After his Opinion in Essent, Bot AG has turned on the heat on the ECJ in a case with many similarities, Case C-573/12 Vindkraft. Catherine Banet, a former student of mine, has excellent analysis linking the two cases here. Vindkraft concerns the successor to Directive 2001/77 (at stake in Essent), i.e. Directive 2009/28. The Advocate General essentially argues that the new Directive itself is contrary to EU primary law in allowing Member States to discriminate against foreign produced renewable electricity by limiting access to their national support scheme to electricity generated on their territory; and that such illegality is not backed by the environmental exceptions to the Treaty.
The ECJ has not yet held in Essent. As I have noted, it is far from guaranteed that it will follow all of the AG’s lead. (The Opinion at the time of posting was not yet available in English).