Equation for heat transfer and output of heat from a thermal storage unit

mzc07

Currently we are designing a thermal storage unit to be used in conjunction with a heat pump system. See attachment figure.
The thermal store is a mass of fire bricks with a helical pipe wrapped around it. Water flows through the helical pipe and the mass flow rate of the water is what allows us to control the output temperature. However we are struggling to find an equation to tie in mass flow rate, the temperature output of the thermal store, and the dimensions of the helical pipe.

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