Patent Application
20140345674
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1. Field
This disclosure is generally related to the fabrication of solar modules. More specifically, this disclosure is related to fabrication of a solar module that is resistant to moisture ingress.
2. Related Art
The negative environmental impact of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photovoltaic effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal band gaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple single-junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
For homojunction solar cells, minority-carrier recombination at the cell surface due to the existence of dangling bonds can significantly reduce the solar cell efficiency; thus, a good surface passivation process is needed. In addition, the relatively thick, heavily doped emitter layer, which is formed by dopant diffusion, can drastically reduce the absorption of short wavelength light. Comparatively, heterojunction solar cells, such as Si heterojunction (SHJ) solar cells, are advantageous.
When fabricating solar cells, a layer of transparent conducting oxide (TCO) is often deposited on the a-Si emitter layer to form an ohmic contact. However, compared with traditional diffusion-based solar cells, the TCO-based SHJ solar cells are more susceptible to moisture ingress. Not only do they tend to lose their material properties when exposed to moisture, they may also serve as a medium through which moisture can reach the junction of the solar cell, thereby degrading the cell performance drastically.
One embodiment of the present invention provides a photovoltaic (PV) module. The PV module includes a front-side glass cover facing sunlight, a plurality of interconnected PV cells situated below the glass cover, a plurality of bussing wires electrically coupled to the PV cells, and a back-sheet situated below the PV cells. The back-sheet comprises a metal layer sandwiched between a top insulation layer and a bottom insulation layer. The back-sheet comprises a cut slot to facilitate the bussing wires to thread through the cut slot to reach a junction box situated below the back-sheet. The PV module further comprises one or more insulation layers inserted between the bussing wires and sidewalls of the cut slot in the back-sheet. The insulation layers are configured to insulate the bussing wires to the metal layer in the back-sheet.
In a variation on the embodiment, the insulation layers in the back-sheet include one or more of: polyethylene terephthalate (PET), Fluoropolymer, polyvinyl fluoride (PVF), and polyamide; and the metal layer in the back-sheet comprises Al.
In a further variation, the back-sheet includes one or more of: a dyMat APYE® (registered trademark of Coveme of Bologna, Italy) back-sheet, a Protekt® Al back-sheet made by Madico, Inc., and an Al-based back-sheet made by Isovolta Group or Dunmore Corporation.
In a variation on the embodiment, the one or more insulation layers include at least one of: dielectric tape, a tube made of dielectric materials, a non-metal partial back-sheet, and a partial back-sheet with a metal interlayer.
In a further variation, the dielectric tape includes Kapton® tape.
In a further variation, the tube includes at least one of: a polyethylene terephthalate (PET) tube and a polyvinyl fluoride (PVF) tube.
In a further variation, the non-metal partial back-sheet includes a Protekt® (registered trademark of Madico, Inc. of Woburn, Mass.) back-sheet or a Tedlar® (registered trademark of E. I. du Pont de Nemours and Company of Wilmington, Del.) back-sheet.
In a variation on the embodiment, the PV module further comprises an additional partial back-sheet situated between the PV cells and bussing wires at a location where the bussing wires thread through the cut slot. The additional partial back-sheet includes a metal interlayer situated between a top insulation layer and a bottom insulation layer, and the additional partial back-sheet is configured to: insulate the bussing wires to the backside of the solar cells and block potential moisture ingress from the cut slot in the back-sheet.
In a further variation, the additional partial back-sheet includes an Al interlayer.
In a variation on the embodiment, the PV cells include at least one double-sided tunneling junction solar cell.
In a variation on the embodiment, the PV cells and the bussing wires are encapsulated between the front-side glass cover and the back-sheet during a lamination process, forming a laminated structure.
In a further variation, encapsulating the PV cells and the bussing wires involves using a low moisture vapor transmission rate (MVTR) encapsulant that comprises one or more of: polyolefin and ionomer.
In a further variation, the PV module further comprises a metal frame configured to hold the laminated structure.
In a further variation, the metal frame is sufficiently large to ensure a predetermined minimum distance is maintained between corners and edges of the laminated structure and the metal frame, thereby facilitating application of insulation materials with sufficient thickness.
In a further variation, corners of the laminated structure are wrapped with one or more layers of dielectric tape.
In a variation on the embodiment, the PV cells include one or more of: a transparent conducting oxide (TCO) layer acting as an electrode and an anti-reflecting coating (ARC) layer.
In the figures, like reference numerals refer to the same figure elements.
