Vapor Compression Cycle Research Articles

A review on the operational instability of vapor compression system

Operational instability, or hunting, is widely observed in vapor compression systems (VCSs), negatively affecting the operational safety and efficiency. As one of the most significant challenges present in VCS operation and control, operational instability has been extensively studied for decades. This paper reviews the current research state on the operational instability of VCSs, including the mechanisms for triggering hunting, the measures to mitigate hunting and the related unsolved issues. Two different views, i.e. the inherent characteristics of two-phase evaporating flow and dynamic characteristics of expansion (EV)-evaporator control loop, were classified for explaining the causes of hunting. As the typical characteristics of two-phase evaporating flow, the present of slug flow upstream in an evaporator, the sudden variation of heat transfer mechanism or two-phase flow instability would trigger the unexpected change of refrigerant temperature at evaporator exit. Consequently, operational instability will be resulted in. The dynamic behaviours of the EV-evaporator control loop in terms of its nonlinearities were considered as another essential factor that would cause system instability. Superheat setpoint adjustment and adaptive superheat control were suggested as two effective measures for mitigating hunting. With the increasing applications of vapor compression cycle for electronic cooling and the multi-evaporator VCSs, more effects should be done to investigate their operational instability with the consideration of two-phase flow instability and the coupling influences of each EV-evaporator control loop.

Fuzzy modeling approach for transient vapor compression and expansion cycle simulation

Abstract Previous mode switching algorithms for heat exchanger moving boundary models in the literature are composed of a set of IF-THEN rules. These representations could lead to numerical challenges due to the inherited discontinuities associated with IF-THEN rules. This paper presents an alternative mode switching methodology which results in a continuous moving boundary heat exchanger model over all possible mode changes. Numerical performance of the proposed method for a heat exchanger has been tested using simulations and sample results are compared with a moving boundary model with switching based on IF-THEN rules and also a finite-volume method. The proposed switching algorithm was implemented within a complete vapor compression cycle model and results were compared with experimental data for a start-up transient.

Performance comparison of low GWP refrigerants for a miniature vapor compression system integrated with enhanced phase change material

An innovative personal cooling system (PCS) called the Roving Comforter (RoCo) uses an R134a-based vapor compression cycle (VCC) to provide cooling. To reduce environmental impact, it is necessary to evaluate RoCo performance with low GWP refrigerant alternatives. The condenser heat of the VCC is stored in a phase change material (PCM)-based latent thermal storage, while a serpentine micro-channel heat exchanger (MCHX) is used as the evaporator in the system. This article discusses the development of dynamic models for both PCM-condenser and MCHX-evaporator. A system model is then developed and validated for RoCo using Modelica. The validated model is used to compare the system performance for RoCo with R32, R1234yf, R1234ze(E) and R290 so that they provide the same amount of cooling. Compared to R134a, the change in COP is +8%, −12%, −3% and −5% with R32, R1234yf, R1234ze (E) and R290, respectively. The reduction in system charge with R32, R1234yf, R1234ze (E) and R290 is 1%, 11%, 30% and 52%, respectively. The compressor RPMs for R134a, R32, R1234yf, R1234ze (E) and R290 are 2100, 900, 2400, 2900 and 1700 respectively.

Review of vapor compression refrigeration in microgravity environments

Abstract Using a vapor compression cycle for cooling in microgravity environments was already suggested in the 1970s to leverage the high coefficient of performance. Since then, only a few systems have operated in microgravity with scarce documentation of these flights. The lack of measured data and detailed documentation makes identifying the necessary next steps difficult for researchers entering the field of refrigeration in space. This paper provides a review of available literature for vapor compression systems flown in microgravity by outlining the history of vapor compression devices in space and presenting performance data. Moreover, gaps in the literature are highlighted and open questions are posed based on the reviewed material. Next steps of research are suggested to support and ultimately achieve reliable vapor compression refrigeration in space. Calculating equivalent masses for a fair comparison of different microgravity cooling technologies is proposed by capturing both energy consumption and used volume.

