Why does the heliostat field layout not directly face the south direction

I have noticed that some of the heliostat field layouts generated through SAM are not directely facing the South direction. This can be seen in the attached figure and from the layout reported in previous entries.

I have not studied the Power Tower for long. In my opinion, many heliostat field layouts are South facing field. Would you please tell me the reason that not arrage the heliostat field directely facing the South direction?

Thank you very much!

Best Regards!

Harry Han

Dear Harry,

For a system in the northern hemisphere, the majority of heliostat mirrors are likely to be north of the tower and facing south so that they can reflect the summer solar radiation toward the top of the tower.

Best regards,
Paul.

Thank you Paul. I really appreciate of your response. Would you please tell me why the heliostats in the western field is more than those in the easten field?

Dear Harry,

That may be caused by data in the weather file. For example, if the solar irradiance data shows the effect of cloudy afternoons in the summer with clouds accumulating on the western horizon, the field layout routine might place more heliostats to the west of the tower (and pointing east) to maximize incident irradiance.

You can see that with the default power tower case if you generate a heliostat layout using the Boulder CO and Los Angeles CA weather files. Boulder has summer afternoon thunderstorms that form in the mountains to the west of the city, so there are more heliostats on the eastern side of the tower. Los Angeles has summer morning fog to the east of the city, so there are more heliostats on the western side of the tower. The difference is subtle, but visible. Neither Boulder nor Los Angeles are likely to be viable locations for a power tower system, but I chose those locations to illustrate the effect of weather data on the layout.

Best regards,
Paul.

Thank you Paul. This is really helpful! Are there any papers I can turn to to help my understanding of the mechanism that the SAM using to calculate the heliostat field layouts.

Best regards

Harry Han

Dear Harry,

SAM uses a model called SolarPILOT for the heliostat field layout and tower and receiver optimization. There is a basic description of the model in SAM's Help system. You can find descriptions in the table of contents under Power Tower, Heliostat Field Design, and more under Molten Salt Tower, Heliostat Field.

The CSP team is working on a published description of SolarPILOT, but it is not available yet.

Best regards,
Paul.