Patent Application
20060193066
The invention generally relates to concentrating solar power and, more particularly, to concentrating solar power using one or more films to form a collector cone or a collector with multiple conical sections or frustums.
Focusing solar energy to high intensity can provide high temperatures at the target (focal point) in order to drive high-efficiency heat engines. Parabolic trough reflectors have been used effectively in this role. Parabolic dish mirrors can achieve even higher temperatures.
With parabolic trough and dish mirrors, considerable precision is required to construct and maintain them. The mirror facets of a parabolic dish concentrator are fairly expensive to manufacture. Each facet (approximately one square meter) must be mounted on a very rigid structure and must be precisely aligned to keep the sun's image on the target. About once a week, each mirror must be re-aligned. For a 100 square meter dish (1 m2 per facet), 100 mirrors must be realigned. Realignment can be done by electronically-controlled actuators, but that requires two motors per mirror facet in addition to sophisticated electronics.
Parabolic dish reflectors have been known to start fires in grass when accidentally pointed in the wrong direction. They can also cause damage to human eyes if the mirror points in a direction that causes sunlight reflection toward a person or if the person looks at the target (focal point).
Some concentrated solar power collectors use plastic films that are inflated and held in place by internal air pressure that is greater than the air pressure outside the inflated collectors. In U.S. Pat. Nos. 3,364,676, 4,033,676, 4,136,123, 4,352,112, and 4,432,342, for example, inflation is used to form and hold the reflecting surfaces in shapes that are or that approximate parabolic surfaces. Some of these designs require a boom to support the target (focus), have expensive support frameworks, require precise focus on the sun, and/or have poor collection efficiency due to the geometry. U.S. Pat. No. 4,267,824 describes a solar concentrator inflated to a cone shape and having a transparent end covering. The inflated shape is supported by its narrow end, and thus wind could blow the inflated shape to the side.
Some other U.S. patents relating to solar power are U.S. Pat. Nos. 4,088,121, 4,612,914, 4,543,945, 4,212,290, 4,744,644, 4,108,158, 3,899,672, 4,161,942, and 4,496,787. Also, see W. P. Teagan's “Review: Status for Markets for Solar Thermal Power Systems” (Arthur D. Little, Acorn Park, Cambridge, Mass., May 2001) which is a document that was prepared for Sandia National Laboratories, and “Direct Solar Reduction of CO to Fuel: First Prototype Results” (Ind. & Eng. Chem. Res., Vol. 41, Number 8, 2002, pp. 1935-1939) by A. J. Traynor and R. J. Jensen.
The present invention generally relates to an inexpensive method of producing a high-temperature solar energy collection system and/or device that uses thin flexible reflective films. The term “Suncone” is sometimes used herein to refer to various illustrative embodiments of systems and/or devices according to the invention.
In some embodiments according to the invention, one or more films (typically inexpensive) are formed into cone frustums that reflect and concentrate solar rays as the rays travel from the larger aperture of the collector toward its narrow end. At the narrow end, the lowest frustum of the reflective film(s) can be connected to a metal reflector shaped so that it continues to concentrate the sunrays. These rays are reflected toward an energy absorber (target), which can be coated with a selective coating that readily absorbs the light rays but typically is a poor radiator of infrared energy. A fluid can flow inside the absorber to extract the heat from the absorber. The fluid can then flow away to its point of use. Alternatively, the reflective film can be connected to a metal absorber that is coated with a selective layer to absorb sunlight and heat a working fluid. A plurality of these cone collectors can be used together and housed in a single enclosure unit, and a plurality of such enclosed units (each containing a plurality of the cones) can be used in concert with each other.
The reflective surfaces ideally should be designed so that all or most of the rays from the sun reflect only once before striking the target. For example, if the reflectivity of the cone surface is 0.9 and there is only one reflection before the ray hits the target, then 90% of the energy from the sun will hit the target. With the same reflectivity, if the rays reflect from one part of the cone and then reflect again from another part of the cone, only 81% of the sun's energy will reach the target.
In use, Suncone is pointed toward the sun. Suncone requires less focus precision than a parabolic dish or a parabolic trough in order to concentrate a large portion of the incident sunlight on the target. The reflection method used by Suncone is referred to as non-imaging optics.