A review on independent and integrated/coupled two-phase loop thermosyphons

Two-phase loop thermosyphon is a natural phase-change heat transfer cycle widely used for thermodynamic applications. In additon to low manufacturing and maintenance costs, the primary advantages of such device include high heat transfer capability, reliability, and sustainability. Significant progress has been made in the two-phase loop thermosyphon and its integrated/coupled systems for energy conservation and environment protection. This paper represents a comprehensive review that discusses the physical mechanisms, characteristics and research progress of the two-phase loop thermosyphon based on recent experimental and simulative studies. The review extends to worldwide applications of independent two-phase loop thermosyphon in the renewable and sustainable energy fields and its utilisation for performance enhancement of vapour compression cycles by system integration and coupling. Challenges and recommendations for future research and application of two-phase loop thermosyphon are identified. This review will help researchers and engineers better understand the mechanisms, features and potential of the two-phase loop thermosyphon technology and promote its development.

Energy Analysis of a Novel Ejector-Compressor Cooling Cycle Driven by Electricity and Heat (Waste Heat or Solar Energy)

Low-grade heat is abundantly available as solar thermal energy and as industrial waste heat. Non concentrating solar collectors can provide heat with temperatures 75–100 °C. In this paper, a new system is proposed and analyzed which enhances the electrical coefficient of performance (COP) of vapour compression cycle (VCC) by incorporating low-temperature heat-driven ejectors. This novel system, ejector enhanced vapour compression refrigeration cycle (EEVCRC), significantly increases the electrical COP of the system while utilizing abundantly available low-temperature solar or waste heat (below 100 °C). This system uses two ejectors in an innovative way such that the higher-pressure ejector is used at the downstream of the electrically driven compressor to help reduce the delivery pressure for the electrical compressor. The lower pressure ejector is used to reduce the quality of wet vapour at the entrance of the evaporator. This system has been modelled in Engineering Equation Solver (EES) and its performance is theoretically compared with conventional VCC, enhanced ejector refrigeration system (EERS), and ejection-compression system (ECS). The proposed EEVCRC gives better electrical COP as compared to all the three systems. The parametric study has been conducted and it is found that the COP of the proposed system increases exponentially at lower condensation temperature and higher evaporator temperature. At 50 °C condenser temperature, the electrical COP of EEVCRC is 50% higher than conventional VCC while at 35 °C, the electrical COP of EEVCRC is 90% higher than conventional VCC. For the higher temperature heat source, and hence the higher generator temperatures, the electrical COP of EEVCRC increases linearly while there is no increase in the electrical COP for ECS. The better global COP indicates that a small solar collector will be needed if this system is driven by solar thermal energy. It is found that by using the second ejector at the upstream of the electrical compressor, the electrical COP is increased by 49.2% as compared to a single ejector system.

Optimisation of Control Input Allocation Maps for Electric Vehicle Heat Pump-based Cabin Heating Systems

The heating, ventilation and air conditioning (HVAC) system negatively affects the electric vehicle (EV) driving range, especially under cold ambient conditions. Modern HVAC systems based on the vapour-compression cycle can be rearranged to operate in the heat pump mode to improve the overall system efficiency compared to conventional electrical/resistive heaters. Since such an HVAC system is typically equipped with multiple actuators (compressor, pumps, fans, valves), with the majority of them being controlled in open loop, an optimisation-based control input allocation is necessary to achieve the highest efficiency. This paper presents a genetic algorithm optimisation-based HVAC control input allocation method, which utilises a multi-physical HVAC system model implemented in Dymola/Modelica. The considered control inputs include the cabin inlet air temperature reference, blower and radiator fan air mass flows and secondary coolant loop pumps’ speeds. The optimal allocation is subject to specified, target cabin air temperatures and heating power. Additional constraints include actuator hardware limits and safety functions, such as maintaining the superheat temperature at its reference level. The optimisation objective is to maximise the system efficiency defined by the coefficient of performance (COP). The optimised allocation maps are fitted by proper mathematical functions to facilitate the control strategy implementation and calibration. The overall control strategy consists of superimposed cabin air temperature controller that commands heating power, control input allocation functions, and low-level controllers that ensure cabin inlet air and superheat temperature regulation. The control system performance is verified through Dymola simulations for the heat pump mode in a heat-up scenario. Control input allocation map optimisation results are presented for air-conditioning (A/C) mode, as well.

