Volume 8 ocean energy 8 01 – generating electrical power from ocean resources

Volume 8 ocean energy 8 01 – generating electrical power from ocean resources Volume 8 ocean energy 8 01 – generating electrical power from ocean resources Volume 8 ocean energy 8 01 – generating electrical power from ocean resources Volume 8 ocean energy 8 01 – generating electrical power from ocean resources Volume 8 ocean energy 8 01 – generating electrical power from ocean resources Volume 8 ocean energy 8 01 – generating electrical power from ocean resources Volume 8 ocean energy 8 01 – generating electrical power from ocean resources

8.01 Generating Electrical Power from Ocean Resources AS Bahaj, The University of Southampton, Southampton, UK © 2012 Elsevier Ltd 8.01.1 Introduction 8.01.2 Wave Energy Conversion 8.01.3 Marine Current Energy Conversion 8.01.4 Technology Development Assessment 8.01.5 Prototype Device Development and Commercial Farms 8.01.6 Future Prospects Acknowledgments References 1 6 8.01.1 Introduction The current global energy needs, their associated security of supply, and price fluctuations are at the forefront of the energy debate In addition, environmental issues associated with the current energy generation, which is centered on the utilization of fossil fuels especially for the production of our electricity needs, and the impact of climate change will require new thinking in terms of energy efficiency and implementation of policies to exploit nonpolluting natural resources Inherently, the utilization of such resources implies the development of low carbon technologies, which can also be perceived as the current cornerstone in many of the developed and developing countries aimed at reducing our dependence on fossil fuels This reduces emissions and creates new jobs (green revolution) Nevertheless, such aims will need strong support in terms of investment and policies targeting support to enhance energy-generating capacities through the development of low carbon technologies, especially those from renewable resources It is also clear that the current economic climate offers a window of opportunity for new growth through targeted elements of the various stimuli packages initiated by many governments around the world for the expansion of renewable energy technologies Ocean energy resources derived from wind, waves, tidal, or marine currents can be utilized and converted to large-scale sustainable electrical power Conversion systems are easily adaptable and can be integrated within the current utility power supply infrastructure and networks However, in the development of renewable energy technologies, many countries have embarked on policies that are highly reliant on the expansion of large-scale offshore wind energy to electrical power, with only little attention being directed at other areas of renewable energy Although this is understandable, as wind technologies are far more mature, it is important not to marginalize the utilization of other ocean resources by concentrating effort on and diverting the available financial resources to offshore wind only Ocean energy has many forms – tides, surface waves, ocean circulation, salinity, and thermal gradients The focus of this volume of Comprehensive Renewable Energy is dedicated to two of these Those found in tidal or marine currents, driven by gravitational effects, and wind-driven waves, derived ultimately from solar energy Globally, tidal dissipation on continental shelves has been estimated at 2.5 TW [1] The United Kingdom, which is currently considered the world’s leader in the technological conversion of ocean energy resources, has estimated shoreline resources of approximately 10% (0.25 TW) of the global tidal resource If one-tenth of this figure could be tapped for power generation (which would undoubtedly require a very large capital investment), tidal stream or marine current power could deliver around 220 TWh a−1, which roughly equates to half of the United Kingdom’s current electricity consumption Whilst most of the incident wave energy is dissipated in deep water, where economic exploitation is yet to be demonstrated, there is nevertheless a significant nearshore resource estimated at 1.3 TW globally by the European Thematic Network on Wave Energy, with a technically exploitable resource of 100–800 TWh a−1 [2] In addition, the United Kingdom is also one of the countries that has one of the most energetic of wave climates, which could provide up to 50 TWh a−1 [3] This volume of Comprehensive Renewable Energy gives an analysis of the current state of art of generating electricity from wave and tidal currents (termed ocean energy) This chapter gives an overview of ocean wave and marine current energy conversion with more emphasis on the latter; Chapters 8.02, 8.03, 8.04, and 8.05 address the history of wave energy, wave resource assessment, wave device development, and air turbines, respectively; and Chapter 8.06 reviews the economics of