An Overview of Ocean Renewable Energy Technologies

          The untapped potential of ocean renewable energy is vast like the ocean’s uncharted depths.  And like the deep ocean, it is also mysterious —since most technologies that capitalize on the sea as an energy resource are still in the early stages of development and testing.  Therefore, Bedard et al. (2010) state that it is unclear which technologies will be the most cost efficient and reliable while producing the fewest environmental effects.  The technologies currently being developed include ocean wave, thermal, tidal/open-ocean current, tidal barrage, salinity gradient and shallow/deepwater offshore wind energy.  Among these, only shallow water offshore wind energy has reached the status of a fully-deployed commercial technology.  Part of the reason the other technologies are not yet commercial is because of the time it takes for development and testing.  This process, from initial concept to deployment of a full-scale model in natural waters is estimated by Bedard et al. to take at least 5 to 10 years. Consequently, the future for ocean energy technology is bright since only a glimmer of its potential impact has been seen. —Juliet Archer
Bedard, R., Jacobson, P., Previsic, M., Musial W., Varley, R., 2010. An overview of ocean renewable energy technologies. Oceanography 23, 22–31.

In their review, Bedard et al. explain the history, current status and different concepts or designs of each of the above mentioned ocean energy technologies.  As of their writing, only 4 mega-watts (MW) of wave energy have been installed worldwide.  Most of the deployments of concepts such as point absorbers, overtopping terminators, linear absorbers, and oscillating water column terminators (OWC) have been small-scale prototypes.  In the United States, further installation of wave energy technology has been hindered by a lack of standardized facilities in which to test wave energy devices in the open ocean.  However, these and other challenges are recognized and are being met by private and public groups, such as the Northwest National Marine Renewable Energy Center (NNMREC) funded by the United States Department of Energy (DOE), and Pacific Gas and Electric (PG&E).  In Portugal these problems have already been confronted, resulting in the first commercial wave energy plant being deployed in 2008.  Furthermore, demonstration projects are continuing and planned at other sites around the world, including Australia and Ireland.  If these early projects are successful, Bedard et al. predict that wave energy technologies with a total of 100 MW of capacity will be deployed in five to ten years. 
Tidal current or hydrokinetic ocean energy technologies have only been installed in rivers or less than one kilometer from coasts.  The three types of water turbines that exploit the kinetic energy of moving water are axial, cross-flow and combination axial and cross-flow turbines.  In addition there are also non-turbine designs such as oscillatory hydrofoils, hydro venturi and vortex induced motion devices.  In the United States, hydrokinetic technology has been tested in New York’s East River but like wave energy, hydrokinetic technologies lack proper infrastructure to deploy and test devices in tidal passages.  However, in-stream demonstrations in rivers continue around the world.   If results are successful, the authors expect that tidal energy capacity will increase by 1–10 MW within five to ten years.  Despite studies in the 1970s determining minimal adverse environmental effects, technology has not yet been developed to harness the power of open-ocean currents, like the Gulf Stream. 
Offshore wind energy is a promising resource because of the location of high-wind areas near some of the world’s largest cities.  In comparison to onshore wind energy technology, offshore technology does not suffer from the same transportation and installation restrictions.  However, the cost of infrastructure and logistical support for offshore units is significant and capital costs are typically double those of onshore turbines.  Therefore, offshore units tend to be larger in order to maximize the value of the infrastructure.  In 2008, the worldwide capacity of offshore wind energy was 1,471 MW.  This is insignificant in comparison to the almost 121,000 MW of total installed wind power in the same year.[1]  Overall, the industry is challenged by high costs, especially of operation and maintenance, in comparison to mature land-based wind technology.  However, Germany, China and the United States, among other countries, are all in processes of adding new capacity.  Furthermore, the first full-scale floating turbine was deployed in 2009 and exemplifies the infancy and potential growth of the industry.
Ocean thermal energy conversion technologies (OTEC) extract energy from the difference in temperature between cold deep ocean water (less than 40°F) and warm surface water (more than 80°F).  The challenge of developing commercially viable OTEC energy technologies is currently being undertaken by a number of small companies, some of which are funded by the DOE.  The major difficulties in the commercialization of OTEC are that the capital costs are high and the resource has a low-energy density.  At the time of writing, no major commercial OTEC technologies have been installed. 
Salinity gradient energy conversion technology, like OTEC, extracts energy from a physical gradient.  Salinity gradient power or osmotic power exploits the differing salt concentrations in fresh and sea water.   The methods for this process are reverse electro dialysis (RED) and pressure-retarded osmosis (PRO).  Currently, the Netherlands and Norway are the only countries developing salinity gradient technologies for commercial use.  In the Netherlands, plans have been proposed to exploit the salinity differential between the Afsluitdijk dike and the ocean with RED technology.  In 2009, Norway installed the world’s first salinity gradient power plant using PRO technology, designed for 10 kW of capacity.  If this plant is successful, then the construction of commercial osmotic power plants is probable in the next few years.  Based on the current state of ocean renewable energy technologies, rapid growth in the use of ocean–based renewable energy can be expected. 


[1] “Global Wind 2008 Report,” Global Wind Energy Council, accessed January 23, 2011, http://www.gwec.net/index.php?id=153&L=0. 

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