The advantage of targeting the IR window is that it would work even with clouds and water vapor in the atmosphere whereas, plain old thermal radiation needs a c

The advantage of targeting the IR window is that it would work even with clouds and water vapor in the atmosphere whereas, plain old thermal radiation needs a clear night to be effective. Of course it won't pump heat backwards into the sun but they propose a water circulation system, which I imagine could store some daytime heat to release it at night. That and masonry of the walls of the building would store even more.