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March 31, 2014

Batteries have become smaller and more powerful in recent years, but power-hungry mobile device displays have become larger at the same time. I'm amazed that my cellphone, tablet computer and netbook work so long on a single charge; but, sometimes, it would be nice if the battery charge would last just a little longer.

The problem is the limited energy density possible in an electrochemical cell (a.k.a., battery). As the table shows, the energy density of today's battery technology is much less than that of other energy sources.

Principle Type kJ/gram
Electrochemical Lead Acid Battery 0.1
Electrochemical NiMH Battery 0.22
Electrochemical Lithium Battery 2.5
Chemical (Combustion) Ethanol 30
Chemical (Combustion) Gasoline 46.9
Chemical (Combustion) Butane 49.5
Chemical (Combustion) Propane 50.3
Chemical (Combustion) Methane 55.5
Chemical (Combustion) Hydrogen 142

A competent engineer will strive to increase the energy conversion efficiency of a system to extract as much useful energy as possible; and, as the table shows, switching from one energy system to another can have a big affect. If you power your mobile electronic devices with the electrochemistry of lithium ion battery, you get just 2.5 kJ/gram. If it were possible to use butane combustion, the energy density jumps twenty-fold.

This is the approach taken by Lilliputian Systems, Inc., which has developed a USB recharger fueled by butane. This MIT spin-off is manufacturing its "Nectar" device, somewhat larger than a cellphone, that incorporates a solid oxide fuel cell for conversion of butane and the oxygen in air to electrical power.[1] Some major engineering and regulatory problems needed to be addressed in the design of this device.

Butane will combust only at temperatures above 400°C (750°F), and a fuel cell needs to operate at about 500°C (900°F) to use butane.[1] As can be imagined, thermally insulating the fuel cell from its surroundings is important for both safety and for extracting as much useful energy as possible.

Silicon is not a useful electronic material above about 125°C, but it's an inexpensive and useful structural material beyond that, since the melting point of silicon is 1,414°C. The Nectar fuel cell is micromachined from silicon, and the silicon is nitrided to Si3N4 to reduce the thermal conductivity.[1] Silicon has a thermal conductivity of about 150 W/m-K, while silicon nitride's thermal conductivity can be as low as 10 W/m-K.[2]

Boulton Watt steam engine, 1784

Old school energy conversion - The Boulton Watt steam engine, circa 1784.

This engine incorporated a water-cooled condenser, which increased the efficiency of steam engines of the time from 1% to 3%.

(Illustration from Robert H. Thurston, "A History of the Growth of the Steam Engine," D. Appleton and Company (New York, 1878), p. 119, via Wikimedia Commons.)

The fuel cell approach is useful if you want electricity, but there are applications for which a small, mechanical internal combustion engine would be useful. In 2011, Yiguang Ju of Princeton University (Princeton, NJ) and Kaoru Maruta of Tohoku University (Sendai, Japan) published a review article, "Microscale combustion: Technology development and fundamental research," in the journal, Progress in Energy and Combustion Science.[3] This 47-page paper is a useful resource on this topic that now includes engines for micro-unmanned aerial vehicles.

One difficulty is scaling combustion motors to very small dimension is that it's difficult to create a combustion reaction in a small volume. The reaction products quench the reaction and heat flows too quickly to the environment. A team of scientists from the University of Twente (Enschede, the Netherlands), the Russian Academy of Sciences, and the University of Freiburg (Freiburg, Germany) have overcome this limitation, and they've demonstrated a combustion actuator with the dimension 100 x 100 x 5 micrometers. It's based on the internal combustion of hydrogen and oxygen created by an in situ electrolysis of water.[4-6]

The actuator is formed as a 530 nanometer thick membrane of silicon-rich nitride capping a small chamber through which water is allowed to flow (see photograph).[4-5] Electrical current flow through the interdigital electrodes electrolytically dissociates the water into oxygen and hydrogen, forming nanobubbles. The nanobubbles create a slight pressure of a few atmospheres that moves the membrane.[4-5]

Electrolytic combustion actuator

The electrolytic combustion microactuator.

(Fig. 1B of ref. 5, via arXiv.)

The interesting aspect of the engine cycle is that turning off the electrical current causes the bubble to rapidly collapse, an effect attributed to a recombination of the hydrogen and oxygen. Combustion is presumed, since the time of collapse is too short for the gases to be dissolving back into the water.[4-5] Optimal operation for the device occurred at a 50 kHz alternating current, which might be related to the resonance of the membrane.


  1. Brian Dodson, "Miniaturized butane fuel cell system enables new USB battery charger," Gizmag, July 15, 2013.
  2. Xiang Zhang and Costas P. Grigoropoulosa, "Thermal Conductivity And Diffusivity Of Free-Standing Silicon Nitride Thin Films," Rev. Sci. Instrum., vol. 66, no. 2 (February 1995) pp. 1115-1120. A PDF file of this paper is available here.
  3. Yiguang Ju, Kaoru Maruta, "Microscale combustion: Technology development and fundamental research," Progress in Energy and Combustion Science, vol. 37, no. 6 (December 2011), pp. 669-715. A PDF file of this paper is available here.
  4. Vitaly B. Svetovoy, Remco G. P. Sanders, Kechun Ma and Miko C. Elwenspoek, "New type of microengine using internal combustion of hydrogen and oxygen," Scientific Reports, vol. 4, Article no. 4296 (doi:10.1038/srep04296, March 6, 2014.
  5. Vitaly B. Svetovoy, Remco G. P. Sanders, Kechun Ma and Miko C. Elwenspoek, "New type of microengine using internal combustion of hydrogen and oxygen," arXiv Preprint Server, February 27, 2014.
  6. The Powerful Promise of a Puzzling New Microscopic Combustion Engine, Technology Review, March 12, 2014.

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