1,721,029 research outputs found
Thermodynamic and environmental comparison of R744 booster supermarket refrigeration systems operating in Southern Europe
Full text linkSupermarket applications are characterized by enormous direct and indirect greenhouse gas emissions. In an attempt to substantially reduce the carbon footprint associated with the food retail industry, carbon dioxide (or R744) seems to be the most suitable long-term working fluid. In fact, R744 possesses a negligible Global Warming Potential (GWP) and advantageous thermo-physical properties, besides being non-toxic, non-flammable, inexpensive and readily available. However, the energy efficiency of the pure R744 (i.e. transcritical R744 or “CO2 only”) systems still needs to be considerably enhanced when it comes to units featuring the basic architecture and operating in warm regions. The reason for this lies in the occurrence of transcritical operating conditions at which very poor performance can be ascribed to the aforementioned solutions. The full deployment on the part of the commercial refrigeration sector in high ambient temperature countries towards “CO2 only” systems is also strongly related to the presence of some remaining non-technological barriers.
In this study, the performance of the state-of-the-art arrangements for pure CO2 supermarket applications was theoretically compared with that of the most currently used solutions (i.e. HFC-based systems). The comparison was based on both energy and environmental assessments. Also, the actual improvement potential achievable by a conventional R744 booster refrigeration configuration, as well as some enhancement strategies were evaluated by applying the advanced exergy analysis. All these assessments were carried out by opting for the typical running modes of retail stores located in Southern Europe. In relation to the studies available in the open literature, this work involved at least four relevant and peculiar scientific merits, such as the application of the multi-ejector concept, the evaluation of the performance of CO2 refrigeration systems integrated with the air-conditioning (AC) unit, the selection of various warm climate cities and the implementation of the advanced exergy analysis.
It could be concluded that nowadays transcritical R744 ejector supported parallel systems are the most highly efficient and eco-friendly technologies for food retail industry in Mediterranean Europe, being able to successfully replace HFC-based solutions
Valutazione delle prestazioni di macchine frigorifere a CO2 per applicazioni commerciali in climi caldi
No full textLe prestazioni delle macchine frigorifere a CO2 sono notevolmente influenzate dalla temperatura dell’aria esterna. La configurazione base, infatti, si caratterizza per efficienze molto più basse rispetto alle soluzioni attualmente impiegate nei climi caldi. Al fine di migliorarne le prestazioni, differenti espedienti tecnologici sono stati recentemente proposti. In questo articolo, i coefficienti di prestazione (COP) ed i consumi energetici annuali di diverse soluzioni impiantistiche a CO2 vengono comparati a quelli di un sistema convenzionale ad espansione diretta a R404A. Tali valutazioni sono state implementate considerando le tipiche condizioni operative di un supermercato localizzato in diverse città dell’Europa meridionale
Theoretical evaluation of supermarket refrigeration systems using R1234ze(E) as an alternative to high-global warming potential refrigerants
No full textHydro-fluoro-olefin-1234ze(E) features an insignificant global warming potential and it could be successfully used in supermarket
applications in an attempt to foster solutions which are more eco-friendly than the currently employed ones. In this article, four
indirect refrigeration systems using R1234ze(E) as the primary working fluid were theoretically compared with both a R404A multiplex
solution and a R744/R134a cascade configuration. The annual electricity consumption and the total equivalent warming impact
were assessed in three different American locations and in two European cities. The outcomes obtained demonstrate that R1234ze(E)
can suitably replace R404A in commercial refrigerating plants, especially in warm climates. In fact, R1234ze(E)-based solutions
exhibits similar energy saving and at least a decrement by 26.7% in total equivalent warming impact beside the configurations
employing R404A
Thermodynamic Performance Evaluation of R744 Supermarket Refrigeration Systems by employing Advanced Exergy Analysis
No full textIt is a well-known fact that, at high outdoor temperatures, it is necessary to adopt some expedients in order to make the basic R744 refrigeration cycle energetically competitive with the solutions using synthetic refrigerants. The most effective design procedures to apply to any refrigerating plant can be suitably revealed by employing some advanced thermodynamic tools.
This paper deals with the application of the advanced exergy analysis to a transcritical CO2 refrigeration system using an auxiliary compressor and a subcooler and to the one employing a dedicated mechanical subcooling. The evaluation was carried out by selecting the typical running modes of a supermarket and a design external temperature of 30 °C. The results obtained showed that, in both the investigated cases, the avoidable irreversibilities associated with the evaporator and with the main compressor were completely and mainly endogenous, respectively. On the other hand, a large amount of the inefficiencies related to the subcooling loop, the auxiliary compressor and the gas coolers could be dropped by improving the other component
Comparative Exergoeconomic Analysis of Various Transcritical R744 Commercial Refrigeration Systems
No full textThe interest in carbon dioxide (R744) as a refrigerant for supermarket applications has been remarkably rising thanks to its favourable thermo-physical, safety and environmental properties. Furthermore, R744 refrigerating plants are still prone to accomplishing large improvements in terms of cost-effectiveness, capability of heat recovery and energy efficiency.
