A scalable solution to harvest kinetic energy

Harvesting kinetic energy from sources such as wind, waves, and human activity is a challenge. The production of electrical power from kinetic energy sources requires a physical structure to capture the energy and an electromechanical transducer to convert it to electricity. Large-scale applications, such as those based on the energy of ocean waves, require low-cost and efficient energy-converting devices. Smaller-scale applications, for example, using human activity to power portable devices, need to be lightweight and to easily and comfortably integrate with clothing or personal accessories. Accordingly, the answer to improved kinetic energy conversion lies in new materials. Materials science already plays a key role in making energy collection from other sources more cost-effective, for example, through low-cost photovoltaic materials to exploit solar energy or new super capacitors and improved low-cost composite turbine blades to harness the wind.

Among the new materials available to convert kinetic energy, electroactive polymers—which produce an electrical current from a change in shape or size as they are stretched and relaxed—display higher performance in terms of energy density and efficiency than traditional transducer materials and have a lower production cost. This contrasts with alternative kinetic energy harvesting means such as electromagnetic generators, which require relatively sophisticated mechanical transmission in order to operate at the speeds needed to efficiently produce power due to their low energy density. Other materials, such as piezoelectric ceramics—which produce electricity resulting from mechanical pressure—have different limitations. Not only do they often contain undesirable lead compounds. They are also relatively stiff and require a heavy and rigid connecting structure linking them to energy sources.

Figure 1. Heel-strike generator based on a dielectric elastomer. Photo of the device installed in a boot (top left) and cross section (top right and bottom). The fluid or gel coupling medium forces the polymer to stretch when the heel is compressed, thus producing electricity.

The great potential of electroactive polymers was reflected at the recent SPIE 2011 Smart Structures/NDE conference.¹ The type of electroactive polymer known most commonly in the literature as dielectric elastomers² were the most popular. These are thin sheets of elastic insulators coated with stretchy electrodes. They generate energy by mechanically separating the electric charge—and thereby increasing the electrical energy—as the stretch condition of the elastomer is relaxed and the thickness of the sheet increases.³ This makes them, in effect, stretchable capacitors. Here, we report our work and that of colleagues at SRI International related to the use of dielectric elastomers in kinetic energy harvesting applications⁴ at different scales, including a small-scale shoe generator and a large-scale wave energy harvesting buoy.

Figure 2. Dielectric elastomer ocean wave power generator based on an articulated, multibody system buoy at sea trial site (top) and concatenated rolls in a generator module (bottom). When a wave passes, the outriggers move relative to the buoy and stretch the rolls using a lever arm. The black material between the rings with green edges (visible in the bottom photo) is the electrode-coated dielectric elastomer material. (Photos courtesy of SRI International.)

The proliferation of mobile communication devices among the general public, military personnel, and emergency services has increased the need for extending the life of batteries and for simplifying chargers. Exploiting the properties of dielectric elastomers, researchers at SRI developed a so-called heel-strike generator that can be fitted in a shoe or a boot. This device uses compression of the heel during walking to harvest power from human motion without adding any physical burden to the wearer. What gives this device the means of producing electricity is a transducer consisting of 20 stacked layers of dielectric elastomer films. The generator works by using a fluid (or gel) coupling to transfer the compression of the heel to stretching of the elastomer films, thus generating electricity (see Figure 1). This device can produce an electrical output of 0.8 joules (J) per step, equivalent to a power of 1 Watt. Intended primarily to charge batteries, this shoe generator was also able to power night-vision goggles. It achieved a maximum energy density of about 0.3J/g. This power level far exceeded outputs of other more complex, more costly, and heavier heel-strike generators based on direct deformation of piezoelectric elements⁵ as well as direct-drive electromagnetic devices such as voice coils or linear induction generators. This device offers the added advantage of being energy efficient. A person weighing 80kg who exerts a maximum deflection of their heel of 3mm releases a total available energy per step estimated to be 2.4J. Thus, our device, which generates 0.8J per step, represents 33% overall efficiency: a good value for such a simple device.

Further examples of applications of dielectric elastomers at larger scale include devices that harvest the power of ocean waves. They have the potential to produce clean renewable energy in an environmentally sound manner. They also offer greater reliability than solar or wind energy generators and lower visual and auditory impact than wind turbines. Moreover, they can be made available near many centers of population and industry. Despite these benefits, widespread adoption of wave power harvesting has been hampered by engineering, economic, and logistical factors. For example, conventional systems have proven expensive due to a need to overengineer their structure to deal with storms.

To address these drawbacks, we have done research in partnership with the HYPER DRIVE Corporation (Japan) to develop harvesting systems that can effectively convert hydrodynamic energy into electrical power through simple and low-cost solutions. This work included two sea trials in which a complete energy-harvesting system was deployed at sea. The first such system was based on a suspended proof mass that stretched its springlike muscle material as the buoy heaved on the waves. The system was a proof-of-principle demonstration of how a buoy, such as a navigation buoy, might use ocean waves to power its onboard lighting or instrumentation and communications systems. However, this was not practical for large-scale power generation to feed the electricity grid because of the large-size proof mass that would then be needed.

Instead, we developed a proof-of-principle system directly using hydrodynamic energy to mechanically stretch and contract dielectric elastomers (see Figure 2). For logistical convenience we used the same oceanographic buoy as the proof-mass system, which would ultimately not be suitable in an optimized system. It was tested at sea in the Pacific Ocean near Santa Cruz, CA. This device produced an output of more than 25J in laboratory testing. Namely, it used about 220g of active dielectric elastomer material with an energy density of more than 0.1J/g. At sea, half this energy density was measured, thus yielding about 11J. The energy-harvesting circuit used in the sea trial was 78% efficient. Such performance levels suggest that dielectric elastomers may indeed be practical for large-scale power generation.

We have shown that dielectric elastomers can effectively harvest kinetic energy in both small- and large-scale applications. In the future, we hope to leverage our experience to further develop these and other kinetic energy harvesting applications by addressing the technical challenges related to system design and lifetime.