8.8 Wave Energy Technologies

Introduction

Wave energy technologies aim to capture the energy contained in surface ocean waves and convert it into useful power, usually electricity. While hydropower and other marine technologies deal with rivers, tides, or temperature gradients, wave systems are designed specifically around the motion and characteristics of wind-driven waves at the sea surface. This chapter focuses on how wave energy is harvested, the main types of devices, and the technical and practical issues that are unique to this resource.

The Nature Of Wave Energy

Waves are created when wind transfers energy to the ocean surface. As waves travel across the sea, they carry energy both in their height and in their period, which is the time between successive crests. The power in waves is not the movement of water from one place to another, but the movement of wave forms and the oscillatory motion of water particles.

An important feature of waves is that the available power increases very quickly with wave height. In deep water, the power per meter of wave front is roughly proportional to the square of the wave height and directly proportional to the wave period. A simplified expression for the wave power per unit width of wave front is

$$P \propto H^2 T,$$

where $P$ is wave power per meter of crest, $H$ is significant wave height, and $T$ is wave period.

This strong dependence on wave height means that energetic ocean regions can contain very large power densities, but it also means that devices must tolerate extreme forces during storms.

Basic Principles Of Wave Energy Conversion

Wave energy converters attempt to turn the oscillatory motion of waves into rotation or linear motion that can drive a generator. In general, they rely on one or more of the following physical principles.

The first is oscillating structures. In these systems, part of the device moves relative to another part due to wave forces. This relative motion can be used to drive hydraulic systems, mechanical linkages, or directly coupled generators.

The second is oscillating water columns. Here, waves cause the water level in a chamber to rise and fall. This motion compresses and decompresses the air above the water, forcing it back and forth through a turbine.

The third is overtopping. In these devices, waves run up a ramp and spill into a raised reservoir. The collected water then flows back to the sea through low head turbines, similar to a small hydropower plant.

In all cases, the key technical challenge is to absorb as much wave energy as possible while maintaining structural integrity in a harsh marine environment and keeping the system efficient and reliable.

Main Types Of Wave Energy Converters

Although many designs exist, most wave energy converters fall into a few broad classes based on how they are oriented relative to the wave direction and how they interact with the waves.

Point Absorbers

Point absorbers are devices that are small relative to the wavelength. They typically float on the surface and move up and down as waves pass. Some designs also allow motion in several directions. The device usually consists of a buoy and a reference structure, such as a weighted body or a moored element.

Power is generated from the relative vertical motion between the buoy and the reference body. This can be done with hydraulic pistons, mechanical gear systems, or linear electrical generators. Because point absorbers are relatively compact, many units can be deployed in an array to capture significant power from a given area.

Point absorbers are attractive because they can absorb energy from waves coming from multiple directions and do not require precise alignment with the prevailing waves. However, they must be carefully designed to resonate with typical wave conditions to maximize energy capture, while not becoming dangerously overstressed in storms.

Attenuators

Attenuators are long, multi-segment floating devices aligned roughly parallel to the direction of wave travel. As waves pass along their length, the segments bend at the joints. This flexing motion is converted into power, often through hydraulic systems in the joints that drive generators.

Because of their elongated shape, attenuators interact with the wave field along their entire length, which can give them a relatively high energy capture per device. Their performance depends on the alignment with the wave direction and the detailed design of the joints and power take-off system.

Attenuators must accommodate complex bending, twisting, and heaving motions. They require robust mechanical design and advanced control systems to operate effectively in a wide range of sea states.

Oscillating Water Columns

Oscillating water columns, often abbreviated as OWCs, are structures that use an air chamber partially submerged in the sea. Waves cause the water surface inside the chamber to rise and fall, which alternately compresses and decompresses the enclosed air. This airflow is directed through a special type of turbine that can rotate in the same direction regardless of the direction of flow.

