Limiting Velocities in the solar field of PTC plants

Good morning,
I'd like to have some more insight on how the defaults for the limiting velocities for the HTF in the solar field of PTC plants are selected. I believe the impact of such numbers is pretty high, and maybe some "optimal" value might be found instead of prescribing numbers.
For the max velocity for instance (max mass flow), I guess the optimum could be found analyzing the trade-off between pressure drops (pumping power) and augmented electricity production (within technical limits of power block over-capacity).

Are those number deriving from practice?
have you performed any sensitivity analysis?
do you have some related reference?

Thanks in advance

Emiliano Casati

Dear Emiliano,

The maximum velocity (a function of the user-defined maximum flow rate) is dependent on the design of a single loop. At the design point, there a design mass flow rate that achieves the design outlet temperature. The maximum mass flow rate should be higher than the design. The extent of the overdesign is a function of design points, expected operating range, and HTF pump considerations. Considering these parameters, the value in the default case is a value we consider "representative" for that case. Although this may not be the optimal maximum flow rate with respect to maximizing annual net energy output, the difference in maximum output between the default and that optimal value should be negligible.

The minimum mass flow rate is an estimate of the ability of the HTF pump to operate below its design point.

Best regards,
Paul.

Dear Dr Gilman,
thanks a lot for your answer,
this point is clearer now!

Emiliano