The present invention relates to a method and apparatus for concentrating a fluid with improved energy efficiency. The method comprises the steps of: providing a membrane distillation unit having an evaporation side in fluid communication with a first reservoir for containing the fluid, and a condensation side being in fluid communication with a second reservoir for containing a coolant； evaporating at least a portion of the fluid and condensing the fluid in the second reservoir. The method further comprises the steps of controllably transferring heat from the coolant to the fluid such that the temperature of the fluid is maintained at a predetermined temperature.
