Patent Application
20110197968
The present invention relates to solar energy conversion systems and methods to collect, redirect, and concentrate sunlight to enhance the efficiency of energy production.
There are two main classes of solar energy conversion systems, thermal and photovoltaic. Thermal systems use a thermal transfer medium, such as water to absorb heat from solar radiation, and require a secondary generator to convert the heat into electrical power. Photovoltaic systems generate electrical current directly, and most commonly are comprised of arrays of photodiodes cells made from silicon. Because photovoltaic cells produce electricity directly, they are suitable for both small and large installations and the electrical power can be easily connected to the distribution grid. Thermal systems can be used to heat water on a small scale, but typically are only suitable for large installations for power generation because of the need to convert to electricity using a secondary generator.
Despite impressive improvements in efficiency in the past 30 years, the main drawback of photovoltaic systems is the high cost of silicon photocells. Intending to lower costs, the prior art includes many examples of optical concentrators that capture sunlight from a large aperture, using less expensive materials such as Fresnel lenses and curved mirrors, and focus the light on to a smaller photocell surface. Although the concentration factors can be impressive, focusing optics introduce other problems that are difficult and costly to overcome. There is generally a trade off between the field of view and the concentration factor, which means that high magnification concentrator systems need to incorporate mechanical tracking systems to follow the sun's movement. Furthermore, many such concentrator systems require a specialized form factor of solar cells which makes these systems difficult to integrate with commercially available large format photovoltaic panels.
The prior art also includes a number of examples of sunlight collector systems that do not involve tracking mechanisms, but instead have a wide field of view for capturing sunlight as the sun moves across the sky. In general, they do not concentrate the sunlight to the same extent as systems which track the sun, but are desirable because of the inherent simplicity of a system with no moving parts. However, many of the non-tracking collector systems described in the prior art also require specialized form factors and are not suitable for use with large format solar panels.
In an attempt to further improve the efficiency of photovoltaic solar panels, several companies have developed or have announced they are developing so-called bifacial solar cells that can be illuminated from both sides, including Hitachi, Sanyo, Gamma Solar, Sun Power Corporation, and Origen Energy. Bifacial solar cells can be mounted vertically to receive sunlight directly on the side facing the sun as well as albedo sunlight reflected from the surrounding terrain on the reverse side. Reportedly, the solar collection efficiency can be boosted by as much as 35% compared to a single-sided panel in such a configuration, but the benefit depends on the terrain of the particular installation and the proximity of other panels if a number of them are arranged in a collection array.
Sunlight deflecting panels have also been used to reduce glare by reflecting the sunlight entering buildings at a relatively steep vertical angle towards the ceiling, while allowing light passing horizontally to transmit directly to allow a viewer to see through the window. Although these panels have been demonstrated to be effective for building lighting management, it is not clear if they would be suitable for solar energy applications or how to optimize them accordingly.
Gellert (U.S. Pat. No. 4,074,704) describes the use of a lenticular vertical panel comprised of an array of elongated prisms combined with reflector panels used to capture sunlight at low angles in winter for the purpose of heating a fluid in a pipe, but the system has a limited field of view and does not work efficiently when the sun is higher in the sky.
Butler (U.S. Pat. No. 4,513,734) describes a system of simple plane mirrors used to reflect light on to both sides of a bifacial flat plate photoreceptor, which receives sunlight over a wide field of view, but the efficiency of the collecting aperture is very low. Another example is described by Finkl (U.S. Pat. No. 5,538,563), using simple flat reflectors, but the field of view is restricted and it requires a tracking mechanism to maintain efficiency.
There are a number of examples of transparent panels intended for vertical building windows that incorporate arrays of horizontal reflecting surfaces to reflect overhead sunlight on to the ceiling to prevent glaring sunlight from entering directly while allowing viewers to look through at a near horizontal angle. None of these inventions includes a description of how the panels may be used for the purpose of efficient solar energy conversion, but are relevant because they describe sunlight deflector panels with similar construction to the present invention. Wadsworth describes a transparent panel with thin slots that reflect light by total internal reflection, but the width and draft of the slots would create significant scatter and losses for solar power collection applications. Milner describes an alternative design incorporating two panels with interlocking ridges which could reduce the width of the air gaps to insignificant dimensions.
Thus there is a need for a low cost and effective solar deflecting system to direct the sun's rays efficiently onto conventional large format photovoltaic panels.
