Patent Application
20140345670
The present invention relates to solar panels generally, and to solar panels that feature the ability to passively self-concentrate impinging light on a fixed portion of the underlying photovoltaic (PV) material during varying times of the day particularly, to yield a solar panel that features higher conversion efficiency and incrementally higher output when compared to an equivalent sized conventional solar panel constructed in the same manner and of the same PV material. On an “equivalent power” basis, i.e. when comparing a conventional solar panel and an improved solar panel constructed in accordance with the teachings of the preferred embodiment of the present invention, the improved panel can be constructed at far lower cost and obviates the need for an active tracking system.
Increasing the output per unit area of PV-based solar panels is of vital importance if the goal of reducing man's reliance on consumable carbon-based fuels to generate electricity is to ever be realized. Indeed, in some parts of the world the only electricity available is generated from comparatively small, highly inefficient diesel-powered generators thus leading to the paradox that people living in the poorest locales pay some of the highest prices for electricity. Lowering electricity prices and the resulting benefits flowing therefrom (e.g. light, refrigeration, and access to computer technology) for these individuals is a recognized human need.
Harnessing the power of the sun, and in particular improving the conversion efficiency of photovoltaic (PV) solar collectors, is thus an important societal goal. Thus far, efforts to improve the conversion efficiency of PV solar collectors have fallen into two broad areas: 1) “Endogenous” improvements in the nature of the PV material that comprises the assembly; and, 2) “Exogenous” improvements that deal broadly with methods of causing more sunlight to fall onto the PV material as the Earth daily rotates on its axis and as it yearly rotates around the sun.
In the “endogenous” area, the evolution of PV material has proceeded slowly—from costly single crystal silicon cells, to cheaper polycrystalline cells, through modern thin film cells with multiple layers of different materials stacked one on top of the other to capture photons in multiple energy band gaps. While thin film cells promise the highest conversion ratios they are the most expensive to manufacture. At the present time, conversion efficiencies of approximately 20% at one sun are routinely achievable in monocrystalline and polycrystalline applications, with polycrystalline panels available at installed prices below $1.00 per watt generated.
In the “exogenous” area, efforts have evolved along two lines: 1) Tracking—i.e. mechanical devices that ensure that the panel is optimally positioned with respect to the sun; and, 2) Concentration—i.e. the use of lenses or mirrors to concentrate more light into a smaller area of PV material—thus taking advantage of the fact that within limits, PV materials are more efficient in terms of energy conversion when used in concentrated light. Tracking and concentration techniques are unsurprisingly somewhat costly and thus were of more importance in the past when the underlying PV material was a limiting factor in terms of installed cost. Today, however, with installed prices below $1.00 per watt, most small to medium-sized installations feature simple arrays of low-cost static, non-concentrating panels. Because of this, realizing a means of creating a statically mounted solar panel that incorporates a low cost intrinsic concentrating ability without the need for costly directional control systems would be of great utility.
a is an example of a partially exploded view of a passively tracking, partially concentrating photovoltaic solar panel in which the ridges and grooves that form the profile of each linear cylindrical Fresnel lens comprising a monolithic array of linear cylindrical Fresnel lenses are aligned along the east-west axis and lie parallel to the longitudinal center lines of the strips of photovoltaic material. This figure shows the resulting linear focal zone of concentrated light cast on the underlying photovoltaic material by the southernmost element of the monolithic array of linear cylindrical Fresnel lenses at local A.M.
b is an example of a partially exploded view of a passively tracking, partially concentrating photovoltaic solar panel in which the ridges and grooves that form the profile of each linear cylindrical Fresnel lens comprising a monolithic array of linear cylindrical Fresnel lenses are aligned along the east-west axis and lie parallel to the longitudinal center lines of the strips of photovoltaic material. This figure shows the resulting linear focal zone of concentrated light cast on the underlying photovoltaic material by the southernmost element of the monolithic array of linear cylindrical Fresnel lenses at local noon.
c is an example of a partially exploded view of a passively tracking, partially concentrating photovoltaic solar panel in which the ridges and grooves that form the profile of each linear cylindrical Fresnel lens comprising a monolithic array of linear cylindrical Fresnel lenses are aligned along the east-west axis and lie parallel to the longitudinal center lines of the strips of photovoltaic material. This figure shows the resulting linear focal zone of concentrated light cast on the underlying photovoltaic material by the southernmost element of the monolithic array of linear cylindrical Fresnel lenses at local P.M.
