Hydraulic Pressure Energy Harvesting for Wireless Sensing
Professor Kenneth A. Cunefare, The Georgia Institute of Technology
The pressure ripple present within most hydraulic systems is commonly viewed as an
annoyance or a detriment to system performance. However, the pressure ripple also
represents a high-intensity power source for energy harvesting. Researchers at the
Georgia Institute of Technology have developed devices to generate electricity from
pressure ripple at power levels that enable wireless sensing and communications,
among other applications.
Energy harvesting is the term used to describe technologies that seek to generate
electricity from ambient, low-energy-density sources such as light, wind, flow, vibrations,
and acoustics. The main goal for energy harvesting has been to enable a variety of selfpowered wireless electronic sensing and communication systems. Motivations for the
technology have been elimination of maintenance required to replace batteries,
elimination of the chemical waste associated with conventional batteries, elimination of
wiring, enabling the installation of sensors in remote or difficult to access areas, etc.
Typically, the amount of power being generated is much less than a Watt, commonly
much less than a micro-watt. The common devices for energy conversion employ a
piezoelectric beam or membrane, an electroactive polymer, or an inductive coil and
magnet arrangement.
Energy Harvesting in Hydraulics
In a hydraulic system, an energy harvesting technology might be integrated with healthmonitoring sensors and eliminate the need for batteries or wires providing power to
individual sensors; this would reduce maintenance contact and eliminate potential
points of failure. An example of a health-monitoring technology that is currently available
in the hydraulics industry is Eaton’s “LifeSense” hydraulic hoses, which are designed to
monitor hose integrity and generate an alert of impending failure. Energy harvesting
technology could be used to power sensors embedded within components and devices
where it is impractical or impossible to run wires, such as directly within high-speed
rotating internal parts. On mobile hydraulic equipment, the pressure ripple generated by
track motors beyond the rotating track-frame/cab joint may be used to power sensors on
the track frame, such as individual track speed sensors.
Researchers at the Georgia Institute of Technology have been conducting research on
and developing Hydraulic Pressure Energy Harvesters (HPEH) to generate electric
power from pressure ripple. An HPEH, schematically depicted in Figure 1, comprises a
piezoelectric element, such as an off-the-shelf multi-layer cofired stack, contained within
a housing and exposed to the pressure ripple in a fluid system.
A complete HPEH-powered wireless sensing node, schematically depicted in Figure 2,
would use the energy produced by the HPEH components to power sensors and
communications. The HPEH serves as the “battery” within the sensor node. Power
conditioning electronics are required between the piezoelectric element in the HPEH
and the particular sensor and communications electronics.
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Figure 1 – Simplified schematic of hydraulic pressure energy harvester, where the
interface implements fluid-mechanical coupling between a piezoelectric stack and
pressure ripple in a pressurized fluid.
Figure 2 – Schematic of HPEH-powered wireless sensor node.
Concept Feasibility
Pressure ripple in hydraulic systems is a form of acoustic noise. A challenge of
harvesting energy from common acoustic noise is the very low energy density available.
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For example, in air, a 60 dB plane wave has an intensity of approximately 1 W/m2, a
100 dB plane wave has an intensity of 10 mW/m2, and the intensity of a 140 dB plane
wave is approximately 100 W/m2. These sound fields correspond to a conversational
level, an uncomfortable loud level which would cause hearing damage from continuous
exposure and a level beyond the threshold of pain. Nonetheless, the actual intensities
are quite low, and would require large devices to capture the necessary amount of
power for sensing and wireless communications.
In pumped fluids, however, the situation can be quite different, with intensities on the
order of kW/m2 possible. Consider Figure 3a which depicts the power per unit area
(intensity, milli-Watt/cm2) conveyed in the pressure ripple in a hydraulic system as may
be caused by an axial piston pump at 270 Hz; Figure 3b depicts the corresponding
power conveyed in the pressure ripple in different size hydraulic pipe. From these
figures it is clear that significant energy is available in the pressure ripple, and the issue
then becomes how to perform an effective pressure-to-electrical-power conversion.
Note that the vertical axis scales in Figure 3a and 3b are in units of mW, since a wireless
sensor node typically requires mW level powers. For larger diameter pipes, the power
available in the ripple could be at the Watt level and higher, enabling energy uses
beyond just sensors and communications.
Pressure ripple, psi, peak‐to‐peak
0 10 20 30 40 50 60 70
Pressure ripple, psi, peak‐to‐peak
0 10 20 30 40 50 60 70
1E+6
1E+5
1000
100
Power, mW
Intensity, mW/cm2
10000
‐20
1E+4
1E+3
‐8
1E+2
10
1E+1
0
100 200 300 400 500
0
100 200 300 400 500
Pressure ripple, kPa, peak‐to‐peak
Pressure ripple, kPa, peak‐to‐peak
a)
b)
Figure 3 – Intensity and power in pressure ripple; a) intensity mW/cm2, b) power conveyed by
pressure ripple in hydraulic hoses from -8 to -20 size, mW
Proof of Concept
Multiple generations of prototype devices, depicted in Figure 4, have been developed
and tested. The devices have been designed to withstand static pressures up to 5000
psi. Individual prototypes have generated from 3.2 mW to 150 W of power from
pressure ripple ranging from 100 kPa to 300 kPa (15 to 45 psi). These power outputs
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are sufficient for a broad range of sensing and communication applications,
demonstrating that the underlying HPEH concept is flexible and viable. A prototype
wireless temperature sensor has demonstrated the viability of the concept, where the
sensor and its communication circuit were completely powered by electricity generated
from pressure ripple.
Figure 4 – Prototype HPEH devices. Devices designed for up to 5000 psi service, with power
generation capabilities from 150 W to 3.3 mW.
Other feasibility demonstration efforts have produced a prototype wireless hydraulic
fluid temperature sensing system. The system, with components depicted in Figure 5,
used a HPEH prototype to power an off-the-shelf Cymbet energy harvesting power
condition and communications board.
Figure 5 – Components for HPEH-powered wireless temperature sensor. System comprises an
HPEH prototype with integral temperature sensor, off-the-shelf Cymbet board implementing
energy harvester power and signal conditioning as well as wireless communication functions,
and a wireless USB receiver module.
On-Going Development
The HPEH concept has demonstrated feasibility of extracting useful levels of energy
from the noise ripple present in hydraulic systems. The focus of further development of
the concept is on extending the range of pressures, both static and dynamic, for which
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the HPEH can be used. This work entails maximizing the energy production for a given
pressure ripple input, through appropriate design of the device and its power
conditioning circuits. In addition, the work is exploring the use of advanced piezoelectric
materials with higher energy density potential than can be achieved with currently
available multi-layer co-fired stacks. Finally, work is on-going for additional feasibility
demonstrations of various sensing technologies in different applications of relevance to
the hydraulics industry. We are interested in partnering for development of applications
and devices.
Acknowledgements
Collaborators on this work include Professor Alper Erturk, and Ellen Skow.
This research has been supported through the Center for Compact and Efficient Fluid
Power. The CCEFP is a National Science Foundation Engineering Research Center,
and includes industrial sponsors from such major companies as Danfoss and ParkerHannefin (www.ccefp.org).
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