Marine Hydrokinetic Energy in the Gulf Stream Off North Carolina: An Assessment Using Observations and Ocean Circulation Models
There has been global interest in renewable energy for meeting energy demands, and as these demands increase, it will become of greater importance to utilize low-carbon energy sources to mitigate anthropogenic impact on the environment. Onshore hydropower is responsible for half of the electricity generated by a renewable source in the USA. In the ocean, marine hydrokinetic (MHK) energy in western boundary currents (WBCs) can be considered for electricity generation by submarine turbines. WBCs are a continuous and sustainable source of energy that could be transmitted to shore to support coastal communities in future years. The Gulf Stream is the WBC of the North Atlantic subtropical gyre, and it flows for part of its course along the upper continental slope off the southeastern USA. This large-scale current has maximum flow speeds exceeding 2 m s−1, and this together with its proximity to the coastline distinguishes it as a potential source of MHK energy. Using current data from a moored acoustic Doppler current profiler (ADCP) and a regional ocean circulation model, MHK power densities offshore of North Carolina were found to average 798 W m−2 for the ADCP and 641 W m−2 for the model during a nine-month period at a potential turbine site, a difference of about 20%. The model was shown to have similar current speeds to the ADCP for slowly varying currents (fluctuations of weeks to months due to Gulf Stream path shifts), and lower speeds for higher frequency current variations (fluctuations of several days to a couple of weeks due to wavelike Gulf Stream meanders). This article considers the Gulf Stream as a prospective renewable energy source and assesses the power density of this WBC at multiple locations offshore of North Carolina. Understanding the Stream’s power density character, including its spatial and temporal variations along the North Carolina coast, is essential in considering the Gulf Stream as a future alternative energy resource.
KeywordsPower density Alternative energy Ocean turbine Gulf Stream Cape Hatteras
Marine hydrokinetic (MHK) energy, the kinetic energy in moving seawater, is one potential renewable energy source for electricity generation. Almost all MHK energy is in ocean surface waves, tidal motions, and large-scale (non-tidal) currents. In this chapter, we consider the potential for harvesting MHK energy from large-scale ocean currents to generate electricity using moored submarine turbines. A worldwide survey of ocean currents using Hybrid Coordinate Ocean Model (HYCOM) simulations shows that the western boundary currents (WBCs) in the wind-driven subtropical gyres are essentially the only large-scale currents with flow speeds sufficiently fast to be considered for commercial installation of electricity-generating turbines (VanZwieten et al. 2013; Bane et al. 2017). These WBCs include the Gulf Stream, Kuroshio Current, North Brazil Current, Agulhas Current, Somali Current, and East Australian Current (Imawaki et al. 2013).
To examine the power (kinetic energy per unit time) in the Gulf Stream flow off North Carolina, we use current observations from moored and boat-mounted acoustic Doppler current profilers (ADCPs) and ocean surface-current radars, and simulations of ocean currents by a regional ocean circulation model. This model-data approach gives estimates of how much power is available for the resource in this region, delineates how the power varies in space and time, and will help determine which areas off North Carolina are optimal for future deployments of MHK devices. This approach also provides for model-observation comparisons of current speed and power, thereby revealing the confidence that may be placed in model predictions of ocean power at times and at locations where observations are unavailable. In the following discussion, we present the characteristics of Gulf Stream power. The engineering, environmental, legal, financial, and other use considerations that will likely affect the development of commercial MHK energy harvesting facilities are not our focus here and thus are not discussed. Details about these subjects may be found in other publications, for example, Boehlert and Gill (2010), Neary et al. (2014), Brown et al. (2015), and Li et al. (2017). In the next section, we describe pertinent aspects of MHK energy, and in section “The Gulf Stream”, we give a description of the Gulf Stream off the southeastern US coast. Section “Assessing MHK Energy and Power Density” presents the available data sets to determine MHK energy, and section “MHK Energy from the Gulf Stream Along North Carolina” discusses the speed and power at particular locations off the coast of North Carolina, including their temporal and spatial variations.
