I don't think EnergyPlus can currently model variable speed closed circuit cooling tower. The only work around I can think of is using PlantComponent:UserDefinedto define own variable speed closed circuit tower. This will be a very time consuming approach though. As the documentation says, open and closed circuit tower models for single and two speed are very similar. Since variable speed cooling tower models are based on empirical curve fits of performance data, first thing I would try is to run this model and see how it compares with the closed circuit tower data that I have. If variable speed tower can be used, that would be the best approach.