US20130118559A1

US20130118559A1 - Busses for bifacial photovoltaic cells

Info

Publication number: US20130118559A1
Application number: US13/676,173
Authority: US
Inventors: Jose E. Castillo; Paul S. Hauser
Original assignee: Prism Solar Technologies Inc
Current assignee: Prism Solar Technologies Inc
Priority date: 2011-11-14
Filing date: 2012-11-14
Publication date: 2013-05-16
Also published as: US8614842B2; WO2013074611A1; WO2013074607A1; US20130120815A1; US20160155873A1

Abstract

PV module composed of individual PV cells oriented and electrically connected according to a methodology that is viable for at least i) bifacial cells with substantially equal solar-energy conversion efficiency achievable on each side of each cell, and ii) PV modules with low operating current. Embodiments of the invention facilitate the use of different busing technologies to reduce cost and complexity of the resulting PV module while increasing the electrical energy harvested by the PV module.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of and priority from U.S. Provisional Patent Application Nos. 61/559,425 filed on Nov. 14, 2011 and titled “Advanced Bussing Options for Equal Efficiency Bifacial Cells”; 61/559,980 filed on Nov. 15, 2011 and titled “Flexible Crystalline PV Module Configurations; 61/560,381 filed on Nov. 16, 2011 and titled “Volume Hologram Replicator for Transmission Type Gratings”; and 61/562,654 filed on Nov. 22, 2011 and titled “Linear Scan Modification to Step and Repeat Holographic Replicator”. The disclosure of each of the abovementioned Provisional Patent Applications is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to conversion of solar energy to electrical energy and, more particularly, to ways of orienting and electrically coupling of bifacial photovoltaic (PV) cells.