ocean energy 8.01.2 Wave Energy Conversion Wave energy is inherently stochastic, being a consequence of airstreams or wind energy Wave energy stems from wave motion and as can be seen below, its power is related to the wave height and period There are many technological variants, each of which is aimed at exploiting the various properties of wave action Deep-water sea waves offer large energy fluxes under predictable conditions over periods of days Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00807-6 Generating Electrical Power from Ocean Resources The power P per unit width of a wavefront is given by � �� � ρg � 2 Pẳ Hs Te 64 ẵ1 where P is in (W m−1), ρ is the density of sea water (kg m−3), g is the acceleration due to gravity (m s−1), Hs is the significant wave height (m), and Te is the wave period (s−1) [4] More in-depth theoretical analysis of the fundamentals can be found in the now standard textbook on wave energy conversion [5] The conversion of wave energy into usable energy is extremely complex due to the hydrodynamic processes present in the diffraction and radiation of the waves as they propagate to shore In essence, the conversion to electrical power is subject to hugely varying energy fluxes and time scales (few seconds in wave action and hour or days in sea states) that the conversion technology needs to cope with and, once conversion occurs, conditioning the generated power to the 50 or 60 Hz of the electrical utility grid is also challenging The conversion is established through what is known as a power take-off (PTO) system, such as an air turbine, power hydraulics, electrical generator, and other variants that can be exploited for the production of usable energy Wave energy conversion has its roots in our response to the oil crises of 1973 (see Chapter 8.02) and consequently this area is much older than the relatively new marine current energy conversion An in-depth review of the status of energy conversion technologies is given in Reference It should be noted that both wave energy and marine current converters (Section 8.01.3) will need to be designed not only to generate power but also to ensure survivability of the device The designs will need to withstand the most severe conditions expected in their lifetime For instance, the concepts being developed vary greatly and since it is not possible to predict with great accuracy the severity of a storm at a certain location, a probabilistic approach will be needed to determine design conditions and the acceptable level of risk for the device being developed [6] 8.01.3 Marine Current Energy Conversion Marine currents are a form of kinetic energy and are generally diffuse, but concentrated at a number of potential sites around the world where sea flows are channeled around or through constraining topographies (e.g., islands and straits) The tides, which drive such currents, are highly predictable, being a consequence of the gravitational effects of the planetary motion of the earth, the moon, and the sun Such characteristics offer marine current energy converters (MCECs) an advantage over other renewable energy resources – such as wind or wave energy [7, 8] This is because power generation from developed projects that rely on marine currents will have quantifiable and firmly foreseeable future energy yields that can be planned for and managed appropriately within the normal utility grid Additionally, the quantifiable long-term energy yields offer a particular advantage to project developers for negotiating, with electrical utility companies, a better power purchasing agreement compared with other renewables The utilization of marine currents is akin to that of wind energy resource conversion; that is, the kinetic energy of the moving fluid can similarly be extracted and applied using a suitable type of turbine rotor It does not, therefore, require water-impounding structures such as dams used in conventional hydropower but some sort of anchoring system within the flow stream In addition, and in most cases, the fundamental understanding needed has similar basis as those used to predict the energy in a moving fluid as employed in wind energy conversion The analysis offered for the consideration of wind turbines can be extended to marine current turbines The power Po (W) available from a stream of water (in the absence of significant changes in depth or elevation) is given by Po ẳ Avo3 ẵ2 where ρ (kg m−3) is the density of fluid, A (m2) is the cross-sectional area of the rotor under consideration, and vo (m s−1) is the unperturbed fluid speed As can be seen from Reference 2, Po is proportional to the cube of the fluid velocity; hence, the energy produced is highly sensitive to variations in the fluid velocity In addition, the power in the flow is also proportional to the fluid density, which for water is about 800 times that of air This indicates that the power density or flux (kW m−2) for marine current energy resource will be appreciably