Vapor Compression Cycle Systems Research Articles

Vapor Compression Cycle: A State-of-the-Art Review on Cycle Improvements, Water and Other Natural Refrigerants

Air conditioning and refrigeration have become necessary in modern life, accounting for more than 7.8% of greenhouse gases (GHG) emitted globally. Reducing the environmental impact of these systems is crucial for meeting the global GHG emission targets. Two principal directions must be considered to reduce the environmental impact of air conditioning systems. Firstly, reducing the direct effect by looking at less harmful refrigerants and secondly, reducing the indirect effect by searching for options to improve the system efficiency. This study presents the latest developments in the vapor compression cycle and natural refrigerants, focusing on water as a refrigerant. Natural refrigerants, and especially water, could be the ultimate solution for the environmental problems associated with the operation of vapor compression cycle (VCC) cooling systems, including ozone depletion (OD) and global warming (GW). Reducing the environmental impact of building cooling systems is essential, and the recent system improvements made to enhance the system coefficient of performance (COP) are thoroughly discussed in this paper. Though the cycle improvements discussed in this work are essential and could increase the system efficiency, they still need to solve the direct environmental impact of refrigerants. Accordingly, this paper suggests that natural refrigerants, including water, are the most suitable strategic choice to replace the current refrigerants in the refrigeration and air conditioning industry. Finally, this study reviews the latest VCC system improvements and natural refrigerants in order to guide interested researchers with solutions that may reduce the environmental impact of VCC systems and suggest future research areas.

Data-driven modeling of high-speed centrifugal compressors for aircraft Environmental Control Systems

The Environmental Control System (ECS) is the main consumer of non-propulsive power onboard aircraft. The use of an electrically-driven Vapor Compression Cycle (VCC) system, in place of the conventional air cycle machine, can lead to a substantial increase of the coefficient of performance. This work documents the development of an integrated design optimization method for VCC-based aircraft ECS, where the sizing of the system is performed along with the conceptual design of the compact heat exchangers and the high-speed centrifugal compressor. A data-driven model of the compressor has been developed to reduce the complexity of the VCC system model and the computational cost of the associated optimization problem. The model is based on artificial neural networks and has been trained on a synthetic dataset of 165k centrifugal compressor designs, generated with an in-house tool. The case study selected to demonstrate the capabilities of the proposed methodology is the multi-objective design optimization of an electrically-driven VCC system for the ECS of a single-aisle, short-haul aircraft, flying at cruise conditions. The results show that the number of function evaluations needed to identify the Pareto front reduces by a factor of three when using the data-driven model, in place of a meanline method. At the same time, the robustness of the numerical solver is improved, leading to the identification of optimal solutions covering a wider design space. Finally, the proposed methodology enables the analysis of the trends established between the system performance metrics and the design of the individual components.

Thermodynamic Performance Comparison of CCHP System Based on Organic Rankine Cycle and Two-Stage Vapor Compression Cycle

Owing to different temperature rages of power generation and refrigeration in the cogeneration system, for the sake of selecting the working fluids that are suitable for both power generation and refrigeration simultaneously, 17 commonly used working fluids are evaluated in this paper, based on an organic Rankine cycle coupled with a two-stage vapor compression cycle system in different geothermal fluid temperatures. The performances of working fluids under different working conditions, and the maximum power generation as well as cooling capacity are analyzed. Additionally, the main parameters are analyzed to optimize the system performance. The results indicate that net power output has a local maximum where it corresponds to the optimal evaporation temperature. Besides, the lower the critical temperature, the greater the thermal conductance, and the pressure ratio decreases with evaporation temperature. Hydrocarbons all have higher total heat source recovery efficiency. R1234yf, propane and R1234ze, R152a have excellent maximum net power output when the geothermal fluid temperature is low and high, respectively. R134a always has better maximum net power output and cooling capacity. The net power output is used for cooling, and the COP is closed, therefore, maximum net power output results in the maximum cooling capacity. In addition, that of propane and R1234yf are excellent until the geothermal fluid temperature are 140 °C and 120 °C separately. R1234ze and R152a are good when the geothermal fluid temperatures are 140 °C and 150 °C, respectively.