This need is met by a solar deflector panel assembly for directing sunlight on to a photoreceptor surface including at least an approximately rectangular solar deflector panel constructed of a transparent material having a length L, a width W, a top surface, a bottom surface, and an inner and outer edge along length L; an array of spaced planar reflecting surfaces extending over the length L of the solar deflector panel substantially parallel to the inner and outer edges, the planer reflecting surfaces being substantially perpendicular to the top and bottom surfaces; an approximately rectangular photoreceptor surface having a top edge and a bottom edge positioned so the top edge of the photoreceptor surface is oriented towards the sky and the bottom edge oriented towards the ground; and wherein the solar deflector panel, is positioned so that the inner edge is held in proximity and approximately level with the top edge of the photoreceptor surface and the outer edge of the solar deflector panel is extended away from the photoreceptor surface.
Another aspect is a solar deflector panel as described above, wherein the solar deflector panel is constructed of a stack of identical elongated strips of transparent material, each strip with parallel top and bottom surfaces and two opposing parallel reflective side surfaces substantially perpendicular to the top and bottom surface, affixed together with the top and bottom surfaces of the elongated strips forming flat top and bottom surfaces of the deflector panel.
Another aspect is a solar deflector panel and photo receptor surface as described wherein the array of reflecting surfaces are tilted with respect to the surface normal of the top and bottom deflector panel surfaces.
Another aspect is a solar deflector panel as described wherein the reflective surfaces are composed of metallic or dielectric coatings.
Another aspect is a solar deflector panel as described wherein the transparent material has a refractive index of greater than 1.414 and the reflecting surfaces are achieved by total internal reflection by providing a small gap between the reflecting. surfaces.
Another aspect is a solar deflector panel as described, wherein separate top and/or bottom sheets of transparent material are laminated and affixed to the stack of the elongated strips.
Another aspect is a solar collection assembly including at least the solar deflector panel described above, and an additional rectangular reflector panel fit between the bottom edge of the photoreceptor surface and the outer edge of the solar deflector panel.
Another aspect is a solar deflector panel and photoreceptor surface as described, wherein the photoreceptor surface is bifacial and an additional solar deflector panel is disposed on the opposite side of the photoreceptor surface.
Another aspect is a solar deflector panel and photoreceptor surface as described wherein the photoreceptor surface is bifacial and an additional solar deflector panel and an additional rectangular reflector panel is disposed on the opposite side of the photoreceptor surface.
Another aspect is a solar deflector panel as described, wherein the solar deflector panel is comprised of a stack of identical elongated strips of transparent material, each strip with parallel top and bottom surfaces and two opposing parallel reflective side surfaces tilted with respect to the surface normal's of the top and bottom surfaces, affixed together with the top and bottom surfaces of the elongated strips forming flat top and bottom surfaces of the deflector panel.
Another aspect is a solar deflector panel and photoreceptor surface as described, wherein the photoreceptor surface is bifacial and an additional solar deflector panel is disposed on the opposite side of the photoreceptor surface.
Another aspect is a solar deflector/reflector panel and photoreceptor surface as described wherein the photoreceptor surface is bifacial and an additional solar deflector panel and an additional rectangular reflector panel is disposed on the opposite side of the photoreceptor surface.
Another aspect is a solar deflector/reflector panel configuration and photoreceptor surface wherein the photoreceptor surface is deployed around one or more cylinders configured within a solar deflector/reflector configuration such as shown in
The present invention involves the use of a thin solar deflector panel to direct sunlight on to a photoreceptor surface such as a photovoltaic solar panel. The photovoltaic panel is held on edge as compared to conventional panels that are generally laid flat facing the sky. The solar deflector is extended outward from the top edge of the photovoltaic panel to present a large collection aperture to the sun, and either transmits or reflects incident sunlight on to the photovoltaic panel. Two deflector panels can be placed on opposite sides of the photovoltaic panel and therefore the present invention is suitable for use with bifacial solar cells. In addition to the solar deflector panel, a secondary reflector panel may also be incorporated into the system to enhance the collection efficiency further, positioned with one edge proximal to the bottom edge of the solar cell panel and the other edge in close contact with the outer edge of the solar deflector panel.
