Comprehensive investigation of numerical methods in simulating a steady-state vapor compression system

Computer simulation has become a required tool in the design phase of vapor compression systems; however with relatively few exceptions most simulations focus on the basic four component systems. With an increasing focus being placed on energy efficiency, the simulation of multi-component vapor compression systems (having multiple evaporators, condenser or compressors) will become essential to assist in the design of these more complicated systems. The implementation of a component-based framework will facilitate the simulation of multi-component systems. This paper describes three algorithms used to simulate a component-based vapor compression system. A test matrix of 6174 sample runs covering a wide range of operating conditions was constructed to determine the robustness and speed of each method when using three different types of nonlinear equation solvers. Each method was tested by simulating a basic four component cycle and a more advanced multiple evaporator system. The results are presented in such a format as to describe the reasons that contribute to any instability of the solvers and the computational efficiency of each method is discussed.     

Experimental development of an intelligent refrigeration system

In this study, an alternative solution to reduce energy consumption in industrial refrigeration systems is proposed and introduced. A typical industrial refrigeration system was conceived, built and modified in the laboratory, receiving a novel power law control system, which utilizes a frequency inverter. The operation and energy consumption of the system operating either with the new control system or with the traditional on–off control were compared to realistically quantify the obtained gains. In this manner, the measured temperature data acquired from several points of both systems and the energy consumption in kW h during a 24 h experimental run period are compared. The closed-loop power law controlled system shows a much smaller variation of the cold chamber internal temperature and electrical energy consumption economy of 35.24% in comparison with the traditional on–off system, under the same operating conditions.     

Performance of compression/absorption hybrid refrigeration cycle with propane/mineral oil combination

This paper discusses the feasibility of a vapor compression/absorption hybrid refrigeration cycle for energy saving and utilization of waste heat. The cycle employs propane as a natural refrigerant and a refrigeration oil as an absorbent. A prototype of the cycle is constructed, in which a compressor and an absorption unit are combined in series. The performance of the cycle is examined both theoretically and experimentally. Although the solubility of the propane with the oil is not enough as a working pair in the absorption unit, the theoretical calculation shows that the hybrid cycle has a potential to achieve a higher performance in comparison with a simple vapor compression cycle by using the waste heat. In the experiment, the prototype cycle is operated successfully and it is found that an improvement of an absorber is necessary to achieve the good performance close to the theoretical one. The application of an AHE (absorber heat exchanger) can reduce the heat input to a generator. Further examinations on some other combinations of refrigerant/refrigeration oil and additives are desirable.     

Use of ejectors in a multi-evaporator refrigeration system for performance enhancement

In this study, an improved cooling cycle for a conventional multi-evaporators simple compression system utilizing ejector for vapour precompression is analyzed. The ejector-enhanced refrigeration cycle consists of multi-evaporators that operate at different pressure and temperature levels. A one-dimensional mathematical model of the ejector was developed using the equations governing the flow and thermodynamics based on the constant-area ejector flow model. The model includes effects of friction at the constant-area mixing chamber. The energy efficiency and the performance characteristics of the novel cycle are theoretically investigated. The comparison between the novel and conventional system was made under the same operating conditions. Also, a comparison of the system performances with environment friendly refrigerants (R290, R600a, R717, R134a, R152a, and R141b) is made. The theoretical results show that the COP of the novel cycle is better than the conventional system.     

Hybrid liquid desiccant air-conditioning systems: A conceptual study with respect to energy saving potentials

In conventional air-conditioning systems with vapour compression cycles the dehumidification is realised by cooling the air below the dew point of the supply air. One possibility to avoid cooling the air below the dew point and thus to reduce the electric energy demand of air-conditioning systems is hybrid liquid desiccant air-conditioning systems (HLDACS) which use an open absorption cycle for dehumidification of the air. This conceptual study examines four different HLDACS with respect to their electric energy demand and shows energy saving potentials compared to a conventional air-conditioning system for three different climatic design conditions. All considered systems consist of an open absorption system in combination with either a vapour compression system (VCS) or an indirect evaporative cooling system. The results show that electric energy savings of 30–60% depending on the HLDACS and climates are possible.     

