Economical Polar-Axis Solar Tracker for a Circular Reflective Dish
The most urgent need in the field of solar energy is to lower the final cost per watt of all involved components. Some of the most expensive components within a solar tracker go into the precision drive system which accurately follows the motion of the sun. This invention reduces the complexity and the required number of drive components in a tracker optimized for reflective dishes. With this invention a single drive motor can keep 20 or more large reflective dishes accurately tracking the sun, whereas 40 drive motors with more complex control systems would typically be required for the same number of dishes. In addition this invention allows for complete inversion of the dishes, which helps reduce dust accumulation on the optical surfaces and lowers wind resistance during storms.
This application is a continuation-in-part of Indian patent application No. 2010/DEL/2007 filed on Sep. 24, 2007 and entitled “An Economical Solar Tracker for a Concentrating Reflective Dish”.
FIELD OF THE INVENTIONThe invention relates to solar energy, and more specifically to methods for tracking the motion of the sun as it moves through the sky, enabling more efficient concentration and utilization of the energy coming from the sun.
BACKGROUND OF THE INVENTIONIn order to accurately track the motion of the sun through the sky, it's necessary to somehow replicate that motion. The majority of solar trackers deal with the sun's motion as if it were an arbitrary series of azimuth-elevation (az-el) coordinates. They generally depend on complex computer software to predict those coordinates, and/or a sophisticated, closed-loop, two-axis control system to move the tracker so that it accurately follows the motion of the sun. Such solar trackers are called az-el trackers, and two examples of them are illustrated in
One distinct disadvantage of az-el trackers is that difficulties arise when they are installed in tropical locations, that is, anywhere between the tropic of capricorn and the tropic of cancer; a huge belt around the planet from 23.5 degrees south to 23.5 degrees north. In this region, the sun can and will pass directly overhead, which means that the elevation at that moment will be 90 degrees and the azimuth will be briefly undefined. In this situation the best that an az-el tracker can do is a quick turn-around at noon, since there is a discontinuity of as much as 180 degrees between the data it was following as the sun rose in the east, in comparison to the data it must follow for the descent of the sun in the west. Simplistic tracking schemes could easily be confused to the point of no longer working when confronted with such an extreme discontinuity.
The problem is considerably simplified with a polar-axis tracker. Such a tracker works because in truth the motion of the sun is not at all arbitrary, rather it is very simple and predictable. The apparent motion of the sun is better understood as the earth's motion relative to the sun. The primary apparent motion of the sun is due to a constant rate rotation of the earth about its polar (rotational) axis. In order to accurately account for this motion with a tracker, a stable rotational axis must be created in accurate alignment with the earth's polar axis. Rotating the tracker's rotational axis at a rate which is equal but opposite to the rotational rate of the earth allows anything attached to that axis to stay at a fixed orientation relative to the sun. Solar trackers which take this approach are generally called polar-axis solar trackers, to better distinguish them from the previously mentioned az-el solar trackers.
Note also that in
Note that the elliptical Scheffler reflective dishes in the tracker of
The elliptical Scheffler reflective dishes of
It is an objective of this invention to bring together all of the best qualities of the Scheffler/Gadhia solar tracker, and improve on them where possible, for circular reflective dishes rather than for elliptic Scheffler reflective dishes.
It is an objective of this invention to have the weights of the solar tracker balance each other out, minimizing the torques required to maneuver the reflective dish.
It is an objective of this invention to have a single control motor drive multiple reflective dishes, thus reducing the cost of the system.
It is an objective of this invention to minimize the size of the solar tracker relative to the size of the reflective dish, thus collecting the most possible sunlight with a minimum tracker cost.
It is an objective of this invention for the solar tracker to accommodate the seasonal motion of the sun in a manner which can be either manual or automatic, to permit a further cost reduction (by omitting the automatic drive system) in cases where it is possible and appropriate.
It is an objective of this invention to keep the reflective dish as close as possible to the ground, so as to improve the system's ability to withstand wind storms.
