|
OK, so this is a little off-topic and its most certainly a flight-of-fantasy but it’s too cold and wet to go flying or to work on my plane so cut me some slack. I apologize in advance for its length.
The inspiration for the treatise that follows was an announcement by a researcher at Stanford University just before Christmas. He claims to have invented a new type of lithium-ion battery based on a silicon anode which has 10 times the energy density compared to existing, carbon-anode, lithium-ion batteries. A factor of 10! That's huge! Normally, advancements in battery technology are measured a few percentage points at a time. You can read about it here and decide for yourself about the validity:
Needless to say, the blogs and newsgroups dealing with alternative energy and electric cars have lit up like a Las Vegas night but I was wondering how this technology might affect an airplane. In particular, what if these batteries were used to power my still-under-construction LNC2. A "slippery" airplane like a Lancair should be ideal as an electric powered airplane due to its inherent efficiency in the air.
Everything which follows is based on the assumption that the announcement is accurate, that the energy density of the anode will be reflected in the energy density of the whole battery and that the technology will proceed to commercialization without any problems. These are major assumptions. We all remember cold fusion.
As a point of reference, Tesla Motors, Inc. ( www.teslamotors.com) has developed a 53 kWh lithium-ion battery pack to power their slick new electric sports car, the Tesla Roadster. This battery pack uses existing technology and only weighs 900 lbs!!!
We can calculate the energy density of an existing technology battery pack by dividing 53 kWh by 900 lbs. The result is about 0.06 kWh /lb. If the new technology has an energy density 10 times the old, then the new technology is 10 times 0.06 kWh/lb or 0.6 kWh/lb.
The weight of batteries which can be accommodated in the LNC2 is equal to the weight of the stuff we are removing including the IO-360 engine and accessories and the weight of avgas less the weight we are adding back in such as the electric motor, gearbox and associated electronics.
The total avgas in a typical LNC2-320/360 is 21 gals in each wing plus 10 in the header tank for a total of 52 gals times 6 lbs per gal resulting in a total avgas weight of 312 lbs. Assuming the engine installation is 350 lbs (I'm going by memory here) gives a total weight of 662 lbs (312 plus 350) which can be eliminated from the LNC2 if you are converting it to electric. Calculating the weight of an electric motor and controls is a little less precise. Again turning to Tesla Motors, they have developed a 185 kW (about 250 HP) electric motor for their roadster which weighs only about 70 lbs. This is a bigger motor than we actually need compared to the 160 –200 HP engines usually found in LNC2s but this isn't all that accurate so lets go with it. To that we have to add the weight of the gearbox (the motor turns at 13,000 rpm) and the electronic controls. I don't have a handle on those but lets assume that the whole package of motor, gearbox and controls weighs 162 lbs. Subtracting the motor, gearbox and controls (162 lbs) from the weight of the avgas and Lycoming (662 lbs) leaves us with 500 lbs available for batteries (funny how these things work out). With batteries storing 0.6 kWh/lb we have a total energy storage of 300 kWh (500 lbs times 0.6 kWh/ lb). I suppose someone on this list will insist that a backup set of batteries is required! They will be ignored for now.
Assuming an LNC2 with a 180 HP engine running at 65% power cruises, at altitude, at 200 kts (230 mph) we see that there is 117 HP (180 HP times 0.65) going to the prop. This is equal to about 88 kW (0.75 kW per HP). Assuming that the electric motor and controls are about 85% efficient we find that the electric motor will be drawing about 103 kW (88 kW divided by 0.85) of electric power from the batteries to maintain the same speed.
Endurance is the energy stored (300 kWh) divided by the rate of usage (103 kW) or 2.9 hours. Range is endurance (2.9 hours) times the speed (230 mph) or about 670 statue miles without reserves. This performance isn't as good as the existing avgas powered version but this is still a very usable, practical, airplane. The range can be extended by going to higher altitudes or by flying at a lower speed. Alternatively, redesigning the airplane for say, another 200 lbs of wing mounted batteries would put it in the same league as the avgas version of the LNC2 in terms of range and endurance at the same speed.