A vapour compression chiller fault detection technique based on adaptative algorithms. Application to on-line refrigerant leakage detection

The existence of faults in vapour compression chillers plays a significant role in terms of energy efficiency loss, performance degradations, and even environmental implications. In this paper, a dynamic model-based fault detection technique suitable for real-time implementation is proposed. The main objective is to obtain a reliable and automated tool for fault detection in vapour compression chillers, which can be applied in steady-state or transient operation. The fault detection methodology is based on comparing actual and expected performance using an adaptative model and operating variables dynamic thresholds. The technique has been successful applied for on-line refrigerant leakage detection with experimental tests involving the artificial introduction of the fault in a laboratory vapour compression plant, showing the results its capability of detecting incipient leakage failure conditions avoiding false alarms.     

A theoretical study of an innovative ejector powered absorption–recompression cycle refrigerator

This paper describes a novel cycle which uses a steam ejector to enhance the concentration process by compressing the vapour from the lithium bromide solution to a state that it can be used to re-heat the solution from which it came. The energy efficiency and the performance characteristics of the novel cycle are theoretically investigated in this paper. The theoretical results show that the coefficient of performance (COP) of the novel cycle is better than the conventional single-effect absorption cycle. The characteristics of the cycle performance show its promise in using high temperature heat source at low cost.     

Distillation column configurations in ammonia–water absorption refrigeration systems

In ammonia–water absorption refrigeration systems a purification process to reduce the water content in the vapour leaving the generator is required. During this process the water content in the vapour must be reduced to a minimum, otherwise it tends to accumulate in the evaporator and strongly deteriorates the efficiency of the system. The vapour purification can be carried out by partial condensation, by establishing a liquid–vapour counter flow or by combining both methods. In systems with partial condensation, the distillation column can be composed of one or more rectifiers using different cooling mediums, and the rectifying and stripping sections. In complete condensation systems only the rectifying and stripping sections can be used. Therefore different distillation column arrangements should be considered. This paper presents a study of several distillation column configurations for single stage ammonia–water absorption refrigeration systems with partial and complete condensation. In order to evaluate and compare the different configurations, a parameter that indicates the ratio of the ammonia vapour concentration increase in each part of the column to the total ammonia purification has been defined. The analysis has been based on the system COP. Finally the efficiency in each part of the column has been calculated to estimate its design requirements.     

Optimisation of the throttling system in a CO2 refrigerating machine

This paper is an answer to the need of finding the optimal solution for the throttling system in refrigerating machines using CO₂ as working fluid; such a solution must combine reliability, low installation cost and high energy efficiency. To this purpose, different expansion systems are compared by means of a simulation programme, including a new one, operating with a differential valve, a liquid receiver and a thermostatic valve. The typical compression refrigerating cycle performed by CO₂ involves transcritical operations and therefore the upper pressure needs to be adjusted to the optimal value, that, unlike the traditional cycle, is not determined by heat transfer. The innovative system here proposed shows an intrinsic self-adjusting capability that leads to COP values quite close to the maximum ones when a fixed suitable value of the differential pressure is chosen, even if the temperature of the secondary fluid varies to a large extent.     

The effect of oil in refrigeration: Current research issues and critical review of thermodynamic aspects

A lubrication agent is necessary in almost all the refrigeration vapour compression systems, particularly for the correct operation of the compressor. However, a certain portion of the oil always circulates with the refrigerant through the cycle. This circulation is at the origin of a deviation from the theoretical behaviour (i.e. based on pure refrigerant) of the components. This article aims at reviewing the oil-related researches in the field of refrigeration. Previous reviews in the literature focused on the thermo-hydraulic consequences of the presence of oil; we will analyse here its thermodynamical consequences. In a first part, a brief literature review will give an overview of current scientific and technological issues concerning the impact of oil on components or on whole refrigeration systems. The typical approaches and methods employed to address this problem will be described. These researches require sound tools for the evaluation of thermodynamic properties of refrigerant–oil mixtures. The second part of this article is hence a critical review of these tools, and focuses particularly on liquid–vapour equilibrium, absorption–diffusion, and mixture enthalpy calculation.     

