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 (WC_HPQ)
• Statistical peak of mooring forces (MF_HPQ)
• Statistical peak-to-average ratio of absorbed power (AP_P2A,_HPQ)
• End-stop impact events (ES_HPQ)
• Absorbed power in realistic seas (RS_HPQ)
• Adaptive control effort (AC_HPQ)

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 * ( MF_HPQ * WC_HPQ * AP_P2A,_HPQ * ES_HPQ * RS_HPQ * AC_HPQ )

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.

What is the Average Climate Capture Width per Characteristic Capital Expenditure (ACE) metric and how will it be used in the Wave Energy Prize?

By Philip Michael, Ricardo Inc.

What is ACE?

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 wave energy converter (WEC) technologies that are at low TRLs.

The Wave Energy Prize needs a more fundamental metric that expresses the effort to benefit ratio using data generated in a wave tank testing program, and derived from technical analyses of designs.

While some existing metrics that have been developed by the wave energy research community express an effort to benefit ratio that can be determined at low TRLs, these existing metrics were derived using a body of research and experience on traditional approaches to exploiting wave power. Thus, they may not be wholly valid for the kinds of novel designs and materials that we can foresee as contestants in the Wave Energy Prize.

With support from Sandia National Laboratories and the National Renewable Energy Laboratory, the Prize team derived a new metric that is a modification of existing metrics. Importantly, this metric allows for robust analysis of innovative WEC devices using novel materials. The ACE metric 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.

ACCW and CCE—the most important LCOE drivers for WEC devices—are calculated values derived from measurements in the wave tank at 1/20^th scale (scaled up to full scale), and from structural and cost analyses of full scale drawings.

Other parameters that are ultimately influential in determining LCOE of a WEC are accounted for in the Prize through the Technology Performance Level (TPL) Assessment of the Technical Submission and in the Hydrodynamic Performance Quality (more details coming in the next newsletter).

How will ACE be used in the Wave Energy Prize?

The goal of the Wave Energy Prize is to stimulate the development of innovative WEC concepts that have the prospect for becoming commercially competitive with other forms of electricity generation.

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. Thus, only WECs that achieve this threshold value for ACE following 1/20^th wave tank testing at Carderock’s MASK Basin will be considered as candidates for prizes. Prize winners from among these candidates will then be decided using the Hydrodynamic Performance Quality metric.