The idea is not new: for instance around 1800 some “twin” mills have been used to pump water in Denmark and there are quite a few similar concepts and international patents dating the first half of 1900 – see for instance the drawing below.
What is interesting here is the size of the company testing the idea – Vestas has installed approximately 1 year ago a multi rotor wind turbine, using old refurbished V29 nacelles equipped with new sensors and electronics. Blades tip distance is less than 2 meters.
I’m sure that people who think that wind turbines are ugly will think that this solution is atrocious. For me it’s a quite interesting “out of the box” exercise in a business where rotor diameter has been relentlessly growing in the last decades (V29 is from the ’90… twenty-something years later we have V112 and bigger models).
The benefits are self-evident: greater swept area, saving in tower and foundation, less land use, etc.
The challenges are equally impressive: a whole new set of loads to consider, a system that can easily become unbalanced, increased turbulence and so on. It’s early to say if it will ever become commercially viable but it is for sure an interesting experiment.