1-axis and 2-axis solar trackers

Abstract

A one-axis sun position tracking device with its rotation axis parallel to the rotation axis of the Earth, rotates perpetually at a constant speed in the opposite direction of the Earth's rotation. This device comprises a shaft that is aligned to the Earth's polar axis, one or more crossbars are rigidly attached to and perpendicular to the shaft, solar energy collectors are mounted on the crossbar and could rotate around the crossbar that defines declination angle. A self-latched declination angle adjustment mechanism keeps the declination angle constant at most of time. A drive mechanism keeps this solar tracker to rotate perpetually. An automatic and abrupt declination angle change will keep the declination angle updated to correct value each day. A similarly configured two-axis tracker that continuously updates its declination angle by a mechanism derived from a differential coaxial rotation. Two independent driving mechanisms control the speed and/or duration of the two coaxial rotations, and are programmed to eliminate all tracking errors from various sources.

Description

FEDERALLY SPONSORED RESEARCH

Not applicable

SEQUENCE LISTING

None

BACKGROUND

1. Field

This invention relates to solar energy collections, specifically to sun position tracking that is used in sunlight concentration and collection.

2. Prior Art

Sun position tracking is very important to solar energy collection, especially for solar concentrators. Different implementations have been invented, they could be categorized as 1-axis tracking and 2-axis tracking. 1-axis tracking is simple, however, it is commonly believed that 1-axis tracking has a poor tracking capability, that is not true in a special case; 2-axis tracking can have a very good tracking accuracy, however, it is usually very complicated because of the coupled rotation of 2-axis.

Most 2-axis tracking methods are using local coordinate system, or called horizontal mount. It offers convenience of low profile of installation, however, the angular motion control is very complicated. The first rotation axis is typically perpendicular to the local horizontal plane, a platform is built rotating around this axis, which sets the azimuth of the solar tracker; and the second rotation axis is built on this platform and determines the elevation of the solar tracker. Rotation around both axes are nonlinear motions, usually servo motors with input from sun position sensor are used to drive the tracker.

2-axis tracking can also be done in polar coordinate system, or called equatorial mount. It offers convenience of simple tracking. The first rotation axis is typically parallel to the rotation axis of the Earth, a platform is built rotating around this axis; and the second rotation axis is built on this platform and follows the seasonal change of the declination of the Sun. Rotation of the first axis is very close to constant speed of one turn per day, usually a clock motor is good enough to drive this axis; rotation of the second axis is sinusoidal and very slow, oscillates once per year, occasional manual adjustment or some kind of automatic rotation were proposed.

In U.S. Pat. No. 4,202,321, Volna described a hybrid solar tracking device, which he used both local and polar coordinate systems. He used azimuth (axis 13) and elevation (axis 16) angles in a local coordinate system to define the orientation of his solar energy collector 29, however, he used polar axis 23 and declination “point axis 27” to describe his drive mechanism in a polar coordinate system, and used spindle 26 to connect both as “A coordinate transformation apparatus” (claim 8). In this way, he avoided the complex rotation control in the local coordinate system. All four axes: azimuth axis 13, elevation axis 16, polar axis 23, and declination “point axis 27” need to intersect at the same point so that he could use it as an axis converter. In the polar coordinate system, he proposed a generic concept of manual or automatic adjustment of the declination angle by sliding the “spindle 26”, hence the “pointing axis 27”, back and forth by ±23½°. In column 4, line 49, “Alternatively, in a more sophisticated and automatic embodiment of the invention, appropriate drive mechanisms may be connected to automatically slide bearing blocking 25 over datum surface 24 in timed relationship with the days of the year.” However, he did not give any specific method to implement such automatic adjustment of the declination angle. Also, Volna's implementation is an axis converter, not a polar tracking device, it has mechanical limitation of rotations that the device he proposed could not rotate over 360°, or rotate perpetually since the circular sector arm 20 collides with the pedestal 11 at certain angle. It has to rotate back and forth each day, which defeats one of the major advantages of polar tracking.

In U.S. Pat. No. 4,402,582, Rhodes described a polar tracking device and a method to automatically adjust the declination angle. However, the continuous declination angle adjustment mechanism is only approximately sinusoidal, there is no way mentioned to correct such error. In addition, the gear ratio is set to be 365:1, which will lead to accumulation of error since a year is not exact 365 days, and it is not easy to implement a more accurate gear ratio into such parasitic driven apparatus. Also, the parasitically powered adjustment only works when it rotates continuously, it cannot have the option of rotating back and forth.

