Patent Application
20120167949
Solar radiation is an abundant energy source. However, attempts to harness solar power on large scales have so far failed to be economically competitive with most fossil-fuel energy sources.
One reason for the lack of adoption of solar energy sources on a large scale is that fossil-fuel energy sources have the advantage of economic externalities, such as government subsidies including low-cost or cost-free pollution and emission. Another reason for the lack of adoption of solar energy sources on a large scale is that the solar flux is not intense enough for direct conversion at one solar flux to be cost effective. Solar energy concentrator technology has sought to address this issue. For example, solar radiation energy is easily manipulated and concentrated using refraction, diffraction, or reflection to produce solar radiation energy having many thousands of times the initial flux. This can be done using only modest materials such as refractors, diffractors and reflectors.
With so many possible approaches, there have been a multitude of previous attempts to implement low cost solar energy concentrators. So far, however, solar concentrator systems cost too much to compete unsubsidized with fossil fuels, in part because of large amounts of material and large areas that that solar collectors occupy. The large amounts of materials used to make solar concentration systems and the large areas that are occupied by solar concentration systems render solar concentration systems unsuitable for large-scale solar farming.
Attempts at reducing the amount of materials used in solar concentration systems and the large areas that they occupy include using flat reflective films that assume a smooth concave shape under inflation pressure. Thus in certain approaches, inflation air may be used to impart a curved profile to a reflective component of a concentrator for a solar collector structure. Such inflatable solar concentrators may offer certain benefits over conventional concentrator designs because they employ common structural elements and therefore help in reducing cost of the solar concentrator. Additionally, since inflatable concentrators use air as a structural member, lower cost thin plastic membranes (here referred to as films) can be used as a primary reflector. This can yield significant weight advantages in a system deployed in the field. The weight advantages in the concentrator itself can in turn reduce the complexity of structures used for mounting and tracking systems, thereby reducing the overall mass of the system and hence its cost.
Inflatable concentrators can be more cost effective, but inflatable concentrators are subject to the shape the inflation pressure produces, which can produce non-uniform concentrated light as compared with non-inflatable concentrators. In particular, the shape of the inflated primary mirror may result in an irregular illumination profile on the receiver. This irregular illumination profile in turn may yield lower efficiency of the solar receiver and overall lower system efficiency.
Accordingly, there exists a need in the art for improved methods for optimizing reflectance profiles of concentrator designs while maintaining low cost.
Embodiments of the present invention include structures and methods that enhance irradiation uniformity from solar concentrators, and in particular, inflatable solar concentrators. Certain embodiments may use features formed on a reflective layer of the solar concentrator to globally correct for deviation in reflective behavior from a desired shape. According to one example, the features may correct reflective behavior of a Hencky-type surface of an inflated thin film, to match a specific desired surface that creates a specific desired reflected light distribution across a receiver. In some embodiments, local features such as facets may be formed on a reflective layer, such that the resulting illumination profile represents a superposition of multiple facets. The latter approach minimizes non-uniform illumination resulting from shading. In certain embodiments, the features may be formed by embossing a film. In some embodiments features may be formed on a front film to correct the reflectance of a back film. In certain embodiments the features may correct the profile of point focus concentrators or line focus concentrators. Global and local correction techniques may be used together, and may be used on front film or reflective film(s) together or separately, on point focus or line focus systems. Global and/or local correction may also be used in combination with other approaches, such as secondary optic receiver compensation.
Some embodiments of the present invention provide an apparatus. The apparatus comprises a reflective solar light concentrator having a physical shape and a feature formed in or on the reflective solar light concentrator to match optical behavior of the physical shape to optical behavior of a desired shape. The apparatus may also include an upper transparent portion that allows light to penetrate and reach the reflective solar light concentrator.
Other embodiments of the present invention provide an apparatus comprising an upper transparent portion that allows light to penetrate and a lower portion coupled to the upper transparent portion. The lower portion may include a reflective concentrator that has a physical shape and reflects the light that penetrates the upper transparent portion. The apparatus may also include a feature formed in or on the reflective concentrator that is configured to modify an optical characteristic of the physical shape to be substantially similar to an optical characteristic of a desired shape.
Certain embodiment of the present invention provide a method including forming a reflective solar light concentrator having a physical shape and forming a feature in or on the reflective solar light concentrator to modify an optical characteristic of the physical shape to be substantially similar to an optical characteristic of a desired shape. The method may further include forming an upper transparent portion that allows light to penetrate and reach the reflective solar light concentrator and coupling the upper transparent portion to the reflective solar light concentrator.
