I have been contemplating a similar system. My solution was to store hot water for the radiant floor system in underground tanks, buried inside. For the evacuated tube heaters, I am going to use a separate set of tanks, that will heat the water tanks with coils. That way I can just let the evacuated tubes run all day, and heat their tanks as hot as they want. Then when the (underground) water tanks start to cool, they can feed the coils. Of course I am building the whole system inside a "greenhouse" so I don't have to worry about freezing or much in the way of heat loss at night.
I posted a thread about it but here is a diagram that explains it better.
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