MHK Energy and Power Density
Harvesting the Gulf Stream’s MHK energy with moored submarine turbines is a type of “in-stream” power generation, meaning it does not use water impoundment like a hydroelectric power plant that stores water behind a dam (VanZwieten et al. 2014). Kinetic energy flux in an oceanic current is the amount of MHK energy that flows through a unit cross-sectional area oriented perpendicular to the current direction per unit time. Kinetic energy flux is equivalent to the power density, P (in W m−2), of the current, and it is given by
The Gulf Stream
The Gulf Stream, like most WBCs, is a narrow (~100 km in width), deep (~1 km in vertical extent) jetlike flow that has a maximum surface-current speed often in excess of 2 m s−1 (Imawaki et al. 2013). Current speed decreases in any direction away from this surface maximum. These characteristics are shown schematically in Fig. 3. Off Cape Hatteras, the inshore edge of the Gulf Stream is approximately 40 km (~25 miles) from shore, where surface-current speeds along the edge are roughly 0.2 m s−1. To reach the core, where the fastest speeds can be found, is an additional 30–40 km. This is based on the cross-current extent of the Stream, and the fact that its cyclonic shear zone is roughly the inner third of the Gulf Stream and the anticyclonic shear zone is the other two-thirds, thus leading to an asymmetry in the current’s jetlike shape. Cape Hatteras is the closest location to the Gulf Stream along the US east coast north of the Florida Straits where it is only several km from Fort Lauderdale. The volume transport in the Straits is approximately 30 Sv (Barringer and Larsen 2001), and the transport off Cape Hatteras is typically about 90 Sv (Halkin and Rossby 1985) (1 Sv = 106 m3 s−1). Another apparent feature in Figs. 1 and 3 is the wavelike, meandering path of the Stream (Webster 1961). This meandering pattern propagates poleward, and each meander wave has an alongshore wavelength between 100 km and a few hundred km (Tracey and Watts 1986). At a fixed location, it will take from around 3 days to as many as 20 days for one meander wavelength to propagate past (the “meander period”) (Bane et al. 1981). This means that the Gulf Stream jet will move closer to shore for about half of the meander period, then move offshore again for the next half-period. This lateral meandering motion of the Stream’s narrow jet will cause the current speed at a fixed turbine location to increase and decrease with a meander’s period. Another type of motion that the path of the Gulf Stream undergoes from time to time is a longer term lateral shift of the jet’s path (Bane and Dewar 1988; Quattrocchi et al. 2012). This can move the Stream laterally relative to a moored turbine site, either decreasing or increasing the power generated by that turbine for weeks to months (Bane et al. 2017). In section “MHK Energy from the Gulf Stream Along North Carolina”, we show observed and modeled power variations at a fixed turbine site that are caused by meanders and path shifts. These Gulf Stream motions will be the dominant causes for power generation variations over time.
Assessing MHK Energy and Power Density
Cape Hatteras (CH) transect stations
Cape Lookout (CL) transect stations
Onslow Bay (OB) transect stations
The Gulf Stream power assessment presented here uses data sets from two different sources. One is the Regional Ocean Model System (ROMS) that has been run for an area that encompasses the Cape Hatteras region. The other is a 150 kHz ADCP that was moored on the ocean floor on the upper continental slope off Cape Hatteras. Each data source is described below.
