SHW with dual-coil tanks [RE-wrenches]

[Nicholas Ponzio]

Solar Hot Water Wrenches,

I've noticed several manufacturers are offering dual-coil solar
storage tanks. The idea is that you can combine your backup boiler
storage and solar storage all in one tank. Has anyone adopted this
strategy? How does this effect system efficiency? I understand that
stratification in the tank will allow a certain volume of "cooler"
water in the bottom where the solar HEX is but does that mean your
usable storage volume is diminished? And how do these internal coils
compare to tube-in-shell HEX?

Thanks in advance for any insights.

-Nik Ponzio