The previous sections have shown the potential for the feasibility of Ocean energy devices and an outlook that SKF will have to judge its bearing prices in the future. The clear indications that there still are major engineering and economic works and challenges ahead to overcome before ocean energy device are deemed to be commercially deployable have been based on a lot of assumptions and uncertainties. These challenges can be considered to hinder the development of the marine energy sector. In this following section, the look into how the challenges can be tackled by all manufacturers can help accelerate the technology [37]:
Copied (if possible only couple of sentences per point)
-Design variety and consensus: marine energy innovation activity is spread over a wide variety of concepts and components. Over the shorter term, the lack of design consensus in both wave and tidal current energy technology fields is likely to restrict the pace of development and learning. At the same time, there may be significant longer-term advantages from retaining design variety.
-Parallel support for incremental and radical innovation: closest-to-market large-scale wave and tidal current prototypes (of around 1 MW units) using more conventional designs and components receive the bulk of financial resources and innovation efforts across the sector as a whole - especially from the private sector. While the testing of these more mature prototype designs is vital to capture learning-by-experience, there is a parallel need to support innovation in more radical options which may enable step-change performance improvements and/or cost reductions over the longer term. Given the longer timescales for this more radical innovation, public funding has a continued leading role to play here.
-Feedbacks between learning-by-doing and learning-by-research: because of the early stage of marine energy technology development, there is currently only limited experience in real operating conditions. One aspect of accelerated development is the feeding-back of data and experience on prototype performance and operating experience into earlier phases of the innovation chain. In practice, the transfer of experience is likely to be limited by commercial competition.
-Shared learning for generic technologies: there are a number of 'generic' technologies and components which have application across the sector, such as foundations, moorings, marine operations and resource assessment. While these offer opportunities for shared/collaborative learning, the support and transfer of generic knowledge and components are limited by commercial competition.
-Knowledge and technology transfer from other industries: other industry sectors, such as, but not limited to, offshore engineering and offshore wind, offer potentially important opportunities for knowledge and technology transfer. Enabling this transfer depends in part on a better understanding of the 'adaption costs' - the costs of transferring components and methods to the marine context - and also, identifying and taking advantage of specific opportunities for collaboration with other industries or supply chain partners
The progress of marine energy technology is highly uncertain, and like all scenarios describe in the previous section, the scenario of accelerated marine development devised here is highly sensitive to assumptions regarding capital cost and technical performance. As such, the Marine scenario provides one possible development pathway, assuming high but plausible levels of technological progress. Restating that to achieve the high and sustained levels of innovation and learning assumed depends on creating and sustaining a highly effective marine energy innovation system, the main details of this system will not be elaborated further. But may serve as guidelines to support SKF in the future decision making, and how it will impact its implication and growth in the market. In practice there are a significant technical, economic and institutional challenges involved in providing this.
Copied: In the short term (up to 2020) there are considerable deployment challenges, including planning and legislation, skills shortages and availability of installation vessels. Another challenge may be related to intellectual property protection. Despite a level of headroom, grid reinforcement may also become a significant challenge during this period. Over the next decade, the cost reductions embedded in the Marine scenario are predicated on niche-learning, with progressive cost reduction and design consensus, as a small number of 'first generation' wave and tidal device designs become de facto 'industry standards'. In this scenario, there is also likely to be a consolidation of developer firms, as mergers and acquisitions bring together some small developers and allow hybrids of the best technologies to emerge and reduce costs. Over the longer term (after 2035), it is implausible to describe the direction of marine energy innovation in any detail, but achieving the kinds of sustained learning that are embedded in the Marine scenario is likely to require the introduction of 'second generation' technologies capable of more efficient resource extraction and conversion. In the meantime, there is a need for support measures and policy strategies which allow more unconventional and disruptive technologies to be researched, developed and tested. Supporting RD&D on more radical and higher risk technologies is an important enabler of innovation over the longer term, and there is an important role here for publicly funded long-term R&D programmes.
Sustaining the learning assumed in the Marine scenario over the long term will also require the development of a much more internationalised marine energy industry - and associated innovation system - over the medium and longer terms.
Institutional and infrastructure barriers (such as supply chain constraints, planning constraints and grid reinforcement) may have been largely addressed in the long term. However, resources in deeper waters or more difficult locations may become exploitable by this time, presenting new technical or infrastructure challenges. In addition, competition for material and financial resources from other energy and non-energy sectors could impose longer-term constraints on the sector [37].
