Hydro Review

Ocean/Tidal/Stream Power: Wave Power's Path to Commercial Acceptance - A Comparison with Deepwater Wind

Wave energy has seen a wealth of development and may be a technology that can offer a viable option and compete, at least as an equal, with deepwater wind. Current wave energy technology is close to commercial viability.

By Timothy R. Mundon

Wind power has an enormous amount of momentum in the U.S., much of which stems from a large investment in, and development of, on-shore resources. Due to promises of a vastly higher resource, lower visual profile and a proximity to high demand centers, U.S. wind is likely to start moving into shallow coastal waters.

Development of wind in Europe emphasizes the feasibility of large scale offshore wind. Unfortunately, the U.S. coastline, with a relatively small continental shelf, is not quite as amenable to these developments as Europe’s North Sea.

Wave power nearing commercial viability

As a result of these geographical challenges and spurred by the increased resource, there is a growing desire to develop deep water, floating wind options. The U.S. would benefit from not only looking at floating offshore wind but also wave energy, which may be equally capable of economically extracting energy from the marine environment.

Today, conventional offshore wind in shallow waters is marginally viable. Thus the move into deeper waters, with more complex platforms and foundations, implies even more tentative economics. Wave energy has seen a wealth of development and may be a technology that can offer a viable alternative option and compete, at least as an equal, with deep water wind.

There are surprising similarities between deepwater wind and wave energy and in most cases, wave energy can show significant practical advantages. While there are still obstacles wave energy must overcome, devices exist now at a near-commercial level.

This article does not address a specific technology solution or provider, but it does provide a realistic comparison between these two resources and explains why wave energy is not more readily accepted.

Unconventional wind

Conventional deep water wind turbine foundation technology is limited to a depth of around 50 meters using jacket type foundations. In these depths, foundations are typically open truss frame structures, constructed from steel and solidly anchored to the seabed using piles or suction anchors. These typically expensive structures are aimed at supporting large 5-MW class turbines in order to keep the overall cost per kilowatt-hour down.

As the water depth increases, foundation costs increase such that floating structures derived from the deepwater oil and gas industry are the only option. Structures such as Spar platforms, tension leg platforms or semi-submersible hulls are the primary choice.

For the purposes of this paper, deep water wind relates to the use of floating platforms in depths where it is not viable to install a fixed foundation. There are a number of companies moving forward with products in this arena, most notably Statoil with their Hywind concept, but also others such as Blue H, Principle Power and Sway. These developers publicly appreciate that a significant amount of work is still needed before floating wind can be achieved commercially. Nevertheless, investment is still high with Statoil developing a full scale prototype Spar fitted with a 2.3-MW turbine for a reported investment of $70 million.

Resource availability

It has been stated that there is a potential resource of 3,270 terawatt-hours of wind in water deeper than 30 meters (Musial 08), with nearly 2,000 TWh of this within 50 nautical miles from shore (Musial 10). This large figure is most likely the reason why such attention is given to deep water wind technologies.

Also, in a 2008 report by the National Renewable Energy Laboratory, the potential resource for wave energy in the U.S. is estimated at 252 TWh, from a total resource of 2,120 TWh. This magnitude level difference at first glance may seem to preclude the development of the wave resource and suggest that we should invest heavily to develop deepwater wind.

However, it is important to appreciate what these resource figures actually mean. These are estimates of maximum resource, to be realized only if the whole available area was covered with extraction devices. These figures do not fully attempt to assess the realistically recoverable power as this is influenced by a number of difficult-to-quantify issues, including permitting, environmental and economic considerations.

Furthermore, it is important to appreciate that these potential resource figures are huge.

With the 2009 U.S. electricity usage of around 4,000 TWh (EIA Statistics), the deepwater offshore wind resource could supply nearly the whole of the U.S., even a potential resource of 252 TWh in wave energy could still supply 15 percent of the national electricity requirements.

These figures, however, become less relevant as we consider a realistic future energy mix. Renewable portfolio standards set for the next 20 to 30 years are optimistically in the order of 20 percent to 30 percent and these will likely comprise a combination of onshore wind, nearshore wind, solar, biomass, etc. The introduction of deep water wind or wave energy into the energy mix is unlikely to significantly change the proportions of renewables in the foreseeable future. Therefore, in practice, we are likely to be able to employ only a tiny fraction of these available resources. The problem we truly face is not the magnitude of the resource; it is the practical deployment and the economical conversion into electricity.

