No, this doesn't rely on the nighttime being colder than daytime. So long as you were using a wavelength in which the atmosphere is transparent, you could run

No, this doesn't rely on the nighttime being colder than daytime. So long as you were using a wavelength in which the atmosphere is transparent, you could run the same system at any time of day even if you were in a temperature-controlled bubble that had no daily heat fluctuations. The only reason this system operates at night is because the atmosphere is not actually transparent to the frequencies used; during the day, the sky is just too hot to use as a cold sink.