High-temperature heat pumps (HTHP) are essential for improving the efficiency of industrial heating processes. Increasing the HTHP supply temperatures can enable a broader impact of the technology on different industrial sectors, but also has challenges associated with cycle efficiency, compressor discharge temperatures, and material compatibility. This study presents a refrigerant screening and compressor isentropic efficiency sensitivity study for steam-generating heat pumps (SGHP) at a variety of operating steam supply conditions. Additionally, modeling efforts are presented for the design and development of a state-of-the-art test facility for a novel HTHP architecture, aiming to produce heat at 200 degrees C in a subcritical cycle. Specifically, the cycle features refrigerant economization coupled with internally cooled rotors in an additively manufactured twin-screw compressor for enhanced discharge temperature control and efficiency. Based on a previous refrigerant screening, the refrigerant that produced the best results on a COP, discharge temperature, and critical temperature basis, was pure cyclopentane. A key advancement is the use of moving boundary heat exchanger models, which calculate phase change regions based on local thermodynamic conditions. Compared to conventional fixed temperature difference models, the moving boundary models of the evaporator and condenser revealed pinch variations of up to 3 K over a 40 K range of condensing temperatures, resulting in variations in compressor power and heating capacity of up to 13 % and 33 %, respectively. The resulting system model predicts a COP of 2.85, a second law efficiency of 48.7 %, and a heat sink outlet temperature of 201 degrees C given a 120 degrees C heat source temperature. The modeling efforts presented in this work thus demonstrate the feasibility of this cycle architecture in maximizing HTHP efficiency for industrial applications.

The global phase-down of high-Global Warming Potential (GWP) refrigerants has created a need for compressors optimized for alternatives such as R-290 and R-454C. Although compressors using these refrigerants exist commercially, the open literature offers limited guidance on how their thermophysical properties, including vapor density, volumetric capacity, and leakage sensitivity, should inform geometric design. Additionally, it lacks a clear, systematic methodology for adapting existing compressor platforms to alternative refrigerants. This work addresses these gaps by building on a validated experimental and numerical framework that matched manufacturer and experimental data within +/- 5 % and incorporated detailed leakage correlations, a motor map, and a six-lump thermal network in the mechanistic model. Using this foundation, a soft optimization procedure is applied to redesign an R-410A scroll compressor for use with R-290 and R-454C. The optimization establishes clear relationship between geometry and performance, showing that the displacement volume governs capacity for low-density refrigerants, the built-in volume ratio controls over-/under-compression losses and strongly affects efficiency, scroll height primarily affects flank leakage and volumetric efficiency, and the base radius balances leakage reduction against higher frictional and thrust-bearing losses. Optimized R-290 design achieves 74.4 % peak overall isentropic efficiency, a seasonal COP of 5.11, and about 3.4 % lower annual power use than the R-410A baseline. Optimized R-454C design reaches 73.9 % peak efficiency at matched capacity, and the R-410A baseline delivers 74.4 % peak overall isentropic efficiency. Overall, this methodology provides a transferable pathway for adapting legacy HFC-based compressors to emerging next-generation refrigerants and to different compressor architectures.

Meeting stringent carbon reduction goals in the commercial transport sector requires the rapid deployment of zero-emission solutions such as hydrogen fuel cell-powered heavy-duty vehicles. While proton exchange membrane fuel cells (PEMFCs) have demonstrated their viability in this context, significant parasitic losses in the air supply subsystem remain a barrier to further gains in overall system efficiency. The integration of a recuperator presents a substantial opportunity for improvement by enhancing expander energy recovery, thus reducing the net power consumption of the air supply system. This study systematically evaluates the impact of recuperator integration on heavy-duty vehicle PEMFC air system performance. Conceptual modeling and multiobjective optimization of two recuperator types—chevron-type plate heat exchanger and plate fin heat exchanger with offset strip fins—have been performed, using the non-dominated sorting genetic algorithm-II, to minimize pressure drop and maximize effectiveness under a predefined volume constraint. A steady-state model of an air system has been assessed under three distinct load conditions, integrating six optimized recuperator designs, along with one off-the-shelf, experimentally tested shell-and-tube unit. The results demonstrate potential for air system efficiency improvement through recuperator integration, particularly under high-pressure operating conditions. Among all evaluated configurations, one of the optimized plate fin heat exchanger designs achieves the greatest reduction in system power consumption, offering up to 5.29 % improvement at full load. Beyond confirming the efficiency benefits of recuperator integration, the findings underline the essential role of system-specific design optimization in realizing the full benefits of such integration.

