Patent Grant
8056555
Regular parabolic trough solar collectors are typically built with rugged structural members to provide rigidity so that when one part of the trough is rotated by a sun-tracking mechanism, the whole trough rotates together. The parabolic reflectors are typically glass or highly polished metal. U.S. Pat. Nos. 4,114,694 and 5,058,565 show trough structures that can be rotated by external means, but the troughs have to be rotationally rigid.
The rigid structures of these troughs are expensive, and the reflecting surfaces are expensive.
The tubes that carry the cooling fluid are typically located somewhat above the parabolic reflector. These designs are typically quite sensitive to the accuracy of the sun-tracking system. That is, if the troughs are not pointed fairly accurately toward the sun, the reflected rays tend to miss the target (the pipe carrying the cooling fluid). U.S. patents such as U.S. Pat. Nos. 3,923,381 and 6,676,263 overcome this problem by having a fairly narrow channel for concentrating the sunrays, and the fluid pipe is located deep within the cavity. These troughs also require rigid and expensive structures to maintain the geometry of the troughs.
This invention, called “Suntrof” herein, is a design for a solar trough collector which has the target fluid pipe deep within the cavity formed by a parabolic reflector. The reflector is an aluminized plastic film or other flexible reflective material, which is held in place by being stretched horizontally (longitudinally) between rigid ribs and cemented to the inside of the ribs. A protective sheet of tough plastic film is cemented to the outside of the ribs to prevent wind from hitting the reflective film. The ribs can be metal or fiber-reinforced plastic and can be molded in a factory inexpensively.
Since many of the aluminized plastic films have high tensile strength, if a sheet of the film is stretched tightly between two curved ribs, the film between the ribs will have a strong tendency to maintain the same curvature as the ribs over the whole length of the film. The tension of the sheet is transmitted from rib to rib until the sheet reaches the end of the trough, where the force of the tension is countered by a rigid end framework. By this means, the ribs do not have to sustain any large lateral or longitudinal forces.
One of the main features of the Suntrof structure is the concept that rigidity of the trough is provided mainly by the sun-tracking system. By having a set of sun-tracking cables connected to the tops of a set of troughs on a field, rigidity is provided, and all parts of each trough are pointed toward the sun. The cables are attached to tracking pipes at the east and west sides of the field, which rotate to cause the cables to move east and west and thus move the troughs east and west to track the sun. In this sense, it is similar to the tracking method presented in the U.S. patent application No. 60/648,865, “Solar Power Concentrator Using Reflective Films” by the present inventor. That system goes by the acronym of “Suncone.” The tracking system of Suncone is similar to the one presented here, except that Suncone has a two-axis tracking, while Suntrof has single axis tracking. But the major difference is that the Suntrof tracking system is designed to provide rigidity to the troughs, whereas the tracking system of Suncone does not provide rigidity to the reflective surface geometries.
Since the troughs are held in place by leverage of the cables from above, concrete foundations are not necessary, as they are with regular solar troughs and dishes. The Suntrof supports may simply be pipes driven into the ground.
The plastic film reflectors are much less expensive than glass or polished metal reflectors. Since strong metal beams are not required to provide the rigidity, additional expense is saved.
By having the fluid pipe down near the bottom of a fairly narrow trough, the system requires less tracking accuracy, and the cable tracking method will function properly.
It is therefore an object of the present invention to efficiently collect solar energy by a parabolic trough design that requires less sun-tracking accuracy than widely used parabolic troughs.
It is another object of the present invention to provide a method that utilizes inexpensive plastic films as parabolic reflectors in order to provide a less expensive method of collecting solar energy.
It is another object of the present invention to provide a sun-tracking system that not only causes a field of troughs to track the sun but also provides rigidity to the troughs without having to have heavy metal structures.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
An important feature of this invention is that the whole trough is held rigid by the tracking cable system. At first, one might think that a long trough system built with lightweight plastic components would tend to twist along its length. That is, parts of it would point in different directions. But the tracking cable system is attached to the tracking arch 5, and that provides it with a long lever arm that pivots about the fluid pipe 11. Even a small cable can hold the system rigid in windy conditions, due to the long lever arm. Archimedes said, “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” The ribs 2 and the tracking arches 5 form levers that rotate the trough from the top, rather than having the trough rotated from the bottom as with regular troughs and dishes.
In
The trough is supported by a support post 10. The support post also supports the fluid pipe 11 and the glass enclosure tube 12.
The trough is covered by a transparent cover 16 to prevent wind and dust from entering the trough. The cover is sealed to the protective enclosure film along the top sides.
The tracking arch 5 is a piece of rigid material that is a circular arc as though it were a portion of a rim of a wheel with its radius equal to the distance from the rim to the pivot line of the rotation of the collector. (The pivot line is the center of the fluid pipe). The tracking cables fit into grooves 17.
If the forces necessary to counter gravity and wind loads were provided by a pivot at the bottom of the trough (as it is with current dish and trough solar collectors) the structure would have to be very robust, and heavy gear boxes would be required to point the trough toward the sun.