Numerical simulation and experimental validation of vapour compression refrigeration systems. Special emphasis on CO2 trans-critical cycles

A numerical and experimental comparative study of a carbon dioxide trans-critical refrigerating system and a conventional sub-critical refrigerating cycle is presented. Attention is focussed not only on the whole refrigeration cycle, but also on the behaviour of the hermetic reciprocating compressors used in these systems. The comparative cases presented have been specially designed for small cooling capacity units with an evaporation temperature around 0 °C. A detailed numerical simulation model for hermetic reciprocating compressors performance, widely validated under conventional fluid refrigerants, has been extended to numerically obtain the CO₂ compressor prototypes behaviour. Two CO₂ compressor prototypes working with CO₂ have been experimentally tested in a specific unit, specially designed and built to analyse high-pressure single stage vapour compression trans-critical refrigerating equipments. This set-up has allowed validating a detailed numerical simulation code for the thermal and fluid-dynamic behaviour of single stage vapour compression refrigeration system working with CO₂ as fluid refrigerant. The numerical results and the experimental data obtained to validate compressors, heat exchangers and whole cycle behaviour have shown a really good agreement. Finally, the numerical and experimental comparison between the carbon dioxide system and the sub-critical conventional cycle has shown the possibility of CO₂ as fluid refrigerant under the studied working conditions.     

Refrigeration Carnot-type cycle based on isothermal vapour compression

A refrigeration Carnot-type cycle based on isothermal compression and two reversible expansions is proposed. Although ideal, this cycle is close to a realistic one which could be designed with existing hardware.     

A study on effective use of heat transfer additives in the process of steam condensation

It is well known that the additives in absorption chillers play a significant role in increasing absorber performance. Realizing that the additives in absorption chillers circulate throughout the system including the condensers, we investigated the effect of additives in the condenser. Reported herein are the results of the experimental and theoretical investigations done by using effective heat transfer additives for enhancing heat transfer coefficient in condensation of steam over a horizontal copper (99.9% Cu, 0.1% P) tube surface. By using effective additives, the condensation heat transfer coefficient can be enhanced as much as 1.47 times when compared to filmwise condensation. The steam condensation, which occurred in our experiments while using effective additives, was mostly pseudo-dropwise like. In our experiments, we noted that the use of heat transfer additive such as 2-ethoxyethanol for steam condensation was highly effective. This increase in heat transfer coefficient can be attributed to concept of Marangoni effect. It is understood that this surface convection is caused by local variations in the interfacial tension. So far there has been very little noted literature available on the theoretical aspect of surface tension effect on enhancing heat transfer rate in steam condensation. In the current research we try to explain the surface tension effect for enhancing heat transfer rate in steam condensation using effective heat transfer additives.     

斯特林制冷机用于冰箱技术的发展优势

相对于常用的蒸汽压缩节流制冷冰箱系统,采用逆向斯特林循环自由活塞斯特林制冷机的新型冰箱具有高效率采用"绿色"制冷剂制冷温区广启动电流低制冷量易控等特点,在环保及节能方面具有明显的优势.论文结合自由活塞式斯特林制冷机自身特点,分析其用于小容量家用冰箱以及低温冷柜的优越性;介绍了国外斯特林冰箱的研究进展;展望了这种极具潜力的冰箱制冷系统在我国的发展前景.     

