The builders of a hydropower plant at an old Welsh quarry want to double the amount of electricity it produces.
Snowdonia Pumped Storage already bagged permission in 2013 for a pumped water plant which can generate 49.9 megawatts (MW) of power.
The firm hopes to double this to 99.9MW after recent law changes mean energy storage is included the new scheme to pay for power generation, called the capacity mechanism.
An application has gone to the UK Planning Inspectorate which is responsible for power plants of 50MW or more in size.
The Glyn Rhonwy project will let water from a reservoir flow through a turbine down to a second reservoir when electricity is needed, using cheap, excess power at times of low demand to pump water back up to the reservoirs.
SPH managing director Dave Holmes described it as “hugely frustrating” they have to re-submit planning application. He said it is only underground equipment which will be bigger.
He claims boosting the power output would increase the payments to the local council in Gwynedd by around £600,000 a year.