Design Article
A design to generate UAV electrical power in flight
Guy Wagner, Electronic Cooling Solutions, and Travis Mikjaniec, Mentor Graphics
6/6/2012 5:50 AM EDT
Mechanical design
The mechanical design of the EHTEG had to be able to support the thermal components and allow adequate flow of cold air around the external heatsinks. It also had to be lightweight with minimal frontal area to reduce aerodynamic drag. They used a structure that was fabricated from two flanged aluminum channel sections, made of 0.032 in. 5052H32, which formed the top and bottom. Two aluminum side panels (0.063 in. 6061T6) were riveted to the top and bottom channels, and the seams welded to form a gas-tight seal. The forward and aft bulkheads were machined from 6061T6. The bulkheads at each end were retained by screws along their peripheries. They used thin-wall alloy steel inlet and outlet pipes.
The heatsinks were fastened by screws from the outer (cold side) to the inner (hot side) forgings. The TEG modules were sandwiched between the outer heatsinks and the EHTEG sidewalls. They used a high-temperature thermal interface material at each interface in the thermal path to maximize heat transfer from the inner heatsinks through the thermoelectric generator modules and the outer heatsinks. Figure 8 shows an exploded view of the EHTEG.
Converting to usable electricity
The thermal environment of each thermoelectric module was slightly different because of its location on the EHTEG, so each module’s output was also different from its neighbors. The input interface modules received the output from a pair of modules and converted the input voltage to 12 V. The input module also automatically adjusted its input impedance to match the source impedance, thus operating at the maximum power transfer point.
The outputs of the input modules were combined and fed to a single 12-V bus regulator that provided a regulated 12-V output to external loads. An electronic load was included in the power conditioning electronics for testing purposes. The electronic load automatically adjusts its resistance to extract the maximum power available from the EHTEG system. The data I/O board provided voltage levels proportional to selected voltage and current levels for input to the onboard data-logging system. For example, the total power delivered by EHTEG system could be computed by monitoring the output voltage and current flowing into the electronic regulator.
In summary
The engineer team used FloTHERM to create virtual models of the exhaust system, analyzing various design configurations quickly before building any physical prototypes. And the results of the CFD models correlated well with those obtained on the engineering test bed.
Because the TEGs are actually thermoelectric coolers run in reverse, their efficiency is only around 5 percent; that is, 5 percent of the heat energy flowing through is turned into electricity. If this efficiency rate can be doubled, the technology could be used in many practical and profitable applications. New commercial opportunities are spurring interest in thermoelectric power generation. The design techniques described here could be used to develop much higher power output thermal energy harvesting power systems.
Credit: Ambient Micro
Click on image to watch the video
References
1. FloTHERM,
2. J. Langley, M. Taylor, G. Wagner, and S. Morris, “Thermoelectric Energy Harvesting from Small Aircraft Engines,” SAE International, 2009.
3. Marco Nuti, Emissions from Two-Stroke Engines, Society of Automotive Engineers, Inc., Chapters 7 and 9.
4. Combustion Products Applet: Allan T. Kirkpatrick, Colorado State University, Fort Collins.
About the authors
Guy Wagner has more than 39 years of experience in the electronics industry. His experience includes: IC and system cooling and packaging technology, disk drive design, computer system design, and design of telephone switching systems. Wagner, an expert in cooling of electronics systems and high-power ICs, has authored 17 papers at international conferences on this subject and has 26 patents. He has been doing thermal consulting since 2001. Before joining Electronic Cooling Solutions, he held positions as a Director of Engineering at Cornice Inc.; member of technical staff at Storage Genetics; Chief Scientist for the HP/Agilent Technologies PolarLogic business unit; and member of technical staff at AT&T Bell Laboratories. Wagner has a Master of Science degree in Mechanical Engineering from Iowa State University.
Travis Mikjaniec has a Master of Science degree in Aerospace Engineering from Carleton University, with a research focus on rotorcraft aerodynamics and aeroelasticity. Mikjaniec has spent the past five years working for Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd.), specializing in the application of CFD to the design of electronics equipment.
----------------------
If you found this article to be of interest, visit Military/Aerospace Designline where you will find the latest and greatest design, technology, product, and news articles with regard to all aspects of military, defense and aerospace. And, to register to our weekly newsletter, click here.