Benchmarking of Ocean Energy Devices
Referring back to the SKF model, the next step in our strategy was to identify all the technical aspects surrounding the Ocean Energy technology. In fact in this section the benchmarking of the devices will enable SKF to identify leading device manufacturers in both Tidal and Wave energy technology.
Figure 2: Challenges for long-term survival of Ocean energy devices
The following section will enable to shed some light on criteria that will be used for the benchmarking of wave and tidal devices coupled with the assumptions presented in the previous sections. Figure 31 illustrates the main challenges that device manufacturers need to face for a possible long-term survival in the industry. The challenges are not meant to be exhaustive nor a strict guideline, however coping with these challenges will facilitate the successful long life of the devices [45].
Copied: The primary wave-body interface is a good wave-maker:
It is important that there is a strong coupling between the fluid motion in the near field boundary around the device and the far field fluid motion which is associated with wave action in the most commonly occurring seas. This results in an efficient wave power extractor as there is a reciprocal relationship between wave generation and absorption. However, as the motion of the body becomes greater it should progressively reduce its ability to generate waves. This means that as the seas get larger the moving body progressively decouples from the wave induced fluid particle motion thus limiting the amount of power that has to be converted.
The device can avoid extreme laoding in storms:
Apart from progressive decoupling as the sea state increases the device needs to be able to move to a 'fail safe' condition in which it completely avoids the extremes of wave loading in storms. This is a 'last resort' scenario as ideally it is desirable for a device to continue production in storms due to substantial decoupling from the waves. It is not economic to provide structure to withstand extreme loads as it is only required for a very small percentage of time and mainly lies redundant.
The device has an appropriate broad bandwidth response:
The device should have a good power capture over the range of most commonly occurring incident wave frequencies it is subjected to. In a physical system, reactive energy is stored as kinetic and potential energy, whilst the active power is related to power capture and radiated power. At a system's natural frequency the variation in reactive energy is zero, as the incident wave force and the velocity of the working surface are in phase. Thus for a broad bandwidth response the device dynamics must ensure that this is largely achieved over a range of frequencies and there is a variety of means to do this. For example, by having two or more natural frequencies within the wave frequency range the responses from them can merge to give a broad bandwidth. This can be achieved with 'harbours' in front of oscillating water columns (Count & Evans 1984). Alternatively 'slow tuning' can be adopted where the stored kinetic or potential energy is adjusted with sea-state so that even with a narrow bandwidth the natural frequency of response is centred on the incident wave frequency to maximise performance. Finally so called 'phase control' or 'complex-conjugate control' can be used in which the kinetic or potential energy is manipulated on a wave-by-wave basis to maximise performance (Budal & Falnes 1980; Salter et al. 2002).
the device is not site specific and can be mass produced
These factors minimise production and design costs. From experience of the two prototype wave energy converters on Islay the amounts of expenditure required to design and certify bespoke components are substantial, making site-specific adaptations extremely undesirable. The use of mass production techniques has the potential for dramatic reductions in cost, particularly in the power take-off components. This implies that other device elements should be modified so that they are sized suitably for a mass-produced module. Reliability of the components will also increase with mass-production because of the increased effort in design and experience gained in their use.
the device has short direct load paths:
The use of short, direct load paths is a well known design principle and is clearly of relevance to the design of wave energy converters where large forces have to be transmitted. This influences the size and cost of structural elements in the device. For wave energy converters the loading scenario is complicated because of the inherently oscillatory and distributed character of the incident wave force.
Either the whole device or serviceable components are easily removed.
Working at sea is fundamentally more expensive and more hazardous than working ashore. Moreover, the sea-state may severely limit the times that the device is accessible for servicing thus reducing availability. With sea bed mounted devices it is desirable that all the components which are likely to require attention are demountable for servicing back at base. This implies a static non-serviceable part of the device remains at site. This is likely to have the additional benefit of making installation an easier operation. Floating devices should be easy to uncouple from the moorings and power take-off connection and be towed into dock.
The challenges set above should be considered as a guideline in future developments of new devices and as a partial assessment of leading manufacturers of ocean energy devices to develop and insure the long-term survivability of their devices. However the judgment based on these criteria's is very complicated as publications from manufacturers is very limited, due to the precautionary measures and the competition regarding this breakthrough technology. In fact, the investigation lead by the Renewable Segment at SKF has lead to the discovery of more then 240 device developers for both Tidal and Wave energy devices [46]. Further these investigations lead to show that all the researchers and developers have different concept and design and are in different testing and developments stages [2][47]. Thus to be able to compare those devices according to the simplified benchmarking technique, the set of common grounds for evaluation were established based on the economical and development of those devices. With the Renewable Segment at SKF, "success factors" were determined have a first overview of leading devices manufacturers and as well as to what type of device they are developing according to the list described in the previous chapter [2][46][47].
