Described herein is a utility-grade power buoy.  The device is submerged below the ocean surface at approximately three meters below mean low tide.  The buoy’s buoyancy chambers maintain a firm lifting force that  keeps the anchor chain taught at all times to avoid any heaving motion.  The buoy captures power from alternating pressure differentials generated by passing waves above the buoy and the relatively passive pressure at the buoy’s lowest point within the water column.  The device is out of view from shore and is able to be anchored within the Neritic or Sublittoral Zone, i.e., relatively shallow depths from 80 feet to the edge of the continental shelf.  The buoy's internal oscillating air/water interface creates an efficient pneumatic energy transfer to drive the patented PTO, a mechanical direct-drive converter known as the Hanna MultiDrive (U.S. patent 8,745,981).  As an alternative PTO, the oscillating air/water interface will also drive a closed loop, self-rectified, Monoradial air turbine.  The MultiDrive mechanical PTO or Monoradial turbine PTO, operate in real time synchronicity within the variable ranges of incident wave periods and amplitude.  A wide range of power extraction within regular and irregular wave domains can be accomplished by utilizing various length and diameter intake ducts attached to the buoy’s primary intake, called the Acceleration Tube. 


  The subsurface direct-drive buoy operates on long-established principles concerning pressure gradients on a floating or submerged object as described by Archimedes.  The buoy's excitation force derives from laws of fluid dynamics and pressure differentials, i.e., a volume of water is displaced within a partly air-filled container as the cyclic, differential pressures of passing waves act upon a hollow container's upper and lower extremities.  As a low trough or valley between passing waves pass over the submerged buoy, the pressure decreases at the buoy's upper portion but the pressure at the buoy's lowest point is greater.  The free surface of water that is contained inside the Acceleration Tube will then rise, providing a lifting force to move an internal Float upwards like a piston.  The oscillating differential pressures between these two points cause a cyclic up and down movement of the free surface of the air/water interface inside the hollow buoy.


  The buoy is a circular, hollow, structure comprised of two vertical axis components.  The upper component, the Mantle, is open at its lower circumference.  The second component, the Acceleration Tube, has two openings, one at its top and one at its bottom.  A pocket of air is contained between both components (see FIGURE 1).  A precise volume of air is pumped into the Mantle when the buoy is launched.  Because the submerged buoy is in a rigid vertical position, the precise volume of air remains trapped within the structure.  When there are no waves, the free surface of water inside the Mantle and Acceleration Tube are at the same level and the pressure is equal in both components.  When a passing wave crests above the buoy, the pressure differentials between the upper and lower portions of the buoy are altered and this creates a downward force that pushes out the water from the lower opening in the Acceleration Tube while, simultaneously, water is drawn into the circumferential opening at the Mantle.  This causes the free surface air/water interface to rise inside the Mantle and lower inside the Acceleration Tube.  When the wave crest lowers, an inverse action takes place which draws water into the Acceleration Tube and, simultaneously, forces out water at the Mantle's opening.  This bi-directional cycle allows either a stacked set of internal Three-blade Propellers or a single internal Float within the Acceleration Tube to be moved by the reciprocating flows passing inside the Acceleration Tube.  The self-rectified Propeller Blades rotate two coaxial shafts connected to the mechanical direct-drive PTO.  Alternatively, an internal Float will rise and fall with the free surface of water inside the Acceleration Tube.  The Float is attached to a lightweight Shaft which extends upward to drive the mechanical direct-drive PTO.  As a third primary drive option, the rise and fall of the water inside the Acceleration Tube can also be utilized to create bi-directional air flows with which to spin a self-rectified, closed loop, mono-radial air turbine.  The direct-drive PTO or turbine PTO are sealed inside the Power Pod and drive a one-way generator.  The sealed Power Pod can be removed independently from the main buoy for routine maintenance operations while the buoy remains in place.


  The direct-drive PTO (FIGURES 1 and 2) mechanically converts the up and down linear action of an internal Float and Shaft into a continuous one-way rotary movement.  The PTO’s rotary force spins a flywheel which in turn spins generators or sea water pumps.  A second alternative drive system does not use the Float.  Instead, it uses two sets of stacked, self-rectifying Propeller Blades (FIGURE 3) to spin coaxial rotary Shafts which are attached to the same PTO used with the internal Float system.  A third alternative PTO is a self-rectifying turbine PTO (FIGURE 4 and 5) which is a closed loop, mono-radial impulse-type drive.  A full description of the turbine can be viewed by clicking HERE.  


   The current invention is a continuation of the patent mentioned in the Abstract section above.  In this new iteration, the buoy is now disposed below the ocean surface.  The primary excitation force is now the pressure differentials which act upon the upper and lower extremities of the buoy.

  The use of pressure differentials has been employed in various wave energy converter (WEC) designs.  A prior art search for subsurface WEC's did find examples that use pressure differential excitation to drive an internal float and shaft.  The earliest reference to such a subsurface power buoy is a conference paper by Markus Mueller, et al. titled: “Conventional and TFPM Generators for Direct-Drive Wave Energy Conversion” in the IEEE Transactions on Energy Conversion, Volume 20, number 2, June 2005.  None of these early designs specify or claim the type of PTO as disclosed in the present invention. 