The mechanical design of the EHTEG had to be able to support the thermal components and allow adequate flow of cold air around the external heatsinks. It also had to be lightweight with minimal frontal area to reduce aerodynamic drag. They used a structure that was fabricated from two flanged aluminum channel sections, made of 0.032 in. 5052H32, which formed the top and bottom. Two aluminum side panels (0.063 in. 6061T6) were riveted to the top and bottom channels, and the seams welded to form a gas-tight seal. The forward and aft bulkheads were machined from 6061T6. The bulkheads at each end were retained by screws along their peripheries. They used thin-wall alloy steel inlet and outlet pipes.
The heatsinks were fastened by screws from the outer (cold side) to the inner (hot side) forgings. The TEG modules were sandwiched between the outer heatsinks and the EHTEG sidewalls. They used a high-temperature thermal interface material at each interface in the thermal path to maximize heat transfer from the inner heatsinks through the thermoelectric generator modules and the outer heatsinks. Figure 8 shows an exploded view of the EHTEG.
Figure 8: The inside and outside of the EHTEG.
Converting to usable electricity
The thermal environment of each thermoelectric module was slightly different because of its location on the EHTEG, so each module’s output was also different from its neighbors. The input interface modules received the output from a pair of modules and converted the input voltage to 12 V. The input module also automatically adjusted its input impedance to match the source impedance, thus operating at the maximum power transfer point.
The outputs of the input modules were combined and fed to a single 12-V bus regulator that provided a regulated 12-V output to external loads. An electronic load was included in the power conditioning electronics for testing purposes. The electronic load automatically adjusts its resistance to extract the maximum power available from the EHTEG system. The data I/O board provided voltage levels proportional to selected voltage and current levels for input to the onboard data-logging system. For example, the total power delivered by EHTEG system could be computed by monitoring the output voltage and current flowing into the electronic regulator.
In summary
The engineer team used FloTHERM to create virtual models of the exhaust system, analyzing various design configurations quickly before building any physical prototypes. And the results of the CFD models correlated well with those obtained on the engineering test bed.
Because the TEGs are actually thermoelectric coolers run in reverse, their efficiency is only around 5 percent; that is, 5 percent of the heat energy flowing through is turned into electricity. If this efficiency rate can be doubled, the technology could be used in many practical and profitable applications. New commercial opportunities are spurring interest in thermoelectric power generation. The design techniques described here could be used to develop much higher power output thermal energy harvesting power systems.
Credit: Ambient Micro
Click on image to watch the video
References
1. FloTHERM,
2. J. Langley, M. Taylor, G. Wagner, and S. Morris, “Thermoelectric Energy Harvesting from Small Aircraft Engines,” SAE International, 2009.
3. Marco Nuti, Emissions from Two-Stroke Engines, Society of Automotive Engineers, Inc., Chapters 7 and 9.
4. Combustion Products Applet: Allan T. Kirkpatrick, Colorado State University, Fort Collins.
About the authors
Guy Wagner has more than 39 years of experience in the electronics industry. His experience includes: IC and system cooling and packaging technology, disk drive design, computer system design, and design of telephone switching systems. Wagner, an expert in cooling of electronics systems and high-power ICs, has authored 17 papers at international conferences on this subject and has 26 patents. He has been doing thermal consulting since 2001. Before joining Electronic Cooling Solutions, he held positions as a Director of Engineering at Cornice Inc.; member of technical staff at Storage Genetics; Chief Scientist for the HP/Agilent Technologies PolarLogic business unit; and member of technical staff at AT&T Bell Laboratories. Wagner has a Master of Science degree in Mechanical Engineering from Iowa State University.
Travis Mikjaniec has a Master of Science degree in Aerospace Engineering from Carleton University, with a research focus on rotorcraft aerodynamics and aeroelasticity. Mikjaniec has spent the past five years working for Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd.), specializing in the application of CFD to the design of electronics equipment.
----------------------
If you found this article to be of interest, visit Military/Aerospace Designline where you will find the latest and greatest design, technology, product, and news articles with regard to all aspects of military, defense and aerospace. And, to register to our weekly newsletter, click here.
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Dr DSP
6/6/2012 12:39 PM EDT
Thanx for the details on the mechanical, electrical and thermal aspects of the design. Too often we only see one aspect, and in this case the interactions were very critical.
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prabhakar_deosthali
6/7/2012 3:11 AM EDT
This idea of harvesting the waste energy is really good.
By seeing the roof top mounting of this generator, I thought - why cannot be a wind energy convertor be installed for generating electricity? When the vehicle is cruising at some constant speed , we will be able generate a stable voltage using a small wind mill.