BACKGROUND OF THE INVENTION

Solar energy will satisfy an important part of future energy needs. While the need in solar energy output has grown dramatically in recent years, the total output from all solar installations worldwide still remains around 7 gigawatts, which is only a tiny fraction of the world's energy requirement. High material and manufacturing costs, low solar module efficiency, and shortage of refined silicon limit the scale of solar power development required to effectively compete with the use of coal and liquid fossil fuels.
The key issue currently faced by the solar industry is how to reduce system cost. The main-stream technologies that are being explored to improve the cost-per-kilowatt of solar power are directed to (i) improving the efficiency of a solar cells that comprise solar modules, and (ii) delivering greater amounts of solar radiation onto the solar cell. In particular, these technologies include developing thin-film, polymer, and dye-sensitized photovoltaic (PV) cells to replace expensive semiconductor material based solar cells, the use high-efficiency smaller-area photovoltaic devices, and implementation of low-cost collectors and concentrators of solar energy.
While the reduction of use of semiconductor-based solar cells is showing great promise, for example, in central power station applications, challenges for the use of conventional solar cells remain for residential applications due to the form factor and significantly higher initial costs. Indeed, today's residential solar arrays are typically fabricated with silicon photovoltaic cells, and the silicon material constitutes the major cost of the module. Therefore techniques that can reduce the amount of silicon used in the module without reducing output power will lower the cost of the modules.
The use of devices adapted to concentrate solar radiation on a solar cell is one of such techniques. Various light concentrators have been disclosed in related art, for example a compound parabolic concentrator (CPC); a planar concentrator such as, for example, a holographic planar concentrator (HPC) including a planar highly transparent plate and a holographically-recorded optical element mounted on its surface; and a spectrum-splitting concentrator (SSC) that includes multiple, single junction PV cells that are separately optimized for high efficiency operation in respectively-corresponding distinct spectral bands. A conventionally-used HPC is deficient in that the collection angle, within which the incident solar light is diffracted to illuminate the solar cell, is limited to about 45 degrees. Production of a typical SSC, on the other hand, requires the use of complex fabrication techniques.
Historically, PV cells have been monofacial, meaning that they have a single active surface capable of converting incident solar radiation to electric potential. Historically, monofacial solar cells are fabricated with a film stack including an anti-reflective/hard coating optimized for transmittance at wavelengths for which silicon has the highest quantum efficiency, passivation, n doped and p doped silicon forming a single p-n junction, and a back electrode. Conventionally, the back electrode is a layer of metal such as aluminum. The front electrode is conventionally provided by a layer of transparent conductive material such as Indium Tin Oxide (ITO) in contact with a higher conductivity small area electrode made of Al, Ag, or some other metal or alloy. Since each conventional monofacial solar cell generates about 0.5V under illumination, conventional monofacial solar cells are typically arranged in electrical series, with the back electrode of a first cell electrically coupled to the front electrode of a second (i.e., adjacent) cell (or vice-versa). This series connection is repeated until the desired voltage is obtained.
Relatively recently, bifacial solar cells have been fabricated, which have photovoltaically active regions on both the front and back sides. Certain conventional bifacial solar cells are fabricated as an n+−p−p+ stack between front and back electrodes. Other configurations are possible, so long as there are two junction regions proximate to a front active surface and back active surface, where each junction region forms an electron-hole pair. The front and back electrodes are conventionally fabricated from a transparent conductor like ITO in electrical contact with small area metal electrode (i.e., a bus bar or finger). Historically bifacial solar cells have had unequal efficiency between the front and back sides of the cells. Accordingly, conventional individual bifacial cells, when assembled into panels or series, are all oriented such that the “front” or high efficiency side is oriented to intercept direct sunlight, while the lower efficiency or “back” side is oriented to receive indirect sunlight from scatter, reflection off the ground or mounting surface, for example. Such orientation and associated electrical connection between and among the cells does not allow to maximize the electrical energy output from the resulting panels. PV modules or panels that take advantage of different orientation of and electrical connections among the individual bifacial PV cells is, therefore, required.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed toward systems and methods for bussing bifacial solar (or photovoltaic, PV) cells, and/or solar cells having relatively low operating current. In one embodiment, bifacial solar cells having substantially equal conversion efficiency are laid out in an alternating fashion to form an array, with sides of these cells corresponding to alternating electrical polarity facing the same direction. Moreover, the fronts and the backs of so laid out alternating cells are bussed together to form an array of the PV cells. In related embodiments of such PV-cell arrays, the component bifacial cells are cut or diced into smaller areas, such that each reduced area cell has an associated current level reduced in comparison with a bigger size bifacial cell.
Embodiments of the invention provide a solar module comprising (i) a first bifacial solar cell having a first front active side and a first back active side, wherein the first front and back active sides have substantially equal solar conversion efficiencies, the first front active side having a first electrical polarity, the first back active side having a second electrical polarity that is opposite to the first electrical polarity; (ii) a second bifacial solar cell having a second front active side and a second back active side, wherein the second front and back active sides have substantially equal solar conversion efficiencies, the second front active side having the second electrical polarity and the second back active side having the first electrical polarity. The orientation of the first and second bifacial cells in the module is such that the first and second bifacial solar cells oriented in series with the first front active side facing in substantially the same direction as the second front active side. The module further includes a bus bar electrically coupling the first front active side to second front active side. In one embodiment, the first and second bifacial solar cells are arranged adjacent to one other, and the first front active side is substantially coplanar with the second front active side. In a related embodiment, the module may additionally include a third bifacial solar cell having a third front active side and a third back active side, wherein the second front and back active sides have substantially equal solar conversion efficiencies, the third front active side having the first electrical polarity, the third back active side having the second electrical polarity. The third bifacial solar cell is oriented, in the module, such that the third front active side faces substantially the same direction as the first front active side. A bus bar is added to electrically couple the second back active side to the third back active side. Furthermore, an encapsulant layer may be disposed on the first front active side and the second front active side such as to cover the bus bar.
Embodiments of the invention also provide a method for fabrication of a photovoltaic (PV) module. The method includes at least (i) separating a PV cell having an original size into a plurality of PV sub-cells, each sub-cell having a size smaller than the original size; and (ii) electrically coupling the sub-cells in series such that a first side of the first sub-cell having a first electrical polarity is electrically connected to a first side of the second sub-cell having a second electrical polarity, the first side of the first sub-cell and the first side of the second sub-cell oriented to face substantially the same direction. The method may additionally include positioning the first side of the firs sub-cell to be substantially co-planar with the first side of the second sub-cell. In one embodiment, the step of electrically coupling includes a) depositing a conformable electrically conductive material on a first surface of an optically-transparent encapsulation layer; and b) covering the first and second sub-cells with the optically-transparent encapsulation layer such that the first surface of the optically transparent encapsulation layer carrying the conformable electrically-conductive material faces the first side of the first sub-cell and the first side of the second sub-cell. Alternatively or in addition, the step of depositing may include depositing at least one of conductive epoxy, wire mesh, or charge collection tape. Furthermore, the method optionally comprises disposing a holographic element in optical communication with at least one of the first and second sub-cells between the optically-transparent encapsulation layer and a surface of the at least one of the first and second sub-cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a holographic planar concentrator.

