Performance evaluation and optimal design of supermarket refrigeration systems with supermarket model “SuperSim”. Part II: Model applications

As described in Part I, the supermarket simulation software “SuperSim” with its integrated refrigeration, building and HVAC system models, can be used to evaluate, compare and optimize alternative supermarket refrigeration systems. In Part II the model was used to evaluate and compare the performance of a CO₂ booster refrigeration system with that of a conventional R404A multiplex system in a supermarket application. Floating head pressure control was implemented for both systems when they were in subcritical cycles. For the CO₂ system, when the system was in transcritical cycle due to higher ambient air temperature, the head pressure was optimized through extensive thermodynamic cycle analysis as a function of ambient air temperature. The performance of the CO₂ booster system in the supermarket was then simulated during a one year period and compared with that of the R404A system. As a result, the system performance will benefit from a lower ambient temperature and a sizeable heat recovery for the CO₂ system.     

Simulation model for complex refrigeration systems based on two-phase fluid network – Part I: Model development

Some complex refrigeration and heat pump systems with several condensers and evaporators have been developed for different kinds of application. Traditional simulation models were developed for systems in certain operating modes and they failed in modeling the complex refrigeration systems with uncertainties of heat exchangers function and refrigerant flowing direction. In order to predict the performance of complex refrigeration systems, a simulation model is presented based on the two-phase fluid network. The model is consisted of distributed-parameter model of heat exchangers and connecting tubes, map-based model of inverter compressor and electronic expansion valve (EEV). Based on the characteristic of refrigeration system and fluid network, the three conservation equations, i.e. energy, momentum and mass equations, are solved iteratively. This model can deal with the uncertainty of refrigerant flow direction by separating the solving process of the components and the fluid network model, and therefore can simulate different kinds of complex refrigeration systems in different operating modes and conditions. The model is validated by the experimental data of an inverter air conditioner in heating/cooling operating modes and it shows the error of the model is mainly determined by the error of submodels of components in calculating heat transfer and pressure loss. The model is applied for performance analysis of three kinds of complex refrigeration systems in the accompanying article [Shi W.X., Shao, S.Q., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network: part II – model applications, International Journal of Refrigeration 31 (3), 500–509.].     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions

Using CO₂ trans-critical system solutions in supermarket refrigeration is gaining interest with several installations already running in different European countries. Using a computer simulation model, this study investigates the performance of two main system solutions: centralized with accumulation tank at the medium temperature level and parallel with two separate circuits for low and medium temperature levels. Both system solutions are presented and the simulation model is described in details. Calculations have been performed to design the systems and optimize their performances where basic layout and size of each solution have been defined. For ambient temperature range of 10–40 °C, the reference centralized system solution shows higher COP of about 4–21% than the reference parallel solution. Using two-stage compression in the centralized system solution instead of single stage will result in total COP which is about 5–22% higher than that of the reference centralized system and 13–17% higher than that of the improved two-stage parallel system. The two-stage centralized system solution gives the highest COP for the selected ambient temperature range.     

Simulation model for complex refrigeration systems based on two-phase fluid network – Part II: Model application

A simulation model of complex refrigeration system based on two-phase fluid network is developed in part I of the paper [Shao, S.Q., Shi, W.X., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network – part I: model development. International Journal of Refrigeration 31 (3), 490–499.]. One same fluid network is used in the model to describe the refrigeration system in different operating modes with the method of the factual and fictitious branches. When there are many heat exchangers with concurrent cooling and heating, the traditional models are not able to define the flowing directions. The developed model, however, whether the initial flowing direction of the connecting branches is right or not, is able to predict the flow and heat transfer. Three typical complex refrigeration systems such as the multi-unit inverter air conditioner, heat pump with domestic hot water and multi-unit heat pump dehumidifier are simulated with the developed model as demonstrations on how to use it. The model can evaluate the influences of one or more parameters on the system performance, which can be used for the optimization of the system by all-condition performance analysis. It is shown that the two-phase fluid network model is effective and convenient for the simulation of different complex refrigeration system, especially for performance analysis, system evaluation, and optimization based on the performance prediction.     

