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.
Wave energy plants use either nearshore or offshore ocean waves and can be so-called attenuators, point absorbers, surge converters, oscillating water columns, overtopping devices and submerged pressure differential. Several prototypes or pre-commercial demonstrators (up to 750kW capacity per device) have been deployed around the world and a number of larger-scale projects are under development but these larger projects generally depend on further cost reductions to be viable
Wave energy converters (WECs) capture the energy found in ocean surface waves. These waves result from the action of wind blowing over long stretches (or fetches) of open ocean surface. Like wind power the availability of wave power is variable and will depend on the prevailing wave conditions at any particular time, sometimes producing maximum power and sometime producing none. The predictability of waves is slightly better than wind forecasting as the incoming swell heights can be measured by buoys or satellites far from the coast meaning that reasonable forecasts of wave conditions can be made up to 5 days in advance. Although waves are created by winds, these inducing winds occur at distant locations. By the time the waves reach the coast for power generation there is not necessarily any correlation between the amount of local wind on a particular day and the energy in the waves in the same region. There are a number of different approaches as to how the power in these waves is converted into useful work and at this stage of development there are many competing designs of WEC. The European Wave Energy Centre (EMEC, 2010) defines 6 main categories of WEC plus a seventh ‘catch-all’ category for designs that cannot be classified as belonging to one of the 6 main types.
This is a device that floats on the surface of the waves and extracts energy in parallel to the direction of the wave front by changing shape along its length. These types of devices are typically deployed in deeper water using a number of catenary mooring lines fixed to points on the ocean floor. The wave power is usually converted to electricity in the device using hydraulics and induction generators. Pelamis (PWP, 2010) is the most well known example of an attenuator type WEC.
Point absorbers also float on the ocean surface but are typically axisymmetrical and absorb energy in all directions through their vertical movements. These types of devices are seen to have multiple competing designs for the power conversion / power take-off (PTO) including remote (at sea) direct conversion with linear generators or onshore conversion using high pressure water and hydroelectric equipment. Power Buoy (OPT, 2010) and Wave Bob (WaveBob Ltd, 2010) are prominent devices that use the point absorber principle.
Oscillating Wave Surge Converter
As waves approach the shore, the shallower water causes them to shoal; elongating the previously circular motion of the water particles. WECs in this category extract the energy from this surging motion using either a linear or rotational oscillator. The nature of surging waves means that devices of this type need to be deployed in shallow water depths, typically with a solid foundation. Oyster (APL, 2010) is the most well recognised device of this type.
Oscillating Water Column
Oscillating water column (OWC) devices use the semi-submerged volume of air open to the sea to extract energy. As the waves pass the device the air in the device is compressed and decompressed; the resulting out or in rushing air is passed through a turbine that is designed to operate in the same direction, irrespective of the flow direction. Devices of this kind may be mounted on the shoreline or breakwaters but they have also been demonstrated in deeper water. Limpet (VHW, 2010) and Oceanlinx (2010) are examples of onshore and offshore OWC devices respectively.
This type of device uses the height of a wave to fill a reservoir by spilling over a barrier and the water in the reservoir is drained back to the sea level through conventional low-head turbines. An overtopping device may use physical reflectors to concentrate the wave energy. There are limited devices of this type, with Wave Dragon (2005) being the most well known.
Submerged pressure differential
These devices are typically located in the nearshore region and mounted to the seabed. The waves cause the sea level to rise and fall over the device, inducing a pressure differential. The alternating pressure difference can then be used to create motion and generate electricity, typically by pumping high pressure fluid. The Archimedes Waveswing (AWSOE, 2010) and CETO (Carnegie, 2010) devices use this principle of operation.
There are a small number of devices that do not fit into the above categories including turbines that are directly turned by the waves (Ecofys, 2002), flexible tubes that bulge (Checkmate, 2010) and devices using electroactive polymers to convert the wave energy into electricity (SRI, 2008).
