Tikalon Header

Wind-Up Toys

August 5, 2011

In the days before small, powerful, permanent magnet motors powered by small batteries with high energy capacity, children's toys used mechanical energy storage. The toy motors could be as simple as the rubber bands in throw-away toys found in cereal boxes, or coiled steel springs in store-bought toys.

A coiled steel spring for use as a wind-up energy storage unit was patented in 1910 as an automobile engine starter,[1] and springs like these were even used to spin-up gyroscopes in short term applications, such as missiles.[2] They were still used in gyroscopes at least to about 1995, since I knew a young mechanical engineer who did some gyroscope spring calculations at that time.

Figure captionFigure two from US Patent No. 950,848, "Gasolene-Engine Starter," by Edwin A. Gardner, March 1, 1910.

In the automobile engine application, the spring and the engine drive could share the same shaft. Clocks and phonographs needed a different arrangement that was patented in 1936, as shown below.

Figure captionFigure one from US Patent No. 2,063,799, "Spring Motor," by Henry Axel F. Fornelius and Henry A.G. Fornelius, December 8, 1936.

The idea of using mechanical deformation as a means of energy storage can be taken to extremes. One US patent application looks forward to the time at which we can store mechanical energy at a molecular level, in carbon nanotubes; viz,[4]
"An energy storage device comprising: at least one nanotube; and an energy storage and recovery mechanism for applying the appropriate levels of strain on said at least one nanotube to produce stored energy and relaxing said at least one nanotube to recover said energy for use external to said energy storage device."

This patent application states that steel springs have a volumetric energy density of about 600 Joule per liter, whereas the value for a nanotube spring is a phenomenal 11.25 mega-Joule per liter.

This estimate follows from the early work of S. A. Chesnokov, et al.,[5] who reversibly compressed purified, unoriented single-wall carbon nanotubes at 29 kbar pressure to a density that approached that of graphite. They determined that the reversible work done in flattening the tube cross section from circular to elliptical was 0.18 electronvolt (eV) per carbon atom. Cranking through the numbers for this random assemblage gives about 3 mega-Joule/liter.

Another means of mechanical energy storage is the flywheel. I wrote about flywheel energy storage in an article about "green" braking systems (Green Braking, October 20, 2010). I mentioned flywheels also in another article (Portable Power Challenge, September 17, 2007) about a 2007 US Defense Department challenge contest for a wearable 20-watt average power source, operable over four days, and weighing less than four kilograms (8.8 pounds).

One thing that the DOD challenge shows is how chemical energy storage bests all other kinds. Twenty watts over four days is 6,912,000 watt-sec, or 6,912 kilojoule (kJ). Listed below are the ultimate energy storage capabilities of various physical or chemical systems (ignoring external components),[6] and the mass required to store 6,912 kJ.

Principle Type kJ/gram kg
Mechanical Flywheel 1 6.91
Mechanical Torsion Spring 0.0003 23,040
Electrochemical Lead Acid Battery 0.1 69.12
Electrochemical NiMH Battery 0.22 31.42
Electrochemical Lithium Battery 2.5 2.76
Capacitor Supercapacitor 0.01 691.2
Chemical (Reaction) Thermite

 4 1.73
Chemical (Combustion) Gasoline 46.9 0.15
Chemical (Combustion) Ethanol 30 0.23
Chemical (Combustion) Hydrogen 143 0.05
Chemical (Combustion) Lithium 43.1 0.16
Chemical (Combustion) Magnesium 24.7 0.28
Chemical (Combustion) Sodium 9.1 0.76

Obviously, when a country exports gasoline or ethanol, it's actually exporting energy. Less obvious is the idea that export of aluminum is also an export of a huge amount of energy. Synthesis of aluminum by the Hall–Héroult process is energy intensive. The feedstock, which is aluminum oxide, is dissolved in molten sodium hexafluoroaluminate (Na3AlF6) at about 1000°C. The resulting liquid is electrolyzed to produce molten aluminum.

Aluminum is recycled because of the energy needed to refine it, not because of its scarcity. The energy required to refine aluminum is about 14 kilowatt-hour per kilogram.[7] Thus, each metric ton of aluminum is the equivalent of 14 megawatt-hours of electricity! Think twice before discarding that aluminum beverage can!

References:

  1. Edwin A. Gardner, "Gasolene-Engine Starter," US Patent No. 950,848, March 1, 1910.
  2. Joseph J. Moravek and Rocco N. Figalora, "Gyroscope Apparatus," US Patent No. 3,270,568, September 6, 1966.
  3. Henry Axel F. Fornelius and Henry A.G. Fornelius, "Spring Motor," US Patent No. 2,063,799, December 8, 1936
  4. Timothy F. Havel and Carol Livermore-Clifford, "Devices For Storing Energy In The Mechanical Deformation Of Nanotube Molecules And Recovering The Energy From Mechanically Deformed Nanotube Molecules," US Patent Application No. 20080305386, December 11, 2008.
  5. S. A. Chesnokov, V. A. Nalimova, A. G. Rinzler, R. E. Smalley and J. E. Fischer, "Mechanical Energy Storage in Carbon Nanotube Springs," Physical Review Letters, vol. 82, no. 2 (January 11, 1999), pp. 343-346.
  6. Energy Density page on Wikipedia.
  7. Pieter van Pelt, "Cheap electricity from Iceland - Giant Aluminum Batteries to transport electricity," ZPEnergy, April 24, 2004