It should also be possible for Helicopters to harvest energy from their big rotating blades
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jeremybirch
6/7/2012 9:51 AM EDT
This system is trying to generate electricity from waste heat in the engine exhaust - not from the force of the "air" flow in the exhaust.
The system was originally only 20% efficient meaning 80% was lost as heat, and some fraction of that is in the exhaust, say 30% of the overall energy. This system can capture 5% of that ie 5% of 30% which is only 1.5% of the total energy in the fuel. So at best this raises the thermal efficiency to 21.5%. As the heatsinks etc weigh quite a lot this in itself might increase the fuel consumed. I am not convinced at this level of energy extraction that it has much merit over having a generator linked to the shaft.
Presumably these are TEG's are using the Peltier effect? There may be much more efficient ways to extract energy eg using Sterling engines etc
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bharat solanki
6/8/2012 10:48 AM EDT
i think in car, also we lost good amount of heat energy, instead of wastage something for that also can be done...
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katgod
6/13/2012 11:25 AM EDT
Yes, precisely my thought also.
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Kevin.Jackson
6/13/2012 4:11 PM EDT
When I was nine I connected a motor's shaft to a generator and wired the generator's output to the motor.
With great anticipation and excitement, I wrapped a rope around the shafts and gave it a good tug.
While the shaft rotated a little longer with the wires connected than without, I clearly didn't have the perpetual motion machine I had imagined.
Some years later I learned why.
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Abrahim
6/28/2012 3:01 AM EDT
wind mill convrtr on top of a plane ! ! !
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@NEALETHOMASnet
6/7/2012 6:46 AM EDT
Amazing that this thing got editorial endorsement considering its amateur adoption of aerothermal analysis relying on CFD (C=Contrived) that cannot conceivably capture complexities conveyed in pictures, not just unfaired facets of equilibrium orientation but more importantly abrupt departures encountered as gust response never mind comment "curl back very symmetrically" without conceding punitive loss factor in all U-turned flow. Reinforced by remarks on meagre recovery energetics from chemical potential unqualified by reference to maximal reality cap 40-50% dictated by law of diminishing returns aka 2nd Law! It's a software sales pitch for a caricaturisation code, one of many in the market and all liable to mislead inexperienced individuals when used away from calibration closure conditions, almost invariably inevitable when any complexity is encountered. I've seen consequences at first hand following fatal accidents when commissioned to critiquise such code variants adapted as nuclear and hydrocarbon safety simulators, also persistence of flawed scalings for years after errors and omissions were documented in more sophisticated literature. Overall then, worth no more than gamma as an auldskule english engineering finalist hons project! @NEALETHOMASnet
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arturi
6/7/2012 6:53 AM EDT
prabhakar, as soon as current starts to flow the rotating blades have to overcome a magnetic field and therefore the mechanical resistance is magnified. Hence more fuel is consumed. There's no free lunch there.
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Kevin.Jackson
6/13/2012 4:07 PM EDT
I've had an interest in this approach to energy harvesting for years but Peltier devices are particularly difficult to deal with from a design standpoint (not to mention a little pricey and very inefficient).
I'm happy to read of this research. I expect higher temperature devices may come out of it, which would expand the potential applications of these interesting devices.
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Paulo540
6/13/2012 8:11 PM EDT
I am curious what the NET power output difference is between the untouched 2-stroke motor and the one with exhaust mod... Generally, cooling off exhaust, especially of a motor with very specific exhaust parameters, will change the powerband.
Speaking of motors, why no 4-stroke?
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Linux28
6/14/2012 8:10 AM EDT
I've used TEGs in the past, biggest problem is reliability. They seem to have a relatively short lifetime.
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Abrahim
6/28/2012 3:06 AM EDT
while its possible to attain good temperature differential in such a system for a therocouple but i think that a high efficiency solar cell fitted to utilize the wing span (without destroying the aerodynamic) can be a much less cumbersome and more efficient option.
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GEM
7/10/2012 6:12 PM EDT
jeremybirch you win the prize, this whole science experiment could have been summarized in a few paragraphs as a waste of time.
I assume someone was being paid by a arm of the government to perform this demonstration, and throw away way our money. One other technical comment the aero drag caused by the heat exchanger (i.e TEG) most likely wasted significantly more energy than the TEG captured, making net energy captured negative.
How about capturing the heat and use it in a closed loop vapor cycle engine (i.e. similar to air conditioner in reverse, you will get much higher efficiency than 5%)
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