Vapor Compression Cycle Research Articles

Thermo-Economic Analysis of a Hybrid Ejector Refrigerating System Based on a Low Grade Heat Source

The rising of the global energy demand requires the use of alternative energy conversion systems employing renewable sources. In the refrigeration and air conditioning fields, heat driven ejector systems represent a promising way to produce the cooling effect by using available low-grade temperature sources. In this paper, a thermo-economic analysis of a waste heat recovery hybrid ejector cycle (WHRHEC) was carried out. A thermodynamic model was firstly developed to simulate a WHRHEC able to obtain chilled water with a cooling load of 20 kW, by varying the working fluids and the pinch point values in the heat exchangers. Specific single- and two-phase heat transfer correlations were used to estimate the heat transfer surface and therefore the investment costs. The operative ranges that provide a reasonable compromise between the set-up costs and the cycle performances were then defined and compared to the current waste heat-driven technologies, such as absorption chillers and organic Rankine cycles (ORCs) coupled with vapor compression cycles (VCCs). The last part of the paper presents an economic analysis providing the map of the design (plant size) and contingent (specific cost of energy, waste heat availability) variables that lead to the economic convenience of a WHRHEC system when integrated to a conventional VCC plant.

Effect of multi –stages vapor compression refrigeration cycle using refrigerant R32 for Air-Conditioning unit

Vapor compression cycle is mainly employed as a refrigeration cycle in the Air-Conditioning (AC) unit. In order to save energy, the Coefficient of Performance (COP) of the need to be improved. One of the potential solutions is to modify the system into the multi-stages vapor compression cycle. The present work deals with the investigation of single-stage and multi-stage vapor compression refrigeration cycle. A typical vapor compression cycle that is used in the AC unit is taken into consideration. The used refrigerants are difluoromethane R32. The analysis using the simulation process by utilizing software Aspen One. The performance parameters includes COP, the mass flow rate of the refrigerant and compressor power. It was shown that there exists a maximum COP for multi-stage. By taking these performance simulation results the multi-stage is the efficient method for the air conditioning system.

Matching Testing Possibilities and Needed Experiments for Successful Vapor Compression Cycles in Microgravity

The literature shows five decades of interest in vapor compression cooling for spacecraft, while only very few conclusions can be drawn because of either non-systematic approaches or sparsely documented experiments. In contrast, the demand for high COP refrigerators and freezers has increased with the emerging plans of long duration manned space travel. Research is needed exploiting all available testing approaches to investigate two-phase cycles in microgravity environments. This paper presents relevant testing possibilities with their characteristics and outlines open questions regarding vapor compression cycles in space. Beneficial experiments are derived from open questions and matched with available testing methods to prescribe a path towards reliable and efficient refrigeration systems in microgravity.

Habitat selection in a peri-urban area by a large mammal indicates a low potential for human–wildlife conflict

Abstract ContextIn Australia, various species of macropods (family Macropodidae) are known to occur within peri-urban areas, where they can be a source of human–wildlife conflict. Some species, such as the eastern grey kangaroo (Macropus giganteus), have received considerable research attention over the past few years following demands from land managers for evidence-based management guidelines; however, the ecology of other macropod species found in peri-urban areas, such as the eastern wallaroo (Osphranter robustus robustus), remains poorly understood. AimsThe aims were to determine the home range of male and female eastern wallaroos and assess habitat selection in order to define whether wallaroos in a peri-urban environment should be viewed as thriving (‘matrix-occupying’), persisting (‘matrix-sensitive’) or struggling (‘urban-sensitive’). MethodsHome range and habitat use of six adult male, five adult female and one subadult male eastern wallaroo were investigated using GPS telemetry between October 2017 and May 2018 in the south-west of Sydney. Key resultsHome ranges (mean±s.e.) of males (63.1±10.2ha) were significantly larger than those of females (31.1±3.3ha). Every adult wallaroo had highly overlapping monthly home ranges, indicating strong site fidelity in all individuals. Eastern wallaroos selected habitats based on vegetation composition during the night and canopy cover during the day. Grassland and open native woodlands were preferred during foraging activities at night. By contrast, human-modified habitats, including hard surfaces and lawns, were avoided at all times by all individuals. ConclusionThe results indicate that eastern wallaroos avoid human-modified features in the landscape, so they could be viewed as persisting (‘matrix-sensitive’) in peri-urban areas. ImplicationsCompared with matrix-occupying macropods, such as the eastern grey kangaroo, the eastern wallaroo is less likely to cause human–wildlife conflicts – a result of its avoidance of human-modified habitat. Land-use planning, involving green corridors linking remnant vegetation, should be implemented as part of urban planning to enable the persistence of diverse mammal populations in urban areas, particularly matrix-sensitive species.