So instead of concentrating on the magnitude of the resource, we should look at the most viable methods of capture at the lowest cost per kWh.

Ocean Power Technologies’ PowerBuoy Wave Energy Converter. The system can generate power with waves between 5 feet and 23 feet high. The PowerBuoy also has a fiber-optic communications and Supervisory Control and Data Acquisition (SCADA) capabilities.

Wave energy extraction

The energy present in waves is defined as the power per linear meter of wave crest and this is given by:

(in deep water, for a regular wave)

Where ρ=Density of water (998kg/m3), g=acceleration due to gravity (9.81ms-2), Tp = Wave Period, H=RMS Wave height.

It can be seen from this that the energy present in an ocean wave is dependent upon both its period and its height.

One way to appreciate this is to consider the wave period as a “tuning” parameter and the height as the carrier of the power. Therefore, a device needs to be “tuned” to match the wave period for maximum efficiency, while extracting energy from the wave height. This is not straightforward, however, as real sea states comprise a mixture of many different waves of varying periods and wave heights. Furthermore, the density of the medium (compared to wind) results in a very high power density. As a result, survivability requirements need to be considered very seriously. These basic factors are certainly one reason why wave devices have not developed as quickly as wind.

Wave energy history and current status

Research on wave energy extraction has been underway in earnest since the late 1960s and 1970s, spurred by the oil crises of the ‘70s. During this time, much of the groundwork for today’s wave energy devices was laid. Possibly due to the complexity in extraction, a considerable number of ways to harness wave energy have evolved. As a result of this complexity, it seems unlikely that a single concept will emerge.

Development of wave energy devices did continue after oil pricing stabilized and nuclear power was adopted as a primary source, although funding decreased considerably over this period. During this time, development and research in this field was far more active in Europe than in the U.S.

As a result, a considerable body of knowledge, including a number of full scale prototypes, have been developed to date in Europe. More recently, interest and funding has increased both in the U.S. and Europe althoughthe U.S. continues to trail in development. Nevertheless, work at some U.S. institutions such as OSU/UW and development by companies such as Ocean Power Technologies have continued to advance wave energy in the U.S.

At the moment, there are a number of wave energy devices that are at or on the edge of commercialization, including devices by Pelamis Wave Power (P2), Aquamarine Power (Oyster), and Ocean Power Technologies (PowerBuoy), plus more generic technologies, such as oscillating water columns (Oceanlinx, Voith Hydro, etc).

There are a number of other devices being developed that are nearing the pre-commercial demonstrator level and a number of concepts being developed all over the world.

How does wave compare with deepwater wind?

While both deep water wind and wave power are both somewhat unproven at full utility scale, wave energy devices clearly have a number of advantages, especially in regard to the amount of research completed.

Both types of devices have to be designed to operate in effectively the same environment and logically both types have taken advice from the offshore oil and gas industry to develop survivable designs.

This survivability is a critical element in the design of any offshore structure, and the nature of the marine environment means the engineering involved is not straightforward. To provide some perspective, the energy present in an average “active” sea that these devices (wave or wind) may be deployed into would be in the region of 20 kWm-1. But in the case of a storm, this may rise to as high as 2,000 kWm-1. Therefore, any device must find a strategy to limit the incident wave power. This is where it is important to highlight some differences between floating offshore wind and wave power devices in their response to waves: “One is aiming for stability, the other for activity.”

The objective for a floating offshore wind platform is to provide a solid, stable base for the turbine, which will extend 80 to 100 meters above the sea. This structure will need to absorb and dissipate any high energy waves that will be incident. Moreover, the system will also need to handle the combined wind effects on the turbine that may well be incident at the time of very high seas. At present, this coupled response for floating wind is not well understood.

Conversely, a wave energy device typically needs to move in response to the waves with an optimum motion, generating power from these movements. When faced with very high energy waves, as there tends to be no large above water structure (i.e. wind turbine), the wave energy device may have the ability to employ additional techniques to deal with excess energies. Adaptive methods such as hydrostatic power limiting (larger waves passing around or over the top of the device), active damping or possibly geometry alteration1, among others, can be used to lower imparted forces and thus lower structural costs.

