What's impact of GT evaporative coolers operation on heat rate at partial load? In case of open cycle and combine cycle mode?
First--and most important of all--the heat rate of most gas turbines is rarely, if ever, guaranteed at part load. Heat rate is only ever guaranteed at Base Load, when the IGVs are full open and the unit is exhaust temperature control. Part load heat rate is NOT good, because the IGVs are not fully open. In simple or combined cycle.
In the early days of evaporative coolers, they could not be started until the unit was at Base Load. I think that's changed--but primarily because of the perception that performance improves because of inlet air cooling through the evaporative cooler. But, I believe that there is no "performance" improvement--at least as far as heat rate is concerned.
You can go the Mee Fogging website and there are LOTS of white papers there about performance and heat rate--albeit for fogging, but the concept is similar (cooling the inlet air, while also increasing mass flow). I've never read all of them, but as per my understanding (which I hope otised, a valued contributor to control.com, will comment on) heat rate is poor at part load, and I don't believe that using any kind of inlet air cooling will necessarily improve that.
Hope this helps!