Simulated ocean currents were generated by a regional ocean circulation model implemented for the Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB), hereafter MABSAB, covering the area between 81.8–69.8° W and 28.4–41.8° N (Bane et al. 2017). The model is based on the ROMS (Shchepetkin and McWilliams 2005), a free-surface, terrain-following, primitive equations ocean model in widespread use for estuarine, coastal, and basin-scale applications. The horizontal resolution of this model is 2 km, sufficient to resolve the Gulf Stream’s variability. Model bathymetry was interpolated from the National Geophysical Data Center (NGDC) 2-Minute Gridded Global Relief Data. Vertically, there are 36 terrain-following layers that have high resolution near the surface and bottom in order to better resolve ocean boundary layers. Momentum advection equations were solved using a third-order upstream bias scheme for three-dimensional (3D) velocity and a fourth-order centered scheme for two-dimensional (2-D) transport, whereas tracer (temperature and salinity) advections were solved with a third-order upstream scheme in the horizontal direction and a fourth-order centered scheme in the vertical direction. The horizontal mixing for both the momentum and tracer used the harmonic formulation with 100 and 20 m2 s−1 as the momentum and tracer mixing coefficients, respectively. Turbulent mixing for both momentum and tracers was computed using the Mellor/Yamada Level-2.5 closure scheme (Mellor and Yamada 1982). For open boundary conditions, the model was nested inside the 1/12 degree global data assimilative HYCOM/Navy Coupled Ocean Data Assimilation (Chassignet et al. 2007; Gong et al. 2015) output superimposed with tidal forcing of 6 major tidal constitutes (M2, S2, N2, O1, K1, and Q1) derived from an Advanced Circulation (ADCIRC) tidal model (Luettich et al. 1991) simulation of the western Atlantic. The MABSAB hindcast ran from January 1, 2009 through December 31, 2014, and it did not incorporate the observations from the ADCP into the model computations of currents. Hourly model output was used for this study.
Moored ADCP Measurements
The moored 150 kHz ADCP location was selected based on its position within a region where Gulf Stream meanders have relatively low lateral amplitudes (Miller 1994). Higher power density levels at a fixed location tend to result from less lateral movements of the Stream. The ADCP was deployed for nine months, from August 1, 2013 through May 29, 2014. It was deployed in a water depth of 228 m at 35.1° N and 75.1° W. The instrument measures currents from 9 m above the bottom to 30 m below the surface with 4 m vertical resolution every 10 min. Measurements within 30 m of the surface are unreliable because of overwhelming acoustic reflection from the air–sea interface. The ADCP pod also contained a conductivity, temperature, and depth sensor.
In addition to the moored ADCP measurements, hourly high-frequency (HF) radar surface-current measurements were collected, and sporadic (as weather allowed) vessel-mounted ADCP current measurements were made with downward-looking ADCPs along the CH transect. As these data get processed and analyzed, future studies will be able to make model comparisons with them in order to improve the spatial coverage for power density computations. As confidence is gained in the quality of the radar surface currents and their ability to accurately determine the variability in Stream location, they will serve as a valuable tool to infer MHK energy variability in this region. Because the structure of the Gulf Stream jet is remarkably consistent (Halkin and Rossby 1985), surface currents alone may provide accurate inferences about the MHK resource beneath them.
HF Radar Surface Currents
ADCP Vessel Transects
MHK Energy from the Gulf Stream Along North Carolina
The average model current velocities from 2013 at 75-m depth are displayed in Fig. 4. The average Gulf Stream jet is apparent in this figure; the inshore edge of the average jet is roughly along the 100-m isobath, while the offshore edge of the Stream is close to the 4000-m isobath. Near the center of each of the CH, CL, and OB transects, average 75 m velocities are slightly greater than 1 m s−1. Average velocities diminish toward either edge of each transect.
Current Speed and Power Density Time Series
Year-to-Year Power Variations
The maximum power density reached at the Cape Hatteras transect (Fig. 11) during these 5 years is 1,169 W m−2 at Station 13 (yellow bar) in 2013. In 2009, there is small lateral variation in power density, with greater lateral variation in the subsequent 4 years. The greatest annual-average power density is at Station 13 for most of the years. In 2011–2013, the power density peaks are greater and more pronounced, while stations not within the peaks experienced much less power and greater horizontal current shear. Comparing our ADCP location in 2010 to the other stations in the transect, we note that the ADCP station lies along the inshore flank of the Gulf Stream, within its cyclonic shear zone, and greater power is found at more offshore stations. Again in 2012, the Gulf Stream had shifted onshore as indicated by the skewed power envelope of the annual averages, the result being much higher power at the ADCP station.
