Session: SYMP 7-3: Hydrokinetic Energy Harvesting

Paper Number: 140091

140091 - Investigating Hasel Transducers for Underwater Energy Generation 

In our transition to renewable energy sources to meet the rising global demand, ocean wave energy is under-utilized
compared to wind and solar because of the engineering challenges from the harsh ocean environment. Ocean wave
energy converters (WECs) are systems which convert the mechanical energy of ocean waves into electrical energy.
Traditional WECs use power take-off systems where a large load is concentrated at a single location for energy
conversion such as a rotary electric generator. Concentrated loads coupled with the requirement to operate at
the highest sea states means that conventional WECs are over-designed, heavy, and expensive. Replacing heavy,
rigid components with flexible smart materials can reduce the size, weight, and cost of WECs while enabling
distributed energy conversion along the body to eliminate concentrated loads. FlexWECs are a developing area
of research where a WEC is made from networks of small transducers which create a compliant structure that
converts mechanical to electrical energy. Several transducers have been proposed for flexWECs including rotary
generators and dielectric elastomers, but further research is needed to identify the best transducers and system-level
designs.

Hydraulically-amplified, self-healing electrostatic (HASELs) are a new type of dielectric fluid transducer
which shows promise for flexWECs. HASELs are sealed pouches of dielectric fluid in an inextensible polymer
membrane with flexible electrodes painted on the outside. Their working principle is variable capacitance, whereby
holding constant voltage and physically changing capacitance, HASEL’s charge can be changed. The benefits of
HASELs as electrostatic generators are that they leverage hydraulic amplification of force/displacement, are capable
of self-healing from dielectric breakdown, and are structurally compliant and stackable. Furthermore, their self-
contained pouch of fluid eliminates viscous losses which pumping dielectric fluid transducers are subject to. Despite
these benefits, the energy harvesting performance of HASEL transducers is unexplored.

We aim to identify the energy density and conversion efficiency of HASELs through analytic modeling and
experimental efforts. Through analytic modeling we found that the energy generated per cycle depends on the
pouch geometry and the square of priming voltage. The maximum theoretical electrical energy generated by a single
pouch with the geometry of a commercially available HASEL (Artimus Robotics, C-5015-02-01-B-ACAC-50-096) is
0.98 mJ per cycle, with an energy density of 1.36 mJ cm−3 normalized by fluid volume. We plan to experimentally
determine the energy harvesting performance of HASELs by examining the relationship between cyclic loading,
voltage priming, and electric energy produced. Cyclic loading is representative of the loads experienced by a
flexWEC due to ocean waves. We propose the following energy harvesting loop for contracting Peano-HASEL
transducers: (1-2) Priming and capacitance increase: starting from rest, apply a priming voltage to the HASEL which
causes the electrodes to zip together and capacitance to increase; (2-3) Generation: hold at constant voltage and apply a tensile force to unzip the HASEL, decreasing capacitance and causing charge to leave the HASEL and flow through a load; (3-4) Discharge: While under tension, discharge the HASEL to zero voltage; (4-1) Reset: Release tension force while discharged. The independent variables involved are the magnitude and frequency of the applied force and the priming voltage
applied across its electrodes, which influence the HASEL’s instantaneous capacitance and charge. To find the
relationships between these variables, we will measure the energy generated per cycle while applying cyclic loads in
universal testing machine. We expect that the practical energy harvested per cycle will be less than the theoretical
maximum due to charge leakage losses and because capacitance increase occurs during voltage priming. Additionally,
we expect the amount of energy generated and the conversion efficiency will increase with cycle frequency, which has
been shown for other dielectric fluid generators. This work will evaluate the efficacy of HASEL transducers as electrostatic generators to lay the foundation for HASEL adoption in flexWECs. Further research could investigate
other energy harvesting processes, such as a constant-charge process, and evaluate system-level performance of
HASELs in a particular arrangement within a flexWEC.

Presenting Author: Isabel Hess University of Florida

Presenting Author Biography: Isabel is a Ph.D. candidate of mechanical engineering at the University of Florida in the Fluids and Adaptive Structures Laboratory. She received her Bachelor's degree in mechanical engineering in 2019 from the University of Florida and achieved candidacy in her doctoral program in 2022. Her research interests include soft robotics, bioinspiration, and smart material transducers.

Authors:

Isabel Hess
Patrick Musgrave