activity of the dominant chain and increase the activity of the dominant chain termination reaction in the global combustion kinetics. Results of this study provide theoretical guidance for the safe application of CO2/C3H6 mixture.

Virtual design on the heat pump refrigeration cycle: challenges and approaches

Virtual product design (VPD) of the refrigeration cycle has long been a major subject of many engineers and manufacturers. Issues should be the question of how simulation gets close to the real world in virtual ways. It is especially true when we consider variable refrigerant flow (VRF) heat pumps, having various types of indoor unit combinations, long and complicated piping configurations, and simultaneous cooling and heating functions under extreme weather conditions. In this regard, model-based systems engineering (MBSE) or model-based design (MBD) guides a methodology to make a virtual model easily. Now, there are plenty of engineering platforms and tools to make a strong simulation model, which provides interfacing technology among multiple physics, different dimensions, and codes in different languages. In the study, the authors share their experience in the virtual design for a VRF heat pump system including the architecture, multiple physics, and the connection with the control SW. Modelica, an acausal and object-oriented modeling language, constructs the backbone of the model to describe the dynamic behavior of refrigerants and to combine external components in the form of functional mock-up units (FMUs). The developed virtual model is validated against measured data, showing it can reproduce all the essential physics of interest both in qualitative and quantitative ways.

CO2 heat pump integrated thermal storage for domestic hot water in hotels

Gas/diesel-fired water heaters are widely used in Indian hotels to deliver domestic hot water at 45–50 °C. These are substantial fossil energy consumers and pollute the environment. A CO2 (natural refrigerant) heat pump integrated thermal storage unit is an energy-efficient and eco-friendly solution for meeting heating demands. In tropical ambient conditions like in India, the energy efficiency of such heat pumps can further be improved when the heat source is the chilled water used for air conditioning purposes of the same hotel, thereby reducing the load on existing chillers. However, studies on the design of the proposed system and assessment of its potential for hotel application in tropical climates are still scarce. This information assists energy engineers and hotel management in evaluating new buildings or refurbishments. This paper demonstrates a design strategy to retrofit the conventional system with the heat pump integrated thermal storage unit and assess its energy, economic, and environmental benefits for a typical Indian hotel. Four different heat pump configurations, simple and ejector-assisted transcritical CO2 systems with and without internal heat exchangers, are analyzed and compared. The ejector-assisted CO2 heat pump without IHX having the minimum heating capacity (operates all time) integrated with a thermal storage tank of maximum capacity is observed to be an economical combination. In Chennai, where the average annual water temperature is 28 °C, the operating cost of the proposed system is only 7 % of that of a diesel-fired water heater. Also, CO2 emissions are reduced by 80 %. Thus, the proposed system is seen as an efficient and clean alternative solution for providing hot water and free AC in hotels.

Dynamic characteristics of refrigerant evaporation temperature in air-water heat source heat pump

In order to improve the problem of reduced heat capacity and unstable heating ability of air source heat pump (ASHP) in the heating mode during the cold season, a heat pump evaporator using air and surface water as a heat source is designed in this paper. A heat transfer unit's calculation model of an air–water heat source evaporator is established. Experimental tests and simulation studies are conducted on the refrigerant evaporation temperature and heat pump heating performance. On this basis, a refrigerant evaporation temperature prediction model is constructed using a multiple linear regression (MLR) model. The trend of refrigerant evaporation temperature under air–water heat source is analyzed by taking the winter meteorological parameters in Xiangtan City as an example. The results show that the air–water heat source heat pump (A-WSHP) has a higher refrigerant evaporation temperature and a smaller temperature fluctuation range than the ASHP. The average values of refrigerant evaporation temperatures under the experimental conditions were 5.3 °C and 4.6 °C, respectively. The temperature fluctuation ranges were 3.8 °C-6.7 °C and 2.8 °C-7.0 °C, respectively. The A-WSHP has higher heating capacity and heating stability. Air temperature, air velocity, water spray temperature and water spray flow rate are all important parameters that affect the refrigerant evaporation temperature of a heat pump. Evaporation temperature changes are positively correlated with these factors. In the refrigerant evaporation temperature prediction model, the air temperature change has the most significant effect on the evaporation temperature, followed by spray water temperature, air velocity, and spray flow rate, whose influence weights are 82.5 %, 38.8 %, 29.4 %, and 25.3 %, respectively. Selecting different temperatures of spray water and adjusting the evaporator's inlet spray flow rate can improve the refrigerant evaporation temperature and heat pump heating stability.

