Patent Application
20090044848
The present invention relates to power generators, and more particularly, to electric power generators using thermoelectric effect associated with nanostructured material arrays.
Thermal electric generators are usually made from semiconductor “n” and “p” type elements arranged in series “n” to “p”, and can be attached on one side to a hot plate or heat source, and on the other side to a cold plate or heat sink. The efficiency of these generators includes fundamentally the Carnot efficiency and secondarily the device efficiency, with overall energy conversion values of less than about 10% and usually less than about 5%.
These devices typically rely on semiconductor materials having, among other things, a relatively high Seebeck coefficient, S, a change in voltage with temperature, a high electrical conductivity, σ, and a low thermal conductivity, λ.
The figure of merit, therefore, can be expressed as:
ZT=S
2
*σ*ΔT/λ (1)
so that materials with a high thermal conductivity λ tend to behave poorly as thermoelectric generators, because they can leak away thermal energy that otherwise can contribute to power generation.
It should be noted that the weight of these materials, in many instances, typically does not come into consideration. However, for many practical considerations, weight may be important. For example, Bi2Te3, an often used material in the manufacturing of thermoelectric devices because its ZT value is about 1, has a density of about 7.4 g/cc to about 7.7 g/cc. As such, devices made of this high performace material can be relatively heavy.
Moreover, many of the applications for which the use of a thermoelectric generator can be contemplated requires a thermoelectric device that has a substantially high specific power. As an example, for single junction solar cell based arrays, a specific power of from about 25 W/kg to about 100 W/kg needs to be achieved. In addition, for future applications using, for instance, multi-junction GaAs arrays, a specific power of from about 200 W/kg to about 1000 W/kg may be needed.
However, thermoelectric devices or systems that utilize Bi2Te3, SiGe alloys, or other similar materials can only generate a specific power at a level of from about 1-5 W/kg. Furthermore, in many of the contemplated applications, the temperatures to which the thermoelectric devices can be exposed can be substantially high. Unfortunately, Bi2Te3, SiGe alloys, or other similar materials used in presently available thermoelectric devices or systems tend to melt as the temperature approaches about 400° C.
Accordingly, it would be desirable to provide thermoelectric devices that are efficient, yet lightweight, that can operate at substantially high temperature, and that can generate the necessary voltage to permit useful applications.
The present invention provides, in accordance with one embodiment, a thermoelectric device for use in the generation of power, as well as other applications.
In one embodiment, the thermoelectric device includes a first member designed to collect heat from a heat source. The first member can be designed to withstand temperatures ranging from below 0° C. up to about 600° C. and above. The thermoelectric device can also include a second member in spaced relations from the first member for dissipating heat from the first member. The first and second member, in an embodiment, may be made from a thermally conductive material, such a aluminum nitride. The thermoelectric device further includes a core positioned between the first member and a second member for converting heat from the first member to useful energy. In one embodiment, the core includes a nanotube thermal element exhibiting a relatively high Seebeck coefficient that increases with an increase in temperature, and a conductive element exhibiting a relatively high transition temperature. The thermal element, in an embodiment, may have a density range of from about 0.1 g/cc to about 1.0 g/cc, which can result in weight saving over traditional materials used in a thermoelectric device. The thermal element and conductive element may be coupled to one another, so as to allow the core to operate within in a substantially high temperature range, for example up to about 600° C. and above. In addition, the core may be designed to achieve a relatively high specific power up to and exceeding about 3 W/g at a ΔT of about 400° C.