Some embodiments of the present invention provide a method that includes forming an upper transparent portion that allows light to penetrate, forming a lower portion comprising a reflective concentrator that has a physical shape and that reflects the light that penetrates the transparent portion, and forming a feature in a reflective concentrator to modify optical behavior of the physical shape to match optical behavior of a desired shape. The method further includes directly embossing the feature onto a film and adding a reflective material to the film after the embossing to form the reflective concentrator. In some embodiments, the method includes adding a material to a film, embossing in the material, and adding a reflective material to the film to form the reflective concentrator. In still other embodiments, the method includes embossing in a material, forming a reflective film from the material, and adding another material to the reflective film to form the reflective concentrator. In yet another embodiment, the method further includes embossing a material to form a embossed material, adding another material to the embossed material, and forming a reflective surface using the embossed material after adding the other material to form the reflective concentrator.
Certain embodiments of the present invention provide a method that includes providing an optic element having a shape, measuring a reflectance profile of the optic element, comparing the measured reflectance profile with a desired reflectance profile, and modifying the shape of the optic element to generate a modified optic element having the desired reflectance profile.
Certain embodiments of the present invention provide a method that includes measuring the reflectance profile of the modified optic element, determining whether the reflectance profile of the modified optic element is substantially similar to the desired reflectance profile, and upon determining that the reflectance profile of the modified optic element is not substantially similar to the desired reflectance profile, further modifying the optic element.
Certain embodiments of the present invention provide a method that includes transmitting light through a transparent portion of a solar collector, reflecting the transmitted light off a first reflective film having features disposed thereon, the features configured to modify the optical properties of the first reflective film to match optical properties of a second reflective film having a desired shape, and capturing a substantial portion of the reflected light using a receiver that converts the captured reflected light into electrical energy.
Other embodiments of the present invention provide an apparatus that includes an upper transparent film, a lower non-reflective film coupled to the upper transparent film to form an inflatable structure, and one or more features located on a surface of the upper transparent film, the one or more features configured to focus incoming light onto a receiver located in an inflation space defined by the inflatable structure.
Certain embodiments of the present invention provide a method that includes forming an upper transparent film, forming a lower non-reflective film, coupling the upper transparent film to the lower non-reflective film to form an inflatable structure, and forming one or more features on a surface of the upper transparent film, the one or more features being configured to focus incoming light onto a receiver.
The following detailed description, together with the accompanying drawings will provide a better understanding of the nature and advantages of the present invention.
Certain embodiments of the present invention seek to reduce the levelized cost of energy, which is the cost of generating electricity, of a solar power plant, and to maximize the scale at which such plants can be deployed. Various embodiments of power plants are described in U.S. patent application Ser. No. 12/782,932 filed on May 19, 2010, which is incorporated by reference herein for all purposes. Embodiments of solar collector devices and methods in accordance with the present invention may be utilized in conjunction with power plants having one or more of the attributes described in that patent application.
The objectives of reduced levelized cost of a solar power plant, can be achieved through the use of elements employing minimal and low-cost materials that are able to be mass produced. Potentially desirable attributes of various elements of such a solar power plant, include simple, rapid and accurate installation and assembly, ease of maintenance, robustness, favorable performance at and below certain environmental conditions such as a design wind speed, and survivability at and below a higher maximum wind speed.
According to certain embodiments of the present invention, inflation air may be used to impart a curved profile to a reflective component of a concentrator for a solar collector structure. U.S. patent application Ser. No. 11/843,531 filed on Aug. 22, 2007, which is incorporated by reference in its entirety herein for all purposes, discloses an inflatable solar concentrator balloon method and apparatus. U.S. patent application Ser. No. 13/015,339 filed Jan. 27, 2011, which is incorporated by reference in its entirety herein for all purposes, describes various configurations for inflatable balloon structures. In some embodiments, inflation air may be used to impart a liner (or trough-type) profile to the solar concentrator. U.S. Provisional Application No. 61/560,547, filed on Nov. 16, 2011, which is incorporated by reference in its entirety herein for all purposes, describes a trough-type inflatable solar concentrator. Embodiments of the present invention may share one or more characteristics in common with the apparatuses disclosed in above referenced patent applications.
