Discussed here:

A Norwegian town built a giant mirror to deflect the sun (2017) - https://news.ycombinator.com/item?id=19910305 - May 2019 (97 comments)

Sort of related: https://news.ycombinator.com/item?id=34056620

Others?

There are also two systems in Sydney, the big tall one on Rhodes Central [1] and the slightly smaller on One Central Park [2].

I think the idea is that by removing some of the shadowing from the high-rise buildings, the local authorities allow the buildings to be taller, which makes this type of system pay off.

[1] https://www.abc.net.au/news/2021-09-13/heliostats-bringing-l... [2] https://good-design.org/projects/one-central-park-heliostat-...

There was also a planned (but scrapped) project in Rattenberg, Austria:

https://en.wikipedia.org/wiki/Rattenberg