Friday, March 11, 2011

Array orientation: N-S or E-W?

In your book, you mention N-S orientation vs E-W orientation.  Is it correct that with an E-W orientation, the tracking is not as critical as N-S and could be adjusted manually each morning to track the seasonal changes in the sun's angle?
Best regards,

I am not sure that I would agree that the tracking is less critical. I think that the main issue with orientation is the efficiency over an entire day.

Near solar noon it does not make much difference which way the array is oriented. The sun is highest in the sky at this time and it's rays fall very near perpendicular to the array in either orientation.

When it makes a difference is at higher latitudes (like mine) or if you have a wide unobstructed sky view and you want to get more out of the extremes of the day (morning and afternoon). In these cases, N-S orientation is better, more efficient.

Because home made collectors are not infinitely long (like the ones in the Mojave) we should expect a certain amount of "spill" at the ends if the array is not north-south. If you imagine an E-W array in the morning, the sunlight hitting the reflector at the end nearest the sun will focus some distance down the collector, not at the very end. The part of the collector that does not receive focused light is "wasted" during that time. At the other end of the collector, because of the sun low in the sky, the focused light at that far end does not hit the collector and is also "wasted".

Whether the controller and drive motor works less or not is to me a less important issue since I have adjusted the controller to a very low duty rate so it does not work very hard anyway, even though my array is mostly N-S.

In either case, the controller must accurately position the tilt of the array towards the sun.

I hope this makes sense?


1 comment:

Vector said...

A trough collector collects along the plane parallel to it's axis and the direction it's pointed.

But the sun traces a plane in the sky only on the equinox. On other days it effectively traces a cone.

This means that if the trough is oriented E-W and is perfectly pointed for solar noon, then in the morning and afternoon, it will not be correctly pointed. Intra-day tracking may still be required.

That said, my guess is that the focal length is short enough and the "field of view" is big enough that this effect will be minor compared to the spillage.