The reflective film 104 can expand into the bottom portion of a balloon shape when the inflation space 112, enclosed by the reflective film 104 and transparent film 106, is filled with gas. The film 104 can be made of aluminum, mylar, or another reflective materials. When gas is provided into the inflation space 112 between the sealed films, a balloon structure is formed.
The upper transparent film 106 may comprise a polymer. Many different types of polymers can be used to form the upper transparent film. One form of polymer which may be suitable is polyester, examples of which include but are not limited to polyethylene terephthalate (PET) and similar or derivative polyesters such as polyethylene napthalate (PEN), or polyesters made from isophthalic acid, or other diols such as but not limited to butyl, 2,2,4,4 tetramethylcyclobutyl or cyclohexane.
The transparent upper film 106 may be formed from poly(meth methacrylate) (PMMA) and co-, ter-, tetra-, or other multimonomeric polymers of methacrylates or acrylates including but not limited to monomers of ethyl, propyl and butyl acrylate and methacrylates. Other examples of polymers forming the upper transparent film include but are not limited to polycarbonate (PC), polymethylpentane (TPX), silicones, cyclic olefin derived polymers such as Cyclic olefin co-polymers (COC) and cyclic olefin polymer (COP), fluorinated polymers such as polyvinilidene fluoride and difluoride (PVF and PVDF), ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), fluorinated ethylene propylene (FEP), THV, derivatives of fluorinated polymers, fluorinated derivatives of the above polymers, and co-extruded, coated, adhered, or laminated species of the above. The thickness of the upper transparent film 106 ranges from approximately 0.012 mm to 20 mm, depending on material strength and collector diameter. In addition, the upper transparent film may be formed from one or more polymers in a film stack. The reference to films here on refers to bulk polymers made into films, films stacks, embossed polymers on films, directly embossed films and the like.
In operation of the collector of
Light incident from the sun passes through the upper transparent film 106, is reflected off of the lower reflective film 104, and is accordingly focused and concentrated on a receiver 120. In the embodiment of
The receiver 120 is configured to convert the reflected and concentrated solar energy into other form(s) of energy. According to some embodiments, the receiver may comprise a photovoltaic (PV) structure that is configured to convert solar energy into electrical energy. Such a PV receiver may be cooled using water, glycol, air, or combination thereof.
In certain embodiments, the receiver 120 may comprise a concentrated solar power (CSP) structure that is configured to convert solar energy into thermal energy through a working fluid having desirable properties. For example, the working fluid may be input to a heat engine such as Sterling engine or micro-turbine. Such working fluids can include air, nitrogen, helium, hydrogen, water, molten salts or oils.
U.S. patent application Ser. No. 11/844,888 filed on Aug. 24, 2007, which is incorporated by reference herein in its entirety for all purposes, discloses photovoltaic or thermal receivers for cost-effective solar energy conversion of concentrated light. U.S. patent application Ser. No. 11/843,549 filed on Aug. 22, 2007, which is also incorporated by reference herein for all purposes, discloses interconnection systems for solar energy modules and ancillary equipment, including fluid conduits to a receiver.
The shape of an inflated lower reflective film 104 may result in an irregular illumination profile on the receiver. This irregular illumination profile can in turn reduce the efficiency of the solar receiver and the overall efficiency of the system. In addition, the irregular illumination profile restricts the maximum concentration achievable by the system.
Thus according to embodiments of the present invention, the lower reflective film 104, which serves as an inflated primary mirror, may include features 116 that are configured to globally correct the optical behavior of the concentrator. In particular, the features 116 are designed to correct the reflective profile of the surface of the lower reflective film 104, to a desired shape (for example in some embodiments a parabolic shape).
In some embodiments, the features 116 may be impressions that can be formed by embossing. Such embossing may be of a film directly, or may be of material added to a film prior to or after a reflective structure is created or a reflective material is added. For example, the impressions can be formed directly on a film to which the reflective material is added to form reflective film 104 or can be formed on substrate onto which the reflective material is added and then attached to a film thus forming the reflective film 104. In one application, the features 116 are designed such that the difference in slope of the actual inflated surface of reflective film 104 and that of a desired surface is compensated for. The difference between achieved and desired slope, for example, is illustrated in
In embodiments where the perimeter of the reflective film 104 is circular in shape, corrective features 116, may be annular shaped and may be used to achieve the global correction. The annularly shaped corrective features follow the shape of the circular perimeter. In embodiments where the perimeter of the reflective film 104 is other than circular, e.g., linear, corrective features that continuously follow the outline of the film can be used to achieve the global correction.