The main target of this paper is to compare the final cost of the product of different enhanced CO2 refrigeration supermarket configurations, with a marked emphasis on the one employing a two-phase ejector as an expansion device. The outcomes showed that the use of such technology leads to a drop in the final cost of the product at least by 22.7% over a basic single-stage solution for cooling medium temperatures ranging from 30 °C to 42 °C. The assessment was based on the assumption of three different values of the purchased equipment cost associated with the ejector. The efficiency of the diffuser was observed to influence the costeffectiveness of the system quite substantially. It could be concluded that the higher the heat sink design temperature, the more cost-effective an ejector as a refrigerant expander is
Advanced exergy analysis of a R744 booster refrigeration system with parallel compression
Full text linkIn this paper, the advanced exergy analysis was applied to a R744 booster refrigeration system with parallel compression taking into account the design external temperatures of 25 degrees C and 35 degrees C, as well as the operating conditions of a conventional European supermarket. The global efficiencies of all the chosen compressors were extrapolated from some manufactures' data and appropriated optimization procedures of the performance of the investigated solution were implemented.According to the results associated with the conventional exergy evaluation, the gas cooler/condenser, the HS (high stage) compressor and the MT (medium temperature) display cabinet exhibited the highest enhancement potential. The further splitting of their corresponding exergy destruction rates into their different parts and the following assessment of the interactions among the components allowed figuring out the real achievable improvements. The avoidable irreversibilities of the HS compressor and that of the MT evaporator were mainly and completely endogenous, respectively. On the other hand, the gas cooler/condenser could be predominantly improved by decreasing the inefficiencies of the MT evaporator. As regards the auxiliary compressor, large enhancements were attainable through the drop in the irreversibilities occurring in the remaining components. (C) 2016 Elsevier Ltd. All rights reserved
Multi-ejector R744 booster refrigerating plant and air conditioning system integration – A theoretical evaluation of energy benefits for supermarket applications
Full text linkThe multi-ejector rack is the most promising technology to push the so-called “CO2 equator” further south
and improve the global energy efficiency of R744 supermarket refrigeration systems.
This paper theoretically compares the energy consumption of a CO2 refrigerating plant equipped with a multiejector
unit with that of a R404A direct expansion system (DXS), of a conventional CO2 booster configuration
and of two CO2 solutions using parallel compression. The energy benefits related to the adoption of low
temperature (LT) overfed display cabinets were also assessed. Furthermore, various scenarios involving
different sizes of the supermarket, integration and capacity of the air conditioning (AC) system and efficiency
of the parallel compressors were investigated. The evaluations were carried out by considering different
locations in Southern Europe. The results showed that, as a function of the selected boundary conditions,
energy savings ranging from 15.6% to 27.3% could be accomplished with the multi-ejector concept over
DXS
Energetic, Exergetic and Exergoeconomic Analysis of CO2 Refrigeration Systems Operating in Hot Climates
No full textCarbon dioxide is one of the most interesting refrigerants in terms of both environmental impact and efficiency at low outdoor temperatures. Its use in warm climates still needs for some improvements as well as some economic evaluations in order to analyse its real potential in such weather conditions. CO2 refrigeration system with parallel compression represents one of the solutions which have been proposed in the last few years in order to enhance the performance of a single-stage refrigeration system.
The main target of this study is to compare the thermodynamic efficiency and the final cost of the product of a R744 refrigeration solution with auxiliary compressor with those of a R744 conventional system, both of them operating in transcritical conditions.
The results pointed out that the adoption of an auxiliary compressor resulted in an increase of the COP by approximately 18.7% over the investigated temperatures range. Furthermore, the final cost of the product associated with this solution was on average 6.7% lower than the one of the conventional solution
Water storage to improve the efficiency of CO2 commercial refrigeration systems
No full textCarbon dioxide is being more and more widely used in commercial refrigeration, due to its negligible environmental impact. However up to now its application as the only refrigerant is especially devoted to cold climates, which allow for the best exploitation of this refrigerant. Several solutions are being identified to extend a convenient application of CO2 also to mild climate conditions, through modifications of the refrigerating cycle and the adoption of various innovative configurations. Quite often such configurations require significant extra costs or involve the adoption of solutions not yet well established in the market. In some cases the promotion of interactions between the refrigeration systems and other systems available in the building could allow for the exploitation of simple solutions which could be easily adopted with low extra costs.
In this paper a basic CO2 transcritical/subcritical commercial refrigeration system is considered, applied to cold rooms and display cabinets in a supermarket located inside a large shopping mall. Subcooling of the refrigerant is performed, taking advantage of a large fire prevention water tank.
The whole refrigeration system has been modelled in Trnsys environment. Simulations have been carried out taking into account the hourly weather data and the daily profile of the cooling load demand from the refrigerated food storage equipment.
Different configurations are examined, for the best exploitation of this cold storage. The technical feasibility of this solution is investigated
Modelling commercial refrigeration systems coupled with water storage to improve energy efficiency and perform heat recovery
Full text linkA 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 storag
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