Fresnel lenses have been used with solar collectors and solar cells in various “concentration configurations” for many years. For example, U.S. application Ser. No. 12/036,825 discloses a Fresnel lens as the primary focusing element for concentrating solar radiation on a single multi-junction solar cell. This is a classic solar concentrator, capable of providing a very high solar flux focused on a unitary discrete cell. Numerous other concentrator applications have been constructed throughout the years. As the market for photovoltaic devices has matured, the most common photovoltaic devices deployed are conventional monocrystalline or polycrystalline silicon-based “sheet” panels wherein individual solar cells are connected together into strips and the resulting strips are placed side-by-side into a rectangular panel. The entire assembly is then encased in plastic and typically covered by a sheet of glass. These devices are intrinsically non-concentrating: i.e. the amount of solar flux impinging on the panel approximates 1 sun (1 kW/m2) depending on the time of day and the time of year. These flat panels are usually mounted statically with no means of tracking or otherwise controlling the position of the panel with respect to the position of the sun.
While there has been experimentation combining Fresnel lenses with the types of solar panels now widely available, these experiments have tended to deal with affixing one or more circular Fresnel lenses to standard solar panels. Unknown in the prior art are applications in which a planar cylindrical Fresnel lens is affixed to a flat solar panel. The main reason Fresnel lenses are infrequently used in photovoltaic applications, in general, is that they must be molded or machined from some kind of plastic material. As a result, these lenses are generally unsuitable when exposed to the elements. Obviously, these lenses may be overlaid by glass to increase their viability in exposed applications, but this decreases the optical transmittance of the resulting assembly and attenuates the light provided to the surface of the photovoltaic element by at least 8% (4% per surface of the glass overlay). While such an assembly may, in theory, be constructed, recent technical advances have made it possible to mold Fresnel lenses out of glass, providing at one stroke a lens that is both durable and features intrinsically high transmittance at visible and infrared wavelengths. Such a lens is disclosed in U.S. patent application Ser. No. 13/047,768 (Fresnel Lens Array With Novel Lens Element Profile). Such lenses are far more suitable for applications involving solar panels than their plastic counterparts. Thus, the goal of the present invention is to provide a monolithic array of planar cylindrical Fresnel lenses that are used as a covering element for linear strips of photovoltaic material and serve to increase the conversion efficiency and electrical output of the underlying linear strips of photovoltaic material thus providing a solar panel with greater output and far lower construction costs when compared to conventional solar panels.
In a first, preferred, embodiment, a monolithic array of linear cylindrical Fresnel lenses is mounted such that both the ridges and grooves that form the profile of the lenses and the underlying strips of photovoltaic material are aligned east to west such that the linear focal zone of concentrated light each lens generates falls longitudinally along the center longitudinal line of a strip of photovoltaic material and appears to remain motionless as the sun traces its daily path along the ecliptic. In a second embodiment, a monolithic array of linear cylindrical Fresnel lenses is mounted such that the ridges and grooves that form the profile of the lenses are aligned east to west and the strips of photovoltaic material are aligned north to south such that the linear focal zone of concentrated light each lens generates falls perpendicularly across a multiplicity of strips of photovoltaic material and appears to remain motionless as the sun traces its daily path along the ecliptic. In a third embodiment, a monolithic array of linear cylindrical Fresnel lenses is mounted such that both the ridges and grooves that form the profile of the lenses and the strips of photovoltaic material are aligned north to south such that the linear focal zone of concentrated light each lens generates falls longitudinally along a strip of photovoltaic material and appears to move from west to east, possibly transitioning to a neighboring strip, as the sun traces its daily path along the ecliptic. In a fourth embodiment, a monolithic array of linear cylindrical Fresnel lenses is mounted such that the ridges and grooves that form the profile of the lenses are aligned north to south and the strips of photovoltaic material are aligned east to west such that the linear focal zone of concentrated light each lens generates falls perpendicularly across a multiplicity of strips of photovoltaic material and moves from west to east as the sun traces its daily path along the ecliptic. In addition, it will be apparent that an infinite number of other configurations are possible in which a monolithic array of linear cylindrical Fresnel lenses is mounted such that the linear focal points of concentrated light they generate fall across one or more strips of photovoltaic material at some angle between a line parallel to the longitudinal center lines of the photovoltaic strips and a line laying at right angles to the longitudinal center lines of the photovoltaic strips.
