Identification and control of multi-evaporator air-conditioning systems

Modern multi-evaporator air-conditioners (MEACs) incorporate variable-speed compressors and variable-opening expansion valves as the actuators for improving cooling performance and energy efficiency. These actuators have to be properly feedback-controlled; otherwise the systems may exhibit even poorer performance than the conventional machines which use fixed-speed compressors and conventional expansion valves. In this paper, feedback controller design for the MEAC system is first addressed through experimental identification. The identification produces a low-order, linear model suitable for controller design. The feedback controller employed is multi-input–multi-output-based and possesses a cascade structure for dealing with the fast and slow dynamics in the system. To determine appropriate control parameters, conditions that establish the stability for the cascade design are given. Due to the deficiency in control inputs, the proposed control structure exhibits steady-state errors in the superheat responses which in turn can produce unacceptable steady-state superheats. To resolve this issue, the reference superheat settings are determined via an optimization procedure so that the resultant steady-state superheats become acceptable. The control experiments indicate that the proposed controller can successfully regulate the indoor temperatures and maintain the steady-state superheat temperatures at acceptable levels.     

Refrigerants and energy efficiency

Environmental concerns (i.e. ozone depletion and greenhouse warming) are forcing major shifts from traditional choices of refrigerant working fluids for individual systems and for specific applications. Energy efficiency has taken on new urgency to mitigate fossil fuel demands and the associated effects on greenhouse warming while allowing for future growth of refrigerant demand. Energy efficiency has thus become an increasingly important parameter and a key focus in the refrigerant selection process. Efforts to generically rank refrigerants with respect to their inherent thermodynamic efficiencies have thus far not proven to be very productive. Much conflicting information has surfaced as to the true impacts on energy demand of various candidate fluids. It is apparent that much more than a simple thermodynamic cycle efficiency is involved. Other factors may have an equal or even greater impact on the ultimate energy demands associated with any particular working fluid and/or system design. Even calorimeter testing may prove misleading in the absence of appropriate optimization. This paper discusses individual sources of inefficiency inherent in every real system and attempts to define key fluid characteristics associated with optimizing or degrading the effectiveness of individual system components and the overall system. Relative compressor efficiencies, heat transfer effectiveness, pressure drop impacts, etc., will be addressed.     

System dynamic model and temperature control of a thermoelectric coolerModèle dynamique d'un système et régulation de la température d'un refroidisseur thermoélectrique

A linear dynamic model of the thermoelectric cooler including the heat sink and the cooling-load heat exchanger was derived using small-signal linearization method. It shows that the dynamic model of a thermoelectric cooler has two poles and one zero. The linear dynamic model is shown to vary with operating conditions. A linear feedback system is designed for the cold-end temperature control of a thermoelectric cooler using the average linear dynamic model of the thermoelectric cooler and a PDF controller structure. The step response tests show that the controller has a very satisfactory performance. Some tests under variable cooling load and ambient temperature are also performed to examine the disturbance-rejection property of the controller. Experimental results show that the cold-end temperature can be maintained at the fixed value within ±0.1°C irrespective of the variations of the cooling load and the ambient conditions.     

A general dynamic simulation model for evaporators and condensers in refrigeration. Part II: simulation and control of an evaporator

A mathematical model of an evaporator based on one-dimensional partial differential equations representing mass conservation, and tube wall energy has been formulated. These equations are then restructured and linked to a program data base of all major refrigerants and refrigerant mixtures. The result is a simulation model of an evaporator that is general and flexible. The model is tested over a wide range of operating conditions and a simple controller is implemented to demonstrate the effectiveness of the model for controller and systems design.

Résumé

On a établi un modèle mathématique d'un évaporateur basé sur des équations aux dérivés partielles unidimensionnelles qui représentent la conservation de masse et l'énergie de la paroi du tube. Ces équations ont été restructurées ensuite, puis reliées à une base de données sur les principaux frigorigènes purs et en mélanges. De cette manière, on obtient un modèle d'évaporateur d'application générale et souple. Ce modèle a été éprouvé dans des conditions de fonctionnement très variées et on a employé un système de régulation simple pour montrer l'efficacité du modèle pour la conception et la régulation des systèmes.     

