A thermoelectric module is a device that exploits the thermoelectric effect exhibited by many materials.
The thermoelectric effect is reversible, such that when the two sides of a thermoelectric module are held at different temperatures, the module can generate electric power. For example, in
According to one aspect, a system for generating electricity from a temperature differential includes at least one thermoelectric module having a hot side and a cold side, and a thermal element in contact with one side of the thermoelectric module, to supply heat to or to receive heat from the thermoelectric module. The system also includes a fluid flowing through the thermal element, to supply heat to or to receive heat from the thermal element, and a plurality of nanoparticles suspended in the fluid. The suspended nanoparticles enhance the transfer of heat between the fluid containing the nanoparticles and the thermal element, as compared with a similar fluid not containing the nanoparticles. In some embodiments, the thermal element is a hot thermal element in contact with the hot side of the thermoelectric module to supply heat to the thermoelectric module, the fluid is a hot fluid flowing through the hot thermal element to supply heat to the hot thermal element, the nanoparticles are a first plurality of nanoparticles suspended in the hot fluid, and the system further includes a cold thermal element in contact with the cold side of the thermoelectric module, to receive heat from the thermoelectric module, and a cold fluid flowing through the cold thermal element to receive heat from the cold thermal element. A second plurality of nanoparticles is suspended in the cold fluid, and the suspended nanoparticles enhance the transfer of heat between the cold fluid containing the nanoparticles and the thermal element, as compared with a similar fluid not containing the nanoparticles. In some embodiments, the fluid is water, and the suspended nanoparticles comprise copper ions, silver ions, or both copper and silver ions. The ions may be less than 2 nanometers in diameter.
The ions may be non-colloidal. In some embodiments, the fluid contains copper ions in a concentration of between 250 and 450 micrograms per liter. In some embodiments, the fluid contains silver ions in a concentration of between 150 and 350 micrograms per liter. The fluid may be contained within a closed loop. In some embodiments, the system further includes a heat exchanger that transfers heat from an external source to the fluid to heat the fluid, or transfers heat from the fluid to an external sink to cool the fluid. The system may further include an ion generator that generates the nanoparticles. In some embodiments, the ion generator includes a spaced apart pair of electrodes in contact with the fluid, and a drive circuit that applies an alternating voltage between the electrodes. The alternating voltage may be a chopped alternating voltage. Each electrode may be made of sterling silver. The alternating voltage may have a peak-to-peak amplitude of between 4 and 6 volts. The alternating voltage may have a frequency of between 6 and 8 Hz. The alternating voltage may have a frequency greater than 20 kHz. In some embodiments, the system further includes a plurality of thermoelectric modules having hot and cold sides and a plurality of thermal elements in contact the thermoelectric modules, to supply heat to or to receive heat from the thermoelectric module, and the fluid flows through the plurality of thermal elements, to supply heat to or to receive heat from at least some of the thermal elements.
According to another aspect, an ion generator for generating ions in a fluid includes a spaced apart pair of electrodes in contact with the fluid, and a drive circuit that applies an alternating voltage between the electrodes, the alternating voltage having a frequency of at least 6 Hz. In some embodiments, the alternating voltage is a chopped alternating voltage. In some embodiments, each electrode comprises copper, silver, or both. Both electrodes may be made of sterling silver. In some embodiments, the alternating voltage alternates at a frequency between 6 Hz and 8 Hz. In some embodiments, the alternating voltage alternates at a frequency greater than 20 kHz.
According to another aspect, a method of generating electricity from a temperature differential includes placing a thermal element in contact with a side of a thermoelectric module, passing a fluid through the thermal element to supply heat to or to receive heat from the thermoelectric module, and suspending nanoparticles within the fluid. The nanoparticles enhance the transfer of heat between the fluid containing the nanoparticles and the thermal element as compared with a similar fluid not containing the nanoparticles. In some embodiments, the thermal element is a hot thermal element in contact with a hot side of the thermoelectric module and the fluid is a hot fluid that supplies heat to the thermoelectric module, and the method further includes placing a cold thermal element in contact with a cold side of the thermoelectric module, passing a cold fluid through the cold thermal element to receive heat from the thermoelectric module, and suspending nanoparticles within the cold fluid, the nanoparticles enhancing the transfer of heat between the cold fluid containing the nanoparticles and the cold thermal element as compared with a similar fluid not containing the nanoparticles. In some embodiments, suspending nanoparticles within the fluid comprises providing a pair of spaced apart electrodes in contact with the fluid, and impressing an alternating voltage between the electrodes to generate nanoparticles via electrolysis. In some embodiments, providing a pair of spaced apart electrodes in contact with the fluid comprises providing at least one electrode that comprises silver, copper, or both silver and copper. In some embodiments, providing a pair of spaced apart electrodes in contact with the fluid comprises providing a pair of sterling silver electrodes. In some embodiments, impressing the alternating voltage between the electrodes comprises impressing a chopped alternating voltage between the electrodes. The method may further include impressing the alternating voltage between the electrodes continuously during the generation of electricity by the thermoelectric module. In some embodiments, impressing the alternating voltage between the electrodes comprises impressing an alternating voltage having a frequency between 6 and 8 Hz between the electrodes.
