Method and apparatus for a passive solar day lighting system

A method and apparatus for a passive, fiber-optic day-lighting system collects and transports sunlight as a cost-effective technology solution for day-lighting applications. The system utilizes a low concentration ratio sunlight collection system, in expensive optical fibers, and an inexpensive passive solar thermal tracker. The sun-light collection system uses an array of conical compound parabolic concentrators with concentration ratio in the range of 50-500. The sun-light collection system may also use an array of square or rectangular shaped Fresnel lenses with circular concentric grooves. The array of Fresnel lenses can be formed on a single sheet of plastic, which will minimize the cost of manufacturing and reduce the cost of assembly of individual lenses into an array. The sun-light collection may also use arrays of two concentrators in tandem.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention is directed to a method and apparatus for a passive
 solar day lighting system using sunlight transported into a structure
 through an optical conduit. More particularly, the present invention is
 directed to a passive lighting system that does not require active
 tracking of the sun and utilizes non-imaging optical collectors for
 concentrating collected sunlight and optical fibers for transporting light
 to a light fixture in a room.
 2. Description of Prior Art
 The use of natural light in buildings can significantly reduce the energy
 consumption and improve the quality of life. But interior rooms cannot
 benefit from a window or a sky light and only rooms immediately below a
 roof can enjoy a skylight. There have been several efforts to capture
 sunlight and deliver it to remote parts of buildings using fiber optics or
 light pipes or optical conduits.
 Many of these previous efforts used some type of active sun tracking system
 coupled with complicated lens and mirror systems. For example, U.S. Pat.
 Nos. 4,246,477; 4,297,000 and 4,409,963 disclose various sunlight
 collectors, containing Fresnel lens systems. The collectors are mounted on
 the roof of a building and actively track the sun during daylight hours.
 Other systems for collecting and transporting sunlight are disclosed in
 U.S. Pat. Nos. 4,307,936; 4,541,414; 4,539,625; 5,548,490 and 5,709,456.
 These patents disclose various devices to collect sunlight without
 actively tracking the sun. The U.S. Pat. Nos. 4,307,936 and 4,541,414 are
 directed to sunlight collecting devices that use a parabolic collector to
 capture sunlight and a complex arrangement of lenses to concentrate the
 captured sunlight. The U.S. Pat. Nos. 4,539,625; 5,548,490 and 5,709,456
 are directed to large stack luminescent or fluorescent sheets arranged to
 collect sunlight and convert that collected light into concentrated light.
 Some of these approaches have used complex, delicate systems that track the
 sun actively. The other systems, require either complicated mirror and
 lens assemblies or specialized glass collectors having luminescent or
 fluorescent dyes therein, making the systems so expensive that they cannot
 be justified by the electric energy it saves.
 Passive solar tracking systems are known in the prior art. These passive
 tracking systems generally use solar thermal energy to track the sun's
 path. Solar thermal powered tracking systems are described in U.S. Pat.
 Nos. 4,332,240; 4,262,654; 5,600,124; 4,175,391; 4,275,712; 4,306,541 and
 4,476,854. In each of these patents, a parabolic trough collects solar
 energy and heats fluid-containing reservoirs to cause differential
 vaporization and shifting of fluid to rotate the apparatus. These passive
 tracking systems offer a cheaper means for tracking the sun, however, the
 size of the collectors renders them generally unsuitable for a large scale
 device.
 SUMMARY OF THE INVENTION
 The present invention contemplates a simple sunlight collection system
 based on the premise that the cost of collecting and transporting sunlight
 should be low. Therefore, a high-precision tracking feature is not part of
 the present invention. Important elements required for a day-lighting
 system are the means to collect sunlight and the means to transport the
 collected light to the building interior space. The present invention is a
 passive system without active tracking of the sun and utilizes well
 established non-imaging optical collectors for concentrating the sunlight
 and optical fibers for transporting light to a light fixture in a room.
 The use of optical fibers (plastic or comparatively less expensive glass)
 that are flexible, minimizes cost of installation in comparison to light
 pipes or other optical guides.
 To meet these and other objectives, the present invention is directed to a
 passive solar day-lighting system using a low light concentration ratio
 sun-light collection system. The collection system includes an array of
 non-imaging concentrators housed in an enclosure with at least one
 transparent surface. The sun-light collection system is mounted on a
 passive solar thermal tracking system and is connected to an optical
 conduit which transports collected and concentrated light from the
 sun-light collection system to an interior building space.
 The solar collector system as contemplated in the present invention has a
 low light concentration ratio of at least 50, and no greater than 700. The
 concentration ratio or factor is the ratio of the inlet area to the exit
 area of the collector. The higher the concentration ratio, the lower is
 the half-angle of acceptance and hence the higher is the precision
 required for tracking the sun. Because the array of non-imaging
 concentrators has a relatively low concentration ratio, the system does
 not require precision tracking.
