CO2 refrigeration system with high pressure valve control based on coefficient of performance

A controller configured to perform operations comprising:

• automatically generating a setpoint for a variable of a refrigeration system based on a measured temperature of a refrigerant at an outlet of a gas cooler/condenser, the variable comprising a coefficient of performance (COP) of the refrigeration system, the setpoint generated using a stored relationship between the measured temperature and a maximum estimated COP at the measured temperature;
• and
• calculating the COP of the refrigeration system during operation of the refrigeration system as a function of:
• (i) a change in enthalpy of the refrigerant between at least one first evaporator operating at a first evaporator state and at least one second evaporator operating at a second evaporator state different than the first evaporator state and (ii) a change in enthalpy of the refrigerant between at least one first compressor operating at a first compressor state and at least one second compressor in series with the at least one first compressor and operating at a second compressor state different than the first compressor state.