Testing the power performance of marine energy devices

Oceans offer a massive source of energy; the power harnessed from waves, tides, currents and ocean temperature differentials can be used to power homes, transport and industry.

Current profiler at EMEC (Photo: EMEC)

Within this past decade, the cumulative energy produced from wave and tidal stream energy alone has increased from less than 5 GWh in 2009 to approximately 45 GWh in 2019, according to the Ocean Energy Systems (OES) report.

This energy can be captured using different technologies, such as wave energy converters (WECs) and tidal energy converters (TECs). Marine energy devices and systems can be complex. Manufacturers and developers must ensure that their products function safely and reliably and can withstand the environments in which they operate. This can be achieved by running different tests to ensure that international standards have been followed throughout the marine energy equipment’s development.

IECRE, the IEC System for Certification to Standards Relating to Equipment for Use in Renewable Energy Applications, offers testing and certification for the marine, solar photovoltaic and wind energy sectors.

e-tech caught up with Elaine Buck who manages the technical team at the European Marine Energy Centre (EMEC), recently designated as the first IECRE renewable energy testing laboratory (RETL) for the marine sector, to find out how power performance assessments for electricity producing tidal energy converters are carried out.

Buck is also Co-convenor of the IECRE working group for the recognition of marine renewable energy certification bodies (RECBs) and RETLs, which is responsible for rules and procedures for peer assessment and assessment of RETLs and renewable energy certification bodies (RECBs).

What is a power performance assessment?

A power performance assessment (PPA) aims to provide an agreed methodology for the measurement of the power output of the tidal energy convertors in a range of sea states and an agreed framework for the reporting of the results of these measurements. It enables the estimation of the energy production of a wave or tidal energy conversion system at a prospective site, where wave or tidal power resource information of sufficient detail and quality exists.

EMEC is accredited to ISO/IEC 17025, which allows the Centre to provide independently verified performance assessments. It is also ISO/IEC 17020 accredited and as such, offers independent technology inspection on marine energy converters and their sub-systems.

When developers are ready to test their devices in water, we provide an accredited test report. This will be used in the final conformity statement, which together with a series of other conformity statements, will provide a certificate to that developer that their complete device has been certified.

What are the key phases in a power performance test?

There are four phases.

Initiation phase

The initiation phase aims to gather as much information as possible from the test site and developer to put an agreed proposed scope of work together, after reviewing the site and developer information a recommendations report is produced.

During this stage, we outline the requirements for testing which we discuss with the developer and/or test site. If testing is at a remote site (not at EMEC) we speak with both parties. We create the test device information form, which is filled out by the test site and developer.

We then evaluate the form against IEC Technical Specification 62600-200 for tidal or -100 for wave.

Planning phase

A test plan will be produced in this phase, which outlines the details of conducting a pre-survey of the test site, including the data acquisition for the test period of the performance test.

The presurvey evaluates whether the site has existing metocean data, such as wave resources, tidal currents, symmetry data or any other metocean surveys already done at the site, based on the TS.

If there isn’t any data close enough to where their device will be installed, we plan a new data collection endeavour with the developer. Depending on the type of marine device, we must ensure the deployment of the acoustic doppler current profiler (ADCP), used to collect current data, velocities through the water column. If the test is for wave power performance, a wave measurement instrument (WMI), which measures the significant wave height, and energy period for example will be deployed. This measurement is taken to establish the wave characterization energetically at a particular site and used to estimate the power from a device.

ISO/IEC 17025 requires documenting that the instruments used in PPAs have been calibrated, in accordance with national or international standards of measurements, which we check.

We also make sure that electrical and metocean instruments have been synchronized.

We define where the instrumentation (ADCPs or WMIs) in relation to the device would be deployed according to the TS.

Acquisition phase

Metocean data collection is vitally important for a PPA. The acquisition period covers the schedule of installation, how to deploy the device and instruments, planned generation period, how to do the data analysis and the report and is clearly outlined in the test plan, to ensure a quick turnaround of the final report.

Developers must provide a test log, which gives the operational status of the device, such as when it shuts down because of weather or for maintenance, and a log of those signal data types. Equally, if it is operating, it must be clear what the data flow through a supervisory control and data acquisition (SCADA) system will look like.

During this phase, the developer must follow IEC TS 62600- 200/-100 requirements closely, or if unable to adhere to the TS a deviation will be noted. In this case, a deviations table is created with an adequate description of the deviation and the impact it has on the power performance.

Once the preplanning and presurvey are complete, testing begins. The tidal device minimum testing period is three months and wave, six.

During test acquisition we witness the developer’s test methods, follow the test schedule, start the data quality analysis and control procedures to ultimately gather enough data for generating a tidal power curve or wave power matrix.

EMEC has developed several internal tools used to carry out the metocean data quality control of evaluating and flagging any anomalies in the data sets. The metocean data together with the electrical power data is put into our tidal or wave power performance tool to calculate the wave energy flux for each of the sea states, average it and integrate it with the electrical power data within the sea state. This enables the calculation of a capture length for each sea state for wave energy and power factor for tidal energy. Ultimately, the developers and investors have the power matrix for a wave device and the power curve for a tidal one.

Reporting phase

During this phase EMEC finalizes the test report, data analysis and results, including deviations table against the TS. An accredited certificate is also issued with the report.

How long does the testing take?

In a perfect world if the site is well characterized, and there are no presurvey requirements, the developer has installed and optimized the device and is ready to start the test, without needing to modify the design, or change the control algorithms, a tidal project would take four months; three months for data acquisition and a month for planning and reporting.

A wave project would be about eight months because the test period is longer. Depending on the site and required seasonality capture of all sea states a wave project could be longer.   

What are the projects you have been working on?

EMEC is completing the work done within MET CERTIFIED, where we delivered test reports for tidal energy developers. We are engaged in a number of wider Interreg funded programmes such as Encore and TIGER preparing for power performance assessments. The marine sector especially tidal is posed to install and prove performance over the next four to five years, so activity as a RETL is increasing.

Find out more about IECRE and the services provided by EMEC.