While
While
In some embodiments, the concentrator may include an inflated structure having a substantially lenticular shape (which may result in a focal point lying outside of the inflation space). Again, optical features formed according to embodiments of the present invention could provide for correction of the illumination profile of such a concentrator structure.
In some embodiments, the front (transparent) surface of an inflatable concentrator can be substantially planar while the reflective film 104 may be curved, for example where the front film 106 has sufficient thickness to impart rigidity that resists bowing in response to the internal inflation pressure. Optical features according to embodiments of the present invention can also be used to correct the illumination profile provided by such a structure.
In some embodiments, the correction features 116 can also be used to correct the illumination profile of other alternate concentrator shapes. Examples of such alternative concentrator shapes include spherical-shaped, or pillow-shaped for square or rectangular balloons, or linear concentrators.
Moreover, while
Returning to
A secondary optical structure can also be used to compensate for the non-uniform nature of the primary reflectance. U.S. patent application Ser. No. 12/720,429 filed on Mar. 9, 2010, which is incorporated for reference herein in its entirety, describes optical structures including secondary optics. Embodiments of apparatuses according to the present invention may share one or more aspects in common with this patent application. However, secondary optical structures may require complex optics such as total internal reflectance (TIR) structures, adding expense. In addition, the increased range of angles of rays reflected from inflated structures makes optical secondary designs utilizing TIR more challenging.
It is desirable to correct any inefficiencies in the primary optic, e.g., reflective layer 104, to ensure uniform illumination of the receiver 120. This makes the entire system more robust and less vulnerable to tracking errors. This is especially true given that such correction of illumination uniformity according to the present invention can be employed in conjunction with passive compensation in the receiver and/or correction in a secondary optic, in order to enhance collection efficiency.
According to one embodiment, a typical primary concentrator profile is a parabolic reflector that causes reflected rays to converge at a single point, and causes regions near the focus to exhibit a high degree of uniformity. However, since (a) a parabola does not create a perfectly uniform profile outside of its focal point, (b) the sun is not a perfect point source, and (c) tracking error considerations may drive a reflective shape that differs from a parabola, modifications to a parabolic reflector are made in some embodiments. For example, the desired primary concentrator profile can be a faceted reflector. In a faceted reflector, the reflector comprises an array of reflectors that can individually direct light to a same place, with superposition from the individual reflector facets.
According to certain embodiments, a primary optic element that is larger than a single facet may be used. A primary optic element that is larger than a single facet may be desirable for one or more reasons. One reason is that a large primary optic allows certain components to be amortized over a smaller number of receivers. For example, inflating one large primary optic would require less supporting equipment versus multiple small primary optics that may each require a set of supporting equipment, e.g., wires, hoses, and a multitude of tracking motors or actuators, wheels, cooling systems, etc. In addition, some support structures may not scale linearly with the area of the optic, though the solar collection will so that the average support structure mass may be minimized by a larger primary optic. Further, a large primary optic can make maintenance easier. In some embodiments, a single, large primary optic can simplify alignment of the optic and maintenance.
Solar structures may trade off the efficient use of land, which would prefer higher packing density of structures. Higher packing density, however, can cause problems with one concentrator shading another. Shading may cause problems with production of electricity if the photovoltaic receiver is shaded non-uniformly. Shading issues, however, can indicate in favor of low aerial density of structures. For an imaging primary optic (such as a parabolic primary) the phenomenon of one balloon shading another increases illumination non-uniformity on the receiver. A non-uniformly illuminated receiver may non-linearly drop in electrical output because the photovoltaic elements may be connected in a string, or limit the operating range of a thermal receiver due to non-uniform heat distribution. Due to the superposition of the individual elements, a receiver collecting light from a faceted reflector will lose energy proportional to shading, but will substantially maintain uniformity in the illuminated region. Hence, a faceted primary optic can yield better land use or better overall system efficiency, in addition to the other possible advantages yielded by primaries that are substantially larger than a single facet, as described above. In addition, facetted primaries can normalize out individual reflector abnormalities or imperfections also improving the uniformity on the receiver.
According to embodiments, the optical performance of an inflated film may be corrected to a parabolic or other shape reflector behavior, and/or may be corrected to faceted reflector behavior. Some embodiments may relate to such correction in CPV systems where solar energy is converted into electric power. Other embodiments may relate to correction in CSP systems where solar energy is converted into heat energy.
