Faults and energy consumption of CO2 refrigeration systems in supermarkets
Food products refrigeration accounts for the largest share of energy consumption in supermarkets. In a recent study, the most common faults in supermarket CO2 refrigeration systems were investigated, along with their impact on system performance.
Energy consumption in supermarkets.
In Europe and the USA, the average energy consumption of supermarkets has been evaluated between 320 and 800 kWh/m2 per year. For example, the average energy consumption in Spain is 327 kWh/m2 per year, whereas it is 800 kWh/m2 per year in Poland. In the UK, it varies between 400 and 740 kWh/m2 per year and in the USA it is about 600 kWh/m2 per year.  Products refrigeration is responsible for the largest share of energy consumption, evaluated at 29% - 48% according to various studies in Europe.  In the USA, the refrigeration system accounts for 40%–60% of total supermarket electricity usage annually. 
According to the “Renovation Measure Catalogue for Supermarkets” of the EU-funded Super-HEERO project, the adoption of a regular maintenance of refrigeration systems allows keeping them at the best operating conditions in terms of efficiency and minimisation of power consumption, at a low investment cost.  Therefore, fault detection and diagnosis (FDD) techniques can be used to support supermarket refrigeration system operators in achieving these objectives. 
Impact of six common faults on the performance of a CO2 supermarket refrigeration system
In a recent study, the authors identified common faults in a CO2 supermarket refrigeration system and compiled experimental datasets. The data sets were generated using a laboratory-scale commercial refrigeration system, consisting of a transcritical CO2 compressor rack, one medium-temperature (MT) refrigerated display case and one low-temperature (LT) refrigerated display case, an air-cooled gas cooler, and “false” MT and LT loads.
The authors studied the impact on system performance of six common faults which included: open LT display case door, ice accumulation on a LT evaporator coil, LT evaporator expansion valve failure, MT evaporator fan motor failure, condenser air path blockage, and MT evaporator air path blockage.
The six selected faults affected the LT/MT compressor operation through changing the LT/MT compressor suction and discharge refrigerant states. The LT evaporator operation characteristics were primarily influenced by the following faults: open LT display case door, ice accumulation, and LT evaporator expansion valve failure. The MT evaporator performance was significantly affected by the MT evaporator fan motor failure and the air path blockage. The condenser air path blockage mainly impacted the condenser operation.
For instance, the authors observed that the “open door” fault evidently required more cooling load and resulted in an increase in power consumption. In addition, the compressor had to work harder to meet the demand, resulting in higher discharge temperature and pressure.
 Super-HEERO project. Renovation measure catalogue for supermarkets. https://super-heero.eu/wp-content/uploads/2021/01/RENOVATION-MEASURE-CATALOGUE-FOR-SUPERMARKETS.pdf
 Sun, J., Im, P., Bae, Y. et al. Dataset of low global warming potential refrigerant refrigeration system for fault detection and diagnostics. Sci Data 8, 144 (2021). https://doi.org/10.1038/s41597-021-00927-6