The questions for solar field parameters of parabolic trough

I have two questions.
one is that I don't understand why we should choose the maximum direct radiation incident as the irradiance at design value? Please explain more details.Why should not we use the min or average value?
tow is that how can I confirm the value of 'min single loop flow rate'and 'max single loop flow rate'? is there some method of caculation? Please details more. Thanks.

Our recommendation for choosing reference weather conditions for the parabolic trough models are described in the Help topic for the trough solar field page, under the heading "Reference Weather Conditions for Field Sizing." SAM uses the reference DNI value and other variables to calculate the field aperture area when you choose to specify the field size using the solar multiple (Option 1 Solar Multiple) instead of explicitly specifying the field aperture area.

We recommend using a reference DNI value close to the maximum incident DNI value for the field so that the design values represent the maximum energy levels you would expect the field to experience. The actual parameters are typically for a field larger than the design value because the number of hours of the year that the maximum DNI occurs is small. As explained in Help, if you use the average or minimum incident DNI value, the field will generate more energy than the power block can handle, and SAM will report large amounts of "dumped energy" in the results.

As for the solar field single loop minimum and maximum flow rates, in general you can use the default input values unless you have data to support other values, or some other reason to change them. You can always do a parametric study on the variables to get a sense of how sensitive your output metrics of interest are to the input values. There are some physical constraints to the flow rate limits that you should also consider.

Your choice of a minimum flow rate should: 1) Minimize overnight freeze protection parasitic power, and 2) be appropriate for the solar field pumps' design minimum flow rate. The HTF in the field needs to be circulated at night to avoid "freezing" or congealing of the oil, which makes startup very difficult in the morning. If you choose too low of a minimum flow rate the HTF to move too slowly through the field. This will cause the fossil heat freeze protection system to heat up the fluid to excessive temperatures at night just to keep the solar field outlet section above the freeze temperature. The excessive temperatures result in high thermal losses, and may over-heat the fluid.

You should choose a maximum flow rate to 1) be appropriate for the field pumps maximum design parameters, and 2) to minimize corrosion and vibration n the pipes.

Best regards,
Paul.