 In certain preferred embodiments of the invention, the array of non-imaging
 concentrators is a plurality of conical compound parabolic concentrators,
 each of the concentrators having a predetermined input half-angle and a
 predetermined exit-half angle. Each of the plurality of conical compound
 parabolic concentrators also has a predetermined input diameter and a
 predetermined exit diameter, and the sun-light collection system is
 designed by optimizing the predetermined input half-angle, the
 predetermined exit-half angle, the predetermined input diameter, and the
 predetermined exit diameter of each of the plurality of conical compound
 parabolic concentrators.
 In certain preferred embodiments of the invention, the array of non-imaging
 concentrators alternatively is an array of Fresnel lenses of square or
 rectangular shape, and may be manufactured singly or on integrally on a
 single sheet of plastic. It is also contemplated that the array of
 non-imaging concentrators may be comprised of a plurality of sets of two
 concentrators in tandem with a Fresnel lens as a primary concentrator and
 a conical compound parabolic concentrator as a secondary concentrator.
 The optical conduit according to certain preferred embodiments of the
 present invention is a series of optical fibers coupled to exit ends of
 each non-imaging concentrator. The optical fibers can be made of a plastic
 material or an inexpensive glass.
 The passive solar thermal tracking system contemplated by certain preferred
 embodiments of the invention includes a non-electric tracking device that
 uses solar thermal energy to power a device to move the collector thereby
 tracking sun movement. The contemplated solar thermal tracking system
 should have an inaccuracy of no more than 5.degree. while tracking the
 sun. This requires a half angle of acceptance of no more than 5.degree.
 for the primary collector.
 The present invention is also directed to a method for passively supplying
 sunlight to a day-lighting system. The contemplated method includes
 forming a low light concentration ratio sunlight collection system by
 arranging an array of non-imaging concentrators, each having an input end
 and an exit end in an enclosure with at least one transparent surface. The
 method also includes mounting the array of non-imaging concentrators on a
 passive solar thermal tracking system.
 Preferred embodiments of the method according to the invention further
 include connecting an optical conduit to each exit end of the array of
 concentrators, and passively tracking the sun using the solar thermal
 tracking system, thereby collecting sunlight in the array of non-imaging
 concentrators. The method further contemplates transporting the collected
 sunlight through the optical conduit to an interior portion of a
 structure.
 The significance of passive solar powered tracking system is it does not
 require external power and it is less costly than powered active systems.
 Another advantage is solar powered tracking systems is proven and is
 commercially available for photovoltaic applications (e.g., from Zome
 Works Corp. Albuquerque, N. Mex. 87125). With existing controls, electric
 lighting can be easily integrated with the day-lighting based on the
 present invention, and can be controlled based on sensed lighting level in
 a room. This assures un-interrupted, desired lighting level in the
 interior building spaces during cloudy periods.
 These together with other objects of the invention, along with the various
 features of novelty which characterize the invention, are pointed out with
 particularity in the claims annexed to and forming a part of this
 disclosure. For a better understanding of the invention, its operating
 advantages and the specific objects attained by its uses, reference should
 be had to the accompanying drawings and descriptive matter in which there
 is illustrated preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS
 The first component of the present invention is the concentrator. FIG. 1
 illustrates a non-imaging concentrator according to one aspect of the
 present invention. FIG. 1 illustrates a conical compound parabolic
 concentrator 1 (CPC). The conical CPC contemplated in the present
 invention is not a trough-type collector. Rather, the conical CPC of the
 present invention has specified inlet and exit half-angles, .theta..sub.1
 and .theta..sub.2 respectively, as seen in FIG. 1. The CPC 1 can be
 reflective and can be filled with a dielectric medium or it can be
 dielectric totally internally reflective. The maximum concentration ratio
 or factor, C, for such a CPC concentrator is given by the expression:
EQU C=(n.sub.2 sin(.theta..sub.2)/n.sub.1 sin(.theta..sub.1)).sup.2 =(d.sub.1
 /d.sub.2).sup.2 (1)
 where,
 n.sub.1 and n.sub.2 are the refractive indices of the mediums at the inlet
 and the exit; and
 d.sub.1 and d.sub.2 are the inlet and out diameters of the concentrator.
 If the exit half-angle, .theta..sub.2 is less than 90.degree., the device
 is typically referred to as a .theta..sub.1 -.theta..sub.2 transformer.