In U.S. Pat. No. 4,368,962, Hultberg described a polar tracking device and a more sophisticate method to adjust the declination angle, and he further implemented error correction mechanism to correct various errors which compensates the errors from the imperfect drive train, eccentricity of the earth's orbit around the sun, etc. It is a very complicate implementation with too many gears, and multi-section coaxial rotations. The mechanical link “space crank” requires that four axes to intersect at one point. All make it difficult to practice.

3. Objects and Advantages

To overcome the limitation of solar tracker in these prior arts, and to simplify the implementation method, first, a 1-axis solar tracker in polar orientation with a latched declination angle is invented, and this declination angle is only updated abruptly periodically and automatically, it offers the simplicity of implementation and reasonably good tracking accuracy. Second, a solar tracker with slight different implementation that incorporates differential coaxial rotation is invented, it is particularly applicable to polar tracking, and could be further generalized to any 2-axis solar tracking configuration.

SUMMARY

As known in the prior arts, polar tracking has advantage of simple driving and control requirement, while the rotation around the polar axis is almost at steady speed: 360 degrees per day, the declination angle change is very slow, only changes ±23½° back and forth per year. It would be a good approximation that the declination angle could be kept constant for a day. As a matter of fact, the declination angle change rate varies, its fastest change rate is less than ±0.1° for ±6 hour, while the sun itself has a ±0.267° angular size. So if we could set the declination angle at correct value, keep it fixed through the day while we track the sun using only one axis polar tracking, we still have good enough tracking accuracy. Of course we have to set the next correct declination angle for the next day, a manual adjustment is tedious, a continuous adjustment is an overkill as others already proposed in the prior arts and is unnecessary. An automatic, non-continuous, and abrupt adjustment of the declination angle of the polar tracker is an object of the present invention.

The proposed apparatus has a rotatable shaft orientated substantially parallel to the rotation axis of the earth. One or more crossbars are rigidly and perpendicularly attached to this shaft, solar energy collectors are mounted on these crossbars and could rotate around these crossbars to define different declination angle. Those solar energy collectors are connected to a set of gears by a mechanical link, and different position of the gears determines the declination angle of the solar energy collectors. One gear in the gear chain is usually latched, for example, by a spring load ball against a notch, which causes the whole gear chain and the declination angle to be kept at a fixed position. When correctly forced at the correct time, the latched gear will be unlatched, turned to the next correct position, and reapply the latch. In this example, the spring will be compressed, the ball yields its way of blocking the notch so that the gear could rotate until the ball falls into the next notch; at that time, the external force is removed and the spring loaded ball latches the gear at the new position, which defines the next declination angle for the solar tracker.

This 1-axis polar tracking with automatic and non-continuous declination angle update should provide enough accurate sun position tracking with simple implementation requirement. In order to further improve sun position tracking, this non-continuous declination angle adjustment is not enough. This basic tracking apparatus can be modified to implement more error correction mechanism, a coaxial differential rotation method could achieve that goal, with both rotations near constant speeds. While the main shaft with solar energy collector(s) rotates at the constant speed of one turn a day, the same as mentioned above, a gear that rotates coaxially either faster or slower than the main shaft, this relative motion, goes though mechanical link, can continuously turn the declination angle. Both rotation speeds and durations can be controlled by simple counters, that small adjustments of rotation speeds and/or durations could easily provide error correction for earth's eccentric orbit around the sun and for all other causes, both yearly and daily.

