Posible error in SuperHeating Heat Loss model for SAM Fresnel Example

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luiscoco
Posible error in SuperHeating Heat Loss model for SAM Fresnel Example

Dear Sirs,

I don´t understand why in Linear Fresnel Novatec Solar Boiler example for SAM, Heat Losses in Superheating sections are 52.08 W/m, much lower than heat losses in boiling section 541.7 W/m. In Novatec brochure attached, both heat losses are similar.

Please confirm me this fact.

Best Regards LUIS COCO

Paul Gilman

Dear Luis,

There are two separate issues here. The first is that the SAM 2013.1.15 user interface does not correctly account for the quartic term when evaluating the heat loss polynomial. SAM displayed an incorrect result for the heat loss "calculated value" that appears in blue in the user interface. Impacts of this error include (1) incorrect thermal loss value in the user interface if the quartic term is used and (2) error in the solar field sizing calculations (since heat loss is included in the mirror aperture area sizing calculations). Note that the quartic term is correctly accounted for in all of the performance calculations, so the simulation results are still correct. We will correct the error in the interface in future releases.

(As a side note, it’s important to understand the difference between SAM user interface calculations and the annual performance calculations. The interface calculations are simplistic and used to size the plant and provide initial information to the user while the annual performance calculations are detailed and are much more accurate than the interface.)

The second issue is our method for calculating the average thermal loss from the receivers in user interface calculations. In the interface, we estimate the average thermal loss from the solar field using the unweighted "average" solar field temperature difference: [(T_out + T_in)/2 – Tamb]. This neglects the phase change that occurs in boiling systems, so the boiler section appears to have a much higher thermal loss than it actually does in the performance calculations. Note again that the average thermal loss is only used to size the solar field, and the performance calculations more accurately model the thermal loss from each receiver. We are considering an alternative method for the sizing calculations that more accurately estimates the average heat loss from the boiler and superheater, but for now, understand that these estimates are only accurate to the first order. If you are concerned about the field sizing issue, you can modify the solar multiple to increase or decrease the aperture area as you’d like.

You also emailed a question about a discrepancy in thermal losses:

c) There is a discrepancy of 27% to 30% between the Thermal Losses in Polynomial Heat Losses Model Coefficients considers in SAM example in comparison with the information included in Novatec Fresnel brochure attached #2

The coefficients in the SAM sample file were provided to us directly by Novatec based on their most up-to-date information. To our knowledge, this information supersedes prior published data. If you have questions about the sample file values, please contact Novatec.

Best regards,
Paul.

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