Abstract:
An optical system for a solar lighting device to provide highly concentrated sunlight to interior spaces with minimal disruption of building envelope. The optical system includes an aplanatic optical imaging system mounted on a dual-axis sun tracker, a non-imaging solar concentrator coupled to the aplanatic system, a liquid light pipe to convey the very intense solar flux to the interior of a building, a diffusing light fixture to spread the daylight into the interior space, and a control system to regulate the light output to a constant and desired level.

Description:
[0001]    The present invention is concerned with solar daylighting employing an optical system which provides extremely high solar flux to produce very efficient light output. More particularly, the invention is directed to a solar energy system which combines a non-imaging light concentrator, or angle transformer, with an aplanatic primary and secondary mirror subsystem wherein the non-imaging concentrator is efficiently coupled to the mirrors such that imaging conditions are achieved for high intensity light concentration onto a light pipe. 
       BACKGROUND OF THE INVENTION 
       [0002]    Daylighting systems are very well known in the form of sky lights which are low cost but can only be used in the top floor of buildings and have the disadvantage of requiring large roof penetrations which lead to undesirable heat loss in winter and heat gain in summer and are prone to rain water infiltration. Daylighting systems employing fiber optics which do not require large roof penetrations and can be used in all floors of a building have been developed, but are not in wide use due to the high cost of optical fiber and the use of large, cumbersome optical systems. In order to even compete with sky lights or other daylighting systems, the compactness and economics must be drastically improved. 
         [0003]    One example of prior art of concentrating daylighting is the Hybrid Solar Lighting technology developed by Oak Ridge National Laboratory and commercialized by Sunlight Direct, LLC. It uses a solar concentrator to collect and distribute sunlight into the interior of a building via plastic optical fibers. According to the US Department of Energy, Office of Energy Efficiency and Renewable Energy, this system is the most recent technology: “The most recent technology, hybrid solar lighting, collects sunlight and routs it through optical fibers into buildings where it is combined with electric light in ‘hybrid’ light fixtures. Sensors keep the room at a steady lighting level by adjusting the electric lights based on the sunlight available. This new generation of solar lighting combines both electric and solar power. Hybrid solar lighting pipes sunlight directly to the light fixture and no energy conversions are necessary, therefore the process is much more efficient. It is currently being developed and tested by Oak Ridge National Laboratory in collaboration with the Department of Energy and several industry partners.” 
         [0004]    Another example of prior art is a system from Parans Daylight AB in Gothenburg, Sweden employing Fresnel lenses and fiber optics. A similar system was also developed at the University of Nottingham in the United Kingdom. 
         [0005]    Another example of prior art is the “Himawari” system of La Foret Engineering Co., Ltd. in Tokyo, Japan. The system employs Fresnel lenses and large diameter quartz glass fibers. 
       SUMMARY OF THE INVENTION 
       [0006]    Aplanatic optical imaging designs are combined with a liquid light pipe and optionally a non-imaging optical system for numerical aperture (NA) matching to produce an ultra-compact light concentrator that performs near the etendue limits. In a solar daylighting system the aplanatic optics along with a coupled liquid light pipe and optionally a non-imaging concentrator angle transformer for NA matching produce light output with very high efficiency. 
         [0007]    A variety of aplanatic and planar optical systems can provide the necessary components to deliver light to a liquid light pipe, which forms a highly concentrated light output to an interior space. In one embodiment a secondary mirror is co-planar with the entrance aperture, and the location of the focal plane is chosen to accommodate the NA of the liquid light pipe. Alternatively, a nonimaging light concentrator may be introduced to the optical system to transform the NA of the two mirror system to the NA of the liquid light pipe. The non-imaging light concentrator is disposed at the focal plane of the two mirror system wherein the non-imaging concentrator is a θ 1 /θ 2  angular transformer with θ 1  chosen to match the NA of the two mirror system (sin θ 1 =NA 1 ) while θ 2  is chosen to match the NA of the liquid light pipe (sin θ 2 =NA 2 ). Also, θ 2  is chosen to satisfy a subsidiary condition, such as maintaining total internal reflection (“TIR”). In most cases these two conditions are compatible. 
         [0008]    It is readily shown on general grounds that for the most compact imaging system with a primary and secondary mirror the ratio of depth to diameter is 1:4.  FIG. 1  exemplifies this relation. In a preferred embodiment, such as depicted in  FIG. 1 , the NA of the aplanatic imaging system matches the NA of the liquid light pipe obviating the need for a nonimaging angle transformer. 
