Marine and Hydrokinetic Energy: A New Wave of Renewable Energy

Daniel Stewart for Zondits, June 25, 2015. Image credit: Unsplash

Marine and hydrokinetic (MHK) energy technologies generate renewable power from our oceans and have been under development since the mid-1900s. However, advances were slow until the turn of the 21st century, when improvements in manufacturing techniques and materials began to accelerate the industry’s growth. Along with these technological advances, recent societal pushes to broaden our portfolio of sustainable energy technologies have brought the MHK industry to the brink of commercial viability in a number of markets. Island populations in particular face high costs to import fuels used to generate energy and often have little or no available land resources to dedicate to wind or solar installations, making them ideal candidates for early MHK installations as the industry continues to lower costs.

MHK technologies generally fall into two categories: tidal and wave. Tidal devices harvest ocean currents or tides, usually by translating the linear flow of water into rotational movement that drives the device’s generator. MHK turbines (or, more generally, kinetic water turbines) differ from turbines used in hydroelectric dams in that they capture kinetic energy instead of head pressure to generate electricity. Similar to wind turbines, some units rotate about the axis of water flow while others rotate perpendicular to that axis. Units can be installed on the ocean floor, suspended between the floor and surface, or integrated into other structures, such as bridges or controlled floodways. Other, less developed types of tidal devices include oscillating hydrofoils, Archimedes screws, and tidal kites. One major advantage to tidal devices is that, since ocean tides shift approximately every 6 hours and other offshore ocean currents have unidirectional flow year-round, they tap into a predictable source of energy.

Wave devices convert ocean swell, or the actual waves traveling through ocean waters, into electricity. While wave forecasts are not as consistent as tidal energy, they can accurately predict conditions weeks in advance. Many wave energy devices operate at the ocean’s surface by harvesting the up-and-down heaving motion of waves.

Ocean Power Technologies, a company based in New Jersey, is developing a tall, slender buoy that houses a linear generator. As the buoy rises and falls with each passing wave, the generator’s piston moves back and forth relative to the stator, generating an alternating current. Pelamis Wave Power of Scotland is developing a snake-like device with cylindrical sections that move relative to one another as waves pass underneath them; this motion moves hydraulic pistons that pump fluid through a hydraulic generator onboard.

There are multiple devices under development that are installed near-shore on the ocean bottom where passing waves cause a side-to-side surge in water and pressure differentials, either or both of which can be captured. Some devices even pump fluid to shore or a remote generation station before generating electricity. One of the biggest challenges for wave energy devices is efficiently translating the somewhat irregular oscillatory motion of ocean waves into consistent power output. From the variety of designs that are currently under development, it is clear that there may not be one optimal approach to harvesting ocean wave energy.

Water is a relatively dense fluid and moving water is a relatively energy-dense source. By comparison, a horizontal wind turbine with a rotor diameter of 15 feet will generate approximately 4 kW at wind speeds of 10 mph while an ocean turbine of the same diameter submersed in water flowing at 10 mph will generate approximately 800 kW. Wave and tidal devices can be installed as stand-alone systems, but similar to solar and wind farms, MHK devices will typically be installed in arrays so that multiple devices share a single power take-off system to reduce the total cost of the power produced.

Common challenges to the technical and economic viability of MHK devices tend to be shared with related technologies, and MHK developers have borrowed construction and deployment techniques used when constructing oil rigs, near-shore transportation infrastructure, and other types of marine buoys to enable their devices to survive for decades in a harsh ocean environment. Lessons and adaptations from these related industries have been also applied to MHK devices to allow them to survive a potential “100-year storm.” In fact, the need to over-engineer devices to withstand hurricane-force winds and massive storm swell is one of the biggest cost challenges for many developers. MHK arrays use power take-off and conditioning systems similar to those used by wind farms, and they rely on underwater transmission cables, which cost around $10MM per mile, to transmit their power to shore and ultimately to their end use or their grid tie-in.

A 2011 study by the Electric Power Research Institute (EPRI) estimates that the waters along the continental shelf surrounding the US present up to 2,640 TWh/yr of MHK energy, where an estimated 1,170 TWh/yr, or enough to satisfy nearly one-third of all US electric consumption. In the face of rising energy costs, climate change, and concerns over the security of our energy future, the tide is ever turning in favor of renewable energy technologies such as MHK. Over the past decade, the waves of funding to develop viable MHK devices have grown. Wave Energy Scotland, a research and development body funded by the Scottish government, announced the availability of up to £7MM in total funding for MHK projects in March of 2015, followed by the US Department of Energy’s announcement of $10.5MM in funding towards the testing and development of MHK devices focused on survivability and reliability in April. The resource is abundant and the basic technology has been in place for decades; the biggest feasibility challenges remaining revolve around cost-effective installation design for installation and survivability.


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