Commercial Refrigeration Systems Research Articles

Facilitating static firm frequency response with aggregated networks of commercial food refrigeration systems

Aggregated electrical loads from massive numbers of distributed retail refrigeration systems could have a significant role in frequency balancing services. To date, no study has realised effective engineering applications of static firm frequency response to these aggregated networks. Here, the authors present a novel and validated approach that enables large scale control of distributed retail refrigeration assets. The authors show a validated model that simulates the operation of retail refrigerators comprising centralised compressor packs feeding multiple in-store display cases. The model was used to determine an optimal control strategy that both minimised the engineering risk to the pack during shut down and potential impacts to food safety. The authors show that following a load shedding frequency response trigger the pack should be allowed to maintain operation but with increased suction pressure set-point. This reduces compressor load whilst enabling a continuous flow of refrigerant to food cases. In addition, the authors simulated an aggregated response of up to three hundred compressor packs (over 2 MW capacity), with refrigeration cases on hysteresis and modulation control. Hysteresis control, compared to modulation, led to undesired load oscillations when the system recovers after a frequency balancing event. Transient responses of the system during the event showed significant fluctuations of active power when compressor network responds to both primary and secondary parts of a frequency balancing event. Enabling frequency response within this system is demonstrated by linking the aggregated refrigeration loads with a simplified power grid model that simulates a power loss incident.

Control of compression refrigeration processes with superheat or saturated boiling

Cryogenic separation processes typically use compression refrigeration systems to achieve the desired low temperature levels for heat removal in the condenser. Many compression refrigeration systems are designed to produce slightly superheated refrigerant vapor leaving the evaporator. The liquid expansion valve is manipulated to control the amount of superheating. Commercial refrigeration systems must often handle drastic changes in operating conditions of refrigerant load and condenser temperature, which subject the superheat control loop to severe rangeability issues that complicate control due to nonlinearity and saturation.Some compression refrigeration systems have boiling liquid refrigerant in the evaporator with saturated vapor fed to the compressor. The liquid expansion valve controls the liquid level in the evaporator. The control structure is simplified and more robust.The contribution of this paper is to provide a rigorous quantitative comparison of the design and control of these alternative configurations. The fluid mechanics (plumbing) problems encountered when the superheated design is employed are demonstrated. Realistic simulations of the desuperheating, condensing and subcooling in heat exchangers are used.

Upgrading of Performance of Diffusion Absorption Refrigeration System

This paper deals with the effects of absorbent used on the thermal performance of commercial diffusion absorption refrigeration systems (DARSs). For this purpose, an ammonia solution with added lithium bromide was used in the DARS system instead of a commercial ammonia solution of NH3-H2O (NH4OH) (25% ammonia and 75% water), and all comparison experiments used a fixed amount of refrigerant to experimentally investigate the effects on performance. The system cooling performance was calculated by means of experimental data. The performance of all the solutions prepared was compared to each other. The added lithium bromide increased the amount of water absorbed by the coolant, increasing the amount of refrigerant carried to the generator and condenser. Consequently, the thermal efficiency of the system was increased. By using a 5% solution of lithium bromide with the ammonia solution, the thermal performance increased by up to 12%. It was concluded that lithium bromide has a higher refrigerant absorption capability than water in diffusion absorption refrigeration systems.

Analysis of a closed-loop water-cooled refrigeration system in the food retail industry: A UK case study

Abstract Refrigeration in supermarkets accounts between 30% and 60% of total electricity demand in UK stores. The aim of this study is to conduct a pre-feasibility analysis of whether the use of a water-cooled configuration rejecting heat to the soil can improve the overall cooling performance of commercial refrigeration systems against air-cooled designs. In this work, a model simulating the operation of an existing refrigeration system is presented and validated against field data measurements taken from a supermarket. The examined system is used as a baseline and then modified to evaluate the impact of installing a water-cooled gas cooler. Results indicate that the use of water-cooled gas coolers has the potential to reduce electrical consumption of refrigeration systems by up to a factor of 5 when external temperatures are high. Overall, annual operation indicates the water-cooled alternative uses 3% less electricity than the air-cooled approach. A hybrid system is also considered consisting of coupled air-cooled and water-cooled units operating in parallel, for which an energy reduction of 6% is obtained compared against the baseline system. An economic evaluation of these systems shows promising results with a payback period of about 5 years for systems installed in new stores, although retrofits are costlier.