Generally, the first and second embodiments are favored because they have a much higher intrinsic solar acceptance angle—almost the full 180° arc that the sun traces daily across the sky. By contrast, the third and fourth embodiments are disfavored because the useful solar acceptance angle is low—perhaps only plus or minus 25° from the point at which the sun is highest on the ecliptic (i.e. local noon when the sun's rays are perpendicular to the plane of the solar panel). This is because linear cylindrical Fresnel lenses are incapable of providing a sharply focused line of concentrated sunlight at greater input angles (i.e. nearer each horizon). Of the first embodiment and the second embodiment, however, the first embodiment is preferred, because each strip of photovoltaic material may be placed at any necessary distance from its neighbor to ensure that each photovoltaic strip is evenly illuminated along its entire length. By this means, when used in areas with continuous, uninterrupted sunlight, photovoltaic strips may be omitted from the surface of the panel thus lowering manufacturing costs. Alternately, in areas with more diffuse sunlight, the entire surface of the panel may be covered with strips and the entire panel will perform like a conventional panel in similar conditions. However, when the sun does shine, the panel provides increased electrical output compared to a conventional panel because some fraction of the photovoltaic strips are illuminated by concentrated sunlight. In the second embodiment, however, in continuous, uninterrupted sunlight, a multiplicity of transverse portions of each photovoltaic strip are brightly illuminated and a multiplicity of transverse portions of each photovoltaic strip are poorly illuminated. As a result, the entire photovoltaic strip operates at a very low output. Only in areas with more diffuse sunlight does the output of the photovoltaic strip (and panel) equal the output of a photovoltaic strip (and panel) in the first embodiment. As a result, while the latter three embodiments are disclosed, they will not be discussed in detail.
When constructed and installed in accordance with the teachings of the first, preferred, embodiment, improved conversion efficiency and electrical output result because at any one time some longitudinal linear portion of an entire photovoltaic strip is exposed to concentrated sunlight. Since the earth rotates on its axis east to west, the linear focal zones of concentrated sunlight appear to remain motionless along each strip of photovoltaic material until the sun sets and rises the next morning whereupon the linear focal point of concentrated sunlight reappears at essentially the same location as the day before and the cycle repeats.
Solar panels constructed in accordance with the teachings of the first, preferred, embodiment are said to be “partially concentrating” because only a portion of the surface of the solar panel is exposed to concentrated sunlight. As a result, only those areas of the surface of the solar panel need have photovoltaic materials affixed. Also, because of its intrinsically high solar acceptance angle, such panels provide an essentially static area of concentrated sunlight during all times of the day. While this static area of concentrated light does move slowly north and south throughout the seasons as the Earth rotates around the sun, such a solar panel can be constructed with photovoltaic strips wide enough that some portion of each photovoltaic strip is subjected to concentrated light throughout the year. Because of this, such panels are said to be “passively tracking” in both azimuth and elevation. Also, because the monolithic array of cylindrical Fresnel lenses concentrates light on the underlying photovoltaic material only in a range between greater than 1 and 10 suns (>1 kW/m2 and 10 kW/m2) the heat flux in the photovoltaic material is kept to a minimum. In most applications (with a solar flux of less than about 3 suns [3 kW/m2]) active cooling systems are thus not required. Finally, when used in a cloudy or hazy environment a solar panel constructed in accordance with the teachings of the present invention generates an electrical current equal to that generated by a solar panel with an equivalent area of the same photovoltaic material and a planar glass or plastic covering.