         [0009]    This system with its combination of elements enables employment of the highly efficient and low cost liquid light pipe such that a very intense solar flux can be conveyed to an interior space. With concentrated solar flux of 300-500 times ambient insolation, the roof penetration required for the same light output is reduced in area by the same factor. The system described herein can provide highly concentrated sunlight to interior spaces with minimal disruption of building envelope. This feature combined with the low cost high efficiency liquid light pipe makes this system very attractive commercially. The optical system therefore provides the light intensity needed to achieve commercial effectiveness for solar day lighting. Moreover, the concentrating daylighting system can be integrated into buildings in an esthetically pleasing and minimally intrusive way. This feature makes the system attractive for retrofitting of existing buildings, which is problematic for prior art daylighting systems. Another benefit of the concentrating daylighting system is that the liquid light pipe can be easily integrated into conventional light fixtures producing daylight illumination without the glare that is a frequent problem of standard daylighting systems. Moreover, augmentation with electric light is desirable because of the variability of daylight. In our system this is easily achievable by adding complementary electric light sources into the same light fixture. This facilitates control of the overall illumination. 
         [0010]    Objectives and advantages of the invention will become apparent from the following detailed description and drawings described hereinbelow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates an aplanatic optical system coupled to a liquid light pipe. 
           [0012]      FIG. 2  illustrates an aplanatic optical system coupled to a nonimaging angle transformer which, in turn is coupled to a liquid light pipe; and 
           [0013]      FIG. 3  illustrates an aplanatic optical system coupled to a liquid light pipe which in the interior space is coupled to a light diffuser or luminaire. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    An optical system  10  constructed in accordance with one embodiment of the invention is shown in  FIG. 1 . A primary mirror  20  and a secondary mirror  14  comprise a two mirror system. The secondary mirror  14  is co-planar with the entrance aperture  12  of the primary mirror  20 . A protective window  13  covers the entrance aperture  12  and supports the secondary mirror  14 . The focal plane of the two mirror system resides at a location  40  intermediate between the vertex  18  of the primary mirror  20  and the vertex  19  of the secondary mirror  14  such that the angle  20 , subtended by the secondary mirror accommodates the NA of the liquid light pipe  26 , which is equal to sin(θ 2 ). Solar radiation incident over angle  2 θ 1  (the convolution of the solar disk with optical errors) is concentrated to the focal plane  40  where it is distributed over angle  2 θ 1 . 
         [0015]    In an alternative embodiment, as shown in  FIG. 2 , a non-imaging concentrator  30  is disposed at the focal plane  41  of the two mirror system, which is located near the vertex  18  of the primary mirror  20 . This concentrator  30  is most preferably θ 1 /θ 2  angular transformer with θ 1 , chosen to match the NA of the two mirror system (sin θ 1 =NA 1 ) while θ 2  is chosen match the NA of the liquid light pipe (sin θ 2 =NA 2 ). θ 2  is also chosen to satisfy a subsidiary condition, such as maintaining total internal reflection (“TIR”). In most cases these two conditions are compatible. The concentration or flux boost of the terminal stage approaches the fundamental limit of (sin θ 2 /sin θ 1 ) 2 . The overall concentration can approach the etendue limit of (sin θ 2 /sin θ 0 ) 2 . Alternatively, the non-imaging concentrator  30  can be a known tailored non-imaging concentrator. 
         [0016]    In the angle transformer  30 , both the entrance aperture  33  and the exit aperture  35  are substantially flat, making this a straightforward case to analyze. In fact, the preferred angle transformer  30  has a design which falls under the category of well-known θ 1 /θ 2  non-imaging concentrators. The condition for TIR is 
         [0000]      θ 1 +θ 2 ≦π=2θ c    (1) 
         [0000]    where θ c  is the critical angle, arc sin (1/n). n is the index of refraction of the angle transformer  30  and is typically about 1.5. 
         [0017]    In many cases of practical importance the TIR condition is compatible with limiting the exit angle θ 2  to reasonable prescribed values. Since the overall optical system  10  is near ideal, the overall NA is NA 2 =sin (θ 2 ). NA 2  is chosen to match the NA of the liquid light pipe  26 . In an alternative embodiment a reflective surface  31  of the concentrator  30  need not be such that TIR occurs. In this case the exterior of the θ 1 /θ 2  concentrator  31  can be a silvered surface incurring an optical loss of approximately one additional reflection (˜4%). The coupling between the exit aperture  35  of the angle transformer  30  and the entrance window  24  of the liquid light pipe  26  is preferably index matched, however, a small air gap, which introduces about 10% Fresnel reflection losses, is tolerable. 
         [0018]    The overall optical system  10  is near-ideal in that raytraces of both imaging and nonimaging forms of the concentrator  30  reveal that skew ray rejection does not exceed a few %. Co-planar designs can reach the minimum aspect ratio (f-number) of ¼ for the two mirror system that satisfies Fermat&#39;s principle of constant optical path length. 