Thermoelectric module 100 is an example of a thermoelectric device usable in embodiments. Module 100 is made up of a number of thermoelectric elements 104, each of which is a length of conductive or semiconductive material with favorable thermoelectric properties. For example, the elements may be pieces of n-type and p-type semiconductor material, labeled “N” and “P” in
Preferably, thermoelectric modules used in embodiments are optimized for power generation. Research has shown that the total power available is maximized when the length “L” of the thermoelectric elements is quite short—for example about 0.5 millimeters. However, the conversion efficiency of a thermoelectric module (the fraction of available thermal energy actually converted to electrical energy) increases with increasing length L. For example, a thermoelectric element with a length of 5.0 millimeters may be several times more efficient than one with a length of 0.5 millimeters. The optimum length for a particular application (providing the minimum cost per expected unit of electrical energy) will be a function of the cost of the thermoelectric modules and associated hardware, the cost of the thermal energy supplied to the thermoelectric generator, and the expected life of the thermoelectric generator. A more complete discussion of the factors involved in optimizing the performance of a thermoelectric module may be found in D. M. Rowe and Gao Min, Evaluation of thermoelectric modules for power generation, Journal of Power Sources 73 (1998) 193-198.
It is to be understood that a temperature differential may be provided by any of many, many different media and apparatus. For example, heated fluid 202 may be produced specifically for the purpose of generating electricity, for example by heating water using conventional fossil fuels, solar energy, or by some other means. Alternatively, heated fluid 202 may be the by-product of an industrial process, waste water from an establishment such as a car wash or laundry, naturally occurring hot spring water, or another kind of fluid.
The “hot” side of a temperature differential may be provided by another medium besides a liquid, for example, air exhausted from a building air conditioning system, exhaust gasses from an engine, the surface of any component such as a vehicle exhaust pipe, oven exterior, blast furnace environment, or other suitable heat source.
Similarly, cold fluid 203 may be obtained specifically for the purpose of power generation, or may be the by-product of some other process. For example, cold fluid 203 may be water that is circulated through an underground pipe to cool the water to the temperature of the ground—typically about 54-57° F. (12-14° C.) in many parts of the United States. Or cold fluid 203 may be any naturally-occurring relatively cold fluid, for example water diverted from a river or stream. The “cold” side of a temperature differential may be provided by media and materials other than fluids, for example ambient air, a metallic object, or some other suitable “cold” source.
For maximum electric power output, it is advantageous to supply heat to the hot side of each thermoelectric module as efficiently as possible, and to remove heat from the cold side as efficiently as possible.
For additional power, it may be desirable to combine the power provided by a number of thermoelectric modules 101.
Hot fluid 202 enters thermoelectric generator 400 via hot fluid inlet manifold 401 and exits via hot fluid outlet manifold 402. Cold fluid enters thermoelectric generator 400 via cold fluid inlet manifold 403 and exits via cold fluid outlet manifold 404. The net result is that each of thermoelectric modules 100 is exposed to a temperature differential, by virtue of being between one of hot thermal elements 301 and one of cold thermal elements 302. Thermal energy flowing through each thermoelectric module 100 is converted to electrical energy, and a voltage is developed across each set of electrical leads 101. In some embodiments, leads 101 may be interconnected such that thermoelectric generator 400 produces a single voltage on a single set of leads. For example, thermoelectric modules 100 may be connected in series, so that thermoelectric generator 400 produces a voltage that is the sum of the voltages produced by the individual thermoelectric modules 100.
