Oscillating Water Column Wave Energy for Desalination

Oscillating water column (OWC) wave energy technology has been gaining attention in recent years as a sustainable and renewable energy source for desalination. Desalination is the process of removing salts and other impurities from sea or brackish water to make it suitable for human consumption or agricultural use. With the increasing scarcity of clean freshwater sources around the world, desalination has become a critical solution to meet the growing demand for potable water.

Among emerging desalination technologies, wave energy for desalination is attractive for a simple geographic reason: seawater desalination plants are built on the coast, exactly where wave energy is available. Pairing the two can shorten transmission distances and, at exposed sites, provide a renewable supply matched to a coastal load. This article focuses on the oscillating water column, one of the most mature wave energy converter designs, and on what it takes to couple it to a desalination process.

OWC wave energy technology harnesses the power of ocean waves to generate electricity, which can then be used to power desalination plants. The principle behind OWC technology is quite simple: as ocean waves rise and fall, they cause the water level inside a chamber to oscillate. This oscillating water column drives a column of air in and out of the chamber, which in turn drives a turbine or generator to produce electricity.

How an Oscillating Water Column Works

An OWC consists of a partially submerged, hollow chamber that is open to the sea below the waterline and trapped air above it. As a wave crest arrives, water rises inside the chamber and pushes air out through a duct at the top; as the trough passes, the water falls and draws air back in. Because the airflow reverses direction every half-cycle, OWC plants typically use self-rectifying turbines, most commonly the Wells turbine or impulse-type turbines, which spin in the same direction regardless of airflow direction. A key design advantage is that the turbine and generator sit above the water, away from direct seawater contact, which simplifies maintenance compared with fully submerged devices.

OWCs can be built as shoreline structures cut into rock, integrated into breakwaters, or installed as floating units offshore. The Mutriku breakwater plant in Spain, for example, has operated since 2011 with 16 Wells turbine units rated at about 18.5 kW each, for a total of roughly 296 kW, and the LIMPET shoreline plant on Islay, Scotland, was an earlier grid-connected demonstration rated at around 500 kW. Breakwater integration is particularly relevant to desalination because it allows the energy converter to share coastal infrastructure with seawater intakes and harbor protection.

One of the key advantages of OWC wave energy technology is its reliability and consistency. Unlike solar or wind energy, which can be intermittent and variable, ocean waves are consistent and predictable. More precisely, wave conditions can typically be forecast several days ahead with better accuracy than wind, and wave power tends to vary more smoothly from hour to hour, although it still changes substantially with weather systems and seasons. This makes OWC technology a reliable source of renewable energy for desalination plants, which typically require a continuous supply of electricity to operate efficiently.

Another advantage of OWC wave energy technology is its low environmental impact. Wave energy is a clean and renewable energy source that does not produce greenhouse gas emissions or other harmful pollutants. By using OWC technology to power desalination plants, we can reduce our reliance on fossil fuels and help mitigate the effects of climate change.

In addition to its environmental benefits, OWC wave energy technology can also help reduce the operational costs of desalination plants. By generating electricity on-site using wave energy, desalination plants can reduce their reliance on grid electricity and lower their overall energy costs. This can make desalination more affordable and accessible to communities that are struggling with water scarcity.

There are several ways in which OWC wave energy technology can be integrated with desalination plants. One option is to use the electricity generated by OWC technology to power the entire desalination process, from pumping seawater to treating and distributing freshwater. This can reduce the overall energy consumption of the desalination plant and make the process more efficient.

Another option is to use OWC technology to power specific components of the desalination plant, such as pumps or membranes. By using wave energy to supplement the electrical needs of the plant, desalination operators can reduce their reliance on grid electricity and lower their operating costs.

In some cases, OWC wave energy technology can even be used to directly power the desalination process itself. For example, the oscillating water column can be used to drive pumps or other mechanical components of the desalination plant, eliminating the need for additional electricity sources.