         [0019]    The performance of the two mirror system is not affected by chromatic aberration typical of lens systems. All dielectrics that are transparent in some wavelength range will have dispersion, a consequence of absorption outside the transparent window. Even for glass or acrylic, where the dispersion is only a few percent, this significantly limits the solar flux concentration achievable by a well-designed Fresnel lens. For a planar form of the two mirror system, the only relevant refracting interfaces are the two surfaces of the window  13 , normal to an incident beam  28 . At the interface (the entrance aperture  12 ) angular dispersion is, 
         [0000]      δθ=−tan(θ)δ n/n    (2) 
         [0000]    which is completely negligible since the angular spread of the incident beam  28  is &lt;&lt;1 radian. The optical system  10  is for practical purposes achromatic. In fact, Equation (2) indicates some flexibility in design. The dielectric/air interface (the entrance aperture  12 ) need not be strictly normal to the beam. A modest inclination is allowable, just as long as chromatic effects, as determined by Equation (2) are kept in reasonable bounds. 
         [0020]    While the light pipe is a very effective flux homogenizer, it may be useful to homogenize the input flux to mitigate hot spots. The aplanat is an imaging design, imaging the sun and causing hot spots at the exit of approximately (sin(θ 0 /sin(θ 5 )) 2  where θ 0  is the angular acceptance of the system and θ s  is the semi-angle of the solar disc which is approximately ¼ degree. For materials reasons, because thermal and/or flux excursions are potentially problematic for long term operation, this may be undesirable. 
         [0021]    A variety of Kohler homogenizer and planar optical systems formed by two mirrors can provide the necessary components to deliver light to a light pipe or nonimaging concentrator. In the Kohler homogenizer, radial symmetric mirror segments on both primary and secondary mirrors are pair-wise correlated so that the segment on the primary images the field of view onto the secondary segment, while the secondary segment in turn, images the primary segment on the target. Alternatively, the Kohler homogenization can be done in both the radial and saggital directions so that the mirror segments in both primary and secondary are disposed in either in a rectangular or hexagonal pattern. This embodiment is sometimes referred to as a “free form” design. In one embodiment a secondary mirror is co-planar with the entrance aperture, and the exit aperture is co-planar with the vertex of the primary mirror. 
         [0022]    For illumination of interior spaces it may be beneficial to tailor the spectrum. Removing the infra-red component mitigates heating the interior space. Removing the ultra-violet component is beneficial to avoid damage to materials. The use of reflective optics facilitates this function. In particular, the small secondary mirror may have a multi layer coating to achieve this result. 
         [0023]    Liquid light pipes are a relatively new technology that offers an attractive alternative to fiber optics. They are much less costly, replacing expensive fiber with inexpensive liquid and they are efficient. There is no loss of efficiency due to the packing loss typical of fiber bundles, and the liquid medium has low attenuation and very low cost. They are an ideal complement to the compact optical system that characterizes this invention. 
         [0024]    The following non-limiting examples are merely illustrative of the design of the system. 
       EXAMPLE 1 
       [0025]    Primary mirror  20  combined with secondary mirror  14  are elements of an aplanatic design of maximum compactness where 2R/s ≈4, as shown in  FIG. 1 . The liquid light pipe  26  is positioned with entrance aperture  24  at the focal plane  40  of the two mirror system. The focal plane location  40  is chosen to match the NA of the liquid light pipe, for which 0.42 is a typical value. A transparent cover  13  encases the optics providing protection against the elements. The unit is mounted on a dual axis sun tracker with sufficient angular accuracy to accommodate  28  the angular acceptance (θ 0 ≈1 degree) of the optical system. Notice that sin θ 0  ≈NA 2 /(C) 1/2  where C is the geometrical concentration as befitting an etendue limited system. 
       EXAMPLE 2 
       [0026]    In another embodiment, which is depicted in  FIG. 2 , the focal plane of the two mirror system  41  is placed at the vertex  18  of the primary mirror  20  so that the NA 1 =sin θ 1 ≈0.25. To accommodate the liquid light pipe NA 2 =0.42 a non-imaging optical concentrator (or angle transformer)  30  is used with θ 1 =15 degrees, θ 2 =25 degrees. The nonimaging optical element can operate by total internal reflection. 
       EXAMPLE 3 
       [0027]    As shown in  FIG. 3 , the optical system  10 , which is mounted on a dual-axis sun tracker  11  that is positioned on the roof of a building  50 , concentrates sunlight into the liquid light pipe  26 , which conveys the concentrated sunlight through a small roof penetration or an existing duct to a diff-using light fixture  40  that can be mounted on the ceiling or a wall of the room. The concentrated sunlight can be augmented by an electric light source  42  that can be integrated into the diffusing light fixture  40 . The light emittance from the diffusing light fixture can be controlled to a constant value with a lighting control system that regulates the light emittance from the electric light source in response to the available sunlight or to the total light output of both the concentrating daylighting and the complementary electric light system.