Assessing Ocean Energy Resources

Based on the analysis, the city concluded that tidal power resources are limited. However, two sites were identified for installation of wave power systems that might produce significant electrical power. Those sites justify further analysis to more accurately determine the available power, environmental effects, and constructability.  (See Figure 1, below for a better view.  Fig 1. Two sites, one east and one west of the city of King Cove, Alaska, show potential for installation of a wave energy conversion device. (This figure is arrived at by taking 15 percent of the total mean power, which is the theoretical maximum that could be used before major upstream and downstream effects to the environment could occur (Hagerman, George, and B. Polagye, “Methodology for Estimating Tidal Current Energy Resources and Power Production by Tidal In-Stream Energy Conversion (TISEC) Devices,” EPRI-TP-001 NA Rev 3., EPRI, Palo Alto, Calif., 2006.).)

The city of King Cove, population 800, is in Aleutians East Borough, which stretches from the tip of the Alaska Peninsula to the easternmost Aleutian Islands. This region is bordered on one side by the Pacific Ocean and on the other by the Bering Sea. The city has a lagoon on the west side that could be promising for installation of an in-stream tidal energy conversion system.

In July 2007, New Energy Corp. Inc. approached the city about the possibility of installing its EnCurrent tidal technology at the inlet to King Cove Lagoon. At the time, the city had not investigated potential ocean energy sites. Todd Bethard with HDR Engineering was under contract to provide engineering services for the city of King Cove. The city asked Bethard to do a preliminary assessment of the proposal. As part of that assessment, HDR decided to also evaluate potential wave energy sites near the city.

Because of its remote location, the city of King Cove relies heavily on expensive diesel generation. The city has four diesel plants with a total capacity of almost 2.5 MW. To reduce its dependence on diesel, the city of King Cove developed the 800 kW Delta Creek hydro project. This project began operating in December 1994. The city also is investigating the possibility of developing a 300 kW hydro plant on nearby Waterfall Creek. The city of King Cove is interested in pursuing opportunities to add generating capacity that would reduce its dependence on diesel, save money, and minimize greenhouse gas emissions.

Thus, the city was interested in determining whether the tidal project proposed would be feasible. In July 2007, HDR Engineering began work needed to identify potential sites for an ocean energy project and to provide an estimate of available resources. This assessment needed to be conducted with a minimum level of effort, as it was intended to provide only an initial indication of the feasibility of the tidal project. City decision-makers would use the resulting information to determine whether ocean energy could provide additional renewable generation.

Four-step Process

The assessment process HDR used consists of four steps. The first step is to identify project goals. The second is to conduct a preliminary-level assessment of available ocean energy resources that will identify which systems, if any, might provide substantial power generating capacity. The third step is to take a preliminary look at potential environmental effects. In the fourth step, each assessed location is analyzed for suitability with respect to available power, environmental restrictions, and many other factors. While these analyses are not intended to comprehensively address all issues, they better refine understanding of critical items that warrant further investigation. The following sections describe each step in detail.

Step 1: Identify project goals–The first part of any work is determining the goal. For the city of King Cove, additional electric generating capacity will help reduce its dependence on expensive diesel generation. The goal of this assessment was to determine sites with enough potential to warrant further investigation into the installation of ocean energy conversion devices.

Step 2: Analyze available power–This step involves an estimate of the potential energy available in the area. Generally, there is an obviously preferential form of available power. In the case of King Cove, two obvious sources of ocean energy were available: tidal at the inlet to King Cove Lagoon and waves along the Pacific Coast.

Site conditions ultimately control the amount of available power. Usually, the data needed to directly predict the potential power of a site is not available. As a result, it is necessary to collect readily available data and to extend the value of that data with numerical and statistical modeling. The best alternative is always to use as much measured data as possible and to perform statistical analysis to predict future conditions. Collecting data is expensive, and a considerable amount of data is required to make accurate estimations of conditions. A simple method can be used to reduce the number of sites that must be investigated in detail.

