Conventional practice is to highly insulate a solar storage tank.
But, when this solar storage tank sources a W/W heat pump it may be more efficient & cheaper to NOT insulate the tank.
Heat moves from an uninsulated tank into the surronding earth MUCH faster
(10-20X) or so depending on soil type & ground water..
But it's not all lost, when the HP draws the tank temperature down to below the surronding earth temperature, heat moves back into tank, possibly 25% of it.
This still sounds like a loss, but consider that MY solar panel array will be WAY oversized (10-15X) vs. summer DHW use.
The surplus, solar heat in warm weather can be dumped into the earth with part of it being used in cold months. Plus a cooler tank will allow the panels to be more efficient.
Edit: They insulate the top only.
See:
Laramie, Wyoming - Thermal Battery Systems | Thermal Battery Systems