FIG. 2A shows an embodiment of a holographic spectrum-splitting device.

FIG. 2B shows an alternative embodiment of a holographic spectrum-splitting device.

FIG. 3 is a schematic cross sectional diagram showing the arrangement of two bifacial solar cells arranged according to an embodiment of the invention

FIG. 4 is a top-down plan view of an arrangement of bifacial solar cells mutually electrically coupled according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

References throughout this specification to “one embodiment,” “an embodiment,” “a related embodiment,” or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to “embodiment” is included in at least one embodiment of the present invention. Thus, appearances of the phrases“in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. It is to be understood that no portion of disclosure, taken on its own and in possible connection with a figure, is intended to provide a complete description of all features of the invention.
In addition, the following disclosure may describe features of the invention with reference to corresponding drawings, in which like numbers represent the same or similar elements wherever possible. In the drawings, the depicted structural elements are generally not to scale, and certain components are enlarged relative to the other components for purposes of emphasis and understanding. It is to be understood that no single drawing is intended to support a complete description of all features of the invention. In other words, a given drawing is generally descriptive of only some, and generally not all, features of the invention. A given drawing and an associated portion of the disclosure containing a description referencing such drawing do not, generally, contain all elements of a particular view or all features that can be presented is this view, for purposes of simplifying the given drawing and discussion, and to direct the discussion to particular elements that are featured in this drawing. A skilled artisan will recognize that the invention may possibly be practiced without one or more of the specific features, elements, components, structures, details, or characteristics, or with the use of other methods, components, materials, and so forth. Therefore, although a particular detail of an embodiment of the invention may not be necessarily shown in each and every drawing describing such embodiment, the presence of this detail in the drawing may be implied unless the context of the description requires otherwise. In other instances, well known structures, details, materials, or operations may be not shown in a given drawing or described in detail to avoid obscuring aspects of an embodiment of the invention that are being discussed. Furthermore, the described single features, structures, or characteristics of the invention may be combined in any suitable manner in one or more further embodiments.
For example, to simplify a particular drawing of an electro-optical device of the invention not all coatings or layers (whether electrically conductive, reflective, or absorptive or other functional coatings such as alignment coatings or passivation coatings), electrical interconnections between or among various elements or coating layers, elements of structural support (such as holders, clips, supporting plates, or elements of housing, for example), or auxiliary devices (such as sensors, for example) may be depicted in a single drawing. It is understood, however, that practical implementations of discussed embodiments may contain some or all of these features and, therefore, such coatings, interconnections, structural support elements, or auxiliary devices are implied in a particular drawing, unless stated otherwise, as they may be required for proper operation of the particular embodiment.
Moreover, if the schematic flow chart diagram is included, it is generally set forth as a logical flow-chart diagram. As such, the depicted order and labeled steps of the logical flow are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow-chart diagrams, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Without loss of generality, the order in which processing steps or particular methods occur may or may not strictly adhere to the order of the corresponding steps shown.
The invention as recited in claims appended to this disclosure is intended to be assessed in light of the disclosure as a whole, including features disclosed in prior art to which reference is made.
A “laminate” refers generally to a compound material fabricated through the union of two or more components, while a term “lamination” refers to a process of fabricating such a material. Within the meaning of the term “laminate,” the individual components may share a material composition, or not, and may undergo distinct forms of processing such as directional stretching, embossing, or coating. Examples of laminates using different materials include the application of a plastic film to a supporting material such as glass, or sealing a plastic layer between two supporting layers, where the supporting layers may include glass, plastic, or any other suitable material.
