[cwillu]

At the cost of very efficiently radiating that heat back out into space at night.

Making electricity and then using that electricity to heat something elsewhere lets you insulate, effectively allowing you to create a box that heat energy can only pass one way.

[jaggederest]

We have a one-way diode technology for heat, it's called "glass", and it'll bump your efficiency by about 25% versus uncovered flat plates on a still day. More in windy conditions etc, lots of hand waving assumptions about spherical cows in a vacuum etc.