"For an ideal regenerator, it contributes
nothing to the energy budget and entropy budget when summed
over a cycle. Whatever it takes it gives back over the
course of a cycle."
That's very clever. I can see indeed that we could undo in the isochoric
depressurization what we did in the isochoric pressurization step, at least
for an ideal device.
Now what is such a device (I'll accept a real one and idealize it in my
mind), and why don't intro textbooks mention it given that this seems a
pretty essential part of non-Carnot cycles?