Centrifugal compressor design and cycle analysis of large-scale high temperature heat pumps using hydrocarbons

Using natural refrigerants in heat pumps, including hydrocarbons, is becoming attractive because of the tightening regulations concerning synthetic refrigerants. In the literature there is lack of studies investigating detailed component design, including compressor design when using hydrocarbons. This study investigates numerically centrifugal compressor design with hydrocarbons for large-scale high-temperature heat pumps. The compressor analysis was combined with a thermodynamic analysis of a two-stage heat pump. The effect of temperature lift and flash intercooler temperature on compressor sizing, rotational speed, efficiency, axial force, and Mach number were investigated. Generally, the use of hydrocarbons with five carbon atoms resulted in higher coefficient of performance and required lower compressor rotational speeds and larger compressor dimensions when compared to the hydrocarbons with four carbon atoms. The simulated rotational speeds were ranging from 8 krpm to 30 krpm, impeller diameters from 0.15 m to over 0.7 m and compressor efficiencies from 77% to over 85%. The use of cyclic molecules, namely cyclopentane and cyclobutene resulted in the highest heat pump coefficient of performance. Also, the flash intercooler temperature had a notable effect on the compressor design and axial force which has to be taken into account in heat pump cycle design.

Cooling performance and optimization of a thermal management system based on CO2 heat pump for electric vehicles

Efficient integrated thermal management scheme is crucial for improving the driving range, thermal comfort, and safety of electric vehicles. Compared with the synthetic refrigerant heat pump indirect cooling/heating on the vehicle electrical system, the natural refrigerant CO2 heat pump direct thermal controlling integrated into the thermal management system has advantages in the thermal cycle performance and environmental friendliness. The present study proposed a thermal management system based on CO2 heat hump with multiple working modes for enhancing vehicle energy utilization coefficient. The system thermal performance under the World Light Vehicle Test Cycle was investigated via energy and exergy analyses. The operational control strategies of the thermal management system were optimized for the thermal controls of the cabin, battery and motor. The results show that the minimum exergy destruction of the thermal system occurs at the optimal discharge pressure corresponding to the highest coefficient of performance. Ensuring two-phase latent heat transfer for battery cooling is the most effective measure to maintain battery temperature uniformity via dynamic collaborative regulation of expansion valves’ openings and compressor speed. Compared with constant expansion valve’s opening, the coefficient of performance of the system can be improved by a maximum of 10% with the saturated CO2 vapor at the outlet of battery cooling plate. Due to the low thermal capacitance and minimal thermal inertia of driving motor, its cooling performance is significantly influenced by the heat generation rate and refrigerant flow. Optimal motor cooling efficiency is achieved when controlling the refrigerant quality to 0.85 at the cooling passage outlet. Under the World Light Vehicle Test Cycle, the thermal management system effectively regulated the temperatures across different operating modes while operating at optimal discharge pressures.

Comparison and selection of experimental designs for the characterization of scroll and reciprocating compressors: Adjustment of polynomial models with compact experimental samples

The use of refrigeration equipment and heat pumps is a widely used solution by manufacturers in the industry. Selecting components for this equipment and simulating their behavior require a thorough understanding and effective characterization of the compressor. In this sense, the standard AHRI-540 is the most used one to characterize compressor performance (Ẇc and ṁref) considering polynomial models of 10 coefficients. Unfortunately, this standard does not offer information about where to perform the required experimental measurements for the model adjustment. Drawing on these precedents, this paper investigates various Design of Experiments methodologies to identify the most efficient approaches for characterizing both scroll and reciprocating compressors with minimal experimental points. The study aims to develop a straightforward methodology for optimal point selection, ensuring precise compressor characterization while minimizing experimental costs. For this purpose, two datasets from two Copeland scroll compressors – ZP21K5E-PFV and ZS21KAE-PFV – with massive test campaigns have been employed. Both compressors were subjected to a substantial number of experimental points (1097 and 866) across different suction conditions (SH=11 K, SH=22 K, Tsuc=18 °C) and different refrigerants (R134a, R32, R410A, R404a, …). In a first step, this study presents the design of experimental matrices for characterizing efficiently the energy consumption and mass flow rate in scroll compressors. Additionally, the energy consumption is also characterized from the specific energy consumption (Ẇesp), i.e., the energy consumption divided by the mass flow rate. Considering that the specific energy consumption obtains equal dependencies between scroll and reciprocating compressors, the matrices performed for the specific energy consumption are also suitable for the characterization of reciprocating compressors, also allowing to obtain compact experimental matrices with a low number of points.

