trough solar collectors

[David Williams]

Okay. Here's an argument that I think proves that the east-west
"trough" collector cannot focus sunlight accurately onto its tube
throughout the day, except at the equinoxes.

The trough essentially defines a plane in which the sun must be in
order for its light to be properly focussed. The plane includes the
focal line, in which the collector tube is located, and projects into
the sky in the direction in which the parabolic trough is aimed. If the
aim of the trough is not exactly horizontal - which would never happen
in practice at temperate latitudes - the plane intersects the horizon
exactly in the east and west directions. Therefore, in order for
sunlight to be properly focussed when the sun is close to the horizon,
at sunrise and sunset, the sun must be due east or west.

The sun does rise and set in the east and west at the equinoxes, but
*not* at other times of the year. In June, it rises and sets well to
the north of east and west, and in December it rises and sets to the
south of due east and west. (This is true for observers in both
hemispheres.) Therefore, at times well removed from noon, and at dates
that are not the equinoxes, the trough *cannot* properly focus sunlight
onto its collector tube.

Which is exactly what my calculations showed.

dow

 

[no spam]

The sun does rise and set in the east and west at the equinoxes, but

*not* at other times of the year. In June, it rises and sets well to
the north of east and west, and in December it rises and sets to the
south of due east and west. (This is true for observers in both
hemispheres.) Therefore, at times well removed from noon, and at dates
that are not the equinoxes, the trough *cannot* properly focus sunlight
onto its collector tube.

In theory it may not "properly focus sunlight" but it seems that it works
fairly well in practice. A quick search of the internet will give you
several hits on people who have made just such collectors and are using
them.