Skylight

Abstract

An improved skylight is described that includes a plurality of compound parabolic concentrator like structures into the functional layers of the skylight to provide more visual light transmittance with less solar heat gain—more light with less heat—to wider areas of building interiors.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 62/435,978 filed 2016 Dec. 19 by the present inventor.

BACKGROUND OF THE INVENTION—PRIOR ART

About quarter of all electricity use in the U.S. is for lighting in buildings. While the indoor lighting need in homes or offices is about 500-1000 lux or lumens per square meter—about the level available outdoors at sunrise or sunset—sunlight when incident from directly overhead at noon can provide up to 100000 lux. Skylights can reduce lighting costs by providing daylighting to homes and offices but when installed on commercial roofs for instance they are typically limited to cover less than 5% of the roof area, since to add more also increases solar heat load to the building interior. Most of the available solar power, up to 1000 Watts per square meter, is then wastefully absorbed as heat in building exteriors, even with artificial lighting turned on inside for much of the day. Since cooling and ventilation costs are about 20% of such building energy usage the current tradeoff is between savings in lighting and HVAC costs.

The main challenge for designing efficient skylights is then to maximize the light provided over widest floor area below while minimizing heat gain to the building. In common terminology, this is to maximize visual light transmittance—VLT (or VT) of the skylight while minimizing the fraction of the sun's energy incident on the skylight that is absorbed as heat in the building interior, typically characterized by the solar heat gain coefficient—SHGC.

Sunlight is incident as an intense beam of nearly parallel rays, with small angular spread of only about 0.5°. The area of an opening or aperture which intercepts a beam of solar rays is proportional to the cosine of the angle of incidence and therefore the solar power per unit area is maximum at normal incidence (0°) and minimized at oblique angles. This amount of solar radiation per unit area affects how much is seen as comfortable illumination versus how much is absorbed as uncomfortable heat. This is why noontime in summer, when the sun is high up in the sky, feels hotter than when the same sun is lower in the sky during a winter afternoon. To maximize daylighting for comfortable building illumination while minimizing solar heat gain from intense glare of direct sunlight the ideal skylight must collect as much light as possible from all the angles of the sun but directing the light downwards as a gently diverging or distributed beam to a wide area below.

As is well known, the sun's daily path across the sky, from the eastern to the western horizon, also shifts with the yearly seasons. At equinox—March or September—the sun's track is tilted from zenith (directly overhead) at a solar altitude angle equal to the local latitude (38° towards the south in San Francisco, for example). Over the seasons, this sun's daily arc shifts ±23.5° about the equinox track, from south in December to north in June. To maximize visual light transmittance and minimize solar heat gain—more light with less heat—the ideal skylight must not only collect light efficiently from the low angles of a winter morning but also spread out over a wider floor area the more intense sunlight of a summer noon.

Current skylights, such as the pyramid or dome skylights from SunOptics or Solatube [ Reference 1] include refracting prisms that are designed as a compromise: they admit more light from the sun at oblique angles such as during early or late in the day or in winter with less light being allowed in from high angles of the sun as during mid-day in summer. This avoids a narrow, intense light glare and heat from the sun when it is high in the sky but also means that up to 50% of the available light is lost through total internal reflection (TIR) from the lower surface of the prisms. Sunlight enters through the first entrance surface or top surface of the skylight and exits the medium, after refraction (or total internal reflection) through such prisms, through a second or bottom surface of the skylight. In refraction, light is deflected according to Snell's law by an angle sin γ=n sin γ′ where γ′ is the angle to the normal in the medium with refractive index n. For materials like glass, acrylic or polycarbonate plastics, n is around 1.5, which means that for sufficiently small angles, 30° for instance, are deflected to about 20°, or by about one-third. However, this deflection is reversed when the refracted ray is transmitted from the refracting medium back into air, so there is no net deflection of the ray unless the second exiting surface is at an (acute) angle relative to the first entrance surface. This angle cannot exceed ˜45° (the critical angle at refractive index of 1.5 is about 42°) to avoid total internal reflection TIR; since the seasonal variation of the sun's altitude angle is around 46°, the daily variation being even larger, this means that much of incident sunlight is not collected or optimally distributed through skylights using refractive prisms. This limitation of skylights with refracting prisms applies even when the lower surface of the prisms is curved, with varying slope, to enable a divergence of angles in the light beam transmitted to the building floor below. Such skylights not optimized to admitting light when it is most abundant instead only provides a surface for undesirable heat transfer from the hotter air outside, increasing cooling costs.

