The oceans cover some 72% of our planet. We need not look further than the constant ebb and flow of tides to recognise that there’s an enormous amount of energy locked up in our oceans.
Tides, waves, currents and thermal dynamics of oceans represent energy dense resources yet to be harnessed. And that’s the thing – we’re really no where even close to capitalising on the energy stored in the waters of our planet’s oceans. It is perhaps an irony – after all, humankind has always sought proximity to the sea. Oceans have provided bountiful harvests, means for sanitation, and ways of transport. In more recent times, since the industrial revolution our reliance on placing cities and towns close to sources of water was necessitated by the role of water in industrial techniques.
To this day, and some 60% of the world’s population still live within just 60km of a coastline. Our kinship with the ocean is absolute. And yet, despite our endeavours for clean, renewable energy we haven’t yet managed to bring the power of the oceans into the energy fold.
A plethora of concepts for marine energy systems exist at various stages of development across the world – but few have left the drawing board. With the exception of tidal systems, almost all are conceptual, undergoing research and development, or are in the pre-commercial prototype and demonstration stage.
We can’t cover them all, but in the interests of conveying some sense of the current landscape of marine energy, its various forms and the technologies involved, we consider some of the most promising case studies and note what’s standing in the way of our harnessing the power of the oceans.
Forms of Marine Energy
The oceans represent an energy resource in several ways, each of which requires particular technologies to harness and convert into electricity. The main forms of marine energy include wave power, tidal power, salinity gradients (osmotic power), thermal energy, and marine current power.
Albeit that they have distinct forces driving them, waves, tides and marine currents are similar in that they represent forms of kinetic energy determined by the movement of large bodies of water. As such all three may be tapped using marine turbines which are driven by the force of water.
Wave power is applicable in virtually any open coastal region with significant waves; while tidal power systems work well in coastal regions or the mouths of rivers (estuarine regions) featuring large tidal flows.
Marine current power involves harnessing energy of ocean currents such as the Gulf Stream. At present there aren’t any such demonstration systems in operation.
Osmotic power derives from differences in the salinity levels of oceans that results from the mixing of fresh water and salt water. The salinity gradient that is created may be used to create energy using pressure-retarded reverse osmosis processes and associated conversion technologies.
The thermal energy of the oceans is largely a result of the sun, which heats waters to varying degrees depending on how far it may penetrate its surface and the intensity of sunlight. Areas with the greatest thermal differential – typically in the tropics – represent the most suitable regions for thermal energy conversion. The differential is harnessed using special fluids, which vaporise at higher temperatures to produce high pressure steam that can be used to drive a turbine and generate electricity.
A Picture of Emerging Marine Energy Industry
According to IHS Emerging Energy Research (IHS), worldwide there are ocean projects in development across 16 countries, producing a combined total of 1.8GW. Relative to the estimated energy potential of marine environments, this is a very small figure.
At face value it’s difficult to imagine how one might calculate the energy potential of the marine energy. Nevertheless, although estimates vary (massively) by every indication the amount of energy which could be harnessed using foreseeable technologies vastly exceeds current human energy demands. Marine energy around the United States alone could eventually supply 10% of US energy, with 50,000 MW off of the Northwest coast – an amount equal to the output of 50 nuclear plants (Hebling’s Research). Meanwhile it is widely agreed that global tidal stream energy capacity could exceed 120GW (Marine Current Turbines).
While we’ve known about techniques for harnessing some forms of marine energy for decades, it’s really only been in the last ten years that significant progress has been made in developing large scale marine energy systems.
This is a landscape poised for great change though. While there are considerable challenges to this development, in particular high costs, it’s reasonable to predict that marine energy will come to be a major asset in the world’s renewable energy mix in the twenty years.
Globally, the UK currently holds the lead in marine energy if we are to gauge success in terms of research, investment and active projects. This is due in part to government policy geared at fostering marine energy technologies and industry that has been motivated by recognition that the UK is so ideally suited to multiple forms for marine energy.
