TIDAL ENERGY FOR ISLAND NATIONS Roger Bason President, Natural Currents Energy New England LLC. For email correspondence with the author: rbason@naturalcurrents.com INTRODUCTION Emerging technologies to extract electric power from the movements of tides and kinetic water flows are now being developed by almost a score of companies around the world. The pressures of climate change, peak oil, competition for scarce energy resources, global security concerns and the race to rapidly rollout and develop a sustainable global culture have combined to increase technology innovation in the field of ocean energy systems. This article identifies the nature of the tidal energy resource, evaluates its potential, overviews the technology available and its general state of development, and reviews some of the key concepts and innovations presently being explored by visionaries who look to the world’s oceans as a power source for island nations. TIDAL RESOURCE The moon is the dominant factor controlling the period and height of tides, with the effect of the sun providing a significant but lesser impact. The gravitational forces on the earth’s oceans that produce tides also come from over 100 celestial bodies that, in addition to the diurnal period of tidal flux, influence a long cycle that repeats itself every 18.6 years.1 The prime mover of tidal energy systems is the clockwork nature and gravitational impacts of highly predictable patterns of celestial movements within the cosmos. The moon exerts the greatest impact, which is more than twice that of the sun. The moon’s gravitational attraction generates a standing wave of water that becomes the diurnal flood and ebb tide cycles common in many parts of the world. While the height of this daily tide wave is very small when traversing the depths of the open ocean; the height of the tide wave is amplified when the wave approaches the continental shelf. It is significantly influenced by the shallow geometries of bays, harbors and estuaries that increase the height and velocity of tidal movements, as water depth decreases. The global ocean supports tide ranges that can provide extreme changes in water levels in various parts of the world and can range from 10 to greater than 16 meters and estuaries with tidal currents that can exceed 6 m-sec.2 While the power of tidal currents have been used for centuries to drive water mills for mechanical energy to grind grains, it was only in the last fifty years that electric power has been generated from tidal movement. Most of this has been in the form of tidal barrages that block estuaries by means of dams to capture the height difference developed during the high tide cycle and exploit the hydraulic head using conventional power turbines during low tide conditions. Such barrages exist in four countries (France, Russia, China and Canada) but have been known to provide such significant 1 2
White, 2007. Gorlov, 2001.
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negative environmental impacts (such as blockage of circulation and fish migrations) that similar barrages are unlikely to be developed anywhere in the future. New kinetic hydro technologies currently in development however, now hold great promise for tidal energy for countries around the world, especially island nations. These systems generally utilize the un-confined velocity or speed of the tidal flow to generate electric power with no dam. Kinetic hydro systems are distinguished from dams that create hydraulic height differences to generate power by blocking the free flow of tidal currents to create a waterfall, and which impede vital water circulation in sensitive estuarine and near shore environments. TIDAL POTENTIAL A study published by the British House of Commons in 1999 stated that if one-tenth, of one percent of the energy in the world’s oceans, was transformed into electricity, it would supply the world’s demand for electricity five times over.3 A key problem with ocean energy is that the resource must be harnessed in such a way that it can serve the needs of large sectors of the world’s population in order for the global impact to appreciably reduce Green House Gas (GHG) emissions. It is in this way that tidal energy systems can offer the promise of a sufficient impact on global sustainability through clean power generation. When smaller community-scale tidal power systems are installed, they are most effective when established close to nearby electric load centers to reduce power line losses, costs, and impacts. Preliminary evaluations of tidal power potential at a number of island sites worldwide support the view that tidal power offers promise due to such factors as;
Large off shore and near shore areas with energy production potential, High level of predictability of tidal processes, Very accurate repeatability of tides, and Many tidal flows located close to grid connections or shoreline infrastructure can facilitate system installation.
More detailed evaluations of current velocities in nations, regions and specific sites must be done in greater detail to assess the magnitude of the resource and determine the best locations to tap it. This is particularly true with respect to the growing body of site development concerns such as potential impacts on marine ecology, site geology and sedimentation, navigational and marine safety issues, as well as community acceptance and grid interconnection concerns. Estimates of the global potential of the tidal energy resource for power generation vary and are at a fairly early stage of sophistication. Tester and others (2005) predict maximum potential of 20 GW of global kinetic tidal power capacity. Installed capacity of existing systems now consists of approximately 0.1 GW (1 TW/hr per year) globally.4 Several specific studies evaluate tidal power potential of the UK, a leader in tidal power development. These include a recent evaluation by the N-power Juice Fund in 2007 that estimated 3 4
British House of Commons, 1999. Tester and others, 2005.