Toward sustainable refrigeration systems: Life cycle assessment of a bench-scale solar-thermal adsorption refrigerator

Abstract Adsorptive refrigerators are an unconventional alternative to modern refrigerators employing the vapor compression cycle. Adsorptive refrigerators do not require electricity and can be constructed from widely available and environmentally benign materials, making them a particularly appealing approach to creating refrigeration in remote, impoverished areas where electricity is unavailable or unreliable. This paper describes a prototype that uses ethanol and activated carbon, two widely available and environmentally friendly materials, as the key components of a solar-powered adsorptive refrigerator. The Solar Thermal Adsorption Refrigerator (STAR) prototype obtained temperatures between 2⁰C and 8⁰C on the external wall of the vacuum tube. Results showed that a significant amount of life cycle environmental benefits can be achieved from STAR compared to a conventional refrigerator. Successfully replacing a conventional refrigerator with STAR can annually reduce: 292 ~ 1170 kg CO2-eq of global warming potential (GWP); 104 ~ 418 H+ moles-eq. of acidification; 1.15 ~ 4.61 kg benzene- eq. of carcinogenic; 6850 ~ 27,400 kg toluene-eq. of non-carcinogenic; 1.2 ~ 4.78 kg N-eq. of eutrophication; 0.556 ~ 2.22 kg PM2.5-eq. of respiratory effects; 0.0078 ~ 0.031 g of CFC-11 eq. of ozone depletion; 0.564~2.26 g NOx-eq. of smog. Considering the typical lifetime of a conventional fridge of 13~19 years, the reduction of the aforementioned environmental impacts of the STAR can be scaled significantly.

The fuel cell and atmospheric water generator hybrid system for supplying grid-independent power and freshwater

Despite the success of atmospheric water generators for providing drinking water to remote regions, this technology has a high specific energy consumption. This paper proposes to reuse the electrochemical water of the fuel cell for the vapor compression cycle based atmospheric water generator (VCC-AWG); After passing through an ambient heat exchanger to remove the electrochemical waste heat, the fuel cell flue gas that enters the VCC-AWG is at a higher relative humidity than natural atmospheric air, thus the freshwater yield per energy input can be significantly increased. Hence, the FC-VCC-AWG hybrid system is proposed to be a power and freshwater supply of a grid-independent home. The fuel cell model, the vapor compression cycle’s thermodynamic model, and humid air physics are coupled to analyze the overall system in terms of the fuel cell’s working condition, incoming airflow rate, compressor power consumption, and the ambient relative humidity. When RH = 0.75, adding a 2 kW fuel cell generated up to 3 kg/hr of freshwater, which is 50% higher than excluding the FC. The specific energy consumption can be 200 Wh/kg, so the VCC-AWG can be integrated with small sacrifices to the FC power output.

Parametric analysis of a solar-driven trigeneration system with an organic Rankine cycle and a vapor compression cycle

The objective of this paper is the parametric analysis of a solar-fed trigeneration system ideal for the building sector that produces useful heat, electricity and cooling. The examined unit is driven by 100 m2 of parabolic trough collectors which are combined with a sensible storage tank with thermal oil. An organic Rankine cycle is fed by solar useful heat production and it produces electricity while a part of its power drives a vapor compression cycle with R290. Heating is also produced by separate heat exchangers in the solar loop. The parametric analysis is conducted in steady-state conditions with a developed model in Engineering Equation Solver. The examined parameters are the following: superheating degree in the turbine inlet, cooling production, heating production, solar beam irradiation intense, sun angle, pressure level in the turbine inlet and heat source temperature level. For the nominal scenario of 10 kW cooling production at 5°C and 10 kW heating production at 60°C, the system produces 6.14 kW electricity, while the exergy and energy efficiencies are found 12.14% and 37.34% respectively. Assuming that the system operates 2500 h yearly, the simple payback period of the investment is calculated at 8.5 years. The maximum examined values for both heating and cooling production are at 20 kW.

The Impact of Operating Parameters on the Performance of a New ORC–VCC Combination for Cogeneration

This manuscript presents a new combination based on a thermodynamic conversion, the idea is to combine the Organic Rankine Cycle (ORC) with the Vapor Compression Cycle (VCC). The novelty of the system appears essentially in: the development of new ORC–VCC combination architecture, the lowering of the ORC cycle temperature, the possibility of cold production by the ORC cycle upstream of the pumping phase, preheating of ORC cycle using VCC cycle fluid and new configurations based on the integration of heat recovery systems to improve overall system performance. In addition, each installation mode has several configurations depending on the recovery points that will be integrated later, besides its adaptation to any energy source, where we can use biomass, solar and heat rejects of industry at low temperatures (80‒160°C). This system can produce a cold with negative and positive temperatures. Although, thanks to its architecture, it is also characterized by many combination of selection fluid for the ORC and VCC cycles it is not necessarily to have the same working fluid as the classic systems. The main purpose of this study is to analyze the performance of a new system which combines Rankine-vapor compression cycle for the cogeneration of electricity and refrigeration. Coefficient of performance (COP) will be compared with other cooling systems and power system, such as the system turbo compressor Rankine. The fluids we used in the work are ammonia for the ORC and R600a for the VCC.

