CO 2 refrigeration system with external coolant control

A refrigeration system includes a refrigeration subsystem and a coolant subsystem. The refrigeration subsystem is configured to circulate a refrigerant between an evaporator ( 12,22 ) within which a refrigerant absorbs heat and a gas cooler/condenser ( 2 ) within which the refrigerant rejects heat to provide cooling to a temperature-controlled space. The coolant subsystem includes a heat exchanger ( 61 ) coupled to the refrigeration subsystem at an outlet of the gas cooler/condenser and configured to transfer heat from the refrigerant exiting the gas cooler/condenser to an external coolant when the external coolant flows through the heat exchanger, a control valve ( 64 ), and a controller ( 50 ) configured to operate the control valve to control a flow of at least one of the refrigerant or the external coolant through the heat exchanger based on a temperature of the external coolant relative to a temperature of the refrigerant exiting the gas cooler/condenser.