It is an objective of this invention to enable the complete inversion of the reflective dish, since that will reduce the dust build-up on the reflective surfaces, and further improve the system's ability to withstand wind storms.
Table 1 shows the declination of the sun for each day of the year.
Table 2 shows the daily changes in the declination of the sun for each day of the year.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTIn
The fundamental details of the preferred embodiment are shown schematically in
Leveling bolts 402 at each of the four corners help to bring the support base accurately into level. To better withstand high winds, these bolts can be made much longer and anchored firmly in cement. Clamping assembly 403 firmly grasps tracker frame 406 and holds it in place, pushing it against two fixed stops which form the other half of the clamping assembly, and which also hold the two halves of the support base together. These fixed stops are visible in
Tracker frame 406 is a novel and unique aspect of this invention. Because it is in the shape of a 180 degree arc, clamping assemblies 403 (and optionally 405) can hold it firm at virtually any angle. This allows polar axle 407, supported by tracker frame 406, to be held at an angle such that it will be in parallel with the earth's polar axis.
For higher latitudes which will require the tracker frame to be further tilted, optional triangular supports 404 hold an additional clamping assembly 405 which provides additional stability to tracker frame 406.
Note that all other known polar-axis trackers require a base or support system which either pivots and lifts the dish high in the air, or else is custom-built according to the latitude of the installation site, which complicates keeping the parts in stock for those trackers. In contrast, tracker frame 406 keeps the dish quite low to the ground, reducing exposure to winds, while simultaneously allowing a single set of tracker parts to work well at almost any installation site on earth.
In order to account for and follow the primary motion of the sun, the polar axle must be turned at a rate of 1 revolution per day by the polar axle rotation means. In this embodiment, the polar axle rotation means includes a gear motor connected through pulleys to polar axle drive pulley 408, which in turn rotates the polar axle. Bearings 409 firmly hold the ends of the polar axle in place and minimize the rotational friction, thus minimizing the torques involved. The dish assembly consisting of reflective dish 412, dish support member 411 and solar energy receiver 413 are very nearly balanced about pivot rod 410 and hence polar axle 407, which also helps to minimize the torques involved in the polar axle rotation. Note that solar energy receiver 413 can take a number of forms, as there are several types of technologies for converting solar energy into other useful forms. If electricity is immediately desired, it could take the form of concentrating photovoltaic (CPV) cells. Or if there are price breakthroughs in heat engines such as Stirling Engines, that could be used to create electricity. Alternatively, the heat could be absorbed with some kind of thermal transfer fluid, and transported and/or stored for later use, or later conversion into electric energy.
Besides the apparent daily motion of the sun, there is also a seasonal motion. In astronomical terms, the declination of the sun describes the apparent north-south motion of the sun as seen from the earth. A declination angle of zero means that the sun is in alignment with the equator, which occurs at particular times on March 21 st and September 23rd. The declination of the sun peaks on about June 22nd at an angle of 23.43 degrees north of the equator, and reaches its minimum on about December 22nd at an angle of 23.43 degrees south of the equator.
Within this document, the term declination angle is used not only for the declination of the sun above or below the equator, but also for the angle formed by the dish assembly of this tracker, which mimics that celestial angle. In
Note that in this embodiment, all components of the dish assembly are constructed such that they cannot collide with tracker frame 406 at any angle of motion about polar axle 407, nor at any declination angle between plus or minus 23.43 degrees. Circular reflective dish 412 is also illustrated in a side view, 414, to better illustrate the split-dish construction, and the all-around clearance that is another result of having the tracker frame shaped as a circular arc. The split in the dish is required in order to allow room for the polar axle as the dish pivots for variations in the declination angle.
Before moving on to other figures, note that
In
Note that all of the other illustrations (excepting 5d and 5e) show an embodiment of the tracker configured for a latitude of 28.5°, corresponding to parts of India, China, Northern Africa, Mexico, Australia and many other locations. All of the configurations shown assume the same basic set of parts.
