Abstract:
A dye-sensitized solar cell including ZnO nanowire arrays grown of a flat substrate for harvesting solar energy is integrated with a piezoelectric nanogenerator for harvesting ultrasonic wave energy. The two energy harvesting approaches work simultaneously or individually and can be integrated in parallel or serial for raising the output current, voltage or power, respectively. A solar cell employs an optical fiber and semiconductor nanowires grown around the fiber. A p-n junction based design, organic-inorganic heterojunction, or a dye-sensitized structure is built at the surfaces of the nanowires. Light entering the fiber from a tip propagates through the fiber until it enters a nanowire where it reaches a photovoltaic element. Light entering the fiber cannot escape until it interacts with a photovoltaic element, thereby increasing the solar conversion efficiency. The fiber can transmit light, while the nanowires around the fibers increase the surface area of light exposure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/056,214, filed on May 27, 2008 the entirety of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to electric power generation systems and, more specifically, to a hybrid solar power and mechanical power generating system. 
         [0004]    2. Description of the Prior Art 
         [0005]    There are generally three different sources for scavenging energy from the environment: solar energy, thermal energy and mechanical energy (such as wind energy). Solar cells are typically used to collect solar energy and transform it into electrical energy. However, solar cells cannot produce electricity at times when there is insufficient ambient light, such as in the evening. 
         [0006]    Mechanical energy, from large-scale winds to micro-scale vibration, is almost always available. Thus, a system for converting mechanical energy to electricity would be able to produce electricity almost anywhere at almost any time. 
         [0007]    Recently, a ZnO nanowire-based nanogenerator that can effectively convert small scale mechanical vibration energy into electricity has been demonstrated. However, the power output of the nanogenerator was relatively low in some applications. 
         [0008]    The highest efficiency solar cells to date (40.7%) employ optical lenses to focus light onto the photovoltaic materials. These concentrators are expensive and have therefore been limited in scalability. 
         [0009]    Driven by the telecommunications industry, there has been a tremendous amount of research in the past two decades into fiber optic cables as a medium for transporting data in the form of light. At present, a mature infrastructure is in place for mass production of optical fibers. State-of-the-art fibers can transport light up to distances of 500-800 km with low signal attenuation owing to the physical principle of total internal reflection. Given these characteristics, fiber optic cables are potentially an ideal medium for directing light towards a photovoltaic material system for solar energy harvesting. 
         [0010]    Therefore, there is a need for a hybrid nanogenerator that combines a highly efficient solar cell with a piezoelectric nanogenerator that can generate power continuously in a range of different and changing environments. 
         [0011]    There is also a need for a scalable optical system which can transport the light energy to the photovoltaic elements. 
       SUMMARY OF THE INVENTION 
       [0012]    The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a hybrid solar and mechanical power generator that includes a solar power generating portion and a piezoelectric nanowire vibrational power generating portion. The solar power generating portion electrically coupled to a first electrode. The piezoelectric nanowire vibrational power generating portion includes an electrical contact structure electrically coupled to and extending downwardly from the first electrode and disposed adjacent to the solar power generating portion. A plurality of piezoelectric semiconductor nanorods extends upwardly from a second electrode that is spaced apart from the first electrode so as to be directed toward the electrical contact structure. The plurality of piezoelectric semiconductor nanorods are configured to release electrons across a Schottky barrier formed between the piezoelectric semiconductor nanorods and the electrical contact structure when mechanical energy is applied to the piezoelectric nanowire vibrational power generating portion. 
         [0013]    In another aspect, the invention is a hybrid solar-mechanical power generator that includes a solar power generating portion electrically coupled to a first electrode and a piezoelectric nanowire vibrational power generating portion. The piezoelectric nanowire vibrational power generating portion includes a plurality of piezoelectric semiconductor nanorods extending downwardly from the first electrode and an electrical contact structure electrically coupled to and extending upwardly from a second electrode and spaced apart from the first electrode. The electrical contact structure is disposed so as to be directed toward the plurality of piezoelectric semiconductor nanorods and configured so that when mechanical energy is applied to the piezoelectric nanowire vibrational power generating portion the piezoelectric semiconductor nanorods contact the electrical contact structure and the nanorods release electrons across a Schottky barrier formed between the piezoelectric semiconductor nanorods and the electrical contact structure. 
