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Environmental Energy Harvesting

April 19, 2011

Our natural environment abounds with energy, and much of it can be harvested to power electronic devices. I've written about energy harvesting technologies is some recent articles, as listed below:
• Green Braking, October 20, 2010
• Pyroelectric Energy Harvesting, October 15, 2010
• Flutter Power, August 17, 2010
• Hot Bodies, July 22, 2010

The following table summarizes the sources of environmental energy, and how much of this energy we can harvest.[1] It's not surprising that solar energy tops the list. Solar energy has powered all the biological processes on Earth for more than three billion years.

Energy SourcePower Density(μW/cm2)
Solar (Direct Sunlight)15,000
Solar (Cloudy Day)150
Solar (Indoors)6
Vibration100-200
Acoustic Noise (75 dB)0.003
Diurnal Temperature Cycle10
Temperature Gradient (10°C)15
Reference (One year Lithium Battery)89

Our energy demands are decidedly anthropocentric, cellphones stuck in our ears and the like, so a logical application of energy harvesting is to derive energy from our bodies and our own movements. Our metabolic chemical factory processes a lot of energy; for example, the energy obtained from a jelly doughnut (330 "food calories") is 1.38 MJ. Estimates of harvestable energy content for various human activities were listed in a 1996 paper by Thad Starner, who was then at the MIT Media Laboratory.[2] These included
• Body Heat 2.4 - 4.8 W
• Blood Pressure 370 mW
• Exhalation 400 mW
• Breathing (Total) 830 mW
• Chest Band (Breathing) 0.42 W
• Arm Motion 0.33 W
• Finger Motion 0.76 - 2.1 mW
• Footfalls 5.0 - 8.3 W

One human energy harvesting application that's been around for as long as I can remember is the "self-winding" mechanical watch. This technology was tweaked with the advent of digital watches to comprise a voltage generator powered by the same wrist movements. An alternative energy harvesting design was the Seiko Thermic wristwatch, which used ten thermoelectric modules to generate a microwatt or so from the small thermal gradient between body and ambient temperature.[3]

Some proof-of-concept systems have been done for shoes,[3] and the less obvious case of mechanical stress on the fabric in the clothes that we wear.[4-6] In the later case, researchers at the University of California, Berkeley, prepared nanofibers of the piezoelectric polyvinylidene fluoride (PVDF) that could be woven into garments. The fibers, which had diameters as small as 500 nanometers, were found to generate up to 30 millivolts and 3 nanoamps under mechanical strain at a conversion efficiency of about twelve percent.[4-5] These data correspond to a maximum power of
P = EI = (3 x 10-2)(3 x 10-9) = 9 x 10-11 watt,

or about 100 picowatts. If we can sum the power of a thousand of these fibers, we could obtain a power of 100 nanowatts, or 0.1 microwatt. Actually, the most likely architecture would be many longer fibers that would accomplish the same task with fewer interconnects. This is still a very small amount of power, so more work needs to be done. In contrast, the shoe generator gives us about five watts.

Recently, researchers from the Biomimetics Laboratory and Department of Engineering Science of the The University of Auckland (New Zealand) and Industrial Research Limited, also of Auckland, have published a different approach to human energy harvesting in Applied Physics Letters.[7-8] They use dielectric elastomers, stretchable materials that are called "artificial muscles."

The Auckland elastomer devices had a generating capacity of about 10 mJ/g, and they were able to convert mechanical work to electricity with a 12% efficiency. These energy harvesters are inexpensive, and their combination of softness, flexibility and low mass make them ideal for many environmental energy harvesting applications. They can be inserted into clothing to provide energy from human movement, and it appears they can generate more power than the piezoelectric nanofibers.

Schematic of the physical layout of the soft generatorSchematic of the physical layout of the soft generator.

(Image: American Institute of Physics)

References:

  1. Haluk Külah and Khalil Najaf, "Energy Scavenging From Low-Frequency Vibrations by Using Frequency Up-Conversion for Wireless Sensor Applications," IEEE Sensors Journal, vol.. 8, no. 3 (March, 2008), pp. 261-268.
  2. Starner, T., “Human-Powered Wearable Computing,” IBM Systems Journal, Vol. 35, No. 3&4, 1996, pp. 618-629. Also available here.
  3. Joseph A. Paradiso, "Systems for Human-Powered Mobile Computing," IEEE Design Automation Conference (San Francisco, CA, July 24-26), pp. 645-650.
  4. Sarah Yang, "New fiber nanogenerators could lead to electric clothing," University of California at Berkeley Press Release, February 12, 2010.
  5. Chieh Chang, Van H. Tran, Junbo Wang, Yiin-Kuen Fuh and Liwei Lin, "Direct-Write Piezoelectric Polymeric Nanogenerator with High Energy Conversion Efficiency," Nano Letters, vol. 10, no. 2 (February 10, 2010), pp. 363-750).
  6. Raghu Das, "Applying energy harvesters to textiles," Energy Harvesting Journal, March 31, 2011.
  7. Charles Blue, "Replacing batteries may become a thing of the past, thanks to soft generators," American Institute of Physics Press Release, April 6, 2011.
  8. Thomas G. McKay, Benjamin M. O’Brien, Emilio P. Calius and Iain A. Anderson, "Soft generators using dielectric elastomers," Applied Physics Letters, vol. 98, no. 14 (April 5, 2011), Document 142903 (3 pages).