Boeing 787 Dreamliner - Mark Wagner [50]
Conventional pneumatic systems on commercial aircraft also had this problem and frequently dumped spare power overboard. Boeing therefore reasonably assumed that the more-electric systems would extract as much as 35 percent less power from the engines. It also expected lots of weight saving, as the pressurized air ducted around the aircraft through valves and precoolers weighed hundreds of pounds.
But the savings were in more than just weight. In the no-bleed architecture, electrically driven compressors provide cabin pressurization, with fresh air brought on board via dedicated cabin air inlets. Boeing predicted that this approach would be significantly more efficient because it avoided excessive energy extraction from engines with the associated energy waste by precoolers and regulating valves. Instead, the compressed air was produced by adjustable speed compressors at the required pressure without significant energy waste. That resulted in significant improvements in engine fuel consumption. The engines also would be started electrically rather than pnueumatically. “We have a unique way to tailor torque for the starter motors, and use it for the power that the engines want to see,” said Sinnett. “Each needs a different torque profile, and with conventional engine starting coming from the high-pressure bleed valve we just give the engine everything we’ve got. This way the torque can be softer, quicker, and will also require less maintenance.”
But if this was such a great idea, why hadn’t it been done before? The answer largely lay in the state of the art of the generators that extracted power from the engines. Previously these were relatively big, cumbersome devices, but improvements in “power density” over twenty years had changed that. The two 250kVA generators fitted to each 787 engine, for instance, took up only slightly more room than the single 120kVA unit installed on a 767 engine.
As well as being used to start the engines, electrical power on the 787 replaced virtually all the traditional pneumatic system and drove the environmental and cooling systems, moved the undercarriage legs, controlled the brakes, and ran the anti-icing system. Power sources for the electrical system were engine-driven and APU-driven generators, while the power sources for the hydraulic system were engine-driven and electric-motor-driven hydraulic pumps similar to those in previous aircraft. A small amount of engine bleed air survived, however, and was used for engine cowl anti-ice and nacelle heating as well as to help maintain operational stability in the engine.
Because it had to perform so many tasks, the 787’s hybrid electrical system was designed to handle several voltage types: 235 volts alternating current (VAC), 115 VAC, 28 volts direct current (VDC), and ±270 VDC. All Boeing’s conventional aircraft had 115 VAC and 28 VDC electrical systems, but the company was going into relatively new territory with 235 VAC and ±270 VDC. Both were the consequence of the decision to go to the “no bleed” electrical architecture.
Overall, the 787 generated twice as much electricity as previous Boeing airliners, including the 747. It did this using a gang of six generators: two per engine and two on the tail-mounted APU. Although the four engine-mounted were rated at 250kVA and the two APU-mounted at 225kVA, all operated at 235 VAC for reduced weight. The generators were directly connected to the engine gearboxes and, to cope with the widely varying engine speeds on the ground and in flight, operated at a variable frequency (360 to 800 Hz). Variable frequency was chosen over the traditional integrated drive generator (IDG) concept because it was simpler, and therefore likely to be less of a maintenance burden on the airlines that had pressed Boeing on the point.
With so much electrical power to share, the electrical system was split between two electrical/electronics (E/E) bays, one forward and one aft. Just as importantly,