  Examples of prior art subsurface WEC's are: M3 Wave; CalWave Power Technologies; 40South Energy; Bombora Wave Power; a submerged buoy that uses a turbine PTO by Seung Kwan Song called SPA-OWC: US 9,518,556 B2 (now expired); another expired patent by W.W. Hirsch: US 7,199,481 B2; international patents by AWS Ocean Energy Ltd: WO2017/025765Al and US 10,711,760 B2; a patent by Marine Power Systems called WaveSub, WO20010007418A2;  and patents held by Ocean Power Technologies: US 6,768,217 B2, 6,933,623 B2 and 6,768,216 B1.

  An example of a prior art surface WEC is the WaveEL buoy developed by the Swedish company Waves4Power.  Like the Hanna design, Waves4Power uses a float (piston) and shaft that moves up and down inside the Acceleration Tube to power their hydraulic PTO.  Even when using biologic-type hydraulic fluids, the manufacturer warns: "Do not discharge into drains, soil or water".  The use of hydraulic fluids could pose a threat to the marine environment.  

  Another surface buoy is undergoing sea trials by the Swedish company CorPower Ocean.  Two CorPower PTO patent applications were filed after the Hanna patent had been granted and published.  While some CorPower embodiments are similar to the Hanna MultiDrive, they are far more complex.  Like the Waves4Power buoy, the CorPower PTO drives a hydraulic motor to generate electricity.  Their design uses an expensive electro-mechanical phase control methodology to enhance low energy capture.  Hydraulics and phase control strategies are unnecessary with the Hanna subsurface design.                                          

  A patent by Professor Lei Zuo at Virginia Tech (US 9,394,876), is for a PTO similar to the Hanna MultiDrive.  His patent was issued on 7/19/2016.  The Hanna patent was issued on 6/10/14.  The U.S. DOE and NSF have given a great deal of support to advance Professor Zuo’s PTO design. Despite this, the Hanna PTO enjoys a distinct performance advantage over the Zuo design; the Hanna patent claims the exclusive use of free-wheeling flywheels which sustain angular momentum at the top and bottom of each stroke.

  To read a comparison of the main competing designs that have similarities to the Hanna Subsurface Buoy, click HERE.


  The submerged structure is hidden from view from shore.  The primary driving force is the wave-activated pressure differentials that exist between the upper and lower extremities of the submerged buoy.  By integrating the patented Hanna PTO's with the proven subsurface buoy design, a new, greatly improved point absorber has been created.  The submerged buoy enjoys many advantages over floating point absorbers: It is protected from storms and heavy seas; consequently, there is little stress placed on its single-point mooring line.  It uses a seawater ballast system to adjust bouncy; the system is used to raise and lower the buoy for servicing.  The buoy is less expensive to build because the structure can be much lighter than surface-type power buoys.  The buoy is vertical and stable with no pitch, heave and surge motions.  Being stationary, the device is not sensitive to wave direction.  It does not require complex and expensive phase-control or latching mechanisms; which lowers the LCOE and presents a significant cost saving over surface buoys.  The system is versatile and can use three PTO options: When using the first optional PTO, the internal Float, with its linear motion, rises and falls in direct synchronicity to all types of incident wave spectra for optimum one to one power capture.  When using the second optional PTO, the self-rectified Propeller Blades, the PTO spins without using an internal Float so there are no harmful and noisy "end stops".  The third PTO option is the mono-radial, closed loop turbine which also functions without "end stops".  The subsurface buoy uses multi-length and varied diameter Acceleration Tubes for more sensitive energy capture over a wide bandwidth.  All three types of PTO's and their respective generators are housed within a sealed pod.  The modular pod can be accessed at sea for servicing by allowing the buoy to rise to the surface.   The buoy will scale from a micro grid to utility-grade sizes.  The direct-drive PTO is versatile: it can also spin centrifugal pumps to deliver water to onshore storage tanks.  The stored water will drive generators to power desalination plants.  The mechanical, direct-drive PTO’s one-way flywheel maintains momentum at the top and bottom of each wave cycle for a smooth and sustained power output.

  To read the Hanna MultiDrive patent and view videos, go to: http://www.wetgen.com/PTO_videos.htm.  Subsurface Buoy wave tank demo:  https://youtu.be/Z2chGWLS1IA.  Proof-of-concept Hanna direct-drive PTO demo: https://youtu.be/uwsm-FWjlQE.  Proof-of-concept turbine demo:  https://youtu.be/E1TWdQkGeU4. 



About John Hanna: Interim Director of Hanna Wave  Energy Power Drives.  Navy veteran.  Coast Guard documented mariner.  Former skipper of the research vessel "Ed Ricketts" for California State University's Moss Landing Marine Laboratory.  Mr. Hanna has twenty eight years' experience in QA/QC management on multi-million dollar government and private sector projects for structural steel and welding.  He was contracted by Ocean Power Technologies as the QC manager for their PB-150 PowerBuoy built in Oregon.  Hanna Wave Energy Technologies has been a licensed business in the City of Coos Bay, Oregon since 2010 (No. 9098).  Hanna is registered with Dun & Bradstreet under the name of WETGEN. 

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