The solar deflector panel is comprised of a transparent material with an embedded array of evenly spaced planar reflecting surfaces, approximately perpendicular to the top and bottom surfaces of the deflector panel, and extending over its length. The panel may be constructed of a stack of identical elongated strips, with rectangular or parallelogram cross-sections, and planar reflecting surfaces at the boundary between each strip. Sunlight is both reflected by the reflecting surfaces and transmitted through the transparent material without deviation, depending on the angle of the incident sunlight. The proportion of light that is reflected or transmitted without deviation varies as the sun moves across the sky such that an optimal amount of light is directed at the photoreceptor.
The aspect ratio of the rectangular cross section of the elongated strips, defined as the ratio of the width divided by the height, can be adjusted to optimize the solar collection efficiency over a very wide field of view and therefore does not need tracking mechanisms to operate effectively. This also means that diffuse light can be captured efficiently, which is particularly important when there is significant obscuration of direct sunlight due to cloud cover. The aspect ratio can be optimized to provide uniform collection efficiency over a wide range of sun angles, or alternatively optimized to produce greater efficiency when the sun is either high in the sky or low in the sky.
In another preferred embodiment, the elongated reflective strips are tilted towards the sky that concentrates the sunlight upon reflection when the sun is high in the sky, at the sacrifice of efficiency when the sun is lower in the sky. In this configuration, concentration factors of up to 7× can be achieved over relatively small fields of view. Such a system would be suitable for use with a mechanical tracking mechanism that follows the sun's movement during the day.
The performance of the deflector panel relies solely on the relative geometry of the deflecting strips and therefore can be scaled to whatever thickness of panel is desired. Thin lightweight panels can be constructed by shrinking the geometry of the deflector strip cross sections, or thicker and more robust panels can be constructed accordingly by making the strip cross sections larger.
It is therefore an object of the invention to provide a thin and lightweight solar deflector panel, constructed of inexpensive materials, which is easy to install and maintain and which can be scaled to large dimensions suitable for use with large format photovoltaic panels.
It is a further object of the invention to provide a solar deflector panel optimized to collect light over a large field of view so that no tracking mechanisms are required. Accordingly, the invention would be suitable for collection of diffuse sunlight. It is a further object of the invention to provide a solar deflector optimized to concentrate the sunlight on to a photoreceptor surface to improve the collection efficiency over a relatively narrow field of view.
It is yet another object of the invention to provide a solar deflector panel which is optimized for use with bifacial photovoltaic panels.
It is a further object of the invention to provide a solar deflector panel that is optimized for use with cylindrical photovoltaic panels.
Solar deflector panel 110 thus contains a set spaced planar reflecting surfaces extending over the length L of the panel. The solar deflector panel shown in
Referring now to
The sunlight is incident on the deflector panels 140 at varying angles throughout the day as the sun traverses the sky, from east to west shown as left to right in numerals 130 and 135. Numeral 130 shows the rays of light from the sun when it is low in the sky during the early morning. A majority of the sunlight is incident upon the eastern solar deflector panel, which transmits a preferentially high amount of the incident light without deviation, indicated by rays 143. These transmitted rays subsequently impinge upon the eastern side of the photoreceptor 145, either directly or upon reflection from the bottom reflector panel, as shown by rays 146. The remaining sunlight 147 incident on the western deflector panel reflects a preferentially high amount of the incident light back towards the western side of the bifacial photoreceptor surface 145. It is obvious with such a symmetric geometry that the same description holds for incident rays during the late afternoon, except that the preferential rays are reversed and the majority of the solar rays are transmitted through the western deflector panel and reflected by the eastern panel, on to the respective sides of the photoreceptor panel.
Numeral 135 of
To further understand the operation of the deflector panel assembly and how to optimize its performance, the following analysis is provided. Referring again to
ηA=cos φs 1.
where φs is the angle of incidence of the sun relative to the ground normal.
Furthermore, the proportion of the sunlight incident on the collecting aperture 2B which impinges on the eastern deflector panel is determined by the following equations:
ηi=½(1+tan θ·tan φs); for |φs|≦90 −θ 2a.
ηi=2/(1+tan θ·tan φs); for φs>90−θ 2b.
θi=0; for φs<−(90−θ) 2c.
where θ is the angle of inclination of the deflector panel 140 relative to the surface of the ground.
Referring again to
ηd=(sin φo·sin θ)/sin(90+θ−φo); for φo>0 3a.
ηd=0; for φo≦0 3b.
where φo is the angle of incidence relative to the ground normal of a ray that exits from the eastern deflector panel with or without deviation.
Furthermore, the proportion of light that passes through the eastern deflector panel and is reflected by the bottom reflector onto the photoreceptor surface is determined by the following equations:
ηm=ηd; for nd≦0.5 4a.
