787 Electrical load

achace · Post by **achace** » 11 Mar 2007, 02:56

I hope someone in our forum can come up with an answer to my following thoughts that tells me I am wrong!

Typical electrical loads on airliners with bleed air systems in very simple terms are about 1kW per seat Example the 747 uses about 300kW.

Having just read the Flight International article on the GEnx, I was amazed to learn that each engine has in excess of 1000kW of electric power to satisfy the all electric 787. Obviously the Trent would be the same.

The extra fuel burn required for this power isnt going to do anything for global warming!

This is more than three times the power demand of a conventional bleed system, which by extrapolation is the equivalent of 3 extra tonnes of fuel over a 15 hour flight after making an allowance for the electrical load of the conventional system which as already stated is about 1kW per seat.

This may explain in part why the 350XWB claims a greater specific range than the 787.

I hope someone can tell me I am wrong.

Cheers
Achace

bits44 · Post by **bits44** » 11 Mar 2007, 03:24

A extract from GE's technical release:

The GEnx will deliver 15 percent better specific fuel consumption than the engines it replaces, helping operators save whenever they fly. It is designed to stay on wing 30 percent longer, while using 30 percent fewer parts, greatly reducing maintenance. The GEnx's emissions will be as much as 95 percent below current regulatory limits, ensuring clean compliance for years to come, and it will be the quietest, most passenger-friendly commercial engine ever produced.

95 percent below current emissions limits sounds pretty clean to me! with 15 percent better fuel consumption than comparable engines makes it an easy choice for airlines.

Homo Aeroportus · Post by **Homo Aeroportus** » 11 Mar 2007, 21:46

Hello Achace,

Have also seen the load bank under the test A/C. What a toaster !

There is a difference between installed power and consumption. 5 years ago in Toulouse I have been shown an internal presentation titled "the 1 MegaWatt Airplane". Here we are and even exceed this value with Boeing.

The 787-8 according to the preliminary data released to date will have 2 Gnd Pwr Receptacle (each 90kVA) just under the door 1L (under the boarding bridge !) and another couple also on the LH side just aft the wing. This one won't be easy to reach.

This means that the MAX you can pull is 4 x 90kVA, same as the A380.
Usually you must flip a lot of switches in the cockpit to come not even close to this theorethical max consumption. Best is to switch on the coffee machine in the galley!

Now, as the 787 will have an electric engine start, no pneumatic via the APU or ASU, maybe we could see some cables glowing in the dark at start up.

Next is electric de-icing?

achace · Post by **achace** » 12 Mar 2007, 00:48

Thanks Homo Aeroportus,

Very "enlightening". Still puzzled as to why they need 1000kW on each wing!

Cheers
Achace

mmciau · Post by **mmciau** » 12 Mar 2007, 01:45

achace wrote:Thanks Homo Aeroportus,

Very "enlightening". Still puzzled as to why they need 1000kW on each wing!

Cheers
Achace

Redundancies????

Mike McInerney

achace · Post by **achace** » 12 Mar 2007, 02:19

For the man from Marion,

On a twin, you are correct, each engine has to be capable of carrying the full load of the aircraft.

The gearbox to drive a 1000kW generator is pretty substantial, and as well as being heavy has its own efficiency losses, which would be greater than for instance a 200kW generator, as the losses relate to the generator load.

I guess we have to assume the 787 has some pretty hefty motor starting duties or an awful lot of coffee pots and toasters.

Cheers
Achace (used to live in Adelaide)

smokejumper · Post by **smokejumper** » 12 Mar 2007, 14:55

I beleive that the main advantages are lighter weight and greater efficiency.

Engine bleed air takes high-pressure air from the engine (alternators use a power take-off shaft) that take the air before combustion process and reduces engine efficiency more than takiing power via a shaft.

Also, the added weight of the air ducts and associated plumbing and control valves, etc. to carry the bleed air is more than the weight of the electrical cables.

RC20 · Post by **RC20** » 13 Mar 2007, 04:20

You might want to read it all again. The APU also has a big generator attached to it.

Bleed air is a loss of engine output, electricity is more efficient, and you do not have all the ducts running around. A lot easier routing wires, and less room.

