Temperature Cycle Energy-Harvesting
September 29, 2014
Now that
digital communication,
sensing, and
computing devices can operate at very low
power,
environmental energy-harvesting is being pursued as a means to enable
wireless sensors and other devices. I've written quite a few articles about environmental
energy-harvesting.
• Thermogalvanic Cell, June 25, 2014
• Tidal and Wave Energy, March 24, 2014
• Energy Harvesting Cantilevers, January 14, 2013
• Triboelectric Generators, July 25, 2012
• Electret Energy Harvester, December 7, 2011
• Harvesting Radio Frequency Energy, November 22, 2011
• Cantilever Energy Harvesting, August 16, 2011
• Multiferroic Energy, July 11, 2011
• Acoustic Energy Harvesting, May 5, 2011
• Environmental Energy Harvesting, April 19, 2011
• Green Braking, October 20, 2010
• Pyroelectric Energy Harvesting, October 15, 2010
• Flutter Power, August 17, 2010
• Hot Bodies, July 22, 2010
• Rain Power, January 30, 2008
The most important environmental energy source is
solar energy. Solar energy has powered all the
biological processes on
Earth for more than
three billion years. Since
photovoltaics generally convert only 20% of solar energy into
electricity, direct
sunlight will produce about 20
mW/
cm2 of
electrical power. A
cloudy day will give you about 200 μW/cm
2, while
indoor lighting will produce just about 10 μW/cm
2. Solar energy is not available at
night, so you need to
store some daytime energy or look to other environmental energy sources.
Our environment is populated by
machines as well as
people, and most of these
vibrate. Noisy machines can give you about a 100 μW/cm
2.
Temperature cycling between day and night will produce about 10 μW/cm
2, while a
temperature gradient of 10°C will produce 15μW/cm
2.
Acoustic noise is not a good source of energy, since a 75
dB noise source, equivalent to
road traffic, will produce only 0.003μW/cm
2.
The
human body can be a source of energy for powering
medical implants and other devices. A 1996
paper by
Thad Starner, who was then at the
MIT Media Laboratory, estimated the harvestable energy content for various human activities, as follow:[1]
• 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
Scientists at
MIT have developed a
thermoelectric energy harvester that extracts up to 100 microwatts of power from the temperature difference between
body temperature and
room temperature.[2] A
piezoelectric airflow energy harvester has produced 296 microwatts from an airflow of 8.0
meters per second,[3] and piezoelectrics have also been used to harvest energy from
raindrops. The raindrop energy harvester produces about one Watt-hr per square meter per year for rainfall typical in
France.[4]
Recently,
electrical engineers and
computer scientists at the
University of Washington (Seattle, Washington) have developed a novel energy harvester for environmental temperature change. Instead of a direct temperature to electrical conversion, as in thermoelectrics, their device uses the
vapor pressure change associated with temperature to expand and contract a
bellows. Linear motion harvesters then convert the motion of the bellows to an electrical signal.[5-8]
The research was done by University of Washington
associate professors Shwetak Patel and
Joshua Smith, Sam Yisrael, an electrical engineering
undergraduate student,
Sidhant Gupta, a former University of Washington
doctoral student, and
Eric Larson, a former University of Washington doctoral student and now an
assistant professor at
Southern Methodist University.[6] The team presented this
research at the
Association for Computing Machinery's International Joint Conference on Pervasive and Ubiquitous Computing in
Seattle, September 13-17, 2014.[6]
As can be seen in the figure, the bellows, which is filled with a
gas with a high temperature sensitivity of its vapor pressure, expands considerably, and nearly
linearly, when heated through a 31
°C temperature range. In the device, the bellows expansion activates
cantilever energy-harvesters to power a wireless sensor. A temperature change of only a quarter of a degree
Celsius is enough to wirelessly
transmit data over a five
meter range. This means that even the normal room temperature changes in a room's
air conditioning can power the sensor. Outside temperature changes are generally greater.[6]
Bellows expansion from heating. Shown are actuations at 0°C, 15°C, and 31°C. (Still images from a University of Washington YouTube video.)[8)]
In their
experiments, the research team found the following gases to be effective in the bellows actuator (Table I of ref. 5).
The device was shown to generate up to 21
millijoule of energy per cycle for a temperature change from 5-25 °C.[5] The Washington research team has filed
patents on this idea, but they've placed detailed construction information on a web site.[8] Their future plans include making the device smaller, and using a combination of gases so that the device will operate in an extended temperature range.[6]
Typical applications would be
water leak detectors and
structural integrity monitors that would signal a central monitoring
node. For a wireless water detector I designed to operate many years on a single
battery, see ref. 9.[9] This research was funded by the
Sloan Foundation, and the
Intel Science and Technology Center for Pervasive Computing at the University of Washington.[6]
The bellows-powered sensor at an outdoor location.
(University of Washington Image.[6]
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References:
- Starner, T., “Human-Powered Wearable Computing,” IBM Systems Journal, vol. 35, no. 3&4, 1996, pp. 618-629. Also available here.
- David L. Chandler, "Self-powered sensors," MIT News Office, February 11, 2010.
- Shuguang Li and Hod Lipson, "Vertical-Stalk Flapping-Leaf Generator for Wind Energy Harvesting," Paper SMASIS2009-1276 of the Proceedings of the ASME 2009 Conference on Smart Materials, Adaptive Structures and Intelligent Systems (Oxnard, California), September 20-24, 2009.
- Romain Guigon, Jean-Jacques Chaillout, Thomas Jager and Ghislain Despesse, "Harvesting raindrop energy: experimental study," Smart Mater. Struct. vol. 17 (2008) 015038-9.
- C. Zhao, S. Yisrael, J.R. Smith, and S.N. Patel, "Powering Wireless Sensor Nodes with Ambient Temperature Changes," Proceedings of the 13th International Conference on Ubiquitous Computing (UbiComp 2014), Seattle, USA, Sep 13-17, 2014 (PDF file).
- Michelle Ma, "Changing temperature powers sensors in hard-to-reach places," University of Washington Press Release, September 3, 2014.
- Temperature Power Harvester Web Site, University of Washington.
- Powering Wireless Sensor Nodes with Ambient Temperature Changes, YouTube video, August 29, 2014.
- Build an Inexpensive Wireless Water Alarm, Circuit Cellar Magazine, February, 2014.
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