Skylight with compound parabolic diffusers

Abstract

A skylight to provide daylighting is described that includes a plurality of compound parabolic diffusers into one or more layers that may be dome or pyramid-shaped to enable more efficient collection of sunlight and its distribution through wider angles resulting in more comfortable illumination with less glare and heat gainmore light less heatover wider areas of building interiors.

Claims

1. A skylight configured to direct and distribute sunlight into the interior of a building; said skylight including at least one layer comprised of a plurality of compound parabolic diffusers; each said compound parabolic diffuser formed from a transparent material; each said compound parabolic diffuser being associated with an axis of rotation; each said compound parabolic diffuser being defined as the three-dimensional body of revolution of a compound parabolic collector around said axis of rotation; each said compound parabolic collector being a two-dimensional figure defined by an entrance aperture, an exit aperture, a height of the parabolic sections in between said apertures and a centerline axis of symmetry; wherein each said compound parabolic diffuser having its entrance aperture substantially positioned towards the exterior of building and its exit aperture substantially positioned towards the interior of the building.

2. The skylight of claim 1 with said axis of rotation of each compound parabolic diffuser being the centerline axis of symmetry of the compound parabolic collector.

3. The skylight of claim 1 with said layer being in the form of a hemisphere or dome; said hemisphere or dome defined by an axis of cylindrical symmetry.

4. The skylight of claim 1 with said axis of rotation of each compound parabolic diffuser being the axis of cylindrical symmetry of claim 3.

5. The skylight of claim 1 with said layer being in the form of pyramid or a polyhedron.

6. The skylight of claim 1 with said transparent material being chosen from the class including plastics such as acrylic, poly methyl metha acrylate, polycarbonate or from the class of glasses including tempered low-iron glass.

7. The skylight of claim 2 or 5 with each said compound parabolic diffuser having an entrance aperture that circumscribes a hexagonal figure whereby the plurality of compound parabolic diffusers are arranged contiguously to tile the layer of the skylight in a honeycomb pattern.

8. The skylight of claim 1 wherein the said compound parabolic diffusers are formed from a transparent material having a refractive index of about 1.5 and having a height such that the ratio exit aperture diameter:entrance aperture diameter: height are about 1.25:2.7:3.75.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Figures provide explanatory details referenced in the following detailed description. Embodiments depicted in the drawings are illustrative but do not limit the scope of the invention as will be evident to those familiar with the art. Reference numbers are provided to indicate correspondence between reference elements.

(2) FIG. 1shows a lateral cross-section of the dome layer 101 in one embodiment of the invention. The axis of rotationaxis of cylindrical symmetryis shown as 100

(3) FIG. 2enlarged cross-section 1-1 of FIG. 1, depicting the cross-section of a compound parabolic diffuser 303 with entrance aperture 301 and exit aperture 302.

(4) FIG. 3 depicts a typical skylight that is an embodiment of the invention with dome-shaped layer 101, curb 102, installed on sloped building roof 103.

(5) FIG. 4Detailed view of the 2-D figure of a compound parabolic collector, or cross-section of the compound parabolic diffuser, with acceptance angle and with diameter 403 of entrance aperture 301, diameter 404 of exit aperture 302 and height 405. The axis of rotationthe centerline axis of symmetryof the 2-D CPD figure is shown as 410.

(6) FIG. 5shows a narrow angle beam of sun light rays 501 incident on the entrance aperture being total internally reflected on the curved surfaces of the compound parabolic diffuser to produce a divergent beam of rays 502 at the exit aperture.

(7) FIG. 6shows an embodiment of the invention in which the compound parabolic diffusers are arranged in hexagonal or honeycomb like tiling pattern on the dome layer 101 with hexagonal entrance apertures 601 and exit apertures 602. Section view along 603 corresponds to the enlarged cross-section shown in FIG. 2

(8) FIG. 7 Shows another embodiment of the invention with the compound parabolic diffuser structures rendered in circumferential grooves 701 in the skylight dome layer 101 with the section 702 also being depicted by FIG. 1. The shape of this embodiment may be generated as the body of revolution of the 2-D FIG. 1 about the central dome rotation axis 100.

(9) FIG. 8 shows the result of an optical simulations showing sunlight incident from a oblique angle (10 above horizontal horizon) onto a section of a skylight layer at a (typical) slope of 30 to horizontal. The resulting light distribution 803 from typical skylight 801 with typical refracting prisms is very narrow, leading to glare, while the same incident sun rays on a skylight layer with CPDs 802 results in a much broader angular distribution of day lighting 804 provided to wider area below. Thus the current invention is able to provide the ideal skylight function of providing light from overhead into a wide angle distribution for glare-free comfort below.

