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Marine Renewable Energy: A Florida Reality Check
Uncertainties and Challenges Of Converting Florida Current Energy Into Renewable Electricity

Dr. Howard P. Hanson
Scientific Director
Southeast National Marine Renewable Energy Center
Florida Atlantic University
Boca Raton, Florida

Economics, national security and environmental stewardship all call for the development of reliable sources of renewable energy for the future. Because such resources are inherently regional in nature, it is to be expected that a diversified portfolio of renewable energy sources will emerge, and in coastal regions, various modes of marine renewable energy (MRE) have strong roles to play in such a portfolio.

Rendition of the SNMREC experimental turbine, a typical axial-flow configuration with stabilizing buoyancy pods, in operation. This design has a rotor diameter of three meters and is rated at 20 kilowatts.

Broadly speaking, MRE takes two main forms, marine hydrokinetic (MHK) energy—the energy in the motion of waves, tides and ocean currents—and marine thermal energy, the potential energy stored in the ocean's thermocline, which can be recovered using ocean thermal energy conversion (OTEC).

To accelerate the development of regional MRE resources, the U.S. Department of Energy has created several National Marine Renewable Energy Centers (NMREC). The Hawai`i National Marine Renewable Energy Center (at the University of Hawai`i) and the Northwest National Marine Renewable Energy Center (a joint operation of Oregon State University and the University of Washington) focus on wave and tidal power as well as on OTEC. At the Southeast National Marine Renewable Energy Center (SNMREC), open-ocean currents and a local OTEC resource are the focus of activity. Together the three centers, working with government agencies and industry, are developing standards and capabilities for testing industrial prototypes as well as providing experimental platforms to address environmental issues.

The concept of MRE is hardly new. For example, during the oil embargo of the 1970s there was significant interest in the use of the Florida Current—the reach of the Gulf Stream offshore southeast Florida—to generate electricity, including an interesting proposal that was published in these pages (Venezia and Holt, Sea Technology September 1995). With the present-day emphasis on renewable energy and recent advances in underwater technology, many associated with the oil and gas industry, MRE development is once again proceeding apace. Despite these advances and similar progress in understanding the oceanography of the Florida Straits, however, challenges remain. This article provides an overview and scale analysis to address some critical questions related to developing MHK energy in the Florida Current.

Florida's MHK Energy Resource
The power density of fluid flow, or the kinetic energy flux per unit area, is proportional to the product of the fluid's density and its velocity cubed. Because seawater is about 850 times more dense than air, a two meter per second oceanic flow, typical of the fastest parts of the Florida Current, has the power density of a gale-force wind. (It is worth emphasizing that the forces involved, scaling as the square of the flow speed, are a factor of 10 higher in the water.)

The core of the Florida Current—its high-speed region—is a broad river of northward-flowing water about 20 kilometers wide and 200 meters deep centered, more or less, 25 kilometers offshore southeast Florida. On average, the shear zones, across which the higher-speed water slows to nearly a stop, are broad transition regions, but at any one time shears (especially vertical shear) can be much higher and can occur on scales comparable to the sizes of larger equipment.

Earnings potential over a 20-year operational period for a 20-meter turbine wall, assuming a 90 percent duty cycle, for a variety of wholesale energy rates.

Oceanographers have studied the Florida Current for decades, and the current structure in an east-west cross section measured using a variety of instruments has proven to be remarkably consistent. Using a cross section of the current, it is possible to calculate the total power of the flow. Of relevance to MHK energy recovery is that power is a function of the cut-in speed of a turbine system, the flow speed at which a rotor will turn and produce power against internal friction and the generator's load. The cut-in speed is therefore a measure of system capability relative to a particular flow field. Plotting the cumulative power of the flow as a function of cut-in speed provides an estimate of how much power an array of devices of a given capability can be expected to create.

Of course, it is impossible to fill the entire region of flow at and above a given cut-in speed with rotors (impossible for geometric reasons and also impractical, given the areas involved). Furthermore, no system is 100 percent efficient. Indeed, Betz's law shows that even the most efficient turbine can recover only about 60 percent of a flow's kinetic energy flux, and real-world systems do not achieve even that.

Suppose, then, it is possible to place an array, or 'wall,' of traditional axial-flow systems (the typical wind-turbine configuration) across a Florida Current cross section, with the turbine rotors in the wall occupying half of the total area at and above the relevant cut-in speed. If the net efficiency of those systems is 40 percent—i.e., two-thirds of the maximum allowed by Betz's law—then the power that could be generated by that half-coverage wall of systems would be 20 percent of the total power of the flow at and above that cut-in speed.

This, combined with data from the classic current measurements carried out by scientists at the University of Miami, suggests that turbines with a 1.5 meter per second cut-in speed could produce a maximum extractable power from a single cross section of about 1.6 gigawatts. For comparison, it is worth noting that the Turkey Point nuclear power plant near Homestead, Florida, is rated for 1.7 gigawatts. It also should be emphasized however that the assumptions used here are quite optimistic—hence the usage of 'maximum' extractable power.

This single-section analysis begs the question of whether it is feasible to deploy such walls of turbines across more than one cross section. How quickly the disturbed flow can recover its original structure (or even whether it does at all) is unknown. Numerical simulations are in progress to investigate this issue and to provide an estimate of how far downstream additional cross-section deployments might be possible. Different array designs are also under consideration.

