I always have loved this idea, but people were trying this in the “cleantech boom” (~2008-10) and all failed. Why didn’t it work back then and what’s different today?
Hydropower plants are already permitted and connected generators. They are allowed to produce energy for sale onto the grid. From the utility's perspective, adding solar to a hydropower plant is the same as if there was a year with extra rainfall - the maximum (and reserve) power limits will still be adhered to, but more total energy will come from that single source.
(Note that solar on the ground right next to the hydropower plant would have the same benefits without the additional costs of floating, but often hydropower plants are built in geographies that are more challenging for ground-mounted solar - they're either extremely hilly or built out.)
Wikipedia lists early deployments and the ramp up of production seems to mirror other solar uptake suggesting the early ones proved the concept enough to get the next generation built.
This particular deployment in Indonesia is a knock-on side effect of how cheap it's getting for everyone!
The argument for floating is and was: available, generally unshadowed land, already flat, with power connection often already there, and lower deployment.
So what’s different now? I’d love for this to succeed.
PV now costs less for everyone. That can and seems to make the difference in whether industrial scale deployment is cheap enough for commercial entities to invest.
Or maybe this was just a novel although simple idea: Use the pool at powerplants to put the cells there. Maybe powerplants didn't think of it because they were opposed to solar ideologically.
I’d also guess that shear will to think outside the box has increased. We’ve seen some things the last 15 years when it comes to the actual effects of climate change. The 15 years prior to that the climate change story could still be denied (by some) and I’d argue that now we’ve all seen and felt it and it’s pretty undeniable (without some bias).