         [0014]    In another aspect, the invention is a solar power element that includes an optical fiber. A conductive outer cladding surrounds the optical fiber. A plurality of nanorods extends radially outwardly from the conductive outer cladding. 
         [0015]    In yet another aspect, the invention is a method of making a hybrid solar and mechanical power generator, in which a conductive material layer is applied to a first substrate. A nanorod seed material is applied to a portion of both the conductive layer and to the first substrate. A first plurality of nanorods is grown from the nanorod seed material. A liquid is applied to the nanorods grown from the substrate. The liquid is a liquid that will cause nanorods in contact therewith to agglomerate into a plurality of pointed structures. 
         [0016]    A metal layer is applied to the pointed structures. A second plurality of nanorods is grown from a second substrate. The second substrate is disposed so as to be parallel to the first substrate and so that the second plurality of nanorods extends toward the plurality of pointed structures. A flexible spacer is placed between the first substrate and the second substrate. 
         [0017]    These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic elevational view of a first representative embodiment of a hybrid solar and mechanical electrical power generator. 
           [0019]      FIG. 2A  is a schematic elevational view of a second representative embodiment of a hybrid solar and mechanical electrical power generator. 
           [0020]      FIG. 2B  is a schematic elevational view of an embodiment disposed on textile threads and intertwined. 
           [0021]      FIGS. 3A-3G  are schematic diagrams demonstrating a first method of assembling a hybrid solar and mechanical electrical power generator. 
           [0022]      FIGS. 4A-4F  are schematic diagrams demonstrating a second method of assembling a hybrid solar and mechanical electrical power generator. 
           [0023]      FIG. 5A  is a top plan view of a fiber-mounted solar power generator. 
           [0024]      FIG. 5B  is a cross-sectional view of the fiber-mounted solar power generator shown in  FIG. 5A , taken along line  5 B- 5 B. 
           [0025]      FIG. 5C  is a cross-sectional view of an encapsulated fiber-mounted solar power generator. 
           [0026]      FIG. 5D  is a top plan view of a dye-sensitized encapsulated fiber-mounted solar power generator. 
           [0027]      FIG. 5E  is a cross-sectional view of the dye-sensitized encapsulated fiber-mounted solar power generator shown in  FIG. 5D , taken along line  5 E- 5 E. 
           [0028]      FIG. 6  is an elevational view of a fiber-mounted solar power generator. 
           [0029]      FIG. 7  is a top plan view of a bundle of fiber-mounted solar power generators. 
           [0030]      FIG. 8  is an elevational view of an array of fiber-mounted solar power generators. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
         [0032]    As shown in  FIG. 1 , one embodiment of a hybrid solar and mechanical power generator  100  includes a dye-sensitized solar power generating portion  110  that is electrically coupled to a first electrode  120 . The solar power generating portion  110  includes a transparent substrate  112  (such as an ITO substrate) affixed to the first electrode  120 . A plurality of semiconductor nanorods  122  (such as ZnO nanorods), also referred to as “nanowires,” extend from the first electrode. A light absorbing material having a predetermined optical absorption range is applied to the plurality of nanorods  122 . In one embodiment, the light absorbing material includes a plurality of ruthenium-based dye particles  130 , such as cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium, also referred to as N719 Dye(B2). The dye particles  130  should have an optical absorption range that includes the wavelengths of light to be converted into electrical energy. The growth of ZnO nanorods is disclosed in more detail in U.S. patent application Ser. No. 11/608,865, filed on Dec. 11, 2006 by Wang et al. and U.S. Pat. No. 7,220,310, issued on May 22, 2007 to Wang et al. and U.S. Pat. No. 7,351,607, issued on Apr. 1, 2008 to Wang et al., the entirety of each of which is hereby incorporated by reference. The nanorods  122  can be grown, for example, by physical vapor deposition process or with a wet chemical process. 
         [0033]    A transparent housing  134  (which can include a layer of a metal such as gold) surrounds the nanorods  122  and an electrolyte  132  (such as an iodide based electrolyte) is disposed in the housing  134  and the nanorods  122 . The housing  134  acts as an electrical contact structure that is electrically coupled to the first electrode  120 . 