Table 1: Grading Table
Points
Development experience and status
Technology features
Financial support and future prospects
Joint Venture/Co-operations
Expected Generation Costs
10 to 8
Developer with ocean energy experience for more than 10 years, full scale prototype developed or commercial possibility (small farms) available
Maintainability, rapid installation, readily available parts, withstands conditions, special mooring systems and appropriate generation costs
High backing from private investors, government funds and incentives, large scale or small scale planned for operation till 2020
High profile partners in development, joint ventures for deployment and testing available
Demonstration of prototype, CoE can reach level compared to other renewable after deployment of scaled project
8 to 6
Developer with ocean energy experience for more than 5 years (<10), full scale prototype developed or commercial possibility (small farms) in the next 3 years
Technological features are less prominent then leading devices
Available support from private investors and backing from government and planning to deployment operation in the next 3 years
A notable partner for their development and joint ventures for deployment and testing available
CoE is expected to compete with off-shore wind from early generation
6 to 4
Developer with ocean energy experience for less than 5 years, and is in advanced stage of design
A couple of technological features constitute the device
Investigating financial support possibilities and planning of demonstration of prototype
Manufacturer seeking development partners
CoE after deployment mite be able to compete with high CoE prices of off-shore wind energy
4 to 0
New developer with very little experience in the ocean energy market and is still in concept stage or the development of the technology has come to halt
Technological features minimal or unknown/not disclosed
Almost no financial support and future projects are not planned.
No development partners
CoE can hardly be determined and can be expected to be much greater the other renewable energy devices.According to the factors listed in the table above, weighting have been set to distinguish the importance of factors relative to others. The weighting follow extensive discussion within SKF and literature published to optimize judgment on leading or top performing device developers (Table 10).
Table 2: Criteria and their respective factors
Factors
Weight
Development Status and experiences
0.2
Financial support and future prospects
0.3
Technology features
0.2
Joint Ventures
0.15
Expected generation costs
0.15
Benchmarking Of Wave Energy Devices:
The scores and weightings for each device are found in the Appendix, the benchmarking of wave energy devices has been done on the basis of evaluation of top performing devices. The key importance of this benchmarking is to reflect the type of devices that SKF has to respond to for direct involvement in the market. The benchmarking should be used as one key factor for evaluation of possible supply of SKF plain bearings at first glance and as a five-platform supplier.
Table 3: Top Wave Energy Converters
Rank
Manufacturer
Score
1
Pelamis
7.91
2
Aquamarine Power
7.34
3
Ocean Power Technology (OPT)
6.92
4
Ocean linx
6.84
5
Wave Star
6.76
6
Wave Bob
6.12
Table 11 shows Pelamis and Aquamarine as main developers as they are the only two manufacturers that are proceeding in the commercial phase, and are developing new technologies to stay at the forefront of the market of wave energy [30][32]. Another point to be drawn out which complies with all the previous models and arguments is that out of the top six leading devices four are part of the greater family of point absorbers (table 12).
Table 4: Classification of top Wave Converters
Attenuators
Terminator
Point Absorbers
- Pelamis
- Aquamarine Power Oyster
- OPT
- Ocean Linx
-Wave Star
-Wave Bob
The potential of point absorbers can be evaluated based on the definition of each device at first. The definition of each category has shown that attenuators are influenced by the length of the incident (period T) wave to generate energy, whilst terminators are widely influenced by the amplitude of the incident wave to generate energy. However for the point absorber the influence is both the period and the amplitude of the wave that will influence the energy generation. This assessment is based on the definition of each category. Furthermore, the potential of point absorbers can be reflected as to their CoE, since their haul structure and size is relatively much smaller in comparison with terminators and point absorbers, the CapEx would relatively be much smaller, having a proportional influence in the reduction of the cost of electricity. The goal is not to show the advantages of a category against the other, but as a supplier to this industry it is vital to know where the focus on development should be on short term, for direct implication and setting a trend for long term.
Benchmarking of Tidal Energy Devices:
The same weightings have been used for the benchmarking of tidal energy devices and the scores are attached in the Appendix. The top developers in the field of tidal energy are listed in Table 13 below.
Table 5: Leading Tidal device manufacturers
Rank
Manufacturer
Score
1
Marine Current Turbines
9.70
2
Open Hydro
9.51
2
Atlantis Resources Corp.
