Marine renewables, also known as ocean energy, refers to a broad range of technologies that extract energy from the ocean; this energy can be in the form of ocean waves, tidal movements or thermal gradients. Marine renewables are, in general, at a relatively early stage in their development and, as such, the methods of converting these potential energy sources into useful electrical power are still highly diversified, with many technologies competing for commercial viability.
Tidal stream power plants directly extract energy from the flow of water due to the tides. They typically use submerged horizontal axis turbines but designs with vertical axis turbines and oscillating hydrofoils are also possible. Up to ten demonstration projects have been deployed (up to 1.2MW capacity), many for a number of years. A few larger-scale projects are under development but generally dependent on further cost reductions/financing for viability.
Tidal stream technologies exploit the movement of coastal tidal waters induced by the interaction of the gravitational fields of the earth, moon and sun. The necessary fast flowing currents are produced by features, such as headlands, inlets and straits, that constrict the flow and act to magnify its speed. Tidal flows have the benefit that they are extremely predictable: the movements of the sun and moon, and thus tides, can be accurately determined many hundreds of years into the future. This makes it relatively simple to determine the amount of power that can be generated at any particular time. However, it must also be noted that tidal streams only flow during periods of tidal movement, i.e. when changing from low to high tide in adjacent regions, and that they reverse the direction of their flow 4 times a day (corresponding to two high tides and two low tides). This means that most devices are designed to operate for flow in either direction and that for reasonable periods each day the devices produce little or no power while the flow slows and switches direction. This must be considered in any study of the economic feasibility of a tidal power project.
Most tidal stream devices are broadly similar to submerged wind turbines and are used to exploit the kinetic energy in tidal currents. The higher density of water means that the blades can be significantly smaller and turn more slowly than wind turbines yet they still deliver a potentially comparable amount of power. However the issue of water cavitation at the tip of the turbine blades may present problems and normally acts as an upper limit to turbine rotational speeds. In an attempt to increase the flow and power output from the turbine, concentrators (or shrouds) may be used around the blades to streamline and concentrate the flow towards the rotor.
Similarly to wave energy converters, this technology is still in the relatively early stages of development and, as such, the design space for tidal stream devices is crowded with more than 70 different developers listed on the EMEC (2010) database alone. These can be grouped into the following three broad categories:
Horizontal axis turbine (HAT)
HATs extract energy from tidal streams in much the same way as wind turbines extract energy from the prevailing wind; however the methodology for determining the possible level of energy extraction differs due to the constrained surface, the incompressibility of the water and the amount of energy that can be extracted without unduly affecting the flow characteristics (Bryden and Couch, 2006). Many developers choose to house their devices within ducts in an attempt to concentrate the flow and increase performance. Seagen (MCT, 2010), Hammerfest Strom (2010), Openhydro (2010), Clean Current Systems (2010) and Rotech Turbine (Lunar Energy, 2009) are prominent examples of HATs.
Vertical axis turbine
This type of device extracts energy from a tidal flow in a similar fashion to the HAT described above, however the turbine is mounted on a vertical axis. These devices may be floating on the surface (where tidal flows are fastest yet unwanted wave loading is more prevalent) or mounted on the seabed. There are fewer vertical axis designs seen in the market, possibly due to the technical familiarity with HATs that many companies and suppliers would have from the wind industry. An example of a vertical axis turbine is the device under development by Blue Energy (2009).
A less common design uses a hydrofoil attached to an oscillating arm wherein the motion is caused by the tidal current producing lift on the hydrofoil. A potential benefit of such a design is that it could use a hydraulic power conversion system if this was deemed to be more reliable than a direct drive generator or gearbox. Pulse Tidal (2010) is an example of this type of oscillating design.
The World Offshore Renewable Energy Report 2004-2008 (DTI, 2004) determines that although there is in the order of 3000GW of gross tidal power in the oceans, less than 3% is located in areas suitable for power generation. This is because tidal stream energy extraction is extremely site specific and is only feasible when tidal flow speeds are amplified by factors such as favourable bathymetry, constriction due to surface features or reflections by large peninsulas. Countries such as the UK, France, eastern Canada, the Pacific coast of Russia, South Korea, China, Mexico, Chile, western India, and the Patagonian coast of Argentina amongst others, have been determined to have significant tidal stream resource. Thus the availability of suitable regions for deployment is the primary constraint on tidal stream power generation.
In determining the suitability of a site for tidal stream power generation it is necessary to first measure the tidal speeds that are achieved over a typical two week cycle as this covers both ‘spring’ flows (the largest/fastest tides when the moon and sun are aligned) and ‘neap’ flows (when the moon and sun are out of phase). These speeds can be used to provide a very rough estimate of the suitability of a site. Typically spring peak speeds in the order of more than 3 metres per second at the surface are desirable but the exact specification is device dependant and can also depend on the neap speeds at a location. In order to more fully understand the ability of a site to provide feasible power generation it is necessary to model the flow through the area of interest and study the effects of energy extraction; a highly specialised task that could, for example, be conducted by the technology developer that was being considered for the particular location.
The design of a tidal stream device will determine the constraints on location. As they are deployed in areas of high flow in channels or river mouths, there must be careful attention paid to more traditional stakeholders such as shipping/transport vessels, fisheries and zones reserved for environmental conservation. Many designs are deployed on the sea floor at significant depth and may thus have little impact on surface activities such as shipping; however this is something that must be considered during consultations in order to determine the feasibility of a particular site.
