Harvesting mechanical and thermal energy by combining nanowires and phase change materials

A system is disclosed for harvesting at least one of mechanical or thermal energy. The system may have a flexible substrate, a plurality of electrically conductive nanowires secured to the substrate, and a plurality of electrically conductive metal layers. The metal layers may be disposed on the substrate and spaced apart from one another along a length of the substrate. The metal layers may be in electrically conductive contact with various ones of the nanowires. At least two of the metal layers may be attachable to an external device. At least one of movement or flexing of the substrate produces an output voltage across the metal layers.

FIELD

The present application relates to energy harvesting systems and methods, and more particularly to systems and methods for harvesting mechanical and thermal energy by the use of nanowires and/or phase change materials.

BACKGROUND

U.S. Pat. No. 7,397,169 by Nersesse Nersessian, Gregory P. Carman, and Harry B. Radousky for energy harvesting using a thermoelectric material includes the state of technology information reproduced below.

Waste heat is always generated whenever work is done. Harvesting such waste heat can increase the efficiency of engines, be used to power numerous devices (eliminating the need for auxiliary power sources), and in general, significantly reduce power requirements. Various methods have been used to try and harvest such waste heat and mechanical energy. For waste heat the most important of which is through thermoelectric materials. In order to efficiently convert waste heat to usable electrical energy, thermoelectric materials generally requires a large Seebeck coefficient having a “figure of merit” or Z, defined as,

Z=s2⁢σk
where S is the thermoelectric power, i.e., the Seebeck coefficient, σ is the electrical conductivity (=1/ρ), σ is the electrical resistivity and k is the thermal conductivity and. A dimensionless number ZT is often used as a figure of merit for the TE material. For conventional materials the ZT<1 at T=300 K. The higher the ZT, the more efficient the TE. ρ is the electrical resistivity, and K is the thermal conductivity. The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference (δT) between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or S=V/(δT). Therefore, in searching for a good thermoelectric material, materials with large values of S, and low values of p and k are beneficial.

However, current state of the art thermoelectric materials utilized to harvest waste heat and convert such heat to a useful energy, for example, devices that use a combination of n-type and p-type materials, generally have Z values which are prohibitively low for harvesting energy from low quality waste heat, where δT is on the order of 100° C. or less.

Additional background information on thermoelectric devices is described in U.S. Pat. No. 5,550,387, entitled “Superlattice Quantum Well Material,” issued Aug. 27, 1996 to Elsner et al. (hereinafter the “Elsner '387 patent”). The Elsner '387 patent involves thermoelectric elements having a very large number of alternating layers of semiconductor material. The alternating layers all have the same crystalline structure. This makes the vapor deposition process easy because the exact ratio of the materials in the layers is not critical. The Elsner '387 patent demonstrates that materials produced in accordance with the teachings thereof provide figures of merit more than six times that of prior art thermoelectric materials. A preferred embodiment discussed in the Elsner '387 patent is a superlattice of Si, as a barrier material, and SiGe, as a conducting material. Both of these materials have the same cubic structure. Another preferred embodiment which is discussed is a superlattice of B—C alloys, the layers of which would be different stoichiometric forms of B—C but in all cases the crystalline structure would be alpha rhombohedral. In a described preferred embodiment the layers are grown under conditions as to cause them to be strained at their operating temperature range in order to improve the thermoelectric properties.

Background information on an energy harvesting system is described and claimed in U.S. Pat. No. 2004/0238022 A1, entitled “Thermoelectric Power From Environmental Temperature Cycles,” issued Dec. 2, 2004 to Hiller et al. (hereinafter the “Hiller '022 application”). The Hiller '022 application involves an electric generator system for producing electric power from the environmental temperature changes such as occur during a normal summer day on Earth or Mars. In a described preferred embodiment a phase-change mass is provided which partially or completely freezes during the relatively cold part of a cycle and partially or completely melts during the relatively hot part of the cycle. A thermoelectric module is positioned between the phase-change mass and the environment. The temperature of the phase-change mass remains relatively constant throughout the cycle. During the hot part of the cycle heat flows from the environment through the thermoelectric module into the phase change mass generating electric power which is stored in an electric power storage device such as a capacitor or battery. During the cold part of the cycle heat flows from the phase change mass back through the module and out to the environment also generating electric power that also is similarly stored. An electric circuit is provided with appropriate diodes to switch the direction of the current between the hot and cold parts of the cycle. A preferred phase change mass is a solution of water and ammonia that has freeze points between about 270° K to about 145° K depending on the water ammonia ratio. It is further described that, preferably, a finned unit is provided to efficiently transfer heat from a module surface to the environment.

In view of the foregoing, it will be appreciated that there presently is significant interest in systems and/or methods which enable energy to be harvested. The successful harvesting of energy could be used for a wide variety of beneficial purposes, for example to power sensors, photovoltaic cells, and other electronic components. However, the complexity and drawbacks of present day systems makes many present day systems either too expensive or too inefficient for many energy harvesting applications.