9.51
3
Voith Hydro
9.30
4
Tidal Energy
8.60
5
Hammerfest Storm
8.16
In the case of tidal energy devices, no distinguishable family can be drawn out, apart from the fact that all devices are horizontal axis turbines, as they seem to outclass vertical turbines in their output efficiency as well as in their ability to operate in much greater size and withstand the conditions of the ocean. This can be closely related to the initial developments on wind energy turbines, in its early stages the abundant number of turbines have been designed from different types of vertical axis turbines to the commonly used horizontal three blade turbines of today [48].
Energy Devices Viewed by SKF
The following section sits within the SKF management strategy and helps explore and identify further the technical segment established. The topic of this section is to try and bring out the possible implication of SKF, and more specifically plain bearings in such applications. This identification is the result of the understanding that no single device operates and possesses the same technology as any other [3][13]. However as a first assessment regarding the categories can bring out a valuation of requirements for each category.
Bearing Applications in Ocean Energy
In this section an overview of each category is reviewed with an example of device manufacturer for this application, as an attempt to bring out the bearing applications required for each category. The study is in no way conclusive and applicable to all devices to each category but as an inductive reasoning through the leading devices a trend can be evaluated as requirements for SKF as well as needs for the device manufacturers.
Wave Energy Converters
Terminators: Aquamarine Power Oyster:
Aquamarine Power is in the development of their third generation wave energy converter device, called the Oyster 800 (Fig.32) [49]. The converter will generate an estimate 800kW as its name suggests by extracting the energy from the surge of the incident wave [32].
Figure 3: Oyster Wave Energy Converter
The Oyster energy converter from Aquamarine Power captures the energy from the incident waves in near shore through a pump that is controlled by the wave surge motion and enables to compress water. The high-pressure water is driven into a hydroelectric turbine situated onshore [32] that will convert that energy into electricity. The Oyster can be considered as a mega structure (Fig.32) hinged to the seabed, it is a buoyant structure where a hinged flap is attached to the anchored structure at depth around 15 meters. The hinged flap is almost entirely submerged under water and reciprocates backward and forward in response to the wave spectrum and amplitude.
Figure 4: Simplified Drawing of Oyster device
The link between the flap and the main rigid structure is our key focus, the reciprocating movement can only be operate threw a bearing that enables the motion of the flap. Figure 33 shows the location of the bearing location at the baseline of the flaps. The bearing seems crucial for the proper functionality of the application, as the structure should withstand high impact loads of the incident waves.
Figure 5: Loads subjected on the Bearing
The bearing will have to deal with mainly radial loads, and some sway loads. As the loads on the flap are not uniformly distributed, the bearing will have to enable the smooth running of the one-degree of motion device.
The general observations on terminator devices show similarities in working principle from the first original design of the Pendulor in Japan (Fig. 35) [50].
Figure 6: Preliminary Design of the Pendulor
The size and operating conditions of the bearing are crucial and usually require bearing supplier to work outside their offered catalogue.
A proposed solution to the high loads and twisting moments that are created on the hinges due to the none uniform loads that the bearing are subjected to is to use a spherical plain bearing that will be able to tilt and rectify twisting moments alignment both hydraulic rams for smooth operations whilst withstanding the heavy loads of the structure. The use of bushings in this type of application can be considered due to the large concentrated loadings on the hinges, due to the weight of the structure and the impact loads on the flaps, however it is key to estimate the loadings on the extremities of the bushings as they cannot withstands the same conditions as they center counterpart, the use of washers can help reduce fatigue on the extremities of the bushings however the other issue would be to take into consideration the twisting moments caused by the flap on the hinge and if the bushing could tolerate a certain bending for smooth operation.
Attenuators: Pelamis:
The Pelamis wave energy converter is the leading device in the attenuator category. The root of the word Pelamis is Latin for serpent. The haul structure resembles to a floating sea snake [30].
Figure 7: Schematic of Pelamis (2 degrees of freedom)
COPIED: The Pelamis machine is made up of five tube sections linked by universal joints which allow flexing in two directions. The machine floats semi-submerged on the surface of the water and inherently faces into the direction of the waves. As waves pass down the length of the machine and the sections bend in the water, the movement is converted into electricity via hydraulic power take-off systems housed inside each joint of the machine tubes, and power is transmitted to shore using standard subsea cables and equipment [30].
