The turbine is an upgrade of the company's existing 6.2MW offshore machine but will be fitted with a 152-metre rotor, compared with the 126-metre rotor of the previous model.
The greater blade length means an increase in the swept area of about 50% and, the company claims, a 20% increase in yield at wind speeds of 9.5m/s.
Senvion has started work at the site in Langen-Neuenwalde in Lower Saxony, Germany, with the access roads currently under construction.
The manufacturer expects to enter commercial production of the new model in 2015. Senvion announced last November that it was developing the turbine.
The prototype has already been sold to German developer EWE, which also operates a 5MW Repower prototype nearby.