Evaluation of gray box thermostatic expansion valve mass flow models

Thermostatic expansion valves (TXVs) are widely used in air-conditioning and commercial refrigeration systems. Accurate predictions of the refrigerant mass flow rate across a refrigeration cycle are desired in many engineering processes and applications such as a computer simulation model, for control purposes, or for fault detection and diagnostics (FDD). Several researchers have developed mass flow models for TXVs. This paper reviews and analyzes four gray-box TXV mass flow models in the literature and evaluates and compares their performance. Three of the models were developed for air-conditioners, but in the current work their applicability to refrigeration system modeling is assessed, with the use of normal and faulted laboratory data from a commercial walk-in cooler and a freezer. Model regression parameters are calculated during the training process with the use of parameter estimation techniques and a training dataset. The models are then used to predict the mass flow rates for several validation data points. Each of the models is found to perform well for the majority of the data points for the freezer, typically with errors below 10%, but application of the models to the cooler's dataset resulted in relatively large errors. The source of the model deviations includes the difficulty associated with the calculation of the TXV diaphragm force and the thermodynamic states of a two-phase refrigerant at the inlet of the valve. The presence of a liquid line receiver, which are common in refrigeration equipment, appears to cause some model deviation.

Determination of Optimum Insulation Thickness Distribution for Refrigerators

Most of electricity is consumed in either commercial or domestic refrigeration systems. Since the outer volume is determined, inner volume of a refrigeration system is important for a specified energy consumption. Therefore, the optimum distribution of insulation material according to inside and outside conditions for on and off time of a refrigerator is very important. Uniform distribution of insulation material is useful only convection and conduction resistances are the same for all sides and also on and off periods. In this study, a general solution of the optimum distribution of thermal insulation material for a given insulation material volume or given inner volume is suggested for refrigeration systems and also explained by a case study.

Conversion of a Direct to an Indirect Refrigeration System at Medium Temperature Using R-134a and R-507A: An Energy Impact Analysis

This work presents the experimental evaluation of energy consumption and refrigerant charge reduction when a commercial direct expansion refrigeration system is converted into an indirect system. The evaluation (with R-134a and R-507A) used a commercial cabinet with doors for medium temperature and a single-stage refrigeration cycle using a semi-hermetic compressor and electronic expansion valve; 24-h energy consumption tests were performed at laboratory conditions for each refrigerant and configuration at three heat rejection levels (23.3, 32.8 and 43.6 °C), maintaining an average product temperature inside the cabinet of 2 °C. The work analyses the impact of the conversion on temperature and pressure indicators, as well as, in the energy performance of each element. For R-134a the refrigerant charge was reduced in a 42.9%, but the energy consumption rose by 22.0%–22.8%; for R-507A the charge reduction was of 32.8% with an increase in energy consumption of between 27.7% and 38.7%.

Seasonal energy efficiency ratio for remote condensing units in commercial refrigeration systems

Different technological solutions are nowadays available to improve energy efficiency in centralized commercial refrigeration packs. Energy assessment of possible solutions is nowadays performed with different computational tools and comparison between alternative solutions is often difficult. In this paper, a methodology following the European Standards EN 14825 and EN 13215 is proposed for discussion. Sensitivity analysis with respect to climatic categorization of European and Mediterranean Countries is presented. The accuracy of the Seasonal Energy Performance Ratio (SEPR) estimations in comparison to traditional hourly based calculation is discussed using both absolute and relative energy figures. The reliability and usefulness of the presented method as a tool for a fair and accurate comparison between different systems is presented. Finally, next steps needed for the method full development are identified and possible further steps meeting the technology trend in commercial refrigeration are highlighted.

Constrained optimal high pressure equation of CO2 transcritical cycle

CO2 transcritical cycle has been widely applied in commercial refrigeration and heat pump water heating systems. Its cycle COP (coefficient of performance) heavily depends on the high pressure optimization and control. In theory, the optimal high pressure increases quickly with the increasing gas cooler outlet temperature or ambient temperature. However, due to reliability and cost concerns, there is an upper limit of the high pressure in compressor design and system operation. In this work, we proposed a constrained optimization method for getting constrained optimal high pressure equation of CO2 transcritical cycle. With the new method, the high pressure and its upper bound were simultaneously optimized at a given level of COP loss. Two specific approaches were developed to get the constrained optimal high pressure equation. Up to 1MPa high pressure reduction was achieved with about maximum 5% and average 1% COP losses.

Experimental analysis of R-450A and R-513A as replacements of R-134a and R-507A in a medium temperature commercial refrigeration system

This work presents the experimental evaluation of R-513A (GWP = 573) and R-450A (GWP = 547) as R-134a (GWP = 1301) drop-in replacements and as R-507A (GWP = 3987) retrofits in a commercial direct expansion refrigeration system for medium temperature applications (2 °C). The evaluation covered 24-hour tests using a single-stage cycle with semi-hermetic compressor, an electronic expansion valve customized for each refrigerant and a commercial vertical cabinet with doors placed inside a climatic chamber. The tests were performed at three water dissipation temperatures (23.3, 32.8 and 43.6 °C). Experimental results indicate that R-513A and R-450A can operate with R-134a plants, with increments in energy consumption between −1.6 to +1.2% for R-513A and from +1.3 to +6.8% for R-450A, whereas in comparison with R-507A, R-513A offered reductions in energy consumption between 4.4 to 8.2% and R-450A between 0 to 3.3%. The paper analyzes the modification of the operating pressures/temperatures and the energy indicators using the four refrigerants.

