HERO-WEC/Subsystems

Subsystems of the Hydraulic and Electric Reverse Osmosis Wave Energy Converter

Built following many of the same guidelines that the Waves to Water Prize contestants were required to follow, the HERO WEC features a modular power take-off and is capable of demonstrating how the deployment process might vary between both hydraulic and electric marine-energy-powered desalination devices. The HERO WEC was designed using predominantly commercial, off-the-shelf parts; a few critical components, including the winch, frame, and mounting points, were custom built by NREL. The system was designed with one end result in mind: producing a WEC that is reliable, easily transportable and installable, while being simple enough to minimize typical points of failure. The system comprises several parts that enable it to transform seawater into clean drinking water:

Graphic by Tara Smith, National Renewable Energy Laboratory

The HERO WEC can desalinate seawater using a hydraulic or electric power conversion system. The hydraulic system relies on the simple up-and-down motion of waves to create pressure and pump ocean water through a hose and into a land-based reverse osmosis system.

Graphic by Tara Smith, National Renewable Energy Laboratory

Alternatively, the electric system uses the motion of the waves to engage an onboard rotary generator to create electricity. The electricity then travels through a subsea cable to power an onshore pump that draws water from the ocean and pushes it through the same reverse osmosis desalination system.

WEC

The HERO WEC is a single-body, point-absorber-style wave energy converter that is anchored to the seabed. The device has three major subsystems (described in detail below): a float, a frame, and a modular power take-off.

The power take-off is a modular unit that can be configured to spin a generator for the purpose of producing electricity, or it can spin a seawater pump for applications where operating without the use of electricity is desirable. In both configurations, the WEC was designed for the purpose of providing the needed water input into a reverse osmosis (RO) desalination unit.

The seawater reverse osmosis process requires a significant amount of energy to pump enough water at a desired pressure so that the reverse osmosis membranes can separate the salt from the water. In the electric configuration, this is done by producing electricity to power an electric submersible pump, which then feeds the saltwater into the RO system; in the hydraulic configuration, the WEC pumps the water directly to the RO system.

The WEC is anchored using a cable that is spooled around a winch-actuated power take-off. As waves pass the WEC, it moves the WEC up—or away—from the seafloor, spinning the winch, which allows the WEC to extract energy from the waves. Once the WEC reaches the top of the wave, the winch is rewound by a pneumatic spring attached to the winch, so that the cycle can start over and continue to generate power on each subsequent wave.

Float

The HERO WEC’s octagonal, inflatable outer structure enables the device to both float atop the waves, as well as be transported with ease. Credit: Andrew Simms, NREL

The float, which was designed as an octagonal raft around the mechanical and electrical components of the HERO WEC, keeps the device on the surface of the water. This component is critical to ensuring that the WEC has enough buoyancy to spin the winch and generate clean drinking water from the motion of the waves.

This integral piece of the WEC was designed by the NREL team and fabricated by Inflatable Technologies in Denver, CO. The inflatable raft design was key to ensuring that the entire WEC could be packed down to fit within the standard shipping container in which each Waves to Water competitor was required to transport their device.

Frame

A metal frame, which can be broken down into four packable segments, serves as the HERO WEC’s backbone. Credit: Andrew Simms, NREL

The HERO WEC team built an octagonal frame to keep the power take-off system contained within the float. Constructed in four-quadrant sections, the frame was designed to be broken down to fit inside a standard shipping container.

The primary structure was fabricated from rectangular steel tubing coated in an epoxy-based paint. Within the frame, there are designated mounting points for the gearbox, generator/pump, and air spring. Fiber reinforced plastic grates were installed to prevent fish and other marine wildlife from getting trapped inside the WEC. These grates also provided an extra layer of protection for any divers working on the WEC while it was in the water.

Power Take-Off

A closer look at the HERO WEC’s mechanical system. Credit: Andrew Simms, NREL

The power take-off (PTO) is a mechanical system that is used to convert the bi-directional motion of the WEC into unidirectional shaft work, which is needed to drive rotary pumps and generators. The PTO consists of four critical sub-systems; a winch, a one-way clutch, a spring return, and a gearbox. As the WEC moves up and down relative to the seabed, the winch spins. When the WEC moves upward, the one-way clutch is engaged and the winch rotates the gearbox. The gearbox is used to increase the rotational speed, reducing the size of the pump and/or generator required. When the WEC reaches the top of the wave, the one-way clutch disengages, allowing the spring return to rewind the winch.

