Energy-efficient and environmentally friendly integrated CO2 vapour compression units for supermarkets

The aim of the project is to reduce unit costs and thus increase the uptake of energy efficient and environmentally friendly, integrated CO2 refrigeration machines in the supermarket sector, able to provide freezing, chilling and AC. A group of standardized, validated and ready for use design layouts, which covers most of the applications needed for the supermarkets, has been developed together with leading industry players. The Modelica-based simulation tool for transient simulations of supermarket energy systems was upgraded by adapting to new design of integrated compressor racks equipped with multi-ejectors for expansion work recovery, liquid recirculation ejectors included. A full 3D CFD model was upgraded by validation of the turbulence models most appropriate for two-phase transonic flows. The experimental facility newly installed at SINTEF/NTNU laboratory was numerically modelled to evaluate the energy performance of possible system configurations. Three alternative winter operation modes were proposed depending on the ambient conditions. The numerical models in Dymola-Modelica environment for the simulation of supermarket refrigeration systems with integrated loads and systems for the reduction of the power consumption (parallel compressors, ejectors, etc.) have been further developed. These models have been used for the determination of the optimal standardized configurations of refrigeration systems taking into account the requirements (loads, design ambient temperatures, etc.) of the project partners in the project, i.e., Norgesgruppen and Rema1000. The compressor rack provided by Advansor AS as donation, received at the end of the previous year 2016, has been used for experimental tests. Different experimental tests were performed in 2018: (i) Performance maps of the compressor pack - to measure the power consumption of different configurations of CO2 transcritical refrigeration systems with different conditions at the outlet of the gas cooler, discharge pressures, pressures in the liquid receiver and load percentages. (ii) Tests for Rema 1000 and Norgesgruppen - to measure the power consumption of the systems proposed for the partners of the project Rema 1000 and Norgesgruppen. (iii) Pressure drop and performance of the new design of multiejector block by Danfoss with enlarged outlet manifold and integrated particle filter. A POD-RBF reduced-order model of the two-phase ejector was developed and validated to cover a wide range of operating conditions for typical supermarket systems. The hybrid ROM of each ejector was successfully implemented to the refrigeration system simulation programmes i.e. TIL-Suite Modelica by TLK-Thermo GmbH, Coolseletor®2 software by Danfoss. Influence of heat transfer through the ejector block walls on the ejector performance was experimentally and numerically evaluated. Pack controllers for integrated racks were developed, in collaboration with Danfoss, and will be tested in the field after the official project termination.

The achieved effects: - development of Modelica-libraries of ejector geometries (4 high-pressure-lift and 4 low-pressure-lift cartridges for vapor compression, 2 cartridges for liquid pumping) that will enable easier system modelling of CO2-ejector supported systems, - development of advanced CFD codes for modelling/dimensioning of CO2 two-phase ejectors, enabling precise design of ejector geometries and enabling numerical performance mapping for off-design conditions in a broad operational envelope, - system analysis & layout optimisation of compressor racks for Norwegian supermarket chains, enabling future pack adaptation for any climate and load profile. The effects achieved should release further development of CO2-ejector technology, crucial for energy-performance boost in CO2 transcritical systems in warmer climates. The saving potential in the European Retail Sector only of substituting current HFC units with efficient CO2 systems accounts for approximately 9 TWhel/a.

It has already been verified experimentally that the integrated-planning approach to supermarket refrigeration and HVAC systems yields up to 30% reduction in the electricity consumption compared to a classic design. In order to reduce the unit costs and facilitate the uptake of the energy efficient and environmentally friendly integrated CO2 vapour compression racks in the supermarket sector, a group of standardized, 'ready-to-be-applied' design layouts, covering most of the possible supermarket applications, are deeply required. The main research problem addressed in SuperSmart-Rack involves combining the existing components (compressors, heat exchangers, fittings, etc.) and the components that still must be developed (ejector geometries, automation equipment, hardware and software for the controlling system, etc.) into a series of technically and economically optimized standard solutions for integrated CO2 vapour compression units. The optimization, comprising theoretical work and experimental testing of the prototype units, will be performed for a set of load profiles and ambient conditions that are specified by the supermarket chains as typical and are thus worth of having in the newly created portfolio of the ready-to-apply design layouts. A list of research activities comprise: - Development/upgrade of numerical simulation tools. - Development of standardized expansion work recovery modules based on the multi-ejector concept. - Optimization of design layouts for selected cases of integrated CO2 vapour compression units for supermarkets equipped with expansion work recovery system. The benefits for supermarket chains will result from the reduction in investment costs, electricity bills and space occupied by the machinery room as the integrated racks will be more compact than standard solutions due to the elimination of separate HVAC components. The contractors of commercial refrigeration and HVAC systems will gain new market possibilities outside Norway.