A data center cooling system comprising: a cooling tower (104,212); one or more modular refrigeration chiller units; a water loop that can be selectively directed through the cooling tower (104,212), through one or more of the modular refrigeration chiller units or through a combination thereof, wherein the water loop is configured to, at an interface with the data center, receive a heat load from the data center, and to deliver chilled water back to the interface with the data center; a control unit configured to control direction of the water loop through the cooling tower (104,212), through one or more of the modular refrigeration chiller units or through a combination thereof; a first water temperature sensor configured to measure water temperatures at a point in the water loop where the heat load from the data center is received; and a second water temperature sensor configured to measure water temperatures at a point in the water loop where the chilled water is delivered back to the data center, wherein both the first water temperature sensor and the second water temperature sensor are connected to, and monitored by, the control unit, characterised in that, the cooling system further comprises a solar cooling unit through which the water loop can also be directed, wherein the control unit is further configured to: compare available sunlight energy to a minimum amount of sunlight energy needed for solar cooling; and selectively direct the water loop through the solar cooling unit or through a combination of the solar cooling unit and one or more of the modular refrigeration chiller units if the available sunlight energy is greater than the minimum amount of sunlight energy needed for solar cooling, otherwise repeating the comparison of the available sunlight energy to the minimum amount of sunlight energy needed for solar cooling at a given time interval if the available sunlight energy is less than the minimum amount of sunlight energy needed for solar cooling. a cooling tower (104,212); one or more modular refrigeration chiller units; a water loop that can be selectively directed through the cooling tower (104,212), through one or more of the modular refrigeration chiller units or through a combination thereof, wherein the water loop is configured to, at an interface with the data center, receive a heat load from the data center, and to deliver chilled water back to the interface with the data center; a control unit configured to control direction of the water loop through the cooling tower (104,212), through one or more of the modular refrigeration chiller units or through a combination thereof; a first water temperature sensor configured to measure water temperatures at a point in the water loop where the heat load from the data center is received; and a second water temperature sensor configured to measure water temperatures at a point in the water loop where the chilled water is delivered back to the data center, wherein both the first water temperature sensor and the second water temperature sensor are connected to, and monitored by, the control unit, characterised in that, the cooling system further comprises a solar cooling unit through which the water loop can also be directed, wherein the control unit is further configured to: compare available sunlight energy to a minimum amount of sunlight energy needed for solar cooling; and selectively direct the water loop through the solar cooling unit or through a combination of the solar cooling unit and one or more of the modular refrigeration chiller units if the available sunlight energy is greater than the minimum amount of sunlight energy needed for solar cooling, otherwise repeating the comparison of the available sunlight energy to the minimum amount of sunlight energy needed for solar cooling at a given time interval if the available sunlight energy is less than the minimum amount of sunlight energy needed for solar cooling. compare available sunlight energy to a minimum amount of sunlight energy needed for solar cooling; and selectively direct the water loop through the solar cooling unit or through a combination of the solar cooling unit and one or more of the modular refrigeration chiller units if the available sunlight energy is greater than the minimum amount of sunlight energy needed for solar cooling, otherwise repeating the comparison of the available sunlight energy to the minimum amount of sunlight energy needed for solar cooling at a given time interval if the available sunlight energy is less than the minimum amount of sunlight energy needed for solar cooling.