As broadly used and described herein, the reference to an electrode or layer as being “carried” on a surface of an element refers to both electrodes or layers that are disposed directly on the surface of an element or disposed on another coating, layer or layers that are disposed directly on the surface of the element.
An HPC 100, shown schematically in a cross-sectional view in FIG. 1, typically includes a highly-transparent planar substrate 104 of thickness d (such as, for example, substrate made of glass or appropriate polymeric material having the refractive index n₁) at least one diffractive structure 108, having width t, at a surface of the substrate 104. Such diffractive structure may include, for example, a holographic optical film (such as gelatin-on-PET film stack) in which a plurality of multiplexed diffraction gratings have been recorded with the use of laser light. The diffractive structure 108 can be optionally capped with a protective cover layer (not shown). The substrate 104 is typically cooperated with a solar-energy-collecting device 112 such as a PV cell. The diffractive structures 108 diffract wavelengths usable by the PV cell 112, while allowing the light at unusable wavelength to pass through, substantially unabsorbed. The usable energy is guided via the total internal reflection at the glass/air or glass/cover interface to strings of solar cells, resulting in up to a 3× concentration of solar energy per unit area of PV material.
As shown in FIG. 1, the PV cell 112 of width T is juxtaposed with the second surface of the substrate 104 in opposition to the diffractive structures 108 and in such orientation that ambient (sun-) light I, incident onto the structure 108 at an angle θ_I, is diffracted at an angle θ_Donto the cell 112 either directly or upon multiple reflections within the substrate 104. To estimate the range of incident angles that would produce the diffracted light intersecting the surface of the PV cell 112 for different parameters of the HPC 100 such as substrate thickness, the displacement of the PV cell 112 with respect to the edge of the grating 108, other geometrical parameters one can use the grating equation. For example, for a glass substrate 104 and when t=T=d, the range of incident angles (the collection angle) at which the cell 112 is illuminated is about 45 degrees. When t=2T−2d, the collection angle is reduced to about 38 degrees. The angular range within which the corresponding diffracted light is produced is about 10° to 15° for most of the wavelengths. However, the angle-wavelength matching can be used to extend this range for different portions of the available spectral bandwidth of the HPC 100.
The increase in PV-conversion efficiency, in comparison with a use of a conventional PV-cell, is also achieved by using multiple junction cells that create electron-hole pairs at the expense of energy of incident light over a wider spectral range than a single junction cell. The use of holographic grating with such spectrum-splitting devices (SSD) also offers some advantages. The hologram can be designed to diffract light within a specific spectral band in a desired direction (for example, towards one PV-cell) and be multiplexed with another hologram that diffracts light of different wavelength in another direction (for example, towards another PV-cell). One example of such holographic SSD 200, shown in FIG. 2A, includes two holographically-recorded diffractive structures 204 and 208 that are cascaded at a surface 212 of the substrate 216 (i.e., at the input of the SSD 200) and that diffract light of different wavelengths. For example, the upper hologram 204 diffracts light at wavelength λ₂longer than wavelength λ₂diffracted by the hologram 208. Two PV-cells, respectively-corresponding to the holograms 204 and 208—a long-wavelength PV cell 214 and a short-wavelength PV-cell 218—are positioned transversely with respect to the holograms 204, 208 (as shown, at side facets of the substrate 216). Directionally-diffracted towards target PV-cells light 224, 228 reaches the PV-cells via reflections off the surfaces of the substrate 216. A simple light-concentrating reflector can additionally be used. A similar SSD 230, upgraded with cylindrical parabolic reflectors 234, 238 that guide the diffracted light towards target PV-cells, is depicted in FIG. 2B. In both cases, the collection angle is determined by geometry of the system and the diffraction characteristics of the holograms.
Bifacial solar cells are known to have unequal efficiencies of solar energy conversion for the front and back sides of an individual PV cells. It is unexpectedly discovered that, when such PV cells are assembled into conventional panels or series such that the “front” sides (high efficiency sides) of all the cells are oriented to intercept direct sunlight, while the lower efficiency or “back” sides are oriented to receive sunlight delivered indirectly (from scatter, reflection off of the ground, or mounting surface, for example; see examples of FIGS. 2A, 2B above), the electrical energy output from the resulting panels is not optimized.