Design Optimization of a High-Speed Twin-Stage Compressor for Next-Gen Aircraft Environmental Control System

Abstract The environmental control system (ECS) is the largest auxiliary power consumer, i.e., around 75% of non-propulsive power, among the aircraft subsystems. The adoption of a novel ECS architecture, based on an electrically-driven vapor compression cycle system, can enable a twofold increase of coefficient of performance at cruise conditions, as compared to the conventional air cycle machine. The core of this technology is a high-speed, miniature centrifugal compressor, consisting of two impellers mounted in back-to-back configuration, and running on gas bearings, operating with refrigerant. The fluid dynamic design optimization of the twin-stage compressor, to be installed in the vapor compression cycle test rig under realization at Delft University of Technology, is documented in this paper. First, the scaling analysis for centrifugal compressor is extended to provide guidelines for the design of twin-stage machines. Then, a multi-objective conceptual design optimization is performed by resorting to an in-house reduced-order model (ROM), coupled to a genetic algorithm. The fluid dynamic performance and the structural integrity of the optimal design are assessed by means of a hybrid framework, encompassing computational fluid dynamics and ROMs, and by finite element analysis. The results show that it is possible to design a twin-stage compressor for the target application, featuring an average efficiency higher than 70%, a maximum compression ratio exceeding 9, and an operating range of 0.27 at the design rotational speed, despite the detrimental effects of motor cooling and miniature size.

Energy and Exergy Analysis of Solar Organic Rankine Cycle Coupled with Vapor Compression Refrigeration Cycle

This study investigates a combined power generation and cooling system comprising an organic Rankine cycle (ORC) and a vapor compression cycle (VCC). Thermodynamic analyses of the system were conducted for two operation modes, i.e., the basic ORC mode and the ORC–VCC mode, using six different working fluids: R245fa, R114, R600, R142b, R152a, and R1234yf. The results showed that the thermal efficiency of the combined ORC–VCC system was almost twice that of the basic ORC system. The effects of thermodynamic parameters, such as the turbine inlet temperature and pressure and the condensing temperature, on the system performance were discussed. The second-law efficiency, cooling capacity, and coefficient of performance were addressed by varying the condensing temperature.

A hybrid piecewise FDD strategy for refrigeration charge fault of airborne vapor-compression cycle system

Many researchers have carried out study on refrigerant charge fault for variable refrigerant flow (VRF) air-conditioning system, while few pay attention to refrigerant charge fault for vapor-compression refrigerant cycle system (VCS) of enhanced vapor injection (EVI) with a flash tank onboard. To develop a strategy for refrigerant charge fault of the above system, the optimization methods have been implemented to predict the refrigerant charge level (RCL) based on virtual refrigerant charge (VRC) models. Firstly, Pearson correlation coefficient analysis is employed to obtain the characteristic parameters. Secondly, a piecewise modified VRC model with the different characteristic parameters from VRC models is obtained for two cases of RCL < 75% and RCL > 75%. Meanwhile, combined with the principal component analysis (PCA) and the support vector machine (SVM) algorithm, a PCA-SVM model is developed to improve the prediction accuracy, and the average root mean squared error (RMSE) is only 7.8 compared to 14.9 of the modified VRC model when RCL > 75%, meanwhile, the correctly detected ratio is improved from 62.8% to 84.5%. Then, for solving the difficulty of fault detection and diagnosis (FDD) when RCL > 160%, the moving time window average (MA) algorithm is introduced and an online PCA-MA-SVM model is developed. Finally, a hybrid piecewise FDD strategy combined with three models for refrigeration charge fault in airborne VCS of EVI with a flash tank is obtained.