higher than that produced by wind energy when considered at appropriately rated speeds for both technologies [7] The consequence of this is that smaller and hence more manageable MCECs can be installed to exploit local conditions, such as water depth or bathymetry where they are favorable However, water depth in practice places a constraint on the maximum rated power of a marine current turbine Such a constraint does not exist for wind turbines If one considers technology variants that are similar or related to those of wind energy conversion (although other unique design philosophies are being pursued), estimation of energy capture can be undertaken For instance, the classic analysis of power extraction which stems from a wind front intercepted by an actuator disk [9] can be used In the case of a horizontal axis turbine, the analysis states that the maximum power that can be extracted by a single turbine in an unconstrained flow is the fraction 16/27 (=0.59) of the kinetic energy flux through the actuator (rotor) disk area, and the power, in the case of no extraction, is given by eqn [2] In general, this fraction is known as the power coefficient Cp, defined by � � P P ẳ ẵ3 Cp ẳ Po ρAvo Generating Electrical Power from Ocean Resources where P is the power developed by the rotor For all wind turbines currently in operation, Cp < 0.59; however, more sophisticated design methods allowing for the effects of finite numbers of blades predict for typical designs, maximum values of Cp in the range 0.4–0.5 [9, 10] Such analysis also applies to the case of similar turbines in marine current sites or tidal stream channels, provided these are wide and deep compared with the rotor disk diameter and that there is only a small change in free surface elevation across the turbine location [10, 11] In eqn [3], Cp can be thought of as the effectiveness of a device in generating power, regardless of flow speed or capture area of the device In must be noted that there are major differences in the engineering of marine current turbines as compared with those operating in wind energy This is particularly due to the higher density of water compared with air, the closer proximity of the free surface, and the much slower speed of flow and cavitation Furthermore, installations of such converters in fast-flowing seas will clearly present structural engineering challenges for both system integrity and the foundations of the submerged structure [12] 8.01.4 Technology Development Assessment There is a plethora of concepts with different designs targeting energy extraction from wave and marine current resources It is therefore unrealistic to establish a common approach that can arrive at an assessment of the various concepts being developed Nevertheless, one can arrive at a common characterization of operating principles which could be used to establish a class of technology within wave or marine current energy conversion In both cases, energy capture is achieved through a structure that converts the resource into mechanical energy and then through conventional rotating or linear generators – known as the PTO – into electrical power for supply to an electrical grid In wave energy, air, water turbines, as well as hydraulic motors can be used to provide the mechanical coupling that converts the alternate motion of the waves into a continuous one-directional motion (Chapter 8.02 and Reference 6) In marine currents, a rotor such as in a horizontal axis turbine can be used Rotor design encompasses blade design, in which the mechanical motion is transferred to a type suitable for input to a stage where electrical power is produced This subsystem may include a gearbox, a set of lever arms and pistons driving hydraulic fluid, and/or an electrical generator of a fixed or variable speed type In most cases, the generated electricity is conditioned and transmitted to the grid through a subsystem which includes power electronics, transformers, circuit breakers, and cables In order for the technology to succeed, progress will need to span from research and development to project-scale implementa­ tion Currently, there is a global community involved in various phases of wave and marine current energy device and project development To date, there are only a handful of devices that can be classified to be at the commercial pilot demonstration stage, and much effort is concentrated at research and development and testing at scale stages at various sea test sites such as the European Marine Energy Centre (EMEC) [13] However, it is recognized that there are large numbers of concepts being considered or developed and that these efforts seem to dilute the already scarce financial support Hence, there is a need for some consolidation Nevertheless, development of wave and marine current technologies will need to consider the appropriate steps needed to establish reliable devices and coherent projects In Figure 1, such steps are highlighted in terms