By definition thermal mass is equivalent to thermal capacitance heat capacity.
Thermal mass water storage tanks.
A rocket mass heater will burn cleanly no creosote to condense inside flue channels and heat a masonry or cob mass warming the water while absorbing some heat from the water during the day.
It is kept liquid at 288 c 550 f in an insulated cold storage tank.
As rock or masonry.
What is water storage tank and thermal mass.
Water stores 3 to 4 times as many btu s per lb.
Thermal energy chilled water or hot water is produced during periods of off peak electrical demand or usage collected in a thermal energy storage tank then withdrawn and distributed to the facility during on peak periods.
I think you would find it effective and easier to have separate thermal mass in contact with the water tanks and heated by the fire.
Water contained in low cost non pressurized cylinders has proven to be the most practical and effective approach to the capture and storage of thermal energy for space and hot water heating.
The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 c 1 051 f.
Warm and chilled water enters and exits the tank through diffusers located at the top and bottom of the tank.
Thermal storage tanks are used in solar heating systems and wood boilers where you need to store the produced heat in water for later use.
No water coils or piping logistics needed.
With proper insulation of the tank the thermal energy can be usefully stored for up to a week.
If you check the specific thermal capacity value for various materials you will notice that water has a remarkably high thermal mass.
It also releases this heat energy faster when you really need it.