Design and construction of an integrated tetrafluoroethane (R134a) refrigerator-waste heat recovery dryer for fabric drying in tropical regions

A work on the design and construction of an integrated tetrafluoroethane (R134a) refrigerator-waste heat recovery dryer suitable for use in tropical regions is presented. The system comprises of a refrigerator with its condenser unit retrofitted to serve as the heat recovery mechanism and a drying chamber. The refrigerator had a vapour compression cycle driven by environmentally friendly R134a working fluid (refrigerant). The dryer component was powered by heat dissipated by the condenser piping from the exit of the compressor (superheat region) to the entrance of the sub-cooled region. The maximum drying temperature attained during pre-loading test was 49 °C while the evaporator provided cooling at a temperature of 5 °C. The specific moisture extraction rate of the dryer varied over 0.19–0.22 kg/kW.hr while 150W of cooling was produced at the evaporator in all cases. The energy utilization ratio obtained was 0.92, indicating that 92% of the waste heat recovered was actually utilized. The system coefficient of performance was estimated to be 10.09 thus indicating that the energy derived from IRWHRD was 10 times the energy it consumed. Application potentials therefore exist for use of this dual purpose system for simultaneous production of refrigeration and heating. Storage of food and drying of fabrics make the IRWHRD an option for use in both agricultural development and entrepreneurship development in laundry business.

The Attempt on the Seven Londoners

Abstract Historians of the English Revolution are well aware of the infamous events of 4 January 1642, when Charles I tried to arrest one peer and five Members of Parliament on charges of treason. The failed plan, later known as ‘the attempt on the Five Members’, led to an uproar in both Houses of Parliament and days of rioting in London that drove the king and his family to flee for their safety under the cover of night. Scholars are, however, less aware—perhaps even entirely unaware—of the events of January 1643, exactly one year later, when the king made a fateful attempt to arrest seven politically active Londoners for treason: Isaac Pennington, John Fowke, John Venn, Randall Mainwaring, Richard Browne, Edmund Harvey and Robert Tichborne. This article looks closely at the politics surrounding the king’s ‘second mistake’, and the ways in which City militants and the war party in Parliament manipulated events surrounding the affair in order to escalate the conflict. Much more than simply suggesting that Charles I was at best incorrigible, a king incapable of learning from his mistakes, the attempt on the seven Londoners reveals the significance of the struggle for popular mobilisation in the war-torn metropolis, and indeed the degree to which London had, by early 1643, become a cornerstone of Parliament’s war effort.

Thermoeconomic analysis and multi-objective optimization of a novel hybrid absorption/recompression refrigeration system

In the present article, thermodynamic, exergoeconomic, economic, and sustainability investigations of a recently developed environmentally friendly hybrid absorption/recompression refrigeration cycle is proposed to evaluate its feasibility for decision making and marketing. The proposed system is a novel hybridization of the conventional vapor compression and absorption cycles, wherein a booster compressor has been used between the generator and condenser of the single-effect absorption cycle to improve its performance. Also, two separate multi-objective optimization models are developed using a combination of the nondominated-storing-genetic algorithm (NSGA-II) and artificial neural network (ANN) to address the optimum performance values concerning the objective functions. The obtained results approve that the proposed cycle is a promising concept from both thermodynamic and economic viewpoints. The results indicate that the system presents a coefficient of performance and exergy efficiency of 4.88 and 37.43% under the optimum working conditions. The overall rate of exergy destruction of the system is 20.23 kW, and a sustainability index of around 1.53 can be achieved at a cooling capacity of 150 kW. The economic results indicate that the reference system has a payback period of 4.17 years, which is reduced to less than 4 years after doing the optimizations.