DRAWINGS

Figures

FIG. 1 is a perspective view of a 1-axis solar tracker,

FIG. 2A is a zoom in view of the gear with self-latch mechanism,

FIG. 2B shows a cross-section view of the self-latch gear, when it is in latched position,

FIG. 2C shows a cross-section view of the self-latch gear, when the latch starts to yield under force,

FIG. 2D shows a cross-section view of the self-latch gear, when it is in un-latched position,

FIG. 2E shows a cross-section view of the self-latch gear, when it is latched in a new position,

FIG. 3A shows a perspective view of the worm gear starts to engage with the “open worm tooth”,

FIG. 3B shows a bottom view at start of the engagement of the worm gear with the open worm tooth,

FIG. 3C shows a bottom view at the finish of the engagement of the worm gear with the open worm tooth,

FIG. 4 is a perspective view of a 2-axis solar tracker,

REFERENCE NUMERALS

• • 10 rotatable shaft,
  • 11 driving motor,
  • 12 lower support,
  • 14 upper support,
  • 15 polar axis,
  • 20, 22 crossbars,
  • 30 solar energy collector,
  • 35 plate,
  • 37 universal joint,
  • 40 rod,
  • 50, 70 worm gears,
  • 51, 65 gears,
  • 52, 62 beams,
  • 60 worm,
  • 61 motor,
  • 63 slot,
  • 64 spring,
  • 66 ball,
  • 71, 72, 73 inner teeth of a worm gear,
  • 80 open worm tooth,

DETAILED DESCRIPTION

A 1-axis polar solar tracker is shown in FIG. 1, a rotatable shaft 10 is installed between a lower mount 12 and an upper support 14. Both 12 and 14 are fixed on the ground, and are installed in such a way that the shaft 10 can rotate along axis 15, which is essentially parallel to the celestial rotation axis of the Earth. Crossbars 20 and 22 are rigidly attached to the rotatable shaft 10, and the crossbars 20 and 22 are perpendicular to the rotatable shaft 10. There are more support beams 52 and 62 rigidly attached to the rotatable shaft 10. Solar energy collector 30 is mounted on the crossbar 20 and it could rotate around the crossbar 20 for at least ±23½°, which defines the declination angle. If we establish xyz coordinates here, center line of the shaft 10 as y-axis, center line of the crossbar 20 as x-axis, then we clearly know the z-axis is perpendicular to both x and y. the angle between the normal of the solar energy collector surface and the z-axis is the declination angle. More solar energy collectors could be similarly mounted on crossbar 22 and are omitted for illustration simplicity. One side of the solar energy collector 30 is attached to plate 35, which is pivot connected to one end of a rod 40 by a universal joint 37, the other end of the rod 40 is pivot connected to a worm gear 50, which is mounted on the beam 52. As the worm gear 50 turns, when the rod 40 goes down to the lowest point corresponds to 23.5° declination angle of the solar energy collector 30, and when the rod 40 goes up to the highest point corresponds to −23.5° declination angle of the solar energy collector 30. In a very good approximation, a steady rotation of the worm gear 50 translates into a sinusoidal oscillation of the declination angle of the solar energy collector 30 through the mechanic link 40. The worm gear 50 is driven by a worm 60, which is connected to another worm gear 70. The worm gear 70 is mounted on beam 62, and is usually latched to the beam 62 as explained in the next paragraph. Everything mentioned above forms a temporary rigid body on shaft 10, and is driven by motor 11 to rotate at a constant speed along polar axis 15 perpetually. There is an “open worm tooth” 80 which is fixedly mounted on the ground, and is shown not touch any other part.

FIGS. 2A, 2B, 2C, 2D, and 2E illustrate a generic self-latch mechanism of the gear 70 on the beam 62. FIG. 2A is a zoom-in 3D view of the tracker near the gear 70, a latching ball 66 is visible. FIG. 2B shows a cross-section view of the gear 70 on the beam 62. The gear 70 has the same number of outer teeth as that of inner teeth. 20 teeth on the gear 70 are shown for illustration purpose only, exact number of teeth varies by design. A spring 64 is housed inside a slot 63 which is positioned in radial direction of the beam 62, this spring 64 pushes a ball 66 outward, against the notch between two adjacent inner teeth 71 and 72 of the gear 70, prevents the gear 70 from rotating around the beam 62 freely. The strength of the spring 64 and the inner tooth slope determine the workload of this latch. When enough tangential force is applied to gear 70, the inner tooth 72 will push the ball back to the slot 63, as shown in FIG. 2C. If the gear 70 continue to rotate, as shown in FIG. 2D, the spring loaded ball 66 has no latch function to the gear 70 until one tooth interval has been rotated, as shown in FIG. 2E, at this time, the ball 66 will be pushed out by the spring 64 again, positioned between the inner teeth 72 and 73 of the gear 70. If the tangential force is removed, the gear 70 is now latched at a new position.