DETAILED DESCRIPTION OF THE INVENTION

(10) The primary innovation disclosed in this patent is to propose optical structures compound parabolic diffusersthat when included in functional skylight surfaces or layers provide for more efficient collection and more uniform distribution of the sunlight over a wider building floor area below. The three-dimensional shape of the compound parabolic diffusers is defined as the body of revolution of the more commonly known two-dimensional figure of opticsthe compound parabolic concentrator or collector CPCaround a defined axis of rotation. The CPC is well known already in the art for the collection and subsequent concentration or collimation of light but in the present invention their optical properties are used to deflect light from all possible sun angles directing it as if from overhead into a wider angular distribution to floor below. Since the change of direction or deflection of a light ray through the total internal reflection in a CPC is significantly greater than available through refraction CPCs provide an additional advantage over the refractive prisms common in the current art in that they are capable of deflecting light even from oblique angles of the sun directly downwards toward the building floor below.

(11) Instead of losing light through total internal reflection, as with current refracting prisms, we propose to collect, guide and distribute light more efficiently than available in the current art. With any optical structure or element used for optimal radiative transferto collect and transmit light from a source to targetit is recognized that a physical parameter known as etendue, throughput, or phase space volume remains conserved or invariant. From the system point of view, the etendue equals the area of the entrance pupil (or entrance aperture) times the solid angle the source subtends as seen from the pupil or aperture. This presents a simple approach to the problem of maximizing the divergence or solid angle into which the skylight broadcasts the collected sunlight to the building interior. By maximizing the concentration of such rays to a smallest possible area at the exit from the optical structure, this should result, with conservation of throughput, in the widest divergence or solid angle of rays at the exit aperture. This problem has been studied, for the optimizing of collection and concentration of radiation, including solar rays, in the field of non-imaging or anidolic optics as described in Radiative TransferS. Chandrasekhar (Dover, 1960). A solution described in High Collection Non-Imaging OpticsW. T. Welford & R. Winston (Academic Press, New York, 1989)] has been to use compound parabolic concentrators/collectors or CPCs. Compound parabolic concentrators are 3-dimensional structures comprised of the body of revolution generated by the 2-D compound parabola formed from the sections of two parabolas, with each passing through the focus of the other in a common plane. The 2-D cross-section of a compound parabolic diffuser, or its CPC analogue, is shown in FIG. 4, and these are defined by an entrance aperture, an exit aperture and the height of the parabolic sections in between. Alternately, the 2-D cross-section figure of the CPD may be defined by an acceptance angle and the entrance or exit apertures. As with an optical fiber that is ubiquitous in modern telecommunications, if the CPC is filled with a material whose refractive index exceeds ambient air then light rays entering the entrance aperture are reflected on the inner surfaces through total internal reflectionTIR. For a 3-dimensional CPC with a circular lateral cross-section, characterized by an acceptance angle , filled with a medium of refractive index n, the maximum theoretical concentration achieved of rays collected at the entrance pupil or aperture is (n/sin ).sup.2the ratio of the areas of the entrance and the exit apertures; equivalently, for a CPD, this is also the maximum achievable divergence of rays at the exit aperture.

(12) Compound parabolic diffusers, although defined by similar 3-D shape as CPCs here are used here not to collimate or concentrate but to distribute or diffuse light into a wide angular distribution. Further, the total internal reflection is deployed to bend light incident from lower sun angles through a larger deflection angle to provide it as if from overhead to the interior floor below.

(13) By including total internally reflecting CPCs instead of the typical refracting prisms the current invention is able to collect sunlight from a wider range of sun angles and distribute the collected sunlight to a wider area of a building interior below. The current invention makes use not of the convergence of light as it reaches the exit aperture but of the resulting divergence as the light rays leave the exit aperture, and therefore seeks precisely to avoid collimation of light; to not imitate the naturally narrow beam from the Sun, but to disperse, cause significant angular divergence in it, to spread it out to a wider floor area below. The current invention has a plurality of CPC-like structures (Compound Parabolic Diffusers CPDs) which when placed or arranged over a dome or pyramid like surfacethe skylight layercollect light from a wider range of sun angles, from as low as 10 above the horizon to directly overhead, but in all cases, spreading it over a wide area/wide angle to the occupied areas of building below.

(14) As shown in FIG. 5, when curvature, or slope, is included on the reflecting interface (optical materialair), then the sun's rays, although incident nearly parallel, are reflected into a divergent beam with angles varying according to the variation in slope of the optical materialair (e.g. plastic-air) interface.

