A giant water turbine to provide energy for Windsor Castle has been installed on the River Thames.
The 40-tonne Archimedes screw turbine joins another already installed at Romney Weir.
When they are both up and running in the new year they are expected to provide half the electricity needed for the Queen's Berkshire residence.
The £1.7m project will also provide electricity to homes and buildings on the castle estate.
The hydropower scheme is being built by Jackson Civil Engineering for developer Southeast Power Engineering Ltd (Sepel) and is being managed by the Environment Agency.
Project manager Stephen Naylor said: "This is the first one for us, we're going to learn a lot from it."
Mr Naylor said the Environment Agency would work with developers and communities to explore similar schemes at other suitable sites.
A fish pass will be built to allow migration up the Windsor stretch of the river. Another will be created for endangered eels.
The Queen's deputy treasurer Mike Stevens said: "The Royal Household is constantly looking at new ways of saving and supplying energy so as to remain as environmentally friendly as possible well into the future.
"Accordingly, the household was very keen to support this project."