The coastal waters of the UK represents one of the largest marine energy resources in the world, estimated at more than 10GW, representing about 50% of Europe’s tidal energy capacity (Marine Current Turbines). UK tidal energy alone is estimated to be able to power some 15 million homes.
As of 2011, there were already some 300MW of UK projects in development around the UK (Hebling’s Research). The industry has been forecast to be worth £6.1 billion to the UK economy by 2035, and capable of displacing up to half a million tons of CO2 every year by 2020 (Renewable Energy World).
The UK is also host to the world’s first marine energy test facility – the European Marine Energy Centre (EMEC) – in Orkney, Scotland. The centre was established in 2003 to facilitate the development of the marine energy industry both in the UK, and abroad. EMEC has supported the deployment of more wave and tidal energy devices than at any other single site in the world. We covered EMEC previously, so for more insights see ‘On Scotland’s Renewable Energy‘.
Tidal Power
Taken together, tidal and wave power are the more further developed of marine energy technologies. This is largely because the technology is similar to wind power – kinetic energy of water is akin to the kinetic energy of wind, and both may be harnessed through rotor blade techniques. Water, being some 832 times denser than air, contains far greater amounts of energy than wind. This is advantages as it means energy may harnessed using much smaller rotors than are necessary for wind turbines.
At high tide the water flows upstream through the large turbines of the power station, and at low tide it flows downstream again. Turbines may be housed in damn like structures, or on-board devices anchored to the seabed.
The oldest tidal power station is found in La Rance, near St. Malo on the Atlantic coast in northern France; it was built in the 1960s. For many years it was the largest of its kind, with an output of 240MW.
Case Study | Marine Current Turbines
We can look towards Marine Current Turbines (MCT) for insight on the current state of tidal power. MCT is based in Bristol, UK, and although now a business of Siemens, still operates under their original name as a world-leader in tidal power systems. They’ve broken records, developed the world’s most efficient tidal systems and are involved in testing several of the more promising tidal plants in the world today.
Arguably the most successful tidal stream generator to date in the world
Dr Stephanie Merry, Head of Marine at the UK Renewable Energy Association
One of MCT’s most significant developments is situated at Strangford Lough, a small inlet southeast of Belfast, UK. As far back as 2008 the company has been successfully generating electricity using their SeaGen tidal energy system – achieving a peak rating of 1.2MW and surpassing the previous record of 300kW (MCT press release, 2008). As of 2013, the system has contributed more than 7GWh of power into the electricity grid (MCT press release, 2013).
Generating at full power is an important milestone for the company, and in particular our in-house engineering team. We are very pleased with SeaGen’s performance during commissioning. It demonstrates, for the first time, the commercial potential of tidal energy as a viable alternative source of renewable energy.
This work is vital for SeaGen’s long-term commercial deployment in projects elsewhere in the UK and overseas
Martin Wright, Managing Director of Marine Current Turbines
The SeaGen system features twin 16m rotors – each driving 600kW turbines – mounted on a cross-beam fixed to a central support pillar that is anchored to the seabed. The cross-beam isn’t just for passive support; it can be raised out of the water to allow for maintenance of the turbines. When it was installed, the system had the largest swept area (the cross-sectional area covered by the movement of rotors) of all the commercial scale tidal products available.
The SeaGen system’s rated power of 1.2MW can be achieved in waters with tidal flow greater than 2.4m/s – common in the waters of Strangford Lough where twice a day, 350 million cubic metres of tidal water flows through a narrow strait into Strangford Lough and then back out to sea. The force of the water is equivalent to a wind blowing at 555 kilometres per hour, and as it passes it spins the rotors at up to 15rpm. The rotation drives a generator in the same manner as with wind turbines (described in detail here: ‘A Primer on Wind Power ‘).
MCT partnering with Siemens makes good sense. Siemens bring with them capital; much needed when costs involved in marine energy are so very high – a principal reason for many new marine energy projects falling through.
