Testing Update

carderock-collage-lg

Ever since registering more than a year ago for the Wave Energy Prize, our teams have eagerly anticipated the opportunity to test their 1/20th-scale WEC prototypes in the Navy’s Maneuvering and Seakeeping (MASK) Basin, at Carderock, Md. In a building with a footprint of more than five acres, what developer wouldn’t be giddy with excitement to test a new technology in the nation’s premier wave-making facility containing more than 12 million gallons of fresh water?

The beginning of August marked the beginning of the final round testing, and some of our Finalist Teams have already completed their 1/20th scale model tests in the MASK Basin. Others are still awaiting their turn. M3 Wave was first in the tank, followed by Waveswing America, Harvest Wave Energy (Team FLAPPER), AquaHarmonics, CalWave Power Technologies, Oscilla Power and Sea PotentialRTI Wave Power is testing its prototype this week, and SEWEC is onsite building its device.

“Watching our Finalist Teams’ WEC concepts come to life at the MASK Basin has been a thrill,” said Alison LaBonte, Marine and Hydrokinetic Technology Program Manager in DOE’s Water Power Technologies Office. “We’re looking forward to the Judges’ analyses of the nine weeks of testing and learning how many and which technologies surpassed our goal of doubling the energy capture efficiency of wave energy converters.”

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Armed with Science: The Navy’s Indoor Ocean

As many teams mentioned during the Wave Energy Prize Team Summit in April 2016, the Finalists are incredibly excited to test their 1/20th-scale WEC prototypes at the nation’s premier wave-making facility, the Naval Surface Warfare Center’s Maneuvering and Seakeeping (MASK) Basin, at Carderock, Md. With testing beginning in less than two weeks, let’s take a closer look at this unique facility, which was featured in the official U.S. Department of Defense science blog, Armed with Science, last year. More »

From the Wave Energy Prize website: A question about ‘overtopping’

A university student from Texas writes:

“I read the ‘types of devices proposed by these teams include point absorbers, terminators, attenuators, oscillating water columns, and oscillating wave surge converters’ from your website. I am just reviewing some papers about WECs and found that there is another type of WEC which is called overtopping. My question is whether the overtopping type is less efficient so none of the top teams use it.”

(“WEC” means “Wave Energy Converter” or sometimes “Wave Energy Conversion.” It is common to say “WEC device” as well.)

To answer, the Wave Energy Prize provides an avenue to allow teams to develop innovative technologies that have the prospect for achieving a long-term impact of lowering the cost of electricity to make wave energy competitive with other means of generating power.  If someone came forward with a design for an “overtopping device” that shows promise achieving our overall program goal, as outlined in our Technology Performance Level (TPL)* assessment, then it may well have progressed in the competition.

To learn more about TPL, refer to “Details on the Technical Submission” written by Jochem Weber from National Renewable Energy Laboratory (NREL).

Team Technical Summaries

It has been more than two months since the 20 Qualified Teams in the Wave Energy Prize were announced, and you are probably wondering what they have been working on. Well, they have been developing their 1/50th scale wave energy converter (WEC) models; preparing for testing these scale models; and starting to numerically model their innovative WEC designs. Below, in their own words, the Qualified Teams provide a glimpse into the breakthrough technologies they are developing (we present them alphabetically based on team name, and will do so for all Qualified Teams).

Advanced Ocean Energy @ Virginia Tech
Hampton Roads, Va.

AOE@VT WEC Technology Image for Publication FINAL 20150921

Virginia Tech’s MULti-body LinEar Terminator (MULLET) is a self-contained array of Bundled pIpe Terminator Wave Energy Converters (BITWECs). Each BITWEC is a floating bundle of large-diameter high-density polyethylene (HDPE) pipes. The basic MULLET building block is a “tandem BITWEC” whereby two pipe bundles are arrayed one behind the other in the down-wave direction.

Each bundle is tethered by a pair of flexible fiber matrix composite tube pumps to the deeply submerged deck of a pontoon barge, which also is fabricated from large-diameter HDPE pipe sections, as shown in the conceptual diagram above, seawater output from the tube pumps is piped to an underwater habitat housing an accumulator and Pelton turbine-generator.