The spring return uses two stages of chain and sprocket reduction. Each stage creates a 4.5:1 reduction (combined, the two stages create a total reduction of 20.25:1), so that when the winch completely unspools all 15 wraps, the final sprocket only rotates about 270 degrees (3/4 of one rotation). The final sprocket it attached to a pneumatic piston, which is plumbed to a three-gallon air tank. When the WEC is moving up and spinning the gearbox, the pneumatic piston is compressing the air inside the tank. When the clutch disengages, the air in the tank pushes the pneumatic piston back, which rewinds the winch. This PTO is used for both the electric and the hydraulic conversion systems.

Hydraulic System

The hydraulic, or mechanical, subsystem pressurizes ocean water and feeds it into the desalination system to produce clean, safe, drinking water. Credit: Andrew Simms, NREL

In the hydraulic system operation, the output shaft of the gearbox spins a pump that is situated inside of the center of the float. The HERO WEC’s pump is a rotary, six-diaphragm pump that was originally intended for agricultural applications, such as spraying pesticides and herbicides. This style pump was chosen because of its inherent resistance to corrosion and its ability to handle solid particles that are found in seawater (e.g., sand, algae, etc.).

There are two pressure relief valves located on the outlet of the pump to protect it from over-pressurization in large waves, as well as prevent the WEC from picking up the anchor in aggressive wave conditions. The pump is also connected to a hose situated on the outside of the WEC. The hose runs along the seabed and up the pier up to the reverse osmosis system. The pump provides the seawater, at the necessary pressure, for the reverse osmosis system to produce clean, safe, drinking water.

The system operates similarly to a hand-powered bike pump or a well pump, but, in this case, it is the ocean waves that are driving the motion of the pump.

Electric System

The electrical enclosure box remains on dry land and is attached to the floating HERO WEC via an underwater cable. Credit: Andrew Simms, NREL

The team faced several challenges in designing and building the HERO WEC’s electrical system, such as translating highly variable ocean waves into stable, usable energy; designing an electrical system rated for exposure to seawater and the ocean’s elements; and adapting off-the-shelf components not intended for marine energy to ocean fluctuations.

Similar to the hydraulic system, in the electric configuration, the output shaft of the PTO gearbox spins the electric generator. When the generator spins, it sends electricity through an underwater cable that is attached to the seabed and runs up the pier. However, before the electricity can be used for water production, it must be converted from high-voltage three-phase AC power into high-voltage DC power. The high-voltage DC power is then converted into 24-volt DC power through a charge controller, which charges a 24-volt battery bank. The battery bank stores the intermittent energy that comes from the waves and powers an inverter that translates the energy into single-phase 110-volt AC power, just like that which comes from a typical household wall outlet. The energy is then used to power an off-the-shelf submersible pump that produces water at the required pressure and flow necessary for the operation of the reverse osmosis unit/desalination system.

Reverse Osmosis System

The NREL team fashioned the HERO WEC’s reverse osmosis system from two off-the-shelf components joined together. Credit: Andrew Simms, NREL

The reverse osmosis system is how the HERO WEC desalinates water. At the heart of the HERO WEC’s desalination system are two off-the-shelf Spectra reverse osmosis units. These units are commonly used on boats for the purposes of generating fresh drinking water. Each Spectra unit uses semi-permeable membranes coupled with an energy recovery unit, known as a Clark pump, to separate salts from water. The Clark pump is both a hydraulic intensifier and an energy recovery device —it reduces both the pressure inputs and energy consumption required to produce clean water. In addition to coupling the two reverse osmosis units together, the team added a supplementary flow meter and pressure sensor to monitor the seawater that goes into the reverse osmosis system, as well as a small accumulator to smooth out the pressure and flow spikes from the wave oscillations. In this configuration, the accumulator acts like a battery, storing small amounts of energy from each wave so that the reverse osmosis units can operate with fewer fluctuations.

HERO WEC

• Introduction
• Subsystems
• Related Tools

• Design & Docs
• Publications
• Contact