The primary factor that will determine the suitability of a country or site for wave power is the level of wave resource found there. Wave power at a location is normally expressed in units of the average kilowatts per metre of incident wave front over a year, or kW/m. Thus at a sight with an average wave resource of 20kW/m the average amount of power that could be converted by a 10 metre wide device is in the order of 200kW, assuming 100 percent of the energy is captured (although devices will capture less than this in practice). The momentary power production can be significantly higher during periods of larger waves however the yearly average resource at a site gives a good measure of the suitability of a site assuming the selected device characteristics are known. Given the wide range of device types and the conditions they require it is not possible to endorse a certain threshold that makes any single site suitable, instead the overall economic picture must be considered.
The best wave climates, with annual average offshore power levels between 20-70 kW/m of wave front or higher, are found in the temperate zones at 30 to 60 degrees latitude in countries such as Chile, Canada, UK, Ireland, Australia, New Zealand, USA, and Portugal amongst others. The size and nature of the wave resource also changes depending on the water depth; as waves progress to shallower depths they lose energy due to wave breaking and bottom friction effects. While this causes lower gross energy levels in the nearshore, the associated ‘filtering out’ of extreme storm events can potentially compensate for the decreased energy levels (Folley & Whittaker, 2009).
When selecting a final site for deployment the wave resource at that particular location will typically be modelled using historical wind/swell data to give the expected performance over time (and thus economic performance) and this modelling may be supplemented by direct measurements of wave heights and periods using some form of deployed buoy or probe.
Being situated in the ocean, and typically being deployed on or near the surface, WECs must compete for space with more traditional stakeholders such as shipping/transport vessels, fisheries, recreation areas and zones reserved for environmental conservation. The nature of the wave power intensity means that large installations, on the multi-mega watt scale, may require many hundreds of metres of length in which to be deployed, if not kilometres for utility scale installations. Depending on the nature of the device this may be offshore where interactions are generally limited to ocean going vessels, or closer to the nearshore where additional concerned parties must be considered.
The installation and maintenance of WECs typically 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; this can be done in a number of ways and depends on the specific device being considered. However all installation approaches must face the same issue of survivability in extreme sea states. By their very nature WECs are deployed in some of the most hostile ocean conditions making it a significant challenge to secure the device in a cost effective manor.
There is also a large diversification in the maintenance philosophies adopted across the range of devices, with some designs requiring offshore maintenance, some using a ‘tow-to-shore’ strategy and others placing much of their power conversion equipment onshore to decrease the maintenance burden. These activities require skilled offshore workers and vessels as well as the appropriate onshore technical support. Both the installation and maintenance of WECs can be very large cost drivers in the lifetime cost of a device 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 WECs 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 typically deployed far from shore and protrude a limited distance above the sea surface) nature of WECs give them a relatively high level of social acceptance. The main stakeholders that must be considered are other impacted ocean users and environmental groups.
Wave energy is in the demonstration phase of technology development. Without any major evidence to the contrary it is reasonable to say that there are no commercially viable WECs in operation anywhere in the world. However it should be said that a significant number of full scale prototypes have been deployed across a number of countries; certainly all of the companies listed above have full-size or scaled prototypes deployed in representative ocean conditions.
The large number of designs remains highly diversified with more than 100 different developers listed on the EMEC (2010) database alone. The state of development could roughly be compared to the early years of the wind industry when there were a number of competing technologies until the 3 bladed horizontal axis wind turbine came to dominate the market. That is not to say that will be a clear cut ‘winner’ in the wave power market however 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.
To the best of the author’s knowledge there have been no major wave power projects deployed in developing countries at this stage.
Global resource levels are estimated to be very large, however much of this is considered unexploitable due to geographic, grid or environmental constraints. The total global exploitable resource has been roughly estimated as being in excess of 2TW, two million megawatts (WEC, 1993); however more detailed resource maps are available for many countries and these will often distinguish between deeper water and nearshore resource levels.
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).
There is limited information available on the local impacts of WECs 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; however it should be noted that at this time all the available data points to WECs as being relatively benign devices that have minimal environmental impact. Only during the course of further development, through more environmental impact assessment studies and larger constructions will the wave energy community be able to gain a firmer idea of any potential impacts on marine and bird life due to possible noise, motion or wave shadow effects.
WECs 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, as discussed in the following section on marine energy economics, the total installed capacity will very likely remain small for wave energy 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 wave technologies the challenge comes from the difficult installation environment posed by the open ocean and the relatively limited but 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.
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