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Ocean Power

Potential sources for ocean energy include wave, ocean current, tidal current, tidal range, ocean thermal energy conversion, and salinity gradient sources.

At the end of 2017, the operating capacity of global ocean energy amounted to approximately 529 MW, among which more than 90% was generated from tidal power came from tidal barrages across bays and estuaries [REN21, 2018].

Imminent Breakthroughs

  • Ocean energy technologies deployed in open waters (excluding tidal barrage) achieved great progress in 2017. Several projects were launched, adding at least 5MW by the end of the year, bringing a total of 17 MW of tidal stream and 8 MW of wave energy capacity [REN21, 2018].
  • It is reported that the abundant ocean energy has the potential to provide 100% to 400% of current global electricity, estimated ranging from 20,000 terawatt-hours (TWh) to 80,000 TWh of electricity a year [Mofor et al., 2014].
  • Over 50% of global RD&D investments in ocean energy are taking place within the EU. In future, the EU is also planning a €320m funding boost for budding ocean energy industry by 2050 [Magagna & Uihlein, 2015; Neslen, 2016].
  • In-stream turbine technology in tidal barrages have made great progress and are nearing commercialization. The two largest ocean energy projects are the 254 MW Sihwa plant in the Republic of Korea and the 240 MW La Rance tidal power station in France [REN21, 2016].

Wave

  • There are over 50 wave energy devices at various stages of development, including oscillating water systems, oscillating body systems, overtopping devices, and power take off systems (Figures 1, 2) [Lewis et al., 2011].

Figure 1. Generic schematic of wave converters and their operation. From NREL and Lewis et al., 2011.

Figure 2. Generic schematic of oscillating body device and overtopping device. From NREL and Lewis et al., 2011.

Thermal Conversion

  • There are two basic Ocean Thermal Energy Conversion (OTEC) designs: closed cycle plants, in which turbine-generator system is driven by expanding working fluids; and open cycle plants, in which turbine is driven by vaporized seawater. Additionally, hybrid OTEC cycle and advanced OTEC cycle plants are being developed [Soto & Vergara, 2014].
  • In an open cycle (Figure 3), sea water creates steam, which is the cycle's working fluid, and desalinated water can be collected [Lewis et al., 2011].

           Figure 3. Open OTEC cycle. From Soto & Vergara, 2014

  • In closed conversion cycles (Figure 4), a secondary fluid like ammonia creates a high-pressure vapor to drive a turbine. Closed conversion cycles are more efficient [Lewis et al., 2011].

           Figure 4. Closed OTEC cycle. From Soto & Vergara, 2014

  • A hybrid conversion cycle (Figure 5) combines closed and open cycles [Lewis et al., 2011].

           Figure 5.Hybrid OTEC cycle. From Soto & Vergara, 2014

Tidal/Currents

  • Various technologies for ocean energy deployment were being advanced by more than 90 tidal energy technology developers around the world during 2017. About half of them focused on horizontal-axis turbines [REN21, 2018].
  • More than 200 companies made efforts on developing different types of wave energy converters, with point-absorber devices being the most common approach [REN21, 2018].
  •  The first tidal turbine arrays with a cluster of multiple interconnected turbines were deployed in 2017 [REN21, 2018].
  • The Sihwa Lake Tidal Power Station (254 MW) established in South Korea is the world’s largest tidal barrage, opened in 2011. Conventional bulb turbines are employed in its tidal barrages [Mofor et al., 2014].
  • One recent trend of ocean energy development is the interest expressed by large Original Equipment Manufacturers (OEMs) such as Al­stom, DCNS, Hyundai Heavy Industries, Kawasaki Heavy Industries, and Siemens [Mofor et al., 2014].
  • French tidal turbine developer Guinard Energies carried out a demonstration of its 3.5 kW P66 turbine, in the purposes of simplifying installation and maintenance in isolated areas, for example, hybrid applications with solar PV and batteries [REN21, 2018].
  • Tidal range
  • Recent advances focus on single or multiple offshore basins that are located away from estuaries, known as tidal lagoons [Lewis et al., 2011]. Tidal lagoons offer greater flexibility with little impact on delicate estuarine ecosystems [Lewis et al., 2011].
  • Tidal range projects constitute impoundments in the form of barrages or coastal lagoons to facilitate a potential head difference with gates and hydro-turbines for energy conversion purpose [Angeloudis et al., 2018].
  • Tidal range is reported recently to have the potential to make sustainable contributions to national electricity demand [Angeloudis et al., 2018].
  • Tidal and ocean current
    • To allow for optimal and flexible vertical positioning, open-ocean current models will need hydrodynamic lifting designs [Venzia and Holt, 1995; VanZwieten et al., 2005].

