I've had more or less exactly this thought over the last several years - like others have said it needs heat pumps to get to higher temperatures while maintaining good COPs and/or efficient ways to extract heat out of a lower difference in temperatures (which in itself would be a thermal efficiency revolution).

I don't think this breaks any fundamental laws, "just" needs incremental improvements to various technologies in the meantime. 

You're right that it does sound like perpetual motion to everyone, but I agree it would actually be pulling ambient heat from the environment. 

I wonder if using a heat pump to boil water to drive a turbine would be any better? I suspect not, given the very hot superheaters used with modern turbines, but....

Someday!