Siemens also bring important expertise. They are, after all, a world-leading wind power manufacturer – a technology which has significant cross-over with tidal power.
MCT remain at the forefront of the industry, currently involved in several projects which are providing platforms for developing their systems. Of note is the SeaGen S which features a number of design changes following testing of the system in Strangford Lough. The new system features three blades, not two, and increased rotor diameter of 20m. These changes have combined to increase rated power to 2MW. There are plans to deploy an array of five SeaGen S machines – each costing roughly £9 million ($15 million) – off the coast of Wales. It’s hoped that the machines will be installed by 2016.
MCT has also secured lease agreements for three new commercial-scale tidal projects in the UK that speak to higher ambitions of the company. Each of these projects will be rated up to 30MW capacity and will use the forthcoming generation of SeaGen technology (presumably the non-disclosed SeaGen U). The target date for installation for these projects is 2020/21.
It should be noted that tidal power systems needn’t be submerged. MTC are partnered with Nova Scotian project developer Minas Energy to develop a 2MW floating tidal current turbine, called SeaGen F. The turbines will produce enough clean and reliable energy to supply up to 1,800 Nova Scotian households (MTC press release, 2014).
Wave Power
In the context of wave power systems, the key technology is the wave energy converter which converts kinetic energy of waves into electricity. Of these there seems to be no limit on the range of designs and concepts in circulation. That said, few have manifest into anything that actually generates electricity.
One of the more promising endeavours came from a company called Pelamis who reached large-scale testing of their second generation system.
Pelamis’ Wave Energy Converters are a semi-submerged, articulated structures composed of cylindrical sections linked by hinged joints (a set-up which is fairly common among WECs). As units respond to the movement of waves, they build up hydraulic pressure which is used to drive an on-board turbine.
In 2008, Pelamis’ first experimental wave farm was opened in Portugal, at the Aguçadoura Wave Park. The farm featured three first generation Pelamis wave energy converters, and produced 2.25MW in total installed capacity. While there were plans to extend capacity further, the plant shut down just three months after opening.
Pelamis moved on to concentrate efforts on development of a second generation ‘P2’ system at EMEC. Seemingly successful, and having received considerable accolades as well as gathering some 15,000 hours of real grid connected test data, unfortunately the company ran into trouble. Unable to secure backers for advanced testing of the P2, the company went into administration in November 2014.
To read of Pelamis is a sad story – clearly they were a flagship within the young industry. Pelamis were on track to deliver a working system that could have made a significant impact, both as a beacon to fledgling marine energy designers emblematic of the potential of wave power, and as a consequential technology to enter the energy market and showcase the promise of marine energy within a sustainable energy mix. Nevertheless, the decline of Pelamis says more of the harsh financial climate of developing marine energy systems than it does of the feasibility of the technology itself. After all, other floating wave energy converters remain in development.
But there has been great news in recent times in development of commercial-scale wave power. Here we turn attention to the other side of the world, to Australia, and to a concept not based on the water’s surface at all, but a fully submerged unit that’s fully operational.
The Perth Wave Energy Project will be the first demonstration of a complete grid-connected CETO system anywhere in the world, the only wave project to consist of more than one wave energy unit connected together and the only wave project to produce both power and freshwater.
The Perth Wave Power Project (PWEP) is an innovative offshore development located in Garden Island, Western Australia, developed by Carnegie Wave Energy. PWEP is special because it is the world’s first commercial-scale wave energy array connected to a grid – it’s supplying green electricity Department of Defence for HMAS Stirling, Australia’s largest naval base.
PWEP utilises it proprietary CETO wave energy technology. The CETO system – not an acronym but the name of Greek sea Goddess – consists of a fully submerged buoy (technically termed a Buoyant Actuator) that is tethered to a pump on the seabed. The CETO buoy oscillates in response to the ocean’s waves, transferring that energy through its tether and causing a pump to extend and contract. The pump pressurises fluid which is then directed onshore through a subsea pipeline. Once onshore, the high-pressure fluid drives a traditional hydroelectric power plant. The present CETO 5 unit is rated to 240kW.
