PRIMRE/MRE Basics/Ocean Current Energy

From Open Energy Information

Ocean Current Energy

Strong ocean currents are generated from a combination of temperature, wind, salinity, bathymetry, and the rotation of the earth. The sun acts as the primary driving force, causing winds and temperature differences. Because ocean currents are fairly constant in both speed and flow and carry large amounts of energy, the ocean may provide a variety of suitable locations for deploying energy extraction devices such as turbines.

Technology

Turbines capable of harnessing the kinetic energy of ocean currents may be indistinct from other marine current turbines and function according to the same principles. However, they may be designed or optimized for lower flow speeds relative to currents in tidal channels and may not need to account for reversing flow. A major difference is where the devices may be located – both geographically and in the water column. Strong ocean currents tend to be further offshore than tidal currents, which tend to be found in coastal or inland waters. This leads to deployment in deeper water, where a fixed structure supporting a turbine higher in the water column where current is stronger is impractical. Devices may be suspended from moored surface platforms or attached to buoyant structures tethered to the seabed. Electricity is produced by coupling a generator to the turbine. Power is transmitted back to shore via cable.

Horizonal ocean current turbine

Resource Potential

Suitable areas for the development of marine hydrokinetic energy from ocean currents are relatively few in a global context. Feasible locations are typically associated with the western boundary currents of ocean basins, transporting warm water from the tropics towards the poles. These currents feature fast flows over wide areas that can exceed 3 m/s in some cases [1]. A numerical modeling study identified eight such currents with kinetic power density (power per square meter of cross-sectional area) greater than 500 W/m^2 averaged over three years at a depth of 50 m: the Gulf Stream (S.E. US), Agulhas Current (South Africa), Kuroshio Current (Japan), the Indonesian throughflow, Somali Current, Brazil Current, Madagascar Current, and the East Australia Current [2]. Many of these are visible as colors towards the red end of the colorbar in the image below. Though generally consistent over time, there is variation in resource intensity over seasons and years. Ocean currents can meander or even reverse direction locally.

[1] Bane, John M., et al. "Marine hydrokinetic energy from western boundary currents." Annual Review of Marine Science 9 (2017): 105-123.

[2] VanSwieten, James H., Imke Meyer, and Gabriel M. Alsenas. "Evaluation of HYCOM as a tool for ocean current energy assessment." Proceedings of the 2nd Marine Energy Technology Symposium, April 15–18, Seattle, WA (2014).

Global ocean current visualization, courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio

Technical Readiness

Ocean current turbines are still nascent technologies with no open-ocean prototype deployments. Concepts have advanced to model testing at sub-scale, but many of the technical risk areas have not been addressed, such as long mooring and electric transport cables, deployment and maintenance in a high current, fatigue of the structure and blades, stability during slow current and mitigation of local current reversals. Because the Florida Current has less spatial variability, higher annual average energy flux, and is closer to load near Miami and Ft. Lauderdale, these areas are more likely to see the first deployments of ocean current turbines.

Economics

LCOE estimates of ocean current energy are poorly established because of the nascent state of the technology. NREL’s reference model project estimated that a mature ocean current sector (achieved after multiple deployments) could achieve LCOE values near $0.18/kWh for a hypothetical 50 unit 200 MW capacity project and $0.15/kWh for a 100 unit 400 MW capacity project (Neary et al. 2014). This level of technical maturity is unlikely in the near-term even in the Florida straits. With no experience designing, building, deploying, operating and recovering ocean current turbines, these LCOE estimates have a high degree of uncertainty.