As seen in FIG. 1, this latched gear 70 is connected to worm 60, which in turn drives the gear 50. The position of gear 50 determines the declination angle of the solar energy collector 30 through the link rod 40, so the declination angle is latched to a particular value. The whole assembly is rotating together on the shaft 10 along the polar axis 15, one turn per day drive by a single motor 11, as a 1-axis solar tracker. However, at a pre-determined time of the day, the worm gear 70 started to be in contact with the “open worm tooth” 80 which is fixed on the ground. A zoom-in 3D perspective view is shown in FIG. 3A.

The “open worm tooth” 80 is a spiral shaped tooth, its cross section matches that of the worm gear 70 as seen in FIG. 3A. This “open worm tooth” 80 sits roughly in a plane that is perpendicular to the polar axis 15. In this plane, as shown in FIGS. 3B and 3C, the distance between one end of the “open worm tooth” 80 to the center of the shaft 10 is different from the distance between the other end of the “open worm tooth” 80 to the center of the shaft 10, the difference is one tooth pitch of worm gear 70. When the 1-axis solar tracker rotates, the worm gear 70 engages with the “open worm tooth” 80, which forces worm gear 70 to turn one notch during the engagement. Once the worm gear 70 leaves the “open worm tooth” 80, the worm gear 70 is latched at a new position, one tooth advance from the previous position.

We could choose the appropriate gear ratios so that a close approximation of 365.242199 turns of the shaft 10 will result in one turn of the gear 50. In theory one stage of worm gear is enough. A 2-stage worm gear is illustrated in FIG. 1 to provide a stronger latch of the declination angle enhanced by worm 60 to worm gear 50, and this latch strength enhancement is unidirectional which is a bonus feature; and to provide more accuracy and design flexibility of gear ratio choices. More stages of gears could be used for the same purpose; if a ratchet is incorporated, the shaft 10 could rotate back and forth while the declination angle is still updated as if the shaft 10 rotates perpetually. This is a one axis solar tracker in polar mount with its declination angle fixed during each day, and abruptly updates its declination angle daily and automatically.

To further enhance the tracking accuracy, a two-axis tracking method is needed, that means the tracker has to adjust its declination angle continuously. As seen in FIG. 4, a rotatable shaft 10 is installed between a lower mount 12 and an upper support 14. Both 12 and 14 are fixed on the ground, and are installed in such a way that the shaft 10 can rotate along axis 15, which is essentially parallel to the celestial rotation axis of the Earth. Crossbars 20 and 22 are rigidly attached to the rotatable shaft 10, and the crossbars 20 and 22 are perpendicular to the rotatable shaft 10. There is a support beam 52 rigidly attached to the crossbar 10. Solar energy collector 30 is mounted on the crossbar 20 and it could rotate around the crossbar 20 for at least ±23½°, which defines the declination angle. If we establish xyz coordinates here, center line of the shaft 10 as y-axis, center line of the crossbar 20 as x-axis, then we clearly know the z-axis is perpendicular to both x and y. the angle between the normal of the solar energy collector surface and the z-axis is the declination angle. More solar energy collectors could be similarly mounted on crossbar 22 and are omitted for illustration simplicity. One side of the solar energy collector 30 is attached to a plate 35, which is pivot connected to one end of a rod 40 by a universal joint 37, the other end of the rod 40 is pivot connected to a gear 51, which is mounted on the beam 52. As the gear 51 turns, when the rod 40 goes down to the lowest point corresponds to 23.5° declination angle of the solar energy collector 30, and when the rod 40 goes up to the highest point corresponds to −23.5° declination angle of the solar energy collector 30. The gear 51 is driven by another gear 65, which is rotating coaxially with the shaft 10. Motor 11 drive the shaft 10 at a speed of one turn a day; motor 61 drives the gear 65 at a different speed. The differential rotation speed between the shaft 10 and the gear 65, combines with the gear ratio of gears 65 and 51, turns the gear 51 to rotate one turn per year. For example, if the gear 51 has 20 teeth and the gear 65 has 50 teeth, then the gear 65 turns at speed of (365.242199±1)/365.242199*20/50=1.00109516 or 0.9989048 turns per day.

