Marine Energy: How Much Development Potential Is There?

Of all the large natural resources available for generating electricity in the U.S., ocean energy may be one of the last investigated for its potential. To fill this knowledge gap, we performed an assessment of the available and extractable ocean resources. When discussing marine resources, we refer to two forms: wave and kinetic stream.

Although there are other marine energy sources— the thermal energy resulting from the large temperature differences between deep and cold ocean waters and sun-warmed surface waters, the chemical energy in ocean salinity gradients, and marine biomass— we only discuss wave and kinetic stream energy.

There are two forms of tidal energy: potential (i.e., harnessing the potential energy changes associated with the tidal rise and fall of sea level) and kinetic (i.e., harnessing the kinetic energy associated with the motion of the tidal stream). In this article, we only discuss the kinetic form, which can be tapped without building barrages or dams. There are three types of kinetic energy from water: tidal, ocean current, and river streams.

The U.S. has significant wave and kinetic stream energy resources. These are renewable energy resources that can be converted to electricity without greenhouse gas emissions. The technology to convert these resources to electricity, albeit in its infancy, has been deployed in demonstration projects.1 Commercial projects are expected in the next five to ten years. Given proper care in design, deployment, operation, and maintenance, ocean wave and kinetic stream energy could be two of the most environmentally benign electricity generation technologies yet developed.

Understanding the power of marine energy

 To understand the generating potential of a given site, there are two relationships that are important to know. The first is the factors that affect the power contained in a wave. The power fluctuation of a wave is a factor of two variables: the significant wave height (in meters squared) and the mean wave period (in seconds). The annual average wave power fluctuation in deep water that is required for a site to have commercial interest is about 20 to 50 kilowatts per meter.

The second is the kinetic power density of a stream of water. This relationship depends on both the density of the seawater (in kilograms per cubic meter) and the instantaneous speed or velocityof the stream (in meters per second). Kinetic power density varies considerably over a tidal cycle and can vary with depth. To make a relevant comparison
between sites, values for kinetic power density usually are averaged over the year and over total water depth. Annual depth-averaged power densities for sites with commercial interest in the U.S. are about 2 to 5 kilowatts per meter squared.

Putting the numbers into context

Before we quantify the electricity generating potential of the ocean, it is important to understand the backdrop of current annual electricity generation in the U.S. In 2007, about 4.16 million gigawatt-hours (GWh) of electricity was generated (which equates to an average
capacity of 475 gigawatts). Of this, 49 percent came from coal, 23 percent from natural gas/oil, 19 percent from nuclear, 6 percent from conventional hydroelectricity, and 3 percent from other sources (which includes non-hydro renewable energy).2

Wave energy

In 2003, EPRI estimated the U.S. wave energy resource.3 This assessment was performed using as much as 20 years of measurements of wave height and period (the time interval between the arrival of consecutive wave crests at a stationary point). This data was gathered from buoys deployed by the National Oceanic and Atmospheric Administration (NOAA) and
the Scripps Institution of Oceanography. As a result of this assessment, EPRI determined that the available annual wave energy resource in the U.S. is about 2.1 million GWh (for all state coastlines with an average annual wave power flux greater than 10 kilowatts per meter). This energy is divided as follows: — Alaska (Pacific coastline only): 1.25 million GWh; — Northern California, Oregon, and Washington: 440,000 GWh; — Hawaii and Midway Islands (northern border of the exclusive economic zone): 330,000 GWh; and — New England and mid-Atlantic states: 100,000 GWh.

Although the available annual wave energy resource is about 2.1 million GWh, the amount of this energy that can be extracted is unknown. There are uncertainties with regard to societal acceptance of wave energy conversion devices, including the number that could be
deployed and their spacing, conflicting uses of the ocean areas where these devices
would be deployed, and effects of the devices on fish and other animals. To make a preliminary estimate of the extractable wave energy resource in the U.S., we assumed that society would allow us to extract 15 percent of the total available resource. We also assumed a “wave-to-wire” conversion efficiency of 90 percent, based on technical studies
and data from developers, and a plant availability of 90 percent. The annual electricity that would be produced using these assumptions is about 255,000 GWh. This equates to an average annual capacity of about 30,000 MW. Given a typical capacity factor of 33 percent
(derived from EPRI feasibility studies), the rated power is about 90,000 MW. This amount of energy is comparable to the total energy generation from all conventional hydroelectricity.

