Close to a small fishing port in southwestern Japan, the slim white turbines of the country's first commercial-scale floating wind farm glimmer offshore, months before a key project in Tokyo's green-energy strategy begins.

Still heavily reliant on imported fossil fuels, Japan has declared offshore wind energy a "trump card" in its drive to make renewables its top power source by 2040, and reach carbon neutrality a decade later.

That's despite rising project costs and fears over inadequate infrastructure to produce turbines en masse.