In another embodiment, a method of generating power is provided. The method includes initially providing a thermoelectric device having (i) a first member designed to collect heat from a heat source, (ii) a second member in spaced relations from the first member for dissipating heat from the first member, and (iii) a core positioned between the first member and a second member for converting heat from the first member to useful energy, the core having a nanotube thermal element exhibiting a relatively high Seebeck coefficient that increases with an increase in temperature, and a conductive element exhibiting a relatively high transition temperature, the elements coupled to one another allowing the core to operate in a substantially high temperature range. Next the thermoelectric device can be positioned so as to permit the first member to collect heat from a heat source. Thereafter, the collected heat can be driven across the core to the second member due to a temperature differential between the first member and the second member. Subsequently, during the course of heat transfer, the core is allowed to convert the heat transferred across it into power. In one embodiment, once power has been generated, the power can be directed to another to permit such a device to operate. Alternatively, if the thermoelectric device is coupled to a machine or device capable of generating waste heat, so that the waste heat can act as a heat source to be captured, the device can convert the waste heat to power and redirect the power to the machine for further use. To enhance efficiency and power generated, the number of thermal elements and conductive elements in the core can be increased. In addition, the power generated can be up to and exceeding about 3 W/g at a ΔT of about 400° C.
A method of manufacturing a thermoelectric device is also provided. The method includes initially providing at least one nanotube thermal element exhibiting a relatively high Seebeck coefficient that increases with an increase in temperature. In one embodiment, the nanotube thermal element can be provided with a density range of from about 0.1 g/cc to about 1.0 g/cc. In addition, the nanotube thermal element can be doped with one of a p-type dopant, n-type dopant, or both. Next, the thermal element can be coupled to a corresponding conductive element exhibiting a relatively high transition temperature to provide a core member. In one embodiment, the thermal element and the conductive element can withstand a temperature range of from below 0° C. up to about 600° C. and above. Thereafter, the core member may be positioned between a first member designed to collect heat from a heat source, and a second member in spaced relations from the first member for dissipating heat from the first member. To provide the thermoelectric device with the ability to increase the power generated, in one embodiment, the number of nanotube thermal elements on can be increased.
Carbon nanotubes, such as those manufactured in accordance with an embodiment of the present invention, can exhibit a significant Seebeck effect. In particular, these carbon nanotubes can exhibit a Seebeck coefficient that may be substantially linear with temperatures, for instance, from ambient to at least about 600° C. Moreover, the Seebeck coefficient for a structure made with substantially aligned carbon nanotubes of the present invention can be measurably higher.
Furthermore, the carbon nanotubes of the present invention can have lower density than traditional materials used in making thermoelectric generators. As such, significant weight saving can be achieved by replacing the relatively heavy traditional materials with the lighter carbon nanotubes of the present invention. Due to their relatively lower density, relatively higher Seebech effect, and relatively lower thermal conductivity, carbon nanotubes can be designed to achieve relatively high specific power.
Thermoelectric devices or generators of the present invention may be manufactured using, in one embodiment, at least one sheet or one yarn made from single, dual, or multiwall carbon nanotubes. In one embodiment, the sheet or yarn may be doped with p-type or n-type dopants, and subsequently coupled to a conductive material, such as copper or nickel. These affixed elements (i.e., doped sheet or yarn, and conductive material) may, thereafter, be arranged or assembled in various configurations to provide the thermoelectric devices or generators of the present invention. It should be appreciated that the flexibility and low density of carbon nanotubes, and thus the sheet or yarn, permit geometries that would not otherwise be possible with traditional semiconductor materials.
Nanotubes for use in connection with the present invention may be fabricated using a variety of approaches. Presently, there exist multiple processes and variations thereof for growing nanotubes. These include: (1) Chemical Vapor Deposition (CVD), a common process that can occur at near ambient or at high pressures, and at temperatures above about 400° C., (2) Arc Discharge, a high temperature process that can give rise to tubes having a high degree of perfection, (3) Laser ablation, and (4) HIPCO.