ηm=1−ηd; for nd>0.5 4b.
Referring now to
The proportion of the sunlight incident on the entrance face which passes through the rectangular strip without reflection is determined by the following equation:
ηt=(w−|tan φrr|)/w; for |tan φrr|≦w 5a.
ηt=0; for |tan φrr|>w 5b.
where w is the width of the rectangular strip relative to the thickness of the glass panel t, and w/t is the aspect ratio, and φrr is the angle of the refracted ray relative to the panel surface normal 165.
The angle of the refracted beam can be calculated from Snell's law, as follows:
φi=φs+θ 6.
where φi is the angle of the incident ray relative to the panel surface normal 165. It follows that:
φrr=arcsin(sin φi/n) 7.
where n is the refractive index of the transmitting medium.
The numeral 170 in
ηr=1−ηt; for tan φrr≦w 8a.
ηr=(w/tan φrr)−ηt; for tan φrr>w 8b.
The angle of the exiting reflected ray can be determined again from Snell's Law:
φrf=φrr 9.
φro=arcsin(n·sin φrf) 10.
φR=φro+θ 11.
where, φrf is the angle of incidence of the reflected ray relative to the panel surface normal; φro is the angle of the exiting reflecting ray relative to the panel surface normal; and φR is the angle of the reflected ray exiting the panel relative to the ground normal.
The proportion of the light incident on the eastern panel that undergoes a single reflection, and is incident directly or by reflection from the bottom reflector surface on the eastern side of the photoreceptor panel is determined by substituting φR for φo in equations 4 through 7.
It follows that the total proportion of sunlight which passes through the eastern deflector panel, either without deviation or by a single reflection, and is incident on the eastern side of the photoreceptor surface either directly or by reflection from the bottom reflector, is determined by the following equation:
ηE=ηA·ηi·(ηt(ηdt+ηmt)+ηr(ηdr+ηmr)) 12.
where, ηdt, ηmt, ηdr, ηmr are respectively the relative proportions of light that are transmitted directly, transmitted and then reflected off the bottom mirror, reflected directly, and reflected and then reflected from the bottom mirror.
It also follows from the obvious symmetry of the geometry that a similar set of equations can be obtained for the western side panel and the total amount of light collected by both sides is the sum of the two. Therefore
ηw(φs)=ηE(−φs); and 13.
ηT(φs)=ηW(φs)+ηE(φs) 14.
where ηT is the total efficiency collected by the east and west panels.
The total amount of sunlight collected in the day is normalized by the following equation:
N
eff=2·B·Σ ηT(φs)/Σ cos(φs) 15.
where Neff is the normalized total integrated daily sunlight collected, expressed in the relative number of single sided panels lying horizontally on the ground.
Equations 1 through 15 can be solved for any combination of the parametric variables η, w, and n, as a function of the solar angle φs, to determine the proportion of sunlight incident on the collection aperture that is directed on to both sides of the photoreceptor surface.
The example shown in
In a similar manner the configuration can be optimized for peak efficiencies during the early or late hours in the day when the sun is lower in the sky by increasing the aspect ratio of the rectangular strip cross section as shown in
The calculated strip width aspect ratios for both maximum integrated efficiency and maximum peak efficiency are shown in Table 1, for various panel angles. Referring to the second and third columns, the efficiency improves as the panel angle is reduced from 45 degrees, but it reaches a plateau of 2.11 effective panels at angles less than 30 degrees. Therefore, there would be diminishing benefit to reducing the angle further for the purpose of improving the total integrated efficiency.
The fourth, fifth and sixth columns show respectively the calculated strip aspect ratio corresponding to maximum center efficiency for various panel angles, the corresponding efficiency when the solar angle is zero, and the normalized total integrated efficiency at that panel angle. The preferred panel angles for such optimization are in the range of 30 to 40 degrees, where the normalized total efficiency falls within a range of 1.88 to 1.97. The relationship between the panel angle and the strip cross-section aspect ratio is noted to be very linear, with a slope equal to 0.012 per degree.
For steeper panel angles, the collection aperture previously defined, as 2B for the panel configuration depicted in
ηA=cos φs; for |φs|<(90−θ) 1a.
ηA=cos(|φs|−(90−θ)/(2·cos(θ)); for |φs|>(90−θ) 1b.
