Minimum DNI for start up MS SPT model

Hi Paul/NREL

When running the annual simulations, I believe I am correct in saying that the model varies HTF flow rate to achieve the desired outlet temperature based on the DNI and resultant incident flux on the receiver.

However, how does the model specify a minimum DNI for production of hot salt? if so, what is this value and can I change this?

Kind regards

Harry

Hi Harry,

I'm not sure I understand your question. Can you clarify what you mean by "for production of hot salt?" The model attempts to maintain the HTF (molten salt in this case) temperature between the hot and cold temperature inputs on the System Design page.

Best regards,
Paul.

Hi Paul, 

Thanks for getting back to me. Sorry about that, what I meant was, how does the performance model "decide" when to start/stop the heating of HTF. I had assumed it was based on a minimum instantaneous DNI, but I had a more thorough look through, and I believe it is done by specifying a minimum flow rate, is that correct?

IF so I am assuming the model calculates the flowrate of HTF required to yield the outlet temperature, and if the DNI and hence flux on the receiver are so low that the flowrate would be below the specified lower bound, then the process is non operational for that hour?

IS that at all correct Paul? if not, please could you correct me?

Thank you,

Harry

Hi Harry,

One way to control the minimum receiver operation is by adjusting the input “Minimum receiver turndown fraction” on the Tower and Receiver page.

SAM applies this fraction to the incident (after reflection losses) flux:

Minimum incident flux = Design thermal power absorbed by HTF * Min Receiver Turndown Fraction / 0.9

Where 0.9 is an estimate for receiver thermal efficiency. 

The effect of this is very similar to setting a minimum on the HTF mass flow rate.

Best regards,
Paul.