         [0034]    A piezoelectric nanowire vibrational power generating portion  150  is disposed parallel to the solar power generating portion  110 . The piezoelectric nanowire vibrational power generating portion  150  includes a second transparent substrate  152  (such as an ITO substrate) and a plurality of aligned piezoelectric semiconductor nanorods  162  (such as ZnO nanorods) extending upwardly from a second electrode  160 , which are directed toward the housing  134 . 
         [0035]    The solar power generating portion  110  is positioned relative to the vibrational power generating portion  150  so that the electrical contact structure  120  contacts the piezoelectric semiconductor nanorods  162  when mechanical force is applied to either portion (e.g., due to vibrational energy being applied thereto), the piezoelectric semiconductor nanorods  162  will contact the housing  134  and bend. Once bent, the piezoelectric semiconductor nanorods  162  will release electrons across a Schottky barrier formed between the piezoelectric semiconductor nanorods  162  and the housing  134 . 
         [0036]    As shown in  FIGS. 2A and 2B , in one embodiment the solar power generating portion  220  may be mounted on a first textile fiber  212  and the piezoelectric nanowire vibrational power generating portion  250  may be mounted on a second textile fiber  252  that is intertwined with the first textile fiber  212 . In this design, the relative movement of the two fibers  220  and  250  will generate electricity due to the principle of piezo-electronics. Meanwhile, shining of light will excite the solar cell portion on the back of the contact region to generate electricity as well. These fibers can also be interwoven into a fabric. 
         [0037]    As shown in  FIGS. 3A-3G , one method of making a hybrid solar and mechanical power generator  300 . Initially, as shown in  FIG. 3A , a plurality of nanorods  322  is grown from an electrode  320  affixed to a transparent substrate  310 . The nanorods  322  are then coated with a layer  322  of dye molecules, as shown in  FIG. 3B , to form a solar collecting unit  306 . 
         [0038]    As shown in  FIG. 3C , a second plurality of nanorods  344  is grown from a substrate  342 . An electrolyte  346 , such as a p-type polymer, is then applied to bury the nanorods  344 . As shown in  FIG. 3D , due to the high aspect ratio of the nanorods, pyramid-shaped extrusions  350  form as a result of the addition of the electrolyte  346 . As shown in  FIG. 3E , a metal layer  360  (such as a layer of gold, or any other metal that can form a Schottky junction with ZnO) is deposited on the electrolyte pyramid-shaped extrusions  350 , thereby forming an array  340  of conductive extrusions  362 . The metal layer  360  plays two roles: it acts as a cathode of the solar portion and as an electron collector for the piezoelectric nanogenerator portion. 
         [0039]    As shown in  FIG. 3F , a third plurality of nanorods  374  is grown from a substrate  372 . These form a piezoelectric semiconducting nanorod unit  370 . 
         [0040]    The hybrid generator  300  is then assembled by stacking the array  340  of conductive extrusions  362  on top of the piezoelectric semiconducting nanorod unit  370 , separating the with a spacer  380 , and placing the solar collecting unit  306  on the array  340  of conductive extrusions  362 , separating them with sealing spacer  382  and then injecting an electrolyte  384  into the solar collecting unit  306 . Alternately, the electrolyte  384  is applied to the solar collecting unit  306  prior to the application of the sealing spacer  382 . 
         [0041]    In an alternate embodiment, as shown in  FIGS. 4A-4F , a hybrid generator  440  can be formed by generating a solar collecting unit  306  as described above. A mechanical piezoelectric nanorod unit  400  is generated by growing a plurality of nanorods  412  from a substrate  410  and coating the substrate  410  with a conductive layer  414  (such as a metal, e.g., gold or platinum). The solar unit  306  is placed above the mechanical piezoelectric nanorod unit  400  and separated by a sealing spacer  380  and an electrolyte  382  is injected into the solar collecting unit  306 . 
         [0042]    A corrugated contact unit  420 , as shown in  FIG. 4E , is generated by patterning a substrate  122  with a plurality of corrugations (such as an array of pyramids, an array of trenches, an array of corrugations, an array of crenulations, an array of nano-bowls or combinations thereof) and depositing a metal layer  424  thereon. The solar collecting unit  306 , the mechanical piezoelectric nanorod unit  400  and the corrugated contact unit  420  are then stacked upon each other, as shown in  FIG. 4F . 