of their relationship both to the technology (device) and to the project development (site and any associated environmental impacts) The former is related to the conception of the original idea and its development whilst the latter is more related to deployment at a pilot/commercial scale in the sea For instance, in order to provide reliability of the systems being developed, conversion devices will need to be subjected to technology readiness level (TRL) or assessment protocols [14–17] An introduction to TRLs and the step-by-step development of WECs are addressed in Chapter 8.04 Further analysis of the various sections of Figure can be found in Reference 12 Devicespecific issues Energy capture Power take-off Control systems Electrical conversion Electrical transmission Fixing/moorings, including cabling Economics Modeling/energy price Financing/insurance Vessels/ports Grid Projectspecific issues Site resource/device Consent and permitting Infrastructure/layout Arrays Deployment Maintenance Environment Site and resource assessment Environmental impact Other impacts Stakeholders consultations Risk issue Infrastructure proximity-ports, etc Technical capabilities People and safety Figure Developmental issues for wave and marine current energy conversion technologies spanning device and project development Adapted from Bahaj AS (2011) Generating electricity from the oceans Renewable and Sustainable Energy Reviews 15(7): 3399–3416 4 Generating Electrical Power from Ocean Resources The technology can be considered to be mature when multiple devices at the tens of megawatts are deployed in the sea and the full reliability of the systems and their maintenance needs are quantified Such deployment will require a developer to identify a project site, undertake the various steps needed to obtain consent, and permit and have a robust plan for deployment and maintenance The investment in such a site is dependent on the perceived energy production This in turn will depend on the intensities of the resources, be it wave characteristics or water flow velocities Generally, and as can be seen from Figure 1, the overall cost of an ocean energy project – wave or a marine current – will be likely dominated by capital and operating costs including upfront cost of consenting, surveys, and so on (Chapter 8.06) Since the revenue is mainly dependent on the flow or wave conditions, the profitability of a project is highly dependent on a clear understanding of site conditions including wave and flow field characteristics Hence, resource and site assessments will be crucial in any economic analysis of the viability of an ocean energy project In addition, there are many factors which also need consideration: weather windows for deployment, understanding of seabed conditions, availability of vessels, proximity to a grid connection, and ports In most cases, especially in the United Kingdom, the grid connection issue is similar to that encountered with other renewable energy technologies such as offshore wind and hence will not be discussed here 8.01.5 Prototype Device Development and Commercial Farms In both wave and marine current energy conversion, there is currently, to the author’s knowledge, no commercially operated capacity in the world However, in terms of prototypes operating at anticipated future commercial device capacities, there are notable installations within the United Kingdom and elsewhere A survey of publicly available information on prototype device development and deployment as well as commercial aspirations can be found in Reference 12 What will underpin the expansion of commercial activities in wave and marine current energy conversion will be the deploy­ ment and installation at array or farm scale Currently, there are no such farms or arrays of multiple devices deployed and operating in the sea However, a notable exception has been the installation in northern Portugal in 2008 of a set of three Pelamis P1 wave energy devices representing the world’s first grid-connected wave farm having a capacity of 2.25 MW (Figure 2) This Aguỗadoura wave energy project was supported by a generous feed-in tariff of approximately €0.23 kWh−1, but was unfortunately abandoned for various reasons attributed mainly to the financial collapse of one of the partners in the project [18, 19] Nevertheless, the technology developer Pelamis Wave Power Ltd [20] is currently progressing with the testing of their P2 converter at EMEC, and plans for larger projects of farms within the United Kingdom are underway [12, 20] For technology progression and for gaining operational experience in the sea, it will be imperative that the next phase in the development of wave and marine current energy devices is at the multimegawatt scale of multiple devices in array or farm configurations This is a necessary step that will allow project developers to gain the required experience at all stages of deployment; quantify the production of power at farm scale; and