The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
Embodiments of the present invention provide a solar module that is moisture resistant. More specifically, the solar module includes a glass front cover and an Al-based back-sheet that has a low moisture vapor transmission rate (MVTR). The Al-based back-sheet typically includes a slot to allow bussing wires to pass through to be connected to the junction box located at the back of the solar module. To minimize moisture leakage through the slot, an additional Al-based partial back-sheet can be inserted directly beneath the slot. Moreover, to prevent possible shorting of the internal circuits of the solar cells due to the Al layer included in the Al-based back-sheet, in some embodiments of the present invention, additional insulating layers, which can include insulating tapes or polyethylene terephthalate (PET) tubes, are wrapped around the bussing wires where they pass through the Al-based back-sheet. Special attention is also paid at the corners of the solar module to prevent arcing. In some embodiments, additional insulating materials (such as tapes or frame sealant) are applied at the corners. Additionally, low-MVTR materials, such as polyolefin or ionomer, can be used as encapsulant in the laminated module to ensure the moisture resistant capability of the module.
It has been shown that tunneling junction solar cells can provide superior performance because the quantum-tunneling barrier (QTB) layers can effectively passivate the surfaces of the base layer without compromising the carrier collection efficiency.
Compared with traditional diffusion-based solar cells, the tunneling junction solar cells that include a-Si and TCO layers are more susceptible to moisture. More specifically, it is well known that many TCO materials, such as ZnO or Al:ZnO, are moisture-sensitive. They may lose their material properties. For example, a ZnO film may become rough or porous when exposed to moisture for a prolonged time. On the other hand, although indium-tin-oxide (ITO) can outperform ZnO in terms of being degraded less under moisture exposure, it still experiences certain levels of degradation when exposed to both heat and moisture. Note that once the TCO film becomes porous, it allows the moisture to reach the solar cell junction, thus degrading the solar cell performance drastically.
To prevent penetration of the moisture to the solar cells, a photovoltaic (PV) module should provide moisture-resistant packaging. To assess the quality of the PV module, the International Electrotechnical Commission (IEC) and the Underwriters Laboratories (UL) standards introduce testing protocols that involve damp heat (DH) tests and humidity freeze (HF) tests. A damp heat DH1000 test specifies a 1000-hour exposure to damp heat (DH) at 85° C. and 85% relative humidity (RH). A typical HF test specifies 10 temperature cycles from −40° C. to 85° C. at 85% RH. Moreover, recent emphasis on potential induced degradation (PID), which can also be affected by heat and moisture, also puts pressure on the control of moisture ingress, because charged ions from the superstrate (such as a glass cover) would require moisture as a medium to migrate to the solar cell to degrade the quality of the solar cell junction.
To meet the IEC and/or UL standards for PV modules, the PV modules need to have reliable electrical interconnects and packaging. Due to their sensitivity to moisture, special care is needed for PV modules with TCO-based solar cells, such as the one shown in
A possible low-MVTR packaging scheme involves using glass for both front and back covers. However, the increased weight of such modules may make them unsuitable for certain applications. For example, they may not be suitable for installation on roofs with limited load-bearing capacity. A different low-MVTR packaging approach may involve using encapsulant materials with lower MVTR, such as polyolefin. However, such an encapulant material does not have a proven 25 years of field reliability track record. A more desirable low-MVTR packaging scheme involves using a glass front cover and a back-sheet with a low MVTR, such as a back-sheet with an aluminum interlayer. The Al interlayer provides a high-quality vapor barrier. An exemplary lamination back-sheet that includes an Al interlay can be a dyMat APYE® (registered trademark of Coveme of Bologna, Italy) back-sheet. Other vendors, notably Madico Inc. of Woburn, Mass., Isovolta Group of Austria, and Dunmore Corporation of Bristol, Pa., can also provide Al-based back-sheet.
In
To prevent any potential contact between the bussing wires and the Al interlayer, in some embodiments of the present invention, an additional insulation layer is introduced between the bussing wires and back-sheet 420. Various ways can be used to insert the insulation layer, including but not limited to: wrapping each individual bussing wire with an insulating tape or film, inserting each individual bussing wire into a tubing made of insulating materials (such as a PET tubing), laminating the bussing wires into one or two layers of back-sheets that contain only insulating materials, etc. Note that special attention is needed when selecting materials for the additional insulation layer to make sure that it has sufficient dielectric strength to meet the IEC and UL insulation requirements, that it is compatible with the subsequent lamination process (which may be performed under high temperature, such as around 130-150° C. for EVA-based lamination), and that it is flexible enough to survive the required DH and HF testing cycles.
In some embodiments of the present invention, one or more layers of polyimide film, such as Kapton® (registered trademark of E. I. du Pont de Nemours and Company of Wilmington, Del.) tapes, are used to wrap around each bussing wire at locations where the bussing wires may potentially be in contact with the Al interlayer. Note that the Kapton® tapes have sufficient dielectric strength and a thermal operating range that can be up to 400° C. Moreover, Kapton® tapes can maintain good adhesion during lamination; hence, they are less likely to peel off.
In addition to dielectric tape and partial non-Al back-sheets, it is also possible to use insulation tubing, such as a PET tube, to slip over each individual bussing wire. Similarly, EVA can be used to bond the insulation tubing with the bussing wire.