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there is 11 GW (94 TWhr/year) available from tidal kinetic power in all of the UK. It also estimated that of that total about 25 percent of the best tidal resources, or 3.2 GW, may be extractable during the next five to ten years based on the contemporary understanding of tidal technology efficiency and the rate of investment in the ocean power industry.5 According to a 2005 Black and Veatch Consultants study commissioned by the Carbon Trust, approximately six percent (22 TWhr/year) of the UK power needs could be extracted from the total tidal resource of 110 TWhr/ year based on the present understanding of technical feasibility. Black and Veatch estimate that the tidal power potential for the UK represents approximately half of the European Technically Extractable Resource, while the non-European global Technically Extractable Resource is estimated to be approximately 120 TWhr/year.6 While estimates vary considerably and technology is at an early stage of development, a general approximation for the overall global potential for tidal power could approach a value comparable to the existing power output of conventional ‘land-based’ hydropower, which is approximately 10 per cent of the world’s present electric generation capacity. TIDAL TECHNOLOGY Kinetic hydro tidal turbines contain rotating blades that turn in flowing water and convert mechanical energy into electricity using a gearbox and electrical generator. These systems are generally two primary types, horizontal axis or vertical axis turbines. Another known system generates power from a third type of movement which is oscillatory movement that imitates the fluttering oscillating movement like that of a whale’s or a dolphin’s tail. Horizontal axis – This class of turbines most closely resembles a modern propeller type wind turbine in design, with blades rotating in a plane perpendicular to the axis, which must be oriented into the direction of the flow or tidal current. Examples include Sea Gen’s Marine Current Turbines and the Verdant Power demonstration in the East River near Roosevelt Island, NY, USA. Vertical Axis – Vertical axis turbines have their blades oriented parallel to the axis of rotation rather than perpendicular to it. An early example of this was the Darrieus turbine, which looks like an eggbeater. A more recent variation is the Gorlov Helical Turbine. Oscillatory – Pulse Generation is a single company known to be developing tidal electric power from the oscillatory movement of a hydrofoil inspired by the tail of a whale or dolphin. A 100 kW prototype is now being developed. Most kinetic hydro systems operate like underwater windmills. They operate purely by the conversion of the kinetic energy of the natural water current. For power production, the output of such turbines can be estimated by the power equation below;7
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ABP Marine Environment Research, Ltd. 2007. Black and Veatch Consulting Company, 2005. 7 Bason, 2007. 6
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P (kW) = 0.5*η*ρ*A*v3 η = mechanical turbine efficiency (%/100) ρ = density of seawater (1000 kg/m3 ~1,000) A = area swept by turbine cross-section (m2) v = water current velocity (m/sec.)
For a small tidal turbine with a cross sectional area of 2.5 meters and an efficiency of 35 percent operating in a 2 m/second current (4 knots), the water to wire electrical output would be approximately 3.5 kW. The same turbine operating in a 4 m/second current (8 knots) would approach an electrical output of approximately 28 kW or eight times the output. Many different designs have been proposed for tidal power technologies. Typical components of these systems include: (1) rotor blades, which convert energy from water currents into rotational motion, (2) the drive train, usually consisting of a gear box and generator that converts the rotational shaft motion to electricity, and (3) a structure which supports the rotor blades and gear train. Other key elements that distinguish various types of devices include: Support Structure Types – can be (1) bottom mounted, (2) supported by pylons and resemble underwater windmills, and (3) barge / dock mounted systems. Rotors – can be (1) shrouded (ducted) or (2) open to the water flow. Blades - can be (1) fixed or (2) variable pitch. Yaw – or directional adjustment to the flow can be (1) fixed or (2) controllable angle. Vertical Axis Turbines – can be affected by (1) Drag and / or (2) Lift forces.
