Power from the Florida Current A New Perspective on an Old Vision BY HOWARD P. HANSON , SUSAN H. SKEMP , GABRIEL M. ALSENAS , AND CAMILLE E. COLEY enerating power for the electrical grid from wind chemical inertias can also play roles) provides the op- has become a familiar concept. On large scales, portunity for true base-load power generation that is G “wind farms” with hundreds of megawatt-level sustainable and about as green as can be imagined. turbine systems are being constructed at various breezy One mechanism for oceanic rectification is the locations around the world; on small scales, individuals generation of swell, which is particularly evident on are erecting kilowatt-scale systems on their rooftops the eastern sides of the large ocean basins; the en- to power their homes and to take advantage of “net ergy of this can be captured in a variety of ingenious metering” protocols that let them sell excess power to ways. Another mechanism is the wind-driven ocean their utility companies. However, the wind is fickle: circulation systems in the various basins. We focus even in locations with favorable wind climatologies and here on the western boundary current system of the even with generating systems that can adapt to vary- North Atlantic’s circulation, the Gulf Stream, which ing wind speeds and directions, there are lulls during flows northward at speeds up to about 2 m s-1 just off which power generation all but ceases. the east coast of Florida (e.g., Fig. 1). The proximity Climatologists have long embraced the metaphor of this power-dense resource to the large base-load of the oceans’ acting as the flywheel of the climate demand of southeast Florida’s metropolitan region— system. Because of their large mechanical, chemical, the seventh largest in the United States, according and thermal inertias, the oceans rectify Earth-system to the Bureau of the Census—makes hydrokinetic processes that operate on short time scales (e.g., ba- energy extraction from the Gulf Stream potentially roclinic instability in the atmosphere) by integrating attractive as a component of a diversified renewable short-term variations into longer time scales through energy portfolio for the future. ocean–atmosphere exchange mechanisms and inter- This potential has been recognized for decades, nal circulation processes. A familiar example is the and it is easily understood with a simple scaling ar- observed lag of global temperature increases in recent gument. To first order, the power per unit area swept decades behind the observed increase in atmospheric by a turbine’s rotor (the power density) that can be carbon dioxide concentrations. extracted from a fluid flow depends on the rate at It should be no surprise that the same flywheel which the kinetic energy of the fluid flows past the metaphor applies to describing the oceans’ role in rotor; that is, the power density is proportional to renewable energy. Rectification of the atmosphere’s the product of the fluid’s density and the cube of its variability by the oceans’ inertia (our concern here flow speed. Because, in terms of orders of magnitude, is mostly with mechanical inertia, but thermal and seawater is 1,000 times more dense than air, whereas the Gulf Stream and other vigorous western bound- ary current systems flow at about a tenth the speed AFFILIATIONS: HANSON , SKEMP , ALSENAS , AND COLEY —Center of (brisk) low-level winds, these currents offer about for Ocean Energy Technology, Florida Atlantic University, Boca the same power density as the wind does. This scaling Raton, Florida implies that, if it could be sufficiently waterproofed CORRESPONDING AUTHOR: Howard P. Hanson, Center for and somehow mounted with its rotor in the Gulf Ocean Energy Technology, Florida Atlantic University, 777 Glades Stream, a megawatt wind turbine would deliver its Road, Boca Raton, FL 33431 megawatt quite faithfully, and it would not be subject E-mail: [email protected] to the whims of the wind’s intermittency. DOI:10.1175/2010BAMS3021.1 Of course, in the real world this is only colorful ©2010 American Meteorological Society hyperbole. From simple drag-law considerations, the forces on the rotor blades scale as the square of the AMERICAN METEOROLOGICAL SOCIETY JULY 2010 | 861 Unauthenticated | Downloaded 10/02/21 05:21 AM UTC FIG . 