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
A mid-size unmanned aerial vehicle (UAV) is powered with a one- or two-cylinder, two-stroke engine. Some of the engine’s mechanical output typically is used to drive an alternator to power onboard electronics. A small two-stroke engine converts the energy output of gasoline at an efficiency rate less than 20 percent on average.
As smaller UAVs are designed with more sensors and communications technology for longer missions, the additional electrical power to run them drives the need to generate onboard electric power. One way to create onboard electrical power would be to harness the remaining 80 percent “waste energy” produced by the two-stroke engine.
A team of engineers at Electronic Cooling Solutions worked with John Langley and engineers at Ambient Micro to build an exhaust-heat thermoelectric generator (EHTEG) that can be incorporated into a UAV design to harvest and convert this waste energy into electrical power in flight [1] (Figure 1). The engineers at Electronic Cooling Solutions did the initial EHTEG design, as well as analyzing and optimizing the thermal design. Then Langley’s team built, tested, and redesigned the generator based on the test results.
Figure 1: UAV with the EHTEG attached on top.
As smaller UAVs are designed with more sensors and communications technology for longer missions, the additional electrical power to run them drives the need to generate onboard electric power. One way to create onboard electrical power would be to harness the remaining 80 percent “waste energy” produced by the two-stroke engine.
A team of engineers at Electronic Cooling Solutions worked with John Langley and engineers at Ambient Micro to build an exhaust-heat thermoelectric generator (EHTEG) that can be incorporated into a UAV design to harvest and convert this waste energy into electrical power in flight [1] (Figure 1). The engineers at Electronic Cooling Solutions did the initial EHTEG design, as well as analyzing and optimizing the thermal design. Then Langley’s team built, tested, and redesigned the generator based on the test results.
Figure 1: UAV with the EHTEG attached on top.
Energy in a small engine is wasted as the heat that is lost to cool the cylinder and cylinder head, loss from friction, and the heat of the exhaust stream. Using the heat from the cylinder and cylinder head is too complicated because it would interfere with the process of keeping the entire engine cooled during operation. Energy lost from friction is difficult to access, and it’s not a significant part of the overall energy loss anyway.
The best choice is the heat that is lost in the exhaust stream because it usually is about the same amount of power, or more, as the power delivered to the shaft, and it’s easy to get to.
The EHTEG had to be mechanically robust and integrate into the aircraft without compromising flight safety. It had to extract the required heat without impairing engine performance. It had to provide the largest possible temperature differential across the thermoelectric modules while operating within the maximum temperature limits of the thermoelectric modules. And it had to be designed with minimal weight and aerodynamic drag.
The best choice is the heat that is lost in the exhaust stream because it usually is about the same amount of power, or more, as the power delivered to the shaft, and it’s easy to get to.
The EHTEG had to be mechanically robust and integrate into the aircraft without compromising flight safety. It had to extract the required heat without impairing engine performance. It had to provide the largest possible temperature differential across the thermoelectric modules while operating within the maximum temperature limits of the thermoelectric modules. And it had to be designed with minimal weight and aerodynamic drag.
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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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