provide the economic evidence of the scale effects (deployment, installation, connectivity, maintenance within the operating environment, etc.) that are deemed necessary for cost reductions in this important field of renewable energy conversion An approach that has been developed around the world, which is hoped to provide seamless project development in wave energy conversion, is the creation of infrastructure zones in the sea to provide facilities to developers These provisions take away from developers the prolonged and costly aspects associated with all the difficulties of permitting, consenting, and planning needed for operation in the ocean Such marked and confined ‘ocean zones’ in effect offer ‘deploy-and-plug’ facilities in which all device-specific issues are taken care of [12] Currently, there are only two globally available zones, one in the United Kingdom – Wave Hub – which is operational (Figure 3) and the other in Portugal – Wave Pilot Zone (WPZ) The United Kingdom’s Wave Hub has an initial capacity of 20 MW that could be scaled up to 50 MW in the future, and plans are afoot to have its first array in 2013 [21] The WPZ, established on the west coast of Portugal about 130 km north of Lisbon, has an initial capacity of 80 MW that could be expanded to 250 MW in the future [22] These sites will undoubtedly help in progressing technology development and allow Figure World’s first wave farm of three Pelamis P1-A machines of 750 kW each installed at Aguỗadoura, Portugal Image courtesy of PWP Ltd [18] Generating Electrical Power from Ocean Resources Figure World’s first deploy-and-plug ocean zone installed at Wave Hub, UK Image courtesy of Wave Hub, www.wavehub.co.uk experience to be gained at both the deployment and the installation phases Overtime, technical, operational, and maintenance data will be generated that will help with the development of second-generation devices 8.01.6 Future Prospects It is clear from current planned activities in wave and marine current energy conversion that the impact of the economic crises has not been as disastrous as for other sectors The impetus for deployment at farm scale is gathering pace Scotland, with support from its independent executive, seems to provide the necessary support to enhance technology prospects for deploying at the required scale Over the past years, several developers and electrical generation utilities have jointly been preparing for commercial-scale deployment projects in the Scottish Pentland Firth In the United Kingdom, the Crown Estate owns the seabed around the coast and has been offering site options and leases for projects for offshore wind and now wave and marine current energy conversion The recent results, specifically for the latter, announced by the Crown Estate for Round Pentland Firth and Orkney waters have created ambitious plans to install up to 1.6 GW of multiple devices in farms These plans encompass an initial 10 demonstration and commercial projects totaling 1.2 GW of potential capacity covering different technologies These are divided as 600 MW wave energy capacity and 600 MW marine current capacity having an estimated total cost of approximately £4.2 billion (US$7 billion) and are planned to be installed by 2020 [23] In addition, the highlighted projects will need a new electrical grid or strengthening of the existing electrical grid and other infrastructure costing around £1 billion (US$1.5 billion) The extra investment is proposed to be covered from public sources to develop and build not only new grid connections but also harbors and other infrastructure in Orkney and Caithness The idea behind the above venture is similar to that proposed for the development of specific ‘ocean zones’ for technology installation mentioned earlier The Crown Estate being a project partner will in effect be offering seamless support transcending bottleneck issues such as permitting, consenting, and in this case including financial support However, a lot more will need to be in place, for example, the long-term incentives needed from the government to bridge the financial investment gap in such projects [12, 24, 25] Another issue that is a major barrier for the wave and marine current energy conversion technologies is the cost per megawatt installed This is estimated to be in the range £7–10 million (US$11–15 million) per megawatt installed, with the lower value for multimegawatt installations and the higher one for a single commercial prototype This will need to be surmounted not only by industry but also the required government support mechanisms to at least achieve parity with offshore wind currently costing around £3 million MW−1 installed 6 Generating Electrical Power from Ocean Resources In summary, given all the above, there are also other important issues that will need addressing, most of which is the need for the technology to prove itself within the operating environment, that is, to gain operational experience in