I would like to know how others have modeled a water-side (hydronic) economizer using OpenStudio. To achieve the best controls with this solution, which method have you taken?

Specifically I am looking to model a cooling tower which will provide either chilled water directly to the AHUs via a fluid-to-fluid heat-exchanger and/or or condenser water to the water-cooled chiller when the chiller is enabled needed to meet the demand. Ideally there would be a modulating valve to allow for part-load economizing and they would be the same coil in the AHUs.AHUs controlled with a modulating valve.