Performance analysis of water refrigerant heat pump with different configurations for high-temperature application

Water is a promising working fluid for high-temperature heat pumps (HTHPs) with the advantages of being cheap, safe, stable, environmentally friendly, non-toxic, and non-flammable. HTHPs using water as a refrigerant not only can reduce greenhouse gas emissions in the industrial sectors by recovering waste heat but also have no adverse effect on the environment if the refrigerant leaks. This article evaluates the safety, energy, and exergy performance of six designed HTHP configurations with water refrigerant by analyzing the discharge superheat, power consumption, heating capacity, COP, and exergy efficiency—at different condensation and evaporation temperatures. The results indicated that water injection and intercooling can effectively decrease the discharge superheat to ensure safety performance. Among these systems, two-stage cycles can supply higher output temperatures and have better system performance compared to single-stage cycles at high-temperature lifts and large compression ratio conditions. The two-stage system with a flash tank (TS-FT) has the biggest heating capacity, the best COP, and the best exergy efficiency. With an evaporation temperature of 80°C and condensation temperature of 140°C, the COP is 4.14 and the exergy efficiency is 70.9% for the TS-FT. Compared with the single-stage ordinary system (SS-OS), the COP of the TS-FT has an increment of 36.6%. The exergy efficiency of the TS-FT is 19.8% higher than that of the SS-OS. Considering the aforementioned theoretical analysis, the TS-FT with the best safety, energy, and exergy performance is the optimal HTHP system with water refrigerant for high-temperature applications.

Using Charge Determination Design of Experiments to Develop A Refrigerant Charge Health Status Model for Heat Pump Systems

Refrigerant based heat pump systems are becoming an integral system in electric vehicle architectures due to their high efficiencies in providing heating and cooling to people and components within the car. An important component in heat pump systems that determines optimal efficiency is the amount of refrigerant. As such, the capability to model refrigerant charge helps quantify the health status of the heat pump system, whereby the lack or over abundance of refrigerant in a heat pump refrigerant system leads to various other component failures, e.g., liquid slugging, compressor overheating, material fatigue in heat exchanger, and degraded/stuck expansion valves. In designing a heat pump system, engineers need to perform a set of design of experiments to determine an optimal refrigerant charge based on a set of performance metrics in the presence of certain noise factors. This optimal refrigerant charge provides conditions where the heat pump system operates efficiently in both heating and cooling, in addition to facilitating operational conditions that will not lead to secondary component degradation or damage. The search for optimal refrigerant charge is classified as refrigerant charge determination, whereby engineers incrementally increase the refrigerant in the heat pump system in operation of heating/cooling and collect data about performance metrics. Some of the key performance metrics used to determine efficiency of a heat pump system include i) compressor inlet superheat temperature, ii) condenser outlet subcool temperature, iii) compressor high side pressure, iv) compressor low side pressure, v) condenser outlet pressure, and vi) condenser quality estimate. Furthermore, this process follows design of experiments concepts and is performed for both heating and cooling modes of operation. In this paper, we leverage refrigerant charge determination as a training data source to develop refrigerant charge models, where several performance metrics are health indicators used as model inputs and the amount of refrigerant added to the heat pump system are ground truth refrigerant charges used as model outputs. In this paper we develop regression models to estimate the total refrigerant charge, which is used to classify different health states of refrigerant based on levels of performance degradation corresponding to specific refrigerant charge thresholds. We trained a robust linear regression model using this charge determination data and found that the worst case estimation error was less than 10% with respect to the refrigerant charge grouth truth.

