A system and method are disclosed for cooling ambient air to be supplied as combustion air to a gas turbine. The system comprises a closed coolant loop direct expansion cooling system including a compressor for compressing a suitable working fluid, an expansion device downstream from the compressor for expanding the working fluid so as to cool a cooling coil. The cooling coil is in heat exchange relation with ambient air flowing to the gas turbine for lowering the temperature of the ambient air to a lower temperature such that combustion air delivered to the gas turbine is below the ambient temperature thereby to increase the efficiency of the gas turbine. A return line is provided for returning the working fluid to the compressor.