The MULLET barge is assembled while floating in calm water at dockside and then towed to its offshore installation site. Once on site, the barge is connected to a pre-set catenary anchor-leg mooring (CALM) buoy. The barge pontoons and CALM buoy are then flooded with seawater, submerging the barge to its mid-water operating position, where the barge deck acts as an inertial reaction plate for the floating bundles to work against, stretching the tube pumps. For inspection and maintenance, the barge and CALM buoy are re-floated to the surface by reversing the installation procedure.

AquaHarmonics
Portland, Ore.

aquah

AquaHarmonics’ Wave Energy Device is a point absorber device consisting of a simple Power Take Off (PTO) system mounted in a cone/cylinder shaped hull with a single mooring line that has a power cable at its core.

The PTO System consists of a sheave fixed to a shaft mounted in bearings within a sealed compartment and directly coupled to a pair of axial flux generators. The device only generates power on the rise of the wave, and during the fall of the wave the generators are operated as motors to reel in the mooring line for the next wave cycle.

During reel in, the control system of the device can provide additional energy input to achieve phase locking with any wave frequency. This control method is known as “de-clutching,” which has been shown to effectively increase the operational bandwidth of a wave energy device.

The generated power is far greater than that consumed during the wave cycle with some energy stored on board for periods of low wave activity. The power is conditioned on board and sent to shore via a slipring on the shaft connected to the power cable located at the core of the mooring line.

AquaHarmonics. Clean.Simple.Energy.

Atlantic Wavepower Partnership
Newport, R.I.

awp

The AWS-III is a large-scale surface floating multi-absorber wave energy converter. Each absorber cell comprises a partially submerged air-filled chamber, one face of which is covered with a rubber diaphragm which flexes in response to the incoming wave actions. The movement of the diaphragm pumps air to and from the cell via an air turbine-generator set where the energy is converted to electricity. The cells are inter-connected via a ring-main such that air is exchanged between cells rather than with the outside environment.

All mechanical moving parts are isolated from the sea and contained within the device, whilst the turbine technology is tried-and-tested and available on the commercial market. The device is moored using traditional systems for offshore structures, either catenary systems and drag-embedment anchors or tension tethers and suction anchors. Accordingly, the device is utility-scale and has low technical risk and is capable of on-board maintenance of all parts with the exception of the diaphragms. Initial systems are expected to be rated at 2.0MW.

Atlas Ocean Systems
Houston, Texas

atlasos

The Atlas Ocean Systems SQ5 Wave Energy Converter is a completely new device that captures energy primarily from pitch using two independent coupled oscillators. The SQ5 consists of three primary elements: 1) a float, 2) a new innovative submerged bag filled with air under pressure, and 3) a large submerged ballast.

The bag and submerged ballast are essentially in static buoyant equilibrium. The catamaran float provides roll stability and resonates in pitch with the waves and transfers vertical motion to the bag system which then excites a vertical oscillation of the submerged ballast. The oscillation of the ballast drives shape changes within the bag to pump air through a reversible flow turbine to generate useful energy. Pitch response of the float is adjustable in real-time using a ballasting system and coupling between the float and ballast is adjusted during design. Coupling between the two oscillating systems provides a wide dual-peak absorption spectrum providing good power output over a variety of sea states. There are no mechanical moving parts, seals, hinges, or pistons requiring lubrication or maintenance. The turbine system is protected within the pressurized pneumatic system and is suspended in a transverse mounting minimizing gyroscopic forces. The nominal system is designed for a 500kW-1MW rating.

Technical Summaries from more of the Wave Energy Prize Qualified Teams will be profiled in alphabetical order in the future. Visit the new Wave Energy Prize Team Updates page for more information.

Why is the development of wave energy converters so challenging? Part 2: Extracting energy from waves

Development of wave energy converters

So, how exactly can we exploit this resource?

The Wave Energy Prize received technical submissions detailing 66 unique WEC concepts.

A very small number of these couldn’t work, but the overwhelming majority were concepts that could exploit, to differing degrees, the fluid motions of water particle motions. While some of these concepts were variations on ideas seen before, that is still a very large number of different ideas. At present, the means of exploiting wave power seems only constrained by the imagination of the inventors.