Unfortunately, it is precisely such non-optimized orientation that is used in fabrication of commercially-available PV modules or panels: Because of the stack-orientation for bifacial cells, the polarity of some bifacial cells is such that the positive pole occurs at the high efficiency side. Accordingly, even though the cells are bifacial in nature they are still strung in a conventional manner in the module, with the highest efficiency side facing the sun, resulting in all the cell polarities being located on the same side.
The idea of the present invention stems from the realization that electrical connection among the individual PV cells in a stack arranged such that the back side of a first cell electrically connected to the front side of the adjacent cell, or vice versa, increases the efficiency of harvesting the solar energy.
An arrangement of mutually electrically coupled bifacial solar cells is shown in cross section in FIG. 3. Each bifacial cell has a positive and negative poll (electrically-positive and electrically-negative sides). In the example of FIG. 3, the positive poll of cell 300 is situated at the “top” active surface, and the negative poll of cell 300 is situated at the “bottom” active surface. The polls of cell 316 are oriented in the opposite manner.
As shown in FIG. 3, the bifacial solar cells 300 and 316 are arranged such that the electrically-positive face of solar cell 300 is substantially coplanar with the electrically-negative face of adjacent solar cell 316. Bifacial solar cells 300 and 316 may be equal efficiency bifacial cells. Where bifacial cells 300, 316 are equal efficiency cells, each of the cells 300, 316 has a front and back active surfaces, where each active surface demonstrates substantially the same level of solar conversion efficiency. In such special case, and from an efficiency standpoint, it does not matter which surface—the one with a negative electrical polarity or the one with a positive electrical polarity—is oriented toward a source of direct illumination.
Each bifacial solar cell 300, 316 is constructed of multiple layers of doped semiconductor material. For example, the cell 300 has a substrate layer 302 sandwiched between differently doped semiconductor layers 304, 306. In one example, the layer 302 is a p-type semiconductor substrate layer, the layer 304 is an n+-type semiconductor material and the layer 306 is a p+-type semiconductor material. The layers 302, 304, 306 are optionally formed of single-crystalline, poly-crystalline or amorphous silicon. Other semiconductor layer stacks can be used to create a bifacial photovoltaic cell, as long as the requirement that the semiconductor stack be capable of producing electron-hole pairs at each junction between its layers (the junction between the layers 304 and 302, and the junction between the layers 302 and 306, in case of the cell 300) when light is incident on a front side and a back side of the cell is satisfied. The bifacial cell 316 is similar to cell 300, but oriented such that its negative pole is facing the same direction and, optionally, is also co-planar with the side of the cell 300 associated with the positive poll. Like cell 300, cell 316 has a substrate layer 318 sandwiched between two differently doped semiconductor layers 320, 322. In an exemplary embodiment, layer 318 is a p-type substrate layer, layer 322 is an n+-type semiconductor material and layer 320 is a p+-type semiconductor material.
On the outside of the active semiconductor layers of the cells 300 and 316 additional layers may be present (shown, in FIG. 3, as layers 308, 310, 326, and 324). Each of these additional layers may carry or be supplemented by at least one auxiliary layer (not shown) such as, for example, a transparent electrode layer (made of a transparent conductive oxide, TCO, such as indium tin oxide ITO or aluminum zinc oxide AZO, for example), a passivation coating, an anti-reflection coating, and a hard protective coatings, to name just a few
The cell 300 is illustrated to be equipped with electrical contacts 312 and 314 that are electrically coupled to semiconductor layers 308 and 310, respectively. Such electrical coupling may be effectuated by soldering, for example, or printing the contacts 312, 314 at the TCO layer (not shown) juxtaposed with the layers 308 and 310 respectively. Electrical contacts 330, 328 are provided in a similar fashion for the adjacent cell 316 in electrical cooperation with the layers 324, 326.
The individual PV cell such as the cell 300, when illuminated with sunlight, generally generates a relatively low level of electric potential, for example about 0.5 V. Accordingly, to optimize the solar energy harvesting, the cell 300 is preferably connected with additional PV cells to form modules generating higher and more practically-useful voltage levels. The arrangement of FIG. 3, showing two electrically connected individual PV cells 300, 316, may be only a small segment of a typical solar module. As shown an electrically-positive side of cell 300 is connected to an electrically-negative side of the adjacent cell 316 via an electrical buss connection 332. The negative side of cell 300 is connected, as shown with an arrow 333, to an electrically-positive side of an adjacent non-illustrated cell by bus connection 334. Unlike conventional bus connections, which connect the back side of a first cell to the front side of an adjacent cell, the connections of the embodiment of FIG. 3 332, 334 are co-planar with their respective connected surfaces.
