Wave Energy Converters: Sustainable Energy Source

Ocean waves contain tremendous energy. Experts estimate the theoretical annual energy potential of waves at the off-coasts of the United States to be as much as 2.64 trillion kilowatt-hours. Wave power has greater energy density than wind or solar. Researchers and engineers have developed various wave energy converter systems in the past few decades. This article briefly overviews wave energy converter devices with their challenges and possible solutions.

Significant challenges of WEC include the high installation cost, deployment location determination, and bidirectional energy flow. The discussion focuses on the use of bidirectional flow turbines and triboelectric nanogenerator for this purpose.

Background

Wave power is the capture of energy of wind waves to do useful work, for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC). The Islay LIMPET connected to the United Kingdom’s National Grid, becoming the world’s first commercial wave power device. Other famous wave power plants, include Ada Foah Wave Farm, Aguçadoura Wave Farm, Azura, Mutriku Breakwater Wave Plant, SDE Sea Waves Power Plant, SINN Power wave energy converter, Sotenäs Wave Power Station, bolt lifesaver and Pico Wave Power Plant.

With excellent energy density, wave power generates up to 24-70 kW per meter of the wave, with peak near-shore power ranging from 40-50 kW per meter. The world’s total wave resource has been estimated to be as much as 2 terawatts (TW) of energy. The exploitable limit is probably at most about 10-25% of the resource; thus, ocean wave energy is potentially a significant contributor to human energy demands, not a panacea.

Classification of Wave Energy Converters

Wave energy converters can generally be classified into one of six different categories or design archetypes:

• Point Absorbers: In a typical point-absorber design, the device remains fixed at one end (or at least fixed relative to the water’s surface), while the other end undergoes vertical movement as the wave crests and troughs lift and lower it.
• Overtopping or Terminator Devices: They utilize a difference in potential energy by storing a volume of water at a height above the ocean’s surface and then draining it to power the turbine.
• Oscillating Water Column: In this design, a chamber traps air between the water’s surfaces, and when additional water enters the chamber, it compresses/decompresses the air, driving a bi-directional air turbine mounted on the top of the platform.
• Attenuators: Wave energy converters orient parallel to the direction of wave travel. They are usually modular in design and rely on the flexing of joints to generate power.
• Oscillating Wave Surge: It has a flap that oscillates like a pendulum due to wave energy to produce electricity or to pressurize a fluid
• Submerged Pressure Differential: The device rests on or near the seafloor and relies on pressure fluctuations as a wave passes overhead to flex a pliable material, such as an air bladder, to squeeze a fluid to drive a turbine.
• Rotating Mass: It utilizes a shift in the hull’s center of buoyancy and center of gravity when waves act to rotate a mass to generate power.

Challenges

The modern scientific pursuit of wave energy started in the 1940s during Yoshio Masuda’s experiments and is still developing. No prominent technology exists in this technology domain due to various technical challenges, which are as follows:

• End-stop problem: The hydraulic actuator exceeds its maximum displacement limit to cause damage to the system during unexpected extreme conditions.
• Bidirectional flow: Oscillating water column (OWC) and breakwater integrated OWCs wave energy convertor use air turbine for generating power. Direct use of conventional turbines is not possible due to the bidirectional flow of air. The design of the turbine becomes complicated and large due to the presence of non-return valve airflow rectification systems.
• Damage to the seals and the valves: Overtopping type of wave energy converters generally use hydro turbines. Ocean water is a dynamic fluid with various unpredictable components that can damage the seals and the valves in a hydro turbine.
• Short lifetime and higher maintenance: Direct mechanical drive devices transmit the wave energy into electrical energy using linear-to-rotary conversion systems by using gear systems. However, the direct mechanical drive system undergoes higher load cycles, resulting in a relatively short lifetime and higher maintenance costs.
• Unequal generated voltage: The irregular wave motion creates an unequal generated voltage, making the power transmission system highly complicated
• Determining the location for wave energy converters: Due to rough conditions of the ocean and less predictable data, the feasibility of extracting wave energy in the marine environment to generate electric power becomes a challenge.
• Cost and efficiency: It is difficult to obtain high energy conversion efficiency over an entire range of excitation parameters. It has a complex design and a high cost of installation^.

Solutions

Mechanical or Hydraulic End Stop Cushioning Devices:

Both ends of said track(s) are equipped with springs or other mechanical or hydraulic end-stop cushioning devices to prevent severe end-stop shocks from large waves. To reduce or eliminate such shocks, the following measures are implemented: a) increasing the resistive load of the onboard generator, b) enabling some pitching motion in the mounting frame with attached vertical frame column(s) in alignment with oncoming waves during wave crests and rebounding toward wavefronts during wave troughs, and c) allowing water in the top cavity to slide out if and when the barrier experiences rapid deceleration towards the end of its upstroke.