Figure 8: Hinges and bearings of Pelamis
The two-degree of freedom motion is controlled by bearings that have to withstand the reciprocating motion of each member leaving the axial loads to be considerable compared to terminators devices. As quoted by Senior Engineer and Bearing Group Leader at Pelamis that their "biggest challenge has always been [on how to] manage the loads and motions from such an active and constantly variable environment, whilst at the same time extracting as much power as possible. The working forces generated across each joint can be several hundred tonnes, which can present huge problems for the bearings as they have to take up the reactive forces coming back through the joints" [52].
The bearings have significant roles in attenuators, as they are key for the efficiency of the device. Smooth running bearing can maximize the output of the Pelamis or any other attenuators by minimizing stick slip and minimizing frictional losses. The effect of reciprocating loads and constant change in direction, the bearing will have to deal with both high radial and axial loads, leaving a suggestion of the favorable use of spherical plain bearings that will enable to withstand to some extend the loads and oscillating frequency of these devices.
Point Absorbers:
It has been defined that point absorbers showcase different working principles then attenuators and terminators. Point absorbers are divided in different subfamilies, the leading devices OPT, WaveBob, WaveStar and OceanLinx all present similarities but use different technologies to harvest the energy from the waves.
Figure 9: WaveStar, Point Aborber
The Wavestar machine draws energy from wave power with floats that rise and fall with the up and down motion of waves. The floats are attached by arms to a platform that stands on legs secured to the sea floor. The motion of the floats is transferred via hydraulics into the rotation of a generator, producing electricity.
The bearing application in such devices show similarities as terminators, as the floating body reciprocates linearly due to the change in wave heights the bearing oscillates accordingly whilst carrying the loads subject from the arm and the floating apparatus. The proposed is the potential use of a bushing for such application as the major load will be the static load of the hinged arm with the hydraulic rams; however this proposal can be only examined if the arms are in the direction of the incident wave. As to say in the case of the wave being perpendicular to the arms shock loads will cause higher bending moments on the arm and create relatively higher axial loads that a bushing will not be able to withstand. The importance to estimate the direction of the waves is important for the cost effective use of bushings, however a spherical plain bearing will be able to withstand all those undesirable conditions on bushings and limiting the need to worry of unwanted radials loads being able to extract wave energy from all types of incident waves.
On the other hand, OPT and WaveBob have similar working principals creates electricity from the vertical motion of the float relative to the stationary spar [54]. This motion drives a mechanical system coupled to generators and produces AC electricity [55]. The electricity is rectified and inverted into grid-compliant AC, which has been certified to international interconnection standards [56].
The bearing standards in such devices depend mainly on the power take-off system (Table 14).
Table 6: Bearing Loadings on different types of Point absorbers take off systems
Group
Machine
Air-gap
Loading
Design
Integration
Shear force
Synchronous
Field Wound
PM
5mm
5mm
Large magnetic attraction
Flat surfaces large active area
In air gap
Outside air gap
Low
Air-Cored
C-GEN
PM Tubular
5mm
5mm
Subjected to own weight
Within air gap along axial length
Long surface between poles
Low
High
Variable Reluctance
SR
VH
TF
~1mm
~1mm
5mm
High magnetic attraction
Long and thin topology
Outside with poles
On stator
Low
High
High
Table 15 summarizes the three different types of electric generators that may be found in point absorbers.
Table 7: Illustration of types of electric generators
Synchronous [58]
Air-Cored [59]
Variable Reluctance [60]
Copied: Linear bearings are used in applications such as transport within factories, milling machines, assembly lines, elevators, forklifts, precision measuring equipment and in many actuators. Linear motion can be catered for using sliding carriages on guide rails. The carriages have built-in reciprocating ball bearings, roller bearings or plain polymer surfaces to run against the steel rail. These varieties of linear guides and profile rails are widely manufactured by companies such as SKF, Rexroth Bosch Group, INA Schaeffler Group, Hepco and IGUS.
The type of bearing mechanism is chosen depending on the torque, load capacity, speed and life cycle of the application. This type of bearing would be applicable to the design of a WEC as the guides could keep the moving translators on a rigid, straight path. A sliding device would operate most effectively, namely due to the developement of hard wearing, abrasive resistant polymers such as those from Glacier Garlock Bearings and Deva-tex, Deva (2010); GGB.
Roller tracks require sealed compartments, good lubrication, a return path and low loads for a long life span. If limitless motion (e.g. milling machines), that is required, linear ball guides, linear bushings and linear rolling guides are the most suitable as they wear slowly. For limited motion, i.e. microscopes or precision measuring devices with high speed motion, crossed roller bearings or stroke rotary bushings provide the very accurate movement due to their tight tolerances but with a lower life span due to maintenance needs [61].