Recharging batteries this large is a major issue. A 240 volt , 200 amp, single phase service would require about 6 hours and 15 minutes (neglecting charger loses) to fully charge a 300 kWh battery pack (300,000 Wh divided by 240 V divided by 200 A). This is probably acceptable for overnight charging in a hangar assuming it has that kind of a power feeder available ;-) Fast recharging, say something like a 5C charge rate ( i.e., 12 minutes) at a public use charging station at a remote airport would be a real technical challenge. That would require a 1.5 MW feed. If we assume that the charging is done at 500 volts, a 5C charge rate is equivalent to 3,000 amps! Oooops! Based on my understanding of the inventor's data, he hasn't tested the new batteries at anything greater than a 1C charge rate (i.e., 1 hour) and that resulted in reduced energy density. However, the battery is in the earliest stages of testing and development.
One last calculation. My local utility has a special off-peak rate of 5 cents per kWh for charging electric cars. Assuming this is also applicable to electric airplanes, we have $0.05 per kWh times 300 kWh used during the above described flight or $15 total fuel cost. That's roughly one-tenth the cost of avgas (at $5/gal) for the same flight. I doubt you can buy a 600 mile bus ticket for $15. Of course this ignores battery replacement cost which is unknown for this new technology but, at worst, is probably comparable to an engine rebuild.
So why would anyone want an electric airplane assuming one could be built with comparable performance and price? The following "Pros" have occurred to me:
- Virtually eliminates noise and vibration.
- Lower operating cost.
- No consumption of petroleum products, foreign or domestic (very little electricity is generated in the US from burning oil).
- No exhaust emissions from the airplane including either greenhouse gases (e.g., CO2) or traditional pollutants (e.g., NOx, SOx, particulates, etc.). Emissions from the electric generating plants that supply the electricity is a whole 'nuther subject.
- No danger from avgas fires or explosions.
- Ultimate altitude engine. An electric motor will continue to produce rated power from sea level to outer space if you figure out a way to cool it.
- No more worries about future supplies of tetraethyl lead or from EPA mandates for cleaner burning engines or fuel.
- No more concerns about fuel contamination by water, ethanol, etc.
- No more worries about fuel supply in general. Just the occasional black- or brown-out.
- No more worries about LOP/ROP, cylinder head cracking, broken crankshafts, galled cams, arcing mags, etc., etc.
- Potential for much higher component reliability. The electric powered plane would probably only have two moving parts, the motor shaft and the prop shaft, as opposed to the myriad of parts going suck, squish, bang, blow hundreds of times a second.
- By combining two half-size electric motors on one shaft in one housing and supplying each set of windings with its own electronics and dedicated battery pack, you have almost the same system level reliability as you do with a twin engine airplane with very little increase in cost or weight and no asymmetric thrust problems.
- Reduced maintenance (e.g., no oil and filter changes, etc.)
- No carbon monoxide to worry about.
- No vapor lock to worry about.
There are, of course, a few reasons why one would not want an electric airplane. Here is a list of the "Cons" that I thought of:
- Virtually eliminates noise. Yes, I know its also in the "Pro" list but have you ever stood near a P-51 or a Corsair doing a high speed, low altitude pass. If you haven't and you like airplanes then that should be items 1 and 2 on your "Bucket List".
- Lack of infrastructure for "slow" recharging in your hangar. No real technical issues so this is a solvable problem. All it takes is time and money.
- Lengthy recharge times at public use facilities. Many technical challenges to "fast" recharging in terms of the infrastructure on the supply side, the interconnection and battery considerations. May have to await the development of suitable ultracapacitors instead of batteries.
- Battery life and replacement cost unknown. May turn out to be a non-issue.
- Potential for electrical shocks, fires and explosions. Hard to quantify relative to the danger of fires and explosions from avgas.
- Increased potential for engine failure due to static discharge or lightning strikes. With increased use of electronics for reciprocating engine control, this may be a moot point determined mostly by the quality of component level design and installation rather than any inherent system level considerations.
- Landing weight equals takeoff weight. A potential problem for the LNC2 although should not be a problem for a clean-sheet-of-paper design.
- Risk of the batteries self-immolating like certain laptop batteries have done. Unknown if this new technology precludes that type of event.
So where does all this leave us? No where for now as the inventor says it will take 5 years to commercialize the new battery design. We will have lots of time to see if it lives up to its hype. Hope you all enjoyed this little exercise and will find it thought provoking. It makes you wonder what general aviation will look like in 50 years.
Yes, I know I should stop daydreaming and get back to work on my plane.
Jim McKibbin
LNC2 – 30% and holding
|