The arrangement of FIG. 3 additionally shows an optional encapsulant layer 336, which in some embodiments is bonded to or deposited on the outside of the outermost layers 308, 310, 324, 326 of cells 300, 316. While in the arrangement of FIG. 3, only a single encapsulant layer 336 is shown, in a related embodiment the encapsulant may be disposed over several sides of the PV cell or the PV module containing several PV cells that are connected according to the idea of the invention such as to embed the PV module within the encapsulant. In certain embodiments, the encapsulant layer 336 is formed of a sheet of EVA that is laminated to solar cells 300, 316 in order to provide enhanced mechanical durability and scratch resistance, for example. In so affixing the encapsulate sheet to the solar cells, the electrical connections 332, 334 (i.e., bus bars) may be pre-deposited onto the surface of the encapsulant layer 336 that would be facing the PV cells prior to assembly or lamination of the encapsulant layer 336 onto the cells 300, 316. This pre-deposition of electrical connectors 332, 334 is enabled and made practical precisely because of the mutual orientation of the individual solar cells 300, 316 in accord with an embodiment of the invention, as such orientation (shown in FIG. 3) obviates the need for front-to-back electrical coupling between adjacent cells that would be required in the case of conventional orientation of the cells. As the electrical connectors 332, 334 are substantially parallel with the front and back surfaces of the cells 300, 316, these connectors can be pre-disposed onto the encapsulant layer 336, which is also substantially co-planar with the surfaces of cells 300, 316 and, therefore, attached to the PV cells simultaneously and in the same processing step with attachment of the encapsulant layer 336 to the cells.
In a related embodiment, additional cells are connected in series to the cells 330, 316 in a similar manner, by orienting and arranging these additional cells such that both the front side and the back side of the resulting module is built of cells having alternating positive and negative polls. An example of the resulting PV module 400 is shown in FIG. 4, which shows in a top plan view a two-dimensional array of bifacial solar cells mutually electrically coupled according to an embodiment of the invention. In the embodiment 400, thirty six (36) equal efficiency bifacial cells (e.g., 401 through 436) are provided. Each cell has positive and a negative face. For example, cell 401 is oriented such that its positive face points “up” out of the xy-plane of FIG. 4. Similarly, cell 402 is oriented such that its negative face points “up” out of the plane of FIG. 4. Individual cells of the embodiment 400 are configured in a string with cells having alternate electrical polarity facing the same direction. Adjacent cells are electrically serially connected with bus bars. Because of the alternate polarity arrangement, bus bars are adapted to electrically connect adjacent cells in a front-to-front, and back-to-back manner, rather than in a conventional back-to-front manner of the related art. In the embodiment 400, a bus bar 444 connects the positive face of cell 401 with the substantially co-planar negative face of adjacent cell 402. The positive face cell 402 is connected to the negative face of the adjacent cell 403 in the string by a bus bar 446, and so on, to result in a multiple serial connection of individual solar cells. The entire string of cells supplies its combined output current through bus bar 448.
According to an embodiment of the invention, high efficiency individual PV cells, whether or not bifacial, are reduced in area as compared to the size of the conventional PV cells. This may be achieved by dicing or scribing-and-breaking off-the-shelf available PV cells. An individual reduced-area cell generates proportionately reduced amount of current. For example, a module constructed of 125×125 mm²uncut PV cells with a single PV cell solar conversion efficiency of about 22% will have a normal operating current of about 3.5 A to about 4 A. As the size of an individual cell is increased to, for example, 150×150 mm², the current provided by such individual PV cell under otherwise identical conditions will increase to about 5-5.8 A while the voltage remains constant. Resistive power losses associated with a solar cell are proportional to the product of the series resistance with the square of the module current and can be assessed as P_LOSS=I²R_SERIES.