Referring now to
Each strip of photovoltaic material 112a, 112b, and 112c is constructed of a multiplicity of unitary solar cells lying next to each other in a row and electrically connected in parallel. In other words, the positive output of one cell is wired to the positive output of its neighboring cell(s) and so on. In this preferred embodiment of the present invention, the strips of photovoltaic material 112a, 112b, and 112c are evenly spaced across the base plate 111 of the solar cell assembly with gaps 113 interposed between the strips to form a rectangular photovoltaic panel. This rectangular photovoltaic panel is supported by rectangular support structure 119. Rectangular support structure 119 may be made of any rigid or semi-rigid material capable of supporting the affixed solar panel and maintaining its planar configuration. Rectangular support structure 119 may be constructed integrally with the rest of the components of the solar panel, or, in the alternative it may be constructed separately as part of the frame to which the solar panel is mounted. In low solar flux applications—ranging between 1 sun (1 kW/m2) and 5 suns (5 kW/m2) inclusive—rectangular support structure 119 is ideally constructed of materials that feature intrinsically high thermal transmission characteristics to provide an integral passive cooling mechanism for the overlying strips of photovoltaic material 112a, 112b, and 112c. In these applications, rectangular support structure 119 may include, for example, cooling fins or projections to increase surface area and thus the ability to passively cool the overlying solar panel. For some intermediate and higher solar flux applications—above about 3 suns (3 kW/m2)—rectangular support structure 119 may include an active cooling system including without limitation: 1) Forced atmospheric air; 2) Forced gas; 3) Pumped cooling liquids; 4) Compressed refrigerant; or, 5) Fan cooled heat pipes or projections.
Supporting monolithic array of lenses 110 above base plate 111 and strips of photovoltaic material 112a, 112b, and 112c are lateral walls 114 through 117. Lateral walls 114 through 117 extend up perpendicularly from the edge of the top surface of rectangular support structure 119 and down perpendicularly from the edge of the bottom surface of array of lenses 110 and are mirrored on their inside aspects such that sunlight impinging on may be cast back into the solar panel and onto strips of photovoltaic material 112a, 112b, and 112c. For example, referring specifically to
The present invention takes advantage of the fact that, within limits, most photovoltaic material exhibits a marginally higher conversion efficiencies when exposed to concentrated sunlight. In other words, when exposed to a solar flux of 5 suns (5 kW/m2), a piece of conventional photovoltaic material generates slightly more than five times the power of an identical piece of photovoltaic material exposed to a solar flux of 1 sun (1 kW/m2). See generally, S. M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 724-725 (3d ed., John Wiley & Sons 2007). As a result, by concentrating sunlight on the underlying photovoltaic material, it is possible to both: 1) Reduce the amount of photovoltaic material necessary to construct a solar panel that generates an equivalent amount of power in direct proportion to the increase in solar flux; and, 2) Reduce the amount of photovoltaic material somewhat further still because the photovoltaic material used is operating somewhat more efficiently. However, as the amount of solar concentration increases, the thermal flux applied to the photovoltaic material increases until the conversion efficiency drops off and ultimately, the photovoltaic material is destroyed. Id. Thus, under intermediate to higher solar concentrations (above about 3 suns (3 kW/m2), it is preferable to use some type of active cooling system to prevent the thermal flux from becoming too high.
Of course, constant direct sunlight is not always available in even the sunniest locations. Conventional non-concentrating solar panels function poorly in applications where clouds are prevalent simply by virtue of the fact that comparatively less sunlight impinges on the underlying photovoltaic device. Advantageously, the present invention performs just as well as a conventional non-concentrating panel in overcast conditions. The diffuse Lambertian sunlight transiting the array of linear cylindrical Fresnel lenses emerges no less attenuated than it would have if a covering of glass or plastic was used instead. As a result, when used in a cloudy or hazy environment, a solar panel constructed in accordance with the teachings of the present invention generates an electrical current equal to a solar panel with an equivalent area of the same photovoltaic material and a planar glass or plastic covering.
