The subject invention relates to photovoltaic systems, thermoelectric systems, and in particular a hybrid thermoelectric/photovoltaic device.
Photovoltaic (PV) systems convert photons into electricity while thermoelectric (TE) systems convert heat into electricity. Several prior art references propose hybrid photovoltaic/thermoelectric systems.
It is possible, for example, to attach a commercially available photovoltaic cell onto the top of a commercially available thermoelectric module. The interface between the photovoltaic cell and the thermoelectric module, typically an adhesive, solder, or other thermal interface material, however, presents a thermal interface which lowers the efficiency of the system.
U.S. Pat. No. 3,956,017 discloses a solar cell and a heat conduction metal layer made of silver or aluminum provided on the rear surface of the solar cell using vacuum deposition technology. A p-type semiconductor and an n-type semiconductor are soldered to the heat conduction metal layer to form a thermoelectric converter. Lead wires, interconnected via a resistor, are soldered to the p-type semiconductor and the n-type semiconductor to electrically interconnect them. The solar cell converts sunlight into electricity via the optoelectric effect. At the same time, the solar cell is heated and this heat is converted to electricity by the thermoelectric module via the Seebeck effect. Published patent application No. 2006/0225783 also discloses adding thermoelectric material to a photovoltaic cell.
Still, those skilled in the art continue attempts at optimizing hybrid photovoltaic/thermoelectric systems. See for example “Photovoltaic/Thermoelectric Hybrid Systems: A General Optimization Methodology,” Applied Physics Letters 92, 243503 (2008).
One issue in such hybrid systems is that the efficiency of the photovoltaic cell decreases as its temperature increases. Thermoelectric efficiency, on the other hand, increases as temperature differences increase. Cost and manufacturability are also issues.
In one aspect of the subject invention, a thermoelectric subsystem is added to a photovoltaic cell to both cool and thus increase the efficiency of the photovoltaic cell and also to increase the electrical output of the overall system. One proposed hybrid system is also cost effective to manufacture. The subject invention results from the partial realization, that in one preferred embodiment, an array of thermoelectric couples and a heat sink can be added directly to a commercially available solar cell using a variety of manufacturing techniques not previously employed in fabricating such hybrid systems.
The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives.
The subject invention features a combined thermoelectric/photovoltaic device comprising a photovoltaic cell with a common electrode and an electrically insulative, thermally conductive layer applied to the common electrode. The hybrid device includes an array of thermoelectric couples each including a p-type semiconductor element and an n-type semiconductor element. There is an electrically conductive bridge for each thermoelectric couple formed on the electrically insulative thermally conductive layer. A more complete device further includes a cold plate, and a second electrically insulative, thermally conductive layer applied to the cold plate. Electrically conductive bridges electrically connect adjacent thermoelectric couples formed on the second electrically insulative thermally conductive layer. The cold plate may be solid, or may include passages such as fins for a fluid. Alternatively, of the cold plate can include a porous structure.
In one version, the electrically insulative thermally conductive layers may include aluminum nitride, aluminum oxide, a ceramic material, glass, or a polymeric material. The electrically insulative thermally conductive layers may also include electrodes electrically connected to the bridges.
Typical p-type semiconductors include materials such as Bismuth Telluride and typical n-type semiconductor elements include materials such as Antimony Telluride. There may also be metallization between the thermoelectric couples and their respective bridges.
The subject invention also features a method of making a combined thermoelectric/photovoltaic device. In one example, the method comprises applying (e.g., via deposition) a first electrically insulative thermally conductive layer to the common electrode of a photovoltaic cell, forming an array of electrically conductive bridges on the first electrically insulative thermally conductive layer, and fabricating p-type semiconductor elements and n-type semiconductor elements. A thermoelectric couple is secured to each bridge. Each thermoelectric couple includes a p-type semiconductor element and an n-type semiconductor element.
Fabricating the semiconductor elements may include dicing plates of the p- and n-type elements. These p- and n-type plates can be metallized prior to dicing. A pick and place mechanism can be used to secure the couples to the respective bridges. The couples can be soldered or adhered to their respective bridges.
In one example, fabricating the couples and securing them to their respective bridges includes growing the thermoelectric couples on their respective bridges. Printing techniques can be used and the thermoelectric couples may be sintered.
A more complete method further includes applying a second electrically insulative thermally conductive layer to a cold plate and forming an array of electrically conductive bridges on the second electrically insulative thermally conductive layer electrically connecting adjacent thermoelectric couples.
In one example, the p-type and n-type semiconductor elements are first assembled on the electrically conductive bridges of the second electrically insulative thermally conductive layer and they are then secured to their respective bridges formed on the first electrically insulative thermally conductive layer applied to the common electrode of the photovoltaic cell. The electrically conductive bridges can be formed on the first electrically insulative thermally conductive layer and the first electrically insulative thermally conductive layer is then applied to the common electrode. A photovoltaic material is then applied to the common electrode. In some examples, electrodes are formed on the insulative thermally conductive layers.