The same analysis of the CL and OB transects is shown in Figs. 12 and 13. The maximum power density at the CL transect is 1,105 W m−2 at Station 5 in 2012. The distribution is skewed more landward. The power peak for each year fluctuates between Stations 5 and 6—the yellow and green bars. These stations both have a 5-year average of about 875 W m−2, and lie over depths of approximately 500 and 2800 m. This is important to note considering the costs of cabling and hardware required to reach greater power availability in the Stream at locations farther offshore. A depth of 2,800 m presents significant challenges for mooring turbines to capture power and then transporting the power back to shore over longer distances.
On the OB transect, the maximum power is 1,008 W m−2 at Station 4 in 2013. The annual OB transect power is more uniform and consistent throughout the 5 years. The lateral shear is apparent, and the greatest available power is at Station 4. The water depth at Station 4 is 654 m, presenting potential engineering challenges. In 4 of the 5 years, Station 3 has the second greatest power with a 5-year average of about 400 W m−2 and a water depth of 334 m. This is much shallower than the depths of Stations 5 and 6 in the CL transect, and based on its consistent power density over these 5 years, this suggests it would be a viable turbine location.
In this chapter, we have presented an observation-and model-based assessment of MHK energy in the Gulf Stream off the coast of North Carolina, as well as demonstrated the additional available observations from land-based HF radars and shipboard ADCPs that will be included in future studies. Data from a moored 150 kHz ADCP and a ROMS model were analyzed. At a depth of 75 m at the ADCP location, the ADCP data and model data yield the same 9-month speed average of 0.94 m s−1, and a fairly uniform 45° current direction. The power density time series shows that the ADCP observations occasionally reach 4,500 W m−2 with an average of nearly 800 W m−2. Recent offshore wind studies in North Carolina have demonstrated that winds produce an average hub-height power density of 600–800 W m−2, close to the ADCP power density average of 798 W m−2. This suggests that Gulf Stream subsurface turbines may become viable as turbine and mooring technology develop.
Kabir et al. (2015) used HYCOM and a grid bounded by 34.9° N to 35.2°N and 74.9–74.5°W to find that more than 50% of the days from November 2003 through December 2012 at 20 m depth exhibit a power density of 500 W m−2 or greater. This coincides with the offshore end of the CH transect near Station 14. From Fig. 10 it can be seen that approximately 48% of the time both the model and ADCP site yield power densities greater than or equal to 500 W m−2. Figure 10 and Kabir et al. (2015) use different models to conclude that nearly the same power densities of 500 W m−2 are available for the same percentage of the time. The small difference between the two percentages is likely due to the analyzed model grid location being farther offshore than the ADCP site.
Yang et al. (2014) define the recoverable resource as the amount that can be extracted within current technological limits. Given the present uncertainty in underwater turbine technology and engineering capabilities, it is difficult to provide with confidence a determination of the recoverable energy resource available from the Gulf Stream. However, here, we have provided a view into the energy resource available and demonstrated its spatial and temporal variations along the North Carolina continental slope. Quantifying the Gulf Stream power characteristics is an essential step for the development of ocean current energy in this region. Accurate approximations of the energy resource and ocean environment will lay the foundation for extracting Gulf Stream energy in the future. As society continues to exploit fossil fuels and non-renewable resources at great cost to the environment, renewable energy increasingly becomes a more attractive long-term alternative. In helping to reach the goal of 80% of US electricity generation from renewable energy resources by 2050 (National Renewable Energy Laboratory 2012), ocean energy can be an essential component of a renewable energy portfolio that takes advantage of continuing technological advances in resource characterization, turbine design, mooring technology, and ocean engineering.
We gratefully acknowledge funding from the North Carolina Renewable Ocean Energy Program for continued support of MHK energy research off North Carolina. We also appreciate Joe DeCarolis, Billy Edge, Mo Gabr, Harvey Seim, and Jim VanZwieten for their suggestions. Thank you to Patterson Taylor for supplying Fig. 6 and to Zhaoqing Yang and Kevin Haas for their comments that helped improve this chapter.
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