The phase-down of high-global warming potential (GWP) refrigerants such as R-410 A, driven by the Kigali Amendment and regional regulations, has motivated the need for sustainable alternatives. Among alternative candidates, R-290 and R-454C offer favorable thermodynamic properties but lead to efficiency and capacity losses when used in compressors designed for hydrofluorocarbons (HFCs). This study establishes an experimental and numerical framework for developing compressors optimized for R-290 and R-454C but applicable for a range of alternative working fluids. Using experimental data collected in the scope of this project, a mechanistic compressor model was developed in the Positive Displacement Simulation (PDSim) environment to capture leakage, mechanical, and thermal losses. The model, featuring a multi-lump thermal network, achieved mean absolute percentage errors of 2.40%, 1.18% and 2.17% for mass flow rate, power consumption and overall isentropic efficiency, respectively, along with mean absolute error of 1.74 °C for discharge temperature. Energy distribution analysis showed that approximately 18% of electrical input power was lost through motor and mechanical frictional losses. The model-inferred energy distribution indicated that the scrolls, motor, and shell components are likely important heat-transfer regions, providing insights on internal thermal pathways. The developed framework provides an experimentally assessed foundation for compressor performance analysis and redesign studies. Future work will employ a soft-optimization approach to systematically adjust the displacement volume, built-in volume ratio, and leakage-sensitive parameters to enhance capacity and overall isentropic efficiency for R-290 and R-454C relative to the baseline R-410 A, and subsequently extend the analysis to other HFCs and their alternatives.

This study presents a comprehensive framework for assessing and enhancing energy efficiency across a fleet of passenger ships through waste heat recovery (WHR) integration. Leveraging operational data from 49 ships of different classes, sizes, ages, and operational locations, typical thermal demand and waste heat availability patterns have been characterized. Six representative scenarios of onboard energy demand have been identified through such analyses, and a scalable thermal demand management strategy has been proposed, considering heat pumps, organic Rankine cycles, and absorption chillers as potential WHR retrofit candidates. The impact of WHR integration has been assessed under varied operating and climatic conditions at both ship and fleet levels. Results show substantial electrical savings by reducing auxiliary boiler use and increasing utilization of recoverable heat. Across the fleet, total WHR improvement potential has been found ranging from approximately 2000 to 33,000 MWh/yr, with corresponding electrical savings up to 186,000 MWh/yr. The findings also highlight significant fleet-wide variability in terms of choice of technology and their impact linked to ship class, size, age, and duty. This emphasizes the need for data-driven retrofit strategies rather than a one-size-fits-all approach. The method and findings from this study provide valuable insights for decision-makers to select optimal technology combinations and deployment strategies to maximize energy efficiency and accelerate maritime decarbonization.

Identifying low-GWP refrigerant mixtures for specific heat pump applications requires coupling working fluid selection with cycle performance evaluation. This work proposes a framework which connects refrigerant screening at the molecular level with heat pump system simulation for a dishwasher application. A multiobjective genetic algorithm simultaneously optimizes binary and ternary mixtures from 27 pure substances, using coefficient of performance (COP) and global warming potential (GWP) as competing objective functions. Each candidate is evaluated in a vapor-compression cycle model that includes pressure drop estimation at an evaporation temperature of 276.15 K and condensation temperatures ranging from 303.15 K to 348.15 K. Mixture properties are calculated from the multi-fluid mixture model; where adjusted binary interaction parameters are unavailable, the model is supplemented by COSMO-SAC-dsp as gE-model in the theoretically based departure function for predictive capability. The algorithm converges to R-290/RE-170 (0.854/0.146 mol/mol) as the optimal blend, achieving a COP equivalent to the pure R-290 reference. The near-pure behavior of the optimum reflects the application-specific constraints that penalize temperature glide, limiting the design space for mixture benefits. The methodology is transferable to applications where glide-tolerant heat exchangers may unlock larger mixture advantages.

