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Eppley Solar Tracker

Eppley ST-1 Eppley Laboratory is the main manufacturer of manually adjusted Solar Trackers used with the Eppley NIP or Cavity Radiometer. The Model ST-1 (see photograph) is electrically driven using a clock-based motor which makes one revolution every 24 hours. These trackers have to be adjusted manually to account for changes in the declination of the sun and the changes in true solar time (see equation of time).
These trackers require sunny weather to accurately align the NIP. During the equinoxes (March and September), the declination is changing rapidly, and alignment should be done at least three times a week. Because true solar time changes most rapidly during December, it is important to watch the alignment during the winter.

Useful alignment tricks

One can make a tube that fits between the flanges of the NIP between the alignment target and the sunlight hole in the top flange. By cutting a window in the tube to see the target, one can more easily align the NIP during periods of intermittent cloudiness.
Since the tracker makes a complete revolution once every 24 hours, the cord between the NIP and the data logger either has to go through a rotary connector or be disconnected every few days to prevent the wires from separating from the connector. By using a coiled phone cord, the stress on the cable can be reduced, and the maximum period during which the connection can be maintained without separating the cable from the connector is approximately doubled.
With a well-aligned tracker, one can watch the direction the sun dot moves on the target from day to day. During cloudy periods, one can slightly over-adjust the light spot so that the next day the sun spot moves closer to the target instead of away. This can extend the period when the NIP is in alignment by a day or two. This is particularly useful during cloudy months.

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© 2022, UO Solar Radiation Monitoring Laboratory.
Last revised: March 29, 2022.

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