A physical model to predict climate dynamics in ventilated bulk-storage of agricultural produce

This paper presents a physical model for predicting climate dynamics in ventilated bulk-storage of agricultural produce. A well-ordered model presentation was obtained by combining an object-oriented zonal decomposition with a process-oriented decomposition through matrix–vector notation. The objective of this paper is twofold: (1) to present the modelling procedure and (2) to present the resulting model, validated with real data. The model is a suitable simulator to assess potential effects of changes in ambient climate, design, and controller tunings. The model predictions fit well to extensive real data from three different cases. The good fit for all three cases was achieved with the five calibration parameters calibrated on the basis of data from one case only.     

Airflow blockage and guide technology on energy saving for spiral quick-freezer

The effect of airflow blockage and guide technology on energy saving for spiral quick-freezers were investigated by simulating and analyzing the airflow field and measuring of the velocity distribution in the freezing zone for different designs. The k– turbulence model was used. The velocities and temperatures of the air in the freezing zone for different designs of airflow blockage and guide boards were measured. The study shows that the airflow pattern plays a key role on energy efficiency, freezing time, and production rate. In the study case, through the optimization of the airflow blocking boards and the guide boards, the average air velocity in the freezing zone would be enhanced to 2.5–2.7 times compared with the original design. Correspondingly for bean curds in a stationary condition, the freezing time would be shortened by 78–85%, energy efficiency and the production rate would be increased by approximately 18–28% individually.     

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.     

PMSM馈能系统硬件在环仿真研究

为对电动汽车动力电机馈能效率进行研究,方便馈能效率的测量,提出一种基于滑模控制算法的PMSM(permanent magnet synchronous motor)馈能研究方法,即通过控制电动机的输入电压与电流来驱动相同性能的电动机发电。首先以馈能理论为基础,建立包含滑模变结构控制、空间矢量、PMSM等模块的PMSM馈能系统仿真模型,并进行离线仿真分析。结果表明:采用所提出的馈能方法可达到70%左右的馈能效率,具有一定的可行性。同时,建立PMSM馈能系统dSPACE硬件在环试验平台,对其进行硬件在环试验验证。试验结果表明:电动机馈能效率在60%~80%的范围内,与离线仿真所得结果较为接近。这不仅验证了所建立的馈能仿真模型的正确性,且进一步证明了纯电动汽车动力电机自身馈能在节能方面的可行性。所提出的馈能方法能够为目前现有馈能方式提供更多选择,硬件在环试验研究也能够为后续的PMSM台架和实车试验研究提供一些技术支持。     

A design method of thermoelectric coolerConception d'un refroidisseur thermoélectrique

A system design method of thermoelectric cooler is developed in the present study. The design calculation utilizes the performance curve of the thermoelectric module that is determined experimentally. An automatic test apparatus was designed and built to illustrate the testing. The performance test results of the module are used to determine the physical properties and derive an empirical relation for the performance of thermoelectric module. These results are then used in the system analysis of a thermoelectric cooler using a thermal network model. The thermal resistance of heat sink is chosen as one of the key parameters in the design of a thermoelectric cooler. The system simulation shows that there exists a cheapest heat sink for the design of a thermoelectric cooler. It is also shown that the system simulation coincides with experimental data of a thermoelectric cooler using an air-cooled heat sink with thermal resistance 0.2515°C/W. An optimal design of thermoelectric cooler at the conditions of optimal COP is also studied. The optimal design can be made either on the basis of the maximum value of the optimal cooling capacity, or on the basis of the best heat sink technology available.     

Thermodynamics of magnetic refrigeration

A comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes is presented. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, and a list of the thermodynamic potentials is given. These are based on the very recent discovery of an alternative Kelvin force. It is shown that this force is compatible with the internal energy proposed by Landau and Lifshitz. New formulas for the specific enthalpies are presented. Cyclic processes are discussed in detail, e.g. the Brayton, Ericsson and Carnot cycles. Magnetic refrigeration and magnetic heat pump cycles are preferably designed by applying the cascade or/and regeneration principle. Cascade systems allow wider temperature ranges to be obtained. The main objective of this article is to yield a theoretical basis for an optimal design of new magnetic refrigeration and heat pump devices.     