SUMMARY

Features and advantages of the disclosed apparatus, systems, and methods will become apparent from the following description. The inventors are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the apparatus, systems, and methods. Various changes and modifications within the spirit and scope of the application will become apparent to those skilled in the art from this description and by practice of the apparatus, systems, and methods. The scope of the apparatus, systems, and methods is not intended to be limited to the particular forms disclosed and the application covers all modifications, equivalents, and alternatives falling within the spirit and scope of the apparatus, systems, and methods as defined by the claims.

The inventors have developed apparatus, systems, and methods that use nanowires to harvest both mechanical and low-quality thermal energy. The inventors apparatus, systems, and methods have been demonstrated through the use of Zinc Oxide (ZnO) nanowires combined with the phase change material NiTi (“Nitinol”). The ZnO nanowires are dense and uniform, with a length of 10-50 um, diameter of 200 nm, and a growth direction along the c-axis. The same growth direction of the nanowires guarantees the alignment of the piezoelectric potentials of all of the nanowires.

The apparatus, systems, and methods described herein have use in providing power generation harvested from low quality waste heat and external motion. The apparatus, systems, and methods described herein also have use in their ability to power small sensors and actuators devices requiring microwatts of power, or infrequent power from energy stored in a supercapacitor.

In one aspect the present disclosure relates to a system for harvesting at least one of mechanical or thermal energy. The system may comprise a flexible substrate, a plurality of electrically conductive nanowires secured to the flexible substrate, and a plurality of electrically conductive metal layers. The plurality of electrically conductive metal layers may be disposed on the flexible substrate and spaced apart from one another along a length of the flexible substrate. The plurality of electrically conductive metal layers may be in electrically conductive contact with various ones of the plurality of nanowires. At least two of the plurality of electrically conductive metal layers may be attachable to an external device. At least one of movement or flexing of the flexible substrate results in an output voltage across the plurality of electrically conductive metal layers.

In another aspect the present disclosure relates to an energy harvesting apparatus which may comprise a flexible substrate, a plurality of electrically conductive nanowires secured to the flexible substrate, and a plurality of electrically conductive metal layers. The electrically conductive metal layers may be disposed on the flexible substrate and may be in electrically conductive contact with various ones of the plurality of nanowires. A phase change material may also be included. The flexible substrate may be secured to the phase change material. Movement of the phase change material results in an output voltage being generated across the electrically conductive metal layers.

In still another aspect the present disclosure relates to a system for harvesting at least one of mechanical or thermal energy. The system may comprise a flexible substrate and a plurality of electrically conductive nanowires. The plurality of electrically conductive nanowires may be secured to the flexible substrate so that the nanowires are generally parallel to one another and spaced out along a length of the flexible substrate. A plurality of electrically conductive metal layers may be disposed on the flexible substrate and spaced apart from one another along a length of the flexible substrate. The plurality of electrically conductive metal layers may further be arranged generally perpendicular to the nanowires and in electrically conductive contact with various ones of the plurality of nanowires to form a series circuit. At least two of the plurality of electrically conductive metal layers may be used to form electrodes which are attachable to an external device. At least one of movement or flexing of the flexible substrate results in an output voltage across the plurality of electrically conductive metal layers.

The apparatus, systems, and methods are susceptible to modifications and alternative forms. Specific embodiments are shown by way of example. It is to be understood that the apparatus, systems, and methods are not limited to the particular forms disclosed. The apparatus, systems, and methods cover all modifications, equivalents, and alternatives falling within the spirit and scope of the application as defined by the claims.

DETAILED DESCRIPTION

Referring to the drawings, to the following detailed description, and to incorporated materials, detailed information about the apparatus, systems, and methods is provided including the description of specific embodiments. The detailed description serves to explain the principles of the apparatus, systems, and methods described herein. The apparatus, systems, and methods described herein are susceptible to modifications and alternative forms. The application is not limited to the particular forms disclosed. The application covers all modifications, equivalents, and alternatives falling within the spirit and scope of the apparatus, systems, and methods as defined by the claims.

An important technical problem the co-inventors of the presently claimed subject matter have solved is the development of apparatus, systems, and methods to provide electrical power to small devices, wherein the electrical power is harvested from the ambient environment. This means turning waste energy, both thermal and mechanical, into useful electrical energy to power a wide variety of small electrical or electronic devices. The teachings of the present disclosure involve the use of nanowires as a new component to achieving an efficient, functioning energy harvesting device.

The co-inventors have demonstrated the feasibility of using nanowires to harvest both mechanical and low-quality thermal energy. This goal has been demonstrated through the use of Zinc Oxide (hereinafter “ZnO”) nanowires combined with the phase change material NiTi (also known as “Nitinol”, a shape memory alloy). As shown inFIGS. 1-3, ZnO nanowires10were grown using a conventional chemical vapor deposition process. The nanowires10produced by this process were dense and generally uniform in shape, and had a length of typically about 10-50 um, a diameter of typically between about 10 nm-500 nm, and more typically about 200 nm, and a growth direction along the c-axis (i.e., out of the paper as best shown inFIGS. 2 and 3). The same growth direction of the nanowires10advantageously guarantees the alignment of the piezoelectric potentials of all of the nanowires10.

