Tidal Currents1

Introduction

Marine Current Turbines Ltd. Artist's Concept.

The Northern Puget Sound project area includes the Inland Sea known as the San Juan Archipelago, the northern part of Puget Sound. There are approximately 172 islands in the San Juan Archipelago. The maximum vertical tidal variation at this latitude is approximately 12 feet. The 172 islands of the San Juan Archipelago constrict this vertical tidal variation to varying degrees, generating tidal power potential in the process.

One of the many renewable energy options that have been discussed over the years is tidal power. Most of this discussion has centered on building dams or pumped storage systems in high latitude tidal estuaries.2 Given the current political and regulatory environment, establishing dams or pumped storage systems of any kind anywhere in the Northwest Straits is not going to happen. However, development of more environmentally benign energy extraction facilities such as in-stream turbines may not only be possible from an environmental impact standpoint, but desirable from a renewable energy production standpoint. There are multitudes of questions that require answers before such an energy production scheme will be taken seriously, not the least of which is the first question, "Is there enough potential ocean current energy to be worth the effort required to extract it?"

The primary advantage of ocean energy technology is the high energy density of ocean currents. Seawater is 832 times as dense as air, providing a 5-knot current with the equivalent kinetic energy of a 162-mph wind. Seawater being a moving fluid like wind, power also increases as the cube of speed. Unlike wind, water is not very compressible. This creates difficulties in adapting wind extraction airfoil lift sections to the marine power environment. Other advantages over wind and solar include predictable energy output as cyclical tidal patterns allow electricity outputs from ocean energy facilities to be accurately predicted far in advance, providing reliable base power for integration with electrical grids.

Marine current turbine technology offers numerous advantages even over other renewable energy technologies:

• It is one of the most intense renewable energy forms; so a typical one megawatt marine current turbine will be less than 50 percent of the size of the equivalent wind turbine and these turbines can also be packed closer together in an array, thereby further reducing costs.
• It is based on the tides and is therefore a completely predictable form of renewable power (unlike weather dependent wind, solar, or wave energy).
• Gaining the necessary permissions and licenses is expected to become easier than for on-shore developments as it involves government departments rather than local planners.
• The environmental impact is negligible; marine current turbines will have little or no visual impact; they are silent, and do not offer any serious hazard to marine life.
• The modular nature of this technology makes the lead time between spending on construction and receiving revenue relatively short.
• The resource is known to be extremely large.

In order to evaluate the economic impacts of ocean current power development in the Northwest Straits it will be necessary to quantify the available power potential and to quantify the efficiency of energy extraction turbines, generators, and associated equipment. To do so, the following tasks have been accomplished:

• Research and evaluation of available marine energy production designs.
• Identification of high tidal current areas in the Northwest Straits.
• Analyses of site-specific annual tidal energy potentials.
• Plotting of three-dimensional depth contours and digital current models on a Geographic Information System (GIS).
• Identification of existing electrical distribution network locations and capacities.
• Articulation of energy production scenarios.
• Identification of environmental impacts to be addressed during Phase II.
• Ground truthing of published tide current tables for identified areas.

Downloading hourly current speed for selected NOAA current stations created the following table for ocean current sites in the Northwest Straits.

Back to Top

Northwest Straits Current Power Sites

Table 1.

*Lat/Long in Degrees Minutes

Hourly current speed was obtained using Tides & Currents Pro software. Hourly potential energy in watts was calculated from current speeds using the standard equation for kinetic energy P(w) = .5pAV3, where p equals the density of seawater (1015 kg/m3); A equals area; and V3 is velocity cubed in meters per second. Multiplying 0.5 times 1015 times current speed cubed resulted in a value of potential energy per meter squared. This hourly value was summed for all hours during 1999 to give a value for annual potential energy per square meter for each site.

A unit area of 314 square meters was chosen as a likely size for a single power production unit. This is a rotor diameter of 20 meters and is roughly the size of a Marine Current turbine. With the exception of Deception Pass, this size unit would rate a maximum generator size of a megawatt or under, which is the current state of the art for wind generator units. It should be noted that one unit at Deception Pass would generate more energy than one unit at each of the other 19 sites combined.

Back to Top

Northwest Straits Ocean Current Power

Table 2.