Tidal stream energy

Kinetic energy resources in the tidal stream are not as well understood as wave energy resources in the U.S. Sites with high tidal kinetic energy typically are located in narrow passageways between the ocean and large estuaries or bays. The total in-stream resource for a particular site is the product of the kinetic power density and the cross-sectional
area of the channel. The kinetic power density varies considerably as you move vertically from the water surface to the channel bottom, as well as across the channel width.

To make an initial estimate of the available energy through a channel transect, you may use single-point current predictions and bathymetric data from NOAA. However, application of this data generally requires extrapolation of stream speeds vertically and horizontally from the reference point. We offer a methodology for estimating the available tidal resource for a single transect.4

EPRI has studied many potential U.S. tidal stream sites. The annual tidal stream resource at all the sites EPRI has evaluated to date is about 115,000 GWh. Of this, 109,000 GWh is in Alaska and only 6,000 GWh at sites in the continental U.S. In Alaska, sites with high power density and large size exist in the southeast, Cook Inlet, and the Aleutian Islands. In the continental U.S., tidal kinetic energy may be an important resource in Maine, New York, San Francisco, and Washington’s Puget Sound.

The 115,000 GWh estimate excludes sites with annual depth-averaged power densities less than 1 kilowatt per meter squared. If the technology to convert in-stream energy were to become economical at power densities less than 1 kilowatt per meter squared, then the
available resource, particularly in the lower 48 states, could be much greater. Therefore, these resource estimates should be considered the lower boundary of the potential because all the possible U.S. tidal sites have not been evaluated.

The amount of the available tidal stream energy resource that can be extracted is not known. Some uncertainties with regard to this resource include the extent to which society will allow this resource to be developed, the physical effects of the technology with regard
to slowing the velocity of the water and decreasing the magnitude of the tides, and the ecological and environmental effects on fish and other animals.

In addition, the kinetic resource across a particular transect at a site is a poor predictor of both the maximum possible level of extraction for that site5 and the environmental effects of extracting kinetic energy.6 One could determine the number of turbines that could be sited within a constrained channel if the maximum packing fraction for the turbines was known. Packing fraction is equal to the turbine area divided by the total cross sectional area of the channel.

The number of turbines that could be installed at the site also would depend on the limitations of seabed space within the high-velocity transects and the requirement to maintain adequate navigation clearance. In addition, the ecological implications of changing the tidal regime by extracting energy from the flow could limit the energy that could be extracted from a site. Finally, there is a self-limiting point at which it will not be economical to add additional turbines to an array, due to reductions in kinetic energy as a result of the extraction. (There is also insufficient understanding to predict how extracting kinetic energy at one site would affect the availability of kinetic energy at another site within the same estuary or bay.) It is unclear whether the packing fraction, available space, or social and environmental pressures will pose the most stringent limits on resource extraction.

Our preliminary assessment of tidal stream energy extraction assumes a conservative
extraction of 15 percent of the total available tidal kinetic resource, a “water-to-wire” efficiency of 90 percent, and plant availability of 90 percent. Using these assumptions, the annual electricity produced at the sites EPRI studied is about 14,000 GWh. This corresponds
to an average annual power of 1,600 MW and a rated power of about 4,800 MW (given a capacity factor of 33 percent). These estimates should be considered as the lower boundary of the tidal stream resource because not all the U.S. sites with potential have been evaluated.

