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State-of-the-art Integrated Refrigeration Systems in Supermarkets

An Energy Efficiency Evaluation Based on Field Measurements Analysis and Computer Simulations

Time: Fri 2021-05-28 13.00

Location:, Stockholm (English)

Subject area: Energy Technology

Doctoral student: Mazyar Karampour , Tillämpad termodynamik och kylteknik

Opponent: Dr. Silvia Minetto,

Supervisor: Associate Professor and Docent Samer Sawalha, Tillämpad termodynamik och kylteknik; Professor Björn Palm, Tillämpad termodynamik och kylteknik

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Supermarkets have become a vital feature of modern society. However, they have significant impact on the environment due to the large amount of refrigerants needed in the refrigeration system, and due to the large energy use in the supermarket, they are the commercial buildings with highest energy intensity. The largest energy-user in the supermarket is typically refrigeration systems; these systems consume and emit a large amount of high global warming potential (GWP) refrigerants. The impacts of the supermarket refrigeration systems on the environment has been under investigation, mainly in the recent decades. The solution to mitigate these two major impacts of high GWP refrigerant emissions, and high energy use is by building more energy efficient refrigeration systems that use environmentally friendly refrigerants. Refrigeration systems using CO2 as a refrigerant has been emerged and applied as a solution in the recent two decades.

This PhD thesis explores methods to increase the energy efficiency of CO2 refrigeration systems in supermarkets. The objective is to present and evaluate technical solutions that make the system competitive with conventional systems in terms of energy, environment, and economics. This thesis focuses on design modifications and energy systems’ integration. This is based on computer simulations and field measurement analysis of a number of CO2 systems installed in Sweden. The conventional HFC solutions, as reference, and alternative cooling and heating systems are included in the comparative performance studies. 

As the starting point of this PhD thesis, the first study was to benchmark and compare the performance of emerging CO2 systems with conventional HFC systems, which was a completion of results from a previous project. To fulfil this objective, the field performance of five CO2 systems are compared with three HFC-based systems, all installed in Swedish supermarkets. The results show that the newer generation of CO2 systems are more, or as efficient as advanced HFC system solutions. Key system parameters that have the highest impact on CO2 system’s energy efficiency are studied by computer simulations, and a CO2 baseline system is defined as the basis for this thesis. This baseline system is used in later studies to examine design and efficiency improvements and, eventually, to define the state-of-the-art CO2 system.    

The second study evaluates modifications in the design and integration of heating and air conditioning into the CO2 baseline system. This integration concept is investigated primarily by analysis of the field performance of a CO2 integrated system. The system is installed in a Swedish supermarket and applied several features to increase the energy efficiency and functionality of the CO2 baseline system. These include space and tap water heating, air conditioning, and parallel compression. This integrated system is also modelled and compared to stand-alone HFC-based cooling and heating systems. The results show that the integrated solution can provide heating, with heating COP values ranging from 4 to 6, comparable to commercial heat pumps. The system can also provide the entire air conditioning demands of the supermarkets. The efficiency of the system AC performance, while assisted by parallel compression, is comparable to or higher than the HFC solutions. Considering all the thermal functions of the system (refrigeration, heating, AC), the results prove that while the integrated solution is an efficient all-in-one unit in cold to moderate climates, it requires efficiency improvements to be a potent candidate in warm climates.

Energy efficiency improvement of the integrated CO2 system, both in cold and warm climates, is the subject of the third study. Design features, and integration with heating and air conditioning systems are evaluated to identify the most promising solutions in terms of energy efficiency. The outcomes of the parametric study led to defining the state-of-the-art (SotA) CO2 system. 

State-of-the-art integrated CO2 system is defined as an energy efficient, environmentally friendly, and all-in-one compact solution able to meet all the thermal demands of supermarkets in cold and warm climates. This system includes flooded evaporation, parallel compression, and heat recovery at two temperature levels. This system is compared to conventional and alternative refrigeration systems in supermarkets. The comparison shows that the SotA CO2 system offers at least 15% annual energy savings compared to the standard CO2 system, and at least 25% compared to conventional systems with HFC’s in Stockholm, Sweden, making the system the most efficient that can be installed in Sweden today. SotA CO2 system is also one of the most efficient system in warm climates.     

Integration of geothermal storage into SotA CO2 system is the subject of the fourth study. This integration gained interest in the recent years in Northern Europe. The ground thermal storage can be used as a heat sink in summer, and a heat source in winter, to improve the overall performance of the CO2 system. Three scenarios of integration, including stand-alone, integrated, and hybrid energy systems in the supermarket building are evaluated. The results of the study shows that annual operating cost of a separate supermarket and district heating consumer for space heating can be decreased by 20-30% if the systems are coupled, geothermal storage is applied, and the supermarket provides heating for the consumer. This integration is advantageous for supermarket with very high heating demands, or supermarkets that share heating systems with a neighbouring building.

Some major contributions are made in this PhD thesis. First, detailed field measurements analysis of the modern CO2 system is presented. This has been missing in the literature to a great extent. Second, a comprehensive screening of the most promising solutions is carried out by computer simulations, which guarantees the CO2 system operates at the highest efficiency possible. This improves and accelerates the decision-making process of supermarket stakeholders on choosing the most energy efficient solutions. Last but not least, this thesis highlights the availability and competitiveness of environmentally friendly refrigeration technologies for supermarkets.