Energetic efficiency of cogeneration systems for combined heat, cold and power productionEfficacité énergétique des systèmes de cogénération utilisés pour produire simultanément de la chaleur, du froid et de la puissance électrique

This paper deals with the problem of energetic efficiency evaluation of cogeneration systems for combined heat, cold and power production. Cogeneration systems have a large potential for energy saving, especially when they simultaneously produce heat, cold and power as useful energy flows. Various cogeneration systems for combined heat, cold and power production are designed by means of computer simulation to minimize consumption of the primary energy. Equations of energetic efficiency of this combined cogeneration systems are presented, that relate the primary energy rate (PER) and comparative primary energy saving (Δq_p) to energy parameters of designed systems. Comparison of energetic efficiency of combined cogeneration systems with contemporary conventional separate production of heat, cold and power shows a large potential for energy saving by designed combined cogeneration systems.     

Liquid–gas heat exchanger for household refrigerator

This paper considers the influence of heat exchangers to the efficiency of a household refrigerating system. A steady state mathematical model is used to compare three most commonly used heat exchanger designs. For each design, an optimal inner diameter of the heat exchanger, subject to the compressor's capacity and the heat exchanger's length is found. The influence of operating conditions, such as condensation and evaporation temperatures, superheat in the evaporator and sub-cooling in the condenser, to optimal dimensions of the heat exchanger is also analyzed in the paper. Presented guidelines and recommendations can be used for design and modernization of household refrigerators and freezers.     

Simple mathematical model for predicting the transient behaviour of an ice-bank system

The time-variable performance of a Refrigerant 22 ice-bank system was simulated by a dynamic model which was derived by assuming that heat transfer was always the limiting process, and which thus ignored hydrodynamic processes. The model comprised four ordinary differential equations describing the position of the ice front, the water temperature, and the refrigerant evaporation and condensation temperatures, each of which was derived by energy balance, plus a number of algebraic equations. Measured plant performance was accurately predicted except immediately after start-up, and in circumstances in which the assumption that the dynamics of refrigerant flow did not exert any controlling influence on the overall process dynamics was inadequate (for example, when the thermostatic expansion valve operation becomes unstable). The model requires only data that should be readily available or can be easily estimated, and thus it is suitable for analyses in the design of ice bank systems to handle time-varying conditions. Simple dynamic models ignoring hydrodynamics can be adequate in circumstances where the main source of variation arises beyond the refrigeration circuit itself.     

Thermodynamic analysis of an R744–R717 cascade refrigeration system

A thermodynamic analysis of carbon dioxide–ammonia (R744–R717) cascade refrigeration system is presented in this paper to optimize the design and operating parameters of the system. The design and operating parameters considered in this study include (1) condensing, subcooling, evaporating and superheating temperatures in the ammonia (R717) high-temperature circuit, (2) temperature difference in the cascade heat exchanger, and (3) evaporating, superheating, condensing and subcooling in the carbon dioxide (R744) low-temperature circuit. A multilinear regression analysis was employed in terms of subcooling, superheating, evaporating, condensing, and cascade heat exchanger temperature difference in order to develop mathematical expressions for maximum COP, an optimum evaporating temperature of R717 and an optimum mass flow ratio of R717 to that of R744 in the cascade system.     

Methodology for thermal design of novel combined refrigeration/power binary fluid systems

Refrigeration cogeneration systems which generate power alongside with cooling improve energy utilization significantly, because such systems offer a more reasonable arrangement of energy and exergy “flows” within the system, which results in lower fuel consumption as compared to the separate generation of power and cooling or heating. This paper proposes several novel systems of that type, based on ammonia–water working fluid. Importantly, general principles for integration of refrigeration and power systems to produce better energy and exergy efficiencies are summarized, based primarily on the reduction of exergy destruction. The proposed plants analyzed here operate in a fully-integrated combined cycle mode with ammonia–water Rankine cycle(s) and an ammonia refrigeration cycle, interconnected by absorption, separation and heat transfer processes. It was found that the cogeneration systems have good performance, with energy and exergy efficiencies of 28% and 55–60%, respectively, for the base-case studied (at maximum heat input temperature of 450 °C). That efficiency is, by itself, excellent for cogeneration cycles using heat sources at these temperatures, with the exergy efficiency comparable to that of nuclear power plants. When using exhaust heat from topping gas turbine power plants, the total plant energy efficiency can rise to the remarkable value of about 57%. The hardware proposed for use is conventional and commercially available; no hardware additional to that needed in conventional power and absorption cycles is needed.     

Energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption refrigeration systems

In this paper, the energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption systems is presented. A computational model has been developed for the parametric investigation of these systems. Newly developed computationally efficient property equations of water–lithium bromide solution have been used in the computer code. The analysis involves the determination of effects of generator, absorber and evaporator temperatures on the energetic and exergetic performance of these systems. The effects of pressure drop between evaporator and absorber, and effectiveness of heat exchangers are also investigated. The performance parameters computed are coefficient of performance, exergy destruction, efficiency defects and exergetic efficiency. The results indicate that coefficient of performance of the single effect system lies in range of 0.6–0.75 and the corresponding value of coefficient of performance for the series flow double effect system lies in the range of 1–1.28. The effect of parameters such as temperature difference between heat source and generator and evaporator and cold room have also been investigated. Irreversibility is highest in the absorber in both systems when compared to other system components.     

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.     

Development of computer program for simulation of an ice bank system operation, Part I: Mathematical modelling

Since the use of standard engineering methods in the process of an ice bank performance evaluation offers neither adequate flexibility nor accuracy, the aim of this research was to provide a powerful tool for an industrial design of an ice storage system allowing to account for the various design parameters and system arrangements over a wide range of time varying operating conditions. In this paper the development of a computer application for the prediction of an ice bank system operation is presented. Static, indirect, cool thermal storage systems with external ice on coil building/melting were considered. The mathematical model was developed by means of energy and mass balance relations for each component of the system and is basically divided into two parts, the model of an ice storage system and the model of a refrigeration unit. Heat transfer processes in an ice silo were modelled by use of empirical correlations while the performance of refrigeration unit components were based on manufacturers data. Programming and application design were made in Fortran 95 language standard. Input of data is enabled through drop down menus and dialog boxes, while the results are presented via figures, diagrams and data (ASCII) files. In addition, to demonstrate the necessity for development of simulation program a case study was performed. Simulation results clearly indicate that no simple engineering methods or rule of thumb principles could be utilised in order to validate performance of an ice bank system properly.     

A design method for mixed gas Joule–Thomson refrigeration cryosurgical probes

A cryosurgical probe is a medical instrument that is designed to kill cancerous tissue by exposure to cryogenic temperatures. The most important figure of merit for a cryosurgical probe of fixed size is the dimension of the cryolesion that can be generated within a specified time during a surgical procedure. The optimization of the cryoprobe, through variation of the geometry, operating conditions, or working fluid composition, should be aimed directly at this clinical goal rather than at maximizing the refrigeration capacity, COP, or minimizing the tip temperature. This unique optimization criterion has created the need for a design method that considers both the probe design and the heat transfer details external to the probe (e.g. the blood perfusion and metabolic heat rate in the patient). This paper outlines a design method that will allow manufacturers of cryosurgical probes to identify an optimum combination of refrigerant mixture, cryoprobe geometry, and other cycle operating conditions. Specifically, a numerical model of the development of a cryolesion has been developed and verified; this model relates the size of the cryolesion, refrigeration required, and tip temperature. A detailed model of the cryoprobe refrigeration cycle relates the refrigeration to the tip temperature that can be achieved. The integration of these models allows the designer to select the optimal cryoprobe based on cryolesion size. This paper focuses on optimizing the mixed gas composition; however, the design technique is generic and can be used to select other system parameters.     

Multivariable control-oriented modeling of a direct expansion (DX) air conditioning (A/C) system

A dynamic mathematical model for a DX A/C system has been developed. The dynamic model, written in state-space representation which was suitable for designing multivariable control, was linearized at steady state operating points. The linearized model has been validated by comparing the model simulation results with the experimental data obtained from an experimental DX A/C system. The simulated results agreed well with the experimental data, suggesting that the model developed was able to capture the transient characteristics of the DX A/C system modeled. It is expected that the model developed can be useful in designing a multi-input multi-output (MIMO) controller to simultaneously control indoor air temperature and humidity in a space served by a DX A/C system.