Image: Sustainable Marine

Ocean energy is set to play a role in decarbonising fossil fuel production and use as the technologies advance.

Ocean energy, potentially the last Earth-bound frontier for the expansion of renewable energies, has been gathering growing headlines in recent months. After a long gestation, which has seen concepts and companies come and go, it seems to be finally emerging as a viable contender to participate in the drive for decarbonisation towards net zero emissions even if not yet fully commercially ready.

What is ocean energy exactly? Ocean energy is essentially any form of energy derived in a marine environment and encompasses a multitude of technologies, including tidal energy, wave energy, salinity gradient and thermal energy conversion. Offshore wind and floating solar PV also are sometimes included, although not within the scope of this article.

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The ocean energy potential is massive, given the extent of the world’s coastlines. IRENA has estimated the potential in the range 45,000TWh to as much as 130,000TWh per year. For context that is more than twice the current global demand for electricity at around 25,000TWh and more than enough into the foreseeable future. The IEA’s new pathway for net zero by 2050 has total electricity supply slightly more than doubling over that period.

Of this the capacity of installed technologies (as of 2020) is only about 535MW, although IRENA projects a potential capacity approaching 10GW by 2030.

A key advantage of ocean energy is that in general the supply is more stable and subject to slower changes than wind or solar, which are strongly locally weather dependent. On the other hand the key challenge has been the nature and harshness of the environment and an internationally recognised staged development pathway has been formulated to mature technologies towards commercialisation.

Inshore – tidal and salinity gradient energy

Like all renewable technologies location is everything and some are more suitable than others.

Tidal and salinity gradient energy require specific inshore locations and as such, their potential is the least at around 1,200TWh and 1,650TWh/year respectively, IRENA data indicates.

Tidal energy comes in two forms. One, known as tidal barrage or tidal range, is basically similar to hydro and is generated in the passage of water through turbines in a barrage, with water captured in the rising tide and released in the outgoing tide.

The second draws on the tidal stream or current with a seabed mounted turbine or other type of oscillating device.

Salinity gradient energy can be generated from the difference in salt concentration between two fluids, with the best locations being riverbeds where freshwater flows into the sea and the concentration difference is the greatest.

Projects of both types are under development but so far the tidal energy category, which is the closest to approaching commercialisation, is dominated by two large tidal barrage installations in France and Korea. The salinity gradient group has a single project in the Netherlands.

Offshore – wave energy and thermal energy conversion

Wave energy and thermal energy conversion have wider application as in theory at least they can be implemented at any distance from the shore, although in practice near shore locations are preferable as the transmission distances are reduced and the infrastructure requirements less demanding and costly.

The estimated potentials are 29,500TWh and 44,000TWh/year respectively.

Wave energy similarly encompasses a range of technologies from sea surface oscillating devices to floating or submerged buoys and seabed mounted generators. Numerous demonstrations up to megawatt scale are in place and while devices are increasing in size, which if any of the technologies will dominate, or more likely some combination, remains to be seen.

Ocean thermal energy conversion (OTEC) generation draws on the temperature difference between the warm surface and colder deeper layers of water and requires relatively near shore deep water locations with a temperature differential around 20oC. Too close to the shore near-shore circulation patterns influence the thermal structure of the water column.

OTEC uses cycles with heat exchangers and turbines and is still in the R&D phase with megawatt-scale demonstrations emerging. Island locations also are emerging as particularly promising with the opportunity to use the water flows for other purposes. For example, the new Puerto Rico Ocean Technology Complex, which is under development, proposes to use sea water for cooling as well as for a range of other ‘blue economy’ products.

Ocean harvesting

The main focus of ocean energy has been on power delivery to the shore but is so far a largely untapped opportunity for powering offshore assets, such as oil and gas platforms. While the push is on to move away from fossil fuels, they will be in use for the foreseeable future – in the IEA’s Net Zero by 2050 report, fossil fuels still account for slightly over one-fifth of total final energy supply – and the suppliers can support the net zero drive by decarbonising their activities.

Nordic oil and gas company Lundin Energy, which claims the world’s first certified carbon neutrally produced oil and is on a drive to achieve carbon neutrality by 2025, is taking a lead in a collaboration with Swedish wave power technology developer Ocean Harvesting Technologies. In a one-year project running to February 2022 the two companies are investigating how wave energy converters could potentially provide clean, stable and cost effective electricity to major offshore operations.

“This case study will provide valuable input in better understanding the requirements for such an installation. The project will guide us through the early validation stages of our commercialisation,” comments Mikael Sidenmark, CEO at Ocean Harvesting Technologies.

The company’s technology is a point absorbing device with force control to enable it to adapt to the sea state and its modular nature allows for scalability. Under the current roadmap sea trials are planned to scale up to 500kW by 2024 with the first commercial installations in 2025.

There also remains the tantalising possibility of powering marine transport from the ocean and whether this will be possible on any scale is a future technology challenge. One area of activity is the potential of OTEC in powering robotic submarines utilising a phase changing material to generate energy but its realisation is still in the future.