Design and performance evaluation of reciprocating refrigeration systems

The characteristic performance curves of vapor-compression refrigeration systems are defined as a plot between the inverse coefficient of performance (1/COP) and inverse cooling capacity (1/ ) of the system. Using the actual data of a simple vapor-compression system, performance curves of the system are obtained. The curves were found to be linear and this linear relation between 1/COP and 1/ is explained in the light of various losses of the system, resulting from the irreversibilities losses due to finite rate of heat transfer in the heat exchangers and non-isentropic compression and expansion in the compressor and expansion valve of the system, respectively. A finite-time thermodynamic model which simulates the working of an actual vapor-compression system is also developed. The model is used to study the performance of a variable-speed refrigeration system in which the evaporator capacity is varied by changing the mass-flow rate of the refrigerant, while keeping the inlet chilled-water temperature as constant. The model is also used for predicting an optimum distribution of heat-exchanger areas between the evaporator and condenser for a given total heat exchanger area. In addition, the effect of subcooling and superheating on the system performance is also investigated.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part II: System modifications and comparisons of different solutions

The performance of CO₂ refrigeration systems strongly depends on the operating conditions. The specific characteristics of low critical temperature and high operating pressure limit its applications and imply the implementation of different control strategies. This study compares the performance of different CO₂ system solutions for supermarket refrigeration with R404A system. Some possible modifications and improvements on the CO₂ system have been investigated. The COP of the investigated CO₂ system solution can be improved by about 3–7% along the ambient temperature range of 10–40 °C. The annual energy consumption calculations in three different climates; cold, moderate and hot, show that the centralized trans-critical CO₂ system is good solution for cold climates whereas the NH₃–CO₂ cascade system has the lowest energy consumption in hot climates. Both systems proved to be good alternatives to R404A DX system for supermarket refrigeration.     

A hybrid transient model for simulation of air-cooled refrigeration systems: Description and experimental validation

In this paper are described a hybrid dynamic model for transient simulation of refrigeration systems as well as dynamic experiments that have been performed on an air/water heap pump. The machine under consideration is made of an evaporator, a condenser, an expansion valve, a variable speed scroll compressor and a receiver. The refrigerant and second fluid flows in heat exchangers are approximated by a cascade of Continuous Stirred Tank Reactors (CSTRs). This model is quite flexible since a unique structure is used for the evaporator and the condenser models according to different boundary conditions. This is due to the use of a switching procedure between different configurations based on a phase stability test that is designed to ensure the continuity of the system simulation. An analytical thermodynamic model of the refrigerant based on an equation of state is used. Good agreement between simulation results and experimental data is achieved.     

Yearly simulation of the interaction between an ice rink and its refrigeration system: A case study

A transient model of airflow and heat transfer in an indoor ice rink and a quasi-steady model of its refrigeration system have been coupled and used to simulate their response to the time dependent ambient conditions and operating schedule for a typical meteorological year. The results for two different cases show that it is possible to reduce significantly the time of operation of the compressors and the energy consumption of the refrigeration system by simultaneously reducing the ceiling emissivity and increasing the secondary coolant temperature without affecting the quality of the ice.     

A comprehensive design and rating study of evaporative coolers and condensers. Part I. Performance evaluation

The mathematical models of evaporative fluid coolers and evaporative condensers are studied in detail to perform a comprehensive design and rating analysis. The mathematical models are validated using experimental as well as numerical data reported in the literature. These models are integrated with the fouling model presented in an earlier paper, using the experimental data on tube fouling. In this paper, we use the fouling model to investigate the risk based thermal performance of these evaporative heat exchangers. It is demonstrated that thermal effectiveness of the evaporative heat exchangers degrades significantly with time indicating that, for a low risk level (p=0.01), there is about 66.7% decrease in effectiveness for the given fouling model. Furthermore, it is noted that there is about 4.7% increase in outlet process fluid temperature of the evaporative fluid cooler. Also, a parametric study is performed to evaluate the effect of elevation and mass flow rate ratio on typical performance parameters such as effectiveness for rating calculations while surface area for design calculations.     