Salinity Gradients

  • The two concepts identified as potential conversion mechanisms for salinity gradient energy are reversed electro dialysis (RED) and pressure-retarded osmosis (PRO) [Lewis et al., 2011].
    • RED is emerging as one of the most promising membrane-based technologies for renewable energy generation. It harnesses energy by mixing two solutions of different salinity. With equal water volume, RED can achieve about 33-44% energy conversion efficiency [Yip et al., 2014; Tufa et al., 2018].
    • Pressure retarded osmosis (PRO) is one feasible technology that can exploit energy from salt gradients caused by the difference in osmotic pressure through a semipermeable membrane [Holt et al., 2018].

Obstacles to TW-Scale Integration

  • Ocean energy technologies face four main bottlenecks: technology development, finance and markets, environmental and administrative issues, and grid availability [Magagna & Uihlein, 2015].
  • Technological challenges are the main obstacles that hinder the commercialization of ocean energy. Most technologies are still in an early stage that has not yet achieved mature commercialization [de Andres et al., 2017]
  • The main obstacle in ocean energy development is to reduce costs and improve the reliability and performance of systems, in order to making ocean energy competitive in a sustainable energy market [Mofor et al., 2014].  
  • Another challenge is that people find it hard to devise efficient and cost-effective Power Take-off Devices (PTOs) that can address the unique demands of ocean energy [REN21, 2018].
  • Other challenges for commercialization of ocean energy include relatively high risk, high upfront costs, the need for improved planning, consenting and licensing procedures [REN21, 2018].

Wave

  • Very little technological convergence makes wave energy hard to harness, partly due to the diversity of wave resource and the complexity energy extraction from waves. Moreover, the demonstration projects of wave energy converter are generally in an earlier stage that haven’t been commercialized [REN21, 2018].
  • Given the early stage of development, costs of wave energy are still prohibitively high, what leads to the hardness for deployment [Chang et al., 2018].
  • Although wave energy has advantages in spatial concentration and good predictability, it is not entirely steady as it is influenced by wind speed, duration, and fetch, as well as seasonal, latitudinal, and annual variations [Lewis et al., 2011; Mofor et al., 2014].
  • To effectively use wave energy, the variability from wave to wave (measured in seconds), wave group to wave group (measured in minutes) and sea state to sea state (hours-days) needs to be understood. Continually monitoring and controlling the wave energy plant based on predictions is important for high plant utilization. Efforts in this direction have not been widespread [EPRI].
  • Geographical constraint is the main technical obstacle to TW scale integration for wave energy. Site specific analysis for wave energy facilities is needed in order to constrain the spatial resolution of model assessments [Lewis et al., 2011].

Thermal Conversion

  • Thermal conversion technologies experience problems such as vacuum maintenance and heat exchanger bio-fouling issues [Lewis et al., 2011].
  • The technologies of ocean thermal conversion are still in pilot stage, which is way too far from commercial deployment [REN21, 2018].
  • One of the obstacles hindering thermal conversion development is low energy density of the OTEC systems [Mofor et al., 2014].

Tidal/Currents

  • Tidal energy has the most advanced technology among the various kinds of ocean energy. However, it is still in pre-commercial status and limited by availability and high dependence on capital investment [Mofor et al., 2014].
  • Tidal range
    • Development of tidal range power often occurs in estuarine ecosystems, which are fragile and vulnerable to development [Lewis et al., 2011].
  • Tidal and ocean current
    • Certain obstacles inherent to the ocean render tidal and ocean current energy slightly more difficult. Marine turbine designers must take into account reversing flows, cavitation, and harsh underwater conditions (salt corrosion, debris, etc.) [Lewis et al., 2011].
    • The biggest obstacle is the high LCOE that current cost of ocean energy is higher than most of other renewable technologies. Furthermore, the prediction of reduction cost in the long term is still difficult [Segura et al., 2017].
    • Tidal energy technologies are still in high risk of both cost risk and revenue risk [Segura et al., 2017]. 