The installation and maintenance of tidal stream devices involves a large proportion of offshore activities. During the installation phase this may involve large specialised vessels for the transport and lifting of devices. However more and more device developers are attempting to simplify installation procedures to reduce or remove the need for costly specialised craft or by building dedicated barges that can be used without competing for vessel time with other industries. However all installation approaches must overcome the issues associated with the extremely fast water flows at tidal sites. Deployment methods must either be extremely robust to survive the fast flows or very rapid in order to deploy a device during a brief period of low flow (slack tide). By their very nature tidal stream devices are deployed in some of the most hostile ocean conditions making it a significant challenge to secure the device in a cost effective manner.
With so many of the designs for tidal devices submerged beneath the ocean, often with no direct method of surface intervention, the maintenance requirements of tidal stream devices are even more critical than for wave energy converters. There is a large diversification in the maintenance philosophies adopted across the range of devices, some designs support offshore surface intervention from a permanent platform, some lift the entire device from the sea floor and some use a “maintenance by replacement” strategy for certain critical modules. These activities require skilled offshore workers and vessels as well as the appropriate onshore technical support. Both the installation and maintenance of tidal stream devices can be very large cost drivers in the lifetime cost of such a scheme and should be considered carefully when comparing designs.
While the regulatory framework for the development and consenting of onshore wind farms (and to a certain extent offshore wind in many countries) is generally well developed, the small number of deployments of tidal devices means that in most countries the process for obtaining seabed leases and the necessary permits would be considered as ad hoc. It is generally necessary to arrange the following broad set of agreements
- A seabed lease for the area of interest for deployment from the relevant government body that controls that region of the ocean
- Possibly an onshore lease for the area of land that is required for the cable/pipeline landing and onshore substation
- Appropriate environmental permits by conducting an environmental impact assessment (EIA) that can take between 1-2 years depending on the level of baseline data demanded by the permitting authority
- Grid connection agreement / power purchase agreement with the relevant body to ensure distribution and a market for the resulting electricity.
The experience with public acceptability in relation to marine energy has so far been very limited. The comparatively benign and non-obtrusive (they are often completely invisible from the ocean surface) nature of tidal stream devices gives them a relatively high level of social acceptance. The main stakeholders that must be considered are other impacted ocean users and environmental groups.
It is fair to say that there are no commercially viable tidal stream generators in operation anywhere in the world. However there are a significant number of full or part scale prototypes that have been deployed across a range of countries; certainly all of the companies listed in the previous section have full-size or scaled prototypes deployed in representative ocean conditions.
As with wave energy technologies the strategies of developers remains highly diversified, with no clear favouritism of any particular design. It is certain that the current level of diversification cannot be sustained and future cost reductions will require the mass production and refinement of a much smaller subset of the currently promoted designs.
The IEA (2009) forecast only a small share of global power generation will be taken by marine renewables in 2050; less than 50GW. However forecasts in specific countries are often more aggressive; for example in the UK between 2 and 5 GW of installed capacity is being proposed in 2020 (UKERC, 2008; Carbon Trust, 2006).
To the best of the author’s knowledge there have been no significant tidal stream power projects deployed in developing countries at this stage.
There is limited information available on the local impacts of tidal stream devices on the environment. The small number of projects, limited deployment times and small scale of these projects means that there is still a reasonable degree of uncertainty about the long term impacts of large scale deployments. In spite of some initial fears being raised regarding the interaction of turbine blades with sea life, all the available data along with the slow blade speeds indicate that tidal stream devices are relatively benign machines that have a minimal impact on the environment. Only during the course of further development, through more environmental impact assessment studies and larger constructions will the tidal energy community be able to gain a firmer idea of any potential impacts on marine life.
Tidal stream devices directly contribute to climate change mitigation by providing a completely renewable energy source free of GHG emissions (beyond the initial GHG gases associated with production and installation that could be expected to be offset in similarly small time frames as wind turbines due to the broadly comparable device sizes and capacity factors). However, due to the early stage of technology deployment and significant costs of tidal stream energy, the total installed capacity will very likely remain small for tidal stream technologies meaning that their overall contribution to mitigation with the next decades will be relatively small.
As marine renewables are still largely at the R&D and demonstration phase, with a corresponding lack of commercial devices, it is very difficult to accurately estimate costs. The IEA (2009) put lifetime delivered energy costs of marine renewables at USD 150/MWh to 300/MWh across the range of technologies (minus tidal barrage), generally well outside the range of current electricity revenues even when current carbon finance incentives (available in certain countries) are considered. They estimate that costs will need to reduce to between a third and a quarter of their current levels to be feasible without significant support.
For tidal stream technologies the challenge comes from the difficult installation environment posed by tidal races and the costly offshore maintenance strategies required; however these are not insurmountable barriers.
Uncertainties about the costs and technical performance of marine energy technologies must be overcome before significant commercial investment can be attracted. Large-scale prototype/demonstration schemes can help in this respect to inform investors regarding the key issues of reliability, efficiency, reparability.
The issues mentioned above do not make marine renewables undesirable; in fact many countries have been supporting research programmes and private sector development. The incentive for such support is the potential for ocean energy to provide a new set of industries and jobs in early adopting countries as well as providing an alternate source of energy security and a diversification of the energy supply to reduce the intermittency associated with the dominance of a single type of renewable.
[This information is kindly provided by the UNEP Risoe Centre Carbon Markets Group.]
As of March 2011, there are no tidal stream projects in the CDM pipeline.
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