Mac Photos App and storage space

I'm clearing up space on my mac. I use both the Photos app and iPhoto.

I'm happy with iPhoto no issues there - it's a better interface IMO and easier to manage!

Photos however...Doing an OmniDiskSweep on my drive it says the Photos app is using 23.1GB. I set the preferences to optimise Mac storage. Is no method of just streaming the photos. I don't want to save anything locally besides the thumbnails...Any thoughts?

How can I delete the download off my mac but keep them in the cloud?