Presumably the store is fed from a heat pump and you want it to be a constant flow system? It would work, but, I think you would find that the reaction times for could be quite slow. A flow switch will trigger as soon as hot water is drawn and the flow rate exceeds the set point.
To give this further thought (for me), which manufacturers valve are you proposing to use and what are the signal outputs it delivers for integration into the control system?
My thermal store will be heated by a gas boiler. I would go with an internal coil for DHW, but they are typically sized too small if you want run the store a bit cooler, say 60-65C. So yes, it would be a similar situation as if the store was heated by a heat pump.
The modulating pump and flow valve should keep the exhanger warm all the time, so there should be no lag when the water is first turned on. As the cold water cools the primary, the balancing valve would open to maintain the return temperature. This would cause the pump to increase the flow.
I would use a Grunfos Alpha2 modulating pump and a valve like this:
JG Speedfit ¾ - https://www.NoLinkingToThis/p/jg-speedfit-themostatic-balancing-valve/276HT
You can get the valves without the 70C bypass function, which I would not need. The pump would be on all the time. I am not sure how far that valve closes. Ideally you would want one that closed right up like a TRV, but based on the water temperature, rather than air temperature. I also wondered if a 3 port TMV would work, with one port blocked, as they are easier to find.
The nice thing is that there would be no wiring or switches, just power to the pump, and it would work even if the hot water flow was very small, which can sometimes play havoc with a flow switch. It might also help to maintain stratification, especially in the case of low flow, as only enough water to keep the exchanger warm would move. I was thinking to set it to keep the return around 45-50.