The inexpensive film(s) that form(s) each of the reflective cones can be a thin (e.g., 2 mil) aluminized plastic, a thin metal foil, or some other type of thin film that might be treated or otherwise coated. For example, a polyester film (such as “Mylar” which is available from DuPont) that is coated or otherwise treated to place aluminum on one side of the film can be used. Instead of using air pressure inside a film-formed cone to inflate it and maintain its shape, some embodiments according to the invention use slight air pressure (e.g., 0.2 psi) inside an enclosure (such as a cylindrical enclosure) and inside each of the cones (which are located inside the enclosure) to achieve equal or substantially equal air pressure on either side of the cone wall (i.e., both inside and outside the cone) and to form the shape of the enclosure, while mechanical tension also can be used to maintain the cone's shape. Thus, in these embodiments, the conical shape of the cones is maintained by tension on the film(s) that form each of the cones, since air pressure is pushing upward on the top and bottom of the unit. Air pressure maintains the shape of the enclosure, and that shape can be cylindrical. The side wall(s) of the enclosure can be made of a plastic or other thin film material that might be coated or otherwise treated, and it too can be inexpensive. The side wall(s) can be made of the same material used to form the cone(s) but typically will be a different material and a thicker (e.g., 10 to 20 mils) material than the cone material. One or more transparent films can cover the larger-opening end of each of the cones, and can be made of clear plastic. This covering transparent film typically should have a transparency of 96%, a tensile strength of about 30,000 psi, be UV resistant, and capable of tolerating weather for decades. A fluoropolymer resin could be used to form the covering transparent film. For example, one fluorocarbon-based polymer that could be used is ethylene tetrafluoroethylene (ETFE) such as the “Tefzel” ETFE that is available from DuPont.
Alternatively, instead of relying on air pressure to push out on the enclosure to create and maintain the rigidity of the (cylindrical) enclosure, rigid structural members may provide the necessary rigidity and support to keep the flexible cones in tension (from the top and bottom of each cone) and in their conical shape.
In some embodiments of the present invention, a cone formed of the reflective collector film(s) may have two or more cone frustums. That is, as opposed to being a pure cone shape, the collector can be formed of multiple cone sections. In such embodiments, separate cone frustums can be used and rigid rings can be mounted at the junction between each of the cone frustums to provide the multi-cone frustum shape.
In general, the invention, in some illustrative embodiments, involves inexpensive, lightweight, and reflective films formed into one or more cone shapes (where each cone shape can have one or more frustums) with the largest diameter opening of each of the cone shapes pointed toward the sun to concentrate the sun's rays as the rays are reflected through the cone shapes to the narrowest end of the cone shapes. The rays are concentrated onto absorbing bodies, and the collected energy can be used, for example, to heat a fluid that flows in channels within the absorbing bodies. The reflecting films can be inexpensive plastic. An enclosing lightweight plastic or other flexible material (which might be coated or otherwise treated) surrounds an assembly of one or more of the concentrating cone shapes, and the entire structure can be made rigid by slight interior air pressure and possibly also by interior diagonal wires, or by lightweight structural members. This system with its enclosure housing one or more cone shapes is less expensive than standard parabolic dish solar collectors and is lighter in weight. It requires less precise sun-tracking systems than dish or trough collectors. It can achieve higher temperatures and higher solar collection efficiency than solar troughs.
In general, in some embodiments, the invention relates to a solar power concentrator that comprises flexible material (e.g., one or more films) maintained in place and shape by tension and disposed within a housing. The flexible material comprises one or more cone frustums, and the cone frustums together define the shape. In one particular embodiment, the shape includes a single cone frustum. In any event, the shape includes a first end, a first opening at the first end, a second end opposing the first end, a second opening at the second end, and a passage extending through the shape from the first opening to the second opening. The first opening is larger than the second opening. The flexible material also comprises an inner surface facing the passage and an outer surface facing away from the passage, and the inner surface reflects solar energy when solar energy is incident upon the inner surface. The housing, within which the flexible material is disposed, comprises a top, a bottom, and at least one side wall, and the tension is provided by the first end of the shape being coupled to the top and by the second end of the shape being coupled to the bottom. The housing defines interior space between the at least one side wall of the housing and the outer surface of the flexible material, and air pressure in the interior space is equal to or substantially equal to air pressure in the passage. This pressure can be caused by air that is supplied into and maintained in the interior space and the passage. A plurality of these flexible material formed shapes can be disposed together within the housing.
