That's the first I've ever heard about using batteries as the primary start energy for the APU. Normally, commercial aircraft run off of external power when on the ground, and that same externally-supplied power is what is used to "kick over" the APU (which is a small gas turbine, usually in the tailcone, which produces power which, among other things, supports starting the main gas turbines in the engine nacelles).
I've never heard that they were using the battery system to provide starter energy for the APU, and abandoning the long-time practice of using ground power.
I do know, as a fact, however, that the primary purpose of the batteries is, in fact, exactly as I described it. The argument has always been about "parasitic losses." Parasitic losses are losses which serve no purpose... such as running a generator unit or hydraulic pump even when it's not being used. The entire argument for the transition from hydraulic to electrical, especially combined with "battery storage," has been to reduce the requirement for the power being tapped off the engines on a continual basis. The idea is, you take a little bit of power, which is used to charge the batteries... and you discharge the batteries (in "lump amounts") as needed.
This is the same thing you see with hybrid vehicles, by the way. I worked on the battery system which went into the "New Bus for London" (the big red double-decker buses which were unveiled just prior to the last Olympics in London). This vehicle had a small engine in the right rear corner, which could run the vehicle at "constant speed" with just a tiny bit of excess power available... with that excess going to charging the battery tray. When you'd brake, "regenerative braking" would drive power back into the battery pack, and when you wanted to accelerate, you'd dump a lot of energy out of the battery pack.
This worked out really well in this application. The vehicle is significantly more fuel-efficient.
Of course, we were talking about a vehicle riding at street level, with multiple exits, and the ability to off-load the vehicle very quickly if necessary, as well as having the battery pack very robustly designed, fully vented, water-cooled, and with both ballistic and shock/vibration protection designed into the enclosure... all in a compartment with no other hardware adjacent, separate from the habitable spaces of the vehicle.
Don't get me wrong... I LOVE this technology. I just recognize the issues... MAJOR issues... with it.
My aerospace activity was all related to auxiliary equipment related to the engines... so I'm pretty familiar with big commercial gas turbine design, even if I wasn't directly responsible for the engines themselves. A big part of that was understanding the startup processes for commercial aircraft, since our equipment was directly dependent on that and was crucial to that as well.
In other words, I'm not tossing out "academic" experience... I'm talking about what the engine and aircraft manufacturers communicated to me and my team as how they do these things. That's my source of info about "what these batteries are used for."
But, at least on these issues, my knowledge is, admittedly, second-hand.
1. The system we have developed is capable of detecting minimal changes in any dimension of any size or shape battery cell with extreme accuracy.
2. There is oxygen expelled from the positive electrode of Lithium-ion cells only if the temperature exceeds a certain value. However our system detects instability before this temperature rise.
3. The system does include levels of security by way of multiple redundant hardware/software and as it is able to detect any unstable cells at an early stage of instability a gas such as Halon 1301 provides an effective suppressant preventing any further temperature increase. Liquid nitrogen would really not be ideal as a coolant in this situation!
4. Our system is designed to minimise the drawbacks and make Lithium-ion a safe technology.
5. The system was originally designed for improved safety within the electric vehicle market. I am unable to comment on the economic decisions made by the aviation industry in their choice of battery technology, I can however say that our system is not only cost effective but most of all makes Lithium-ion battery technology much safer.
Ann, There are countless anecdotes of MBA interference albeit indirectly in the engineering process.
One prime example was the gentleman who declared that a wing spar process would be shipped out to the lowest bidder since the process we were looking at only realized $5 per sq.ft. whereas the aircraft assembly area was (at that time) making $42 per sq.ft. When it was pointed out to him that the wing spar was the most vital part of the wing assembly and that's why the company manufactured it in house...to keep track of material, machining and QA...he said the outsource would look after that. When it was pointed out that the company assumed full responsibility for the wing construction again he argued that he ASSUMED the outsource would do the same...little did he know about engineering reality and yet he had the say-so to even suggest such things.
The 787 goes further towards an "all-electric" plane like the F-16, but the purpose of the batteries isn't what you state. The aft battery (Boston fire) starts the auxilary power unit. The APU is a small gas turbine in the tail of most airliners and provides electrical power, HVAC, engine start, etc. on the ground as well as "hot standby" power during certain flight phases. The APU is shut down in flight. The battery is required to ground-start the APU away from external power or in a flight emergency. The forward battery (Japan incident) powers "essential" items like the glass cockpit as a last layer of power redundancy.
Both batteries exist on "legacy" aircraft...it has nothing to do with the higher electrical load on the 787. This is the first Li-Ion on an airliner although military aircraft have used them for awhile.
Adding to your and other's comments: The 65,000 gallons of fuel on a 747 have far more energy content (by weight and volume) than Li-Ion and of course the fule system has catastrophic failure modes. They've been mitigated to very acceptable levels but not zerop (TWA800 accident).
Boeing either has incredible hubris, or has a truly innovative solution to the problem. Which one will be interesting....
As I have been around for some time I do recall that Li Ion batteries were used in Laptop which did have the noterity of catching fire and it was in the news. Since I have worked with a lot of different type of batteries, Lead, calcuim, NCL, Gel, They all have some enviornmental and physical conditions that would lead you to do a 40 year type of analisys as planes have change in ATM pressures, temperature, gravity, Electrical Stresses, Fire is not the only safety issue for 250 passengers and they should not be exposed to forced exiting for any safety issue, PLAN AHEAD.
Just because they may have come up with a way to stop or contain a fire, it doesn't mean that the aircraft will be safe. The electrical system will still be compromised and that could be a bigger danger, especially with a fly-by-wire aircraft. It's like the containment dome at a nuclear power plant; if a problem develops, it might prevent a radiation leak but that won't restore power.
I think Warren has a good point about the 16-year-old's feeling of invulnerability and immortality. I remember (just barely). But I think this is a different problem, one that goes with increasing amounts of power, and other inevitabilities, as ScotCan points out.
ScotCan, sounds like you've had similar experiences. My perspective was partly from the outside as a journalist, and partly from the inside, both working as a contractor and knowing others who worked at these places. There are also larger historical phases: the MBA/bean-counter plague was not always an inevitable point in a company's lifecycle.
My objection to your statement is not that you haven't implemented an additional level of detection... though "volumetric" detection is notoriously unreliable, since cells vent at different rates, different cans will deform in different ways, etc...
My objection is that you seem to be claiming that "detection" and "block-switch-off" can be sufficient to eliminate this concern. It can't. The cells still decay, with use or with "abuse," and still produce oxygen as the principle byproduct of that decay.
Unless you're also implementing a liquid nitrogen "spray cool" process as part of your system, along with a multiply-redundant detection/control hardware solution... I won't be comfortable with the use of these devices in large quantities in aerospace applications.
Li-Ion batteries are great devices... but they have real, unavoidable drawbacks.
Ultimately, my point is that, in order to save a tiny amount in terms of fuel economy, they've compromised the safety of the aircraft, and thus of all the passengers, crews, and people along the flightpaths... plus, they're adding other costs into the system (such as, perhaps, your own system?) just to deal with the added risks of these systems... which, I strongly suspect, more than offset the "fuel economy" savings.
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