Enabling Technologies

  • The last four decades have seen substantial advances in the fields of corrosion, submarine cables and communications, materials, and construction due to the exploration for oil and gas, and it is expected that many obstacles will be avoided due to these advances [Lewis et al., 2011].

Wave

  • Wave energy conversion systems are classified into four categories: attenuators, oscillating water columns, overtopping systems, and point absorbers [Melikoglu,2018].
  • Oscillating body Wave Energy Conversions (WECs) harness energy by transferring the power from waves to the motion of structures [Mofor et al., 2014]; related technologies include point absorber, attenuator, and inverted pendulum systems [Mofor et al., 2014].
  • Overtopping devices are applied to transfer water into a reservoir over the water level with assistance of the wave movement [Mofor et al., 2014].
  • A large majority of the wave energy converters is based on oscillating water column technique [Khan et al., 2017].
  • Wave energy has high availability and less energy loss. It is reported to be available 90% of the time, while availabilities of solar and wind power are hardly 20–30% times [Khan et al., 2017].

Thermal Conversion

  • The turbine–generator system of the OTEC power cycle can continuously generate electricity, which is an advantage compared to most renewable energy systems that are intermittent. The associated spin-off technologies can be applied commercially to desalination, air-conditioning, aqua-culture and metal productions [Ravindran & Abraham, 2016].
  • A typical OTEC generally uses natural or low global warming potential (GWP) refrigerants such as ammonia, liquid hydrogen, carbon dioxide, or methanol to drive vapor turbines [Khan et al., 2017].

Tidal/Currents

  • There are some innovative technologies leading the trend of tidal stream energy exploration, including seabed-mounted, horizontal-axis, and axial flow turbines [Mofor et al., 2014].
  • A 3-D modeling approach is used to simulate the tidal energy extraction and reduce bias in resource and environmental impact assessments [Brown et al., 2017].
  • There are two approaches to tidal energy conversion: one is tidal range, which captures the potential energy among tides of different sea levels or by hydro turbines; another is a hydrokinetic approach that captures the kinetic energy from the horizontal flow of tidal currents [Mofor et al., 2014].
  • Tidal range
    • There have been several proposed schemes for tidal and ocean current energy, including axial flow-turbines, crow-flow turbines, and reciprocating devices (Figures 4) [Lewis et al., 2011].

Figure 6. (From left to right) Generic schematic of twin turbine horizontal axis device, cross-flow device, and vertical axis device. From NREL and Lewis et al., 2011.

  • The bulb-turbine is the conversion method most widely used to generate electricity from tidal range [Lewis et al. 2011].
  • Tidal range facilities use well-established in-stream turbine technologies that can also be used in run-of-river hydropower projects, for example, Sihwa in Korea and La Ranceiii in France [REN21, 2018].
  • Tidal and ocean currents
    • Open-water technologies like tidal stream and wave energy converters are deployed with various prototypes. These technologies are in an earlier stage of development [REN21, 2018].
    • Energy from tidal current is harnessed by the rise and fall of sea and ocean waters. It is reported that 4–12 m range spring and neap tides have a potential of 1–10 MW/km along the seashores [Khan et al., 2017].
    • Very few active concepts being developed in Deep Ocean Current while laboratory-scale tank testing has occurred [Mofor et al., 2014].
    • The platform of tidal current power station (TCPS) is used in order to support generators, turbines, and other equipment/tools. Different types of TCPS platforms include floating moored systems, pile mounted systems, and seabed mounted/gravity based systems [Melikoglu, 2018].

Salinity Gradients

  • The first prototype of an osmotic power conversion plant was constructed in Norway in 2009 [Lewis et al., 2011].
  • Reversed electro dialysis (RED) and pressure-retarded osmosis (PRO) are the two main approaches in exploring salinity gradient energy [Yip et al., 2014].
  • High performance concentration capacitors with graphene hydrogel electrodes are used to harvest salinity gradient energy [Zhan et al., 2018].