Energy benefit assessment of a water loop heat pump system integrated with a CO2 commercial refrigeration unit

The improvement of energy efficiency and the use of environmentally friendly working fluids are key elements of current European policies. Supermarkets are intensive energy consumers and approximately the 40% of their annual energy consumption is for refrigeration. Direct emissions of greenhouse gases associated with the use of high Global Warming Potential (GWP) refrigerants, and the indirect impact on the environment related to high electrical energy consumption, make shopping malls not sustainable buildings. This paper analyses the energy saving potential of integrated supermarket air conditioning and refrigeration systems using a Water Loop Heat Pump system (WLHP). A basic CO2 booster commercial refrigeration system, applied to cold rooms and display cabinets, is considered. Heat recovery from the refrigeration circuit is performed in the heating season, while in the cooling season a dry cooler on the water loop allows heat rejection to outdoors. The building and all systems are modelled in the Trnsys environment taking into account the hourly weather data, the simulated daily profiles of the cooling and heating load demand and the request from refrigerated food storage equipment. Such a model allows a thorough understanding of the potential for energy savings with heat recovery solutions.

Reinforcement Learning Control of Transcritical Carbon Dioxide Supermarket Refrigeration Systems

Commercial refrigeration systems consume a substantial amount of electrical energy, resulting in high indirect global warming impact due to greenhouse gases emissions. The multitude of different system configurations, system complexity, component wear, and changing operating conditions make efficient operation of this kind of refrigeration systems a difficult task. This paper presents an investigation of machine learning for supervisory control of a supermarket refrigeration system. In particular, a reinforcement learning algorithm for a CO2 booster refrigeration system is designed by exploiting the Matlab-based “SRSim” simulation tool. The reinforcement learning controller learns to operate the refrigeration system based on the interaction with the environment. The analysis shows that learning control is a feasible model-free technique to find a suitable control strategy for demand-side management in a smart grid scenario.

R-407H as drop-in of R-404A. Experimental analysis in a low temperature direct expansion commercial refrigeration system

This work analyses experimentally the possibility of using the reduced GWP refrigerant R-407H (HFC family, GWP = 1378) as drop-in substitute of R-404A (HFC family, GWP = 3945) in low temperature centralized direct expansion refrigeration system. The experimental plant and the test methodology are described. It covered top up or partial drop-in and energy consumption tests at −20 °C of product temperature and 25, 35 and 45 °C of condensing temperature using a small-scale supermarket under laboratory conditions. It has been measured that the use of R-407H instead of R-404A is favourable from the point of view of energy efficiency. Reductions in the energy consumption of up to 7.7% (compressor) and 4.0% (whole system) have been measured. However, there is an increment in the compressor's discharge temperature of up to 13.8 K. In addition, the operation of the system with both refrigerants was correct.

Experimental assessment of the replacement of a conventional fin-and-tube condenser by a minichannel heat exchanger in an air/water chiller for residential air conditioning

This paper analyses the performance of an air/water chiller typically used for residential air conditioning when a minichannel condenser is used in replacement of a conventional fin-and-tube condenser. The experimental facility, the data reduction process and the uncertainty analysis are briefly described and then the experimental results are presented and discussed, comparing the results obtained with both, the minichannel and the fin-and-tube condenser. Additionally, the chiller is numerically modelled using a commercial refrigeration system modelling software (IMST-ART). The good agreement between experimental and numerical results allows validating the model that is then used to calculate the mass of refrigerant contained in all the components of the system, as well as other operational parameters of interest not measured experimentally. The analysis of the results shows that replacing a conventional fin-and-tube condenser by a minichannel condenser allows, under almost any operating condition, reducing the mass of refrigerant increasing efficiency and cooling capacity.

Retrofit of lower GWP alternative R449A into an existing R404A indirect supermarket refrigeration system

R404A is going to be phased out from most of the commercial refrigeration systems due to its high GWP value of 3943. R449A (GWP of 1282) has been proposed to replace R404A with only minor system modifications in supermarkets. This paper presents the measurements of a light retrofit replacement of R404A using R449A in a medium temperature indirect refrigeration system (secondary fluid temperature at the evaporator outlet between −9 and −4 °C). It has been demonstrated that with a slight expansion device adjustment and 4% increase of refrigerant charge, R449A can be used in this refrigeration system designed for R404A because of its suitable thermodynamic properties and acceptable maximum discharge temperature. At a secondary fluid temperature at condenser inlet of 30 °C, the COP of R449A nearly matches that of R404A (both were between 1.9 and 2.2), despite having approximately 13% lower cooling capacity. As a conclusion, attending to the GWP reduction and similar energy performance, it was demonstrated using the TEWI methodology that the use of the recently developed refrigerant R449A in these applications can reduce the total CO2 equivalent emissions of an indirect supermarket refrigeration system designed for R404A refrigerant.