Solar PV power driven cold room storage for Saharan rural area

The use of renewable energy sources is usually a reliable alternative in rural areas and developing countries, where the gridline does not exist or is at a great distance. The present work is to study the possibility of feeding the tomatoes preservation cold room storage in the Sahara region driven by solar photovoltaic generator. An energy-efficient design has been achieved by analytical calculation. A PV field calculation approach yielded the following results: for a storage capacity of about 1 ton of tomatoes, the cooling capacity is 2.5 kW. The use of the design unit gave us in terms of electricity requirements 27 kWh/day by the GPV generator at Adrar province. Electrical power demands can be obtained using an alternative solution, which appears in Vapor Compression Cycles driven by PV modules. This technology considered as promoting concept for the south of Algeria investment.

Experimental analyses of different control strategies of an R-410A split-system heat pump by employing a turbomachinery expansion recovery device

Abstract With increasing need for energy efficient vapor-compression cycles (VCCs), researchers have investigated different aspects of such systems to increase performance. The goal of this research is to experimentally investigate the design and control of a turbomachine expander to recover work and control the VCC. An R-410A split-system heat pump experimental setup has been utilized to compare the performance and control capabilities of a variable nozzle and a fixed nozzle with phase separation combined with evaporator bypass flow metering. A theoretical analysis on the benefit of the expander in heat pumps motivated heat pump operation in cooling mode. The variable nozzle experiments yielded a significant decrease in expander isentropic efficiency, and the evaporator bypass control led to an increase in system coefficient of performance (COP) of 2.3% with an expander overall isentropic efficiency of 18.8%. Compressor suction superheat control was achieved with the evaporator bypass over a range of ambient conditions.

Air conditioning systems for aeronautical applications: a review

ABSTRACTThis paper presents a review of the various aeronautical air conditioning systems that are currently available and discusses possible system configurations in the context of the aeronautical environmental control systems. Descriptions of the standard vapor compression cycle and air cycles are provided. The latter includes, simple-cycle, bootstrap-cycle, simple-bootstrap cycle (3-wheel) and condensing cycle (4-wheel). Water separation and air recirculation systems are also explored. A comparison between vapor compression cycles and air cycles is provided, as well as a comparison between different air cycles. Air cycle units are far less efficient than vapor compression cycle units, but they are lighter and more reliable for an equivalent cooling capacity. Details regarding the aircraft conceptual design phase along with general criteria for the selection of an air conditioning system are provided. Additionally, industry trends and technological advances are examined. Conclusions are compiled to guide the systems engineer in the search for the most appropriate design for a particular application.

Energy performance of next-generation dedicated outdoor air cooling systems in low-energy building operations

A conventional chiller system based on the vapor-compression cycle, widely used in dehumidification and cooling, has its limitations in energy performance and environmental concerns. Previous studies have mainly focused on analyzing one specific type of alternative cooling technology in a certain climate or for a specific building. In this study, we apply thermodynamic models to systematically evaluate the energy efficiencies of next-generation dedicated outdoor air cooling systems, namely those with desiccants and membranes, and compare them against the industry benchmark, revealing their prospects and limitations at different time scales and locations. The innovative systems we propose are coupled with energy recovery and renewable energy systems to further enhance the system energy efficiencies. A sensitivity analysis on equipment performances further informs how to choose equipment parameter values to boost the system energy performances. Results reveal the principles of how to systematically organize cooling equipment to achieve potential energy savings from a thermodynamic point of view, and highlight 100% increase in working-hour average system COP for desiccant and membrane cooling systems exampled in a low-energy office building at Boston. Parametric studies in different climates and working conditions further substantiates the superior system energy performance of next-generation cooling technologies in latent heat removal.

A multistage elastocaloric refrigerator and heat pump with 28\u2009K temperature span

Elastocaloric refrigerators are a promising alternative to the vapor compression cycle because they do not require refrigerants with high global warming potential. However, these coolers have yet to achieve temperatures low enough to — for example — be used as standard household refrigerators. We built one-stage, two-stage, and three-stage elastocaloric cooling systems to determine if staging of the elastocaloric effect could significantly expand temperature span. Our three-stage system achieved 1.5 times the maximum temperature span of our single-stage system, and produced the highest temperature span of any elastocaloric device to-date at 28.3 °C, where previously the record was 19.9 °C. Interestingly, we found that multistage systems can achieve equivalent temperature spans but at higher coefficients of performance compared to similarly-constructed single-stage systems.