Referring now to
For example in
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Of course, the seasonal change in declination of the Earth's polar axis relative to the plane of the ecliptic causes the linear focal zones generated by each one of the cylindrical Fresnel lens comprising monolithic array of lenses 110 to move slowly north and south throughout the year. For example, on the equator at Quito, Ecuador the apparent elevation of the sun at its zenith can vary between approximately 68° north and 112° south with the sun appearing directly overhead twice (at the northward and southward equinoxes). Assuming, for example, a solar panel as described above in [0032], a 2.0 cm wide focal zone 121 moves approximately 2.6 cm north to 2.6 cm south of the longitudinal center line of each of photovoltaic strips 112a, 112b, and 112c during the year. Such a solar panel installed in Quito, Ecuador and pointed at the location of the sun at its zenith at the northward and southward equinoxes, focal zone 121 will be longitudinally centered on photovoltaic strip 112a throughout the day and linear focal zone 121 will be illuminated with a solar flux of approximately 4.0 suns (4 kW/m2). The remaining 1.0 cm strip on either side of focal zone 121 is subjected to Lambertian diffuse solar radiation. However, on the day of the northern solstice, the longitudinal center line of focal zone 121 will have moved 2.6 cm south of the longitudinal center line of photovoltaic strip 112a and focal zone 121 will extend off of photovoltaic strip 112a by approximately 1.6 cm. Similarly, on the day of the southern solstice, the longitudinal center line of focal zone 121 will have moved 2.6 cm north of the longitudinal center line of photovoltaic strip 112a and focal zone 121 will extend off of photovoltaic strip 112a again by approximately 1.6 cm.
Referring now to
1) Exposed to a solar flux of 1 sun (1.0 kW/m2);
2) Three 7.2 cm wide strips of polycrystalline photovoltaic material;
3) Each strip 100 cm long;
4) With a conversion ratio of 15%; and,
5) A resulting theoretical maximum output of 32.4 W.
While both solar panels contain the same amount of photovoltaic material (2,160 cm2) the improved solar panel does not require an active tracking mechanism to keep it pointed at the sun throughout the seasons. On a constant power basis (i.e. both solar panels generating the same 38.4 W theoretical maximum output as the improved panel), a conventional solar panel requires much more photovoltaic material. Specifically, to create a conventional solar panel that generates a theoretical maximum output of 38.4 W, approximately 19% more photovoltaic material is required (˜2,550 cm2 versus ˜2,160 cm2).
While the present invention is intended to be used in applications with no dynamic tracking (i.e. fixed statically pointing in one direction), those having skill in the art will recognize that by means of a single axis tracking system the amount of photovoltaic material required can be reduced dramatically. Specifically, if a single axis tracking system adjusts for variations in the sun's elevation above the horizon throughout the seasons, it is possible to create an improved solar panel with a theoretical maximum output of 38.4 W using 2.0 cm wide strips of photovoltaic material versus a conventional panel requiring 8.5 cm wide strips. Such an improved solar panel equipped with a single axis tracking system requires approximately 76% less photovoltaic material than a conventional solar panel (600 cm2 versus 2,550 cm2).
As discussed above, to lower construction costs, photovoltaic strips 112a, 112b, and 112c are preferably configured with empty zones 113 lying between them. It will be evident that it is possible to install additional photovoltaic strips in these empty zones. By this means, a solar panel may be constructed with a maximal area of photovoltaic material that generates maximal power output in cloudy or overcast conditions, yet returns to its normal, higher output, partially concentrating mode of operation when the sun is bright. For example, when the sun is bright, photovoltaic strips 112a, 112b, and 112c operate in concentrating mode while the photovoltaic strips installed in empty zones 113 are exposed to Lambertian diffuse solar radiation. It will be evident that in this configuration at least two power management systems will be required: One for photovoltaic strips 112a, 112b, and 112c (if all the same size and all wired in parallel) and one for photovoltaic strips installed in empty zones 113 (if all the same size and all wired in parallel).