the sea This experience is paramount as it gives confidence to investors, energy utilities, and governments in the viability of the technology In addition and from a technological point of view, the viability of the technology will also depend on achieving long-term operational reliability of the devices coupled with appropriate maintenance regimes It must also be remembered that marine current and wave energy conversion technologies are still in their infancy However, progress to multimegawatt deployment in farms is likely to be fast as judged by (a) the plans for Pentland Firth mentioned earlier, (b) global activities such as those in Korea and Canada, and (c) the recent entry to the this area of China Additionally, this notion is also supported by the prepared ‘ocean zones’ – Wave Hub and WPZ – which will also provide impetus for technology deployment and the deployment of four grid-connected, large-scale precommercial devices in the sea at EMEC and plans for large-scale test centers in Canada, the United States, and Sweden These activities and consolidation provided by the entry to the market of electrical utilities provide further evidence that the future for wave and marine current energy conversion looks bright and that the large-scale road technology rollout will not be long Acknowledgments This work is part of the research themes on ocean energy conducted by the Sustainable Energy Research Group within the Faculty of Engineering and the Environment at the University of Southampton Funding for ocean energy research is from various sources including the UK’s Engineering and Physical Science Research Council (EPSRC), the Technology Strategy Board (TSB), the European Union (EU), and industry Full details of the group’s program can be found at www.energy.soton.ac.uk References [1] Egbert GD and Ray RD (2003) Semi-diurnal and diurnal tidal dissipation from TOPEX/Poseidon altimetry Geophysical Research Letters 30(17): OCE 9-1–OCE 9-4 [2] Wavenet(2003) Results from the work of the European Thematic Network on Wave Energy European Community http://www.waveenergy.net/Library/WaveNet%20Full% 20Report(11.1).pdf [3] Thorpe TW (1999) A brief review of wave energy Technical Report ETSU-R120, May [4] Twidell J and Weir T (2006) Renewable Energy Resources, 2nd edn London; New York: Taylor & Francis [5] Falnes J (2002) Ocean Waves and Oscillating Systems Cambridge: Cambridge University Press [6] de Falcão AFO (2010) Wave energy utilization: A review of the technologies Renewable and Sustainable Energy Reviews 14: 899–918 [7] Bahaj AS and Myers LE (2003) Fundamentals applicable to the utilisation of tidal current turbines for energy production Renewable Energy 28(14): 2205–2211 [8] Fraenkel PL (1999) New developments in tidal and wave power technologies In: UK-ISES Proceedings C73, pp 137–145 [9] Betz A (1966) Introduction to the Theory of Flow Machines Oxford: Pergamon Press [10] Burton A, Sharpe D, Jenkins N, and Bossanyi E (2001) Wind Energy Handbook Chichester, UK: Wiley [11] Blunden LS (2009) New Approach to Tidal Stream Energy Analysis at Sites in the English Channel PhD Thesis, University of Southampton [12] Bahaj AS (2011) Generating electricity from the oceans Renewable and Sustainable Energy Reviews 15(7): 3399–3416 [13] www.emec.org.uk [14] Bahaj AS, Blunden LS, and Anwar AA (2008) Formulation of the tidal-current energy device development and evaluation protocol Sustainable Energy Series, Report August [15] Department for Business, Enterprise and Regulatory Reform (2008) Tidal-Current Energy Device Development and Evaluation Protocol URN 08/1317 http://www.berr.gov.uk/ files/file48401.pdf [16] http://www.emec.org.uk/national_standards.asp [17] OCEAN ENERGY (2003) Development & Evaluation Protocol, Part 1: Wave Power HMRC September [18] http://www.pelamiswave.com/our-projects/agucadoura [19] http://www.guardian.co.uk/environment/2009/mar/19/pelamis-wave-power-recession [20] www.pelamiswave.com [21] http://www.wavehub.co.uk/news/press_releases/wave_hub_plugged_in_and_open.aspx [22] http://www.wavec.org/client/files/10_02_02_Madrid_Ana_Brito_Melo.pdf [23] www.thecrownestate.co.uk/energy/wave-and-tidal [24] http://www.ofgem.gov.uk/Sustainability/Environment/RenewablObl/Pages/RenewablObl.aspx [25] http://www.decc.gov.uk/en/content/cms/what_we_do/uk_supply/energy_mix/renewable/policy/renew_obs/renew_obs.aspx ... characteristics offer marine current energy converters (MCECs) an advantage over other renewable energy resources – such as wind or wave energy [7, 8] This is because power generation from developed projects... general, this fraction is known as the power coefficient Cp, defined by � � P P � ¼ � ẵ3 Cp ẳ Po Avo Generating Electrical Power from Ocean Resources where P is the power developed by the rotor For... at Aguỗadoura, Portugal Image courtesy of PWP Ltd [ 18] Generating Electrical Power from Ocean Resources Figure World’s first deploy-and-plug ocean zone installed at Wave Hub, UK Image courtesy