Heliostat - Tracking Alternatives

Tracking Alternatives

The movement of most modern heliostats employs a two-axis motorized system, controlled by computer as outlined at the start of this article. Almost always, the primary rotation axis is vertical and the secondary horizontal, so the mirror is on an alt-azimuth mount.

One simple alternative is for the mirror to rotate around a polar aligned primary axis, driven by a mechanical, often clockwork, mechanism at 15 degrees per hour, compensating for the earth's rotation relative to the sun. The mirror is aligned to reflect sunlight along the same polar axis in the direction of one of the celestial poles. There is a perpendicular secondary axis allowing occasional manual adjustment of the mirror (daily or less often as necessary) to compensate for the shift in the sun's declination with the seasons. The setting of the drive clock can also be occasionally adjusted to compensate for changes in the Equation of Time. The target can be located on the same polar axis that is the mirror's primary rotation axis, or a second, stationary mirror can be used to reflect light from the polar axis toward the target, wherever that might be. This kind of mirror mount and drive is often used with solar cookers, such as Scheffler reflectors. For this application, the mirror can be concave, so as to concentrate sunlight onto the cooking vessel.

The alt-azimuth and polar-axis alignments are two of the three orientations for two-axis mounts that are, or have been, commonly used for heliostat mirrors. The third is the target-axis arrangement in which the primary axis points toward the target at which sunlight is to be reflected. The secondary axis is perpendicular to the primary one. Heliostats controlled by light-sensors have used this orientation. A small arm carries sensors that control motors that turn the arm around the two axes, so it points toward the sun. (Thus this design incorporates a solar tracker.) A simple mechanical arrangement bisects the angle between the primary axis, pointing to the target, and the arm, pointing to the sun. The mirror is mounted so its reflective surface is perpendicular to this bisector. This type of heliostat was used for daylighting prior to the availability of cheap computers, but after the initial availability of sensor control hardware.

There are heliostat designs which do not require the rotation axes to have any exact orientation. For example, there may be light-sensors close to the target which send signals to motors so that they correct the alignment of the mirror whenever the beam of reflected light drifts away from the target. The directions of the axes need be only approximately known, since the system is intrinsically self-correcting. However, there are disadvantages, such as that the mirror has to be manually realigned every morning and after any prolonged cloudy spell, since the reflected beam, when it reappears, misses the sensors, so the system cannot correct the orientation of the mirror. There are also geometrical problems which limit the functioning of the heliostat when the directions of the sun and the target, as seen from the mirror, are very different. Because of the disadvantages, this design has never been commonly used, but some people do experiment with it.

Many other types of heliostat have also occasionally been used. In the very earliest heliostats, for example, which were used for daylighting in ancient Egypt, servants or slaves kept the mirrors aligned manually, without using any kind of mechanism. (There are places in Egypt where this is done today, for the benefit of tourists.) Elaborate clockwork heliostats were made during the 19th Century which could reflect sunlight to a target in any direction using only a single mirror, minimizing light losses, and which automatically compensated for the sun's seasonal movements. Some of these devices are still to be seen in museums, but they are not used for practical purposes today. Amateurs sometimes come up with ad hoc designs which work approximately, in some particular location, without any theoretical justification. An essentially limitless number of such designs are possible.