The Suncone design will not start fires in nearby flammable materials. If Suncone is pointed toward the sun, the solar energy target is deep inside the device so that it cannot harm people's eyes, and the bright rays do not strike nearby flammable objects. If Suncone is pointed away from the sun, it does not concentrate the light.
Suncone can produce high temperatures efficiently, so that it can produce high-pressure steam for driving highly efficient heat engines, for example. It is more effective at producing high temperatures than solar trough collectors.
One objective of the invention is to efficiently collect solar energy at high temperature so that high-temperature steam or other fluid may drive highly efficient heat engines.
Another objective of the present invention is to provide a structure for an assembly of conical solar collectors by surrounding the assembly with a film enclosure that is held in place by low air pressure (and possibly also by interior diagonal wires), or by surrounding the assembly with a rigid structure with rigid structural members and guy wires. In either case, the flexible cones are supported and kept in shape by tension and not by inflation.
Another objective of the invention is to provide a solar collector that does not require high precision in tracking the sun.
Another objective of the invention is to provide a solar collector that uses inexpensive materials and inexpensive support structure so that the cost of the collected solar energy is low. Suncone does not require a boom to support the target, and this eliminates the boom cost that is required for solar dishes.
Another object of the present invention is to provide high-temperature solar collectors that can be linked together to a single tracking mechanism for tracking the sun.
Another object of the present invention is to provide a configuration of reflective surfaces so that all or most of the rays from the sun reflect only once before reaching the target.
Another object of the invention is to provide a system that concentrates solar energy onto photovoltaic surfaces.
Other objects, advantages, and features of the invention will become apparent from the following written description to follow, taken in conjunction with the accompanying drawings. The description provides illustrative embodiments according to the invention. Various combinations and changes are contemplated and are part of this disclosure even if not expressly described or otherwise pointed out herein.
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 certain embodiments of the invention and are not to be construed as limiting the invention.
Interior wires 15 run diagonally from a point on the base 6 to the top of the enclosure 4 and connect to the rigid rim 10. The purpose of the diagonal wires is to improve rigidity. Since air pressure is pushing outward on the enclosure 4 and the interior wires 15 are pulling inward, the structure will be quite rigid. From the drawing, it appears that the wires 15 run through the cones, but when looking at
The shape of the metallic reflectors 2 is defined by a curve of revolution (generatrix) about the axis. The curve can be an exponential curve, but is not necessarily a parabolic curve, since a parabolic curve is defined as one in which the distance from the axis to the curve is proportional to the distance along the axis with an exponent of 2. For the metallic reflectors, the exponent can be different than 2, but the exact value will depend on the geometry of the rest of the system and is optimized by a computer ray-tracing program.
The assembly of
Not shown are the connecting pipes that bring the cooling fluid to the target rods 3 and remove the heated fluid. Also not shown are the mechanisms and/or connections needed to inflate the enclosure 4. A small air pump can supply the air. For a field of many Suncone units, a hose from a central pump can supply the air to each unit.
The insides of the cones 1 can be aluminized (or made reflective in some other manner) for high reflectivity. The outer surface of the cones can be coated by flat black, which radiates heat well. Computer simulations show that the cone material remains cool, since the inside reflective layer allows very little solar energy to enter the plastic, but the outside black layer radiates the heat away. The enclosure 4 can be transparent to allow radiation from the cones to pass through the enclosure. Alternatively, the enclosure 4 can be a color (e.g., black) or painted a color (e.g., black) that it will readily absorb radiation from the cones and re-radiate the heat away on the outside. Since the surface of the enclosure ideally is maintained parallel to the sun's rays (as the sun's movement is tracked), it does not get hot from direct sunshine.