A dynamically controllable evaporative cooling system for thermal management of transient heat loads

Evaporation of thin liquid films has the potential to sustain significantly high heat fluxes. This cooling mechanism has been studied extensively to create devices capable of handling large static heat loads for electronic cooling. However, significant and sudden variations in the heat loads can result in the dryout of the thin film, which can jeopardize the performance of the cooling system causing irreversible damage to the electronic component. Hence, a mechanism capable of controlling the performance of evaporative cooling is necessary. This study presents a unique strategy that combines a novel evaporator with a vapor compression cycle, whose performance can be varied to address the transient heat loads. The evaporator is designed to take advantage of simultaneously-occurring multiple modes of heat transfer to dissipate large heat fluxes. This study presents a device and system-level analysis to quantify the effect of different system operating conditions that control the overall cooling performance and efficiency.

Capacity enhancement of air conditioning systems by direct integration of a latent heat storage unit

With increasing use of renewable energy sources, efficient and consistent use of these energies leads to the necessity of storage facilities. These systems allow for uncoupling of energy production and consumption. Besides battery storages linked to electrical heat pumps, thermal storages have huge potential to provide flexibility for heating and cooling purposes. In combination with air/air-based air-conditioning systems, in particular latent heat storages (LHS) offer several advantages. In this publication, the use of a LHS serving as a subcooler for the liquid refrigerant flow leaving the condenser of a vapor compression cycle is investigated. The concept aims at an increase of the peak cooling power of the refrigeration cycle by unloading the LHS. Theoretical and experimental investigations using a LHS containing paraffin as phase change material (PCM) are presented. The results of the calculation model underline the feasibility and predict an increase of the cycle’s refrigeration capacity by about 30% during unloading of the LHS, resulting from a sub-cooling of the refrigerant by about 25 K. The storage concept and theoretical characterization of the thermal behavior of the LHS during loading and unloading are demonstrated and validated by laboratory experiments. With these findings, a practical proof for the implementation of direct heat exchange between refrigerant and phase change material is given, going beyond conceptual considerations presented in literature.

Comparative study of combined organic Rankine cycle and vapor compression cycle for refrigeration: Single fluid or dual fluid?

Combined organic Rankine cycle and vapor compression cycle (ORC-VCC) systems for heat activated cooling are widely concerned because of their simple structures and convenient maintenance. According to the number of the working fluids, there are two different ORC-VCC systems, which are single-fluid and dual-fluid ORC-VCC systems. However, it lacks a deep understanding of their applicability. In this paper, using the geothermal water of 140 °C as heat source, the flash tank vapor injection cycle is coupled with ORC (ORC-FTVIC) in order to improve the efficiency. Meanwhile, four different systems of ORC-VCC and ORC-FTVIC based on single-fluid and dual-fluid are compared. Six kinds of working fluids are selected to analyze the effects of system parameters on the system performance. Taking the second law efficiency of thermodynamic as the objective function, the key parameters and working fluids of four heat activated cooling systems are optimized simultaneously and compared. When the heat source temperature is 140 °C, the cooling capacity of dual-fluid ORC-FTVIC system is maximum (2282.0 kW), followed by single-fluid ORC-FTVIC system (2177.5 kW), dual-fluid ORC-VCC system (2132.8 kW), single-fluid ORC-VCC system (1918.8 kW). The corresponding optimum working fluids are R1234yf-R152a, R1234yf, R1234yf-R152a and R1234yf, respectively.

Comparison of approximation-assisted heat exchanger models for steady-state simulation of vapor compression system

Finite-volume heat exchanger models are generally the most time-consuming component in a Component-based Vapor Compression Cycle simulation. While the approach of Performance Maps is commonly used, this method has its limitations. This study compares 3 approximation-assisted heat exchanger (HX) models for the steady-state simulation of vapor compression system: interpolation black-box model, Kriging black-box model, and Kriging-assisted three-zone model. We applied the 3 models to approximate the pressure drop (ΔP) and enthalpy change (Δh) of five different HXs at ten-thousand refrigerant inlet conditions randomly selected from the respective input domains of the HXs. Moreover, we applied the 3 models to simulate three different vapor compression systems at various test conditions. The approximation results were compared against the baseline finite-volume results. The results show that, for approximation of HX performance, Kriging metamodel gave the most accurate approximation with an overall ΔP and Δh mean absolute error (MAE) of 4.46% and 0.9%, respectively. For system simulations, Kriging metamodel gave the most accurate results. Its largest COP and capacity errors were 2.54% and 1.45%, respectively. The results also show that the three-zone model gave large error for Δh approximation at those operating conditions where the HX had an average two-phase outlet condition (xout,avg < 1.0) but with superheated vapor in part of the coil (vap% > 0). For computational efficiency, by using the approximation-assisted models, the system simulation time was sped up by a factor of 10 to 600, depending on the testing conditions.

