fiasco that was my flame against the downwind faster than the wind
vehicle, you might think that I’d be afraid of touching on more air-powered
perpetual motion. You’d be wrong :-). I’m not afraid to make a fool of myself
if I stand a chance of learning something in the process – and in this case,
it’s so obviously bogus that even if I was afraid, the sheer stupidity here
would be more than enough to paper over my anxieties. Take a look at this –
the good part comes towards the end.
What this video is about is one of the more novel approaches that
I’ve seen towards emission-free vehicles. Instead of using electricity
provided by a battery, an Indian company has developed a car that operates
on compressed air. It drives the pistons of an engine by allowing the
compressed air to decompress, and then it uses the motion produced to drive
the wheels of a car (for motion), and a small electrical generator (for
the vehicles electronics, instruments, and accessories.) It’s a really
clever idea. And there’s absolutely nothing magical about it: a compressed gas
tank is just another way of storing energy: it takes energy to compress the
gas into the tank; some of that energy is given back when the gas is
uncompressed. The neat thing about the compressed air vehicle is that
the gas tank weighs less than the batteries of an electric or hybrid,
and it doesn’t have all of the nasty caustic chemicals that are in
a typical battery. The tank can be filled in under a minute at a
compressed gas station, or overnight by plugging it in and running
an electric compressor. And it can go for around 200 miles at
50-60 mph on a single tank. So it’s a reasonably practical car. Certainly
not what you’d want for long-haul driving, but for commuters, it’s
Of course, marketing drones are never satisfied with a simple good
idea. Just being good isn’t good enough.
Now, I’m all for being creative about mocking idiots. It’s
fun coming up with ways of rephrasing their bullshit to demonstrate
how stupid it is. But in this case, I just can’t do it. I can’t
do anything better than what they’ve done themselves. In the smarmy
tones of the voiceover guy, starting around the 2:45 mark, they
conclude the video with the following:
The air car isn’t cost free to operate, because it does take some energy to
compress air – but interestingly, MDI has created a generator powered
by compressed air. Which presents a tantalizing possibility: what if
that generator was onboard the car? Then one day, perhaps the
compressed air that runs the cars will also run a generator to
compress its own air. A car that runs on air, and constantly
refuels itself! Round a round, a perfect circle. Perpetual motion.
A no-cost fillup ever. Not one iota of pollutants, ever. And a cute
car. All for about $15,000. That’s a future car.
They admit that they’re trying to sell you a perpetual
motion machine! And they don’t understand that there’s anything wrong
with that. How could I possibly say anything to make them look
For the sake of pedantry, I’ll walk you through the problem with
this, and all other perpetual motion machines.
It’s an unfortunate fact of our universe, but any time you change energy
from one form to another, you lose some. No matter what you do, you
always lose some. It comes down to thermodynamics, which is how
physics describes energetic interactions. The laws of thermodynamics can,
non-mathematically, be sumarized as: “You can’t win”, “You can’t even break
even”, and “You can’t quit the game”.
The first rule tells you that you’ll never get out more
energy than you put in. So the perpetual motion story doesn’t work,
even under the first rule. You have some quantity of energy
stored in the compressed gas. You use some of that energy
to make the car move, and you use some of it to run the compressor.
You’re going to run out: you’re putting less energy back in to the
air tank that you’re taking out of it, because part of it is moving
the car. So even if you had a perfectly efficient system,
it wouldn’t matter. In fact, with a perfectly efficient compressor,
running the compressor would be a no-op – it would do exactly nothing.
Every bit of energy that it drew from the compressed air in the tank would
be returned to the tank as compressed air – so it would be as if it weren’t
part of the system. But the car would still be draining energy.
The second rule tells you that you can’t even get out as much energy as
you put in. Any time you use energy to do work, any time you transfer energy
from one from to another, some of the energy gets wasted. So even if you never
wanted to move the car – you just wanted to keep the tank full of compressed
gas – you’d lose. When you take the energy in the compressed gas, and you use
it to spin a generator, what you’re doing is changing energy from its stored
form in the compressed gas, to a mechanical form which pushes the generator,
to an electrical form. Every step of that involves loss: decompressing the
air, pushing the generator, generating electricity from the generator,
using the electricity to produce mechanical energy to drive the compressor,
and using the mechanical energy to compress the air. So just the closed
cycle: cycling the air out of the tank, into the generator, and then using the resulting electricity
to drive a compressor to refill the tank – will end up, in short order, with
an empty tank.
And the third rule? It says you’re stuck: you can’t create a system
which doesn’t have to follow the first two rules. If you’re in this universe,
you’re pretty well stuck with this. There is no free energy. There is no
such thing as perpetual motion. You’ll always get less energy out of a system
than you put in.
If you look at the way we really produce energy, we use systems where
losing a whole of energy is just fine. Part of the reason that oil is so
damned valuable is because it contains a huge amount of energy for
its mass, and it’s easy to extract that energy. The fancy way of saying that
is that gasoline has a tremendous energy density. There’s roughly 46
megajoules of energy per kilogram of gasoline: that’s a hell of lot of energy!
There’s enough energy in it that it more than makes up for the effort of
carting it around, even if we use it very inefficiently. Typical gasoline
engines operate at less than 20% efficiency – meaning that the amount
of energy from the gasoline that actual gets translated into mechanical energy
moving a car – is less than one fifth of the amount of energy released by
burning the gas. Even with 4/5ths of the energy going to waste, gasoline is a
remarkably efficient energy carrier for us.
The challenge in all alternative fuel sources for vehicles is that
we need to find ways of storing energy that give us something like the
energy density of gasoline, and which don’t cost a ridiculous amount
of energy to “fill up”. The very best electrical batteries that we’ve
devised have an energy density of just 2.5 million joules per kilogram – a
bit under 1/20th the energy density of gasoline!
Lots of companies are looking at really interesting ideas for how to do a
better job storing energy for driving vehicles. Compressed gas is a good idea;
at the pressure that’s being looked at for cars (around 4,500 pounds per
square inch), it’s got a respectable energy density: about 4 million joules
per kilogram. And the engines that are built to run off it get around 18-20%
efficiency – roughly the same as gasoline engines. In terms of energy density,
it’s still pretty awful compared to gasoline – but it’s significantly better
than even the best battery! And there’s good reason to believe that we can
work out storage for gas at higher pressures than that, and that we can boost
the efficiency of compressed gas engines by at least a little bit. So this is,
at least potentially, a serious, viable technology.
But it’s not free energy. And when you tell lies about a technology,
getting people to believe that in things that are too good to be true, all you
do is set things up for failure. If someone can produce a really practical
compressed gas car for a reasonable price, and it’s 20% efficient, and
requires you to refill the compressed air tank by plugging it in in your
garage every night, it would be absolutely brilliant. People like me
will beat a path to you door to buy one! But if you’ve told people that once
they buy it, it’ll be free and perfect and will generate its own fuel by
magic, then they’ll be serious pissed off at the electric bills that they’re
paying to refuel their supposedly free-energy car – and you’ll have successfully
transformed it in the minds of your customers from something brilliant to
something incredibly disappointing.