Bidirectional Turbines

Wells turbine is suitable for bidirectional flows but has low efficiency at a high-flow coefficient and poor starting characteristics. Wells turbine with booster turbine can be used for wave energy conversion to improve performance.  Another solution includes a twin-rotor with two rows of rotating blades axially offset from each other and mounted on the same shaft with corresponding guide vanes, as in a conventional axial-flow or radial-flow turbine. The reciprocating airflow between the OWC chamber and the atmosphere takes place as the unidirectional flow alternately through one or the other bladed set.

In the case of a turbine with self-pitch-controlled guide vanes, two sets of guide vanes on either side of the rotor are pivoted with the flow direction adjusted using the aerodynamic moment. Guide vanes can also change orientation in the correct direction for efficient operation.

Triboelectric Nanogenerator (TENG)

A triboelectric Nanogenerator (TENG) is a new energy harvester that converts small-scale mechanical motions into electrical energy by a combination of triboelectrification and electrostatic induction through the periodic contact, separation, and/or sliding movement between two tribo-materials with different abilities to gain or lose electrical charges. It has four modes, i.e., vertical contact-separation mode (VCTENG), lateral sliding mode (LSTENG), single-electrode mode (SETENG), and freestanding triboelectric-layer mode (FTENG). Common structures of TENG in ocean wave energy harvesting are spherical-shell structure, wavy structure, spring-assisted structure, and bionic structure.

The use of a TENG network enables the achievement of high-power output electrical energy. The TENG network consists of thousands of connected TENG units, utilizing a specific connecting method.

Selection Index for Wave Energy Deployments (SIWED)

A recent study describes a novel index that accounts for the interactions of wave climate and wave energy converters, offering an unbiased approach that considers climate’s variability, survivability, and energy production.

Here, Equation 1 shows the SIWED index where CoV_Hm0 is the coefficient of variation, CF is the capacity factor, H_EVA is the value of return waves based on extreme value analysis, and H_max is the maxima value of wave height from the dataset. If SIWED obtains a higher value, it means the site and selected WEC have a better match and can deliver reliable energy production.

Wave Energy: Major Companies & Research Institutes

According to Stellarix research, The global wave energy market is projected to reach USD 107 million by 2025 from an estimated market size of USD 44 million in 2020, at a CAGR of 19.3% during the forecast period. Major companies, startups, and research universities that are leading technology development are shown in Figure 4. Some active companies and universities are discussed below.

Active Companies

SEABASED is developing wave power parks that to produce electricity by absorbing and transforming the kinetic energy of ocean waves into electrical energy suitable for grid use.

Buoys on the water surface connect to magnetized weights, translators, and generators resting on the seafloor, forming the Wave Energy Converters. The buoys raise and lower the weights to create power.

CalWave’s xWave Series transforms the motion of ocean waves into electricity and includes scalable energy farms that can supply coastal communities with anywhere from 100 KW to over 500 MW of local power (for the x100 model). The system operates autonomously, and the device remains fully submerged to provide protection against aggressive swells and storms.

 Active Universities

A recent investment of $6M from the Naval Facilities Engineering Command and Expeditionary Warfare Center was done for the University of Hawaiʻi for wave energy conversion research. The new funding supports researchers in their work on various research projects. One of the projects involves developing a power generation and management system for a floating Oscillating Water Column WEC. Another project focuses on creating a novel breakwater system that incorporates a WEC. The main objective of these projects is to generate power from wave energy while ensuring the protection of coastal regions. Furthermore, the funding aims to facilitate the development of a small-scale WEC. In close proximity to the shore, it has the capability for rapid deployment in power generation and seawater desalination.

Flowave Ocean Energy Research Facility at the University of Edinburgh provides a setup for commercial developers to test their technologies and projects related to wave energy systems.

Conclusion

There are numerous types of wave energy converters, which is part of the reason that wave energy hasn’t grown as rapidly as wind or solar energy. There is still no convergence on a single design archetype in wave energy converters. Progress is slow due to the unique nature of each design and the dispersion of R&D efforts across various archetypes. The major challenges in wave energy converters are the efficient design of bidirectional flow turbines, the high cost of installation, and determining locations for deployment of wave energy converters. A New Triboelectric Nano-generator (TENGS) to harvest blue energy in the ocean is an advanced technology at present, and it still requires further development. In the coming years, wave energy will find its platform like wind energy.