Tidal power research programs in industry, federal and state governments and at universities in the UK, Norway, Ireland, Italy, Sweden, Denmark, Canada, Australia and the US over the past six to eight years, have established an important foundation for the emerging tidal power industry. Most recent evaluations of companies at various stages of tidal power development include eight companies in North America8 and ten international companies9. Technical challenges exist for most the kinetic hydro systems now in various stages of testing and development. These include issues related to deploying electric equipment in the demanding marine environment with respect to durability of system components, bearings, blades, structural supports as well as other concerns such as community acceptance, marine navigation, safety and environmental impacts. 8
North American companies include; Blue Energy Canada – prototype testing with the University of British Columbia; Clean Current – deployed ducted prototype in Pedder Bay, British Columbia; GCK Technology – prototype helical version of Darrieus turbine developed by Gorlov; Oceana Energy Company – have acquired 11 US Federal Energy Regulatory Commission (FERC) sites for power development; Ocean Renewable Power Corp (ORPC) – (Red Circle Systems) with OGen prototype; Verdant Power – tests of axial flow system have failed in the East River, NY 2007; Underwater Electric Kite (UEK) Systems – has no recent published performance data; Natural Currents New England – Seeks to set up Regional Tidal Utility at 7 sites, testing 20 kW RED HAWK in 2008. Proprietary design is scalable to 10 MW and larger 9 Hammerfest Strom – 300 kW system operating in Norway’s Kval Sound since 2003; Hydro helix – single test off the coast of France in early stages of system development; Lunar Energy – produced Rotech Tidal Turbine, with 8 deployments scheduled for 2008; Marine Currents Turbines (MCT) – tested a 300 kW unit called the Seaflow; Open Hydro – Irish firm, tests scheduled / completed in Nova Scotia, and Orkney Islands; Ponte di Archimede – Italian company completed prototype 2.5 kW system; Pulse Generation – generates power using oscillatory motion like whale’s tail; SMD Hydrovision – cable stayed dual turbine recently completed a seven week trial; Swanturbines – consortium of 8 companies with 1 meter diameter prototype; Tidal Generation, LTD – working on a 500 kW prototype.
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INNOVATIVE APPROACHES Kinetic hydro systems can provide electricity to power shoreline infrastructure by deploying systems in areas of high water speed. This may best enable systems that are connected with canals, tidal cuts, bridge footings, marinas and docks. Some systems are designed to be installed from floating barges and can be moored in areas of high water speed that coincide with grid interconnection points or specific shoreline community, commercial or industrial needs. The co-location of tidal sites along with offshore wind facilities may reduce installation and operating costs by sharing permitting, maintenance and power line expenses with interested partners may be another way forward. Other innovations that may bear fruit include the development of hybrid power generation using kinetic tidal systems in tandem with shoreline wind and solar electric systems. Others have considered power storage in cold weather environments capturing power when annual snow melts facilitate seasonal hydropower that can convert water to hydrogen for future use in vehicles or fuel cells. Another approach is to provide tidal power for coral reef recovery systems that utilize small amounts of electricity for the regeneration of coral reef structures so threatened in many areas of the globe. The future use of expended oil platforms stations may renew their economic viability by extending power production by means of tidal power that converts seawater to hydrogen to fuel the sustainable cultures of the future. KEY CONCEPTS While many kinetic hydro systems are being developed throughout the world, no single system has become dominant as a commercially viable, off-the-shelf technology. This condition may remain for several years as the public and the investment community gear up their support of tidal power technologies. The industry may become more segmented as the various needs of the marketplace come into focus. There is a clear need for small, flexibly deployed tidal power systems. There is also a top echelon of the market that seeks to establish large scale, tidal electric utilities that can impact the growing specter of climate change by providing megawatt scale systems that produce clean, renewable power to ever greater sectors of the population. Some kinetic tidal systems are limited by the physical laws of material strength and due to their design can only be made of limited size. Such systems as the cross flow Gorlov Helical Turbine and the Verdant Power axial flow, propeller fan turbines are then considered to be modular and must be deployed in extensive fields consisting of many turbines to produce megawatts of power. Other systems realize an economy of scale due to their design such as the Natural Currents Red Hawk turbines that can be produced like many successful wind turbines in ever larger and larger sizes that subsequently reduce the cost of the system per kW or MW installed. Such systems are considered to be scalable, with the Red Hawk reaching a theoretical limit based on the system design and strength of materials of 75 MW from a single turbine system. TIDAL ENERGY FOR ISLAND NATIONS Tidal energy offers many opportunities for island nations, and particularly provides power near shore communities as well as infrastructure thus serving a variety of development needs. While the map below broadly indicates world wide opportunities based on the intensity of the color scale, due
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to the scale of the map, many passages between islands, reefs, near canals, channel cuts and bridges all present excellent opportunities for system deployment. Map 1 Intensity of Global Tidal Flux
Source: Science Magazine, July, 2000 Note: Darker red indicates more intense tidal flux.
