Backup cooling device of a user site having at least one cold store for the preservation of foodstuffs or perishable goods, said cold store being cooled by a mechanical cooling system, the user site being equipped with a liquid CO 2 tank used on the site for a primary use, the backup system comprising the following elements: - a liquid CO 2 evaporation circuit positioned in the cold store and comprising an evaporator, which is a second evaporator independent from the evaporator of the mechanical cooling system; - a line which supplies, from said tank of liquid CO 2, said second evaporator of the evaporation circuit, in order to cause the evaporation of the CO 2 and to bring frigories to said goods, the line being provided with a device to control the quantity of CO 2 supplied to said second evaporator; - an evacuation towards the exterior of the cold store of the CO 2 vaporised in said second evaporator; - an acquisition means of at least one item of information representative of the functioning or malfunctioning of the mechanical cooling system; - a system ("BOX") for the acquisition and processing of data designed to retrieve said at least one representative item of information and to initiate the supply of CO 2 of said second evaporator from said tank when the retrieved information is representative of a malfunctioning of the mechanical cooling system, and characterised in that it comprises the following elements: - an ambient temperature thermostat located in the cold store; - a calibrated orifice provided on the CO 2 circuit at the output of said second evaporator, designed to receive a controlled flow of CO 2 based on the pressure inside the tank; - a solenoid valve configured to open and close the CO 2 supply to said second evaporator based on the measurement performed by the thermostat; - a pressure regulator, provided on the CO 2 circuit at the output of said second evaporator, capable for making it possible for the pressure in the evaporator and in the circuit to be maintained at a pressure greater than the triple point of the CO 2. - a liquid CO 2 evaporation circuit positioned in the cold store and comprising an evaporator, which is a second evaporator independent from the evaporator of the mechanical cooling system; - a line which supplies, from said tank of liquid CO 2, said second evaporator of the evaporation circuit, in order to cause the evaporation of the CO 2 and to bring frigories to said goods, the line being provided with a device to control the quantity of CO 2 supplied to said second evaporator; - an evacuation towards the exterior of the cold store of the CO 2 vaporised in said second evaporator; - an acquisition means of at least one item of information representative of the functioning or malfunctioning of the mechanical cooling system; - a system ("BOX") for the acquisition and processing of data designed to retrieve said at least one representative item of information and to initiate the supply of CO 2 of said second evaporator from said tank when the retrieved information is representative of a malfunctioning of the mechanical cooling system, - an ambient temperature thermostat located in the cold store; - a calibrated orifice provided on the CO 2 circuit at the output of said second evaporator, designed to receive a controlled flow of CO 2 based on the pressure inside the tank; - a solenoid valve configured to open and close the CO 2 supply to said second evaporator based on the measurement performed by the thermostat; - a pressure regulator, provided on the CO 2 circuit at the output of said second evaporator, capable for making it possible for the pressure in the evaporator and in the circuit to be maintained at a pressure greater than the triple point of the CO 2.