Low-Power Autonomous Wave Energy Capture Device for Remote Sensing and Communications Applications

Deanelle Symonds1 Edward Davis1 IEEE Member IEEE Member R. Cengiz Ertekin2 IEEE Member 1Trex Enterprises Corporation 10455 Pacific Center Court San Diego, CA 92121-4339 dsymonds@trexenterprises.com edavis@trexenterprises.com

2University of Hawaii

Department of Ocean and Resources Engineering School of Ocean and Earth Science and Technology (SOEST)

ertekin@hawaii.edu

Abstract Ocean remote sensing techniques often rely on autonomous buoys to measure and transmit real-time oceanographic and meteorological data. The operating lifetime, payload capacity, and sampling rate of such platforms are limited by onboard battery power. Here, we describe a rotary-drive, wave energy conversion device which utilizes the heaving motion of a surface buoy to generate power over a broad range of sea-states. The device was demonstrated to generate over 50W of power in moderate seas at the Kilo Nalu Nearshore Reef Observatory.

Index Termsenergy harvesting, heaving buoy, marine electronic equipment, marine harbors and ports, marine technology, ocean energy, remote sensing, wave spectra, wave energy conversion

I. INTRODUCTION Remote sensing techniques in the ocean often rely on buoy platforms which utilize instrumentation and communication equipment to measure and transmit real-time oceanographic and meteorological data. For example, NOAA and its international partners maintain over 2000 moored and drifting buoys worldwide for accurate environmental characterization, climate modeling and forecasting, disaster monitoring and warning, and correlation of anthropogenic influence on the ocean environment [1], [2].

Financial support was received from the Navy Small Business

Technology Transfer (STTR) Program; Office of Naval Research (ONR). Additional financial support for the Phase I At-sea Demonstration was received from the Hawaii Small Business Innovation Research Assistance Program; High Technology Development Corporation (Honolulu, HI).

The operating lifetime, payload capacity, and sampling rate of such platforms are limited by onboard battery power, since typical systems can draw between tens to a few hundred Watts of continuous power during operation. It is often expensive to maintain these buoys, especially those that are remotely deployed thousands of miles from shore. Increasing buoy self-sufficiency is highly desirable to reduce logistical and safety costs associated with buoy maintenance, as well as to minimize interruptions in operation and data acquisition. While some buoy platforms utilize solar energy to trickle-charge onboard batteries or provide a small amount of supplementary power (5-10W), solar harvesting is less power dense and ineffective at night or during periods of cloudy or stormy weather. Wave energy conversion (WEC) devices can provide power to increase system reliability, operational availability and payload capacity---complementary, even, to those buoys equipped with solar harvesters. This is because wave energy is the most concentrated form of renewable energy (up to 1000 times more power dense than wind and 2-3 times as dense as solar) [11], [13], [15]. Wave power is also predictable and dependable, with the ability to accurately forecast the wave power spectrum days in advance. Most commercial wave energy capture devices (WEC) are designed to produce power greater than 1kW and have complicated designs that are impractical for downscaling [3]-[5], [11], [13], [15]. At the same time, commercially available low-power (< 1kW) WEC systems, as well as those currently in development, are too large or have demonstrated limited

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power generation capacity to be practically utilized for remote sensing buoy applications [4], [6]-[9]. Recently, a WEC prototype was developed for the Office of Naval Research that generates energy from the wave-induced heaving motion in shallow-water environments [10]. The heaving-body design, simulation of the dynamic response, laboratory characterization and demonstrated at-sea performance are described in this paper. In its first sea demonstration, the WEC device successfully generated between 23-53W of peak power at the Kilo Nalu Nearshore Observatory. The compact design of the prototype system makes it more power dense per unit weight (up to a factor of 5) and volume (up to a factor of 3) than its low-power, commercially available peers across a broader range of sea states and wave periodicities.

II. WAVE ENERGY CAPTURE PROTOTYPE DESIGN The prototype WEC falls into the buoy-type class of systems known as point absorbers, since it harvests energy from all directions at a single point in the ocean [3], [5], [11]-[17]. Typical buoy-type point absorbers consist of a float at or near the ocean surface which oscillates with one or more degrees of freedom to engage a drive belt, hydraulic system, or generator (linear or rotary) to convert mechanical motion to electricity [3]-[17]. The device that we describe here utilizes the wave-induced heaving motion of the surface buoy to capture energy via a rotary-drive power take off system [10]. In recent years, an increased interest in autonomous ocean remote sensing and communication platforms has opened developmental paths for low-power (

Fig. 2. Basic block diagram of the power take-off system, analogous to the

pull-cord mechanism that starts a lawnmower or a chainsaw.

Fig. 3. Measured output power (W) as a function of generator rotational

speed (RPM) for varying resistive loads.