High-flux thermal management at megawatt scale using a double-effect absorption/vapor-compression cascade refrigeration cycle

Abstract A robust, high-performance, cascaded double-effect absorption/vapor-compression cycle with a high temperature lift was conceptualized and analyzed for a naval ship application. A double-effect LiBr–H2O absorption cycle driven by high-temperature (>275 °C) exhaust heat was coupled to a subcritical CO2 vapor-compression cycle to provide low temperature refrigerant (-40 °C) for high-heat flux electronics applications and medium temperature refrigerant (5 °C) for space conditioning and other low-heat flux applications. The double-effect absorption/vapor-compression cycle outperformed the corresponding single-effect cycle, providing 74.4% higher cooling capacity for the same waste heat input; however, it required more heat exchange area. The improved performance of the double-effect configuration was much higher at lower heat rejection temperatures. A parametric study on the exhaust and heat rejection temperatures showed that the cycle exhibited high COPs over a wide range of operating conditions, indicating the robustness of the cycle.

In an Ottoman Holy Land: The Hajj and the Road from Damascus, 1500–1800

In an Ottoman Holy Land: The Hajj and the Road from Damascus, 1500–1800

Performance assessment of a low-grade heat driven dual ejector vapour compression refrigeration cycle

In the present study, an ejector expansion refrigeration cycle is modified by placing a low-grade heat driven ejector between the compressor exit and the condenser inlet to reduce the secondary energy consumption. This modified cycle is designated as dual ejector vapour compression refrigeration cycle. The condenser temperature is assumed to be 308 K. R32 and R1234yf are considered as working fluids due to their lower global warming potentials and lesser flammability compared to hydrocarbons. The result indicates that for each of the considered working fluids, there is an optimum compressor pressure ratio corresponding to which mechanical COP (Coefficient of performance) is a maximum. It is observed that using combinations of the heat driven ejector and the compressor, mechanical COPs are 25.7% and 37.2% higher compared to those of ejector expansion refrigeration cycles operating with R32 and R1234yf respectively for an evaporator temperature of 263 K. Corresponding heater exit temperatures are found to be 338 K (for R32) and 348 K (for R1234yf). Mechanical COP of the proposed dual ejector cycle is also higher than that of the vapour compression cycle operating with single heat driven ejector.

Comparative Experimental Study of Low GWP Alternative for R134a in a Walk-in Cold Room

window.dataLayer = window.dataLayer || []; function gtag(){dataLayer.push(arguments);} gtag("js", new Date()); gtag("config", "UA-72322659-1");

Exergy Investigation of R410A as a ‘Drop In’ Refrigerant in a Water-Cooled Mechanical Vapor Compression Cycle

The urge to replace hydrofluorocarbons which possess high global warming potentials has taken center stage in the air conditioning industry due to both international and local policies such as the Kigali Amendment and Japan’s revised Fluorinated Gas law. This has prompted the exploration of novel refrigerants as well as their mixtures to create high performance environmentally friendly alternatives. These refrigerants can be integrated into existing systems as ‘drop in’ refrigerants, which provide a simpler and low cost substitution process to replace environmentally harmful refrigerants such as R410a. R410a is currently a widely used refrigerant in air conditioning systems, but is to be phased out of use under the Kigali Amendment by the late 2040s. Thus to compare the suitability of replacing this refrigerant with environmentally friendly ‘drop in’ alternatives, a preliminary baseline investigation on a mechanical vapor compression chiller with R410a is conducted via performance and exergy indicators. The testing procedure utilized Air Conditioning, Heating and Refrigeration Standard 551/591 which revealed an optimum charge amount of 0.70 kg with a peak performance near 88% of full capacity. The heat exchanger overall heat transfer coefficients showed varying trends, whilst the exergy destruction was as expected highest for the compressor.

Detailed experimental analysis of the energy performance of a desiccant wheel activated at low temperature

Desiccant dehumidification systems, such as those using desiccant wheels (DW), are an interesting alternative to conventional dehumidification systems based on vapor-compression cycles. The aim of this work was to obtain experimentally the performance of a DW activated at low temperature along the heat and mass transfer surface under different working conditions. The air temperature and humidity were measured at 32 different points, in order to analyse the performance of the DW at different circular sectors of the rotor.The results showed that the surface temperatures of the desiccant material on the process side decreased when the rotation speed decreased. Conversely, the surface temperatures of the desiccant material on the regeneration side increased when the rotation speed decreased.The minimum outlet process air temperatures and the maximum process air dehumidification were obtained in the circular sector closest to the regeneration side. In this circular sector the highest efficiency values were obtained. The number of circular sectors where the highest efficiency values were obtained increased when the rotation speed or air flow rates increased. Using these data, the wheel control strategy could be designed to increase the dehumidification capacity.