The Incredible E-Plane
By Bill Moore
A long-time proponent and electric vehicle experimenter, Jim Dunn said he was always being asked when would he build an electric airplane. As he'll readily admit, its on thing to power a bicycle or even a car with electricity, It's something altogether different to power an airplane with it.
After years of prodding, Dunn said he got serious after developing a device for starting aircraft at his company, Advanced Technology Products. It was called the "Start Stick." It weighed just three pounds and was powered by special "thin metal film batteries".
"We had a lot of fun. . . going around starting airplanes that would normally take a ground cart, including things like a B-17, with this little stick, and ended up winning the outstanding design competition award . . about a three foot high trophy."
Folks in the general aviation industry were so impressed by this nifty piece of modern battery technology that again they urged Dunn to consider the electric airplane idea. Dunn and his partner, Dave Ekstrom, started giving the idea some serious consideration. They first looked at building an electric-powered, self-launching glider, which has been done in Europe. But, as Dunn pointed out, "most of them are limited to eight to ten minutes run time."
"Last year we put a little more effort into it and actually put in a proposal to NASA for the funding to see if we could get something underway so we could do a more detailed study."
With his proposal and calculations in hand, Dunn approached three different light aircraft manufacturers at the annual EAA Fly-In in Oshkosh, Wisconsin the summer of 2000.
"We had looked at Pulsar and Europa. . . the Starlite and quite a range of things including Burt Rutan's canards, the Long EZs and Co-Zs . . . And we found a lot of challenges, but there was one plane that we ran across, which is the plane we ended up -- ironically -- using, the first candidate. We approached Diamond Aircraft and asked them if they'd be willing to work with us." The company makes a range of composite light aircraft including motor gliders. This was Dunn's first choice, initially. He also talked to the American Ghiles Aircraft company, makers of the Lafayette III, as well as a third firm that makes an aircraft called the Eagle 150.
Dunn asked each to come back with a proposal and the best "a really good deal" would participate in the project. It turns out all three came back two days later and gave him an airplane. In the end, however, Dunn took deliver of both the Diamond and the American Ghiles Lafayette III. Currently his efforts are focused on modifying the latter because American Ghiles went to the extra effort of modifying the aircraft's wing to hold the lithium batteries that will help propel the E-Plane.
Dunn is interested in electric power for several reason, one of them which any light aircraft owner will immediately appreciate. The small, 80 hp Rotax engines that power the new generation of kit-built composite aircraft must be overhauled every 800-900 hours. The standard Lycoming or Continental light aircraft engine in the 100 hp and up class has an overhaul schedule of every 1,500 to 2,000 hours. Dunn said that by switching to an electric drive motor, the overhaul period could easily be extended to 10,000 hours. This is comparable to the dramatic increase in reliability that occurred when jet turbine engines replaced piston engines in military and commercial aircraft.
"We've reached the point where we can build incredibly efficient electric motors," Dunn stated. "Today with. . . things we've learned from electric vehicles and hybrid electric vehicles that are emerging, as well as just other improvements in the state-of-the-art, we have electric motors that are ninety-five to ninety-seven percent efficient." The cost of the motors has also dropped, he added.
While switching to an electric-drive motor also results in a decrease in the weight of the engine and increase in the weight to hp ratio, there is also a significant trade off. When the weight of the fuel cell, controller and batteries are added, the overall power plant weight ends up higher than the original reciprocating engine.
Dunn admits that when it comes to energy storage, gasoline -- in the form of aviation fuel -- is hard to beat. He's worked with all types of advanced battery technologies and none of them even come close to gasoline.
"Some of our best lithium batteries measure in the 150-200 watt-hours per kilogram," Dunn explained, "particularly those used in satellite and military applications." By comparison, gasoline is rated at 13,000 watt-hours per kilogram. Even at the a 20% efficiency rate for the best internal combustion engine, this still translates into 2,600 watt-hours per kilogram, nearly 13 times the energy density of our best battery technology.
