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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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.

plate1.png

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.

 

U.S. Department of Energy’s Wave Energy Prize Announces Finalist Teams

Meet the Wave Energy Prize Finalist Teams

The U.S. Department of Energy (DOE) announced Tuesday that nine teams have been named finalists in the Wave Energy Prize, a 20-month design-build-test competition, and will proceed to the next phase of the competition.

The nine finalists and two alternates, identified from the 17 remaining official qualified teams, will continue their quest to double the energy captured from ocean waves and win a prize purse totaling more than $2 million. Each of the finalists and alternates will now receive seed funding from DOE to develop 1/20th scale models of their wave energy converter (WEC). These models will be tested at the nation’s most advanced wave-making facility, the Naval Surface Warfare Center’s Maneuvering and Seakeeping (MASK) Basin at Carderock, Md., beginning in the summer of 2016.

Official finalist teams are:

Alternate teams are:

“The qualified teams’ efforts resulted in some very promising technologies for the judges to evaluate,” said Wes Scharmen, principal investigator at Ricardo, Inc. and chief judge of the Wave Energy Prize. “Based on our preliminary evaluation, the data indicates that many of the teams identified as finalists have the potential to achieve the ACE threshold, and thus the potential to exceed DOE’s program goal. We’re looking forward to further verifying their designs performance at 1/20th scale in the MASK Basin at Carderock this summer.”

ACE—a benefit-to-cost ratio—was selected by the Wave Energy Prize as a metric appropriate for comparing low Technology Readiness Level WEC concepts when there is not enough data to calculate the levelized cost of energy —itself a cost-to-benefit ratio—from a device. ACE is determined by dividing, in essence, the wave energy extraction efficiency of a WEC by its structural cost. Finalists were determined based on their potential to achieve the doubling of the current state-of-the-art ACE value of 1.5 meters per million dollars (m/$M) to 3 m/$M during 1/20th scale tank testing at the MASK Basin, making them eligible to win the grand prize.

A panel of expert judges evaluated each qualified team based on their revised technical submissions, numerical modeling results, Model Design and Construction Plans, and the results of small-scale tank testing of their 1/50th scale models, and determined aggregate scores to identify the finalist pool.

The Wave Energy Prize is encouraging the development of game-changing WECs that will reduce the cost of wave energy, making it more competitive with traditional energy solutions.

Congratulations to the finalist teams, and thanks to all who have participated in theWave Energy Prize to date!

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.

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.

Details on the Hydrodynamic Performance Quality (HPQ) Metric

The Hydrodynamic Performance Quality (HPQ) of a Wave Energy Converter (WEC) Technology

By Diana Bull, Sandia National Laboratories

The two components that comprise the Average Climate Capture Width per Characteristic Capital Expenditure (ACE) metric are the most important levelized cost of energy (LCOE) drivers for WEC devices, however there are many other influential parameters. Although a scaled wave tank test cannot provide information on all influential parameters (system availability, installation, etc.), it can provide substantial useful information beyond ACE.

ACE requires knowledge of the power absorbed by the device in a West Coast deployment climate and the Characteristic Capital Expenditure needed to build the device. By requiring additional sensors to monitor other aspects of the devices performance, processing the data to obtain alternative views beyond averages, and subjecting the devices to additional wave environments, much more can be learned about a device’s overall performance. In addition to monitoring averaged absorbed power, the devices will be outfitted with sensors that measure mooring forces, accelerations, and the position of the device. This data will be processed to reveal statistically significant peak values, ratios between peaks and means, as well as identifying events like end-stop impacts. Lastly, all of the sensors and processing will occur not only for the irregular wave spectra used to establish average climate capture width (ACCW), but also for two large irregular wave spectra (LIWS) and two realistic wind swell spectra (RWS).

This additional data will be processed into six performance-related quantities for each device tested in the MASK basin. These performance-related quantities are:

  • Statistical peak of mooring watch circle (WCHPQ)
  • Statistical peak of mooring forces (MFHPQ)
  • Statistical peak-to-average ratio of absorbed power (APP2A,HPQ)
  • End-stop impact events (ESHPQ)
  • Absorbed power in realistic seas (RSHPQ)
  • Adaptive control effort (ACHPQ)

These quantities relate to aspects of the techno-economic performance not addressed by ACE and will allow devices to distinguish themselves on more levels then the ACE metric alone provides.

Each of these hydrodynamic performance-related quantities will be allocated to a factor (in the range of 0.94 – 1.06) and the HPQ of a device will be established by multiplying the ACE metric by the factors allocated to each performance-related quantity.

HPQ = ACE * ( MFHPQ * WCHPQ * APP2A,HPQ * ESHPQ * RSHPQ * ACHPQ )

Each of these factors may have limited beneficial, non-beneficial or no influence on the HPQ. The allocation of the factors from the performance-related quantities will be the responsibility of the judging panel.

The HPQ will establish that the winners’ designs will more effectively address key aspects of the techno-economic performance. The HPQ continues to encourage teams towards a systems-level engagement through the end of the competition. At the end, the device with the highest HPQ that has surpassed the ACE threshold will be declared the winner of the Wave Energy Prize.

The Wave Energy Prize Purse and Seed Funding

Wave Energy Prize Cash Purse

Why participate in the Wave Energy Prize? Teams will be competing for more than $2 million in prizes and seed funding.

The prize purses available to the winner(s) of the Wave Energy Prize will be:

  • Grand Prize Winner: Team ranked the highest after testing of the 1/20th scale WEC device model at the Carderock MASK Basin – $1,500,000
  • 2nd Place Finisher: Team ranked second after testing of the 1/20th scale WEC device model at the Carderock MASK Basin – $500,000
  • 3rd Place Finisher: Team ranked third after testing of the 1/20ht scale WEC device model at the Carderock MASK Basin – $250,000

To be eligible to win a monetary prize purse, a team’s 1/20th scale device must achieve a threshold of 3m/$M Average Climate Capture Width per Characteristic Capital Expenditure. The judging panel will rank all teams whose devices achieve the threshold and assess their overall performance using the Hydrodynamic Performance Quality (HPQ), outlined in Section 6 of the Wave Energy Prize Rules.

The Wave Energy Prize will also provide seed funding to the Finalists (up to $125,000) and Alternates (up to $25,000) determined at the end of Technology Gate 2. This seed funding will be provided to the Finalists and Alternates for costs associated with the building of the 1/20th scale model to be tested at the MASK Basin, as well as costs associated with the shipment of the 1/20th scale model and participation in the testing process.