TODAY’S STUDY: U.S. TIDAL ENERGY POTENTIAL

Assessment of Energy Production Potential from Tidal Streams in the United States
June 29, 2011 (Georgia Tech Research Corporation)

Executive Summary

Tidal stream energy is one of the alternative energy sources that are renewable and clean. With the constantly increasing effort in promoting alternative energy, tidal streams have become one of the more promising energy sources due to their continuous, predictable and spatially-concentrated characteristics. However, the present lack of a full spatial-temporal assessment of tidal currents for the U.S. coastline down to the scale of individual devices is a barrier to the comprehensive development of tidal current energy technology. This project created a national database of tidal stream energy potential, as well as a GIS tool usable by industry in order to accelerate the market for tidal energy conversion technology.

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Tidal currents are numerically modeled with the Regional Ocean Modeling System and calibrated with the available measurements of tidal current speed and water level surface. The performance of the model in predicting the tidal currents and water levels is assessed with an independent validation. The geodatabase is published at a public domain via a spatial database engine and interactive tools to select, query and download the data are provided. Regions with the maximum of the average kinetic power density larger than 500 W/m2 (corresponding to a current speed of ~1 m/s), surface area larger than 0.5 km2 and depth larger than 5 m are defined as hotspots and list of hotspots along the USA coast is documented. The results of the regional assessment show that the state of Alaska (AK) contains the largest number of locations with considerably high kinetic power density, and is followed by, Maine (ME), Washington (WA), Oregon (OR), California (CA), New Hampshire (NH), Massachusetts (MA), New York (NY), New Jersey (NJ), North and South Carolina (NC, SC), Georgia (GA), and Florida (FL). The average tidal stream power density at some of these locations can be larger than 8 kW/m2 with surface areas on the order of few hundred kilometers squared, and depths larger than 100 meters. The Cook Inlet in AK is found to have a substantially large tidal stream power density sustained over a very large area.

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Background

Tidal streams are high velocity sea currents created by periodic horizontal movement of the tides, often magnified by local topographical features such as headlands, inlets to inland lagoons, and straits. As tides ebb and flow, currents are often generated in coastal waters. In many places the shape of the seabed forces water to flow through narrow channels, or around headlands. Tidal stream energy extraction is derived from the kinetic energy of the moving flow; analogous to the way a wind turbine operates in air, and as such differs from tidal barrages, which create a head of water for energy extraction. A tidal stream energy converter extracts and converts the mechanical energy in the current into a transmittable energy form. A variety of conversion devices are currently being proposed or are under active development, from a water turbine similar to a scaled wind turbine, driving a generator via a gearbox, to an oscillating hydrofoil which drives a hydraulic motor.

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Tidal energy is one of the fastest growing emerging technologies in the renewable sector and is set to make a major contribution to carbon free energy generation.. The key advantage of tidal streams is the deterministic and precise energy production forecast governed by astronomy. In addition, the predictable slack water facilitates deployment and maintenance. In 2005, EPRI was first to study representative sites (Knik Arm, AK; Tacoma Narrows, WA; Golden Gate, CA; Muskeget Channel, MA; Western Passage, ME) without mapping the resources (EPRI, 2006g). Additional favorable sites exist in Puget Sound, New York, Connecticut, Cook Inlet, Southeast Alaska, and the Aleutian Islands among others. Besides large scale power production, tidal streams may serve as local and reliable energy sources for remote and dispersed coastal communities and islands. The extractable resource is not completely known; assuming 15% level of extraction, EPRI has documented 16 TWh/yr in Alaska, 0.6 TWh/yr in Puget Sound, and 0.4 TWh/yr in CA, MA, and ME (EPRI 2006b-f). The selection of location for a tidal stream energy converter farm is made upon assessment of a number of criteria:
Tidal current velocity and flow rate: the direction, speed and volume of water passing through the site in space and time.

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Other site characteristics: bathymetry, water depth, geology of the seabed and environmental impacts will determine the deployment method needed and the cost of installation.

Electrical grid connection and local cost of electricity: the seafloor cable distance from the proposed site to a grid access point and the cost of competing sources of electricity will also help determine the viability of an installation.

Following the guidelines in the EPRI report for estimating tidal current energy resources (EPRI 2006a), preliminary investigations of the tidal currents can be conducted based on the tidal current predictions provided by NOAA tidal current stations (NOAA, 2008b). There are over 2700 of these stations which are sparsely distributed in inlets, rivers, channels and bays. The gauge stations are concentrated along navigation channels, harbors and rivers but widely absent elsewhere along the coast. As an example, the maximum powers at some of these locations around the Savannah River on the coast of Georgia are shown in Figure 1. The kinetic tidal power per unit area, power density, given in this figure were calculated using the equation where ρis the density of water and V is the magnitude of the depth averaged maximum velocity.

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These tidal currents and therefore the available power per unit area can have significant spatial variability (Figure 1); therefore, measurements (or predictions) of currents at one location are generally a poor indicator of conditions at another location, even nearby. It is clear that the majority of the data is available along the navigation channel in the Savannah River, with sparse data within the rest of the tidal area. EPRI (2006a) suggest a methodology using continuity and the Bernoulli equation for determining the flow in different sections of a channel. This is a reasonable approach for flow along a geometrically simple channel, but is not applicable for the flow in the complex network of rivers and creeks along much of the US coastline. Thus we have applied a state-ofthe-art numerical model for simulating the tidal flows along the coast of the entire United States…

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Total theoretical available power estimates

The published maps and the database provide the distribution of the existing kinetic power density of tidal streams in the undisturbed flow conditions. These results do not include any technology assumptions or flow field effects as in the case of device arrays. In order to calculate a theoretical upper bound based on physics only, a simplified method that considers both the kinetic and potential power with the exclusion of any technology specific assumptions is applied. The details of the method is outlined in a recent paper (Garrett and Cummins, 2005). The power calculated with this method is used in estimating the tidal power potential for the entire country with a specific value for each state. The method uses undisturbed flow field from the model with simple analytical methods, accounts for the cumulative effect of dissipating energy and provides information on an estuary scale.
Considering a constricted channel connecting two large bodies of water in which the tides at both ends are assumed to be unaffected by the currents through the channel, a general formula gives the maximum average power as between 20 and 24% of the peak tidal pressure head times the peak of the undisturbed mass flux through the channel. This maximum average power is independent of the location of the turbine fences along the channel…

This upper bound on the available power ignores losses associated with turbine operation and assumes that turbines are deployed in uniform fences, with all the water passing through the turbines at each fence.

This method is applied to the locations bounded between two land masses and has locally increased tidal current speed along the United States coast. A list of these locations grouped by state is given in Table 6. The list displays the coordinates and the name of each location (i.e. the midpoint) together with the width, mean/maximum of the constriction and the total theoretical available power. The totals are given for each state and for the entire country. Once again, Alaska with a total of 47GW constitutes the largest piece of the national total of 50 GW. Cook Inlet has the largest average maximum available power of 18 GW (Figure A22) closely followed by Chatham Strait with 12 GW (Figure A20). Alaska is stands out as an abundant resource of tidal stream…