(15) In one embodiment of the invention, we propose to replace the prisms in typical current skylight layers with compound parabolic diffusers. A lateral cross-section of such a skylight, including such CPD structures into its layer is shown in FIGS. 1 & 2. If the rotation axis of each CPD is chosen as its own symmetry axis or centerline of the 2-D figure FIG. 4 then the CPDs become arranged in an contiguous array in the functional layer of the skylight. A typical embodiment of such a skylight FIG. 3 may have a dome like shape, with a diameter of between 30-100 cm, covering the skylight opening, with CPDs embossed on the layers with dimension ratio of 1.25 mm:2.5 mm:3.75 mm of exit aperture diameter:entrance aperture diameter: height, as in FIG. 4. When formed from a transparent material such as acrylic plastic or polycarbonate or glass with refractive index around 1.5, the CPC analogues of CPD with such dimension ratios provide an acceptance angle of 45.

(16) Since the skylight in the current invention is able to distribute light over a wider area, it achieves the spring afternoon effect of providing more light over more area with less resultant heat gain. Since the skylight in the current invention also is able to deflect light from low, oblique sun angles to be provided as if from overhead it combines the key features of the ideal skylightproviding comfortable illumination from overhead with little or no heat or glare to the eyes of building occupants below.

(17) Since the CPDs in this embodiment are a body of revolution about their own axis of symmetry their cross-section taken laterally, perpendicular to this axis of rotation, will be circular. Circles cannot completely cover or tile the locally planar surface of the dome, hemisphere or polyhedral surface of the skylight. In this case, hexagons, or other polygons may approximate the circular cross-sections of the entrance apertures by having these circles circumscribe each approximating hexagon so that, like a honeycomb, a contiguous arrangement of the CPDs is enabled for complete coverage or tiling of the surface layer of the skylight by the CPDs, as shown in FIG. 6.

(18) In another embodiment of the invention, the CPDs may rendered in circumferential rings or grooves in skylights with cylindrical or axisymmetry, as shown in FIG. 7, with their cross-section being depicted by FIG. 1. In this embodiment, each CPD is the body of revolution of the 2-D CPC (FIG. 4) about the axis of cylindrical symmetry or body axis of the skylight, shown as 100 in FIG. 1.

(19) FIG. 8 summarizes the advantage of this inventionmodeling the result of optical simulations with sunlight incident from an oblique sun angle10 above horizontal horizononto a section of a skylight layer at a typical slope of 30 to horizontal. The resulting light distribution 803 from typical skylight 801 with typical refracting prisms is very narrow, leading to glare, while the same incident sun rays on a skylight layer with CPCs 802 results in a much broader angular distribution of day lighting 804 provided to wider area below. Thus the current invention is able to provide the ideal skylight function of collecting light from even oblique angles of the sun while providing day lighting from overhead into a broad angle distribution for glare-free, comfortable illumination of a wide area of building interior below.
Similar to current prism structures, these compound parabolic diffuser structures may be rendered in polycarbonate or acrylic plastic, also known by the trademark Plexiglas or by the chemical name poly methyl metha acrylate (PMMA). Acrylic plastics are low-cost, and have been used in rugged applications such as combat aircraft windows in the Second World War and, since they are proven sun UV resistant for years, also used widely in building construction, including in current skylights, greenhouses and pavilions. Although these plastics absorb solar UV radiation, visible light is transmitted with high efficiency, of up to 92%.
Also significant is that such transparent plastics can be easily molded into the desired form factors including skylight panelsthrough injection molding, casting or extrusionin manufacturing techniques known, for example, Jungbecker of Germany or K S Manufacturing/Henry Plastics of San Leandro, Calif. The appropriate grades of sun UV resistant plastic are available in pellet form from common acrylic raw suppliers such as Evonik.

(20) Our skylight design may include more than one layer with such CPD structures, like many double-glazed skylights or double-pane windowsone of which, designated the first or outer layer, with upper side facing the sun outside, and a second or inner layer below with lower side facing the building floor. The gap between the inner and outer layers of the skylight can function to collect air heated by the sunlight in or from near the ceiling of the building that anyway absorbs most of the solar heat that is incident upon the building roof. The advantage provided is that this heat may be vented easily, similar to many current skylights, or the heat kept inside for passive solar heating during cooler months. The skylight layers or panels comprising these layers including the compound parabolic diffuser CPD structures may also be placed on top of a light tube or other tubular daylighting device (TDD).

(21) The exit apertures of the CPDs may be provided with texturing, roughening or other means of diffusing the light rays further.

SUMMARY

(22) A skylight to provide daylighting to building interiors is described that includes a plurality of compound parabolic diffusers into one or more layers that may be dome or pyramid-shaped to enable more efficient collection of sunlight and its distribution through wider angles over wider areas of floor below resulting in more comfortable illumination with less glare and heat gain.