 However, the term concentrator will be used throughout the following
 description instead of the term transformer. Since the present invention
 utilizes fiber-optic cables to transport sunlight form the concentrator,
 the required exit angle (half-angle) of the CPC concentrator is determined
 by the half-acceptance angle of the optical fiber. The half-acceptance
 angle for current fibers for lighting application are in the range of
 30.degree. to 40.degree.. Then, the concentration ratio or factor is
 dictated by the inlet angle, i.e., the larger the inlet angle, the smaller
 is the concentration factor. For instance, if the inlet and exit angles
 are 5.degree. and 40.degree. respectively, then the concentration factor
 is about 54, assuming n.sub.1 and n.sub.2 to be 1.
 The present invention stems from the fact that low concentration ratios can
 accommodate higher acceptance angles and hence lower precision for
 tracking the sun. Therefore, a high precision active tracking control is
 not required. The present invention, thus, takes advantage of the existing
 passive solar thermal tracking with less precision in tracking the sun.
 The advantages of passive solar thermal tracking are: it uses solar
 energy; no external power is required; no motors are required; and it is
 therefore less expensive than active tracking systems. Currently,
 single-axis passive trackers are available on the market for photovoltaic
 application. Zome Works Corp. of Albuquerque, N. Mex. 87125 markets
 Universal Track Racks that could be utilized. The present invention can
 use such a tracker, but preferably uses dual-axis tracking since tracking
 of the sun during the course of the day as well as during the course of
 changing seasons is desired. Since the focus of the present invention is
 on the overall system development and not on individual components, the
 passive tracker principle and construction of the same are not discussed
 here. Those skilled in the field can easily construct a dual-axis passive
 tracker. Descriptions of various solar thermal powered trackers can be
 found in U.S. Pat. Nos. 4,332,240; 4,262,654; 5,600,124; 4,175,391;
 4,275,712; 4,306,541; and 4,476,854, the disclosures of which are herein
 incorporated by reference.
 FIG. 2 illustrates a schematic of the fiber-optic day-lighting system
 according to one embodiment of the present invention. As shown in FIG. 2,
 the fiber optic day-lighting system includes a passive solar thermal
 tracker 3, on which a housing 4 is rotatably mounted. The housing 4
 comprises an array of CPCs 1. Each of the CPCs 1 is connected to an
 optical fiber 2. The optical fibers 2 can be bundled into one or more
 bundles depending upon their destination, i.e., whether the whole bundle
 is delivered to a single light fixture or multiple light fixtures within
 the interior building space. An example of the low concentration
 fiber-optic day-lighting system is presented below:
 Assumptions:
 Incident solar input=9100 lumens/ft.sup.2
 Accuracy of the passive tracker: .theta..sub.1 =3.degree.
 (the-half angle of acceptance)
 Overall system efficiency=30%
 Desired output at the fiber-end=5000 lumens
 Fiber half-angle of acceptance: .theta..sub.2 =40.degree.
 Fiber diameter: d.sub.2 =0.5"
 (Exit diameter of the concentrator)
 Calculations:
 Area of the collector required=5000/(9100.times.0.3)=1.83 ft.sup.2
 Concentration ratio: C=150
 (from Eq. (1): C=(d.sub.1 /d.sub.2).sup.2 =150)
 .thrfore.d.sub.1 (inlet diameter of the concentrator)=6.12"
 Number of concentrators=Area required/area of each
 concentrator=1.83.times.144 in.sup.2 /(n.times.d.sub.1.sup.2 /4)=9
 The above calculations illustrate that 9 CPCs are required for delivering
 an output of 5000 lumens. The length or height of the CPC in this case is
 estimated to be 63", which is rather high for practical construction.
 However, there are several strategies to bring the CPC height into
 practical magnitude. If the number of CPCs is increased to 20 (by
 decreasing d.sub.1 and d.sub.2) for example, the CPC height can be reduced
 to 42". Further, the CPC can be truncated without significant loss in
 collection efficiency. From the above discussion it should be evident that
 an optimized sun-light collection system can be designed and developed by
 optimizing the parameters .theta..sub.1, .theta..sub.2, d.sub.1, d.sub.2,
 and the system efficiency.
 FIG. 3 illustrates a further embodiment according to the invention that
 utilizes Fresnel lenses instead of CPCs but still employs the solar
 thermal passive concentrator. FIG. 3 shows a schematic of a Fresnel lens 5
 with incident light rays a, b, and c wherein light ray a is parallel to
 the optic axis OA. The focal point for rays parallel to the optic axis is
 a'. The off-axis rays b and c (at an angle of .+-..theta..sub.1 to the
 optic axis) focus at b' and c', respectively. The angle, .theta..sub.1, is
 the half-angle of acceptance of the Fresnel lens and the precision of the
 passive tracker in tracking the sun. As shown in FIG. 3, r is the radius
 of the Fresnel lens and L is its focal length. Also, d is the minimum
 diameter of the optical fiber 2 needed to accept sun-light at an angle of
 .theta..sub.2. Since the cost of the optical fiber is a driving factor in
 the design of the overall system, the diameter of the fiber, d, and the
 number of fibers should be minimized by optimizing r, L, .theta..sub.1 and
 .theta..sub.2.
