# Can you store energy in an inductor and use it later?

My company uses supercaps to power the device if power is cut. I was wondering if you could do the same thing with an inductor. If you can't, why not?

• What's your definition of "later"? – Connor Wolf Sep 27 '12 at 19:41
• More than a second. – BSEE Sep 29 '12 at 0:45

The magnetic field which stores the energy is a function of the current through the inductor: no current, no field, no energy. You'll need an active circuit to keep that current flowing, once you cut the current the inductor will release the magnetic field's energy also as a current, and the inductor becomes a current source (whereas its dual, the capacitor is a voltage source).

Aspects of the capacitor-inductor duality in energy storage terms:

\begin{array}{ll} \mbox{Capacitor} & \mbox{Inductor} \\ \mbox{* stores energy in electric field} & \mbox{* stores energy in magnetic field} \\ \mbox{* must be open loop (infinite resistance) } & \mbox{* must be closed loop (zero resistance)} \\ \mbox{* loses energy through parallel resistance} & \mbox{* loses energy through series resistance} \end{array}

A superconductor can sustain a magnetic field in a zero resistance current loop, however.

Unfortunately you'll always see the fumes of water vapor caused by the liquid nitrogen in pictures like this, which means temperatures below -183 °C.

• Quibble; what you're seeing there is water condensing out of the air because it's been cooled sufficiently that the relative humidity is above 100%. N2 gas iteself is invisible. – Dan Is Fiddling By Firelight Sep 25 '12 at 19:43
• A nice analogy of an inductor looped in such a fashion would be a flywheel. And those are actually used sometimes in practice to store energy. – Christoph B Sep 27 '12 at 18:31

Problem is that energy in an inductor is due to current, and most all practical conductors have some resistance; this means that energy is continuously drained into heating the coil itself though I^2R loss. This can be overcome by using superconductors, which have no resistance at all, but the problem there is that all presently known superconductors have to be cooled to cryogenic temperatures. Also, while an ideal superconductor would remain superconducting at any arbitrary current, all know superconductors (afaik) have some upper limit to the current density they can support before the effect collapses.

$\begin{array}{lcl} \textbf{Capacitive Storage} & & \textbf{Inductive Storage} \\ \mbox{Must have infinite internal resistance} & | & \mbox{Must have zero internal resistance} \\ \mbox{Voltage must stay in it forever} & | & \mbox{Current must flow through it forever} \\ \mbox{You deal with voltage} & | & \mbox{You deal with current} \\ \mbox{Self discharge may take years} & | & \mbox{Self discharges in very short time} \\ \mbox{Electric field doesn't leak outside much} & | & \mbox{Magnetic field may interfere other components} \\ \mbox{Lighter} & | & \mbox{Very heavy (iron, copper, etc)} \\ \mbox{May be cheaper} & | & \mbox{Fe and Cu may be very expensive in some countries} \\ \end{array}$

Yes, people can and do store energy in an inductor and use it later.

People have built a few superconducting magnetic energy storage units that store a megajoule of energy for a day or so at pretty high efficiency, in an inductor formed from superconducting "wire". I've been told that several electric utilities have bought a few such units and use them to improve power quality.

Most people in the US have dozens of switching voltage converters. Most of those switching voltage converters gradually store up energy at one voltage in an inductor or transformer, then "later" gradually draw that energy out of the inductor or transformer at a more desirable voltage, over and over, often 40,000 or a million times a second.

Many popular electronic parts suppliers allow you to sort inductors by their Q factor. The Q factor rates how well an inductor or a capacitor stores energy. In switching voltage regulators and other energy storage apps, bigger Q is better.

The best off-the-shelf inductors (all non-superconducting) at popular suppliers have a Q factor of 150 @ 25KHz. Most capacitors have an order of magnitude better energy storage (higher Q) than that.

People can and do store some energy in inductors for use later. But in nearly all energy-storage situations we use something else, because that something else either (a) has lower up-front costs or (b) is more efficient or (c) requires less space or (d) some combination of the above.

• A megajoule is not that much: a car battery stores about twice that. And I'm not sure about the scalability of it; people don't always realize that zero resistance doesn't mean that infinite current density should be possible. I also think "later" is meant to be further in the future than 1 microsecond :-). – stevenvh Sep 28 '12 at 16:33