Electriflying

Jim Logajan wrote:

wrote:

100LL Avgas is 44 MJ/kg.

Batteries need to be able to produce better than 20 MJ/kg to be
generally usefull for transportation.

The Avgas energy density is not comparable to battery energy density
because it doesn't factor in thermal to mechanical conversion. Current
automobile engines extract only about 20% of that 44 MJ/kg. Electric
motors are anywhere from 75% to 95% efficient in converting electrical
power to mechanical power.

So a battery with ~11 MJ/kg powering an 80% efficient electric motor
has the same usable energy density as Avgas powering an internal
combustion engine.

A Lycoming is about 27% to 29% percent efficient at 70% power.

Setting the goal for battery energy density at 20 MJ/kg would simply
be wrong because it uses the wrong numbers.

There is also more to battery selection and technology than simply the
energy density of a fully-charged battery pack. Consider that a deep
discharge battery of current technology, such is used in a laptop
computer or a Prius, is rarely discharged below about 20% of capacity to
help extend its life. This is perhaps analagous to the unusable fuel in a
fuel tank of say 5%. That difference needs to be taken into account in
any comparison.

By the same token, to extend life, batteries are also rarely fully
charged, with maybe 10 to 15 percent of theoretical capacity unused.
That makes the usable capacity of a battery in the range of 65 to 70
percent of its theoretical capacity.

Further, there are other realities like the fact that all batteries lose
capacity when they are cold, as they age, and when (not if) cells in the
battery pack fail. That means a reserve calculation is a bit more
difficult, and would likely mean a traditional 5 or 10 percent reserve
would have to be increased to ensure there are no surprises.

Finally, you can also increase your payload with a conventional engine by
reducing the amount of fuel being carried, thereby sacrificing range.
That would not be possible with a battery, since more than likely you
would not be able to reduce cells to reduce weight. The packs would tend
to be a package deal to economize in overall design weight.

Therefore, any calculation using the theoretical energy density of a full
battery pack is wildly optimistic.

On 2011-06-28, wrote:
There is a difference between the theoretical efficiency of a motor of
any type and the obtainable efficiency of a real world motor.

The real-world efficiency of a modern brushless motor and its control
system is around 85%. It's only brushed motors which are merely twice
as efficient as an internal combustion engine. No one today uses
brushed motors for traction outside of golf-carts (and it wouldn't
surprise me if golf carts use brushless motors now).

Dylan Smith wrote:

wrote:

There is a difference between the theoretical efficiency of a motor
of any type and the obtainable efficiency of a real world motor.

The real-world efficiency of a modern brushless motor and its control
system is around 85%. It's only brushed motors which are merely twice
as efficient as an internal combustion engine. No one today uses
brushed motors for traction outside of golf-carts (and it wouldn't
surprise me if golf carts use brushless motors now).

Many railway locomotives still use brushed series-wound DC motors for
traction, and their efficiency exceeds 90 percent in the major part of
their operating range. Their operating efficiency is comparable to that of
AC motors through most of the typical speed range.

"Gatt" wrote in message
...
A few years ago when people were discussing the possibility of
electric-powered flight, you'd get hacked on pretty hard for
suggesting such a thing would be practical or possible in our
lifetime. Obviously if you knew anything about physics or electrical
engineering, why, you'd know it was totally impossible.

It's not totally irrelevant to this thread to note that a manned electric
airplane, the Solar Implulse, recently made a 12 hour, 59 minute international
flight. This also would have been said to be totally impossible not so many
years ago.

I make no claims that the Solar Impulse is a practical airplane. After all, its
average speed of advance was only some 27 knots. That said, gliders and hot air
balloons also generally fail the test of practicality, but there are plenty of
them around.

Vaughn

In rec.aviation.owning vaughn wrote:

"Gatt" wrote in message
...
A few years ago when people were discussing the possibility of
electric-powered flight, you'd get hacked on pretty hard for
suggesting such a thing would be practical or possible in our
lifetime. Obviously if you knew anything about physics or electrical
engineering, why, you'd know it was totally impossible.

It's not totally irrelevant to this thread to note that a manned electric
airplane, the Solar Implulse, recently made a 12 hour, 59 minute international
flight. This also would have been said to be totally impossible not so many
years ago.

I make no claims that the Solar Impulse is a practical airplane. After all, its
average speed of advance was only some 27 knots. That said, gliders and hot air
balloons also generally fail the test of practicality, but there are plenty of
them around.

Vaughn

The word "practical" has implications of purpose and cost.

Recreation is a valid purpose.

If the purpose is to float through the air to sightsee as a recreation, a
balloon is practical for that purpose.

If a balloon costs too much, it becomes economically impractical.

Likewise, electric powered airplanes have already found a niche purpose
where they are practical for the specific purpose, economically practical,
and in production; self launched gliders.


--
Jim Pennino

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Dylan Smith wrote:
On 2011-06-28, wrote:
There is a difference between the theoretical efficiency of a motor of
any type and the obtainable efficiency of a real world motor.

The real-world efficiency of a modern brushless motor and its control
system is around 85%. It's only brushed motors which are merely twice
as efficient as an internal combustion engine. No one today uses
brushed motors for traction outside of golf-carts (and it wouldn't
surprise me if golf carts use brushless motors now).

As best as I can tell, the real world energy efficiency of modern airplane
engines is somewhere in the low 30% range.

We can quibble about the exact number all day long, but it is somewhere
a bit better than 2 and no greater than 3.

Even at 3, the required battery energy density would be 14.666 MJ/kg and
the best of the labratory batteries falls short by a factor of 5.8.



--
Jim Pennino

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