Some knowledge of the flow field is required to estimate available tidal power. The extent of existing data determines the detail with which the preliminary level feasibility can be conducted. In the case of King Cove, available data included limited local bathymetry, shoreline position, and tide. This allowed power to be estimated in a rudimentary way. Calculating available power in this manner provides decision-makers with initial data on which to base decisions about future analyses.

King Cove Lagoon is separated from the city of King Cove by a small inlet with a narrow channel. A complete year of data on the distribution of the tidal range at King Cove was needed to determine the average velocity over each tidal cycle. Therefore, data from 2006 was used. The area of the lagoon and the tidal range1,2 constitute the approximate tidal prism (volume of water flowing into or out of the lagoon) for each tide. The tidal prism was then applied to estimate velocity in order to develop a representative distribution of velocity experienced at the inlet. The distribution of velocity was used to calculate a distribution of power at the inlet. Mean power was then determined from the distribution of power. Results showed that available total mean power at this site was only 17 kW (see Table 1).

Once some knowledge of flow is available, potential systems for extracting tidal power are evaluated. Most of the available systems require relatively high minimum velocity (greater than about 2.3 feet per second) to generate power. The simplified method for analyzing average flow rate used here does not address local peak velocity. It is possible to concentrate flow, causing local acceleration, without significantly increasing head loss or affecting tide propagation upstream. The purpose of this step is to identify systems that could work and eliminate those that will not.

The first technology considered was tidal in-stream energy conversion systems. Some of the systems that will work in this inlet float on the water or are mounted to a structure above water so that the generator is not submerged. Most of these systems were developed for deeper water than is available in the inlet at King Cove, typically requiring at least 30 feet. But a few systems will fit in the channel in the inlet. One such device manufactured by New Energy has a diameter of about 10 feet, allowing only a few to be placed across the 50-foot width of the channel. A system can be purchased from New Energy that provides a maximum power of 25 kW. That system has a design velocity around 9 feet per second. The manufacturer claims it can operate below 5 feet per second, but there will be a considerable amount of the tidal cycle with no power generated. Assuming a minimum velocity of 3 feet per second for any power generation, no electricity could be generated as much as half the time.

Because most of the flow is concentrated in the channel, the velocity presumably is greater than that predicted by the simplified method used but still not sufficient to significantly increase the potential power. Considering the limited space available for mounting tidal current power systems and the relatively low available power, it seems likely that any in-stream tidal power system placed in the inlet will generate average power less than 20 kW.

Another technology considered is a tidal barrage. A tidal barrage traps water behind a dam as the tides rise and fall. Once a sufficient head difference has built up, the trapped water is allowed to flow through hydro turbines to generate power. These systems require large tidal ranges to develop enough power to justify the cost of construction. The 18-MW Annapolis project in Nova Scotia, Canada, is an example of this technology. It has a tidal range in excess of 30 feet.

This type of plant has significant environmental effects, including effects on tidal regime and fish passage. A barrage at this site would produce significantly more electricity than in-stream tidal systems. However, a barrage would not be economically or environmentally viable because of the construction cost of a dam across the inlet, the relatively small tide range, and the environmental effects.

In addition to the tidal inlet at this site, there is a much larger source of energy just offshore. Waves along the southern coast of Alaska are among the largest and most frequent in the U.S. A simplified method for estimating power available from waves was employed to provide sufficient detail to move forward with feasibility without incurring significant cost.

The closest readily-available data for this location comes from a buoy at the Shumagin Islands, about 100 miles southeast the city of King Cove. Preliminary desktop-level analysis of this data indicates that the offshore significant wave height (the average of the largest 1/3 of the waves) between May 2004 and December 2006 was about 16 feet, with an associated peak period of 10 seconds. This buoy is located in about 7,500 feet of water. The maximum height at the buoy during the period of record was 37 feet. Considering wave transformation from deep water to a location near King Cove, less energy likely will be available near King Cove than was measured at the Shumagin Islands.

