Carbon dioxide cooling system with subcooling

A method comprising:

• measuring:
• a temperature external to a first heat exchanger configured to remove heat from a carbon dioxide refrigerant, the first heat exchanger further configured to send the carbon dioxide refrigerant through an expansion valve, wherein the carbon dioxide leaving the expansion valve is fed to a flash tank;
• and
• determining that the measured temperature external to the first heat exchanger, is above a threshold;
• in response to the determination that the measured temperature external to the first heat exchanger, is above the threshold, activating a subcooling system, the subcooling system comprising:
• a condenser configured to condense a second refrigerant;
• a second heat exchanger coupled to an exterior surface of the flash tank, the second heat exchanger configured to receive the second refrigerant from the condenser, the second heat exchanger further configured to remove heat from the carbon dioxide refrigerant stored in the flash tank;
• and
• a compressor configured to compress the second refrigerant from the second heat exchanger, the compressor configured to send the second refrigerant to the condenser.

One or more computer-readable non-transitory storage media embodying software that is operable when executed to:

• measure of:
• a temperature external to a first heat exchanger configured to remove heat from a carbon dioxide refrigerant, the first heat exchanger further configured to send the carbon dioxide refrigerant through an expansion valve, wherein the carbon dioxide refrigerant leaving the expansion valve is fed to a flash tank;
• and
• determine that the measured temperature external to the first heat exchanger, is above a threshold;
• in response to the determination that one or more of the measured temperature external to the first heat exchanger, is above the threshold, activate a subcooling system, the subcooling system comprising:
• a condenser configured to remove heat from a second refrigerant;
• a second heat exchanger coupled to an exterior surface of the flash tank, the second heat exchanger configured to receive the second refrigerant from the condenser, the second heat exchanger further configured to remove heat from the carbon dioxide refrigerant stored in the flash tank;
• and
• a compressor configured to compress the second refrigerant from the second heat exchanger, the compressor configured to send the second refrigerant to the condenser.