Machine learning (ML) methods are becoming commonplace in thermal systems for modeling complex physics-based relationships. These methods enable sensitivity studies to understand the effect of a variable on a system’s non-linear performance and model accuracy. In this study, the performance of a dual-loop diesel generator cooling system is investigated as a case study to understand which ML model most effectively balances accuracy and required training data, using other studies found in the literature as a starting point for the development of this framework. Four generator sets with different sized cooling systems are used as a database for training data of five unique ML algorithms, which are utilized for the prediction of seven cooling system performance metrics. The ML methods used in this study were chosen to represent various types of ML models commonly used in the literature, namely decision tree-based models, such as RF and GB, regression models such as LR and SVR, and KNN as a non-parametric model. This allows assessment and comparison of component and system prediction abilities between models. Additionally, the sensitivity of the model to the amount of training data and the related accuracy of the model outputs is explored and quantified. The results from this study are expected to provide new insights and design methodologies based on existing test data that can be achieved faster than when using traditional 1-D modeling efforts. The results reveal that airflow of the system can be predicted with an average mean absolute percentage error (MAPE) of 1.97% across all models. For temperature measurements, the MAE for each metric measured is under 2 K for nearly all model and performance metric combination, except for the LAT HT prediction, where MAE is less than 4 K. This work provides a framework for comparison of ML models on different performance metrics of a generator cooling system representative of governing principles and equations, the predictability of which by a given model type can ultimately be expanded to the modeling of various thermal systems.

Growing climate awareness has driven legislation to phase-out hydrofluorocarbons ( HFC), accelerating the search for sustainable refrigerants. Low Global Warming Potential (GWP) mixtures of hydrofluoroolefins (HFO), hydrocarbons, and Carbon Dioxide (CO2) are promising alternatives, yet lubricant interactions with certain refrigerant mixtures, and the resulting transport and thermo-physical properties, remain only somewhat understood. Accurate data are essential for component and system performance, especially at extreme temperatures below 233 K and above 393 K. This study presents an experimental setup to measure pressure, temperature, density, and viscosity of A1-A3 refrigerant mixtures with and without lubricants from 203 K to 463 K at up to 100 bar. As a baseline, pure R410A properties are assessed and compared with NIST REFPROP, R134a with PAG is assessed to validate the refrigerant lubricant interaction and high-temperature behaviour of R1336mzz(Z) with POE380 lubricant is also studied up to 413 K for high temperature heat pump applications.

Space cooling and heating systems in buildings use nearly 33% of the world's energy. To make heat pump (HP) systems more energy-efficient, especially during part-load operation, it is important to use variable-speed compressors. However, part-load conditions come with their own set of problems, primarily because the ideal amount of refrigerant varies based on both indoor and outdoor temperatures. Another difficulty is accurately controlling the expansion device when both operating conditions and refrigerant charge levels fluctuate. In this study, we examined an R-410A based 5-ton (17.6 kW) HP system that includes a variable-speed rolling piston compressor and an expander/separator replacing a traditional expansion valve. This expander/separator also recovers some energy during the expansion process. A validated steady-state HP model in cooling mode was used to simulate performance, with refrigerant charge gradually increased from 4.2 kg to 5.6 kg in steps of 0.01 kg across six different operating scenarios. The analysis revealed that the system achieved its highest coefficient of performance (COP) when the charge was around 5 kg, though this optimal point shifted slightly (+/- 0.03) based on operating parameters. While the cooling output rose steadily with added refrigerant, extra charge led to flooding in the expander/separator, which in turn lowered the energy recovery output. Furthermore, using the optimized refrigerant charge of 5 kg, the cycle was simulated to match the building loads for typical residential buildings in a mixed-humid climate (West Lafayette, IN, USA), and in an extreme hot-dry climate (Kuwait City, Kuwait) during summer months. Comparisons were made between an R-410A based 5-ton (17.6 kW) baseline-TXV cycle and the expander cycle. Similar overall hourly COPs were reported from these simulations. However, the cooling capacity from both locations was higher for the expander system than the TXV-system by 0.3 to 2 kW. As the HP was undersized for an extremely hot-dry climate zone, the expander was unable to generate power at peak temperature periods in Kuwait, given the expander over-flooded with refrigerant as opposed to the mixed-humid case. These results highlight the need for an effective system to manage refrigerant charge, as maintaining the right charge level under different environmental conditions is essential, especially with upcoming cooling technologies expected to use low-Global Warming Potential (GWP) refrigerants. In addition, there is a need to optimize the expander designs depending on the climate zones to achieve maximum COP improvements and power generation.