         [0043]    As shown in  FIGS. 5A and 5B , in one embodiment of a fiber photovoltaic collector  500 , an electrode layer  512  (such as ITO) can be applied to an optical fiber  510  (such as an SiO 2  optical fiber) and a plurality of nanorods  524  can be grown radially outwardly therefrom. If the nanorods  524  are made of ZnO, then they act as an n-type semiconductor. In certain embodiments, the nanorods could be made of such materials as ZnO, ZnS, Si, GaN, GaInP, GaInAs, Ge, and combinations thereof. As shown in  FIG. 5B , the nanorods  524  can be coated with a p-type direct gap semiconducting layer  525  (such as a Cu 2 O, Cu 2 S and CuInS 2 ) and then a metal layer  526  (such as a layer of gold, platinum, or combinations thereof) may be applied to form a plurality of photovoltaic elements  520 . 
         [0044]    One potential problem with processing a solar cell on a fiber is the inherent lack of surface area of a cylindrical body. In order to reduce the surface area limitations, nanorods  524  which have a high surface-area-to-volume ratio, are grown radially around the optical fiber  510 . The optical fiber  510  is used to transmit light, while the nanorods  524  around the fibers serve to increase the surface area to which light is exposed. 
         [0045]    In this way, light entering the optical fiber  512  from the tip propagates through the fiber  512  until it reaches a nanorod  524 , at which point it causes e − -h +  pairs to be created, separated, and captured by an external circuit. Light entering the optical fiber  512  cannot escape until it interacts with a photovoltaic element, thereby increasing the solar conversion efficiency. In an ordinary thin film, flat substrate-type solar cell, some incident light is reflected before it can create e − -h + , pairs causing efficiency loss. A fiber optic design solves this problem, and it allows a volume-based three dimensional structure to absorb substantially more solar energy. 
         [0046]    As shown in  FIG. 5C , the photovoltaic elements  520  can be encapsulated in an elongated conductive cladding or housing  530  (such as a platinum coated housing) that serves as a back electrode and also encapsulates an iodide based electrolyte  540 , thereby forming a tubular photovoltaic collector  550 . In this embodiment, light reflects along the walls of the optical fiber  510  until it enters one of the nanorods  524  and hits the junction of the ZnO nanorod  524 , thereby creating an e − -h +  pair. One of the advantages of this embodiment is that light only needs to enter through an end of the fiber  510 , but is trapped until it acts with one of the photovoltaic elements  520 . 
         [0047]    As shown in  FIGS. 5D and 5E , in one embodiment the photovoltaic elements  520  are dye-sensitized using a layer of dye  528 , such as a ruthenium-based dye, applied to the nanorods  524  and then encapsulated in an elongated conductive cladding  530 . In this embodiment, light reflects along the walls of the optical fiber  510  until it enters one of the nanorods  524  and hits the junction of the ZnO nanorod  524  and the dye layer  528 , thereby creating an e − -h +  pair. 
         [0048]    As shown in  FIG. 6 , a fiber photovoltaic collector  500  of the type disclosed is flexible and can be adapted to many shapes, while still maintaining a high transmission of light. As shown in  FIGS. 7 and 8 , several different fiber photovoltaic collectors  500  can be placed together inside a single conductive tubular housing  530  and suspended in an electrolyte  540 . This results in a low volume, high energy and high density device. Another potential advantage is that light can be collected from one location and guided to another location for solar energy conversion. For example, the fiber photovoltaic collector  500  can be buried underground in a dark location while the tip is exposed at the surface and directed towards the sun. This could be an important aspect for generating energy in space-confined areas. 
         [0049]    When ZnO nanorods are subject to deflection, electrons flow from the nanowire to the metal electrode and back to the bottom of nanorods. In the hybrid system, they are sharing the metal electrode. Therefore, the negative electrode of the piezoelectric nanogenerator is directly connected to the positive electrode of the solar cell. These two types of electricity generators can thus be considered as in a serial connection. When functioning together, their outputs add up. When there is only one part working due to the restriction of circumstance, the other part will just be a path for the current flow. The combining of solar cell and piezoelectric nanogenerator will largely enhance the power generation efficiency of a simple piezoelectric nanogenerator or solar cell. Moreover, the environmental restriction for their operation will also be largely reduced. 
         [0050]    The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.