Many variations are possible, for example the hot and cold thermal elements and fluids may be reversed, more or fewer thermoelectric modules may be used, or banks of thermoelectric modules may be combined in more elaborate way. More detail and descriptions of other arrangements for a thermoelectric generator may be found in co-pending U.S. patent application Ser. No. 10/823,353, filed Apr. 13, 2004 and titled “Same Plane Multiple Thermoelectric Mounting System”, and in co-pending U.S. patent application Ser. No. 12/481,741, filed Jun. 10, 2009 and titled “Thermoelectric Generator”, the entire disclosures of which are hereby incorporated herein. Additional information about thermoelectric generators and their application may be found in co-pending U.S. patent application Ser. No. 12/481,745, filed Jun. 10, 2009 and titled “Integrated Energy System for Whole Home or Building”, and co-pending U.S. patent application Ser. No. 12/481,750, filed Jun. 10, 2009 and titled “Automatic Configuration of Thermoelectric Generator to Load Requirements”, the entire disclosures of which are hereby incorporated herein.
In other embodiments, only one side of a thermoelectric module may receive heat from or supply heat to a fluid flowing in a thermal module. For example, one side of a thermoelectric module may be heated by hot fluid flowing through a thermal element, and the cold side of the thermoelectric module may be cooled by ambient air. In other embodiments, the cold side of a thermoelectric module may be cooled by a cold fluid flowing through a thermal module, and the hot side of the thermoelectric module heated by a static heat source such as the outer surface of an oven or furnace. Many variations are possible.
For maximum power generation, it is desirable to enhance the transfer of heat between the fluid or fluids and their respective thermal elements.
One way of enhancing heat transfer to and from a fluid is to utilize a nanofluid. A nanofluid is a fluid containing particles of a size conveniently expressed in nanometers, typically between 1 and 100 nanometers. The particles are called nanoparticles. The nanoparticles may be colloidal, or may be atomic in size. The addition of nanoparticles to a fluid, for example water, can increase both the thermal conductivity of the fluid and the effectiveness of convective heat transfer between the fluid and surrounding structures. Although the physical mechanism accounting for the increases may not be fully understood, the increases may be affected by the size and concentration of the nanoparticles, the material of the nanoparticles, the temperature at which the fluid characteristics are measured, and other factors.
The inclusion of a nanofluid in a thermoelectric generator can result in a significant improvement in the amount of power generated from a given temperature differential. Alternatively, the inclusion of the nanofluid may enable useful power generation from smaller a temperature differential than would otherwise be considered.
Within thermoelectric generator 501, heat from hot fluid 502 is provided to one or more thermoelectric modules, and exhausted to cold fluid 503. The thermoelectric modules generate electricity, which is delivered through leads 509. Internally, thermoelectric generator 501 may include components similar to those discussed above and shown in
In some embodiments, thermoelectric generator 501 may be supplied with fluid or fluids already containing nanoparticles. In other embodiments, the nanoparticles may be generated as needed. In exemplary thermoelectric generator 501, a first ion generator 510 generates ions 511 within hot fluid 502, and a second ion generator 512 generates ions 513 within cold fluid 503. The ions are nanoparticles, which are suspended within the respective fluids so that the fluids are nanofluids. In other embodiments, other kinds of nanoparticles may be used other than ions 511 and 513, but the use of ions 511 and 513 as nanoparticles may provide certain benefits as described below. Although both hot and cold fluids 502 and 503 are illustrated as being nanofluids, this is also not a requirement. In some embodiments, only one nanofluid may be present. For example, if the cold side of thermoelectric generator 501 is air cooled, a nanofluid may be used only on the hot side of thermoelectric generator 501.
Complementary pulse trains 607 and 608 may be fed to an H-bridge circuit 609, which may be for example a TA7291SG Bridge Driver available from Toshiba America, Inc., of New York, N.Y. Control circuitry 610 within H-bridge circuit 609 utilizes complementary pulse trains 607 and 608 to switch a set of transistors to alternately impress the voltage on electrodes 601 and 602. For example, when pulse train 607 is at a high level and pulse train 608 is at a low level, transistors 611 may be switched on and transistors 612 may be switched off. Conversely, when pulse train 607 is at a low level and pulse train 608 is at a high level, transistors 611 may be switched off and transistors 612 may be switched on. As a result, electrodes 601 and 602 alternately plate and disperse ions, until an equilibrium concentration of ions 511 is reached in hot fluid 502. Ion generator 510 may be operated whenever thermoelectric generator 501 is in operation, or may be operated intermittently, may be operated only upon startup to build up a concentration of ions 511 in hot fluid 502, or may be operated based on some other scheme.
