Wave Energy Prize winner(s) to be announced Nov. 16

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It has been a long road for our teams since registering for the Wave Energy Prize back in the spring of 2015. In 18 months, they have written technical submissions, test plans, and build plans; constructed 1/50th-scale models, completed small-scale testing, and numerically modeled their WEC’s performance; and finally, after building 1/20th-scale WEC prototypes, tested their devices at the Navy’s Maneuvering and Seakeeping (MASK) Basin in Carderock, Md. Nothing remains but to find out how many teams were able to meet or exceed the Prize’s goal of doubling the energy captured from ocean waves, and to ultimately announce whose technology met or exceeded the required ACE value and produced the highest HPQ.

The U.S. Department of Energy will announce the winner(s) during the Wave Energy Prize Innovation Showcase to be held at Naval Surface Warfare Center Carderock on Nov. 16. Attendance at this event will be by invitation only.

For our teams, whether they are the winner of the $1.5 million grand prize or not, Nov. 16 is not the end, but rather a new beginning. In the months following the Prize, the teams will analyze the data obtained during testing at the MASK Basin to help bring their innovative technologies to the market.

Testing Update

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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.”

How does the Wave Energy Prize calculate ACE?

The goal of the Wave Energy Prize is to stimulate the development of innovative wave energy converters (WECs) that have the prospect for becoming commercially competitive with other forms of electricity generation.  Specifically, the Prize seeks to double the state-of-the-art performance of WECs. As mentioned in a previous blog, when comparing the economic attractiveness of power generating technologies, levelized cost of energy (LCOE) is a common metric frequently used in the power generation sector.  LCOE is the ultimate expression of the ratio between effort (cost) to benefit (energy generated).

Unfortunately, LCOE cannot be used in the Wave Energy Prize because the data needed to determine LCOE are either not available or are very unreliable at low Technology Readiness Levels (TRLs), and the Wave Energy Prize is fundamentally expecting to be operating with WEC technologies that are at low TRLs.

With support from Sandia National Laboratories and the National Renewable Energy Laboratory, the Prize team derived a new metric to determine effectiveness of low TRL concepts that is a modification of existing WEC metrics. Importantly, this metric allows for robust analysis of innovative WEC devices using novel methods and materials.  ACE is a benefit to cost ratio, and is a proxy for LCOE, appropriate for comparing low TRL WEC designs.

The two components that comprise the ratio ACE are described in full in the Wave Energy Prize Rules. In summary they are:

  • Average Climate Capture Width (ACCW) = a measure of the effectiveness of a WEC at absorbing power from the incident wave energy field.
  • Characteristic Capital Expenditure (CCE) = a measure of the capital expenditure in commercial production of the load bearing device structure.

Wave Energy Prize ACE Metric Slide

Analyses of the current state of the art reveals that existing WEC concepts achieve an ACE value of 1.5 m/$M (meters per million dollars). For WEC technologies that emerge from the Wave Energy Prize to be on a development trajectory to become commercially competitive, our analysis shows that in the Prize, WECs must achieve a minimum threshold value for ACE of 3 m/$M. Below is a detailed description of how to calculate ACE.

Average Climate Capture Width

The average climate capture width (ACCW)—the numerator of ACE—represents an expected yearly average capture width for a WEC operating in typical West Coast wave climates. ACCW is calculated from a set of WEC capture widths for a select set of irregular wave conditions that are either measured in sub-scale physical model testing or calculated from numerical simulations.  The full scale capture widths are weighted by the yearly occurrence of the specified test wave conditions at select locations and summed to yield the ACCW.  This means that a device that performs very well in one sea state but poorly in other sea states may have a relatively low ACCW when compared with the maximum capture width.  Alternatively, a device that has modest performance over a wide range of sea states and wave directions may have a higher ACCW.

Calculating ACCW

ACCW is calculated in two steps, first by calculating the average annual capture width (AACW) for each wave climate of interest through weighted absorbed power measurements in the sea states of each wave climate, and then by averaging the AACW values to give ACCW.  For more details on these calculations, see Appendix I of the Wave Energy Prize Rules.  Below is a description of the approach for determining which tests to perform to determine ACCW, followed by a simple illustration of calculating AACW and ACCW.

Both tank testing and numerical simulations must cover enough sea states to represent a realistic wave climate. Simulations should be performed in enough irregular sea states that the power in every bin of the resource matrix, or joint probability distribution (JPD), at the wave climate can be approximated. For tank testing, testing at every sea state bin at the wave climate would be over burdensome, but enough sea states should be tested to represent the characteristics of that climate.

In both cases, the sea states that are tested should be weighted so that average annual power absorbed for a particular wave climate can be estimated. (This scaling is represented by Ξ in Appendix I of the Wave Energy Prize Rules.)  The resulting dataset will be a power ma­trix of device power absorbed for significant wave heights and energy periods that cover the range of sea states experienced at the wave climate of interest.  Multiplying the power matrix by the JPD and summing the values of the matrix yields the average device power absorbed for a particular wave climate. The average power absorbed is then used to determine the average annual capture width.

