The €50m ($64.2m) plant, which uses waste straw as fuel, will create around 100 jobs.
Briviesca is in the Burgos province, which is part of the central Castilla y León region.
Acciona has signed contracts with more than 100 farmers and 38 companies in the area to supply agricultural waste for the project.
The plant is 85% owned by Acciona and 15% by local power utility Ente Regional de la Energía.
Acciona