Now, this simple analysis uses averaged data, while the real ocean varies on a range of temporal and spatial scales. Numerical simulations can be used to estimate how the extractable power varies—an important factor in system design. For example, results from the Hybrid Coordinate Ocean Model show that the average maximum extractable power for this hypothetical wall of turbines can vary from nearly zero to more than four gigawatts, a factor of 2.5 higher than the 1.6-gigawatt average. Because the forces in these high-power conditions would be some 25 times the average forces, designing systems for average conditions would severely underestimate the loads on the equipment associated with peak flows. Furthermore, the variations on smaller scales (i.e., the turbulence) are as yet unknown and will need to be taken into account.

A Question of Economics
Ultimately it will be necessary for new MRE technologies to succeed in the marketplace. What, then, are the economic prospects for MHK energy development in the Florida Current?

In building relationships with private-sector technology developers, SNMREC has been in contact with more than half a dozen larger organizations (along with a greater number of highly enthusiastic but smaller organizations) that are designing MHK equipment for Florida Current deployment. The variety of the equipment and deployment strategies makes it impossible to perform an across-the-board economic analysis.

But the reverse approach is possible by posing this question: What can our hypothetical wall of turbines earn? For an answer, it is useful to introduce an example system into the calculation; indeed, several examples will be illustrative.

An area/power-weighted averaged effective velocity for the 1.5 meter per second cut-in-speed wall of systems discussed previously is about 1.7 meters per second. This means that, on average, it is possible to calculate the power that can be generated by turbines of a given size and, therefore, the number of such turbines required to reach the 1.6 gigawatt total noted earlier. Under these conditions, 20-meter-diameter axial-flow turbines with 40 percent net efficiency will generate about 305 kilowatts each, on average. To produce the 1.6-gigawatt theoretical maximum, then, would require about 5,245 units.

The number of required units, and therefore the per-unit revenue, scales linearly with the swept area of the rotor. Thus (at $0.04 per kilowatt-hour) the 20-year revenue of a wall of 30-meter systems (2,330 units) would be $4.34 million per unit and, for 40-meter systems (1,260 units), $8.04 million per unit. Because power also scales linearly with the system efficiency (40 percent in the example here), increasing that efficiency would increase both total and per-unit revenues accordingly.

Increasing system capability, however, is another matter. As more capable systems (with lower cut-in speeds) are deployed, the cross-sectional area and total power available both increase because additional flow can be tapped. This implies an increase in the number of units required to fill the larger area. But because the effective (weighted-average) flow velocity is decreased, the average per-unit generation decreases. Consequently, the number of units and the per-unit revenue do not scale linearly with capability. For example, a wall of one-meter-per-second systems would increase the maximum extractable power to about three gigawatts, but to generate this amount with the 20-meter example systems would require some 16,900 units, each of which would earn only about $1.12 million over 20 years at the $0.04 per kilowatt-hour rate.

How these numbers compare to the costs of manufacturing, installation, operations and maintenance, and financing is unknown, as are economies of scale, such as the size of the array, the required underwater grid, cabling to shore, etc. In addition, the market value of wholesale power will undoubtedly increase in the future, but the rate is highly uncertain due to the politics of carbon taxes and to future inflation. Nonetheless, these figures provide a useful foundation for estimates of economic feasibility of deployments.

Environmental Trade-Offs
The late Robert Heinlein, a giant among science fiction writers, contributed to the popularization of the concept of 'TANSTAAFL,' his acronym for 'there ain't no such thing as a free lunch.' That applies in every way to renewable energy development generally and to MRE development in particular. Bird mortality and viewscape issues are examples of the prices to be paid for the development of wind power, although design refinements are helping to mitigate the former and offshore wind deployments may obviate the latter. At this stage, environmental concerns about MRE deployments seem even more daunting, as they extend from the bottom of the ocean to above the sea surface.

All three NMRECs are confronting their various sets of these concerns and have a variety of issues in common, including disturbances to benthic habitats associated with structures on the seabed; the potential for alterations of behavior and even mortality of pelagic species (endangered mammals and turtles are of particular concern); possibly adverse changes to underwater acoustic and electromagnetic environments; and detrimental, in some fashion, modification to the flow and/or wave field of the ocean itself. None of these issues is fully understood, even in particular cases in specific locations, and one role that the NMRECs are playing is to develop a better understanding for their respective regions and MRE modalities. By providing the capability to experiment on small scales and monitor both ecosystem behavior and oceanic responses, estimates of the environmental effects of large-scale deployments can be provided.

The fundamental question concerns the extent to which the trade-offs between whatever disturbances these deployments might cause can be balanced against the environmental advantages that they will provide. For example: MHK anchor systems in the Florida Straits could disturb small patches of deepwater coral beds that have been found there. But electricity generated from such MRE systems could reduce carbon dioxide emissions from fossil-fuel power plants, thereby helping to reduce the rate of oceanic acidification, a process that destroys corals everywhere. Where does the balance lie?

In virtually every case, trade-offs of this nature will need to be considered carefully, and it is these trade-offs that are perhaps the greatest challenge for MRE implementation in the future.

Funding for SNMREC is provided by the State of Florida and the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy through the Wind and Water Power Program.

The author thanks his colleagues at the other national centers, the Hawai`i NMREC and Northwest NMREC, for their encouragement and acknowledges Dr. Bob Weisberg's inspirational ideas for this contribution.

Howard P. Hanson earned his Ph.D. at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. Before joining Florida Atlantic University as professor of geosciences and associate vice president for research, he held research and managerial roles at the University of Colorado at Boulder and the Los Alamos National Laboratory.

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