The sun's rays ideally are concentrated on the target rods 3, which may have channels inside for the flow of water or other working fluid. Since the rods will get quite hot, they are surrounded by metal reflectors 2. Each plastic cone 1 is attached to the metal reflector 2 with an insulating connector (not shown). The metal reflector 2 and the target rod 3 are attached to the base 6, which is shown as a solid circular and cylindrical disk, but it may be any suitable assembly of one or more structural members. The Suncone structure does not have to be as robust as that of a parabolic dish, since it does not have to be as rigid and since it does not have to support a long metal boom that holds a heavy target at the end. In Suncone, the heat absorption is located adjacent to the base 6.
For photovoltaic applications, the target rods 3 could be larger in diameter and coated with photovoltaic films. The metal reflector 2 might also be covered with photovoltaic films. The concentration of light would provide higher energy collection per unit area of photovoltaic material. The target rods in this case could have fluid channels within for the collection of useful solar heat.
It should be noted that the target rods 3 are completely shielded from ground observers when the unit is pointed toward the sun, so that eye damage to passersby is impossible. If Suncone is accidentally pointed toward the ground, it will not be pointed toward the sun, so that it cannot start a grass fire. A parabolic reflector, on the other hand, can intercept sunlight even when it is not pointed directly toward the sun, and the reflected light can ignite fires on the ground. Suncone units could be mounted in parking lots above cars to generate electricity for nearby buildings without concern for the safety of people or property below them. They could also be mounted on tops of buildings. Engineers might be reluctant to place parabolic reflectors in such locations.
For high wind conditions, cables or cords extending from the base 6 to the rigid rim 10 can be reeled in to draw the top downward while the air pressure is reduced. The plastic film portion of the unit would be withdrawn into a sturdy cylinder surrounding the lower part of the enclosure to shield against the wind. Even if the plastic materials are destroyed, they are inexpensive to replace.
A small blower or pump provides the slight air pressure that maintains the shape of the plastic films. We can calculate what the stresses are applied to the plastic films of the assembly. Consider a Suncone unit with a total solar collection area of 50 m2 in which there are 7 cones that are 2 meters long (including metal reflector length) and 1.5 meter radius at the top. If the internal air pressure is 0.2 psi, the total force on the upper end would be 19,700 lbs. Total radial force on the enclosure film would be 17,500 lbs. If we add diagonal cords or wires internally running from the base 6 to the opposite top rim 10 and spiral cords running around the enclosure, the structure will be quite rigid. For additional rigidity, guy wires can be attached to the top rim and connected to extensions from the base. If the enclosure film is 10 mils thick, the stress on it would be 3,500 psi.
The upward force on each of the transparent windows would be 2,200 lbs. Using 10 mil thick clear plastic film, the stress on the plastic would be only 1,800 psi, which is small compared to its tensile strength. This force is applied to the cones, which transmit the stress to the metal reflector. The highest stress on the cone is at its narrow end. If the metal reflector is one foot in radius, the stress on the 5-mil thick plastic film at the connection point will be 5,800 psi. Metallized Dura-Lar film has a tensile strength of 30,000 psi. (Dura-Lar is an oriented polyester film for general purpose use, and generally is less expensive than DuPont's Mylar.) Of course, some of the stress on the cone can be relieved by having wires run from the base to the top where they could connect to rings that are attached to the top of the cones and to the transparent cover.
These calculations were done to show that it is feasible to construct the rigid structure with lightweight plastic films. If there is concern about the effects of wind on such a light structure, the calculations show that there is almost 30 pounds of force exerted outward on each square foot of surface area. By having interior and circumferential cords or wires that counter the surface forces, the structure will be quite rigid.
The advantage of this embodiment is that it is quite insensitive to the accuracy of a tracking mechanism that points the device toward the sun. In this design, the cone is divided into two reflective film cone frustums 20 and 21 in order to more closely match an exponential generatrix for the collector shape. A circumferential rigid ring 22 holds the reflective films 20 and 21 in place. The top of reflective film 20 is held in place by air pressure on the transparent cover (not shown, but like 5 in
Since the arrangement of connectors 39, rod 50, and cables 31 and 32 are difficult to present in a two-dimensional drawing,
Pivot 7 is designed to constrain the orientation of each Suncone unit so that rod 50 is always directly on the north side of the unit, but it allows the Suncone unit to tilt toward the sun. In the Southern Hemisphere, rod 50 would be on the south side of the Suncone unit.