The present invention, in one embodiment, employs a CVD process or similar gas phase pyrolysis procedures known in the industry to generate the appropriate nanostructures, including carbon nanotubes. Growth temperatures for a CVD process can be comparatively low ranging, for instance, from about 400° C. to about 1350° C. Carbon nanotubes, both single wall (SWNT) or multiwall (MWNT), may be grown, in an embodiment of the present invention, by exposing nanoscaled catalyst particles in the presence of reagent carbon-containing gases (i.e., gaseous carbon source). In particular, the nanoscaled catalyst particles may be introduced into the reagent carbon-containing gases, either by addition of existing particles or by in situ synthesis of the particles from a metal-organic precursor, or even non-metallic catalysts. Although both SWNT and MWNT may be grown, in certain instances, SWNT may be selected due to their relatively higher growth rate and tendency to form rope-like structures. These rope-like structures can offer a number of advantages, including handling, lower thermal conductivity which can be a desirable feature for thermoelectric devices, good electronic conductivity, and high strength.
With reference now to
System 10, in one embodiment of the present invention, may also include a housing 12 designed to be substantially airtight, so as to minimize the release of potentially hazardous airborne particulates from within the synthesis chamber 11 into the environment. The housing 12 may also act to prevent oxygen from entering into the system 10 and reaching the synthesis chamber 11. In particular, the presence of oxygen within the synthesis chamber 11 can affect the integrity and compromise the production of the nanotubes 113.
System 10 may also include a moving belt 120, positioned within housing 12, designed for collecting synthesized nanotubes 113 made from a CVD process within synthesis chamber 11 of system 10. In particular, belt 120 may be used to permit nanotubes collected thereon to subsequently form a substantially continuous extensible structure 121, for instance, a non-woven sheet. Such a non-woven sheet may be generated from compacted, substantially non-aligned, and intermingled nanotubes 113, bundles of nanotubes, or intertwined nanotubes (e.g., ropes of nanotubes), with sufficient structural integrity to be handled as a sheet.
To collect the fabricated nanotubes 113, belt 120 may be positioned adjacent the exit end 114 of the synthesis chamber 11 to permit the nanotubes to be deposited on to belt 120. In one embodiment, belt 120 may be positioned substantially parallel to the flow of gas from the exit end 114, as illustrated in
In an alternate embodiment, as illustrated in
Typically, the formation of the yarn 131 results from a bundling of nanotubes 113 that may subsequently be tightly spun into a twisting yarn. Alternatively, a main twist of the yarn 131 may be anchored at some point within system 10 and the collected nanotubes 113 may be wound on to the twisting yarn 131. Both of these growth modes can be implemented in connection with the present invention.
The strength of the individual carbon nanotubes generated in connection with the present invention may be about 30 GPa or more. Strength, as should be noted, is sensitive to defects. However, the elastic modulus of the carbon nanotubes fabricated in the present invention may not be sensitive to defects and can vary from about 1 to about 1.2 TPa. Moreover, the strain to failure of these nanotubes, which generally can be a structure sensitive parameter, may range from a about 10% to a maximum of about 25% in the present invention.
The nanotubes of the present invention can also be provided with relatively small diameter. In an embodiment of the present invention, the nanotubes fabricated in the present invention can be provided with a diameter in a range of from less than 1 nm to about 10 nm.
The carbon nanotubes of the present invention can further demonstrate ballistic conduction as a fundamental means of conductivity. Thus, materials made from nanotubes of the present invention can represent a significant advance over copper and other metallic conducting members under AC current conditions.
Moreover, the carbon nanotubes of the present invention can be provided with a density of from about 0.1 g/cc to about 1.0 g/cc, and more particularly, from about 0.2 g/cc to about 0.5 g/cc. As such, materials made from the nanotubes of the present invention can be substantially lighter in weight. In addition, carbon nanotubes of the present invention can exhibit a Seebeck coefficient that is substantially linear with temperatures, for example, from ambient to at least about 600° C.
It should be noted that although reference is made throughout the application to nanotubes synthesized from carbon, other compound(s), such as boron, MoS2, or a combination thereof may be used in the synthesis of nanotubes in connection with the present invention. For instance, it should be understood that boron nanotubes may also be grown, but with different chemical precursors. In addition, it should be noted that boron may also be used to reduce resistivity in individual carbon nanotubes. Furthermore, other methods, such as plasma CVD or the like can also be used to fabricate the nanotubes of the present invention.