According to an embodiment of the invention, large cells are cut into smaller elements, thereby effectively reducing the current while maintaining the same voltage. In one embodiment, a cell is cut into three equal sized sections, reducing the current to a ⅓ of the current provided by the originally-size cell. If the electrical resistances in a module are maintained, the reduction of a per-cell output current will effectuate an approximate 9 time reduction in electrical power lost due to dissipation on resistance.
In one example, the individual-cell area-reduction approach was implemented with a Sanyo 180 W PV module. Conventionally, such PV module generates about 3.3 A of current at 1.19 Ohm resistance, thereby producing about 13 W in resistive losses. A Sanyo 180 W module was diced into three parts each having ⅓rd the area of the original module. The result was three smaller modules each generating ⅓rd the current of the original module. The electrical assembly of individual smaller modules according to an embodiment of the invention exhibited only about 1/9th of the resistive losses of the original module.
Methods according to embodiments of the invention include reducing the area of solar cells or modules to form sub-cells, followed by applying bus bar or similar electrical connectors made of higher resistivity, but otherwise similar, materials to connect the reduced-area sub-cells. For example, according to embodiments of the invention, conductive epoxy, wire mesh, charge collection tape “charge tape”, flex circuits/flexible PCBs, or other flexible, or more easily assembled bus structures can be used without incurring high resistive losses. In one embodiment, bus structures such as beads of electrically conductive epoxy are applied to an encapsulation film (such as the layer 336 of FIG. 3) prior to the assembly or lamination of the encapsulation film to the diced, reduced area solar sub-cells. In a related embodiment, a charge tape and/or metallic wire mesh was applied to the partially melted or softened encapsulation film prior to the assembly or lamination of the encapsulation film to the diced, reduced area solar sub-cells.
Embodiments according to the invention provide unexpected results that are advantageous over the related art. Bussing together bifacial solar cells in the same plane, without back-to-front or front-to-back connections as done in the related art, simplifies assembly, reduces the amount of handling to which each cell is subjected, and accordingly, reduces the chance of breakage and associated cost. Specifically, arrangements of equal efficient bifacial cells eliminate the conventional need that extra care must be taken not to confuse the front and back sides of cells when constructing a module, to en sure that the high efficiency side of the cells faces the sun. Additionally, the use of in-plane, front-to-front and back-to-back bussing, as opposed to conventional back-to-front/front-to-back bussing, simplifies assembly, makes open circuits easier to detect and repair, and results in a more robust package.
Additionally, by cutting/dicing bifacial cells into smaller pieces, the current associated with individual cell is reduced in proportion to the decrease in area of the cell. Reduced current allows for the use of bus conductors having higher resistance. For example, with low current, the use of charge tape, conductive epoxy, wire meshes, and other higher resistance bussing materials becomes possible. These bussing materials have certain advantages such as increased flexibility, ease of installation, lower cost, and increased durability. In particular, bussing materials can be added to encap sulant layers (EVA, Surlyn, etc.) in predetermined patterns by direct deposition in the case of the epoxy or a partial melt in the case of charge tape and wire meshes. The bussing material pattern can be pre-deposited to match the layout of the cells so as to fully contact the cell bus bars and fingers. The pattern may be deposited on the encapsulant layers on both sides of the cell plane with their corresponding pattern. In alternative embodiments, where the active semiconductor material of the solar cell is embedded in a waveguide, bussing material may be pre-deposited on one surface of the waveguide, which is later adhered to or index matched with the cell.
The invention has been described with reference to certain specific embodiments. Those skilled in the art of mine management and distributed computing systems generally may develop other embodiments of the present invention. The terms and expressions that have been used to describe certain embodiments in the foregoing specification are terms of description, rather than limitation, and, in using such terms, there is no intention to exclude equivalents of the features shown and described. Various configurations of individual PV cells (for example, cells including holograms), cooperation of PV modules in series of PV modules via flexible joints, and additional features of electrical connectors providing electrical communications between individual PV cells and/or individual PV modules of a series of the PV modules are discussed in above-mentioned patent applications incorporated herein by reference in their entirety. It should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention.

