A design to generate UAV electrical power in flight
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  (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.
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.
Designing the interior and exterior TEGs
Heat exchangers on the inside of the muffler absorb heat from the exhaust as it flows through. The heat passes through exchangers that line the inside of the UAV’s aluminum skin to 2-in. square TEGs mounted on the outside of the UAV and finally passes through another row of heat exchangers to the open air. As the TEGs are exposed to the temperature difference between the hot inside exhaust air and the cool outside air, they generate electric current. The greater the temperature difference, the more current is generated.
The team modeled the thermal design of the system using Mentor Graphics FloTHERM computational fluid dynamics (CFD) 3D modeling software . They simulated airflow on the outside (cool air) and the hot exhaust inside to estimate the temperature difference, which enabled them to optimize the internal and external fins of the heat exchanger and the number and location of the TEGs.
They built engineering models of several likely EHTEG configurations and ran them on a test stand using the same 3W80xi engine and propeller that is used in a MLB Company Bat4 UAV. They validated the FloTHERM CFD models for a range of operating parameters that simulate flight conditions by carefully measuring temperatures, electrical output, and exhaust flow rates using a custom-built airflow test chamber. Exhaust gas composition for calculating mass flow and specific heat was derived from previous work [3, 4].
They started with the internal volume and length for the muffler recommended by the manufacturer that was required to make the system act as an efficient expansion chamber exhaust system for the two-stroke engine. These were used to develop the first half-symmetry CFD models that would determine the number of TEGs needed to optimize the electrical output with minimal weight. The model is symmetrical about a longitudinal vertical plane, so building a half-symmetry model reduced the number of elements down to 1.04 million cells with no loss in accuracy.
They kept the fin parameters of the internal and external heat exchangers relatively constant while varying the location of the heat exchangers and the placement of the TEGs. The goal was to maximize the temperature differential across each TEG to extract the most heat energy from the 455°C exhaust gas.
They modeled 13 configurations of heat exchangers and TEGs in the first optimization study. Then they tabulated the power output from each configuration and chose the best configuration. For example, when the heat exchangers were spread out over the full length of the muffler, the hot exhaust flow had difficulty reaching the first set of heat exchangers. The design was improved by placing all the heat exchangers close together toward the midsection of the muffler (Figure 2).
After optimizing the placement of the TEGs, they found that the central fins on the interior heat exchangers disrupted the exhaust flow and the hot exhaust gas wasn’t reaching the front end of the muffler. But if they removed the center fins, the exhaust gas would not channel down the center and the exhaust pulse would not reach the front end of the muffler. So instead of removing the central fins, they placed them in a semicircular pattern (Figure 3). This configuration kept the exhaust pulse moving through the center of the muffler and it was able to curl back very symmetrically as the hot gas flowed back along the outsides and through the fins.
The interior heat exchangers decreased in temperature as the hot exhaust gas flowed from the front of the muffler through the fins to the rear of the muffler and then out of the vertical exhaust pipe. With FloTHERM simulation, the team was able to see the exhaust gas flow pattern for the heat exchanger with the center fins removed (Figure 4). The flow pattern is disrupted before the exhaust stream reaches the front end of the muffler.
Usually outside heat exchangers on TEGs are placed all in a line, which is a problem because the units toward the rear of the external airflow receive more preheated air than the heat exchangers that are upstream. This configuration reduces the delta-T across the heat exchanger and reduces the power output.
So they studied nine configurations to determine the optimum fin parameters for the external heat exchangers. Then they plotted the results against the total power generated by the TEGs. Although all of the custom heatsink designs they analyzed out-performed the stock heatsinks for total power generation, because of time and budget constraints, they used stock heatsinks for the first flight test model. (As a result, they lost about 11 W power output compared to the best possible option.)
The outside TEGs were arranged in four columns by two rows on each side of the muffler. The TEGs maintained a cool side temperature below 58.3°C with external air at 18°C and 22.3 m/s velocity, while keeping the hot side temperature below the maximum allowable temperature of 225°C (Figure 6). The outside heat exchanger temperature ranges from 26.1°C on the leading edge of the front fins to 58.1°C on the base plate next to the hottest TEG.
Some of the first simulations demonstrated that the heat loss through all other surfaces of the muffler, other than the exterior heat exchangers, had to be minimized to maximize heat flow through the TEGs for maximum power generation. They used mineral wool sheet insulation for all the exposed surfaces to minimize the heat loss because it can withstand temperatures of more than 300°C and added little weight to the muffler assembly.
They were able to model the power output of the EHTEG system by summing the power contribution of each pair of TEG modules. By first calculating the hot side and cold side temperatures of each TEG pair, they could then use this data to compute the open circuit voltage. The harvesting and power conditioning circuitry matches the equivalent series resistance for maximum power transfer; thus, the voltage of the load resistance is exactly half the open circuit voltage. This data defines the power harvested per TEG pair. The results for the flight configuration are shown in Figure 7.
Figure 7: Single-side power output (W) for the flight configuration.
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.
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.
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.