The second part of correcting that misalignment is having an easy method of making a fine-tuning adjustment, which is the purpose of the polar axle rotational fine-tuning means, one embodiment of which is shown in
In
Another fine-tuning adjustment is shown in
If an automated declination adjustment is desired, one embodiment of such an adjustment means is also shown in
In this illustration embodiments of three solar trackers are shown, each of which has been configured slightly differently as the system of trackers was installed. The leftmost tracker is configured to be at the end of a string of trackers, thus there's only need for one pulley assembly 702. The middle tracker is configured to be in the middle of the string, with two pulley assemblies 702. For a string of 12 trackers, there would be 10 middle trackers configured like this one. The tracker on the right, finally, is configured to be driven directly from the drive motor, and all the associated drive circuitry (including sensors indicating the position of the trackers relative to the sun's position) would be installed on this tracker. The cables would be connected together as shown, with tensioning springs between them, in order to get all of the trackers in a string to track the sun in unison.
The configuration of optional tensioning springs shown here has two purposes. First, in locations where the system of trackers will undergo large deviations in temperature, the steel cables will alternately undergo thermal expansion and contraction. In the configuration shown, the thermal length shifting in the steel cables is balanced by compensating shifts in the lengths of the tensioning springs which are distributed throughout the length of the cable. This serves to minimize the net angular shifting of any tracker, insuring maximum tracking accuracy in every tracker. Second, in locations with high winds, a sudden gust of wind could act on one dish, or a few dishes, or all of the dishes nearly simultaneously. With the configuration of tensioning springs shown, some of the energy of such gusts is harmlessly absorbed and dissipated by the springs, which would then quickly bring the dishes back to their intended orientations. This energy might otherwise be absorbed by the reflective dish, causing distortions or greater damage. Thus the springs can help to minimize wind damage to the system, as well as reducing the possibility that such gusts would disorient any of the dishes due to cable slippage.
The friction between the steel drive cable 703 and the pulleys it interfaces with (701 & 708) must be sufficiently high to prevent slippage, so the tensioning springs must be adequately stretched via turnbuckle 710 to insure this, and a material with a high coefficient of friction should be used to cover the pulley surfaces. Exorbitant tension is not needed, since the design shown includes large angles of working contact on the drive pulleys, and friction increases exponentially with the coefficient of friction between the two materials and the angle through which there is working contact, in radians.
In
While the tracker described herein is intended primarily for countries in which labor is inexpensive and materials are expensive, it can be readily adapted for other countries, with the simple addition of an automatic declination adjustment, as in
To better understand and weigh this trade-off, tables 1 and 2 are included. Table 1 shows the declination angle of the sun for every day of the year, based on the data published online at: www.wsanford.com/˜wsanford/exo/sundials/DEC_Sun.html. Note that positive numbers indicate that the sun is above the Northern hemisphere, while negative numbers indicate that the sun is above the Southern hemisphere. Table 2 shows how the solar declination angle changes on each day of the year; it is based the data in Table 1.
While one embodiment of this invention with several options has been presented above, many changes can be made without departing from the spirit and scope of the invention. For example, there is no need for the support base to be flat or level, rather it might make sense to incorporate elements of a support base into a new structure which is already under construction for different purposes, but which would be well served by having solar energy collectors mounted on it. Any suitable solar energy receiver may be utilized with this invention, including Stirling engines, concentrating photovoltaic cells, solar-thermal collectors, or others as may be introduced in the future. The shape and size of the dish support member would naturally change so as to better accommodate the needs of those solar energy receivers. The various insights embodied in this invention enable the production of solar trackers for circular reflective dishes at significantly reduced costs, while still attaining tracking accuracies within small fractions of a degree. The scope of this invention should be determined by the appended claims and their legal equivalents, rather than by the explanations or illustrations here presented.
Claims
1. A polar-axis solar tracker comprising:
- a) A strong and stable support base capable of aligning a tracker frame in a true North-South direction, and firmly holding it in that alignment.
- b) A tracker frame in the shape of a letter “C”; substantially like a 180 degree arc, which owing to its shape can be mounted to the support base at any angle, particularly such that the endpoints of the arc are inclined from the horizontal at an angle equal to the geocentric latitude of the installation site, so that said endpoints are in parallel alignment with the earth's polar (rotational) axis.