And that is a challenge; one that the Wave Energy Prize is, in part, endeavoring to address.

If one thinks of the development of modern wind turbines 30 to 40 years ago, there was also a plethora of competing wind turbine designs, each hoping to become commercially competitive.

Over time, as the science, engineering and economic understanding has matured, it became possible to identify optimum approaches to the exploitation of the, essentially, linear wind flow into electricity. All turbines now are generally three-blade, horizontal-axis turbines, with a gear box and generator. More modern designs are now using permanent magnet generators, and maybe even novel hydraulic systems. Even then, the principal power absorption mechanism was essentially the same for all wind turbines – linear fluid flow into horizontal mechanical rotation (we do, however, see some small vertical axis turbines in niche applications).

At present, the means of exploiting wave power seems only constrained by the imagination of the inventors.

The same is true of tidal stream turbines, and also aircraft and car designs. Over time, the science and engineering, along with the design tools and test facilities available, lead to a science-backed consensus regarding optimum configurations. Optimum configurations might change as new materials and components become available, but changes are systematically made through an understanding of their impact on a turbine’s techno-economic performance, arising from good knowledge, understanding and information (this can also relate to cars or planes.)

The exploitation of low-speed fluid oscillations created by ocean waves, imparting very high linear or rotational forces/torques, which need to be converted to (generally) high-speed linear or rotational motion in a generator (doing so cheaply and reliably, while being survivable) is just not as well understood within the science community.

Hence the very large number of wave energy conversion concepts we see in the Wave Energy Prize, and throughout the world.

It is possible that there is no single solution that is optimal, or that there are several optimal solutions, dependent on many factors, such as site conditions, water depth, distance to shore, etc.

The exploitation of wave power faces other very significant challenges, perhaps ones that aren’t faced by wind turbines or tidal stream turbines – the most important of these is survival.

Potential wave power sites off the West Coast of the United States have annual average wave energy fluxes in the region of 20-30 kWm-1 of wave crest length. In severe winter storms, the peak power can be multiple times higher than this, perhaps as high as 1MWm-1 (an increase by a factor of 30-50 over the average wave energy flux). This provides significant challenges to the structural design and consequent capital cost, or requires survival strategies, such as diving, submersion, or perhaps even removal to shore. And these typical winter storms are not as strong as the survival conditions to be met for the “100 year wave” or rogue waves, which will be even more demanding.

Wind turbines operate in a relatively narrow range of wind speeds, and simple survival strategies are possible when winds exceed operational limits; they feather their blades to minimize the forces on the turbine. Furthermore, the forces imparted by water with a density of ~1000kgm-3 are so much larger than when the fluid is air. Tidal stream turbines are also frequently not exposed to the severe storm conditions or wave energy potentially encountered by WECs. Besides, the environmental conditions seen are far more predictable and the ranges are not so extreme.

Interestingly, deep-water waves are also intrinsically more survivable than near-shore ones, as near-shore devices could well be exposed to severe breaking waves in harsh storm conditions, which would likely lead to catastrophic failure. In severe storms, big ships head away from the shore and out to deep water for the same reason.

The exploitation of wave power also creates other challenges that are not normally as onerous for wind turbines and tidal stream turbines. These include:

  • This is straightforward for onshore and relatively straightforward for shallow-water offshore wind turbines. Tidal stream is more challenging, due to the high-speed water currents in commercial sites, but still relatively close to shore, and not at very significant water depths. For WECs in deep water, this requires multiple mooring systems, potentially remotely operated vehicles and divers, possibly operating many miles from the shore base for marine operations.
  • This requires access in potentially hostile and dangerous environments, with weather window constraints.
  • Some WEC concepts are intrinsically directional in their approach to exploiting wave power, whereas wind turbines and potentially tidal stream turbines have easy strategies for orienting the system towards optimum flow conditions for energy extraction.
  • Marine debris and biofouling. With many wave energy concepts located at the surface of the sea where the wave energy is greatest, they are potentially more vulnerable to damage caused by marine debris, such as shipping containers, ropes and fishing nets. Biofouling is also far more prevalent at the surface or near the surface, as this is where the biological activity is greatest.
  • Other uses of the sea. WECs potentially have greater impacts on other users of the sea, such as those in shipping, fishing and recreation. Many wave energy concepts have some freedom to move, but tidal or wind turbines are essentially static devices that are constrained by moorings. This uses up a large surface of the sea, and increases the potential for collisions.
  • Non-linearity. Wave energy theory is linear, which is an approximation that holds only for small waves. In practice, the waves themselves and the interaction of the device with them is likely to be non-linear, potentially requiring very significant computer power to even understand how the device might react to the waves, or accepting uncertainty in performance and maybe life-time of the device.
  • Breaking waves. This is not really a great concern for deep-water WECs, with the possible exception of rogue waves or in survival conditions. However, breaking waves impart huge slamming forces on structures and their moorings, leading to potentially catastrophic failure.