This method may be a little simpler than the arches, but it has the disadvantage that as the troughs point far to the east or west, the cables are pulled downward by their connections to connection rods 32. This would require that the tracking pipes on the east and west rotate at slightly different rates.
When the circular arches are used, the tracking pipes on the east and west would rotate at the same rate.
As the west tracking pipe rotates to pull on the cable and the east tracking pipe rotates to let the cable out, the troughs rotate to the west to follow the sun (in both the embodiments of
Note that the reflective film 1 and the enclosure film 3 approaching from the right are cemented to the rib. However, the support post must pass through the films in order to support the fluid pipe. That means that at the bottom, the films' continuity is interrupted by the penetration of the support post into the trough. The connection assembly affords a method of attaching the plastic films so that dust does not enter the trough. The reflective film 1 and the enclosure film 3 approaching from the left at the bottom are cemented to the connection assembly at points 45 and 46, respectively.
Solar Collection Efficiency of Suntrof Compared to Other Solar Concentrators
Trough collectors have the disadvantage of losing efficiency during the winter due to the fact that the sun is low in the south (in the Northern Hemisphere), and the troughs do not rotate toward the south. On December 21 of each year in southern California, Arizona, and New Mexico, the sun is only about 34 degrees above the horizon at noon, which means that the solar collectors get only 56% as much solar energy as they would if the sunrays were perpendicular to the collectors' apertures. Solar parabolic dishes and Suncone collectors can rotate to the south to keep the sunrays perpendicular to the aperture.
The disadvantage of the dish collectors is that they have to be spaced apart in the north-south direction to prevent shadowing on each other in the winter. (All dish, trough, and Suncone collectors have to be spaced apart in the east-west direction). Consider a Suntrof collector that is 4 meters wide and 100 meters long. On December 21 at noon, it would be receiving 224 kilowatts of solar energy. A row of solar dishes lined up in a north-south row, each having a diameter of 4 meters, would need to have a north-south spacing of about 5.5 meters to reduce shadowing. Each dish would have an aperture of 12.57 square meters. If there were 19 dishes lined up along the 100-meter long field, the total amount of solar energy received would be 239 kW in mid winter, which is only about 7% more than the solar trough that covers the same amount of land. In the summer, the solar dish row would still receive 239 kW of solar energy, but the trough would receive 395 kW. So, for the same amount of land coverage, the troughs would receive much more solar energy. Of course, we must also consider how efficiently each collector transforms the energy into heat in the fluid that flows through it.
As a comparison between Suntrof and standard parabolic solar troughs, Table 1 gives computer ray-trace calculations of efficiency for both types of solar collectors.
If the collector is pointed directly at the sun so that the sunlight is focused on the fluid pipe (containing the heat-collecting fluid), the “degrees off center” is zero. The degrees off center refers to the east-west tracking accuracy, not the north-south position of the sun. The efficiency represents the amount of heat delivered to the fluid compared to the sunlight that is incident on the reflector. In both cases, the collector aperture is two meters wide. However, the dimensions may be scaled up to any size with the same results in efficiency.
Note that the efficiency of both types of solar concentrators drops off as the degrees off center increases, but the efficiency of the regular solar trough drops off faster. In fact, when the regular solar collector is pointed three degrees off dead center of the sun it focuses zero energy onto the fluid pipe for all cases considered in the table.
At higher temperatures, the efficiency drops off, because there is greater radiation from the fluid pipe.
For high temperatures, dish or Suncone collectors have higher efficiency than troughs. Suntrof collectors have higher efficiency, are less expensive, and need less tracking accuracy than regular troughs.
Fabrication
The ribs are constructed with a parabolic shape for the inside surface. Two ribs are bolted together at the bottom to form the ribs on each side. The ribs can be assembled to the trough support system in the field, and then the tracking arches can be attached.
After the fluid pipe with its glass enclosing tube is emplaced and the ribs are connected, the unit is tilted down to one side, and a sheet of reflecting film is placed in the lowered ribs, stretched horizontally, and cemented to the ribs. If desirable, a strip of plastic material can be used to press the film against the rib, and the strip can be bolted in place by bolts that extend through the plastic film into the rib.
After that reflective film is in place, the unit is rotated the opposite direction, and a reflective sheet is placed in the other half, tightened, and cemented in place. The two sheets are sealed together at the bottom. Then the protective enclosing outer film is attached on the outside of the ribs as tension is applied. This protective film prevents the wind from affecting the reflective film.
The frame structure on the ends of the Suntrof modules must be robust enough to sustain the tension of the plastic sheets and the guy wires. End guy wires supply the force to counteract the plastic sheet tension. The lower end of the guy wires are attached to an anchor at a point that is in line with the pivot line (center of the fluid pipe).
This claims priority to and the benefit of Provisional U.S. Patent Application Ser. No. 60/744,675, filed Apr. 12, 2006, the entirety of which is hereby incorporated herein by reference.
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