Simulation and experimental study on dynamic characteristics of R290 split heat pump during start-up

As a new generation of heat pump refrigerant, R290 (propane) not only has advantages in environmental protection, but also has the good performance. However, reliability problems remain in the start-up process of R290 heat pumps, and hence it is necessary to study the dynamic characteristics of the start-up process. This paper establishes a mathematical model based on the physical properties of R290 and the system characteristics of R290 split heat pumps. The model is used to calculate the dynamic start-up characteristics. The proposed mathematical model is validated by experiments, and an error analysis of the heating start-up is carried out, and the mechanism of continuous low suction pressure during start-up is analysed. Taking a variable frequency rated heating condition as an example (indoor and outdoor temperature 20/7 ℃), the maximum deviation between the calculated pressure and the experimental value is within 5% during stable process. The refrigerant mass in the compressor shell is 164.76 g at most and the liquid separation process lasts 370 s during heating start-up process. The continuous suction of low pressure in the R290 split heat pump is attributed to the accumulation of liquid phase refrigerants in the accumulator and the presence of refrigerant condensation in the compressor shell.

Experimental study of relative humidity effect on the edge effect of frosting characteristics on a vertical cold plate surface

As a common phenomenon, frosting often brings negative impacts in various fields. To gain more insight into the frosting process on a vertical cold plate considering the edge effect, a specific experimental system is designed and built to perform frosting experiments under forced convection conditions with relative humidity varying from 40% to 80%. The results show that as the relative humidity increases, the droplet solidification stage period in the unaffected region decreases, but the equivalent width of edge-affected region, area-average equivalent contact diameter of droplets, and coverage area ratio of droplets increases and then decreases. The area-average equivalent diameter of droplets in edge-affected region increases by 41.25%, 214.63%, 137.98%, 159.03%, and 30.30%, when compared to the unaffected region from Case 1 to Case 5, respectively. Besides, the difference between the droplet distribution density in the edge-affected and unaffected regions gradually decreases, but the average frost layer thickness in the edge-affected region increases significantly as the relative humidity increases. The frost layer growth rate decreases first and then increases as relative humidity increases. From Case 1 to Case 5, the frost growth rates at 1,800 s decrease by 90.05%, 70.19%, 56.08%, 62.28%, and 82.39%, respectively, compared to the initial ones. Results of this study can provide a reference for the anti-frosting and defrosting control technologies in the field such as refrigeration and air source heat pump units.

Exergy analysis, coupled heat pump and refrigeration system using waste heat

The paper seeks the optimal configuration of equipment capable of generating heat and cold. For the generation of the two products, for the first time it is considered a coupled system heat pump and refrigeration system. Waste heat from an industrial process is used as the heat source for the heat pump cycle. For the refrigeration system, carbon dioxide is used as the working agent, and for the heat pump the working agent R152a. In order to identify the malfunctions at the level of each device and process, an exergy analysis is used, which identifies the magnitude and location of each malfunction. Based on the information provided by the exergy analysis, it is considered a structural optimization strategy by applying sub-cooling to the refrigeration cycle and heat pump cycle. The analysis shows an increase in the performance indices COPen and COPex when applying the subcooling at the level of the refrigeration cycle. The results of the exergy analysis make it possible to find solutions for improving the functional structure and design of equipment.

Experimental investigation of thermophysical properties of ethylene glycol based secondary fluids

Aqueous solutions of ethylene glycol are commonly used as secondary fluids in different indirect refrigeration systems and heat pumps as well as nanofluids. A very extensive literature review has been done, including more than 90 references published from 1905 to 2023. Despite the wide application and importance, especially in the energy sector, ethylene glycol solutions seem to be less investigated in low temperature ranges and more research is required to improve the quality and quantity of available data. The novelty of this paper is to investigate the most important thermophysical properties of ethylene glycol solutions in low temperatures. In this study a different approach was made and solutions having a specific freezing point temperature (between -5 and -50 ºC) rather than specific concentration were investigates in temperature ranges applicable for different cooling applications. The concentrations giving a certain freezing point temperature seemed to deviate in some cases with 1–2 wt-% between different sources. Nevertheless, the density results were in rather good agreement with all reference data. The viscosity results were lower by up to ±10% compared to reference values. Additionally, the obtained experimental results for thermal conductivity were higher by up to 12% compared to ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) data. The specific heat capacity results were higher by up to 14.6% and 5.4% than current reference data. There is a high probability that the current ASHRAE data for specific heat are actually indirectly calculated values from thermal conductivity data and not validated using differential scanning calorimetry techniques.