Thermodynamic analysis of refrigerants used in ORC-VCC combined power systems for low temperature heat sources

Fossil resources are largely used for energy supply. This situation causes environ?mental pollution. In recent years, studies in the field of more environmentally friend?ly and sustainable energy conversion technologies have increased. In this context, organic Rankine cycle (ORC) technology is combined with RES. In this study, combined ORC and vapor compression cycle (VCC) were investigated. The electricity produced in the combined ORC-VCC system was used both in the compressor of the VCC system and in the plant. The main factor affecting the efficiency of the combined ORC-VCC system is the refrigerant. Therefore, it is necessary to examine the selection of the most suitable refrigerant for an ORC-VCC based system. Fifteen different refrigerants were optimized with the enginering equation solver program, and energy and exergy analyzes of the systems were made separately. According to the results, the best energy efficiency and COP values among the refrigerants was found to be R40 (?ORC = 0.1206) for the ORC system and R113 (COP = 4.405) for the VCC system. For all system components in the VCC, the most exergy destruction occurs in the evaporator, followed by the compressor, condenser, and throttle, respectively. In ORC, the most exergy destruction is in the evaporator, followed by the condenser, tube and pump, respectively. The total efficiency was found to be (? = 0.53) for the combined ORC-VCC system. The total exergetic efficiency was found to be (?glob = 0.26) for the combined ORC-VCC system.

Performance Assessment and Working Fluid Selection for Novel Integrated Vapor Compression Cycle and Organic Rankine Cycle for Ultra Low Grade Waste Heat Recovery

This paper presents the performance assessment and working fluid selection for a novel integrated vapor compression cycle-organic Rankine cycle system (i-VCC-ORC), which recovers ultra-low-temperature waste heat rejected (50 °C) by the condenser of a vapor compression cycle (VCC). The analyses are carried out for a vapor compression cycle of a refrigeration capacity (heat input) of 35kW along with the component sizing of the organic Rankine cycle (ORC). The effects of the operational parameters on integrated system performance were investigated. The integrated system performance is estimated in terms of net COP, cycle thermal efficiency and exergy efficiency by completely utilizing and recovering the heat rejected by the condenser of the VCC system. R600a-R141b with COPnet (3.54) and ORC thermal efficiency (3.05%) is found to be the most suitable VCC-ORC working fluid pair. The integration of the vapor compression refrigeration cycle with the organic Rankine cycle increases the COP of the system by 12.5% as compared to the standalone COP of the vapor compression system. Moreover, the sensitivity analysis results show that there exists an optimum operating condition that maximizes the thermal performance of the integrated system.

Evaluation of hybrid evaporative-vapor compression air conditioners for different global climates

Currently, a majority of global residential air-conditioning requirements are met using cooling systems based on the vapor compression cycle (VCC). To meet the future demand for space cooling and to reduce the corresponding environmental impact, there is a need for alternative cooling systems which require less resources and exhibit a higher coefficient of performance (COP). One of proposed alternatives is called the hybrid evaporative-vapor compression (HEVC) cycle, which utilizes adiabatic latent cooling in combination with the VCC. In this paper, we conducted a feasibility analysis of a HEVC system by comparing its performance to that of VCC for various global climates. To accomplish this, we developed and validated models to simulate the performance of residential VCC and HEVC systems. ASHRAE weather data and design conditions were then used to compare the performance of the two systems. In general, HEVC systems are best suited for hot arid climates with energy reduction greater than 20%. Conversely, humid climates are not suitable for HEVC adoption due to degraded performance of evaporative coolers in these climates. In addition to comparing the energy savings from HEVC systems, it is also critical to analyze their water consumption. Generally, climates that benefit the most from HEVC technology tend to also experience water scarcity issues. Thus, both the energy savings and water consumption of HEVC systems must be analyzed to guide the discussion on their adoption.

Effects of the driving voltage waveform on the performance of vapor compression cycle system driven by the moving coil oil-free linear compressor

The effect of driving voltage waveforms on the performance of refrigeration system driven by a moving coil oil-free linear compressor is experimentally investigated in this study. The tested driving voltage waveforms have three types: square, sine without the crest and trough, sine without the trough. The results show that the displacement curves under these three kinds of driving voltage are similar to that of the standard sine curve, but the current curves are completely different. The motor efficiency for each voltage waveform is lower than that of the sine waveform, especially that the motor efficiency of the square waveform is reduced by 40% at 60 Hz. Compared with the sine waveform, the coefficient of performance (COP) of the sine without the trough waveform is increased by 11% at 60 Hz resulted from the decrease of piston offset, while the COPs of the other two waveforms are lower than that of sine waveform due to the decrease of the motor efficiency. In addition, it is found that the sine without the trough waveform, as the only non-symmetric voltage waveform among the three tested waveforms, can effectively reduce the piston offset and obtain the similar performance to the sine waveform with direct component.