Placing a wave energy conversion system in deep water on the ocean side of Deer Island (see Figure 1) would be prohibitively expensive because of the length of submerged transmission lines required. It seems possible to place systems both east and west of King Cove, outside the shadow zone created as waves propagate past Deer Island. Two sites, 1 mile from shore east of the island and 2 miles from shore west of the island, have sufficient water depth to accommodate a wave power system (see Figure 1).

Detailed wave analyses need to be conducted to determine the most effective location for wave energy conversion systems and the extent to which shadowing will affect power. Wave modeling will allow consideration of the effects of wave transformation as the waves propagate from deep water.

The distribution of offshore wave height at the Shumagin Islands buoy was applied to estimate the distribution of power density offshore, in kilowatts per meter, to determine the average yearly power. The distribution of power density indicates that the yearly average wave power offshore may be about 40 kW per meter of wave crest (or about 4 MW of available power over 330 feet of wave crest). The power density is below 1 kW per meter about 4 percent of the time and below 10 kW per meter about 32 percent of the time.

The percent of wave power a device converts to electricity is a function of the wave height and period and the design of the wave energy conversion system. Available systems range from 40 kW to more than 4 MW. Multiple systems can be deployed along the shoreline to achieve the desired power capability. The waves represented by this analysis are large enough for most of the systems that have been built and field tested to reach the rated capacity. The biggest disadvantage to a wave power system is the early state of the technology, but developing a pilot project at King Cove may reduce the cost.

There are two types of wave technology, floating and bottom-mounted systems. Initially, the most promising floating technology is a buoy system that is moored to the sea floor. Buoy systems range in rated maximum power up to 1 MW per individual floating component. A system of buoys that provide 40 kW to 250 kW could be deployed at either site, depending on the result of wave analyses. A field of enough buoys to reliably generate the desired power would be placed in about 150 feet of water, with a common submerged transmission line bringing the power to shore.

There are several advantages to a system of multiple buoys. The system can be expanded by adding more buoys as the needs of the city grow, at a cost significantly below the initial installation. If one buoy fails, the system still continues to generate electricity. And the buoys can be maintained at sea or towed into port.

Systems designed to be mounted on the ocean bottom convert wave energy into electricity through a device that moves with the waves. The advantages of a bottom-mounted system include increased survivability during storms because of the lower impact loads and less effect to navigation or aesthetics. Two major drawbacks include the increased cost of maintenance and the early state of the technology.

At least one system, the first prototype of the Archimedes Wave Swing, has been deployed offshore. This prototype, deployed in Portugal in 2004, reached a capacity of 2 MW. However, none of the bottom-mounted systems have been applied on a commercial scale, so there are no data on maintenance intervals. Submerged power generation systems will suffer from corrosion and marine fouling. Both are expected with floating systems as well, but the submerged system could require much more costly repairs because of the cost of working below the ocean surface.

Overall, our conclusion is that the early state of the submerged wave energy conversion technology, the unknown potential for failure, and the increased difficulty of servicing subsea systems should probably preclude installation of this technology at King Cove.

Step 3: Assessing environmental effects–Licensing a project is a significant undertaking. Potential environmental effects are far-reaching and can incur a significant cost to evaluate. The preliminary investigation aims to identify likely scenarios requiring further investigation and to identify those effects likely to impede possible projects.

The preliminary environmental assessment focuses on identifying roadblocks that may affect the feasibility of potential wave energy projects, with an eye toward permitting and licensing. Typically this includes a biological evaluation of potential project locations with respect to threatened or endangered species and other critical habitats. The cursory environmental assessment (EA) addresses obvious red flags, poses key questions, and provides direction for the future EA work. Ultimately, a project is likely to require a Federal Energy Regulatory Commission (FERC) license, National Environmental Policy Act (NEPA) documentation, Section 404 and Section 10 approvals, Endangered Species Act compliance, and essential fish habitat review.

