Negative monthly energy (kWh) for months jan, feb, oct, nov and dec.

Hi Paul,

I ran a small model with only 6 modules of parabola (Luz LS-2) total length of 49m. And I set a nameplate capacity of 1 MWe in power block, but the results for monthly energy showed negative values for jan, feb, oct, nov and dec. Actually I am doing a performance analysis of receiver in which i am modelling a receiver of 49 m length in Engineering Equation Solver (EES) and comparing the total useful heat gain output of SAM with EES. Although my main concern is thermal output I'm just curious why monthly energy during those months.

I am attaching the SAM file with this question for better understanding.

Thanks,

Zishan

Dear Zishan,

The system you designed has a solar multiple of about 0.34. That means that at noon on the summer solstice (June 21 for this Northern hemisphere location), the field generates enough thermal energy to drive the power block at about one third of its rated capacity. During the winter months, it does not generate enough thermal energy for the power block to generate any electricity. The negative monthly electricity values represent the parasitic load of the system during those months when the system does not generate any power.

To avoid those negative values, you would need to use a larger solar field (or smaller power block).

Best regards,
Paul.

Paul,

I fixed the solar multiple issue. My next question is related to performance analysis of receiver using EES(Engineering Equation Solver). For my system with a nameplate capacity of 1 MW electricity generation, I would need 3 loop with 6 modules in each loop. I have calculated the useful solar heat gain for one loop. So shall I multiply the result with 3 to get system total useful heat gain? Will the results be in good comparison with what SAM will produce?

Thanks!

Zishan

Dear Zishan,
The thermal conditions of the HTF at the solar field inlet and outlet are affected by the header and runner piping in addition to the collectors in the loop. Please refer to the physical trough reference manual (Wagner 2011) for more details. Specifically, see the general description of the model's energy equations in Section 2.1.1. The manual is available on the Performance Model Documentation page:
sam.nrel.gov/performance
Best regards,

Paul.