I also suspect a lot of "growth" margin, as future versions will be even more electric.

You probably would have to cross 3 major disciplines to fully understand all the reasons and savings. I can follow some of it, but for the rest, I am assuming they know their stuff (I just fix stuff!)

Bilboone · Post by **Bilboone** » 13 Mar 2007, 08:04

Some info about electrical system of the 787.

The APU, fully electrical with two 225 kVA generators.
So , no load compressor , weight saving , more efficient less complex and 20 % more reliable.

Engine : Two 200 kVA generators. No pneumatic system only Cowl anti-ice, so no ducts,precoolers, hp & press regulator valves , weigth saving , less complex , more reliable and much more efficient;
The IDG is replaced by a VF generator (variable freqency) ,higher voltage (230 Vac) , shorter, reduced gage power feeder cables (weight saving : 2000 lbs)).
Electrical engine starter, more reliable.

The 787-8 according to the preliminary data released to date will have 2 Gnd Pwr Receptacle (each 90kVA) just under the door 1L (under the boarding bridge !) and another couple also on the LH side just aft the wing. This one won't be easy to reach.

Flexibility to power aircraft from existing 115 Vac power source and the new 230 Vac power source ( left side , where now the air starter plugs are on coventional aircrafts) , this 230 Vac power source can be used to start engines and airconditioning if APU is not available .

smokejumper · Post by **smokejumper** » 13 Mar 2007, 20:22

RC20 wrote:You might want to read it all again. The APU also has a big generator attached to it.

Bleed air is a loss of engine output, electricity is more efficient, and you do not have all the ducts running around. A lot easier routing wires, and less room.

I also suspect a lot of "growth" margin, as future versions will be even more electric.

You probably would have to cross 3 major disciplines to fully understand all the reasons and savings. I can follow some of it, but for the rest, I am assuming they know their stuff (I just fix stuff!)

Adding to the above, PTO's (Power Take-off) generaly come out the side of the engine case and into a gear box. The PTO can be a shaft or geared off the center shaft. In the case of an APU, the alternator generally is directly driven by the center shaft.

Homo Aeroportus · Post by **Homo Aeroportus** » 14 Mar 2007, 10:28

Bilboone wrote:Some info about electrical system of the 787.

Flexibility to power aircraft from existing 115 Vac power source and the new 230 Vac power source ( left side , where now the air starter plugs are on coventional aircrafts) , this 230 Vac power source can be used to start engines and airconditioning if APU is not available .

Think 230V VF is internal circuitry only.
Ext Pwr remains 115/200V 400Hz, on all 4 plugs.

ELECTRICAL
TWO LOCATIONS WITH TWO
CONNECTIONS EACH
90 KVA , 200/115 V AC 400 HZ,

Would sound logical to move out of the conventional 115 and reduce on copper but I guess no one is ready to upset the Gnd Handling industry with requirement for new type of GPU's.
Exceptions like B1B and F-22 not seen yet on the civilian side.

On the other hand, I read that the engine start is by using 2 x 90kVA connected to the front receptacles.
Any idea then as to why the second set of receptacles at wing level? Thought these where for engine start.

Bilboone · Post by **Bilboone** » 14 Mar 2007, 16:07

Homo Aeroportus wrote:

Think 230V VF is internal circuitry only.
Ext Pwr remains 115/200V 400Hz, on all 4 plugs.

On the other hand, I read that the engine start is by using 2 x 90kVA connected to the front receptacles.
Any idea then as to why the second set of receptacles at wing level? Thought these where for engine start.

You're correct, looks like my technical document I have ,is a bit outdated.
Original there where to 2x 230 Vac connectors. But probably these where changed to 115 V by demand of the Airlines. My excuses for this incorrect info.

So the second set of receptacles at the wheelwell section are just in use when there is a GPU available at that place in the tarmac. But not real necessary, or there is a dedicated GPU available just for engine start or/and powering the air-conditioning. and a normal GPU in use at the front that is not strong enough for engine start.

Also the engine start is done by the reverse use of the VF generator and not a separate engine starter.

http://www.boeing.com/commercial/airpor ... 87sec5.pdf