Experimental evaluation of a cascade refrigeration system prototype with CO2 and NH3 for freezing process applications

A prototype of a cascade refrigeration system using NH₃ and CO₂ as refrigerants has been designed and built. The prototype is used to supply a 9 kW refrigeration capacity horizontal plate freezer at an evaporating temperature of −50 °C as design conditions. The prototype includes a specific control system and a data acq*uisition system. The experimental evaluation started with the real conditions within the design operating parameters. Subsequently, several tests were performed fixing four CO₂ evaporating temperatures (−50, −45, −40 and −35 °C). At each one of the evaporating temperatures evaluated, the CO₂ condensing temperature was varied from −17.5 to −7.5 °C and an experimental optimum value of CO₂ condensing temperature was determined. The discussions on the experimental results include the influence of the operating parameters on the cascade system’s performance. In addition, the experimental results are compared with two common double stage refrigeration systems using NH₃ as refrigerant.     

Emissions and environmental impacts from air-conditioning and refrigeration systems

The impacts of air conditioning and refrigeration systems on stratospheric ozone are primarily linked to release of ozone-depleting refrigerants. Their contributions to global warming stem both from release of refrigerants and from emission of greenhouse gases (GHGs) for associated energy use. Because the energy-related component has a significantly higher warming impact, phaseout of hydrofluorocarbon (HFC) refrigerants with less efficient options will increase net GHG emissions. The same conclusion applies for perfluorocarbon (PFCs), though they are less commonly used as refrigerants. Integrated assessment of ozone depletion, global warming, and atmospheric lifetime provides essential indications in the absence of ideal refrigerants, namely those free of these problems as well as safety, stability, compatibility, cost, and similar burdens. This study examines the trend in refrigerant losses from chiller use. It documents both substantial progress in release reductions and the technical innovations to achieve them. It contrasts the impacts of current refrigerants with alternatives and with the chlorofluorocarbons (CFCs) they replaced. The study examines the sensitivity of efficiency to charge loss. It also summarizes thermodynamic and environmental comparisons of options to show that phaseout decisions based on chemical composition alone, without regard to attributes of individual substances, can result in greater environmental harm than benefit.     

Development and validation of “grey-box” models for refrigeration applications: A review of key concepts

“Grey-box” modelling combines the use of first-principle based “white-box” models and empirical “black-box” models, offering particular benefits when: (a) there is a lack of fundamental theory to describe the system or process modelled; (b) there is a scarcity of suitable experimental data for validation or (c) there is a need to decrease the complexity of the model. The grey-box approach has been used, for example, to create mathematical models to predict the shelf life of chilled products or the thermal behaviour of imperfectly mixed fluids, or to create models that combine artificial neural networks and dynamic differential equations for control-related applications. This paper discusses the main characteristics of white-box, black-box and their integration into grey-box models, the requirements and sourcing of accurate data for model development and important validation concepts and measures.     

Performance evaluation of the ITER Toroidal Field Model Coil Phase I Part 1: current sharing temperature measurement

The International Thermonuclear Experimental Reactor Toroidal Field Model Coil, a large (2.7 m × 3.8 m × 0.8 m) superconducting (Nb₃Sn) DC coil designed and constructed in collaboration between EU industries and laboratories coordinated by European Fusion Development Agreement, has been tested during 2001 in the TOSKA cryogenic facility at Forschungszentrum Karlsruhe, Germany, achieving the nominal 80 kA at 7.8 T peak field and 86 MJ stored energy as a standalone coil (Phase I). Possibly the highlight of the tests was the measurement of the current sharing temperature (T_CS) at different transport currents I in the self-field of the coil. The measurement method is discussed, based also on the two-year long predictive work, which preceded it. The results of the full set of T_CS measurements at I=80, 69 and 57 kA are presented here, and evaluated in a companion paper (part 2).     

A low data requirement model of a variable-speed vapour compression refrigeration system based on neural networks

In this work a model of a vapour compression refrigeration system with a variable-speed compressor, based on a black-box modelling technique, is presented. The kernel of the model consists of a full customized radial basis function network, which has been developed to accurately predict the performance of the system with low cost data requirement in terms of input variables and training data. The work also presents a steady state validation of the model inside and outside the training data set, finding, in both cases, a good agreement between experimental values and those predicted by the model. These results constitute a first step to go through future research on fault detection and energy optimisation in variable-speed refrigeration systems.     