Ocean current energy

The only large ocean current resource in the U.S. is about 30 kilometers off the coast of southern Florida. The total availability of this resource is not known. However, Aeroviroment in the 1970s and recently Florida Atlantic University in 2007 both estimated an annual extractable energy of 50,000 GWh and an average annual power of about 10,000 MW (given a capacity factor of 57 percent).7,8

River kinetic energy

In 1986, New York University studied the overall kinetic power potential of U.S. rivers.9 This study indicated about 12,500 MW. The study involved rivers with discharge rates greater than 113 cubic meters per second and velocities greater than 1.3 meters per second.

Alaska and the Pacific Northwest contain the primary river current resource available to the U.S. However, depending on whether the conversion technology is economical at low power
densities, every state could have a river kinetic resource.

In 2008, EPRI evaluated the available resource at six specific sites in Alaska.10 The total annual energy for those sites was 78 GWh, and the average annual power was 8.9 MW. Because of societal, physical, and environmental limits, it is difficult to quantify the extractable river kinetic resource.

Recommended research

For both wave and kinetic energy in the U.S., we believe further research is needed. First, for wave energy, modeling is required to understand the effects of large scale wave energy extraction on coastal dynamics. This modeling should be performed using measurements taken as the first wave power plants are installed.

Second, for kinetic energy, we believe two research priorities are in order. To more fully understand the magnitude of the available U.S. resource, further evaluations of potential sites is required. In late 2008, EPRI began investigations of several sites in Alabama, Alaska, Florida, Georgia, and Mississippi. In addition, modeling of this resource is required to improve the accuracy and detail of existing maps and the available resource estimates derived from them. Modeling also is required of the effects of large-scale kinetic energy extraction effects. Again, these models should be calibrated with actual measurements as
the first kinetic plants are installed.

In addition, because marine energy resources depend heavily on geography, archiving the resource information in a geographical information system (GIS) database should be explored. EPRI has begun work on this effort, using a database supplied by the National
Renewable Energy Laboratory. This work is being funded, in part, through an award from the U.S. Department of Energy, as part of its program to advance the commercial viability, cost-competitiveness, and market acceptance of new technologies that can harness renewable energy from oceans and rivers.

Acknowledgment

The authors thank George Hagerman for providing the information on wave energy that is included in this article.

Notes

1 Houle, Andree J., and Roger J. Bedard, “An Overview: Development Status of Ocean Wave
and Tidal Technology,” Hydro Review, Volume 27, No. 5, September 2008, pages 30-34.
2 www.eia.doe.gov/cneaf/electricity/epm/table1_1.html.
3 Guidelines for Preliminary Estimation of Power Production by Offshore Wave Energy Conversion Devices, EPRI WP-001, EPRI, Palo Alto, Calif., 2003; http://oceanenergy.epri.com/waveenergy.html.
4 Methodology for Estimating Tidal Current Energy Resources and Power Production by Tidal In-Stream Energy Conversion (TISEC) Devices, EPRI TP-001, EPRI, Palo Alto, Calif., 2006;
http://oceanenergy.epri.com/streamenergy.html#reports.
5 Garret, C., and P. Cummins, “The Power Potential of Tidal Currents in Channels,” Proceedings of the Royal Society A, Volume 461, No. 2060, August 2005, pages 2,563-2,572.
6 Polagye, Brian, P. Malte, M. Kawase, and D. Durran, “Effect of Large-Scale Kinetic Power
Extraction on Time-Dependent Estuaries,” Proceedings of the Institution of Mechanical
Engineering Part A: Journal of Power and Energy, Volume 222, No. A5, August 2008,
pages 471-484.
7 Lissaman, P.B.S., “Coriolis Program: A Review of the Status of the Ocean Turbine Energy
System,” Aerovironment, Monrovia, Calif., 1979.
8 www.fau.edu.
9 Miller, G., et al, “Allocation of Kinetic Hydro Energy Conversion Systems (KHECS) in USA
Drainage Basins,” New York University, 1986, pages 86-151.
10 Alaska River In Stream Energy Conversion Site Survey, EPRI RP-003, EPRI, Palo Alto, Calif., 2008.