Thermodynamic analysis of a combined organic Rankine cycle and vapor compression cycle system activated with low temperature heat sources using low GWP fluids

A combined organic Rankine cycle and vapor compression cycle (ORC–VCC) system activated by low temperature heat sources was studied. Two low GWP fluids were considered as working fluids for the VCC and two different low GWP fluids for the ORC. System performance was evaluated through computational modeling over different operating conditions. The computed thermal COP of the ORC–VCC system varied between 0.30 and 1.10 over the range of operating conditions studied. The computed electrical COP of the ORC–VCC system, defined as the ratio of the rate of cooling and the ORC pump power consumption, varied between 15 and 110. The choice of VCC working fluid had only a limited influence on system thermal or electrical efficiency, with HFO-1234ze(E) presenting slightly better results. Use of HFO-1336mzz(Z) as the ORC working fluid resulted in slightly higher system thermal efficiencies and significantly higher system electrical efficiencies throughout the range of operating conditions studied. Furthermore, the system is evaluated for a typical application and the feasibility study shows good economical results.

System Identification and Control Design of Vapor Compression Cycle Systems

System Identification and Control Design of Vapor Compression Cycle Systems

Energy efficient model predictive building temperature control

Many systems used in buildings for heating, ventilating, and air-conditioning waste energy because of the way they are operated or controlled. This paper explores the application of model predictive control (MPC) to air-conditioning units and demonstrates that the closed-loop performance and energy efficiency can be improved over conventional approaches. This work focuses on the problem of controlling the vapor compression cycle (VCC) in an air-conditioning system, containing refrigerant which is used to provide cooling. The VCC considered in this work has two manipulated variables that affect operation: compressor speed and the position of an electronic expansion valve. The system is subject to constraints, such as the range of permissible superheat, and also needs to regulate temperature variables to set points. An MPC strategy is developed for this type of system based on linear models identified from data obtained from a first-principles model of the VCC. The MPC strategy incorporates economic measures in the objective function as well as control objectives. Tests are carried out on a simulated VCC system that is linked to a simulation of a realistic building that is developed in the U.S. Department of Energy Computer Simulation Program, EnergyPlus. The MPC demonstrated significantly better tracking control relative to conventional approaches (a reduction of 70% in terms of the integral of squared error for step changes in the temperature set-point), while reducing the VCC energy requirements by 16%. The paper describes the control approach in detail and presents results from the tests.

Decentralized Feedback Structures of a Vapor Compression Cycle System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In vapor compression cycle systems, it is desirable to effectively control the thermodynamic cycle by controlling the thermodynamic states of the refrigerant. By controlling the thermodynamic states with an inner loop, supervisory algorithms can manage critical functions and objectives such as maintaining superheat and maximizing the coefficient of performance. In practice, it is generally preferred to tune multiple single-input–single-output (SISO) control inner loops rather than a single multiple-input–multiple-output control inner loop. This paper presents a process by which a simplified feedback control structure, amenable to a decoupled SISO control loop design, may be identified. In particular, the many possible candidate input–output (I/O) pairs for decentralized control are sorted via a decoupling metric, called the relative gain array number. From a reduced set of promising candidate I/O pairs, engineering insight is applied to arrive at the most effective pairings successfully verified on an experimental air-conditioning-and-refrigeration test stand. </para>

Moving-Boundary Heat Exchanger Models With Variable Outlet Phase

Vapor compression cycle systems using accumulators and receivers inherently operate at or near a transition point involving changes of phase at the heat exchanger outlets. This work introduces a condenser/receiver model and an evaporator/accumulator model developed in the moving-boundary framework. These models use a novel extension of physical variable definitions to account for variations in refrigerant exit phase. System-level model validation results, which demonstrate the validity of the new models, are presented. The model accuracy is improved by recognizing the sensitivity of the models to refrigerant mass flow rate. The approach developed and the validated models provide a valuable tool for dynamic analysis and control design for vapor compression cycle systems.