It is a commonly held belief that if there is already one wind-farm in an area, it is therefore OK to stuff a load more in that area. However, as well as the problems already mentioned, the problems from multipath interferences caused by signals reflected from rotors on the same farm, as well as on neighbouring farms, often give rise to the impression of a moving (albeit fleeting) target on ATC radars, that would present itself as indistinguishable from 'pop-up' traffic.
When another farm is added in the same area, the possibilities of such unpredictable multipath returns can increase dramatically, causing a real problem for ATC radars for what might hitherto have been a manageable problem. Before you know it what was once a nice clean picture has been degraded significantly into a mess of spurious ephemeral and semi-permanent returns, which can be indistinguishable from aircraft.
Despite all the funding into studies to overcome these problems, to my knowledge nothing has yet been guaranteed to remove all of the problems; until such time a a technical solution is developed then ATC units should continue to remain extremely cautious of any proposed development.
Wind farm developers (with multi-million pound budgets) are required to consult if within 25km of an airport with radar. However, just because there has been consultation, does not mean that the very powerful, Government backed, wind power industry actually takes any notice of legitimate safety concerns of airports/ATC units.
You are unfortunate if your airport is either:
1) in a hilly area,
2) a coastal area,
3) an area away from MOD radar units (whose veto usually knocks a wind farm proposal on the head in the way that a civil airport ought to be able to)
as your airport will tend to receive a higher proportion of proposals. I have heard that each county council has been given quotas that they have to acheive towards providing a certain percentage of renewable energy, and that this has to be sited in their county, no matter how unsuitable that county may be for wind farm stationing.
This means that some airports are receiving many more such windfarm proposals than other airports, and having to make the resources available for assessing such proposals. (It was with good reason that the CAA dispensed with airport safeguarding and that airports now have to bear this cost on their own!)
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