A cooling unit and cooling method are provided for a container-type data center. The cooling unit includes a heat rejection unit, for rejecting heat from coolant passing through a coolant loop to air passing across the heat rejection unit, and a refrigeration unit controllable to selectively provide auxiliary cooling to at least a portion of the coolant passing through the coolant loop. The heat rejection unit includes a heat exchange assembly, having a first heat exchanger and a second heat exchanger, and one or more air-moving devices providing airflow across the first and second heat exchangers. The first heat exchanger couples in fluid communication with the coolant loop, and the second heat exchanger is coupled in fluid communication with a refrigeration loop of the refrigeration unit for rejecting heat from refrigerant passing through the refrigeration loop to the airflow passing across the second heat exchanger.