To overcome this problem, the E-Plane project has turned to the PEM fuel cell, the same device that automakers are pouring billions of dollars into for use in their future cars and trucks. The fuel cell is, in effect, a battery that runs on oxygen and hydrogen. It produces electricity, heat and water vapor. In phase three of his project, Dunn plans to integrate an 12-15kW PEM fuel cell to provide cruise power for the E-Plane.
The beauty of the fuel cell -- from Dunn's perspective -- is the efficiency with which it converts chemical energy into electricity, which now starting to approach 50%, he stated. This is double the efficiency of the best Carnot-cycle engine.
But just as the problem of hydrogen storage continues to perplex carmakers, so too does it complicate development of a practical E-Plane.
"But the problem you have is we have to figure out a way to come up with hydrogen and hydrogen is a pretty amazing material," Dunn admitted. At the molecular level structure of hydrogen is such that its individual atoms strongly resist being forced together. This means it takes an enormous amount of energy and pressure to store even a small amount of it in a tank. Even with the newly developed 10,000 PSI tanks developed by Quantum and others, compressed hydrogen still doesn't carry the energy comparable to that of liquid hydrogen, the fuel used to power rockets into space.
Although carbon nanotubes and metal hydrides show promise as a storage medium, Dunn pointed out that at a 6% storage volume -- which is considered very good today -- for every 1 pound of hydrogen, you have 16.6 pounds of storage medium you have to lug around.
"That's the dilemma we face today." Cryogenic hydrogen isn't the easiest fuel to work with, especially at an airport, Dunn remarked.
Instead of working with this difficult medium, Dunn and his small team, which includes several world-class test pilots, are looking at making their own hydrogen-on-demand from sodium borohydrite using technology developed by Millennium Cell. The process uses a noble metal catalyst and water to create pure hydrogen gas on demand, neatly solving the problem of hydrogen storage. However, the byproduct of this catalytic process is borate, which is stored in a separate tank. Fortunately, it can be recycled. Dunn added that with this approach, the aircraft will actually have a higher landing weight than take-off weight, something that simply doesn't happen in a conventional aircraft.
The borohydrite system turns out to be equivalent to about 4-5% hydrogen by weight when the total system weight is added up, so it isn't as good as the newer generation pressure tanks, but it is safer and easier to work with. One positive spin-off, however, is the "fuel" can be stored for 25 years, Dunn pointed out.
Fuel cells also require oxygen and Dunn plans to use a combination of ram air and atmospheric oxygen to reduce the need for a fuel cell compressor, yet another component that adds weight and saps power.
"It's not a trivial thing," Dunn explained when considering the challenge ahead of him. "If this were easy, so one would have already done it."
The planned configuration goes something like this. In order to keep the center of gravity (CG) within the necessary margin of safety, the electric motor will be moved slightly further forward than the current reciprocal engine. The lithium batteries will be located in the wings just ahead of the main spare, with a temporary third pack behind the cockpit. Dunn plans to use 300 pounds of state-of-the-art lithium batteries normally used in satellites. Each battery costs $1000 and he needs 150 of them.
In phase two of the project, the 12-15kW fuel cell stack will be located just behind the electric motor and ahead of the cockpit firewall. This fuel cell generates just enough power to cruise, but not enough for take or climb. The batteries will be used for these energy-demanding tasks.
The fuel cells Dunn is looking at are similar to those that will be used in the Helios solar-electric aircraft being developed by AeroVironment and NASA.
Another challenge facing the E-Plane developers is heat. The borohydrite system generates a lot of heat that has to be dealt with, Dunn explained. For every 1kw of electricity produced by the fuel cell, the system also creates 1kw of heat energy. By contrast, an internal combustion engine generates significantly more heat than it uses, sending most of it out the exhaust pipe.
This heat is a "big issue" and getting rid of it creates a lot of parasitic drag. Dunn said he's received countless suggestions from all over the world about how to utilize it, but so far he concludes the best approach is to fly on a cold day.
CONTINUED NEXT WEEK.
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