• Current values in knots come from Tides & Currents Pro 2.5. Changed to Meters/second (Knots*.515).
• Instantaneous current values every hour for selected stations are multiplied by the P(kw) formula to give instantaneous Potential Energy (0.5pAV3) for every hour of the year per Square Meter where seawater density p = 1015 kg/m3.
• PE values are summed to give ANNUAL PE/m2.
• PE MAX is one hour maximum for the year.
• NET SITE Megawatt Hour (MWH) values based on Unit area square meter and overall power efficiency.
• Unit area equal to about 20 meter diameter turbine.
• Overall efficiency is 35 percent for turbine times 85 percent generator = 30 percent

A single unit at each of the above sites would generate approximately 13 megawatt hours annually. Some sites would likely have room for only one unit; other sites could support arrays of single units. A span of 10 times the frontal surface area of a unit along orthogonal axes would be large enough to reduce interference of the turbines with each other. If an array is positioned with a six degree offset to the current stream, additional turbines could be placed on diagonal centers.3 The resultant array for a 10 meter vertical axis turbine would have a surface footprint of approximately 4,000 m2 per turbine (400mX400m = 160,000 m2 = 41 turbines = 4,000 m2/turbine). A 20 meter diameter horizontal axis rotor turbine would have a surface footprint of approximately 15,610 m2 (800mX800m = 640,000 m2 = 41 turbines = 15,600 m2/turbine).

So how many units will each site accommodate? If we construct arrays using the Marine Turbine footprint of 15,600 m2/turbine and apply it to each of the twenty sites listed above we come up with the following number of units per site. Please note that the net site annual power is in Megawatt hours.

Back to Top

Twenty Sites With Horizontal Axis Turbine Arrays

Table 3.

Twenty million-megawatt hours annually is equivalent to a generating capacity of approximately 2,330 MW. This is 30 percent of the Bonneville Power Administration (BPA) hydro capacity4 or 1.6 times the power used by Seattle5 or enough energy to power two million residential customers.6

As an example, using the potential energy values for Lawrence Point, we can compute that the estimated power for a turbine with an overall efficiency of 30 percent would be 1,252 watts/m2/day ((1524 kWh/365 days)*.3) at this site.

It should be noted that the available area estimate for each site listed as well as the area of a hypothetical array footprint is extremely conservative. It should also be noted that only twenty sites were listed from the entire Northwest Straits area for this illustration. Hopefully, the question "Is there enough potential ocean current energy to be worth the effort required to extract it?" has been sufficiently addressed to be answerable in the affirmative.

Back to Top

Ocean Energy Production Designs

Blue Energy 250 kW "Haida Gwaii"

The current world leader in ocean current energy development is Blue Energy Canada Inc.7 Blue Energy Canada is commercializing the Davis Hydro Turbine that converts the energy of moving water in ocean currents, tides, estuaries, and rivers into electricity. Six prototypes (4-100 kW) have been built and tested, and independent assessments have verified feasibility. This is the closest thing to off-the-shelf technology currently available.

The Blue Energy power system can satisfy electricity demands in the multiple gigawatt range by linking "Ocean Class" Davis Hydro Turbines (7-14 MW each) in series across an ocean passage. Smaller energy loads can be met by deploying the 250 kW "Haida Gwaii" power system in off-grid communities, remote industrial sites, and regions with established net metering policies.

Blue Energy's Davis turbine is a vertical axis turbine updated from the original Darrieus patent of 1931. In contrast with wheel-type turbines, it has a barrel shape with a number of straight or curved-in-plane airfoil blades and a shaft that is perpendicular to the fluid flow.

The Darrieus turbine was enthusiastically met by engineers and scientists in both wind and hydro power industries because of its simplicity and because the turbine allowed high speed to develop in slow fluids, maintaining a large passage area without substantially increasing its diameter. However, in spite of numerous intensive attempts for decades to utilize the Darrieus rotor, it has not received wide practical applications, mostly due to the pulsating nature of its rotation and its relatively low efficiency. Fatigue failure of blades is common in this turbine because of its inherent vibration. It also has a problem of self-starting at low rotational speed due to its straight blades which change angles of attack traveling along a circular path.

Blue Energy's turbine is housed in a concrete shell module that provides structural support as well as some bi-directional venturi hydrodynamics. The stand-alone 250 kW "Haida Gwaii" is designed to float on a mooring. It is designed to cut-in at a three-knot current speed and cut out at seven knots.