Although sheets made from carbon nanotubes may be manufactured a similar manner to that described above, sheets of carbon nanotubes may also be made using other processes. For example, Buckey paper may be made by dispersing carbon nanotube “powder” in water with an appropriate surfactant to create a suspension. When this suspension is filtered through a membrane, a type of Buckey paper is created whose properties are illustrated in Table 1 below.
In one embodiment of the present invention, sheets of carbon nanotubes may be stretched to substantially align the carbon nanotubes within each sheet in order to improve properties of the nanotubes. The properties of a carbon nanotube sheet made in accordance with one embodiment of the present invention, and that of a Bucky paper are compared for illustrative purposes in Table 1 below.
It should be note that, in Table 1, the figure of merit does not contain density or weight. However, since carbon nanotubes sheets can be substantially light, the resulting thermoelectric device or generator can nevertheless be designed with very high power to weight ratio.
It should be appreciated that the sheets from which the thermoelectric device may be made can include, in an embodiment, graphite of any type, for example, such as that from pyrograph fibers. Moreover, the sheets from which the thermoelectric device can be made may include traditional particles or microparticles, such as mesoporous carbons, activated carbon, or metal powders, as well as nanoparticles, so long as the material can be electrically and/or thermally conductive.
A strategy for reducing the resistivity, and therefore increasing the conductivity of the nanotube sheets or yarns of the present invention, includes introducing trace amounts of foreign atoms (i.e. doping) during the nanotube growth process. Such an approach, in an embodiment, can employ any known protocols available in the art, and can be incorporated into the growth process of the present invention, as disclosed in U.S. patent application Ser. No. 11/488,387 (incorporated herein by reference).
In an alternate embodiment, post-growth doping of a collected nanotube sheet or yarn can also be utilized to reduce the resistivity. Post-growth doping may be achieved by heating a sample of nanotubes in a N2 environment to about 1500° C. for up to about 4 hours. In addition, placing the carbon nanotube material over a crucible of B2O3 at these temperatures will also allow for boron doping of the material, which can be done concurrently with N2 to create BxNyCz nanotubes.
Examples of foreign elements which have been shown to have an effect in reducing resistivity in individual nanotubes include but are not limited to boron, nitrogen, boron-nitrogen, ozone, potassium and other alkali metals, and bromine.
In one embodiment, potassium-doped nanotubes have about an order of magnitude reduction in resistivity over pristine undoped nanotubes. Boron doping may also alter characteristics of the nanotubes. For example, boron doping can introduce p-type behavior into the inherently n-type nanotube. In particular, boron-mediated growth using BF3/MeOH as the boron source has been observed to have an important effect on the electronic properties of the nanotubes. Other potential sources useful for boron doping of nanotubes include, but are not limited to B(OCH3)3, B2H6, and BCl3.
Another source of dopants for use in connection with an embodiment of the present invention is nitrogen. Nitrogen doping may be done by adding melamine, acetonitrile, benzylamine, or dimethylformamide to the catalyst or carbon source. Carrying out carbon nanotube synthesis in a nitrogen atmosphere can also lead to small amounts of N-doping.
It should be appreciated that when doping the yarn or sheet made from nanotubes with a p-type dopant, such as boron, the Seebeck value and other electrical properties may remain p-type in a vacuum. On the other hand, by doping the yarn or sheet with a strong n-type dopant, such as nitrogen, the nanotubes can exhibit a negative Seebeck value, as well as other n-type electrical characteristics even under ambient conditions.
The resulting doped yarn or sheet of nanotubes can be used as a p-type element or an n-type element in the manufacture of a thermoelectric device or generator of the present invention.
Thermoelectric effect can generally be characterized to as a voltage difference that exists between two places on a conductor exhibiting a temperature difference. This effect, commonly referred to as the Seebeck effect, is defined as that voltage difference between two points when the temperature difference is 1° K.