The nanowires10were transferred to a flexible polyimide film, for example a KAPTON® polyimide thin film12, as shown inFIG. 4a, by gentle scratching. The scratching involves repeatedly moving the KAPTON® polyimide film12(i.e., the substrate) over the nanowires10in a direction generally parallel to a substrate14from which the nanowires10were grown. This causes removal of the nanowires10from the substrate from which they were grown. The nanowires10produced by this method are well-aligned (i.e., oriented substantially parallel to one another) on the receiving KAPTON® polyimide film12, as shown inFIGS. 4band 4c. Following dry-transfer, the KAPTON® polyimide film12may be evaporated, such as with a patterned gold electrode, using conventional microfabrication techniques as shown inFIGS. 5-9. Specifically, as shown inFIG. 6, a spin coat photoresist layer16is formed over the nanowires10and the KAPTON® polyimide film12. InFIG. 7, using a photolithography process, portions of the photoresist material16are etched away to produce parallel channels18between alternating layers of the remaining photoresist material16. InFIG. 8a layer of electrically conductive material such as metal20, for example gold, may be deposited (e.g., by sputtering or any other suitable technique) over the entire area of the KAPTON® polyimide film12.

FIG. 9illustrates a circuit22which is the result of the completed microfabrication process. The areas of the KAPTON® polyimide film12covered by the photoresist/metal16/20have been removed leaving the conductive metal20as generally parallel, spaced apart, conductive metal layers20aalong a length of the KAPTON® polyimide film12. The circuit22is further illustrated in highly simplified schematic form inFIG. 10. InFIG. 10it will be noted that the nanowires10are shown perfectly parallel to one another, and each nanowire10bridges two or more of the conductive metal layers20a. However, in practice, as shown inFIG. 11, the nanowires10are not all perfectly parallel. Nevertheless, the nanowires10collectively form a series circuit comprised of long, generally parallel electrode arrays which are generally perpendicular to the nanowires10. The series circuit22extends between electrode terminals24aand24bof the circuit22, as shown inFIG. 10.FIG. 12shows an actual picture of a portion of the circuit22.

The excellent alignment of the nanowires10that the above fabrication method produces easily enables a scale-up of the output voltage from the circuit22. The total output voltage is approximately the sum of the voltages across individual nanowires10in the vertical direction (i.e., along their lengths) because the nanowires are effectively connected in series as a result of their electrically conductive contact with the conductive metal layers20a. The total output current and total output open circuit voltage scales up with the number of nanowires within each row of electrode pairs.

Mechanical harvesting was demonstrated using a periodic application of force, as shown in highly simplified form inFIG. 13. Tapping the circuit22from the top with a wood stick yielded an open circuit voltage (OCV) of about 0.2V-4.0V, depending on the tap stimulus, as illustrated by waveform26inFIG. 14. Pressing and holding the stimulus produced an oscillating, open circuit voltage waveform28as shown inFIG. 15.FIG. 16shows an open circuit voltage30produced by tapping on different regions of the circuit22. When the bending curvature of the circuit22is qualitatively increased, as shown inFIG. 17, this yields an enhanced voltage, as shown in waveforms32,34,36,38and40ofFIGS. 18-22, respectively. As shown inFIG. 17, the circuit22may also be covered with a suitable protection layer22a, for example Polydimethylsiloxane (“PDMS”).

Referring toFIG. 23, to demonstrate thermal harvesting from low quality heat sources, commercially available Nitinol (Ni—Ti alloy) thin film42, a phase transition material with a transition temperature of 50° C., was attached to the circuit22. In this manner the circuit22essentially acts as a piezoelectric device. The thin film42with the attached circuit22thereto was bent at room temperature. Upon heating above 50° C., the Nitinol thin film42returned to its original flat shape. Accordingly, the nanowire circuit22experienced a drastic shape change. This shape change yielded a peak-to-peak open circuit output voltage/power of about 0.65V, as indicated by waveform44inFIG. 24. Using Gd5Si2Ge2as the phase change material would be expected to provide an event greater peak-to-peak open circuit output voltage. A key feature of the circuit22is the near parallel nanowire10alignment reduces the internal resistance of the nanowires10and allows production of substantial usable power from the circuit22.

The usable power produced by the circuit22is expected to be useful in a wide variety of applications where it is desirable to harvest energy from motion, vibration or changes in shape of an underlying substrate or device on which the circuit22is mounted. The output from the circuit22is expected to be particularly useful in powering low power sensors, motors, optoelectronics including cameras, solar cells, actuators, communications circuits, and a wide variety of other electrically powered devices.

Although the description above contains many details and specifics, these should not be construed as limiting the scope of the application but as merely providing illustrations of some of the presently preferred embodiments of the apparatus, systems, and methods. Other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments.

Therefore, it will be appreciated that the scope of the present application fully encompasses other embodiments which may become obvious to those skilled in the art. In the claims, reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above described embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device to address each and every problem sought to be solved by the present apparatus, systems, and methods, for it to be encompassed by the present claims. Furthermore, no element or component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”

While the apparatus, systems, and methods may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the application is not intended to be limited to the particular forms disclosed. Rather, the application is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the application as defined by the following appended claims.