I suspect the reason is you want hot water stored at ~60C. If you pump water through it slowly enough / enough times to attain that temperature then you are act

I suspect the reason is you want hot water stored at ~60C. If you pump water through it slowly enough / enough times to attain that temperature then you are actually increasing the temperature of the panels (assuming air temperature is less than that). You'd need to use a heat pump to get actual hot water from a small delta T in the coolant.