To generate power efficiently, the conductor typically needs to have substantially good electrical conductivity, while having poor thermal conductivity. A figure of merit commonly known as Z is defined as:
(2)Z=S2*ε/σ. This relationship comes from the consideration of useful power per degree divided by conducted power as shown below.
From the definition of S, the voltage across two points is:
And the current through the conductor would be:
The power generated, not including convection or radiation losses, can be:
(5) Useful Power=I*V=S*ΔT*S*ΔT/(L/ρ*A)=(S*ΔT)2*ρ*A/L≈Constant, where L is the length of the thermoelectric element and A is the cross sectional area and ρ is the resistivity.
(6) The Thermal Power lost down the conductor is given by: Ploss=σ*A*ΔT/L, where σ is the thermal conductivity.
(7) The ratio of electrical power generated to thermal power lost is the figure of merit, ZT: Ratio=(S*ΔT)2*ρ*A/L/σ*A*ΔT/L=S2ΔTρ/σ=Z*T
Heat loss from the conductor can impact energy generation. In particular, the lower the heat loss, due to radiation and/or convection, the higher the ΔT and so power of the device can be. Since both radiation losses and convection losses can be proportional to surface area to volume, the desired geometry for a thermoelectric generator may be that of a cylinder (i.e., yarn of nanotube) of short length. However, if the length is too short, then transmission losses can be high, as will be discussed below. As such, the figure of merit should include these types of losses.
Typically, a ZT value of 1 can indicate that the thermoelectric device is about 50% efficient. A ZT value of 0.1, on the other hand, indicates an efficiency of about 10%. In general, the larger the ZT, the more efficient the device.
Looking at
As noted above, traditional theremoelectric device made with Bi2Te3 has a density ranging from about 7.4 g/cc to about 7.7 g/cc, and may reach over 8 g/cc. The thermoelectric device made from nanotubes of the present invention, on the other hand, has a density range of from about 0.1 g/cc to about 1.0 g/cc, and more particularly, from about 0.2 g/cc to about 0.5 g/cc. As such, there can a factor of about 40 and up to about 80 in weight advantage for the carbon nanotubes of the present invention over Bi2Te3.
In addition, the Seebeck coefficient for a sheet of, for instance, substantially aligned carbon nanotubes may be from about −130 μV/°K to about −140 μV/°K in a combined p-type and n-type element. As such, a maximum voltage at a ΔT of 200° C., for example, can be about:
ΔV=ΔT*S=200×130×10−6=26 mV
Moreover, in addition to the high Seebeck effect and a substantially lower density in comparison to traditional material used in thermoelectric devices, the carbon nanotubes of the present invention can also have substantially lower thermal conductivity due to the existence of dual or multiwall nanotubes, or due to the aggregation of the nanotubes into large bundles. As such, the thermoelectric device made with nanotubes of the present invention can achieve relatively high specific power, for instance, greater than about 1000 W/kg and can exceed about 3000 W/kg at a ΔT of about 400° C.
This specific power compares well with that achieved for single junction solar cell based arrays, which may range from about 25 W/kg to about 100 W/kg, as well as the specific power for future multi-junction GaAs arrays, which may range from about 200 W/kg to about 1000 W/kg.
It should be appreciated that the Seebeck coefficient can exhibit an almost constant curve relative to temperature above 200° K. Such a property can suggest that at relatively high temperatures, for example, at about 600° C. or higher, the thermoelectric device made from nanotubes of the present invention can likely outperform those made with the more traditional semiconductor materials, such as Bi2Te3, since these traditional semiconductor materials can melt at about 556° C.
For most semiconductors, the ZT may vary considerably over a very short temperature interval. However, values of around 1 may be typical. Of the wide variety of semiconductors available, Bi2Te3 is often the most employed because of its relatively high ZT. Table II compares the specific ZT for Bi2Te3 with that for carbon nanotubes of the present invention.