An exemplary method of manufacturing a hybrid thermoelectric/photovoltaic system includes applying a first electrically insulative thermally conductive layer onto the common electrode of a photovoltaic cell, forming, on the first electrically insulative thermally conductive layer, an array of electrically conductive bridges, and securing one end of a thermoelectric couple to each bridge. A second electrically insulative thermally conductive layer is applied to a cold plate. An array of electrically conductive bridges is formed on the second electrically insulative thermally conductive layer. The opposite ends of the thermoelectric elements of each couple are secured to an electrically conductive bridge on the second electrically insulative thermally conductive layer to electrically connect adjacent thermoelectric couples. Forming the array of electrically conductive bridges on the first electrically insulative thermally conductive layer may include photolithography techniques.
In one example, securing one end of each thermoelectric couple to each bridge on the first electrically insulative thermally conductive layer includes growing the p-type and n-type elements on the bridges of the first electrically insulative thermally conductive layer. The opposite ends of the thermoelectric couples may be secured to an electrically conductive bridge on the second electrically insulative thermally conductive layer by employing a pick and place mechanism.
In another example, securing the opposite ends of the thermoelectric elements of each couple to a bridge on the second electrically insulative thermally conductive layer includes growing the p-type and n-type elements on the bridges of the second electrically insulative thermally conductive layer.
Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:
Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. Also, the claims hereof are not to be limited only to the described embodiments. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.
The thermoelectric converter includes an array 36 of thermoelectric couples. Each couple includes a p-type semiconductor element and an n-type semiconductor element. For example, couple 38a includes p-type semiconductor element 40a and n-type semiconductor element 42a and couple 38b includes p-type semiconductor 40b and n-type semiconductor 42b. The p-type elements may be undoped Bismuth Telluride (Bi2Te3) and the n-type elements may be Antimony Telluride (Sb2Te3). Other materials may be used.
Electrically conductive bridge 44a formed on electrically insulated thermally conductive layer 34 electrically connects couple 38a and electrically conductive bridge 44b formed on electrically insulated thermally conductive layer 34 electrically connects couple 38b. These bridge elements electrically connect the p-type and n-type semiconductors elements of each couple. Conductive material such as solder, metal electrodes, conductive adhesives and the like may be used. Photolithography techniques may also be used to pattern the bridges on layer 34. Electrode 45 serves to connect p-type element 40d to a common bus as discussed below.
In one example, square plates of n-type material and p-type material are procured and metallized in an e-beam evaporator or using methods previously described. See metallization 43a for element 40a. Chromium (Cr), Gold (Au), Titanium (Ti), and/or Platinum (Pt) materials can be used in addition to other metals. The plates are then diced to produce the individual p-type and n-type elements. The thermoelectric elements may also be produced individually to near net-shape via injection molding or extruded to near net-shape (cross section) and then diced to length. A pick and place machine is used to attach the array of thermoelectric couples to their respective bridges on layer 34. Solder or a conductive adhesive, glass frit, or other suitable material may be used to secure the individual elements to the respective bridges on layers 34 and 54. Cold plate 50 with layer 54 and bridge 56a and the like may be preassembled and then attached to the opposite end of the semiconductor elements.
In one version, a commercially available PV module (Evergreen Solar 1″×3″ module) is used. The TE module is assembled on the back face of the PV. Prior to processing, the PV module is mounted on a protective surface to shield the PV module during processing. In order to control the flow of charge through the TE, a specific electrode pattern is desired. Typically PV construction utilizes a backside common (ground) electrode which spans the entire back face of the module. This electrode is simultaneously isolated from the TE module and addressable for connecting to adjacent PV modules, step 100.
To provide this insulation, a thin layer of thermally conductive, electrically insulating material is applied via RF sputtering, step 102. The actual thickness may be based on open circuit voltage of PV modules used. The process is conducted using a magnetron sputtering unit. This thin layer provides sufficient electrical isolation while still allowing for adequate conduction of heat from the PV module.
To allow for the addressing of the back face electrode of the PV, photolithography is used to mask a portion of the existing PV electrode, step 100. Prior to the lithography process, the PV module is cleaned and degreased to remove any contaminants that might interfere with subsequent processing. Photoresist is applied to the PV using standard methods and cured. After curing, the back face resist will be exposed on a mask aligner using a mask designed to provide the appropriate electrode patterning. The exposed PV module is immersed in an aqueous solution to develop the exposed resist.
After lithography, the PV module is coated with an insulating layer, via depositions methods previously discussed, such as RF or DC sputtering, step 102. Once the deposition is complete, a solvent based liftoff process may be used to remove the material/resist over the PV module buss bar, step 104. This process may also be used to electrically isolate the cold side substrate of the TE Module, as discussed below.