With the help of two independent rotations of the shaft 10 and the gear 65, error correction for various causes could be done. The earth orbit around the Sun is not a perfect circle, so that the declination angle change is not a strict sinusoidal function of time, neither is every day exact 24 hours. The simple mechanical link 40 between the gear 51 and the solar energy collector 30 (via the plate 35 and the universal joint 37) will not translate the circular motion of the gear 51 into an exact sinusoidal angular motion of the declination angle. All small angular errors for polar axis rotation and for declination angle change, from above mentioned causes and other causes not yet elaborated, can be easily corrected by small adjustment of rotation speed and duration of the shaft 10 and the gear 65, which are in turn driven by motors 11 and 61. A preferred embodiment is to let both motors 11 and 61 operated at constant speeds at most time of a day, say 23 hours, speed up or down a little bit at the remaining 1 hour to compensate any angular errors for that particular day. Those small errors are well known and could be tabulate into control programs.

If this 2-axis solar tracker is mounted in such a way that the shaft 10 is not parallel to the celestial rotation axis of the Earth, polar tracking assumption is no longer valid. However, with the help of two independent rotations of the shaft 10 and the gear 65, accurate sun position tracking still can be achieved when it is operated as a generic 2-axis solar tracker. This is particularly useful when the orientation of the shaft 10 only slightly deviates from the polar axis. In this case, the shaft 10 should rotate close to constant speed, and the “declination angle” of the tracker should change a small amount during the day. All these small variance are well known and can be tabulate into control programs. This optional operation mode may have new applications for this 2-axis tracker.

Operation

For the 1-axis solar tracker that is shown in FIG. 1, once the initial declination angle and polar angle are set correctly, motor 11 starts to drive the shaft 11 at constant speed at one turn per day, in the opposite direction of the Earth's rotation. If the gear ratio is set close enough to 365.242199, perpetual rotation of shaft 10 will provide very good solar tracking. In practice, if the gear ratio is set to 365, then only one manual declination angle adjustment every four years is needed, which is simply to manually turn the self latched gear 70 by one notch once every 4 years; similarly, if the gear ratio is set to 366 or 364, then 3 or 5 manual adjustments every four years are needed, so on and so forth. To correct the error due to non-uniform day length, that is because each day is not exact 24 hour, this driving motor 11 may be programmed to rotate a little faster or slower from day to day to counter such variance, or only to rotate a little faster or slower during a portion of night time is enough to compensate the day length variance problem.

For the 2-axis solar tracker that is shown in FIG. 4, once the initial declination angle and polar angle are set correctly, motor 11 starts to drive the shaft 10 at constant speed at one turn per day, and motor 61 starts to drive the gear 65 at constant speed slightly faster or slower than one turn per day. In the previously mentioned example, with gear ratio of 50 to 20, gear 65 rotates at constant speed of 1.00109516 or 0.9989048 turns per day. Perpetual rotation of shaft 10 and gear 65 will provide very good solar tracking; rotate back and forth will also do the job. To correct the error due to non-uniform day length, that is because each day is not exact 24 hour, the driving motor 11 may be programmed to rotate a little faster or slower from day to day to counter such variance, or rotates a little faster or slower during a portion of night time is enough to compensate the day length problem. To correct non-exact sinusoidal declination angle change, the driving motor 61 may be programmed to rotate a little faster or slower from day to day to counter such variance. The error correction program can be further extended to counter the non-uniform rotations when the shaft 10 is mounted not exactly parallel to the celestial rotation axis of the Earth, the variance is well known.

Claims

1. A 1-axis solar tracking device consists: a rotating shaft with one or multiple crossbars perpendicularly attached to it,this shaft is mounted along the celestial rotation axis of the Earth, rotates continuously and perpetually at constant speed of one turn per day, in the opposite direction of the Earth's rotationsolar energy collectors mounted on the crossbar and can rotate around this crossbar for at least ±23½°, this rotation defines declination angle,a self-latch mechanism keeps the declination angle constant most of the day,an automatic and non-continuous declination angle update mechanism.

2. The 1-axis solar tracking device of claim 1, the mechanism that determines the declination angle of the solar energy collectors consists: a gear, which is mounted on a beam that is rigidly attached to the main rotating shaft,a rod with one end pivot connected to this gear and the other end pivot connected the solar energy collector,a steady rotation of the gear translates into an approximate sinusoidal oscillation of the declination angle of the solar energy collector through the mechanic linking rod,the amplitude of the oscillation of the declination angle is 23½°.