As illustrated in
It has been observed and noted above that sheets made from substantially aligned single wall carbon nanotubes can exhibit a substantially high Seebeck coefficient, for example, on a same order as individual tubes or bundles. Measurements have been obtained ranging from about 325° K to about 600° K. These measurements are shown in
Some of the key thermoelectric parameters for a carbon nanotube material of the present invention in comparison to a semiconductor (Bi2Te3) material are listed in Table III.
The power output from a thermoelectric device made from a sheet of single-walled carbon nanotubes in contact with a high conductivity metal, such as copper, is shown in
Even though the specific power can be relatively high, the practical usable voltage can be low thereby requiring multiple stages or elements or an electronic device that transforms current to voltage.
In this example, a thermoelectric device or generator is provided using at least one carbon nanotube sheet made in accordance with an embodiment of the present invention.
With reference now to
Conducting element 72, on the other hand, may be made from a metallic material, such as copper, nickel, or other similar conductive materials. In one embodiment, the conductive element 72 may be coated (e.g., electroplated) on to the thermal element 71 and subsequently laser cut to provide the segmented pattern as shown. The process of coating and laser etching can be similar to those processes known in the art.
Alternatively, rather than using a metallic material, a glassy carbon material may be used instead as the conducting element 72. In such an embodiment, lines of a glassy carbon precursor may be printed or placed on to the thermal element 71. The thermal element 71 with the glassy carbon precursor material may then be polymerized, in accordance with methods known in the art, to provide a glassy carbon material thereon. This embodiment can act to eliminate contact resistance and enable relatively higher operation temperatures.
To the extent that array 70 requires some stiffness, a high temperature polymer material, such as Torlon, or a polyamide material, may be affixed to the thermal element 71 and conductive element 72. The high temperature polymer or polyamide material, in an embodiment, can be substantially thin and can have a thickness ranging from about, 0.001″ to 0.005″. To affix the polymer or polyamide material to the thermal element 71 and conductive element 72, a thin film of glassy carbon resin, for instance, malic acid catalyzed furfuryl alcohol may be used to coat the polymer or polyamide material, followed by placement of the array 70 thereonto, then curing.
In an alternate embodiment, stiffness may be provided by initially coating one side of a high temperature polymer or polyamide material with copper, nickel or other similar materials to provide the conductive element 72. Next, the coated polymer or polyamide material can be photoprocessed. The polymer or polyamide material, thereafter, can be coated with a thin film of a glassy carbon resin, such as malic acid catalyzed furfuryl alcohol. A sheet or a stack of sheets of substantially aligned carbon nanotubes can then be affixed onto the polymer or polyamide material to provide thermal element 71. After curing, the resulting assembly can be laser cut to form linear array 70 of thermal element 71 and conductive element 72 illustrated in
Voltage for linear array 70 can be calculated from V=n*50×10−6*ΔT. In one example, if n=100, and ΔT=250° C., then V=1.25 volts.
The linear array 70, formed by any of the above embodiments, can then be rolled up about an axis into a disk or core 80 as shown in
Once formed, the core 80 shown in
With the ability to convert heat into power, the thermoelectric device 84 can act as a module that can be used for a wide variety of applications. It should be appreciated that this thermoelectric device is defined by a large cross-sectional area and small hot-cold gap spacing. Such a layout provides a substantially high current with the potential for dense packaging, while utilizing a light weight supporting structure. Moreover, the thermal conductivity through the carbon nanotube sheet can also be substantially high, meaning that for applications with limited thermal power input (e.g., solar collection, waste heat collection, etc.) the efficiency and power can be low. However, with unlimited thermal power, the power to weight ratio can exceed 3 W/g.
In one embodiment, the voltage of device 84 can be characterized by:
V=n*26 mV.
Thus, for example, if V=1.4 V and ΔT=200° C. then n=54, if ΔT=400° C., then n=75 per device.
