Marine Hydrokinetic Energy from Western Boundary Currents

The kinetic energy in ocean currents, or marine hydrokinetic (MHK) energy, is a renewable energy resource that can help meet global energy requirements. An ocean circulation model-based census shows that subtropical surface western boundary currents (WBCs) are the only nearshore, large-scale currents swift enough to drive large, electricity-generating ocean turbines envisioned for future use. We review several WBCs in the context of kinetic energy extraction. The power density in the Gulf Stream off North Carolina at times reaches several thousand watts per square meter at 75 m below the surface, and the annual average power is approximately 500 to 1,000 Watts per square meter - a level that is comparable to wind power density over the nearby continental shelf in an area suitable for marine wind turbine installation. Significant fluctuations in MHK power density occur at a fixed site, with periods of 3 to 20 days (Gulf Stream meanders) and weeks to months (Gulf Stream path shifts). Interannual variations in annual-average power occur because of year-to-year changes in these WBC motions. No large-scale turbines presently exist, and the road to establishing MHK facilities in WBCs will encounter challenges that are similar in many aspects to those associated with the development of offshore wind power. Even so, important differences between wind and ocean current power need to be appreciated. To this end, we mention the two urban legends of ocean current power: "Unlike the wind, the Gulf Stream is always flowing" and "Ocean water is so much denser than air ( 800 times) that slower ocean currents have much more power than faster winds." We propose these new urban truths: "The Gulf Stream is always flowing, but it is not always in the same place" and "The greater density of ocean water as compared with air compensates for speed differences ( 1 m/s for currents and 10 m/s for winds), resulting in comparable power densities for both currents and winds."