Many variations in the operation of ion generator 510 are possible, for example in the voltage used, the frequency of operation, and other parameters. In one embodiment, oscillator 603 produces a pulse train 604 having a frequency of about 14 Hz, resulting in a frequency of switching of the voltage between electrodes 601 and 602 of about 7 Hz. Because exemplary ion generator 510 uses digital circuitry, the voltage between electrodes 601 and 602 may switch essentially instantaneously between its extremes. For the purposes of this disclosure, this kind of alternating voltage will be referred to as a “chopped” alternating voltage. A chopped alternating voltage may also be known as a square wave. Using other drive schemes, the alternating voltage may transition smoothly, for example sinusoidally.
Other frequencies may be used, for example 10 Hz, 60 Hz, 120 Hz, 1 kHz, 5 kHz, 10 kHz, 20 kHz, or another suitable frequency. For example, in some embodiments, the alternating voltage between electrodes 601 and 602 may have a frequency of more than 20 kHz. In some embodiments, the alternating voltage between electrodes 601 and 602 maybe about 5 volts, but other voltages may be used, for example 3 volts, 12 volts, 24 volts, or another suitable voltage.
The use of an alternating voltage may have the beneficial effect of preventing the buildup of scale on electrodes 601 and 602, as during operation, each electrode is a donor of material at least some of the time, so that the surface of each electrode is routinely at least partially shed. This cleaning action may serve to maintain good electrical contact between the electrodes and the fluid.
The use of a nanofluid may improve the performance of a thermoelectric generator considerably, as compared with the performance of the same thermoelectric generator without the use of a nanofluid. This improved performance may be exploited to generate additional power from a given temperature differential. Alternatively, the improved performance may be utilized to generate power from a low-grade waste heat source that may not have been previously considered as useful for power generation.
While the generation of ions in accordance with embodiments has been described in the context of thermoelectric power generation, the ion generation techniques described may be used in other applications as well, wherever enhanced heat transfer characteristics of a suitable fluid would be beneficial. Examples may include hydronic heating or cooling systems, solar energy collection, or other applications.
When silver ions are used as nanoparticles, the presence of silver ions may have additional benefits as well. Silver is known to be a safe and effective biocide, and its presence in a fluid of a thermoelectric generator may reduce or prevent the growth of algae and the presence of microorganisms, for example.
Depending on the nature of the media supplying a temperature differential from which electric power is to be generated, additional components may be desirable.
Experimental results are given below.
In one experimental use, a thermoelectric generator of construction similar to that shown in
About 1 inch of each electrode was in contact with the water. When nanofluids were used, the performance of the thermoelectric generator was measured after 3 hours of operation, the ion generators having operated continuously during the 3 hour interval before performance measurement.
The presence of the nanofluid improved the performance of the thermoelectric generator by a surprising amount, as compared with the performance of the same thermoelectric generator before the introduction of the ions—that is, without the use of a nanofluid. The results of several test runs are given in the table below.
As can be seen in the table by comparing tests numbers 1 and 2, the addition of ions to only the hot fluid resulted in a substantial increase in voltage produced (15.9 vs. 13.1 Volts), even though test number 2 was conducted using a smaller temperature differential between the two fluids. Comparing tests 1 and 3 reveals that using nanofluids for both the hot and cold fluids resulted in an output voltage increase of about 38% (18.1 vs. 13.1 Volts), even though the temperature differentials used in the two tests were nearly identical. Alternatively, comparing tests 1 and 4 reveals that the addition of ions to the working fluids enabled the production of nearly the same output voltage in test 4 (13.0 Volts) as was produced in test 1 (13.1 Volts), even though the temperature differential utilized in test 4 was considerably smaller than was used in test 1.
In another experiment, the system described above, including ion generators in both the hot and cold loops, was allowed to operate for 30 continuous hours. After 30 hours of ion generation, the concentration of silver in one of the closed loops was measured to be about 238 micrograms/liter, and the concentration of copper was measured to be about 362 micrograms/liter. No silver or copper particles were visible via optical or electron microscope, indicating that the nanoparticles were likely non-colloidal atomic silver and copper ions.
The invention has now been described in detail for the purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the appended claims.
This application claims the benefit of U.S. Provisional Patent Application No. 61/440,273, filed Feb. 7, 2011 and titled “Thermoelectric Generation Utilizing Nanofluid”, the entire disclosure of which is hereby incorporated by reference herein.
Number | Date | Country | |
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61440273 | Feb 2011 | US |