Electrode patterning for the hot side and cold side of the TE module is performed using an electron beam evaporator. Photolithography is used to mask the surface of the insulating material for the electrode pattern, using a process already described. A different mask, specific to each electrode pattern is designed and used. The front side electrode is made of a conductive material with an appropriate thickness to allow for a reliable electrode that can be soldered or welded, step 108.
In the case of the cold side electrode, the pattern is deposited onto the heat sink material. This material will be cleaned and degreased to remove any contaminants prior to processing. The insulating layer is deposited on this material prior to electrode deposition as previously described, steps 110 and 112.
In order for the TE module to function both p-type and n-type, TE materials are preferred. Undoped Bismuth Telluride (Bi2Te3) and Antimony Telluride (Sb2Te3) may be used. The material can be purchased in plates and the electrode applied in an ebeam evaporator. After the electrode is applied, step 122, the plates are diced to produce the individual elements.
Once all of the sub-elements have been prepared, the module is assembled using a pick-and place machine. Graphite fixtures are designed and fabricated to ensure proper alignment of the sub-elements during subsequent operations. Graphite combs can be interdigitated between the TE pillars to hold them in place during subsequent processing.
Two methods of fabricating the module are preferred: solder or conductive glass frit attachments and electrically conductive adhesives. Solder attachments provide the ideal thermal and electrical conductivities required but the processing temperatures may not be suitable for all organic PVs. While low temperature solders exist, it is possible that even these temperatures can be too high for some organic PVs.
Solder tabs used for attachments are easily handled by the pick-and-place machine. An automated mix meter system can be used to apply adhesive to the electroded substrates.
Once the module is fully assembled, it is processed to either reflow the solder or cure the adhesive. In the case of the solder, the module is placed in a reflow oven. For adhesive applications, a fixture can be used to apply constant pressure to the module, while it cures in an oven.
Four exemplary methods of fabrication are discussed below including: fabrication of a hybrid module with a commercially available PV as the base, fabrication of a hybrid module with a commercially available PV wherein the TE cold side serves as the base, fabrication of a hybrid module with a polycrystalline PV, formed as part of the process, by starting with the TE cold side, and fabrication of a hybrid module with a polycrystalline PV, formed as part of the process, by starting with the PV.
These methods work for many types of PV materials and TE materials that can be processed by these methods and temperatures. The conductive materials are chosen based on the nature of the PV, i.e., maximum processing temperature, compatibility and chemical resistance.
In
Once the isolation layer is applied, modification to the surface to allow for adhesion of solder or other materials may be required. The surface should be modified such that the TE elements are appropriate electrically isolated. The requirement for this step depends on the method of adhesion. Methods such a conductive glass frit, conductive adhesives, etc. may not require this step or may require different materials. Solders may require a metal pad, while adhesive may require a primer such as an organosilane, organometallic, etc. The adhesion layers may be applied using printing methods (ink, screen, etc) or applied via the use of lithographic methods, where a pattern is created and the materials applied. Application methods include PVD, CVD, sputtering, E-beam deposition, electro plating, chemical reactions, etc (including methods previously discussed).
Once the insulation layer has been deposited and the surface prepared for adhesion, thermoelectric elements 40a, 42a,
An alternate method is to grow the elements. In one example, a TE powder/binder or powder only is applied directly to the PV using techniques such as ink jet printing, screen printing, stereo lithography, and the like. The structure created is a three dimensional interdigitated structure where the current flows from p-type material to n-type material producing electricity.
The cold side plate is then prepared using the methods previously discussed. An AlN or similar material plate 54,
The above structure can also be made using the techniques similar to those discussed above and as illustrated in
Fabrication of a hybrid module from ink jet printing or similar methods is also possible. The following steps can be done in either order, i.e., PV first or TE first. The TE first process,
The tie layers, (e.g., bridge 56a and electrode 47,
Electrically insulating layer 34,
The resulting module is then sintered. Sintering includes pressureless sintering, hot and cold isostatic pressing methods, vacuum sintering, and the like.
In a reverse method, all the steps are the same, except that the hot side electrode and antireflective coating may be applied after sintering if the process used does not allow for bridging of material. These steps are shown in
Setter layer 182,
The result in one preferred embodiment is an integrated system where a thermoelectric array and a heat sink are added to a photovoltaic cell to both cool and thus increase the efficiency of the photovoltaic cell and also to increase the electrical output of the overall system. Cost effective techniques are preferably used to mass manufacture hybrid systems in accordance with the subject invention.
Although specific features of the invention are shown in some drawings and not in others, however, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments.
In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.
Other embodiments will occur to those skilled in the art and are within the following claims.