Energy and exergy performance comparison of different HFC/R1234yf mixtures in vapor-compression cycles

To increase the COP and volumetric capacity of hydrofluoroolefins while reducing the global warming potential (GWP) of hydrofluorocarbons (HFCs), four HFC/R1234yf mixtures with various compositions are compared with verified thermodynamic models. Results show that a lower R1234yf mass fraction leads to a higher mixture latent heat; R32/R1234yf has the highest pressures, the lowest pressure ratios and the biggest temperature glides. As the R1234yf mass fraction increases from 0.0 to 1.0, the cooling coefficient of performance (COP) first increases from 5.25 to 5.52 and later decreases to 5.30 for R32/R1234yf, while it decreases from 5.46, 5.63 and 5.30 to 5.29 for R134a/R1234yf, R152a/R1234yf and R161/R1234yf. The heating COP first increases from 3.90 to 4.00 and later decreases to 3.79 for R32/R1234yf, while it decreases from 3.84, 3.95 and 4.02–3.79 for others. Caused by different volumetric capacities, R32/R1234yf requires a compressor enlarged by 2.8 times, R134a/R1234yf and R152a/R1234yf requires little change on compressor size, while R161/R1234yf requires a compressor enlarged by 1.5 times. R32/R1234yf yields the highest discharge temperature, while R134a/R1234yf yields the lowest. R32/R1234yf shows the highest exergy COPs (ECOPs) when the R1234yf mass fraction is above 60% in cooling mode and 46% in heating mode. Otherwise, R152a/R1234yf performs the best in cooling model and R161/R1234yf performs the best in heating mode. Considering both GWP and efficiency, the optimal composition is 20/80% for R32/R1234yf, 10/90% for R134a/R1234yf and 100/0% for both R152a/R1234yf and R161/R1234yf. This study provides suggestions for the determination of optimal compositions of different HFC/R1234yf mixture refrigerants.

Theoretical thermodynamic performance assessment of various environment-friendly novel refrigerants used in refrigeration systems

The present investigation focuses on theoretical performance of various new environment-friendly refrigerant mixtures as substitutes to high global warming potential refrigerant R22. In this investigation, 34 refrigerants were considered at various composition. In this work, both complex vapor compression cycle (actual cycle) and standard vapor compression cycle (ideal cycle) was considered for the performance assessment of refrigerants. Vital studies such as flammability, toxicity, and environmental impact of various novel refrigerants were also carried out in this study. Results obtained from actual cycle showed that the coefficient of performance of refrigerant mixture RM40 (R1270/R134a 90/10 in mass %) (2.728) was the greatest among 34 investigated alternatives and it was closer to the coefficient of performance of R22 (2.770). Compressor discharge temperature of RM40 was 13.36 ℃ lower when compared with R22. Volumetric refrigeration capacity of RM40 (3335 kJ/m3) was slightly higher than that of R22 (3297 kJ/m3). Power spent per ton of refrigeration of RM40 (1.288 kW/TR) was marginally higher than that of R22 (1.269 kW/TR). Global warming potential (GWP100) of RM40 (133) was very low compared to the GWP100 of R22 (1760). Total equivalent warming index (environmental impact) of RM40 was 5.61% lower than R22. However, performance results obtained from standard cycle for various investigated refrigerants were better than actual cycle, since various losses occur were neglected in the standard cycle. Overall, thermodynamic performance of refrigerant mixture RM40 (R1270/R134a 90/10 in mass %) obtained from both actual and standard cycle was the highest among 34 investigated refrigerants and it was very closer to the performance of R22 and hence, it could be considered as an environment-friendly alternative to replace high GWP refrigerant R22 used in refrigeration systems.

Evaluating Recharge Options for Phase-Change Material Storage of a Personal Conditioning System

The Roving Comforter (RoCo) is a vapor compression-cycle-based personal conditioning system (PCS). It stores its condenser waste heat into a phase-change material (PCM)-based thermal storage. When the PCM is completely melted, the PCM storage needs to be recharged before the next cooling operation. This may be conducted in two different ways: (i) a two-phase thermosiphon operation that consumes much less power and requires a longer recharge time (∼6.5 h), or (ii) a heat pump operation that consumes more power with a much shorter operation time (∼2.5 h). The objective of this article is to analyze the merits and demerits of these two recharge operations. A validated dynamic model of the thermosiphon and heat pump in Modelica is used to evaluate the overall coefficient of performance for combined cooling and recharge operations. The modeling framework discussed here is fairly generic and facilitates investigating vapor compression cycle integration strategies with PCM storage modes. The article provides interesting insights and quantification of benefits obtained from various strategies adopted for improving the PCS.

