“Open loop” cooling means there are evaporative cooling towers being used to cool the chilled water loop water: https://en.wikipedia.org/wiki/Cooling_tower

Adding evaporative cooling towers to a chilled water loop that uses chillers can double the efficiency.

A power plant that draws water from an intake, uses it to reject heat, cools it down, and then discharges it back to the stream/lake/ocean is once-thru cooling.