Instead of losing light through total internal reflection we propose, in the present invention, as in an optical fiber, to collect, guide and distribute light more efficiently than available in the current art. With any optical structure or element used for radiative transfer—to collect and transmit light—it is long recognized that a basic quantity, 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 times the solid angle the source subtends as seen from the pupil. 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 solid angle of rays at the exit aperture. This problem has been studied, for the optimizing of collection and concentration of radiation, including solar power, in the field of non-imaging or anidolic optics [References 2-4] and a common solution described there has been to use compound parabolic concentrators or CPCs for this purpose. Compound parabolic concentrators are 3-dimensional structures comprised of the inner reflecting surfaces of the surface of revolution generated by the sections of two parabolas with each passing through the focus of the other. As with an optical fiber that is ubiquitous in modern telecommunications, light rays entering the entrance aperture are reflected on the inner surfaces through total internal reflection TIR if the CPC is filled with a material whose refractive index exceeds ambient air. Such 3-D CPCs are characterized by an acceptance angle, θ, and if filled with a medium of refractive index n, the maximum theoretical concentration achieved of rays incident at the entrance pupil is (n/sin θ)²)—the ratio of the area of the entrance to the exit apertures—and equivalently, the maximum achievable divergence of rays at the exit aperture.

DESCRIPTION OF THE INVENTION AND DETAILS OF AN EMBODIMENT

The primary innovation disclosed in this patent is to propose optical structures—compound parabolic concentrators—that when included in functional skylight surfaces or layers provide for more efficient collection and distribution of the sunlight over the widest possible building floor area 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. When curvature, or slope, is included on the reflecting interface (optical material—air), 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 material—air (e.g. plastic-air) interface.

A typical dimension aspect ratio of a CPC acceptance angle of 45°—these dimensions—exit aperture diameter: entrance aperture diameter:length of CPC of 1.25:2.7:3.75—in millimeter scale these are not too different from common prisms already rendered in skylight surfaces and therefore presents little challenge for large scale manufacturing.

We propose to replace the prisms in current skylight panels with structures that resemble compound parabolic concentrators. In particular, optical structures that use total internal reflection to deflect sunlight towards floor and with a curved surface to provide angular spread in such reflected rays resulting in a divergent beam for the space below. We note that similar design concepts are deployed in luminaires or lighting using LEDs, to provide uniform, comfortable illumination from spatially and angular localized light emitting chips. Although skylights including a single CPC have been proposed in a large [Reference 5], this is a bulk product that does not render itself to achieving low material or materials costs for installation on small-scale roofs. The present invention is original, non-obvious in being the first to propose including many, a plurality of the CPC-like structures into the functional surfaces of the skylights so that they can provide efficient light collection over the entire sun-path during the day and yearly seasons. A schematic lateral cross-section of such a skylight, including such CPCs structures into its surface layers is shown in FIGS. 1 & 2. A typical embodiment of such a skylight may have a dome like shape with a diameter of between 30-100 cm covering the skylight opening, with CPCs 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. Similar to current prism structures, these CPC-like structures may be rendered in plastic panels comprised of 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 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 panels—through injection molding, casting or even extrusion—in 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.