Major potential effects at King Cove for tidal power systems might primarily be related to upstream effects caused by modification of the flow regime and mechanical or flow-related injuries to marine life. Major effects envisioned for wave power systems might primarily be related to marine mammals.

Step 4: Assessing site suitability–The site suitability analysis is intended to identify potential technically feasible and environmentally acceptable sites that warrant more detailed investigations in future phases of work. Key factors considered include:

– Wave/tidal climate;
– Environmental restrictions;
– Available power generation;
– Conflicts with other users of the sea space;
– Grid interconnection availability;
– Transmission distance;
– Exposure to storm waves, ice, or other risk factors;
– Constructability; and
– Maintenance costs.

Sites are assessed for suitability coincident with the assessment of available power and potential environmental effects. Comprehensive consideration of these concerns allows ranking of potential sites for ocean energy development.

The importance of conditions that dictate suitability of a particular site varies depending on location and community needs. In some places, it might be impossible to construct new transmission lines, making marine cable the only option. In other locations, sensitive environmental conditions might dictate that only a floating wave system be used.

Based on this analysis, it seems likely that a tidal power system at the inlet to King Cove Lagoon will not produce enough electricity to justify its installation. The major advantage of tidal power was the inlet’s close proximity to the city of King Cove and grid interconnection. The analysis suggests that any system placed in the inlet without modifying the inlet will produce, at most, an average power of about 20 kW. If the inlet is modified by constructing a tidal barrage, more power could probably be generated but the cost and environmental effects would likely make a tidal barrage unfeasible.

The analysis suggested there is a considerable supply of wave power available within a relatively short distance from King Cove. The available power could provide a substantial portion of the city’s electricity needs. The brief analysis indicates that a system could be designed to capture as little or as much power as is needed, up to at least a few megawatts. Sheltering from Deer Island will reduce the available power but may increase the system’s survivability. Depending on the results of the detailed wave analysis, the system can be located more or less in the shadow zone behind Deer Island to limit exposure to large waves.

Choosing the right wave energy conversion system would allow the power output to be scaled based on cost and power needs so that capacity could grow with the community.

The major disadvantages to wave power are the required marine transmission cables and additional transmission lines on shore to reach the community.

Looking to the Future

If ocean energy systems are to become commercially feasible in the near term, innovative approaches to project development must be undertaken. The assessment approach described in this article provides a process of quickly generating information to evaluate project viability and guide detailed analyses, permitting, design, and construction.

Based on the results of this analysis, the city of King Cove is determining how to proceed with development of its ocean energy resources. Because of the nascent state of the conversion technology, the city is waiting to determine which devices may emerge that would be applicable to the identified sites. The city also is working to determine if the state of Alaska would assist in funding a demonstration project in the area.

Notes

• NOAA Center for Operational Oceanographic Products and Services (CO-OPS), July 18, 2007, http://tidesandcurrents.noaa.gov.
• NOAA Office of Coast Survey, July 18, 2007, http://chartmaker.ncd.noaa.gov/mcd/ enc/index.htm.

Reference

Rob Thomas, Joel Darnell, Todd Bethard, and Jim Jordan, “Applied Methodology for Assessing Ocean Energy Resources: King Cove, Alaska,” HydroVision 2008 Technical Papers CD-Rom, HCI Publications, Kansas City, Mo., 2008.

Authors: Gary Hennigh is city administrator of the city of King Cove, Alaska. Rob Thomas, coastal engineer with HDR | Shiner Moseley in Corpus Christi, Texas, specializes in analysis of ocean energy systems. Joel Darnell, coastal project manager with HDR Engineering in Seattle, Wash., has been involved in projects ranging from estuarine habitat restoration to offshore energy. Todd Bethard, P.E., water business group leader in the HDR Alaska, Inc. office, is lead engineer for the city of King Cove. Jim Jordan, senior environmental project manager with HDR Engineering in Seattle, is responsible for preparation of facility siting studies, environmental impact statements, project permitting and resource management plans.