1. Now-classic east–west cross sections of northward current: (a) mean and (b) standard deviation for the period April 1982–July 1984, from Pegasus floats in the Florida Straits at about 27°N (from Leaman et al. 1987). flow speed times the fluid density, so the blades of a mand of summertime air conditioning in southeast waterproofed wind turbine, subject to forces a factor Florida. Overall, the mass flux is broadly consistent of 10 greater than their design, would undoubtedly fail with wind-stress curl calculations of the southward almost immediately. In this regard, it is noteworthy that Sverdrup transport across the open Atlantic to the modern underwater systems have rotors much smaller east, as well as with ongoing ship-of-opportunity data than those of wind systems. Further, for use underwater, from acoustic current profilers. wind-turbine gearing would need to be changed signifi- The average flow is northward throughout the cantly, because the rotation rate would be much lower channel (Fig. 1), but the current itself is far from in the ocean. There is also small-scale variability in the barotropic, having instead a core of high-speed flow current to consider. More on all of this follows. (~ 2 m s -1) about 20 km offshore of Florida and within We begin with a very brief overview of this poten- about 50 m (in a ~300-m water column) of the ocean tial renewable energy resource, and then we review surface. The strongest variations are to the west of the history of its attractiveness for electricity genera- this core, and much of this variability can be traced to tion. Subsequently, we will note challenges to bringing the existence of coastline eddies and shelf waves. The these long-held visions to fruition. early years of the Subtropical Atlantic Climate Studies program produced a large number of contributions The Florida Current. Our focus is on the documenting the dynamics and variability of this reach of the Gulf Stream System flowing northward flow and its mesoscale features. between the Florida Peninsula and the Bahamas Because the power density scales as the cube of Archipelago: the Florida Current. This channel is the current velocity, the baroclinic nature of the av- well characterized from a physical oceanographic erage flow shown in Fig. 1a implies that the resource perspective, especially on larger scales, both as to its available for power generation depends strongly on volumetric flow and its distribution and variability. the current speed at which generating systems can The total flow, inferred from voltage measure- operate. Figure 2 shows the power available in the ments on undersea communications cables between average flow (meaning the power densities implied Florida and the Bahamas, varies around 32 Sv (1 Sv by Fig. 1a times the cross-sectional areas occupied by ≡ 106 m3 s-1) or so. From the perspective of power the current at its various speeds) as a function of the generation, it is of interest that these measurements minimum operating speed—the cut-in speed—of a reveal, amid variations on a wide range of frequen- generating system. The total power in this cross sec- cies, an annual cycle with a weak maximum in July, tion, about 20 GW, is about 20% less than previously which is in phase with the electrical base-load de- discussed totals (see below). What is important to 862 | JULY 2010 Unauthenticated | Downloaded 10/02/21 05:21 AM UTC note about Fig. 2 is the additional power that becomes available as generators are improved to operate at lower and lower current speeds. Lowering the cut-in speed from 1.5 to 1 m s-1, for example, would nearly double the power available. Despite decades of progress in understanding the dynamics of the Florida Current, capturing its power, although straightforward conceptually, will require ad- ditional investigations to understand its variability on yet smaller scales. For example, given the blade loading that is a factor-of-10 greater than in the atmosphere noted previously, finescale vertical and meridional shear needs to be well characterized to provide design criteria for underwater turbine systems. Deployments FIG . 2. Power available in the Florida Current, from of commercial-scale systems (as well as arrays of sys- Fig. 1a, as a function of the minimum operating cur- tems) will need to be optimized with respect to both rent speed (cut-in speed) of generating systems. the high-speed core of the flow and the downstream These results depend only on the current data (i.e., the properties of the flow) and not on system efficien- wakes of individual generating units. cies, which reduce power extracted by at least 40% of these values. A Brief HistorY. Although the potential of using the Florida Current to generate power has been recognized for more than a half century, it and convened a workshop that also included ocean, took the sharp hike in the price of oil associated power-system, and heavy-equipment engineers to with the embargo imposed by the Organization of discuss the feasibility of this potential source of Petroleum Exporting Countries (OPEC) in 1973 to power. The participants concluded that there were motivate serious investigation. Early the next year, no insurmountable engineering obstacles to tapping south Florida philanthropist John D. MacArthur the kinetic energy of the Florida Current, and they (whose legacy includes the foundation that offers the agreed on a position statement and a plan forward, MacArthur “genius grants”) brought together senior which was introduced with the following: oceanographers from south Florida and elsewhere— including such luminaries as Harris B.