To reduce the direct global warming impact of refrigerants in HVAC&R applications, low-global warming potential (GWP) refrigerants, including natural refrigerants, have been extensively investigated as alternatives to hydrofluorocarbon (HFC) refrigerants. Among the natural refrigerants, Carbon Dioxide (CO2) offers several advantages, such as excellent transport and thermo-physical properties, being neither toxic nor flammable, and having a low price and high availability around the world. However, the high critical pressure and low critical temperature of CO2 often lead to transcritical operation, resulting in lower efficiency due to the additional compressor power necessary to achieve transcritical operation relative to subcritical HFC cycles. Therefore, a number of cycle modifications are used to enhance the coefficient of performance (COP) of transcritical CO2 cycles to meet or surpass those of HFC cycles. This paper provides a systematic experimental investigation of four such cycle architectures by employing the same multi-stage, two-evaporator CO2 refrigeration cycle test stand, 3 of these configurations in transcritical and 1 in subcritical conditions. The four cycles architectures included intercooling, open economization, an internal heat exchanger and two different ejector control approaches. Specifically, a variable-diameter motive nozzle and a variable-speed liquid CO2 pump located directly upstream of the ejector motive nozzle inlet were analyzed. Based on the experimental data, the maximum COP improvements are 4.64 % and 9.47 % when the ejector and the internal heat exchanger are used, respectively. The CO2 pump, once successfully stabilized, can control the ejector, increase its efficiency by up to 15% and increase the cooling capacity to a maximum of 6.2 %. Nevertheless, a reduction in COP is measured when the pump is in use; however, unlike the other three different configurations, it was only analyzed under subcritical conditions.

Approximately 40% of US energy is consumed by HVAC&R applications. With the push to reduce direct and indirect CO2 emissions, there is a need to adopt environmentally friendly and efficient refrigerants. Zeotropic mixtures represent an alternative to conventional pure refrigerants, meeting efficiency and environmental needs, but with their own challenges. Their heat transfer efficiency is reduced by temperature suppression, and fractionation in phase change can lead to circulating composition shift and thus system and compressor issues. These issues can be compounded through the presence of lubricants. To experimentally assess zeotropic behavior with and without lubricants, this paper details the development of a condenser test stand designed to assess composition shift of zeotropic mixtures of hydrofluorocarbons (HFCs), hydrofluoroolefins (HFOs), hydrocarbons (HCs), and CO2 with lubricants. Additionally, a physics-based model of two-phase zeotropic flow is introduced. The test stand will be later used to improve and validate the model.

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

In this investigated domestic dishwasher a basic heat pump cycle functions as heating source to achieve energy saving. To ensure resource-efficient performance, the working fluid needs to have a global warming potential (GWP) below 150, be non-toxic and have a high coefficient of performance (COP) within the system. For the working fluid screening propane (R290) is chosen as a reference refrigerant. Besides pure refrigerants, zeotropic blends with adapted temperature glides are of particular relevance for energy optimization. In this investigation, the predictive screening for a low-temperature glide blend resulted in a composition of isobutane (R600a) and dimethyl ether (DME, RE170) with a molar ratio of 0.06 to 0.94 (GWP = 1.15). An energy saving of up to 24.8 % comparing propane with the adapted zeotropic blend and an increase of the COP up to 171 % compared to the pure electrical heating system could be achieved.

Aircraft manufacturers are currently developing concepts to bring a hydrogen-powered aircraft to market by 2035. One option to realize this goal is a fuel cell based system, which enables emission-free and efficient flying. In contrast to a conventional aircraft that employs a gas turbine as its primary power source, the heat generated in the fuel cell must be actively rejected into the surrounding environment in order to maintain functioning. In this paper, the potential of two-phase cooling for fuel cell systems in aircraft is compared to liquid cooling systems for a 100 kW-stack. For this purpose, Modelica-based simulations are performed to analyze the operation and function of the proposed cooling systems. Two-phase cooling can greatly improve performance by achieving higher heat transfer coefficients and enthalpies of vaporization. Results indicate that two-phase cooling systems can reduce the required pump power by more than 98 %, and a novel bypass architecture offers further benefits in this regard. Additionally, the two-phase cooling systems can decrease the air volume flow rate needed to dissipate heat to the environment by 36 %, leading to a reduction in drag on the aircraft. The temperature difference between the fluid and the membrane can be reduced to less than 1 K, resulting in a more homogeneous temperature distribution in the fuel cell. Further research is necessary to establish the conditions under which two-phase cooling systems can operate stably without exceeding the maximum membrane temperature.