The resource power density varies significantly between wind and wave, providing the advantage of allowing wave projects to comprise a much lower amount of required sea area for the same rated power output. If we assume that deep water wind farms will use the same, if not greater, turbine spacing than current offshore wind, then this will result in a power density of approximately 10-15 kWh/m2/yr2 whereas wave devices are likely to offer between 50-100 kWh/m2/yr3. This provides a significant difference in the requirements for a permitted area for a project.

Visibility from shore is significantly lower for wave devices when compared to wind. As wind turbines increase in size, larger distances are needed from shore to ensure an acceptable visual impact.

Wave devices feature a much smaller visual profile with some devices extending no more than a couple of meters above the sea. Reducing the visual profile by this magnitude allows for deployment much closer to shore (where resource applicable) with shorter electrical interconnection and allows for deployment in locations with high value visual viewsheds.

Predictability of wave resource is considered to be somewhat more reliable than wind and while instantaneous time-series prediction of waves is extremely difficult, predicting energy a few days into the future can be done accurately. This is not the case for wind, although it is easier for offshore wind than onshore.

Floating wind, with the potential for individual units in the 5-MW range, has the advantage of scale currently as the most advanced wave devices are still in the 1-MW range. It is not yet clear what the cost implication of this will be, but with high installation and Operations and Maintenance costs associated with marine operations, this is in important area.

Similarities

Environmentally, both floating wind and floating wave devices will likely have a very similar profile, as both types of devices will be moored in similar ways. However, wave devices have a considerable advantage in that they typically offer no potential impact to avian species. There is, however, the possibility that, along with wind, they may also need to consider noise emissions during operation for their effect on marine mammals or other aquatic species.

If we remove the wind turbine from the floating foundation, the resulting structure shares many marine engineering considerations and issues with floating wave energy devices. Therefore, it seems likely that the costs for both will be similar before wind turbine costs are included. This seems to be confirmed in that released installed costs put wave energy devices somewhat below those of floating wind, with costs for installed wave capacity at 4-6k/kW and floating wind at $6-14k/kW4. However these figures should be considered with some caution as many developers are very protective of actual costs and these are for prototype installations, which are considerably more expensive than commercial installations.

Further work is still needed for efficient accessibility of both resources. For floating wind, a detailed understanding of the coupled nature of the aerodynamic and hydrodynamic forces is needed to predict how they combine to alter the power output of the turbine. For wave devices, there is a similar problem in understanding device moorings so as to develop systems to ensure position keeping and survivability, but while also allowing freedom of movement so as not to inhibit performance. In addition, installation and anchoring processes are expected to be similar for both wind and wave, possibly employing drag-embedded, suction or GBS anchors to secure the structures.

Pelamis Wave Power’s Wave Energy Converter. The four sections of this linear absorber move relative to each other, and this motion is converted at each hinge point into electricity by a hydraulic power converter system.

Not taking the advantage

Although currently expensive, wave devices are worthy of serious consideration and have a number of advantages over floating wind, including potentially simplified permitting. It seems, however, that these advantages are not fully appreciated as wave energy is not being considered on a level playing field with floating wind.

Wind in general has a huge momentum from the number of installed turbines and as a result is clearly in the public eye.

Also, nearshore wind projects are now underway and receiving much exposure, as are solar and biomass projects, while very little media attention is directed towards developments in wave energy. Also, no full-scale wave energy prototype devices have yet been deployed in U.S. waters.

Wave energy is complex to harness and it is a difficult and extremely long process to develop and validate a feasible wave energy device and development in the U.S. is progressing at a slower rate than seen in Europe. As a result, the US does not appear to possess the same breadth of development in this market area. Much development that has been completed has been progressing in a very disparate manner, with minimal dissemination5 and without the appropriate coordination required to enable the wave energy community as a whole to progress. The effect of this is that there has been a tendency to reinvent the wheel, repeat mistakes and not build upon work already completed, either because information is held as proprietary or possibly due to a lack of awareness of work elsewhere.