All in all, a very difficult environment to design for and survive in.

Whatever the solution is that emerges from the Wave Energy Prize and other U.S.-based and global development activities, to be successful we need to see large increases in the absorbed power, with high bandwidth and adaptability to different sea states, and maybe even wave-to-wave control, with greatly reduced capital expenditure, reliability, survivability, and maintainability.

Why is the development of wave energy converters so challenging? Part 1: The nature of waves

The Wave Energy Prize is not concerned about just developing a wave energy converter (WEC) that can generate power; that’s the easy part (as outlandish as that may seem). Rather, the goal of the Prize is to identify WEC concepts that have the prospect for becoming commercially competitive with existing means of power generation, without unsustainable public subsidies, in the next fifteen or twenty years. While that is an aggressive goal, the Wave Energy Prize is designed to make it achievable. This blog post and the next will describe why developing WECs is so challenging. First, a primer on the nature of waves themselves.

"Deep water wave" by Kraaiennest - Own work. Licensed under GFDL via Commons
“Deep water wave” by Kraaiennest – Own work. Licensed under GFDL via Commons

Real ocean waves are complex, dynamic, and cannot be precisely predicted. They are not neat and well-organized sinusoidal waves with well characterized amplitudes, frequencies, wave lengths and phases that create predictable patterns on the surface of the water through interference, diffraction, and refraction.

Over the deep ocean, winds blowing over many hundreds of kilometers lead to the creation of sea states, which when analyzed spectrally (or, broken down by defining characteristics like frequency, amplitude, and phase), show the superposition of waves of varying frequency, amplitude, and phase. In fact, the spectral distribution almost shows the history of the winds experienced by the sea over the entire journey of the wave.

At a given site in the ocean, the sea state changes over a time span of thirty minutes to one hour. Over the course of a year, a site’s wave energy resource is typically with about 1,000 sea states. Each sea state has different spectral distributions with unique probabilities of occurring – this is the so-called wave scatter diagram.

But the complexity does not end there. Local wind-driven waves can add another “peak” (or high probability of waves of certain characteristics) to the wave spectrum, and the spectral characteristics of the waves change as they move from the deep ocean to shallower depths due to shoaling and focusing effects. Ultimately the waves crest and break as they approach the shoreline.

The Wave Energy Prize is deliberately focusing on deep water applications. This isn’t to avoid the added complexity of shoaling and breaking waves. Rather, it is because it is in the deep water where the wave energy resource is strongest. By the time the waves reach the shore, all the wave energy has dissipated due to friction with the sea bed, resulting in the waves’ energy equaling zero at the shoreline.

It is important to understand how water moves in wave motions to understand how to extract energy from waves (more on this in the next blog). Deep ocean waves are a combination of sinusoidal and longitudinal waves. With wave energy being zero at the sea bed in the deep ocean due to friction, the actual water particle motion varies as a function of water depth, with the water tracing approximately circular paths near the surface, as can be seen in the video below. These paths become more elliptical and eventually linear oscillations as depth increases. The oscillations have smaller and smaller amplitudes as depth increases, as well. The next time you go to the beach, stand in the ocean near the coastline, beyond the breaking waves. You’ll experience your upper body bring pushed to move in circular paths, while your feet don’t really move that much at all.

Thus WECs need to extract the energy from circular, elliptical, or linear fluid oscillations created by waves passing through water, waves that have a large number of spectral distributions as described above, and convert this energy into other usable forms of energy. This is no mean feat, one that the Qualified Teams of the Wave Energy Prize are hoping to achieve; more on this in the next blog post. Stay tuned!

