By George Bower
Advancements made in the electrification of the automobile are just one area in transportation to see such inherent improvements being introduced. For something slightly off-topic, but arguably related at least somewhat to what GM is doing with cars, we’ll take a brief look at what Boeing is doing with a cutting-edge aircraft.
There has been lots of press about the Boeing 787 but usually relative to its advanced lightweight composite design. Little known is that the 787 is the first commercial aircraft to use an “all electric” design instead of the old “bleed air” design.
To see how these old and new designs compare please consider the following schematics.
The old design relied heavily on bleed air (compressed air) as a source of power. What can be lighter than air was the old logic. The bleed air was either bleed off the main engines or supplied by an auxiliary power unit located in the tail cone of the aircraft. Bleed air was used to start the main engines via an air turbine starter (ATS). Its other primary function was to provide air conditioning for the passengers via an environmental control system (ECS). While the air itself is very light, the associated ducting and valves are not. The new all-electric design eliminates the ducting and valves resulting in a lighter, more reliable system.
Starting of the main engines is no longer accomplished with an air turbine starter but with variable speed starter generators.
As Boeing says:
“The generators are directly connected to the engine gearboxes and therefore operate at a variable frequency (360 to 800 hertz) proportional to the engine speed. This type of generator is the simplest and the most efficient generation method because it does not include the complex constant speed drive, which is the key component of an integrated drive generator (IDG). As a result, the generators are expected to be more reliable, require less maintenance, and have lower spare costs than the traditional IDGs.”
The all-electric version relies on a higher voltage “hybrid” system, as Boeing elaborates:
“The 787 uses an electrical system that is a hybrid voltage system consisting of the following voltage types: 235 volts alternating current (VAC), 115 VAC, 28 volts direct current (VDC), and ±270 VDC. The 115 VAC and 28 VDC voltage types are traditional, while the 235 VAC and the ±270 VDC voltage types are the consequence of the no-bleed electrical architecture that results in a greatly expanded electrical system generating twice as much electricity as previous Boeing airplane models. The system includes six generators — two per engine and two per APU — operating at 235 VAC for reduced generator feeder weight.
“The ±270 VDC system is supplied by four auto-transformer-rectifier units that convert 235 VAC power to ±270 VDC. The ±270 VDC system supports a handful of large-rated adjustable speed motors required for the no-bleed architecture. These include cabin pressurization compressor motors, ram air fan motors, the nitrogen-generation-system compressor used for fuel-tank inerting, and large hydraulic pump motors.”
So you see ground transportation isn’t the only place where all electric prevails. It works in airplanes too. Why?… because it is more efficient … just like the Chevy Volt.
This entry was posted on Friday, March 30th, 2012 at 5:55 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.