The result of this is that there are a number of people who are developing many different concepts, a number of which are developed without the required thought given to engineering and scientific validation. In many cases, these developmental short cuts are forced due to funding and IP restrictions and are further compounded by the fact that developers are forced into making ‘difficult to support’ claims about their devices in order to gain financial support. A further side effect of the problem discussed is that in many cases there does not exist the expertise on behalf of reviewers to properly understand these often very specialized and complex devices. This can cause a problem as funding is not being provided to the most appropriate projects.

The future of wave power

Designing any device to withstand the marine environment is difficult. In the case of wave energy devices, this is further complicated by the fact that the devices are inherently complex.

Nevertheless, even with this complexity, there is the marine engineering expertise in the offshore oil and gas industry to construct these devices with little trouble. However, this is typically at a premium cost that only the oil and gas industry can absorb. For marine energy generation, devices will ultimately need to be built at much lower costs.

It is therefore unavoidable that devices will initially be expensive. Economies of scale and further research are required to bring down the cost. Building and installing multiple devices will be needed to reduce the cost per device, but the scaling of individual devices to larger capacities may also be needed in order to offset installation, mooring and operations and maintenance costs.

Development work may be best focused upon improving devices that are already at a high technology readiness level. This is an area where the Department of Energy’s Water Power Program is making significant headway.

However, work should also be concentrated on peripheral systems where developments and improvements could be disseminated and would bring important benefits across a number of devices.

This would include research and development on subsystems such as mooring systems, active control technologies, PTO & gearbox improvements, hydraulics, energy storage and materials technologies. This “subsystem” work should not be wholly constrained to the device developers; rather it should also be completed by 3rd party companies/universities who can later work with multiple device developers to incorporate this new IP into existing devices.

While existing devices are likely to have proprietary technology which can make developing specific “subsystems” difficult, most devices will fit generic device types, which may be sufficient to demonstrate suitable feasibility.

The DOE Water Power Program should also be commended for their recent commitment to develop a series of generic device models that will be placed into the public domain. A useful reference paper by Muller and Wallace (Muller 08) discusses many areas of research that the industry needs.

The route to a viable wave energy device can be extremely long. Pelamis Wave Power took more than 15 years to evolve to their first commercial project6 and spent all of this time thoroughly validating and refining the concept.

Conclusions/Summary

Wave energy should be seen as a viable renewable energy resource, although its success in the U.S. will depend heavily on public support, interest and adequate funding. For wave energy to be successful, the community needs to work together, share information and coordinate to develop in the most efficient manner. It is important that projects get in the water, generate interest and start working towards lowering costs. It is also important that the we spread the technology development across the industry and work together to achieve low per kWh costs through simplicity, reliability and economies of scale.

References

  • W Musial and B Ram, Large-Scale Offshore Wind Power in the United States, NREL Technical report (NREL/TP-500-40745) September 2010
  • W Musial , Status of Wave and Tidal Power Technologies for the United States, NREL Technical Report (NREL/TP-500-43240) August 2008
  • M Muller and A R Wallace, Enabling science and technology for marine renewable energy, Energy Policy, 2008 no.36, pp4376–4382
  • US Energy Information Administration, Summary Electricity Statistics for the United States, http://www.eia.doe.gov/cneaf/electricity/epa/epates.html. Retrieved 03/17/2011.
  • New technology Magazine, “Statoil Draws On Offshore Oil Expertise To Develop World’s First Floating Wind Turbine”, Sep 8, 2009, http://www.newtechmagazine.com/issues/story.aspx?aid=1000340202

Notes

 

1Some devices may be reconfigured of offer a different profile to incident waves in extreme conditions

2Assuming 10D spacing and 25% capacity factor

3Based upon spacing provided by PWP and OPT and assuming a 25% capacity factor

4Cost estimates provided for floating wind assume a 5MW device in the region of $30-70M. For Wave, first commercial deployment (Pelamis) 2.25MW, 3 devices @ $12,8M (approx $4M per device)

5Much of which is caused by investors requirements to keep information proprietary

6Pelamis Wave Power installed three P750 devices (2.25MW) off the coast of Portugal in 2008


Tim Mundon is a Senior Engineer for Kleinschmidt, Energy & Water Resource Consultants.

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