From the known, varying light source and known position of the reflected ray, a least squares routine can be used to iterate for position of the mirror. A polynomial is then fitted to satisfy the entire surface of the mirror. The following table 1 shows exemplary radially symmetric polynomial coefficients for an inflated primary mirror of radius 1.4859 m:
As illustrated in
Since the edge of the balloon is fixed, the example parabola shape has the same clamped outer ring position as that of the inflated structure. That is, the inflated and desired curves share the points (−1.4859, 0) and (+1.4859, 0). A parabola fitting these points is chosen, for which the outer diameter of the parabola images onto the outside diameter of the spot utilized for the inflated structure. That is, the parabola is chosen such that it provides the same diameter spot. Thus, in
The inflated concentrator thus has a greater slope at the far reaches of the radius, as compared with the example parabola. This causes the reflected rays to cross, resulting in non-uniform illumination of the receiver and restricting the maximum concentration allowed by the system.
In
In addition, the incident angles of the rays reflected from the inflated (solid) surface are more varied than the angles of rays reflected from the parabolic (dashed) surface. For example, the reflected ray from the example parabola, which is also shown at x=−1.40 for reference, lands near the edge of the spot.
An image associated with such a situation is shown in
In particular,
To design such an optic, a profile of a typical inflated structure is measured. In certain embodiments, this profile may be measured utilizing proprietary hardware and software as described above in connection with varying light source and known position of a reflected ray, followed by iteration for position of the mirror. The resulting output is a point by point mapping of a typical inflated concentrator under specific sealing conditions. Examples of such conditions include but are not limited to, the pressure within the inflation space of the balloon, fastening conditions, and structure design.
The difference between the mapped points, and points of the desired shape, e.g., a parabola, is then calculated as shown in
In certain embodiments that include a circular balloon, a globally-corrective feature includes an annular shape having a curved surface. This is shown, for example in feature 602 of
In some embodiments, the globally-corrective feature may comprise an annular shape with a flat section that is small in width as compared to the primary optic. For example an annular shape could be 1 mm wide, while the optic is 1.4859 m in radii. The edges of the annular shape could be determined by the allowable error from the assumption of a flat annular shape. This error is estimated to be small for example, on a 1 mm wide annular shape.
According to some embodiments, the thickness of the reflective film itself could be modified. Such modification in the reflective film thickness could be achieved spatially by embossing. For example, if a particular thickness profile is determined to inflate to a desired physical shape (e.g. a parabola) then the embossed structure could simply be used to add thickness to some areas of the film and not to others. An example of such an embodiment is shown in
Using a fixed harness at the edge is less restraining to the radial film strains at the edge than the situation at the apex, where strains occur in both directions. As such, the radial expansion of the balloon near the circumference is higher than near the apex. Hence one may emboss or otherwise alter the film at the edge such that the force needed to radially strain the film at the edge compensates for the fixed ring condition which otherwise preferences radially strains at the edge during expansion.
Initially, the theoretical calculations may be presumed to be correct, and that there is a quadratic thickness variation from center to edge. A film could then be embossed to this shape. The film would be utilized in a balloon structure and then the reflectance of the film can be measured in the manner indicated above. In some embodiments, a picture such as illustrated in
One other embodiment of enhancing the irradiance profile as in
In another embodiment a paint of the appropriate modulus can be spray-painted onto the reflector film. In this example, a clear-coat urethane paint was sprayed onto a 6 foot diameter film using a stationary spray gun and rotating table. The paint showed a substantially linear increase in overall modulus of the film plus coating vs. the thickness of the coating. From this, a first film was sprayed.
While it is shown that various materials can be applied to the film to alter the point focus irradiance profile at the receiver plane by use of screen printing or paint spraying, it is clear that similar techniques can be used to achieve a more optimal line profile and that other techniques to increase the strength of the film spatially are equally beneficial.
Returning to embossing options, the type of optic shown in
A mold master is first formed. In some embodiments this mold master can be a hard electrodeposited alloy formed from a photoresist or other mold. The mold master could alternatively be formed by directly machining the mold onto the roller that is to be used. Once a master stencil is made, submasters may then typically be fabricated from the master and included on the roll apparatus. In certain embodiments, a corrective feature is formed from additional material present on the surface of the film. Thus in some fabrication processes, the film is coated with a material that is easier to emboss.
