Primary refrigeration plant is used

Primary refrigeration plant is used to abstract heat from a secondary coolant circulating through pipes driven into the ground. The primary refrigerant most commonly used will typically be some alternative to Freon, which due to it’s ozone-depleting characteristics had to be phased out until 1996 (see paragraph Refrigerants and the Environment below). Other primary refrigerants are ammonia, NH4 (–33.3°C) and carbon dioxide, CO2 – now not commonly used. The secondary coolant, circulated through the network of tubes in the ground is usually a solution of Calcium Chloride. With a concentration of 30% such as brine has a freezing point well below that of the primary coolant.
The primary refrigeration process is basically the Carnot cycle of compression and expansion reversed. The time required to freeze the ground will obviously depend on the capacity of the freezing plant in relation to the volume of ground to be frozen and on the spacing and size of freezing tubes and water content in the grounds