Air-side heat transfer enhancement in fin-tube heat exchangers using forced vibrations under various conditions

Forced vibrations can occur in fin-tube heat exchangers (FTHEs) by compressors, fans, and the driving conditions of automobiles. Accordingly, the effects of forced vibrations on the heat transfer performance of FTHEs in refrigeration systems and heat pumps should be investigated in detail for their practical design. However, few studies have investigated the improvement in heat transfer by vibrations in FTHEs. In this study, air-side heat transfer enhancement in FTHEs by applying forced vibrations is experimentally investigated. The heat transfer performance of the FTHEs was measured by varying the air-side Reynolds number, vibrational frequency, vibrational amplitude, and fin pitch. For practical purposes, the heat transfer characteristics of the FTHEs were analyzed under constant vibrational power, force, and velocity. The heat transfer enhancement increased with an increase in the vibrational frequency, amplitude, and fin pitch, whereas it decreased with an increase in the air-side Reynolds number. A correlation for heat transfer enhancement using forced vibrations was developed as a function of nondimensional operating and geometrical parameters. The results can be used to predict heat transfer enhancement by vibrations and the FTHE design for heating, ventilating, and air-conditioning systems.

A high-efficiency multi-function system based on thermal desalination and absorption cycle for water, water-cooling or water-heating production

Water-refrigeration cogeneration systems combining thermal desalination and absorption cycle have been identified as an effective and promising way of utilizing low-grade heat. To take full advantage of the synergy that can be achieved by the combination and overcome the disadvantages of the existing systems, this study proposes and analyzes a multi-function system composed of a single-effect water-lithium bromide absorption heat pump, a single-effect water-lithium bromide absorption refrigeration heat pump and a low-temperature multi-effect evaporation desalination unit. Consuming mainly low-grade heat, the system can operate in at least three modes: water-refrigeration mode for summer, water-only mode for spring/autumn and water-heating mode for winter, thus capable of meeting the requirements of different seasons. The mathematical model is presented and validated, and thermodynamic performance is studied. The results show that all the modes have good energy/exergy performance, as demonstrated in a case study where the energy- and exergy-based coefficients of performance of the absorption unit are 1.60 and 0.62, 1.75 and 0.87, and 1.75 and 1.01 for the three modes, respectively, and the energy saving rates are all as high as 42.7% compared with conventional systems. The proposed system also features great operational flexibility. In water-refrigeration mode, the refrigeration load rate can be regulated from 20% to 100% for a water load rate within 55.2%–110%. In water-heating mode, the heating load rate can be regulated between 0 and a top limit allowed which reaches 256% under a water load rate of 50%, peaks at 459% under 90% and decreases to 358% under 110%. Therefore, the system has good internal synergy, reflected in the whole-year multi-purpose operation, efficient energy utilization, and great operational flexibility.

Refrigerant Selection for Heat Pumps: The Compressor Makes the Difference

Increasingly stringent regulations and new applications require selecting improved refrigerants for heat pumps. The selected refrigerants should be environmentally friendly and maximize the performance of the heat pump process. Systematic refrigerant selection methods usually model the compressor with one fixed isentropic efficiency identical for all refrigerants. However, compressor studies indicate that the isentropic efficiency may be highly refrigerant‐dependent. Herein, the need for a refrigerant‐dependent compressor model for refrigerant selection is investigated. For this purpose, a refrigerant‐dependent compressor model is combined with an integrated design of refrigerant and heat pump process. To guarantee a comparable nominal heating power among refrigerants, the compressor design is tailored to the refrigerant by expanding the refrigerant‐dependent compressor model toward compressor sizing. Integrating compressor design into systematic refrigerant selection is enabled by a computationally efficient model implementation. Accounting for refrigerant‐dependent compression substantially changes the resulting refrigerant ranking compared to the widely used assumption of identical isentropic efficiencies for all refrigerants: The best‐performing refrigerant is not even identified among the best ten refrigerants when assuming identical isentropic efficiencies. Consequently, compression is identified as the main driver for differences in refrigerant performance. Therefore, integrating refrigerant‐dependent compressor models is crucial for systematic refrigerant selection.