OWCs can be built onshore, for example integrated into a breakwater or coastal cliff, or they can be mounted on floating structures offshore. Onshore OWCs benefit from easier access for maintenance and the possibility of using conventional construction techniques, but they are limited to sites where suitable coastal topography and wave conditions exist.

Offshore OWCs can access more energetic wave climates, but they must address the full set of offshore engineering challenges, including mooring, structural fatigue, and severe storms. The performance of OWCs is strongly influenced by the design of the chamber geometry and the turbine.

Overtopping Devices

Overtopping devices collect water from incoming waves in a raised reservoir. Waves are guided by a ramp or funnel that encourages them to spill over into the storage basin. The stored water then returns to sea level through low head turbines that generate electricity.

These systems are conceptually similar to small hydropower plants, with the main difference that the reservoir is filled intermittently by waves instead of continuously by river flow. Overtopping devices can be built as floating platforms or as fixed coastal structures.

One advantage of overtopping devices is that some energy smoothing occurs in the reservoir, which can reduce short-term power variability. However, building a raised structure that can withstand constant wave impact and corrosion is technically demanding. Efficiently capturing wave overtopping requires careful design of the ramp and reservoir geometry.

Oscillating Wave Surge Converters

Oscillating wave surge converters are typically flap-like devices that pivot back and forth in response to the horizontal motion of water particles near the seabed in shallow water. They are usually installed close to shore, anchored to the seabed, with a large panel extending toward the surface.

As waves approach the shore, the surge motion causes the panel to oscillate. This motion drives a hydraulic or mechanical system that powers a generator. Because these converters operate in shallow water, installation and maintenance can be more accessible than for deep-water devices. At the same time, they face strong loads from breaking waves and must be carefully designed to cope with nearshore dynamics.

Mooring, Foundations, And Survivability

Most wave energy converters are located offshore, where waves are more energetic. This location requires secure mooring systems or foundations that both keep the device in place and allow the intended motions for energy capture.

Floating devices such as point absorbers and attenuators use mooring lines connected to anchors or seabed weights. The mooring system must accommodate tidal variations, wave-induced motions, and currents, while maintaining the correct orientation and tension to prevent failure.

Fixed or seabed-mounted devices, including some OWCs and oscillating surge converters, rely on foundations similar to those used in coastal engineering and offshore wind. These foundations must transfer loads from wave forces into the seabed and resist long term fatigue, scouring around the base, and corrosion.

Survivability in extreme conditions is a central challenge for all wave technologies. Devices must endure storms with waves far larger than those for which they are optimized for energy capture. Many designs include modes of operation in which the device changes configuration during storms, for example by submerging, locking moving parts, or orienting to minimize loads.

Power Take-Off And Control

The power take-off system is the component that converts device motion or pressure variations into electrical energy. In wave energy converters, power take-off technologies include hydraulic systems, mechanical drivetrains, and direct-drive electrical generators.

Hydraulic systems use fluids under pressure, generated by pistons or pumps driven by device motion, to spin hydraulic motors and generators. These systems can smooth out some of the irregularity of wave motion but add complexity and potential points of leakage.

Mechanical drivetrains involve linkages, gears, and shafts that directly transmit motion from moving parts to a generator. These are similar in principle to systems used in wind turbines, but they must operate under highly variable and often reversing loads.

Direct-drive systems use linear generators or specially designed rotary generators that couple to the device motion without intermediate mechanical or hydraulic stages. These can reduce mechanical complexity but require sophisticated electrical design and robust materials.

Control strategies are essential for maximizing energy capture and protecting equipment. In some designs, active control modifies the device response in real time so that its motion stays in phase with the waves, a condition that can significantly increase power absorption. Control systems also switch devices into survival modes during extreme events.

Arrays And Farm Layout

Just as wind turbines are deployed in wind farms, individual wave energy converters are often planned as arrays or wave farms to produce meaningful quantities of electricity. Placing multiple devices together introduces interactions between them and the surrounding wave field.