Helical Turbine
Alexander Gorlov has developed a similar vertical axis turbine concept at Northeastern University.3 This turbine uses a helical blade design which can be stacked horizontally and vertically to form power arrays. The helical turbine developed in 1994-5 has all the advantages of the Darrieus turbine without its disadvantages, that is, allowing a large mass of slow water to flow through, capturing its kinetic energy, and utilizing a very simple rotor as a major factor of the turbine low cost. The helical arrangement of the rotor blades dramatically changes the performance of the Darrieus-type turbine, resulting in the following characteristics:

• High-speed uniform spinning in relatively slow fluid flow.
• Unidirectional rotation in reversible fluid currents.
• High efficiency.
• No fluctuation in torque.
• No visible signs of cavitation in water for high rotating speed.
• Self-starting in slow waters.

A three-blade helical turbine was thoroughly tested during June-August 1996 in the Cape Cod Canal in Massachusetts. Starting with a firm unidirectional rotation when water velocity was about one knot, the turbine increased its power in proportion to the water velocity cubed to six knots and was 35 percent efficient throughout.

Horizontal Axis Turbine
Marine Current Turbines Ltd. (MCT) out of England has a program of tidal turbine development through research and development and demonstration phases to commercial manufacture.8 An initial grant of $1 million Euro has been received from the European Commission towards R&D costs. The company's plan is to complete the initial R&D phase by 2004, and to start commercial installations at that time.

The technology under development by Marine Current Turbines Ltd. consists of an axial flow rotor of 15 meters to 20 meters in diameter, which drives a generator via a gearbox much like a hydroelectric turbine or a wind turbine. The power unit is mounted on a tubular steel monopile just over 2 meters in diameter that is set into a hole drilled into the seabed from a jack-up barge. The technology for placing monopiles is well developed by Seacore Ltd., a specialist offshore engineering company that is co-operating with MCT in this work. The patented design of the turbine can be installed and maintained entirely without the use of costly underwater operations. The turbine is connected to the shore by a marine cable lying on the seabed that emerges from the base of the pile.

The submerged turbines, which will generally be rated at from 600 to 1000kW, will be grouped in arrays or "farms" under the sea, at places with high currents, in much the same way that wind turbines in a wind farm are set out in rows to catch the wind. The main difference is that marine current turbines of a given power rating are smaller, can be packed closer together, and involve negligible land use or other environmental impact.

Another advantage of this technology is that it is modular, so small batches of machines can be installed with only a small period between investment in the technology and the time when revenue starts to flow. This is in contrast to large hydro-electric schemes, tidal barrages, or other projects involving major civil engineering, where the lead time between investment and gaining a return can be many years.

It is expected that turbines will generally be installed in batches of about 10 machines. Many of the potential sites so far investigated are large enough to accommodate many hundreds of turbines. As a site is developed, the marginal cost of adding more turbines and of maintaining them will decrease, so considerable economies of scale can be envisaged.

No cost estimate is available for the Marine Currents Turbine. Blue Energy envisions a cost of approximately $1250/kW. Gorlov details a cost for his helical turbine array of $2235/kW.

Back to Top

References Cited

1. Northwest Indian College National Indian Center for Marine and Environmental Research and Education. Feasibility Studies for Potential Application of Renewable Energy Technologies at Tribal Colleges and Universities Supplemental Announcement 07. Volume III - Draft Phase 1 Final Report. Submitted: June 30,2001. <http://nwic-research.org/DOE%20PhaseII%20Vol3.htm>

2. Tidal Electric. <http://www.tidalelectric.com>

3. Gorlov, Alexander. "Development of the Helical Reaction Hydraulic Turbine." Final Technical Report, July 1, 1996—June 30,1998. DE-FG01-96EE 15669.

4. Bonneville Power Administration. 1997 BPA Fast Facts. <http://www.bpa.gov>

5. The Bellingham Herald. Thursday, December 28, 2000.

6. Seattle City Light. Customer Guide 1998-99. <http://www.ci.seattle.wa.us>

7. Blue Energy Canada Inc. 1111 Melville St, Vancouver, BC V6E 3V6. (604) 682-2583. <http://www.bluenergy.com>

8. Marine Current Turbines Ltd. <http://www.marineturbines.com>

Back to Top