- c) A polar axle rotatably attached to said endpoints of said tracker frame, which is thereby positioned in parallel alignment with the polar axis of the earth, so that as the earth rotates the polar axle can be rotated in the opposite direction, and thus hold all things mounted on the polar axle in a substantially fixed orientation relative to the sun, despite the rotation of the earth.
- d) A dish support member pivotally attached to the center of the polar axle so that it can pivot through declination angles of plus or minus 23.43 degrees (or more) towards said endpoints of the tracker frame, so that a circular reflective dish mounted on said dish support member can follow the sun's seasonal motion above and below the earth's equator.
- e) A circular reflective dish supported on one end of said dish support member.
- f) A solar energy receiver supported at the other end of said dish support member, positioned substantially at the focal point of said circular reflective dish.
- g) A polar axle rotation means, capable of rotating the polar axle and all components attached to it at a rate of one revolution per day.
- h) A declination adjustment means, capable of adjusting the pivot angle of the dish support member and all other components attached to it, so as to follow the seasonal motion of the sun.
2. A polar-axis solar tracker, as defined in claim 1, in which said circular reflective dish is slotted or split so as to allow passage and clearance for said polar axle when said dish support member is pivoted through its full range of motion.
3. A polar-axis solar tracker, as defined in claim 1, in which said circular reflective dish is sized and shaped so as not to collide with said tracker frame at any angle of the dish support member pivoting motion, nor any angle of the polar axle rotation.
4. A polar-axis solar tracker, as defined in claim 1, in which said solar energy receiver is sized, shaped and positioned so as not to collide with said tracker frame at any angle of the dish support member pivoting motion, nor any angle of the polar axle rotation.
5. A polar-axis solar tracker, as defined in claim 1, in which said solar energy receiver is positioned and/or weighted such that the pivot point of said dish support member is at or very near the center of mass of the dish assembly, comprising the solar energy receiver, the dish support member and the circular reflective dish, thus making it possible to pivot the dish assembly and rotate the polar axle with a minimum of torque.
6. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means is capable of simultaneously rotating the polar axles of multiple solar trackers at the same time, thus eliminating the need for multiple polar axle rotation means, and thus significantly reducing the cost when installing multiple trackers.
7. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means is a system of one or more cables and pulleys, and hence induces zero backlash, as is commonly caused by gears.
8. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means is a system of cables and pulleys, with tensioning springs positioned periodically between said cables in such a way that the thermal expansion or contraction of said cables is absorbed by the springs, and causes minimal or zero misalignment in the driven polar axles.
9. A polar-axis solar tracker, as defined in claim 1, with provisions for the following fine-tuning adjustments:
- a) fine-tuning adjustment(s) for bringing the support base precisely into level.
- b) Horizontal fine-tuning adjustment(s) for bringing the polar axle into alignment with the North-South direction at the installation site.
- c) Vertical fine-tuning adjustment(s) for bringing the inclination or slant of the polar axle into alignment with that of the earth's polar (rotational) axis.
- d) Rotational fine-tuning mean adjustment(s) to fine-tune the rotational angle of the polar axle relative to that prescribed by said polar axle rotation means, so that when multiple trackers are driven by the same rotation means, each of them can be brought independently into alignment with the direction of the sun.
10. A polar-axis solar tracker, as defined in claim 1, in which said declination adjustment means can optionally be a mechanism which is periodically and manually re-aligned with the sun, thus avoiding the costs of an automated adjustment means, and further reducing the overall costs of the system.
11. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means can be driven so as to completely invert said dish assembly at night, or at other selected occasions, for reduced accumulation of dust on said circular reflective dish, or to better withstand high winds or other potentially harmful situations.
Type: Application
Filed: Sep 24, 2008
Publication Date: Mar 26, 2009
Inventor: Daniel Norvin Brown
Application Number: 12/237,134
International Classification: F24J 2/38 (20060101);