For example, for a particular wave climate, if the average power absorbed by a WEC is 90 kW and the average annual wave resource is 30 kW/m, the WEC would have an AACW of 3m.

P average absorbed = 90 kW

P resource = 30 kW/m

AACW = ( P average absorbed (kW) / P resource (kW/m) ) = 3 m

Per Appendix I of the Wave Energy Prize Rules, ACCW will then be given simply by averaging the AACW for all wave climates of interest.

Characteristic Capital Expenditure

Prior analysis performed at NREL shows that the largest contributor to wave energy LCOE is the structural cost of a WEC, and in the Prize, the Characteristic Capital Expenditure (CCE) is used to estimate the structural cost of a device (CCE and RST were discussed in a previous blog post, and are discussed in more detail here). The device structure accounts for the mass of any and all load bearing structures that are critical to the power conversion path. This includes:

  1. Any structure that interacts with the wave environment
  2. Any supporting structures used to resist forces in the power conversion chain in the load path/force flow path
  3. Any significant load-bearing foundation components

This implies that for a heaving buoy, for example, not only must the structure of the buoy be used to calculate CCE, but the structure of the gravity base itself must also be used. For offshore devices that require substantial structures, such as jack up barges, those structures must be included as well.

Once the structure is defined the CCE of a device is calculated using the following equation:

CCE = RST * Asurf * ρ * MMC

where:

  • RST = representative structural thickness [m]
  • Asurf = total structural surface area [m2]
  • ρ = material density [kg/m3]
  • MMC = manufactured material cost [US$/kg]

If more than one structural material is used in a device, the individual CCEs for each material are summed to give a total CCE.  Below are details on calculating each of the variables above for a single material in a device.

Representative Structural Thickness

The representative structural thickness (RST) mentioned in the above equation is a scalar that is used to determine the total structural mass when multiplied by the surface area of the material. The RST can be visualized as a single uniform thickness obtained by “melting down” all of the structural components of a material, and 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.

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Manufactured Material Cost

The last critical variable to calculate CCE is the manufactured material cost (MMC). This value represents the total cost to manufacture the material used in a device at full production scale. Therefore, the MMC includes the raw material cost, any fabrication, forming, and assembly.

In practice, the value of MMC will fluctuate due to material suppliers, complexity of device, number of devices, along with many other market factors. For example, the raw cost of structural steel may be approximately 1 $US/kg but by the time any forming, cutting, or welding is made the MMC may be closer to 3 $US/kg at full production. For a device already built, one can back out the MMC by dividing the total cost to build the device using a particular material by the mass of that material used.

Summary and Example Calculation of RST, CCE, and ACE

Once all the above variables have been defined, one can calculate the RST, CCE, and ACE values for any wave device. Below is an example calculation using cost and performance estimates from the DOE MHK Reference Model #5 which is made of steel and is assumed to operate offshore of Humboldt Bay, Calif.  The absorbed power for Reference Model #5 was simulated at each sea state using the numerical code WEC-Sim developed by the National Renewable Energy Laboratory and Sandia National Labs:

eq1.png

Using this method one can estimate and compare the economic viability of different devices at an early stage. However, one must be careful when employing this method for devices that have different percentage breakdowns with regards to structure, power take-off, mooring, etc. In these situations, and when comparing devices, a more reliable method would be to include all capital costs in the CCE. If all the initial capital costs were included, the CCE would increase from $2.4M to $4.97M, yielding an ACE of 0.84 m/$M.

 

Wave Energy Prize Finalist Teams and Alternates Showcased at Waterpower Week

Photocollage_640x480_V2Wave Energy Prize Finalist and Alternate Teams recently had a unique opportunity to showcase their technologies and network with industry, academic, and government stakeholders during Waterpower Week 2016 in Washington, D.C.

The week’s events kicked off during the National Hydropower Association Annual Conference’s opening plenary session on Monday, April 25 when José Zayas, Director of the U.S. Department of Energy’s Wind and Water Power Technologies Office, highlighted the work of the teams to the more than 700 conference attendees.

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On Monday and Tuesday, the teams had their 1/50th-scale WEC models on display, meeting with Zayas, Cristin Dorgelo, Chief of Staff of the White House Office of Science and Technology, and other event attendees during the conference’s coffee breaks. On Tuesday afternoon, teams switched gears and took part in a Wave Energy Prize Team Summit, a key part of Waterpower Week, where they were able to meet each other and share ideas; learn about the requirements of upcoming Technology Gates 3 and 4; and participate in on-camera interviews discussing their thoughts on the role of government in innovation, their teams’ successes so far, and the challenges they are overcoming in the upcoming final phase of the Prize. The teams then traveled to the MASK Basin at Carderock, Md., on Wednesday morning to better understand the logistical and technical requirements related to 1/20th-scale testing, and to tour the world-class facility where they will test their prototype devices beginning in August.

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Thanks to all those who joined us for Waterpower Week and the Team Summit, as well as all those who helped make the event a success!

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.

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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.

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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

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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 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!