from R&D Innovator Volume 1, Number 4
Getting Warmer: The Nitinol
Banks, an independent machinist and inventor in Point Richmond,
California, develops novel heat engines using a peculiar alloy
called nitinol. He
has taught junior high school music and science, published chamber
music, and worked as a technical associate for Lawrence Berkeley
the past 20 years, I have spent most of my free time working on an
engine that uses low-grade heat and strange mechanical linkages to
create linear or rotary motion.
Although I have devoted more time to this project than to
paying activities, the excitement of reaching a goal has provided
strong incentive to overcome obstacles. Much of this excitement comes from exploring the peculiar
thermal-mechanical properties of the
alloy I use as the power element in my engines.
present invitation to contribute a discovery story is
disconcertingly timely. While
it's more comfortable to write such an account after the
significance of a discovery is substantiated, I suspect that the
very discomforts that actually spur research are the ones that are
most easily forgotten after success is achieved.
Even a painstaking effort at truthful reconstruction is
subject to some unconscious editing when one writes from the
vantage point of certainty.
unwitting ancestor of the nitinol engine was one of a series of
"nonsense machines" I've made over the years as presents
for children -- or for no particular reason.
This toy was an attempt to harness the back-and-forth
action of solenoids into a continuous rotary motion.
built the toy from coat hangers and household junk, and no doubt
it would be long forgotten except for a series of events at the
Lawrence Berkeley Laboratory, where I was working as a technician.
In 1971, after Harry Heckman completed the pioneering run
at the Bevalac particle accelerator, I wanted to give him a
personal memento, and the solenoid motor seemed suitably
toy amused the physicists about as much as it annoyed the
electronics maintenance folks who were forced to repair it.
I guess its appeal lay in its total lack of mechanical
finesse--it rattled and swayed, sparks flew and solenoids groaned
while it ran. Impressive?
Perhaps--you couldn't help but be impressed that the
contraption worked at all!
with that toy combined with another of my interests, small steam
engines, to produce my concept for the nitinol engine.
I thought about integrating the steam engine into an
auxiliary, solar-powered domestic generating system, but a glance
at the economics of solar collectors convinced me to turn my back
on this idea. It's
simply too expensive to produce steam using the diffuse energy of
I searched for a lower-temperature substitute for steam, I
realized that compounds like paraffin have outstanding
coefficients of thermal expansion but relatively poor
Perhaps a paraffin bellows could replace the pistons and
cylinders. At any
rate, during my search for a metallic envelope for paraffin, I
began considering bimetallic components--springs made of two
metals with different coefficients
of thermal expansion--as possible power elements for my engine.
(If this meandering preamble seems self-indulgent, that's
because I want to describe the process, not just the result.
A lifetime's familiarity with creative work convinces me of
the absolute propriety of nonlinear thinking under certain
circumstances. To put
it another way, a premature rush to "get to the point"
can guarantee missing the point altogether.)
Has a "Kick"
played with a bimetallic coil from a kitchen thermometer, then
sketched an engine— based on my solenoid toy—that used 200
bimetallic springs to link a vertical rotating wheel to a
stationary crankshaft. Several
of my co-workers also were designing bimetallic systems to recover
mechanical work from hot water or sunlight.
lab had a sample of nitinol, a homogeneous nickel-titanium alloy
that changes shape due to a change in crystal structure in an
effect unrelated to thermal expansion and contraction.
In its natural state, a wire of this alloy is about as drab
as a metal can be--it has a dull gray surface and bends with an
unenthusiastic sort of elasticity.
But I was intrigued by what I'd read about the material's
Nitinol "remembers" the shape it had when it was
formed at a high temperature.
When cooled, it becomes more malleable (unlike most metals,
which harden). But when the wire is heated, it immediately springs back to
its original form.
to the instructions, shape memory must be imprinted on the metal
by heating it to cherry red.
As someone had borrowed my only torch, I had no convenient
way to heat it and just left the wire on my desk.
After staring at the wire for a few days, I wondered if it
might already have enough shape memory, from heating during
production, to do something interesting.
I made a U-shaped bend and dipped it into my coffeepot.
almost two decades of working with nitinol, I still cannot
adequately describe my reaction at feeling an inanimate piece of
metal spring to life in my hand. Although the force and speed of the response can be measured,
they must be felt to be believed.
I haven't recovered from that experience, nor from the
recognition that nitinol has an infinitely higher specific work
potential than bimetallic materials.
Million Revolutions and Still Going Strong
made a prototype engine in 1973 from loops of nitinol wire.
The engine had two pans, one containing cold water, the
other hot, and it worked from the start.
It's now made over 20 million revolutions and still turns
as briskly as ever, just as long as we maintain the temperature
led to my first patent on a nitinol engine.
the temperature differential between the heat source and the heat
sink need only be 20°C, I can use geothermal hot springs,
solar-heated water, or even Arctic Ocean water combined with
ambient air as sources of energy.
I was certain I could build practical, useful nitinol
engines, particularly since the country was seriously thinking
about energy and environment during the mid-1970s.
My engine, after all, produced usable work from diffuse,
low-grade heat—the most abundant source of energy on earth.
It had few mechanical parts:
The wire was boiler, expander and condenser all in one.
It had neither valves nor seals, and the materials were
cheap. Best of all,
the engine was nonpolluting.
I confidently predicted I'd have a commercial prototype
within a year or two.
Yet Ready to Commercialize--But Soon
bad seeing a model work is not the same as developing an engine
with real-world applications.
Now, two decades later, I've almost reached my goal.
Why the delay? Partly,
it is due to excessive adherence to conventional wisdoms of
machine design. After
my first demonstration appeared in the press, everyone—large
corporations, the government and others—joined in the act.
We all formed the wires into a coil or loop that would
straighten after being heated.
The thermodynamic conversion efficiencies were poor, but
efficiency is not an overriding factor when the energy source is
practically infinite and free.
We made many embarrassing machines in that period, some of
which provided valuable insights.
most of my competitors stopped working with nitinol engines, I
continued because I felt the potential had not been fully
explored. Over the
years, I've tested many designs, continually learning and
latest model has benefited from abandoning some conceptual
blinders that I brought from experience with other engines.
insight that permitted my new generation of nitinol engines
occurred unexpectedly, while I was working alone.
I liken the process to staring at a black box long enough
to allow a hunch to transform itself into a rational course of
action. I now use air
as the cooling medium (heat sink), deforming nitinol wires in pure
linear tension (rather
than the coil or loop configuration) and letting them shrink back
to their original length in solar-heated water.
This uses energy much more efficiently, even if it does not
make a machine that runs "like clockwork."
have finally reached the stage where I "know" my efforts
will be recognized by commercial success.
Early applications are likely to be in the area of on-site
power production for such purposes as agricultural water pumping,
but it is possible to envision (in detail) engines producing many
hundreds of horsepower for applications ranging from refrigeration
to primary power production.
20 years ago I was confident that nitinol would make a
commercially valuable engine, this was a subjective conviction.
It has been a great satisfaction to see this conviction
substantiated by reality.