Having analysed radar recordings from a Windfarm 'vs' Radar trial some years back, my personal views are that windfarms just do NOT mix with aviation.
On the trial, a number of differing aircraft types were flown in pre-determined patterns on the lee side of a windfarm...
The effects on primary only radar are significant, it became impossible to effectively 'track' the aircraft due to the number of false plots/tracks (clutter) created.
Secondary radar was much more resilient and the aircraft could be tracked (about 90% of the time) - however I have seen some 'strange' effects elsewhere in the UK whereby aircraft at a range of 50nm from the radar (the windfarm being about 15nm from the radar) at around FL300 would simply hyperspace/dissapear at random.
Using primary AND secondary does reduce the impact of windfarms but still intoduces false plots/tracks - not particularly usefull if you are trying to control aircraft
A number of companys have attempted to address the windfarm issues by developing advanced tracking techniques.. but they are far from perfect.
I'd be interested if any ATCOs have had any 'experiences' or views on windfarms.