One application for the thermoelectric generator or device 84 is to use it in connection with a small sun collector 90, as shown in
In another embodiment, the collector 90 can be designed to produce a few 10's or 100's of mW for battery charging. Larger configurations, of course, can be designed when more power is desired.
Another application for the thermoelectric device 84 or generator shown in
The thermoelectric device 84 may also be used as a heat or energy engine. In one embodiment, the thermoelectric device 84 can be used as an energy generator from waste heat. In particular, device 84 may be attached so that its hot surface contact a source of waste heat, such as a pipe in a heating system, while its cool surface contact a cold sink, so that heat can be transferred thereto and heat up the cold sink area, and cool down the heat source area. In accordance with one embodiment, if a 1 kg of nonwoven nanotube sheets of the present invention is used to manufacture device 84 for use as a heat or energy engine, such a heat or energy engine can directly convert heat to electrical work, and can put out approximately 1 kW of power. Such a capability allows for a lightweight replacement of, for instance, car and truck alternators, as well as power supplies for marine & aerospace applications. Large scale systems containing a metric ton of nanotubes of the present invention can put out in principle, a megawatt.
The design of such a heat or energy engine can also be used to cool down, for instance a submarine. In particular, the thermoelectric element may be attached to the hot reactor tube of a nuclear submarine on one side, and on the other side to the cold hull of the submarine adjacent to cold ocean water to permit the reactor tube to cool down.
A similar design can be used to incorporate into clothing to transfer heat from the body, which acts as the heat source, to cooler environment, such as air, to cool down the wearer.
In this embodiment, a thermoelectric device is provided using at least one carbon nanotube yarn made in accordance with an embodiment of the present invention.
Looking now at
In this embodiment, a multi-element thermoelectric array is provided using a plurality of carbon nanotube yarns made in accordance with one embodiment of the present invention.
As illustrated in
Although shown with a plurality of panels 110, it should be noted that device 115 can include just one panel 110, and that the device 115, including the thermoelectric panel 110, can be used or designed to have any of a number of other configurations. In addition, nickel wires 113 may be used in place of copper wires 113, or n-type nanotube yarns can be used in place of wires 113.
This design of panel 110 can be mechanically robust. In an embodiment, in order to obtain, for instance, 1.5 volts at about a ΔT of 400° K, the number of thermal elements 111 utilized within panel 110 may be about 58. Moreover, in a vacuum, the panel 110 has the potential for a wide range of operating temperatures, from the highest to perhaps the lowest of operating temperatures. In addition, the highly dense array of thermal elements 111 can give the panel 110 a substantially high operating voltage per unit of heated area in comparison to any of the designs provided above. In an embodiment, if spacing of thermal elements 111 is too close, then cold junctions in panel 110 may need to be heated to raise the temperature.
In space applications, a geometry, such as that shown in
This geometry can also be modified by introducing a reflector on the back surface and doping the nanotubes after growth with boron using a selective masking technique.
Waste heat is essentially a free, readily-available source of energy which can be converted into useful forms through an energy harvesting device of the present invention.
With particular reference now to
Although shown with nine alternating “n” and “p” type elements, it should be appreciated that, if desired, core 134 can be made to have more than or less than the nine alternating “n” and “p” type elements shown. Moreover, rather than just one nanotube sheet, a plurality of nanotube sheets having alternating “n” and “p” type elements may be used. When utilizing a plurality of nanotube sheets, each sheet may be placed on top of one another, or each sheet placed adjacent to and in parallel to one another, or both. Regardless of the arrangement of the sheets, when using a plurality of sheets, the mass of core 134 can increase, which can result in more power output in the thermoelectric device 130.
To provide the doped pattern in array 140, in one embodiment, the n-type elements 142 may be doped (i.e., chemically treated) with chemicals or chemical solutions that can act as electron donors when adsorbed onto the surface of the nanotubes, making the resulting n-type elements 142 electron-doped. Examples of such chemicals or chemical solutions include polyethylenimine (PEI) and hydrazine. Other chemicals or chemical solutions can also be used. Of course, traditional doping protocols may instead be used.