The need for decarbonization of the heat supply has led to increased attention to heat pump technology. When analysing endogenous exergy losses of the Rankine heat pump cycles, it becomes obvious that the expansion losses due to the isenthalpic pressure reduction, have a significant effect, especially for higher temperature lifts. For the first time, this publication presents the heat pump cycle of a novel thermodynamic cycle concept called Recuperative Two-Phase Cycle (RTPC), which intents to reduce these expansion losses using an internal heat exchanger, where an evaporation takes place on the low-pressure side. It is made possible by a non-linear zeotropic mixture composed of a main and an auxiliary working fluid. During the heat rejection to the heat sink, the temperature can have a glide or be nearly constant. In the case of isothermal conditions, the cycle resembles an ideal Ericsson cycle. In this work, the concept is derived from the state-of-the-art, different ideal cycle configurations and the theoretical challenges for a technical realization are presented. Furthermore, a description for a basic calculation is given and the potential for heat pump applications is illustrated by the study case of a heat pump with a temperature lift of 85 K between a latent heat source and sink. The endoreversible cycle approximating the shape of an ideal Ericsson cycle reaches a second law efficiency of 81.3 %, because the expansion losses can be reduced to 2.9 %.

A major part of energy consumption is reported from the residential and commercial buildings due to the large usage of mechanical devices to maintain a comfortable indoor ambiance. Thermal conductivity of the materials used in these buildings can be reduced by using low density materials and an efficient thermal energy storage strategy. In the present paper, the thermal performance of a building envelope was improved by incorporating phase change material (PCM) into the foam concrete mix. At first, PCM was developed by using a light weight material, expanded vermiculite (EV) impregnated with capric acid (CA) and ethyl alcohol (EA) under vacuum conditions. Different foam composite mixes were prepared and experimentally tested. Foam concrete mixtures with EV, CA-EA/EV based PCM, and PCM with added nano silica and coir fiber combinations (PSC) were pre-pared by the replacement of fine aggregate (M-sand) in various weight percentages. This study reveals that PCM with a CA to EV proportion of 55% (wt.) had the maximum adsorption and stability, as assessed through liquid leakage. In addition to improving thermal efficiency, PCM enhanced foam concrete composites showed improved mechanical, hydration, durability, and thermal characteristics. In comparison to the control foam concrete and PCM foam concrete mixes, the addition of nano silica and coir fiber added foam concrete mixtures (PSC) demonstrated improved strength and shrinkage qualities. These improved properties are attributed to the pozzolanic activity of nano silica and the bridging effects resulting from the fiber to micro cracks. Apart from the durability and strength characteristics, scanning electron microscopy (SEM), energy-dispersive X-ray spectros-copy (EDS), differential scanning calorimetry (DSC), and thermogravimetry analysis (TGA) were adopted to characterize the morphological, chemical, and thermal aspects of the different foamed concrete mixtures. The DSC results proved that the PSC-5% foam concrete mixture had latent heat capacities of 43.90 J/g for cooling and 69.16 J/g for heating, with melting and solidification temperatures of 31.51 degrees C and of 21.77 degrees C, respectively. TGA also revealed that the proposed PCM composite has a high thermal resistance. The PSC-5% shows better -compatibility as per SEM and EDS tests by the proper penetration of the nano silica in the foamed composi-tes through the pores. Finally, the thermal analysis tests demonstrated that the PSC-5% has a thermal conductivity of 0.129 W/m-K, which is 49.60% lesser than the foam concrete control sample, and also possesses an improved heat storage capacity, which ensures better thermal comfort in buildings.

Household appliances need to be both efficient and cost-effective. As a result, electrically driven tumble dryers were converted to heat pump driven ones several years ago in order to comply with energy directives, in this case in Germany. In order to increase the efficiency of heat pumps of different performance classes, the use of zeotropic refrigerant mixtures can be useful, since the temperature glide during the phase change of the refrigerant mixture can be adapted to the temperature gradients of the secondary fluids. In order to take advantage of this unique property, a refrigerant mixture selection was carried out by a screening procedure using REFPROP. Within the investigation presented here, two different zeotropic refrigerant blends, propane (R290)/dimethyl ether (DME or RE170) (30 wt.%/70 wt.%) and R152a/carbon dioxide (R744) (90 wt.%/10 wt.%) are focused on as substitutes with low global warming potential (GWP) for R134a in heat pump tumble dryers. The focus is on the interaction between the refrigerant mixtures and the respective lubricant to ensure usability in the heat pump tumble dryer. A specialised polyolefin-based (POE) lubricant was chosen for each refrigerant blend. Because the numerous combinations of possible refrigerant mixtures can lead to unanticipated challenges with oil mixtures, additional experimental investigations are necessary to ensure the compatibility of the lubricant and refrigerant mixture combination in the temperature ranges associated with the investigated application. Therefore, measurements of the vapor pressure, miscibility gaps and thermal conductivity of the refrigerant-lubricant mixture have been carried out and are the focus of this paper.