Devices extract energy from waves, which can create zones of reduced wave height downstream. Nearby devices may then experience different wave conditions depending on their position. Careful layout planning attempts to optimize the arrangement so that each device sees sufficient wave energy while minimizing destructive interference.

Array design considers spacing, alignment with prevailing wave directions, cable routing to shore or substations, and shared infrastructure such as moorings or anchors. Because commercial wave projects are still limited, knowledge of optimal array configurations is less mature than for wind energy, and ongoing studies focus on understanding these complex interactions.

Specific Challenges For Wave Technologies

Wave energy systems face a combination of scientific, technical, and economic challenges that are distinctive among renewable technologies.

One major challenge is the highly variable and often violent nature of the wave environment. Devices must function efficiently in small to moderate waves, yet survive rare storm waves that can be many times larger. This contrast complicates design and cost optimization.

Another challenge lies in durability and maintenance. Corrosion, fouling by marine organisms, and continuous mechanical fatigue all reduce component lifetimes. Accessing offshore devices for inspection and repair is costly and weather dependent, so reliability and maintainability are critical design priorities.

The complexity and diversity of device concepts also slow standardization. Many competing designs exist, with different configurations, materials, and operating principles. This variety reflects ongoing innovation, but it makes it harder to reduce costs through mass production and shared infrastructure.

Grid connection and integration present additional issues. Wave energy is variable and somewhat less predictable than tidal energy, though often more predictable than wind beyond a short time horizon. Matching wave power output to grid needs, choosing suitable sites near existing infrastructure, and ensuring power quality all require careful planning.

Environmental And Coastal Considerations

While detailed environmental assessment is treated elsewhere, wave energy technologies have some particular interactions with the marine environment that influence design and siting decisions.

Wave energy converters extract energy from the wave field, which can slightly alter wave heights and patterns in their vicinity. In some cases, this could lead to minor changes in coastal erosion or sediment transport. Arrays near shorelines may interact with beach dynamics or existing coastal defenses.

At the device level, structures become artificial habitats for marine organisms. This can increase local biodiversity, but also promotes biofouling, which affects performance. Noise from mechanical and electrical equipment must be assessed, especially in areas with sensitive marine life.

Visual impacts from wave devices are usually less significant than from large coastal infrastructure or wind turbines, especially for devices placed further offshore. However, nearshore structures and onshore components, such as OWCs or shore stations, can still affect the coastal landscape and must be carefully integrated into local planning.

Development Status And Future Prospects

Compared with mature technologies such as hydropower, onshore wind, or solar photovoltaics, wave energy remains at an earlier stage of commercialization. Many devices have been tested at small scale in laboratories and nearshore sites. A smaller number have reached full-scale demonstration offshore with grid connection.

Costs for wave energy are currently high, and long term performance data are limited. However, wave energy offers some distinct potential advantages. In certain locations, wave power is more consistent than wind, and waves can continue to deliver energy after storms that generated them have moved away. This can make wave energy a useful complement to wind and solar in diversified renewable portfolios.

Future progress in wave energy technologies depends on advances in materials, corrosion resistance, mooring design, power take-off systems, and control algorithms. It also depends on policy support, access to test sites, and learning from pilot projects to refine designs and reduce costs.

As knowledge accumulates, some device concepts may prove more suitable and become dominant, enabling standardization and economies of scale. In addition, combining wave energy with other marine uses, such as offshore wind platforms, aquaculture, or coastal protection structures, could spread costs and create synergies.

Conclusion

Wave energy technologies aim to harness the mechanical energy stored in ocean waves using a variety of device concepts, including point absorbers, attenuators, oscillating water columns, overtopping devices, and oscillating wave surge converters. All of these systems must cope with a demanding marine environment and the dual requirement of efficient energy capture and robust survivability.

Although commercial deployment is still limited, wave energy represents a promising addition to the family of renewable marine technologies. With continued innovation, testing, and careful integration into coastal and offshore systems, wave energy could contribute to a more diversified and resilient low carbon energy mix.