Representative Structural Thickness (RST) and Characteristic Capital Expenditure (CCE)

In order to compare the potential cost of energy between wave energy conversion (WEC) devices at low Technology Readiness Levels (TRLs), the ACE metric was created as a proxy for Levelized Cost of Energy (LCOE) (see blog post from June 11). The Characteristic Capital Expenditure (CCE) portion of the ACE metric is determined by the following equation:

Characteristic Capital Expenditure (CCE) = Total Surface Area (m2) x Representative Structural Thickness (m) x Density of Material(s) (kg/m3) x Cost of Manufactured Material per unit Mass ($/kg) for all applicable materials.

Representative structural thickness, or RST, is the most crucial of these variables, and is described below.

What is RST?

RST is used to determine the total structural mass when multiplied by the surface area of the device. The RST can be visualized as a single uniform thickness that is obtained by melting down all of the structural components of a WEC then “casting” the shape of the WEC with a constant wall thickness – the RST. This means that all stiffeners and support structures are “lumped” together. A simple representation of the RST is shown below with a flat plate. The original structure includes a grid of stiffeners with a thin hull. That same quantity of material is then represented by a solid plate with the thickness given by the RST.

Plate Thickness

What is considered in determining RST? How is it calculated?

The following components determine RST:

1. Structural Mass

The structural mass of the device accounts for the mass of any and all load bearing structures that are critical to the power conversion path. This includes:

  • Any structure that interacts with the wave environment
  • Any supporting structures used to resist forces in the power conversion chain central to the load path/force flow path
  • Any significant load bearing foundation components

2. Simplified Surface Area

The surface area that is multiplied by the RST is a simplified representation of the WEC device. All stiffeners and support members that do not directly contribute to the power conversion path are excluded. The following examples show how the geometries of two U.S. Department of Energy reference models have been simplified for the RST calculation (reference model reports and documentation can be accessed at http://energy.sandia.gov/rmp). In the first example (point absorber) the surface area (faces in green) is simplified by not including external stiffeners and only accounting for one side of any thin plates. In the second example (oscillating water column), the surface area excludes all external stiffeners and only one surface of any thin plate is considered.

wep-rst-original-rm3-geometry

wep-rst-simplified-rm3-geometry

wep-rst-orig-simp-geometry

How do we calculate RST?

The overall methodology that is used to calculate the RST for all contestants is the same. At the most basic level, the device geometry and wave interactions are used to estimate hydrostatic, hydrodynamic, PTO and structural loads. The loads are calculated using widely used empirical formulas as well as NREL offshore models and first principle approximations where existing standards and guidelines do not exist. These loads are fed into structural models that calculate stresses using existing offshore guidelines and standards. The diagram below represents the process that will be used to calculate the RST for each WEC.

wep-rst-calculation-diagram

What are RST Tables?

As described in Appendix D of the official Wave Energy Prize Rules (5.26.15 R1), each WEC will be assigned an RST Table by September 15, 2015. This table will include the RST value calculated based on the above process, along with additional RST values that represent different load cases. These additional RST values can be used by the Prize judges during the competition to alter the final RST value depending on results from and performance in the 1/50th and/or 1/20th tank tests. For devices that utilize more than one material, an RST table will be supplied for each material.

Why RST?

Now that we’ve described what RST is and how it’s calculated, you’re probably asking yourself, why is the Wave Energy Prize using RST instead of the actual structural design? This simple answer is to maintain consistency during the competition. Every contestant will have a structural mass that is estimated using similar standards and design guidelines, allowing lower TRL devices to be judged against high TRL devices without bias. This also allows for a quicker analysis because devices with similar geometry and similar wave interactions will be viewed as structurally similar devices.

What is Manufactured Material Cost?

Referencing back to the equation for CCE, the last critical variable is manufactured material cost (MMC). This value represents the total cost to manufacture the materials used in the WEC at full production scale. Therefore, the MMC includes the raw material cost, any fabrication, forming, assembly, etc. In addition to the RST tables, the Prize judges will be given a table that includes a range of MMC values that will allow the judges to address designs that have high or low complexity, which will result in a higher or lower MMC value.