The analysis presented thus far has restricted the sun's movement to the equatorial plane that passes directly overhead from east to west as shown in
When the sun deviates from the equatorial plane, as defined by the panel configuration shown in
Referring now to numeral 900 in
Alternatively,
The previous examples of panel configurations have all relied on the use of bifacial photovoltaic panels. The configuration depicted in
Referring now to
(mod eq. 5)
ηt=(w−|(tan δ+tan φrr))/w; for |tan φrr|≦w−tan δ 16a.
ηt=0; for |tan φrr>w−tan δ 16.a
(mod eq. 8)
ηr=1−ηt; for |tan φrr|≦w+tan δ 17a.
φr=|w/(tan φrr−tan δ)|·(1−ηt); for |tan φrr|>w+tan δ 17b.
(mod eq. 9)
φrf=φrr+2·δ 18.
where δ is the tilt angle of the parallelogram strip cross-section.
However, because the tilted reflecting surfaces can cause the reflected ray to exceed the critical angle for total internal reflection at the exit face, an additional boundary condition must be added to equation 17 as follows:
ηr=0; n·sin φrf>1 17c.
Over a large field of view, the benefit of sunlight concentration and improved efficiencies at mid day solar angles is offset by the drop in efficiency due to total internal reflection for solar angles closer to the horizon. However, tilted reflecting surfaces can be useful for certain applications where the field of view is more limited. For shallow panel angles the benefit of tilted strips for the purpose of concentrating the sunlight over a small field of view can be considerable.
The peak efficiency is over 80%, which is the equivalent concentration factor of greater than 6.5 panels. This configuration would be suited for integration with a tracking mechanism, but would still be effective with scattered or diffuse sunlight during overcast weather.
Referring back to
Another embodiment of the present invention is shown generally by the numeral 300 in
The analysis for such a configuration requires further modification to the equations above, as follows:
)modified eq. 2)
ηi =0.5 18.
(modified eq. 4)
ηm=0; for φ<−θ 19a.
ηm=sin(2(θ+φ))·sin θ/(cos φ·cos(θ+φ)) ; for −θ<φ≦0 19b.
ηm=sin(2(θ+φ))·sin θ/(cos φ·cos(θ+φ))−(tan θ·tan φ); for 0<φ≦90−2θ 19c.
ηm=1−ηd; for φ>90−2θ 19d.
(modified eq. 6)
φi=φs
In yet another variation of the preferred embodiment, the photoreceptor panel is oriented vertical relative to the horizontal ground plane and aligned in the east west direction, but with the sun lower in the sky as would be the case in middle northern or southern latitudes. The tilt angle of the parallelogram strips is asymmetric to preferentially transmit light through the side closest the sun on to the photoreceptor and reflect light from the deflector panel on the opposite side of the photoreceptor surface.
The deflector panel and/or the reflector panel can also be curved to improve the collection efficiency.
In the above examples, the optical efficiency of the panel and the bottom mirror were set to 100%, but in real systems the reflectivity of the reflective surfaces and transmittance of the panel will be less than perfect. If good quality mirror coatings are used a reflective efficiency of 96% can be realized, and with good quality anti-reflection coatings on the front and back surface of the panel the Fresnel reflection losses can be reduced to less than 2% per surface. To include the losses in efficiency, equation 12 is modified as follows:
ηE=ηA·ηpt·(ηt(ηdt·ηmt)+ηr·ηpr·(ηdr+ηmr·ηmr))
where npt, npr, nmr are respectively the efficiency of the panel transmission, the panel reflection, and the bottom mirror reflection.
A further embodiment is the case when no bottom reflector is used, which can be analyzed by setting nmr=0.
Considering now the construction of the deflector panel,
The deflector panel may also be constructed from alternative strip profiles that may incorporate other beneficial features such as shown in
As an alternative to assembling multiple strips,
Yet another construction method of the deflector panel would be to cast the panel with embedded thin double-sided reflector strips. The reflector strips could be made from mirror coated polymer strips and held temporarily during the casting process. The casting material could be a UV set polymer resin.
While the present invention has been described in some detail, according to the preferred embodiments illustrated above, it is not meant to be limiting to modifications such as would be obvious to those skilled in the art.
This application claims the benefits of U.S. provisional Ser. Nos. 61/189,092 filed on Aug. 16, 2008 and 61/210,781 filed Mar. 23, 2009.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2009/004668 | 8/14/2009 | WO | 00 | 4/27/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61189092 | Aug 2008 | US | |
| 61210781 | Mar 2009 | US |
