"Closed Loop" Doesn't Answer the Question

The phrase "closed loop" describes how coolant can circulate repeatedly inside a data center, but it does not by itself tell you how the facility ultimately gets rid of the heat carried by that coolant.

One option is dry or air-based heat rejection, where heat is transferred to ambient air through heat exchangers or dry coolers. This can greatly reduce process-water requirements for cooling. Another option is evaporative heat rejection, such as a cooling tower. U.S. Department of Energy guidance explains that cooling towers remove heat primarily by evaporation and therefore require makeup water. They also discharge some water as blowdown to control dissolved minerals.

DOE has also documented high-performance computing systems that use a closed internal water loop connected to dry water coolers. The important distinction is therefore not simply whether the internal coolant loop is closed, but how heat is rejected outside that loop and what water, electricity, and other infrastructure that heat-rejection system requires.