For example, in certain embodiments a polymer is first added onto the film, and then the polymer is embossed. The film with coating can be run through rollers for example as described above, and the relief from the submaster is embossed into the added coating. The polymer coating including the corrective feature can then be cured by exposure to UV radiation, for example. A reflective component (such as a thin layer of Al or another metal) can then be added to the embossed coating in order to produce the reflective structure.
It may also be possible to emboss a polymer film directly, such that the facet may be formed as a shape in the material itself. Thus according to some embodiments, the embossing stamp is utilized to impress directly into the film material into a desired corrective shape (e.g. rounded or faceted) at a temperature. A thin metal (such as Al or Ag) may then be applied/deposited over the relief structures to produce the reflective structure.
According to still other embodiments, embossing may be performed directly into a reflective material to form the optical features. The process of embossing may typically be performed near or proximate to the glass transition temperature of the polymer or thermoplastic with subsequent slow temperature decrease. However, methods which use additional curing steps may perform the shape change on substantially oligomeric species or other liquidus materials at low or near room temperature, and bring the polymer Tg or other mechanical properties up by subsequent UV or other curing step.
According to still other embodiments, a material may be embossed directly but then added to an otherwise structural film by use of an adhesive or thermal lamination techniques. The thin reflective material may be added before or after embossing or before or after the joining of the embossed material with the structural film.
While the above description has focused upon forming features by embossing, the present invention is not limited to using this particular technique. Alternative embodiments could employ other approaches to form the desired optical features.
For example, one alternative approach is the use of a thermoform technique. In the thermoform technique, a mold is created for which the polymer and or mold is heated and pressed onto the mold via vacuum. The mold shape could exhibit the negative of the corresponding feature that is desired to be formed. In some embodiments, only the active or reflective surface of the film retains a substantially modified shape. In other embodiments, both the active surface of the film and the opposite or rear surface of the film may change shape due to the forming process. If both surfaces are allowed to change shape, facets or features much larger than the thickness of the film may be created.
Still other approaches are possible. For example, the optical features according to embodiments of the present invention could be created by techniques such as laser ablation, stencil printing, or other direct write techniques such as inkjet printing. Various approaches that can add or subtract material locally could be used to form the optical features according to embodiments.
As described in detail below in connection with
Embossed features of the primary optic can be fabricated up to pure 90 degree retroreflectors, if necessary. As shown in the
Embodiments may work with square balloons or balloons having other shapes, for example to achieve a higher packing density of collectors. In such embodiments, the receiver could also take on a different shape. A reflective primary optic according to embodiments of the present invention could further include features configured to implement other corrections of the image. Such effects include but are not limited to explicitly minimizing an effect of tracking error, avoiding certain receiver regions, or creating other illumination patterns as desired.
Returning to the embodiment of
It is to be noted that the present invention is not limited to correction to a parabola, but rather to correcting to whatever structure is desired. For example, the general spot irradiance profile for a parabolic reflector is a truncated Gaussian profile. Different shapes can be corrected using commercial optical design software, such as FRED from Photon Engineering of Tucson, Ariz., or ZEMAX from Zemax Corp. of Bellevue, Wash.
In some cases a profile is designed using as a standard asphere of the form:
z(r)=(r2/(R(1+SQRT(1−(1+k)r2/R2))))+a1r2+a2r4+a3r6+a4r8 . . .
This profile deviates from a parabola in such a way that the spot has a uniform distribution, rather than a Gaussian distribution. This would be a special case of an optimized asphere, and an asphere optimized specifically for uniform irradiance in a reflected spot. In other embodiments, the asphere may be optimized for tracking error tolerance, or minimizing receiver cell area for example.
Once the optimized aspheric shape is determined, the piecewise deviation between the shape of the inflated film (the Hencky surface) and the optimized asphere can be determined. The deviation can be determined by performing a point to point subtraction between the optimized asphere and the measured shape of the inflated film. Alternatively, the deviation can be determined by fitting the optimized asphere and the measured shaped of the inflated film to mathematical functions (one function for each), such as nth degree polynomials, and then subtracting the two mathematical functions to obtain a third mathematical function, which represents the deviation. The compliment to this deviation could be embossed piecewise into the plastic film.
Since both shapes (the Hencky surface and the optimized asphere) are axis-symmetric forms, the shape of the embossing would likewise be axis-symmetric. The shapes could take the form of concentric circles, where each annular area has a wedge shaped embossed feature to yield the desired ray angle at the design inflation pressure and corresponding shape.