Due to the low critical temperature of CO2, refrigeration systems utilizing this natural refrigerant often operate transcritically, which often results in lower efficiency values than systems that use conventional refrigerants, caused by the associated high pressure. One common method used to increase the efficiency of CO2 refrigeration systems are ejectors, with which a part of the cycle expansion losses can be recovered. In this work the efficiency increases which can be achieved with a high-lift ejector system have been investigated both numerically and experimentally. Currently used ejectors are designed for a low to medium pressure lift, which results in low efficiency values when being operated outside of their design parameters. Additionally, operation outside of the design conditions leads to the risk of backflow in the ejector. Thus, and due to the complexity of the control strategy, previous ejector systems have only achieved a low level of popularity. In order to further improve the maximum efficiency of the ejector technology based on the investigated high-lift system, the development of a novel ultrahigh-lift-ejector (p(rec) > 1000 kPa) cycle with a subcooling heat exchanger was the focus the work presented in this paper. To carry out this work, measurement data of the high-lift ejector system were taken to validate the developed dynamic simulation results. An ejector map for the tested high-lift ejector was created and validated, which enabled the new ejector cycle with the additional subcooling heat exchanger to be simulated. The potential of the novel cycle to increase the efficiency of the refrigeration system was demonstrated and directly compared to the high-lift cycle, as well to a baseline system without an ejector. Both circuits showed efficiency increases of up to 9 % compared to the baseline circuit, whereby the non-optimized ultrahigh-lift cycle had slightly lower COPs compared to the high-lift cycle (Delta COP < 2 %).

This work presents an approach to the contextual integration of fluid selection and compressor design for the cycle design of efficient industrial heat pumps. The vapor-compression cycle of an air-water heat pump operated at 42 degrees C source and 82 degrees C target temperature is investigated as a theoretical case study. An optimization study is performed, which includes the assessment of suitable refrigerants. Besides well-known single-component refrigerants, various binary mixtures are considered. The cycle optimization aims at simultaneously providing high cycle coefficient of performance and volumetric heating capacity. Cycle operation with the mixtures R-41/trans-2-butene (10, 90mol. %) and CO2/R-161 (40, 60mol. %) yields the highest values of these parameters, respectively. For further evaluation, centrifugal compressors operated with each of the two promising mixtures are designed with an in-house meanline program. In addition, the compressor design for the hydrofluoro-olefin refrigerant R-1234ze(Z) is considered as a reference. All designs are reviewed with respect to cycle as well as compressor design criteria and the applied methodology will assist designers in identifying key decision variables. The comprehensive design assessment suggests that CO2/R-161 (40, 60mol. %) provides the best overall solution for an efficient cycle with a compact compressor design.

Modulating the speed of the compressors and adapting its capacity to the required load has led to a decrease in the annual energy consumption in many applications. However, in the compressor, having adequate lubrication at low speeds usually implies discharging too much lubricant at higher speeds. Although necessary for compressor operation, lubricating oil acts as a contaminant in the rest of the system. Consequently, manufac-turers have to consider the oil circulation rate (OCR), as it limits the speed range of the compressor. In a previous study, a compressor was tested over its speed range (30 - 110 Hz), working with R290 and POE68 as a lubricant. Experimental data confirm the increase of OCR with speed and suggests that working with R290-POE68 could imply higher OCR values compared to other refrigerant-oil mixtures. In this study, the impact of the OCR in the performance of a R290 heat pump was theoretically assessed. Three contributions were studied: The heat transfer coefficient (HTC) reduction in the evaporator, the effect of refrigerant being solved in oil and finally, and the impact of oil in the compressor. It was found that OCR values of 5% could decrease the coefficient of performance (COP) by more than 20%.