Material behavior uncertainty in the design of bonded systems – Part I: Shear displacement and stress prediction

Uncertainties in the performance of engineering systems arise due to idealizations of geometry, material behavior, and loading history. Uncertainties in the geometry are often related to manufacturing processes, while the uncertainty in the material behavior often arises out of materials processing. In this two-part study, we analyze the effect of uncertainty in material behavior on the performance of bonded assemblies in general, but motivated by the example of a fiber-optic system. In the fiber-optic system, the motion of the surface emitting laser relative to the substrate (to which the laser is bonded) due to the viscoelastic behavior of the bond epoxy causes a loss in the light coupled to the fiber. In the first part of the paper, we develop models to describe the shear displacement and the shear stress in bond layer of the fiber-optic assembly. In the second part, we characterize through extensive experimentation the uncertainty in the viscoelastic behavior of the bond epoxy and use it to develop guidelines for the design of the bonded system.     

Drift-controlled design of reinforced concrete frame structures under distant blast conditions—Part I: Theoretical basis

Proper control levels of lateral drifts anticipated for reinforced concrete (RC) frame structures within the predefined performance level become crucial when the frame structure is subjected to distant intense surface explosions. For this purpose, a new design method is presented in a two-part paper based on the transformation of a blast loading into an equivalent static force (ESF). The ESF is calculated in such a manner that the same maximum inter-storey drift ratio (MIDR) under the blast loading will be reproduced. The first part of the two-part paper focuses on the computational model of ESF for a single-degree-of-freedom (SDOF) system and the design method based on ESF with the requirement for controlling its maximum displacement response to achieve the specified target displacement. Numerical examples have been included to illustrate the method while the verifications of the dynamic responses of the designed SDOF system are performed with nonlinear dynamic analyzes. The numerical results indicate that the target displacement is well met for the designed SDOF system in resisting a given blast loading. Extension of the computational model of ESF and the corresponding design method with ESF for a SDOF system into a RC frame structure will be further discussed in the companion paper.     

Study on an activated carbon fiber–ethanol adsorption chiller: Part I – system description and modelling

This article presents the transient modelling for a two-bed, activated carbon fiber (ACF)–ethanol adsorption chiller. This innovative adsorption chiller employs pitch based ACF of type A-20 as adsorbent which is a fibrous adsorbent having the advantages of fast adsorption rate, high porosity and ease of handling when compared with granular adsorbents and powdered adsorbents. Ethanol is used as refrigerant as it has no harm to environment, it is a non-toxic substance, moreover, ethanol has comparatively higher vapor pressure even at low temperature. This innovative system utilizes effectively low-temperature waste heat sources of temperature between 60 and 95 °C along with a coolant at 30 °C. We have found that, regardless of the initial mass distribution, the ACF–ethanol adsorption chiller is able to achieve the same cyclic-steady-state within three cycles or 1890 s.     

Numerical evaluation of harmonic Green's functions for triclinic half‐space with embedded sources—Part I: a 2D model

The Green's functions for a triclinic half‐space for embedded harmonic line load are considered. Corresponding displacement and stress fields are expressed in terms of double Fourier integrals. The first integral was evaluated using contour integration while the second one was computed through the Gauss–Legendre quadrature. The resulting Green's functions algorithm avoids repeated calculations of the same quantities and utilizes the vector computational features within MATLAB environment. Extensive testing of the results has been performed for both displacement and stress fields. The tests demonstrate the accuracy of the proposed procedure for evaluating the Green's functions without any restrictions upon material properties, frequency, and location of the source and observation points. Copyright © 2006 John Wiley & Sons, Ltd.     

Appraising the flammability hazards of hydrocarbon refrigerants using quantitative risk assessment model. Part II. Model evaluation and analysis

Hydrocarbon refrigerants present are fire and explosion hazards due to their flammability. This paper describes a quantitative risk assessment (QRA) model to evaluate the potential for ignition when hydrocarbons are employed in stationary refrigeration and air-conditioning equipment. QRA enables examination of the effects that design, installation of equipment and external conditions on the frequency of ignition of the refrigerant and its consequences. Part I of this study presents the modelling approach for ignition frequencies, sub-models for refrigerant leakage and development of flammable concentration, and the associated consequences, being overpressures and thermal radiation. Part II provides recommended empirical input data and example results generated from the model.     

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.