In addition, the invention is not limited to achieving correction through the use of imprinted or embossed features alone. According to some embodiments, such correction may be implemented in conjunction with corrections applied to the front film by embossing or other techniques, and/or secondary optics, and/or other passive compensation schemes.
Certain embodiments may also achieve correction of non-uniform illumination through local changes to the physical shape of the reflecting surface itself. Some embodiments, which adopt such an approach, involve the formation of facets.
The embodiment of
In such embodiments, portions of the primary optic can uniformly illuminate the receiver, and the receiver illumination profile would therefore be a superposition of multiple facets. In this way the result of shading would be a loss in efficiency proportional to the shade area subtended, though the illumination uniformity on the receiver is not affected.
In one embodiment, a planar facet may uniformly illuminate the receiver. In this instance, the facet could be 0.3 meter diameter size. However, embodiments of the present invention are not limited to this, and facets may be any shape that tessellates or can be tiled to cover the inflated film. Facet size and surface curvature may also be chosen in a variety of ways. Facets may be much smaller than the receiver and still uniformly distribute light over the receiver surface as shown in
Moreover, the receiver may be any shape. A surface may be designed for a shaped facet in order to distribute its reflected light evenly over a receiver of any shape. Thus the receiver shape could be a hexagon or other tessellating polygon or sets of polygons.
Facets may be planar or non-planar. In general, for a facet to evenly distribute light over a receiver, it may be non-planar. However there may be cases where planar facets are desirable, for example because of manufacturing reasons or because or specific schemes of light concentration.
In
Mapping of light rays to a desired distribution at the receiver requires a slight arc or other shape to connect the required surface slopes at each end of the facet. If the facet is smaller than the receiver, the facet surface will be convex if it is continuous. Since embossed depths may typically be between several microns and 100 microns (and typically between 5-50 microns), the depth of the embossing and the desired angular correction sets the length of the facet in this design. Facets can be the longest possible, such that losses due to imperfect molds at the edges of the facet are minimized. The radii of curvature plus a wavelength of light may be lost at the corners.
In
Rays within the radius of curvature of the embossed features, and rays likely within a wavelength of the edge, may be lost to reflection. Specifically, light impinging the corners of the facets 1000 will not reflect to the receiver, either through diffraction or because of the radii of curvature from the embossing mold. Hence, some loss is expected. With long enough facets, however, this loss would be small. For example, for the high slope regime which requires the highest angle of correction, and hence narrowest facet width and an emboss depth of 20 μm, that the facet length would be approximately 125 μm.
For example, with a 1 μm corner radii plus an average wavelength of 0.8 μm, the approximated loss regime would be about 1.3 μm on either side of the 125 μm length, or about 2%. In other words, reflected light from incoming light incident in a location that is approximately 1.3 μm from either end of a facet will be lost and will not impinge on the receiver. In case of shallow slope regimes and the same embossed depth as above, the facet width would be about 1152 μm, resulting in a loss of approximately 0.2%.
A variety of facets can be created based on the information disclosed above. First, the facets could be wider than the examples described above thus minimizing the percentage loss at the edge of the facets. In certain embodiments, the facetted surface could have its own curvature. Second, the facets may illuminate only a portion of the receiver. This can maximize efficiency by avoiding illumination of non-receptive areas between individual solar cells. Third, the facets could illuminate only a portion of the receiver, but be combined with more facets such that the receiver is uniformly illuminated. For example, in an embodiment one hundred facets would each add to uniformly illuminate the receiver, but multiple hundreds of facets may be present on the primary optic or reflective film.
An embodiment of such as system is shown in
According to certain embodiments, the illuminated areas 1105 may correspond to the active areas of photovoltaic cells, and the un-illuminated interstitial areas between illuminated areas 1105 may correspond to non-photoelectrically active areas, for example contact areas, solder areas, printed wire traces, etc. By directing light away from these non-active areas, the efficiency of the system can be increased. Superimposing the plurality of light rays reflected from facet features 1103 on areas 1105 can achieve a high degree of uniformity along with a reduction in non-uniformity attributable to partial shading. While the embossed features 1103 and the corresponding illuminated areas 1105 are shown as square in
Under certain conditions, the system may be partially occluded (as by clouds or by systems on adjacent trackers at the extremes of sun position in the sky). In this situation, rather than certain cells being occluded or partially occluded, the intensity on each cell would stay substantially uniform but decrease with increasing occlusion. In this way, the system becomes more tolerant of both tracking errors and occlusion. This allows the stiffness, accuracy and cost of the tracking system to be optimized for greater total allowable error.