 It is not practical to utilize a single Fresnel lens for collecting
 sun-light to a level for practical use. Therefore, an array of Fresnel
 lenses is needed. U.S. Pat. No. 4,409,963, the disclosure of which is
 herein incorporated by reference, discloses an ideal arrangement of
 Fresnel lenses. In the '963 Patent, each Fresnel lens has a hexagonal
 shape and the lenses are arranged around one lens in a concentric circular
 form. Since cost is an important issue, the present invention provides an
 alternate, cost-effective shape and arrangement of Fresnel lenses. As
 shown in FIG. 4, the preferred shape of Fresnel lens 5 according to the
 invention is a square (or a rectangle). The maximum circle 5' size that
 can be inscribed on a square is shown in FIG. 4. The remaining area of the
 square 5" may be unutilized.
 According to the present invention concentric grooves can be cut in this
 area 5" so that this area also can be utilized to concentrate sun-light.
 It is not difficult to manufacture square or rectangular Fresnel lenses
 and such lenses are currently manufactured by Fresnel Optics, 1300 Mt.
 Reed Blvd., Rochester, N.Y. 14606. These individual square or rectangular
 Fresnel lenses can be assembled into an array. However, an integral
 Fresnel array 7 of such square lenses can be manufactured in one step by
 employing existing manufacturing processes. This approach will eliminate
 the costs associated with assembling individual lenses into an array.
 Since there will be a limitation on size of the largest single array that
 can be manufactured, a modular approach is utilized according to certain
 contemplated embodiments of the invention. For example, an array of 2 feet
 by 3 feet with twelve 6-inch square Fresnel lenses can be one module.
 Depending upon the light collection area required, multiple lens modules
 can be manufactured. These modules can be enclosed in individual housing 6
 as shown in FIG. 4 or multiple Fresnel lens modules can be enclosed in a
 single housing.
 FIG. 4 illustrates additional details of Fresnel lens system according to
 the present invention. Not shown in FIG. 4 is the detail of the fiber
 optic connector or coupler to connect fiber to the collector housing base
 8. There exist commercial connectors and or couplers that can be used for
 this application and are well known to those of ordinary skill in this
 art. Additionally, those skilled in the art can design several variations
 for the connector/coupler.
 Yet another embodiment of the invention provides that the sun-light
 collection system is made from two concentrators in tandem, i.e., a
 primary concentrator and a secondary concentrator are arranged in series.
 The primary concentrator can be a Fresnel lens and the secondary
 concentrator can be a CPC or a transformer that is reflective or
 dielectric totally internally reflective. Such an embodiment is shown FIG.
 5. Such a concentrator is disclosed in Applied Optics, Vol. 26(7), pp.
 1207-1212, by Xiaohui Ning et al., titled "Optics of Two-Stage
 Photovoltaic Concentrators with Dielectric Second Stages."
 The present invention is also directed to a method for passively supplying
 sunlight to a day-lighting system. The contemplated method includes
 forming a low light concentration ratio sunlight collection system by
 arranging an array of non-imaging concentrators, each having an input end
 and an exit end in an enclosure with at least one transparent surface. The
 concentrators may be either the plurality of CPCs described above or the
 plurality of Fresnel lenses.
 The method also includes mounting the array of non-imaging concentrators on
 a passive solar thermal tracking system. As described above, the passive
 thermal tracking system uses solar thermal energy to power the tracking
 device. Although this particular method of tracking sun movement is not as
 accurate as other actively powered systems, the particular arrangement of
 low concentration ratio collecting devices permits a less accurate
 tracking system.
 The method further includes connecting an optical conduit to each exit end
 of the array of concentrators, and passively tracking the sun using the
 solar thermal tracking system, thereby collecting sunlight in the array of
 non-imaging concentrators. The method further contemplates transporting
 the collected sunlight through the optical conduit to an interior portion
 of a structure and integrating with electric lighting.
 With respect to the above description then, it is to be realized that the
 optimum dimensional relationships for the parts of the invention, to
 include variations in size, materials, shape, form, function and manner of
 operation, assembly and use, are deemed readily apparent and obvious to
 one skilled in the art, and all equivalent relationships to those
 illustrated in the drawings and described in the specification are
 intended to be encompassed by the present invention.
 The foregoing disclosure has been set forth merely to illustrate the
 invention and is not intended to be limiting. Since modifications of the
 disclosed embodiments incorporating the spirit and substance of the
 invention may occur to persons skilled in the art, the invention should be
 construed to include everything within the scope of the appended claims
 and equivalents thereof.