Field Optical Focus Fraction

Hello,

I'm doing my dissertation about a CSP Power Tower plant (100 MWe, 8 hours TES), but to characterize losses produced by defocusing, I need to know what is "Field Optical Focus Fraction". Is the defocused area fraction?

Secondly, I have a question: the field optical efficiency is affected by "field optical focus fraction"?

How can I know the losses produced by defocusing?

Best regards,

Manuel.

Dear Manuel,
The "field optical focus fraction" is the fraction of heliostats in the field that are focused. A value of 1 means that 100% of the heliostats are focused. A value of 0.6 means that 60% of the field is focused.
Yes, the "field optical efficiency" includes the effect of defocused heliostats. You can see that clearly in the graph below, which shows the optical efficiency over a period of days when the field defocusses on some afternoons.
SAM does not report defocusing losses as a separate value, but you should be able to analyze the results to calculate an estimate.

Best regards,

Paul.

Dear Paul,

Thanks for your prompt response.
I have one more question. Is related by thermic balance in the plant. In order of appearance, we have:

DNI*effective_Area_heliostats

- Optical Loss
- Defocusing Loss (calculated by "field optical focus fraction")
- Heliostat Availability Loss

= Receiver Thermal Power Absorbed

- Receiver Thermal Power Losses to convection and radiation
- TES Thermal Losses from tanks

= Cycle thermal power input
.
.
.

However, "DNI*effective_Area_heliostats - (Optical Losses + Defocusing Losses + Heliostat Availability Losses)" is not equal to "Receiver Thermal Power Absorbed"

Receiver Thermal Power Absorbed must be equal to "DNI*effective_Area_heliostats - (Optical Losses + Defocusing Losses + Heliostat Availability Losses)".

Is there another loss that SAM is not reporting?


PS: I have calculated "Defocusing Loss":
(1 - field optical focus fraction)*DNI*effective_Area_heliostats*heliostat availability


Best regards,

Manuel.

Dear Paul,

Do you have an explanation?
Thanks.

Best regards,

Manuel.

Dear Manuel,
Please see these posts, and let me know if you still have any questions:

sam.nrel.gov/content/calculating-system-efficiency-trough-and-tower-systems

 

sam.nrel.gov/node/69259
Best regards,

Paul.

I'm having a huge issue with my model. The output Electricity spikes and whenever it does that there is a lowered optical fraction (lower than 1). Does anyone know what reason there could be for this?