This configuration also makes the system more tolerant to partial shading (occlusion). Such greater occlusion tolerance allows systems to be packed more closely together on a given portion of land potentially reducing an overall cost of power even as some systems partially occlude other systems at certain times, such as in the morning, evening, and near the winter solstice or being able to utilize a larger array of potential sites for a given utility maximum power requirement.
Next in operation 1225 the optic element is modified to match the desired illumination profile. The optic element can be modified by changing the thickness profile of the optic element, by fabricating features on the optic element, by fabricating refractive features of the front film, or by doing all of the above. The thickness profile of the optic element can be modified by changing the thickness of the optic, for example, as described with reference to
It should be appreciated that the specific steps illustrated in
While the above figures illustrate the formation of global and local corrective features in separate embodiments, the present invention is not limited to this. According to alternative embodiments, features may be formed to achieve both global and local correction in one structure and on both front transmissive and/or back reflective films.
An example of such an embodiment is shown in
As noted above, corrective optics according to embodiments of the present invention may eliminate the need for a secondary optic. This in turn may relieve the system of a restriction whereby rays from the primary incident on the receiver, must be within an acceptance angle range of the secondary optic. In optical terms, this is the “speed” of the system also known as the focal ratio. Relieved of this restriction, it is possible to decrease the focal ratio of the system. This increases the range of angles incident on the receiver, which may then be limited by the acceptance angle of the PV cells. Since the PV cells typically have a greater acceptance angle than secondary optics, this allows the system to be made much shorter for given power. That is, the system may operate at a lower focal ratio (i.e. F/0.4 instead of F/0.8).
Such a reduction in focal ratio may provide performance advantages for the solar concentrator system. For example, the lateral displacements of the light spot(s) may be reduced for a given tracking error. In addition the system may be shorter in height overall. Such reduced structure size consumes less materials, results in less wind loading, less weight, and ultimately lower cost through direct savings in the concentrator system and indirect savings in the support system.
In addition to forming inflated concentrators, embossed features according to embodiments of the present invention may be employed for any optical system in which the cost of manufacturing a substrate having a desired physical shape exceeds the cost of locally deforming the film to achieve the illumination profile of that physical shape.
For example, certain embodiments may involve the formation of embossed corrective features on an upper transparent film of an inflatable concentrator structure, as illustrated in
Certain embodiments may involve the formation of embossed corrective features on an upper transparent film of an inflatable concentrator without a reflective back surface, as illustrated in
In addition to performing optical compensation, embodiments having embossed features on an upper transparent film may offer other potential benefits. One possible advantage is a reduction in optical losses via increased transmission from structures similar to moths' eyes and hence improved performance of the entire collector device.
An anti-reflective component could serve to reduce reflection of incident light by the upper (transparent) component of the concentrator. The reduced reflection would allow the collection of light that would otherwise be lost to reflection, thereby improving the performance of the device.
For example, the use of an anti-reflective component in an upper transparent portion of a collector, could reduce expected optical losses from around 5.5% per surface (total 11% single pass, 22% double pass) to around 1% per surface (total 2% single pass, 4% double pass) in an embodiment comprising an embossed moth's eye anti-reflection coating on an optically transparent material. In addition, embossed components on the upper surface may produce increased resistance to soiling and super-hydrophobic behavior.
Although specific embodiments of the invention have been described, various modifications, alterations, alternative constructions, and equivalents are also encompassed within the scope of the invention. The described invention is not restricted to operation within certain specific embodiments, but is free to operate within other embodiments and configurations, as it should be apparent to those skilled in the art that the scope of the present invention is not limited to the described series of transactions and steps.
It is understood that material types provided herein are for illustrative purposes only. Accordingly, reflective films can be made of various different reflective materials such as materials comprising polyethylene terephthalate (PET), as described in some embodiments herein. Similarly, transparent films can be made of various transparent materials including but not limited to the polymers described above.
In conclusion, embodiments of the present invention may seek to the reduce costs and maximize scales of solar power plants through the use of elements employing minimal materials and low-cost materials. Elements of the solar power plant are able to be mass produced with existing technology, making them less expensive and better able to compete economically with existing fossil fuels.
The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.
This application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/428,203 filed on Dec. 29, 2010, the contents of which are incorporated by reference herein in their entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 61428203 | Dec 2010 | US |