Claims

What is claimed is:

1. A solar module comprising:

a first bifacial solar cell having a first front active side and a first back active side, wherein the first front and back active sides have substantially equal solar conversion efficiencies, the first front active side having a first electrical polarity, the first back active side having a second electrical polarity that is opposite to the first electrical polarity;

a second bifacial solar cell having a second front active side and a second back active side, wherein the second front and back active sides have substantially equal solar conversion efficiencies, the second front active side having the second electrical polarity and the second back active side having the first electrical polarity,

the first and second bifacial solar cells oriented in series such that the first front active side is facing in substantially the same direction as the second front active side,

and

a bus bar electrically coupling the first front active side to second front active side.

2. A solar module according to claim 1, wherein the first and second bifacial solar cells are arranged adjacent to one other, and the first front active side is substantially coplanar with the second front active side.

3. A solar module according to claim 1, further comprising

a third bifacial solar cell having a third front active side and a third back active side, wherein the second front and back active sides have substantially equal solar conversion efficiencies,

the third front active side having the first electrical polarity, the third back active side having the second electrical polarity,

the third bifacial solar cell oriented such that the third front active side faces substantially the same direction as the first front active side;

and

a bus bar electrically coupling the second back active side to the third back active side.

4. A solar module according to claim 1, further comprising an encapsulant layer disposed on the first front active side and the second front active side such as to cover the bus bar.

5. A method for fabrication of a photovoltaic (PV) module, comprising:

separating a PV cell having an original size into a plurality of PV sub-cells, each sub-cell having a size smaller than the original size;

electrically coupling the sub-cells in series such that a first side of the first sub-cell having a first electrical polarity is electrically connected to a first side of the second sub-cell having a second electrical polarity, the first side of the first sub-cell and the first side of the second sub-cell oriented to face substantially the same direction.

6. A method according to claim 5, wherein the first side of the firs sub-cell is positioned to be substantially co-planar with the first side of the second sub-cell.

7. A method according to claim 5, wherein said electrically coupling includes

depositing a conformable electrically conductive material on a first surface of an optically-transparent encapsulation layer; and

covering the first and second sub-cells with said optically-transparent encapsulation layer such that the first surface of the optically transparent encapsulation layer carrying the conformable electrically-conductive material faces the first side of the first sub-cell and the first side of the second sub-cell.

8. A method according to claim 7, wherein said depositing includes depositing at least one of conductive epoxy, wire mesh, or charge collection tape.

9. A method according to claim 5, further comprising disposing a holographic element in optical communication with at least one of the first and second sub-cells between the optically-transparent encapsulation layer and a surface of the at least one of the first and second sub-cells.