If it is desirable to have a short structure with large solar collection area, the embodiment of one of the “cones” shown schematically in
Alternatively, the rigidity of the structure can be maintained by rigid structural members as was described for
With this design, a single reflector unit with 100-m2 solar collection area can be constructed that has a diameter of 11.3 meters and a height of 4 meters (from base to the cover). Computer simulations calculated its solar collection efficiency to be 70%, which is a less than that of the other embodiments, but it has a single target rod for heat absorption. The target rod can be surrounded by a glass tube, which is evacuated to reduce heat loss. One problem with this design is that it requires greater sun-tracking precision than the other designs.
Partitions inside the pipe 60 divert cool working fluid into the collector target rod and accept heated fluid back into the pipe, to be heated further by the next collector along the pipe 60.
COMPUTER SIMULATIONS: Since it is difficult to determine which reflecting surface geometries will be efficient solar collectors and will be insensitive to sun-tracking accuracy just by examining a drawing, a ray-tracing program called SUNCONE.F was written to simulate the performance of solar concentrators. Several thousand rays per second (of simulated time) are traced from random locations on the sun to random locations at the mouth of the cone. From there, each ray is traced to an intersection with the cone frustums, metal reflector, or rod. At each intersection, part of the ray is reflected, and the rest of the energy is absorbed into the surface. The amount of energy that is reflected and absorbed depends on the reflection coefficient. The ray continues on through multiple reflections until it exits the system. This method is extremely accurate in determining the performance of reflectors and absorbers of various geometries, if the emissivities and reflectivities are properly defined. The surface is assumed to be smooth.
After all the sunrays are traced for a one-second duration, radiation from the film reflectors, metal reflector, and rod (and hohlraum cavity, if present) are simulated. The components are divided into numerical cells. Since each cell receives energy during the sunray simulation, some of that energy is used to heat the cell, and the rest is radiated according to the equation,
P=eAσT4
where P is the radiation power, e is the emissivity, A is the area, a is 5.6699×10−8 in SI units, and T is the absolute temperature in degrees K. The solution of the problem of how much of the energy is used for heating the material and how much is radiated is determined by iteration. The radiated energy for the one-second time interval is emitted from each cell by random rays, which are then followed through multiple reflections. These rays also impart energy to the different component cells. After rays have been emitted from all the cells, we note that the cells now have received more energy, due to the radiation from all the cells. That is, the cells lose energy by emitting radiation but gain energy by radiation from other cells. Thus the process must be repeated in order to determine the balance between radiated and received energy.
SUNCONE.F was used to simulate the performance of existing parabolic dish collectors, and the results of the computer runs were within 5% of the experimental values.
For standard parabolic dish solar collectors, which consist of many mirror facets, the mirrors must be aligned precisely. In the case of a 100-m2 dish with a 13-meter boom on which there is a 22 cm diameter target, if the dish is misaligned by 1 a degree, the target will receive only 39% of the power. If it is misaligned by 1 degree, it will receive no power at all. For some designs of Suncone, if the alignment is 1 degree off, it will still absorb 94% of the sunlight that it would absorb for perfect alignment. Even at 5-degree misalignment, it will still receive over 80% of the power that it would receive at perfect alignment. Since Suncone collectors do not require high sun-tracking precision, systems like that shown in
The cones in Suncone do not have to be precisely constructed. Minor flaws are insignificant. Computer simulations were run with SUNCONE.F in which numerous perturbations (up to a half-centimeter in size) were applied to the cone surface randomly. The energy reaching the target rod was still above 90% of what a perfect cone would provide. The mirror facets on a parabolic mirror must be precise.
Large hailstones can damage glass mirrors. With Suncone, the tough plastic films, supported by slight air pressure, will yield when struck by hailstones and bounce back to their former geometry.
The invention is not to be limited only to the illustrative embodiments shown and described herein. Various changes are possible and will occur to those of ordinary skill without departing from the spirit or scope of the invention. Various combinations not specifically shown or described herein also are possible and are to be considered part of this disclosure.
This claims priority to and the benefit of Provisional U.S. Patent Application Ser. No. 60/648,865, filed Feb. 1, 2005, the entirety of which is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60648865 | Feb 2005 | US |
