CSP IPH Model: Minimum Loop Flowrate

Hi,
I have a pretty basic question. I tried to model an IPH process to produce 1MW thermal power with TES and doing the simulations I noticed that the Minimum Loop Flowrate has a not negligible impact on results. 
So my first question is, let's say, theoretical: why it has such influence and why it is so important?
The second question concerns instead the way the algorithm operates: in the results I see that during hours with absence of radiation, there is still a flowrate of HTF that circulates in the receivers equal to the Minimum Loop Flowrate. I don't understand this thing, wouldn't be more simple to store the HTF fluid in the tanks during night to preserve its temperature? 

Thanks in advance for your answers!

Thank you, Paul!