While, the present invention has been described in what is considered to be the most practical and useful configuration, those having skill in the art will recognize that by rearranging the various elements of the present invention, solar panels can be constructed in an almost infinite variety of sizes and thicknesses using Fresnel lens elements of different widths and focal lengths. Some of these arrangements yield exceptional efficiencies. For example, a theoretical solar panel approximately 1.0 m long, 24.0 cm wide, and 2.5 cm thick comprised of a grooves-out monolithic array of linear cylindrical Fresnel lenses each with a width of 4.0 cm and a focal length of 2.0 cm equipped with 6 strips of photovoltaic material 2.3 cm wide centered 1.7 cm below the monolithic array of linear cylindrical Fresnel lenses would focus a theoretical maximal solar flux of ˜3.9 suns (˜3.9 kW/m2) over its 6, 10 mm wide linear focal zones while functioning year-round as a passively tracking, partially concentrating solar panel. Such a panel would have ˜1380 cm2 of photovoltaic material and would generate a theoretical maximum output of ˜37.4 W (plus any power generated by the additional ˜1080 cm2 of photovoltaic material illuminated by diffuse Lambertian sunlight). A conventional panel with ˜1380 cm2 of photovoltaic material would generate ˜20.7 W.
Moreover, while the present invention has been described in accordance with a preferred embodiment featuring a particular type of lens profile, it will be readily apparent that other lens profiles offer equivalent levels of functionality and that all such combinations of lens profiles are included in the spirit and scope of the present application. For example, U.S. Pat. No. 3,991,741 discloses a roof mount solar collector featuring an array of linear cylindrical Fresnel lenses. While this invention requires an active steering system to maintain the precise location of each focal line so that that it directly impinges on a particular solar collector throughout the day as the sun as it traverses the sky, it will be understood that when such lenses are used in conjunction with the teaching of the present invention to construct a photovoltaic panel, such a steering system is unneeded. Clearly, while not all such lens profiles and equivalent combinations are identified herein, it is intended that all such lens profiles and combinations are included within the spirit and scope of the present disclosure.
Also, while the present invention has been described in accordance with a preferred embodiment featuring a particular type of solar collector, i.e. a solar cell, it will be readily apparent that other types of solar collectors may be substituted. For example, a solar thermal panel in which the photovoltaic material is replaced by solar thermal collectors of the same size and in the same physical location as the photovoltaic collector they replace may be manufactured with minimal variation of the technologies and geometries discussed above. Such thermal collectors may comprise a multiplicity of spaced conduits connected end-to-end and filled with a conveying medium such as fluid or gas that is circulated by means of transfer system such as a pump, compressor, or other arrangement. The transfer system is used to move collected heat to an energy sink such as a thermal reservoir or to a point of application. The principle advantage of the partially concentrating panels of the present invention relative to standard fixed thermal solar collectors is that even though the same amount of thermal energy is collected by both, the partially concentrating effect of the present invention allows considerably higher temperatures to be achieved at the point of the thermal collector and consequently higher temperatures may be achieved in the conveying medium. As a result, higher temperatures in the thermal sink are achieved. This has several benefits. For example, higher temperatures in a thermal reservoir allow a greater amount of energy to be stored for later utilization when the sun is not shining. Similarly, higher temperatures at the point of utilization enable industrial processes not achievable with standard low temperature panels. Of course, even higher temperatures may be achieved when single axis or dual axis steering systems are used. Those having skill in the art will note that while not all equivalent combinations of thermal collectors or thermal sinks are identified herein, it is intended that all such thermal collectors and thermal sinks are included within the spirit and scope of the present disclosure.
The present application takes benefit of U.S. Provisional App. 61/649,934 filed May 22, 2012 and incorporates it in its entirety by reference. The present application incorporates prior U.S. patent application Ser. No. 13/047,768 (Fresnel Lens Array With Novel Lens Element Profile) in its entirety by reference.