Political Considerations

  • Many countries with access to ocean have plans to develop ocean energy for sustainable energy strategies. The EU has implemented support mechanisms to aid the development of ocean energy, and 66 MW of ocean energy projects in the EU are expected to become operational by 2018 [Magagna & Uihlein, 2015].
  • Ocean energy provides an opportunity for niche applications to support the various energy networks. For example, in islands, ocean energy supports the Global Renewable Energy Islands Network (GREIN) clusters, including water desalination [Mofor et al., 2014].
  • Policies that apply to ocean technology fall into six categories: capacity or generation targets; capital grants and financial incentives, including prizes; market incentives; industry development; research and testing facilities and infrastructure; and permitting, space, and resource allocation regimes, standards, and protocols [Lewis et al., 2011].
  • Policies that can help remove technological barriers for ocean energy include creating resource maps, improving capital grand funding, expanding international collaboration, and promoting research, development and demonstration [Mofor et al., 2014].
  • To address economic hurdles, policy makers can provide capital support for technologies at the demonstration stage, and accelerate cost and risk reductions through road-mapping [Mofor et al., 2014].
  • Public subsidization methods such as feed-in tariffs (FIT) can support the development of tidal stream energy as it currently cannot be solely funded privately [Melikoglu,2018].

Social Considerations

  • Ocean energy has the potential to be an environmentally friendly and easily integrated grid-connected energy resource that would be suitable for satisfying RPS and other renewable energy capacity and emissions targets [EPRI].
  • Wave energy does not possess the visual and aesthetic concerns as is typical of offshore wind energy. This would make for easier adoption and expansion [EPRI].
  • Ocean energy would stimulate local job creation owing to the indigenous nature of these resources. EPRI estimates that 25 permanent jobs would be created for every 100 MW of ocean energy commissioned [EPRI].
  • Ocean energy helps reduce dependence on foreign energy sources and would help localize energy independence. This would be very beneficial to vulnerable countries [EPRI].
  • One challenge for ocean energy development is that its infrastructure competes with the space available for various interest, such as fishing, boating, and trade [Lewis et al., 2011]. To remove infrastructure barriers, it is important to ensure ocean energy technology is taken into account in network planning, and ensure its supply chain opportunities [Mofor et al., 2014].  
  • Exploration of ocean energy will create job opportunities. Several estimates on the future potential for employment in the sector are available in the world [Uihlein & Magagna, 2016].

Environmental Considerations

  • Like many other renewable energies, the materials, manufacturing, maintenance and decommissioning of ocean energy devices would have negative effects on the environment, although the utilization of ocean energy is strategic in combatting climate change.
  • Compared with fossil fuels, ocean energy generally has fewer greenhouse gas (GHG) emissions. The GHG emissions from wave and tidal are less than 23 g CO2 eq/kWh, with a median estimate of 8 g CO2 eq/kWh for wave energy from a life cycle perspective. Some projects (such as the Crest project), emit only about 2 g CO2/kWh [Uihlein & Magagna, 2016].
  • Ecological impacts from the construction and operation of ocean power project are main environmental considerations that hinder the implementation, for example, the 320MW Swansea Bay tidal barrage project in the U.K. [REN21, 2018]
  • Local ecosystems may be damaged during construction phases but the impact is dependent on region and technology [Lewis et al., 2011].
  • There is an infrastructural footprint of wave technology that may be prohibitive, and it is recommended to keep the project under 1 GW [Lewis et al., 2011].
  • Wave energy extraction potentially has the adverse effects in modifying sediment suspension and transport, with related changes to beach morphology and coastal habitat [Bonar et al., 2015].
  • One of the potential direct environmental impacts of ocean wave and tidal current technology is on benthic species due to alterations in flow patterns, wave structures, sediment dynamics [Uihlein & Magagna, 2016].
  • Marine mammals, turtles, larger fish and seabirds, are also influenced by ocean energy exploration, including habitat change [Uihlein & Magagna, 2016].
  • OTEC coal plants hybrid systems present great environmental benefits of increasing power production and reducing CO2 by 12.42 kgCO2/MWh by using flash evaporators [Khan et al., 2017].

References:

Angeloudis, A., Kramer, S. C., Avdis, A., & Piggott, M. D. (2018). Optimising tidal range power plant operation. Applied Energy, 212, 680-690.

Bonar, P. A., Bryden, I. G., & Borthwick, A. G. (2015). Social and ecological impacts of marine energy development. Renewable and Sustainable Energy Reviews, 47, 486-495.

Brown, A. J. G., Neill, S. P., & Lewis, M. J. (2017). Tidal energy extraction in three-dimensional ocean models. Renewable Energy, 114, 244-257.

Chang, G., Jones, C. A., Roberts, J. D., & Neary, V. S. (2018). A comprehensive evaluation of factors affecting the levelized cost of wave energy conversion projects. Renewable Energy, 127, 344-354.

de Andres, A., MacGillivray, A., Roberts, O., Guanche, R., & Jeffrey, H. (2017). Beyond LCOE: A study of ocean energy technology development and deployment attractiveness. Sustainable Energy Technologies and Assessments, 19, 1-16.