Theoretical research of the performance of a novel enhanced transcritical CO2 refrigeration cycle for power and cold generation

Three different refrigeration cycles, namely the vapor compression cycle, the combined refrigeration cycle and the new combined refrigeration cycle for power and cold, are investigated. The principles of mass and energy conservation are applied to every component of the systems, and the resulted linear system of equations was numerically solved. It was found that the new proposed cycle had a lower optimum gas cooler pressure than other refrigeration cycles, and this would enhance the lifetime and safety of the system. Besides, it was found that the coefficient of performance of the new cycle is higher than that of the vapor compression and combined refrigeration cycles 110% and 50%, respectively. The exergy efficiency was approximately 78% and 58% higher than that of the vapor and combined refrigeration cycles, respectively. Furthermore, the exergoeconomic results of the new cycle showed that the total unit cost was lower by 16.6% and 13% than that for the vapor compression and combined refrigeration cycles, respectively.

A Waste Heat-Driven Cooling System Based on Combined Organic Rankine and Vapour Compression Refrigeration Cycles

In this paper, a heat driven cooling system that essentially integrated an organic Rankine cycle power plant with a vapour compression cycle refrigerator was investigated, aiming to provide an alternative to absorption refrigeration systems. The organic Rankine cycle (ORC) subsystem recovered energy from the exhaust gases of internal combustion engines to produce mechanical power. Through a transmission unit, the produced mechanical power was directly used to drive the compressor of the vapour compression cycle system to produce a refrigeration effect. Unlike the bulky vapour absorption cooling system, both the ORC power plant and vapour compression refrigerator could be scaled down to a few kilowatts, opening the possibility for developing a small-scale waste heat-driven cooling system that can be widely applied for waste heat recovery from large internal combustion engines of refrigerated ships, lorries, and trains. In this paper, a model was firstly established to simulate the proposed concept, on the basis of which it was optimized to identify the optimum operation condition. The results showed that the proposed concept is very promising for the development of heat-driven cooling systems for recovering waste heat from internal combustion engines’ exhaust gas.

Scale limitations of gas bearing supported turbocompressors for vapor compression cycles

An analysis of single-stage centrifugal compressors supported on dynamic, gas lubricated bearings for driving vapor compression cycles suggests that the maximum feasible compressor power is limited, primarily by the bearing load capacity, in the range from 80 kW to 250 kW. The analysis further suggests that the maximum feasible compressor power depends significantly on working fluid selection. Low-pressure fluids result in large impellers with good efficiency but heavy machines, the power of which is limited by the bearing load capacity. High-pressure fluids require small machines rotating at high speeds, the power of which is limited by the rotordynamic eigenfrequencies. Consequently, there is a family of fluids that allows maximization of the power range of gas bearing supported turbocompressors. Finally, the analysis suggests that the maximum compressor power can be expressed as a function of the reduced evaporation pressure, regardless of the working fluid choice. Maximum COP is reached at a reduced evaporation pressure decreasing nearly linearly from 0.22 to 0.11 as the temperature lift between the evaporator and the condenser increases from 20 K to 50 K. The feasible maximum compressor power increases with increasing specific compressor speed, bearing tip speed (NDm) and rotordynamic stability safety margin.

Effect of internal heat exchanger to the performance of vapor compression cycle using refrigerant R32

Vapor compression refrigeration cycle generally is used in the Air-Conditioning (AC) system. In order to increase the energy efficiency, the Coefficient of Performance (COP) of the cycle need to be enhanced. Many innovations can be used, one is to utilize internal heat exchanger (IHX). In this work, the effect of IHX in the vapor compression cycle with refrigerant R32 is investigated and compared with refrigerant R22. The analyses are carried out at temperature evaporation of -5°C, 0°C, and 5°C by using software ASPEN PLUS. The results show that effect of IHX in the cycle with refrigerant R22 increases the COP from 22.78% to 25.61%. On the other hand, in the cycle with refrigerant R32 the COP increases from 33.79% to 36.80%. The average increase for the cycle with refrigerant R22 and refrigerant R32 is 24.14% and 37.04%, respectively. This fact reveals that IHX in the cycle with R32 is more effective that the cycle with refrigerant R22.