Performance enhancement of a household refrigerator by addition of latent heat storage

This paper studies the effect of adding a phase change material (PCM) slab on the outside face of a refrigerator evaporator. A dynamic model of the vapour compression cycle including the presence of the phase change material and its experimental validation is presented. The simulation results of the system with PCM show that the addition of thermal inertia globally enhances heat transfer from the evaporator and allows a higher evaporating temperature, which increases the energy efficiency of the system. The energy stored in the PCM is yielded to the refrigerator cell during the off cycle and allows for several hours of continuous operation without power supply.     

A generalized analysis for cascading single fluid vapor compression refrigeration cycles using an entropy generation minimization method

This paper focuses on cascading an ideal vapor compression cycle and determining the optimal intermediate temperatures based on the entropy generation minimization method. Only superheating and throttle losses of the cycle are considered since they are inherent to the ideal vapor compression refrigeration cycle. The second law equations have been developed in terms of specific heats and temperature ratios with the intent of reducing involved property modeling. Also the entropy generation was expressed in terms of a single independent variable and minimized to develop an advanced rule for selecting optimum intermediate temperatures. Results for a cascade system operating between reduced temperatures of 0.684 and 0.981 with R-134a as the working fluid are presented. The approximate method presented here predicted the optimum intermediate reduced temperature for a two-stage system to be 0.88, a difference of 2% from the optimum. The method presented was a much better predictor of the optimum temperature than the geometric mean method which was 0.82, a difference of 5% from the optimum. The entropy generation distribution of the optimum solution was investigated. For a two-stage system, the lower stage and higher stage entropy generation was 44% and 56%, respectively. In comparison to the single stage, the two-stage reduced losses by 78%.     

Thermoeconomic optimization of a two stage combined refrigeration system: a finite-time approach

A finite-time thermoeconomic performance analysis based on a new kind of optimization criterion has been carried out for a two-stage endoreversible combined refrigeration cycle model. The optimal performances and design parameters that maximize the objective function (cooling load per total cost) are investigated. In this context, the optimal temperatures of the working fluids, the optimum performance coefficient, the optimum specific cooling load and the optimal distribution of the heat exchanger areas are determined in terms of technical and economical parameters. The effects of the economical parameter that characterizes the investment and energy consumption costs on the general and the optimal performances have been discussed.     

Experimental study on a continuous adsorption water chiller with novel design

A newly developed adsorption water chiller is introduced and tested. In the new adsorption refrigeration system, there are no refrigerant valves, the problem of mass transfer resistance resulting in pressure drop along refrigerant passage in conventional systems when methanol or water is used as refrigerant can be absolutely solved. Silica-gel–water is used as working pair and mass recovery-like process is adopted in order to use low temperature heat source ranging from 70 to 85 °C effectively. The experiment results demonstrate that the chiller (26.4 kg silica-gel in each adsorber) has a cooling capacity of 2–7.3 kW and COP ranging 0.2–0.42 according to different evaporating temperatures. Based on the experimental tests of the first prototype, the second prototype is designed and tested; the experimental data demonstrate that the chiller performance has been greatly improved, with a heat source temperature of 80 °C, a COP over 0.5 and cooling capacity of 9 kW has been achieved at evaporating temperature of 13 °C.     

Carnot-COP for sorption heat pumps working between four temperature levels

The theoretical efficiency limits of heat driven heat pumps operating between three and four temperatures are derived from the fundamental thermodynamical laws, i.e. the energy balance and the entropy balance. While in the three temperatures case the system is fully determined by specification of the three temperatures and the cooling capacity, a four temperature heat pump needs, in addition to the four temperatures and the cooling capacity, specification of an additional operating parameter. This can be, for example, the ratio of the two heat flows which are released at the two different intermediate temperatures. Various assumptions regarding this proportion are discussed with respect to their relevance for both the combination power cycle/vapor compression cycle as well as for single-effect sorption cycles. The present analysis shows that a single-effect sorption heat pump is principally not able to operate reversibly in an environment of four externally specified temperatures unless the four temperatures follow, incidentally, a correlation that is given by the equilibrium properties of the employed working fluids. Therefore, in endo-reversible models for four-temperature sorption cycles only three rather than four operating temperatures may be specified independently.