EPRI. http://faculty.washington.edu/emer/eic/Wave_Energy.pdf

Helfer, F., Lemckert, C., & Anissimov, Y. G. (2014). Osmotic power with pressure retarded osmosis: theory, performance and trends–a review. Journal of Membrane Science, 453, 337-358.

Holt, T., Sivertsen, E., Thelin, W. R., & Brekke, G. (2018). Pressure Dependency of the Membrane Structure Parameter and Implications in Pressure Retarded Osmosis (PRO). In Osmotically Driven Membrane Processes-Approach, Development and Current Status. InTech.

Khan, N., Kalair, A., Abas, N., & Haider, A. (2017). Review of ocean tidal, wave and thermal energy technologies. Renewable and Sustainable Energy Reviews, 72, 590-604.

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Magagna, D., & Uihlein, A. (2015). Ocean energy development in Europe: Current status and future perspectives. International Journal of Marine Energy, 11, 84-104.

Melikoglu, M. (2018). Current status and future of ocean energy sources: A global review. Ocean Engineering, 148, 563-573. 

Mofor, L., Goldsmith, J., & Jones, F. (2014). OCEAN ENERGY: Technology Readiness, Patents, Deployment Status and Outlook; International Renewable Energy Agency (IRENA): Masdar City, United Arab Emirates, 2014.2. Pastor, J.; Liu, Y. Power absorption modeling and optimization of a point absorbing wave energy converter using numerical method. J. Energy Resour. Technol, 136, 021207.

Neslen, A. (2016). EU plans €320m funding boost for budding ocean energy industry. https://www.theguardian.com/environment/2016/nov/09/eu-plans-320m-funding-boost-for-budding-ocean-energy-industry

Openhydro. http://www.openhydro.com/Technology/Open-Centre-Turbine

Ravindran, M., & Abraham, R. (2016). Ocean Thermal Energy Conversion. In Springer Handbook of Ocean Engineering (pp. 1245-1266). Springer International Publishing.

Renewable Energy Policy Network for the 21st Century (REN21). (2016). Renewables 2016 global status report. https://www.ren21.net/wp-content/uploads/2019/05/REN21_GSR2016_FullReport_en_11.pdf

Segura, E., Morales, R., Somolinos, J. A., & López, A. (2017). Techno-economic challenges of tidal energy conversion systems: Current status and trends. Renewable and Sustainable Energy Reviews, 77, 536-550.

Soto, R., & Vergara, J. (2014). Thermal power plant efficiency enhancement with Ocean Thermal Energy Conversion. Applied Thermal Engineering, 62(1), 105-112.

The Renewable Energy Policy Network for the 21st Century (REN21). 2018. Renewables 2018 Global Status Report. Retrieved from: http://www.ren21.net/status-of-renewables/global-status-report/

Tufa, R. A., Pawlowski, S., Veerman, J., Bouzek, K., Fontananova, E., di Profio, G., ... & Curcio, E. (2018). Progress and prospects in reverse electrodialysis for salinity gradient energy conversion and storage. Applied Energy, 225, 290-331.

Uihlein, A., & Magagna, D. (2016). Wave and tidal current energy–a review of the current state of research beyond technology. Renewable and Sustainable Energy Reviews, 58, 1070-1081.

VanZwieten, J., Driscoll, F.R., Leonessa, A. & Deane, G. (2005). Design of a prototype ocean current turbine–Part I: mathematical modeling and dynamics simulation. Ocean Engineering, 33(11-12), pp. 1485-1521

Venezia, W.A., & Holt, J.  (1995). Turbine under Gulf Stream: potential energy source. Sea Technology, 36(9), pp. 10-14

Yip, N. Y., Vermaas, D. A., Nijmeijer, K., & Elimelech, M. (2014). Thermodynamic, energy efficiency, and power density analysis of reverse electrodialysis power generation with natural salinity gradients. Environmental science & technology, 48(9), 4925-4936.

Zhan, F., Wang, Z., Wu, T., Dong, Q., Zhao, C., Wang, G., & Qiu, J. (2018). High Performance Concentration Capacitors with Graphene Hydrogel Electrodes for Harvesting Salinity Gradient Energy. Journal of Materials Chemistry A.

 

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