Coupling of the electrocaloric and electromechanical effects for solid-state refrigeration

Electrocaloric (EC) materials have shown the potential to replace some of the technologies in current commercial refrigeration systems. The key problem when fabricating an efficient EC refrigerator is the small adiabatic temperature change that current bulk materials can achieve. Therefore, such a solid-state EC refrigerator should be engineered to enhance the EC temperature change by rectifying the induced EC heat flow. Here, we present a numerical study of a device that couples the EC and electromechanical (EM) effects in a single active material. The device consists of several elements made from a functional material with coupled EC and EM properties, allowing the elements to bend and change their temperature with the application of an electric field. The periodic excitation of these elements results in a temperature span across the device. By assuming heat exchange with the environment and a low thermal contact resistivity between the elements, we show that a device with 15 elements and an EC effect of 1.2 K achieves a temperature span between the hot and cold sides of the device equal to 12.6 K. Since the temperature span can be controlled by the number of elements in the device, the results suggest that in combination with the so-called “giant” EC effect (ΔTEC ≥ 10 K), a very large temperature span would be possible. The results of this work should motivate the development of efficient EC refrigeration systems based on a coupling of the EC and EM effects.

Conversion of a direct to an indirect commercial (HFC134a/CO2) cascade refrigeration system: Energy impact analysis

Energy impact of conversion of a direct HFC134a/CO2 cascade refrigeration system to an indirect HFC134a-secondary fluid / CO2 cascade for commercial applications is experimentally analysed. Refrigeration systems, fully described, have been tested in laboratory conditions, at three condensing temperatures (25, 35 and 45 °C), providing the cooling needs to a cabinet for fresh product (2 °C) and to another for frozen product (−20 °C), both placed inside a climatic chamber simulating the indoor conditions of a supermarket. Energy evaluation considered 24 hour tests of stable operation in each test condition. A decrease from 1.9 to 3.5 K of the evaporation temperature of the high temperature cycle has been measured when using an indirect system. It resulted in a variation of the energy consumption of the whole system between 7.6 to 14.0% when using propylene-glycol/water and between −0.3 to 11.1% when using Temper −20® as secondary fluid.

Modelling commercial refrigeration systems coupled with water storage to improve energy efficiency and perform heat recovery

A basic CO2 transcritical/subcritical commercial refrigeration system is considered, applied to cold rooms and display cabinets in a supermarket. Subcooling of the refrigerant or heat recovery from condensation can be performed, taking advantage of a large fire prevention water tank. The whole refrigeration system is modelled in a TRNSYS environment, taking into account the hourly weather data and calculating the hourly cooling load demand from display cabinets and cold rooms equipment. New types have been written to describe display cabinets and cold rooms, CO2 refrigerating units and a particular water store.Simulations consider a simple double compression cycle with liquid receiver, and other options among which an auxiliary compressor. Results show that CO2 plants are feasible and energetically acceptable in mild climates, provided that improvements to standard cycle are adopted. Furthermore, heat recovery can be effectively performed through the employment of a heat storage.

Effect of Supermarket Refrigeration Systems on Water Consumption in California

ABSTRACTThis article examines the environmental impact of different commercial supermarket refrigeration systems on the energy and water consumption. The research uses industry standard EnergyPlus building models to compare the environmental impact of air and evaporative cooling methods for the condensers. The energy simulation covers all microclimate regions across California.

Energy and environmental performance assessment of R744 booster supermarket refrigeration systems operating in warm climates

This paper presents a theoretical comparison among different commercial refrigeration systems in terms of annual energy consumption and environmental impact. Eight configurations were studied: a R744/R134a cascade refrigeration system (baseline), a conventional and an improved R744 booster system, two R744 booster solutions with dedicated mechanical subcooling, a R744 booster with parallel compression and two solutions which combined the parallel compression and the mechanical subcooling. The evaluation was based on the weather data in Valencia (Spain) and in Athens (Greece), as well as on the running modes of a conventional European supermarket. A transition zone, which occurred between subcritical and transcritical operations, was adopted.The results showed that all the enhanced configurations may achieve a comparable energy saving to the one of the baseline in both the selected locations. Furthermore, they allow reducing the Total Equivalent Warming Impact (TEWI) by at least 9.6% beside the cascade solution.