Our skylight design, like many ‘double-glazed’ skylights or double-pane windows may include two or more layers—one 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. At least one of these layers will include optical structures similar to the CPCs such the direct parallel beams of sun rays incident from a wide range of angles will be collected and then distributed into a broad divergent lighting pattern to the areas below. 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 concentrator CPC structures may also be placed on top of a light tube or other tubular daylighting device (TDD) which have the advantage of allowing less heat transfer than skylights with less surface area.

The CPCs are a surface of revolution, having cylindrical symmetry, and cross section that is circular. Circles cannot completely cover or tile a planar or polyhedral surface, so the circular cross-section may be approximated by polygons, including hexagons, that, like a honeycomb, completely tile a planar or polyhedral surface of panel. If a hexagonal pattern were chosen then the CPCs shall be rendered with six faces for ease of manufacturing. The CPCs may also be rendered in rings in skylights with cylindrical or axisymmetry. The entrance and exit apertures of the CPCs may be provided with texturing, roughening or other means of diverting or diffusing the light rays to provide more divergence or angular spread to the rays at incidence or when exiting the structure to illuminate wider areas below.

SUMMARY

An improved skylight is described that includes a plurality of compound parabolic concentrator like structures into the functional layers of the skylight to enable more visual light transmittance with less solar heat gain—more light less heat—to wider areas of building interiors.

DRAWINGS—FIGURES

FIG. 1—Schematic of the diametrical section through a cylindrical symmetric skylight

101—the skylight functional layer, a dome in this case, covering the skylight opening

102—the skylight frame and curb

103—roof membrane or structural part of building envelope

FIG. 2—showing detail 1-1 of FIG. 1

104—showing multiple compound parabolic concentrator structures on the inner surface of the functional layer of the skylight

105—a single compound parabolic concentrator structure, typically small dimension˜few mm in size in a skylight about 100 cm wide

106—opening in first frame including an adjustable valve, chimney or linear gap which is means to ventilate to exterior the heated air below first frame

REFERENCES

• 1. U.S. Pat. No. 7,546,709 & U.S. Pat. No. 8,132,375 & U.S. Pat. No. 8,132,375 Solatube
• 2. Radiative Transfer—S. Chandrasekhar (Dover, 1960);
• 3. Principles of Optics—M. Born & E. Wolf, 5^th edition, (Pergamon Press, Oxford, 1975)
• 4. High Collection Non-Imaging Optics—W. T. Welford & R. Winston (Academic Press, New York, 1989)
• 5. U.S. Pat. No. 8,745,838—Replex Mirror Co.

Claims

1. A skylight to provide daylighting to building interiors

including one or more layers,

each layer comprising a panel or plurality of panels,

said panels including a plurality of optical structures which deflect and transmit light through total internal reflection on curved interfaces, examples of such optical structures including compound parabolic concentrators

whereby incident sunlight is substantially directed in a divergent beam distributed towards the building floor.

2. The skylight of claim 1 with said panels being disposed into a polyhedral, hemispherical, parabolic, paraboloidal, pyramid, dome or other similar shape

3. The skylight of claim 1 with said panels comprised of transparent materials including plastics with examples including acrylic, poly methyl metha acrylate, polycarbonate or glasses including tempered low-iron glass.

4. The skylight of claim 1 with the said optical structures having lateral cross-section with a polygonal pattern, examples including hexagonal or honeycomb-like, said pattern chosen for a more complete tiling or coverage of the surfaces of the said panels.

5. The skylight of claim 1 comprising at least two layers including an outer layer located toward the building exterior and an inner layer located toward the building interior,

means provided for adjustable venting to building exterior of heated air between the said layers or from proximity of building envelope whereby solar heat gain to building interior becomes adjustable.

6. The skylight of claim 1 with said panels provided with surface texturing or roughening whereby the diffusion and distribution of light into the building interior is maximized.

7. The skylight of claim 1 including means of thermal insulation provided to one or more surfaces not directly facing the sun whereby solar heat gain to building interior is minimized.

8. The skylight of claim 1 including structural or supporting members for said panels comprising one or more said layers whereby fall protection is provided.