Power can also be generated offshore. In this case, the movement of the buoys can drive pumps and generators that are contained within the buoy itself, with power sent back to shore through subsea cables to a station that connects to the grid.
As of March 2015, three CETO 5 units are in full operation within a multi megawatt system connected to the local energy grid. The first unit was installed back in November 2014, the second in January and the third in March – altogether the units have racked up some 4000 hours operating time.
The project has cost in excess of $31 million, but the various agencies of the Australian government (including the Renewable Energy Agency) have supported the endeavour with with some $20 million.
The Perth Project will operate through 2015, during which time Carnegie will conduct various investigations and calibrations to test and understand the system’s operation under various settings and conditions. These data will inform the design and delivery of the CETO 6 project.
CETO 6 is planned for deployment further offshore, in more powerful waters. Each CETO 6 unit is expected to be capable of generating up to 1MW – approximately four times the generation capacity of the CETO 5 technology currently being used in Carnegie’s Perth Wave Energy Project. The unit’s larger size and capacity will result in lower cost of power production.
Notable as well is that CETO technology has the capacity to produce desalinated water – something much sought after in a country that faces some of the most extreme droughts in the world. Specifically, the high-pressure water output from CETO units can be used to supply a reverse osmosis desalination plant. This is a massive bonus to CETO technology – replacing, or at least reducing, reliance on highly energy intensive processes that are typically fossil-fuel based.
Slow Current Technologies
Of course not all currents are powerful – in fact, slow currents are far more prevalent than high velocity ones. Recognising this, companies are working on harvesting slow current energy too.
Take the Minesto Kite, which is suspended in currents 30-60 meters above the seabed by a tether anchored to the seabed. Because of its tether and design, the kite can actually move dynamically at far greater speeds than the current itself; thereby allowing it to operate cost-efficiently even in slow currents. Operating in slower currents, and moving in harmony with currents rather than remaining stationary, has the added of advantage of inducing less wear and tear, and is predicted to lessen system maintenance costs.
To date five Kite prototypes have been built and tested; electricity was first produced in 2009, and first test in an ocean environment was completed in 2011. Prototypes were deployed in 2013 and is undergoing extensive long-time sea trials in Strangford Lough, Northern Ireland. Minesto’s presently working toward deploying a 1.5MW Deep Green array in 2017, then increasing to a 10MW array thereafter.
Ocean Thermal Energy Conversion
Ocean Thermal Energy Conversion (OTEC) refers to systems which make use of natural temperature differences in the ocean to generate electricity. More specifically the ocean’s warmer surface waters – with temperatures of around 25°C (77°F) – are used to vaporise a working fluid, which has a low-boiling point, such as ammonia. As the vapour expands it can be directed to drive a turbine, which, when coupled to a generator, produces electricity. The vapour is then cooled by colder seawater that is pumped from deeper (+100m) ocean layers, where temperatures of about 5°C (41°F) are common. Cooling condenses the working fluid back into a liquid, so it can be reused in a continuous cycle (www.otecnews.org).
OTEC is thought to have the potential to harness several thousand TWh of electric power each year. Unlike wind and wave energy, this form of electricity production is not subject to fluctuating weather conditions, and would be available at a constant rate.
OTEC is a promising technology, but because of its dependency on temperature differentials, its application is principally limited to tropical waters where there is a sufficient thermal differential year-round of at least 20°C (36°F) to ensure that systems are both viable and efficient.
But while holding massive potential, OTEC is also one of the most expensive of marine energy systems – a fact that has so far constrained its development.
Case Study | Lockheed Martin and Reignwood Group
Lockheed Martin and Reignwood Group have signed a contract to start design of a 10-megawatt OTEC power plant, which would be the largest OTEC project to date. It’s a 10MW offshore plant supplying energy to a Reignwood resort on the island of Hainan, off the southern coast of China. Little information appears available on the project – so it remains to understood how far along development has progressed.