3. The 1-axis solar tracking device of claim 1, the self-latch mechanism that keeps the declination angle constant consists: a spring loaded ball pushes against the notch between two adjacent teeth of a gear, prevents the gear from rotating freely,the strength of the spring and the gear tooth slope determine the workload of this latch,an additional worm gear stage provides a unidirectional and stronger latch of the declination angle,

4. The 1-axis solar tracking device of claim 1, the automatic and non-continuous declination angle update mechanism consists: an open worm tooth which is spiral shaped, its cross section matches that of the worm gear,the open worm tooth which is fixedly mounted on the ground, sits roughly in a plane that is perpendicular to the main rotating shaft,the distance between one end of the open worm tooth to the center of the main rotating shaft is different from the distance between the other end of the open worm tooth to the center of the main rotating shaft, the difference is one tooth pitch of the worm gear,during most time of a day, the open worm tooth does not engage with anything; at a pre-determined time of the day, the open worm tooth engages with the worm gear, and forces the worm gear to turn one notch during the engagement.

5. The 1-axis solar tracking device of claim 1, once the initial declination angle is set correctly, and the gear ratio is set close enough to 365.242199, perpetual rotation of the main rotating shaft will provide a very good solar tracking. However, if the gear ratio is set slightly different from the ideal number 365.242199, periodical manual adjustments is needed to reset the accumulated error in declination angle. For example, if the gear ratio is set to 365, then only one manual declination angle adjustment every four years is needed, which is simply to manually turn the self latched gear by one notch once every four years; similarly, if the gear ratio is set to 366 or 364, then 3 or 5 manual adjustments every four years are needed, so on and so forth.

6. The 1-axis solar tracking device of claim 1, to correct the error due to non-uniform day length, that is because each day is not exact 24 hour, the main rotating shaft may be programmed to rotate a little faster or slower from day to day to counter such variance, or only to rotate a little faster or slower during a portion of night time is enough to compensate the day length variance problem.

7. A variance of the 1-axis solar tracking device of claim 1, if a ratchet is incorporated in the declination angle adjustment mechanism, the main rotating shaft could have an option to rotate back and forth while the declination angle is still updated as if the main rotating shaft was rotate perpetually, with all the benefit of its declination angle fixed during each day, and abruptly updates its declination angle daily and automatically.

8. A 2-axis solar tracking device consists: a rotating shaft with one or multiple crossbars perpendicular attached to it,this shaft is mounted essentially along the celestial rotation axis of the Earth, rotates continuously and perpetually at constant speed of one turn per day, in the opposite direction of the Earth's rotation,solar energy collectors mounted on the crossbar and can rotate around this cross bar for at least ±23½°, this rotation defines declination angle,a co-axial rotation mechanism, though mechanical link, changes the declination angle continuously,an error correction program controls both rotations.

9. The 2-axis solar tracking device of claim 8, the mechanism that determines the declination angle of the solar energy collectors consists: a gear, which is mounted on a beam that is rigidly attached to the main rotating shaft,a rod with one end pivot connected to this gear and the other end pivot connected the solar energy collector,a steady rotation of the gear translates into an approximate sinusoidal oscillation of the declination angle of the solar energy collector through the mechanic linking rod,the amplitude of the oscillation of the declination angle is 23½°.

10. The 2-axis solar tracking device of claim 8, the mechanism that continuously changes the declination angle consists: a gear, which rotates coaxially with the main rotating shaft, engages with the other gear whose rotation causes the oscillation of the declination angle,two independent driving mechanisms that drive this gear and the main rotating shaft,the differential rotation speed between this gear and the main rotating shaft, combines with the gear ratio in the drive train, continuously changes the declination angle.

11. The 2-axis solar tracking device of claim 8, the error correction program controls both rotations can slightly adjust both rotation speed and/or duration; that will compensate polar tracking errors from all sources. Those small errors are well known and could be tabulate into control programs.

12. A variance of the 2-axis solar tracking device of claim 8, the driving mechanism have an option to rotate the main rotating shaft back and forth with all the benefit of accurate polar tracking during the daylight time.

13. A variance of the 2-axis solar tracking device of claim 8, the error correction program can be extended to counter tracking error when this 2-axis solar tracker is not mounted in polar orientation.