Table IV illustrates solutions used and their effect on carbon nanotube materials.
In one embodiment, treatment of n-type elements 142 can be as follows. Strips of copper 143 are electroplated onto the a carbon nanotube sheet to divide it into distinct sections. Every other section, in an embodiment, can be doped to n-type 142, as shown in
This device can also be used as a Peltier device, using the flow of electrons or holes within the thermoelectric material to pump heat from one side of the device to the other. The internal thermoelectric element can be modified slightly from the energy harvesting version to increase the efficiency. The treatment remains the same as above with the exception that a multi-layered piece of nanotube material may be used (thickness of about 1-2 mm) with the nanotube materials placed on top of one another. Short, square elements can then be cut from the treated nanotube material and soldered between the alumina plates, thus increasing the contact area between the thermoelectric material and the alumina.
Advantages of the thermal and conductive elements used in thermoelectric device of the present invention include:
High semiconductor transition temperature of up to 600° C.
High power output of greater than 1 W/g to 3 W/g at a 400° C. difference in temperature.
Substantially light in weight and low cost when compared with the commercially available semiconductor material in large volumes.
Voltages can be tailored by increasing the number of elements in an array.
The thermoelectric device or generator of the present can be utilized for a number of other applications. Among these, devices can be manufactured for applications including: (1) A solar battery charger (2) A high energy light weight transient thermal battery replacement placed in rockets or missiles, (3) A low temperature energy harvester suitable for body heat battery charging or applications used at very low temperatures, such as sub-zero (i.e., below 0° C.) or temperatures in space or in Arctic or Antarctic environments, and (4) a 1 Mega-Watt thermal generator.
Light weight thermoelectric devices can also be manufactured in combination with solar cells to capture the waste heat radiated to space. These devices can be designed to operate at a temperature of about 370° K and radiate to about a 50° K background. This very large ΔT should enable the capture of significant amounts of now wasted power and allow the solar arrays to operate at a reduced temperature thereby improving their efficiency.
Carbon nanotube thermoelectric devices of the present invention can further be used in conjunction with waste heat from satellites, communication electronics, and power systems, for power harvesting and thermal management purposes. An example may be a body heat powered device used for charging batteries. In particular, carbon nanotube thermoelectric blanket power sources could replace delicate, heavy, and expensive GaAs cell and coated cover glass components in photovoltaic arrays, so as to eliminate the costly multi-step assembly. This in turn would permit improved on-station altitude control and reduced propellant usage for either lower launch costs or extended mission operations. Future civil and defense spacecraft may also need more efficient, higher power sources and improved thermal management systems in order to meet escalating mission performance goals. As such, the thermoelectric devices of the present invention can be used for such purposes
Another example may be to use the thermoelectric devices of the present invention in conjunction with various machines, electronic devices, power systems that generate waste heat. The present invention contemplates using the thermoelectric devices to harvest the waste heat, converting the waste heat to power, and redirecting the power to these machines, devices or systems for reused, so as to enhance efficiency and reduce overall power usage.
Moreover, whether used for megawatt-class space-based radar platforms, radio isotope thermoelectric generator (RTG) powered deep space exploration missions, or orbiting nanosat clusters, a high specific power technology such as that offered by the thermoelectric power generators can be a key enabler in each mission area and can provide a strong competitive advantage.
Ground-based devices can also be designed from the thermoelectric element of the present invention.
While the present invention has been described with reference to certain embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt to a particular situation, indication, material and composition of matter, process step or steps, without departing from the spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.
The present invention claims priority to U.S. Provisional Patent Application Nos. 60/964,678, filed Aug. 14, 2007, and 60/987,304, filed Nov. 12, 2007, both of which are hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60964678 | Aug 2007 | US | |
| 60987304 | Nov 2007 | US |