III. MODELING AND SIMULATION The physical model is comprised of a surface buoy, a submerged housing containing the power take off components, and the mooring lines that connect them to each other and to the sea floor [10]. The buoy is modeled as a semi-submerged sphere (15.25 cm. below the surface) with a 68.5 cm diameter. The buoy has a weight of ~90N in air, and a buoyancy of ~220 N. The housing is modeled as a rectangular box that is 20 cm in width, 41cm in height, and 41 cm in length, with a weight of ~270N in air. Pretension forces between the components are assumed to be 133 N (buoy-housing) and 205N (housing-mooring). The tether is a 7-strand, helically-wrapped stainless steel cable of 3/16 diameter. The axial stiffness of the tether is assumed to be 53,950N/m (1085N of tension over 0.2% strain) and is based on actual measured data. The average water depth for the hydrodynamic calculations was assumed to be 10m. The viscous dashpot coefficient of the system was empirically derived to be 1095N/(m/s). This coefficient was calculated from the actual winding velocity of the tether and the corresponding pull force during laboratory experiments.

The motions of the moored buoy and submerged housing were simulated by the computer program OCI/HydroMOB, which solves the wave dynamics of multiple moored bodies [10],[18]. The wave forces are based on linear potential theory. The constant panel, Green function method is used to determine wave exciting forces, added mass, and hydrodynamic damping on the individual buoy, the submerged housing and anchoring mechanism. The module motions are coupled through the mooring lines only. The dynamic analysis is completely linear and it does not take into account any viscous effects that may be present. HYDRAN, another simulation-software which considers the full interaction between the bodies, is used in conjunction with OCI/HydroMOB, to compute the heave motion response amplitude operator (RAO) of the submerged housing for various wave heights and headings [10],[19]. The analysis is carried out in the frequency domain for a range of wave frequencies with significant energy content as defined by the typical shallow water wave spectra. The results of the analysis are RAOs for body motions and mooring line forces. These RAOs are used, together with the wave spectra, to obtain estimates of the short-term extreme response. The Bretschneider spectrum is used in the irregular-sea analysis for varying significant wave height and peak period combinations.

IV. LABORATORY PERFORMANCE CHARACTERIZATION An initial dry lab prototype (Fig. 4) was fabricated and tested in a laboratory setting [10]. During testing, the device was mounted to an overhead scale and the voltage was measured across a 6-ohm load resistor for varying pull-forces (~50-700N, at 50N increments ) on the tether (Fig. 5).

Fig. 4. Lab Prototype attached to overhead scale (not shown).

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Fig. 5. Measured output voltage (V, blue line) and calculated peak power output (W, magenta line) as a function of Pull Force (N).

This set-up consistently yielded up to 160W of peak power (or ~50W average power) for a corresponding pull-force of approximately 660N (Fig. 5). The pull-force during laboratory tests were much less than the ~1800N buoyancy force of the actual A5 buoy. Note, we obtain an average power calculation of 50W since power generation only occurs over half a wave cycle. For example, power is only generated by the pull of the cable from the wave rising and not during cable retraction as the wave falls. Based on successful laboratory experiments, a fully-functional sea-going version of the dry prototype was constructed (Fig. 6).

V. PHASE I: AT-SEA DEMONSTRATION, JANUARY 2009 During January 2009, a sea-going, waterproof prototype was deployed in the Pacific Ocean for 2-day test at the Kilo Nalu Nearshore Reef Observatory (Fig. 6). This test range extends from shore approximately 3 miles off the south shore of Oahu, in the Kewalo Basin region. This test facility is fully-equipped with ocean instrumentation that provides measurements of wave spectra (significant wave height, peak period, wave direction), temperature, and current profiles (surface, subsurface, direction, velocity) at 20-minute intervals. The wave spectra data were used later to relate power generating capacity with sea-state and validate power prediction models [10]. During the sea trial, wave heights ranged from 0.25-0.3m and with periodicities ranging between 6-15 seconds. The device was demonstrated to consistently generate between 20-50W, with a maximum peak power output of ~53W in these low to moderate sea-states. Corresponding time series of measured voltages (ranging between 11-18V) across a 6-ohm load (0.1% tolerance) were used to calculate the peak power output of the WEC device.

Fig. 6. Deploying the WEC prototype at the Kilo Nalu Nearshore Reef

Observatory.

VI. CONCLUSIONS Phase I helped to prove the viability of the rotary-drive design for compact wave energy conversion. Simulations, which analyzed the systems dynamic response in a broad range of sea-states and wave periodicities, were used to size the power take off components to maximize power production. A dry prototype was fabricated and tested to determine the feasibility of the design in achieving a target power rating of 50W. A waterproof prototype was demonstrated at-sea to successfully generate over 50W of peak power in low to moderate sea-states. The success of the Phase I effort led to a Phase II STTR contract to develop a scaled-up 100-200W device for deep water deployment. The Phase II prototype, utilizes two parallel power modules and employs a floating-thruster anchor for pull-resistance and station-keeping. Compact, low-power (

arrays; and small underwater vehicle or watercraft charging stations.

ACKNOWLEDGEMENT The authors gratefully acknowledge the support of our

program officer, Dr. Ron Joslin, of the Office of Naval Research; Prof. Ronald Knapp for providing us with the laboratory data on the stiffness of the steel tether; and Prof. Geno Pawlak, Kimball Millikan, Richard Carter, Laurie Tamura and Jeff Huebotter for their support during the Phase I sea trials.

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