Sizing the Energy Link to Desalination

Seawater reverse osmosis (SWRO) is the most common process considered for wave-powered desalination, and modern SWRO plants with energy recovery devices typically consume on the order of 3–4 kWh per cubic meter of product water, including pretreatment. As an approximate illustration, an OWC installation delivering an average of 100 kW could support roughly 100 kW × 24 h ÷ 3.5 kWh/m³ ≈ 690 m³/day of freshwater. Because an installation’s average output is much lower than its rated capacity, designers should size the desalination plant against realistic long-term average wave power at the site rather than nameplate turbine ratings.

Electrically coupled systems are the most flexible, since wave-generated electricity can feed high-pressure pumps directly, charge storage, or be supplemented by the grid. Mechanically or hydraulically coupled systems avoid conversion losses, but they are harder to implement with an OWC because its primary output is oscillating airflow rather than pressurized water; direct seawater pumping is more often associated with other wave energy converter types, such as submerged point absorbers.

Operating Reverse Osmosis on Variable Power

Reverse osmosis membranes perform best at steady pressure and flow. Frequent starts and stops, or large swings in feed pressure, can reduce permeate quality, increase fouling and scaling risk, and shorten membrane life. Wave-powered systems therefore commonly include one or more buffering measures:

• Energy storage: batteries, flywheels, or pressurized accumulators smooth short-term fluctuations from individual wave groups.
• Variable-speed pumping: variable frequency drives let the high-pressure pumps follow available power within the membrane’s allowable operating window.
• Modular trains: several smaller RO trains can be brought online or taken offline as wave power rises and falls, keeping each operating near its design point.
• Automatic flushing: permeate or low-pressure flushing during shutdowns displaces concentrated brine from the membranes to limit scaling.
• Grid or backup connection: a secondary supply maintains minimum flow during extended calm periods.

Site and Environmental Considerations

The wave resource is strongest on exposed western coastlines at mid to high latitudes, so many water-scarce regions with calmer seas have a weaker resource than these proven sites. OWC turbines generate airborne noise that may require silencers near populated areas, and shoreline or breakwater structures must be designed for extreme storm loading. When the converter is co-located with a desalination plant, brine discharge and intake design still need independent environmental assessment, since the wave energy system does not change the plant’s marine discharge obligations.

One of the challenges of integrating OWC wave energy technology with desalination plants is the variability of wave energy. Ocean waves can be unpredictable and can vary significantly in intensity and frequency. This variability can make it difficult to reliably generate electricity using OWC technology, especially in areas with calm or inconsistent wave patterns.

To address this challenge, researchers and engineers are developing advanced control systems and wave energy converters that can maximize the efficiency of OWC technology. By optimizing the design and operation of OWC systems, we can enhance their reliability and performance, making them more suitable for powering desalination plants.

Another challenge of using OWC wave energy technology for desalination is the high initial capital costs of installing and maintaining OWC systems. While wave energy is a renewable and sustainable resource, the technology required to harness it can be expensive to develop and deploy. This can make it difficult for desalination plants to justify the investment in OWC technology, especially in regions where grid electricity is readily available.

However, as the cost of renewable energy technologies continues to decrease and the benefits of OWC wave energy become more widely recognized, we can expect to see increased adoption of this technology for desalination. Governments, research institutions, and private companies are investing in the development of OWC technology to make it more affordable and accessible for desalination projects around the world.

Related Topics

Professionals evaluating wave energy for desalination may also want to explore triboelectric nanogenerator desalination, which harvests low-frequency motion such as ocean waves at much smaller scales, and reverse electrodialysis, which generates power from the salinity difference between seawater and fresh water. For a biological route to energy-efficient desalting, microbial desalination cells use the electrical current produced by bacteria to drive salt removal.

In conclusion, oscillating water column wave energy technology has the potential to revolutionize the desalination industry by providing a clean, reliable, and sustainable source of electricity. By harnessing the power of ocean waves, we can reduce our reliance on fossil fuels, lower greenhouse gas emissions, and make desalination more affordable and accessible to communities in need of freshwater. With continued research and development, OWC wave energy technology has the potential to play a significant role in addressing the global water crisis and ensuring a sustainable future for generations to come.