Constraints on Industry Development
While solar and wind power have been benefitting from huge reductions in costs of manufacture and installation, and consequent growth in their respective markets, marine energy remains firmly set in its infancy. The case studies presented above do serve as important reminders though that such infancy is almost certainly only a precursor of marine energy’s growth into becoming a legitimate energy solution.
Maturity is only possible though with careful fostering of the technologies. As we’ve seen with solar and wind power, government support, development grants, acquiescent leasing of testing grounds, and subsidies for green energy systems contributing to grids, are all important aspects that serve to nurture growth.
Fortunately there is evidence of governments supporting marine energy in these ways – although not nearly as much as with other renewables. Arguably it’s the high capital costs of marine energy that remains the greatest barrier, limiting research which in turn stalls proving the feasibility of technologies.
There are also inherent challenges in offshore operations: the sea is a tough environment in which to install, operate and maintain any equipment, but most especially complex marine energy power plants. There is certainly a catch-22 situation with marine power in that more powerful waters can generate greater amounts of electricity, but at the same time the strong forces place massive pressures on the structural integrity of systems. Maintenance costs associated with marine based systems are therefore far higher than those of other renewables. It’s foreseeable however that innovation in material engineering may curtail some of the problems associated with harsh marine environments.
The Prize
There are a myriad of advantages to marine energy that render them extremely promising technologies to compliment existing renewable energy solutions.
First and foremost, they produce clean energy with zero emissions, using a perpetual resource that’s accessible across the world.
Given the close proximity of much of the world’s population to potential sites of marine energy (60% within 60km of the coast) there’s limited need for extended transmission lines. Equally, while utility scale systems may be connected to transmission grids, smaller scale deployments may provide electricity and/or drinking water to remote communities.
Waves, tides and currents are both predictable and reliable, meaning that unlike wind or solar power, they suffer less from problems of intermittency. Because of this, marine energy solutions are far more likely to provide base load power (i.e. the mainstay of generation capacity that makes up the bulk of supply) to utility grids than other forms of renewable energy. This is especially the case when marine energy is aggregated over large spatial areas. This aspect of marine power makes it very attractive with respect to efficient utility grid management, because it represents a power source that ameliorates the need for back-up fossil fuel power plants.
Another not insignificant aspect to marine energy is that it’s minimally disruptive to natural landscapes. Especially in the case of submerged systems. Admittedly there’s some debate as to the extent to which marine systems hold a detrimental impact to natural marine ecosystems; but on the whole it seems minimal. There’s even a strong case to be made for the fact that some techniques create artificial reefs, attracting marine life and ecosystems.
An outstanding question remains over how much of an impact may be made on global currents once we begin removing energy from the net system. However, in all likelihood it’s hard to imagine marine energy systems making anything other than a very marginal impact in the near future, and certainly not until such a time when marine energy is a significantly widespread.
Lastly, marine energy techniques can often be developed in such as way as to create useful by-products; in particular desalinated water. In today’s global climate this is an extraordinarily important consideration, and something that likes of OTEC and Carnegie Wave Power are not ignorant of. Delivering not only power, but fresh drinking water, cold water for use in domestic or industrial settings, and fresh water for agricultural practices are all key features of marine energy.
All things considered, while considerable challenges lie before developers, and the learning curve of engineering efficient systems remains steep, the prize of harnessing marine energy is massive. Perhaps even too great to pass up. The technologies already in existence are proving the worth of investing in marine energy however. To be sure, the coming decade will be time of great deal more development in this arena – development that will allow marine energy to move beyond its infancy, and into the forefront of the renewables mix.
Further Reading
Renewable Energy World: Marine Energy Breakthrough: New Technology Multiplies Potential
Renewable Energy World: Wave and Tidal Energy in 2015: Finally Emerging from the Labs
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