China and islands in the East China Sea have a significant, enormous and as yet untapped tidal energy resource. The opportunity to develop clean, renewable energy from this source becomes critically important when viewed as at least a partial alternative to the highly destructive impacts of coal-fired power plants becoming rapidly operational on the Chinese mainland. Image 1 shows a channel cut in south east Majuro, Republic of the Marshall Islands that presents an example of an opportunity to development tidal power near on shore power line infrastructure. Local boaters on Majuro claim tidal currents can reach as high as 10 knots (5 meters per second). Photo 1: Channel Cut from South East Majuro, Rep of the Marshall Islands
Source: Google Earth, 2009
Many islands in the Indian and Pacific Oceans have tidal flows with a range of approximately two meters that will enable sufficient water speeds in many locations to foster tidal electric power. Many island nations in these same areas utilize as much as 50 percent of their foreign exchange earnings 6
importing diesel fuel to power island electric generation while creating pollution problems in both local and global biospheres. Yet almost all of them have tidal channels with enormous untapped flows that in many cases may be sufficient to meet their national energy needs. It is now timely to consider pilot scale, and later utility scale, tidal power systems that can significantly impact both capital and outer island development in a manner consistent with both sustainable planetary stewardship and wise economic growth. SUMMARY The power of kinetic tidal energy systems is a cubic function of water speed. Twice the water speed results in eight times the power output. It is critically important to locate kinetic tidal power systems in water flows that enable both secure deployment and anchoring while maximizing the power output. Almost 20 companies throughout the world are testing tidal power systems based on kinetic water flow. While tidal barrages now operate in four countries in the world, they produce irreparable damage to the ecological systems in which they operate by blocking flow, and so are unlikely to be replicated. The future of tidal power clearly lies with the development of the new kinetic hydro systems that enable recovery of electric power from the speed of natural tidal currents, or winddriven currents. Many island nations throughout the world spend increasing amounts of precious development dollars on fossil fuels to produce electricity. These nations with significant resources of tidal currents could, and should, develop pilot scale and utility scale tidal energy systems that will directly serve their development needs in coordination with other renewable energy sources. In this way, island nations can reduce expensive investments in fossil fuel power plants and invest in clean tidal power and renewable energy systems that will produce electricity and create jobs while simultaneously avoiding environmental, climate change and pollution related risks. Many innovations characterize the emerging tidal electric industry. These include fields of turbines or large scaled systems or both that will significantly impact grid connected power supplies and generation capacity. Likewise smaller systems targeted to residential, community, commercial or industrial use may have acceptance with early adopters such as marinas and island nations all too aware of the need for renewable energy as the basis of future global sustainability.
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Appendix 1: Kinetic Hydro Tidal Technologies
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References ABP Marine Environment Research, Ltd (2007). Quantification of Exploitable Tidal Energy Resources in UK Waters. p. iv.. http://www.abpmer.co.uk/files/report.pdf Bason, Roger (2007). Long Island Tidal and Wave Energy Study. Report for the Long Island Power Authority. Black and Veatch Consulting Company (2005). Tidal Stream Energy Resource and Technology Summary Report. British House of Commons (1999). The Tidal Resource. London, U.K. Gorlov, Alexander M. (2001). Tidal Energy. Academic Press. Northeastern University. Boston, MA. Tester, Drake, Driscoll, Golay, and Peters (2005). Sustainable Energy. MIT Press. MA, USA; p.593. White, Robert Eldridge (2007). Eldridge Tide and Pilot Book. Edited by R.E. White. Eldridge Tide & Pilot Book. Boston, MA.
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