In addition to the insulation problem, another problem needs to be addressed for PV modules with a back-side accessing slot cut in the back-sheet. The slot not only exposes the Al interlayer, as we have explained previously, but may also allow moisture from outside of the PV module to migrate from the back-side of the PV module to the solar cells. In the solutions shown in
Note that a portion of bussing wire 632 that passes through slot 636 is wrapped by an insulation layer 638, which ensures a good electrical insulation between bussing wire 632 and the Al-interlayer included in Al-based back-sheet and exposed by slot 636. Note that insulation layer 638 can include, but is not limited to: layers of tape with high dielectric strength, insulating tubes bonded with EVA, and insulating back-sheets.
PV module 620 further includes an Al-based partial back-sheet 640 situated between terminal bussing wire 632 and the backside of the PV cells. Note that, although not shown in
In another embodiment, instead of EVA, low-MVTR materials, such as Polyolefin (available from the 3M Company of Saint Paul, Minn.) and lonomer (available from E. I. du Pont de Nemours and Company of Wilmington, Del.) can also be used to ensure moisture ingress from the slot 636, or more importantly, from all the edges of the panel.
In addition to the insulation problem and moisture-ingress problem induced by the slot, another problem faces the PV module that implements the Al-based back-sheet. More particularly, along the edges and corners where the Al-based back-sheet is cut, the Al-interlayer may be exposed or not adequately insulated by sealant material, thus causing either shorting or arcing between the encapsulated stack and the metal frame of the PV module. This problem is generally more severe at corners than at the edges because at the four corners the frame sealant tends to spread thinner at the corner in order for the encapsulated stacks to fit snugly in the frame. The inadequate application of the frame sealant, which supposedly serves as both an insulation layer and a moisture blocker, can cause the PV panel to fail the IEC 61215/UL1703 insulation test, which requires Safety Class A panels to have an insulation resistance of 400 MΩ/m2 tested at a voltage of (1000 Volts+2*system voltage). This issue is made worse if the Al frames are shorted to the Al-based back-sheet, causing a large potential drop between the bussing wires and the slotting area in the back-sheet, which makes it more important to ensure good insulation around the bussing wire at the slot area. In addition, the possible arcing due to discharge between the corners or edges of the Al-based back-sheet and the metal frame raises the concern of fire hazards.
To address this corner/edge problem, special care is needed to make sure that the frame sealant is adequately applied. In some embodiments of the present invention, the metal frame holding the laminated layer stack is enlarged (compared with the conventional PV modules) to ensure that sufficient sealant can flow to all corners. For example, one may need to make sure that there is a predetermined minimum distance between the metal frame and edges and/or corners of the laminated layer stack to ensure that insulation material (such as sealant) of a pre-determined thickness can be inserted between the meal frame and the laminated layer stack. In some embodiments, the minimum distance may be between 1 and 3 mm. In some embodiments, one or more layers of dielectric tape, such as Kapton® tapes or other types of tape, are wrapped around the corners of the laminated stacks to ensure sufficient insulation between the back-sheet and the metal frame.
When assembling the PV module, an internal insulation layer with low MVTR is applied between the backside of the PV cells and the bussing wires (operation 706). In some embodiments, the internal insulation layer includes EVA and an Al-based back-sheet material, such as the dyMat APYE® back-sheet. The insulation layer is placed directly above the slot on the back-sheet to prevent possible moisture ingress from the slot.
Subsequently, bussing-wire leads that connect the bussing wires of the solar cell internal circuit assembly and the junction box are prepared, which involves adding additional insulation layers to the bussing-wire leads at locations where the bussing wires may contact the slot edge (operation 708). In some embodiments, tape with high dielectric strength is wrapped around portions of the bussing wires. In some embodiments, one or more layers of back-sheet materials (non-metal based) are pre-laminated onto portions of the bussing wires. In some embodiments, insulating tubes with EVA insertions are slipped on the bussing wires, and are bonded to the bussing wires by the EVA insertions.
Once the additional insulation layers are in place, the bussing-wires leads can be soldered to the bussing wires of the internal circuit assembly (operation 710), and are threaded through the slot to connect to the junction box located at the backside of the PV module (operation 712). Optionally, additional dielectric material can be inserted in the slot, filling any voids left between the bussing wires and the slot, to achieve more robust insulation (operation 714). A lamination process is then performed (operation 716), followed by subsequent trimming and framing of the laminated stack (operation 718), and connection to the junction box (operation 720) to finish the rest of the module fabrication.
The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.
This application claims the benefit of U.S. Provisional Application No. 61/827,429, Attorney Docket Number SSP13-1003PSP, entitled “Photovoltaic Module That Is Moisture Ingress Resistant,” by inventors Bobby Yang, Jiunn Benjamin Heng, Jianming Fu, and Zheng Xu, filed 24 May 2013.
| Number | Date | Country | |
|---|---|---|---|
| 61827429 | May 2013 | US |
