Tank, Flume, and Basin Testing
Free Decay
Purpose: To determine the friction effects (damping) and natural period (frequency) of the marine energy device.
Overview: Free decay tests can be done in multiple DOFs (although it is difficult to get natural frequency in multi-DOF), or a singular DOF of interest. After identifying which DOF(s) is critical to the function of the device, it should be mounted in a way that allows the device to move freely in the DOF chosen to focus on. The body should then be displaced a certain distance or rotated a certain amount of degrees to a still point. Once the still point is reached, the body should be released and allowed to move freely until it reached a state of equilibrium (essentially come to a full stop). This should be repeated at different distances and degrees. During this test, the PTO should not be engaged.
While the body is in motion, position sensors should be recording the body's potion with respect to time to see the natural oscillation period of the marine energy device (the time between each peak as the body oscillates). From this time domain plot, the researcher can get an accurate parameter for decay and get information on damping. This data can improve the accuracy of hydrodynamic coefficients and friction effects
Instrumentation and Measurements: Position sensor measuring the location of the marine energy device as it oscillates to equilibrium.
Quasi-Static Mooring Test
Purpose: Ensure the mooring’s physical properties are correctly modeled in the existing numerical model and for further tank experimentation, such as horizontal stiffness and tension of the mooring lines.
Overview: Quasi-static mooring tests are typically done before conducting dynamic mooring tests. Quasi-static mooring tests occur in a tank flume or basin with still-water. Typically, the buoy or marine energy device attached to the mooring system is pushed or pulled and held using an arbitrary force. The method of enacting the push or pull depends on the testing environment and its capabilities. An actuator can be controlled to apply this force, or simply through a rope tied to the device that is then pulled and held manually. Through this movement of the buoy or marine energy device in still-water conditions, an accurate measurement of the tension in the mooring line(s) as well as the motion of the marine energy device can be documented.
By tracking the buoy’s or marine energy device’s motion and the mooring tension, the horizontal stiffness of the mooring can be extrapolated. From these extrapolated physical parameters, the properties of the mooring within the numerical model can be updated. Doing this ensures the reduction in any unexpected errors since the numerical model is informed with results more representative of the mooring in the real-world.
The draft of the buoy or marine energy device can also be measured once place in the still-water.
Instrumentation and Measurements: 6DOF motion sensor (number of DOFs necessary may vary based on inquired outcome) to measure buoy or marine energy device and a load cell at the fairlead of the mooring line(s) to measure tension.
Dynamic Mooring Test
Purpose: Determine the tension in the mooring line or lines to validate numerical models and ultimately be used to minimize testing costs. This knowledge can also be used to help determine the horizontal excursion and ultimately the watch circle of the device and it’s mooring system once deployed.
Overview: Dynamic mooring tests are typically performed after static tests are done in the tank, flume, or basin. Dynamic mooring tests use regular and irregular (linear to second order) waves to help validate the system’s numerical models. The buoy or marine energy device and its mooring system is placed within the tank, flume, or basin while regular and irregular waves are run. A dot is placed on the mooring line(s) so a mounted motion tracking camera and other devices measure the shape of the mooring line(s) (catenary or straight) to document how the mooring line shape changes based off of where the dot is located in each frame. A load cell is also located at the fairlead of the mooring line(s) and measures the tension in the line throughout the test as well as a 6DOF load cell located on the marine energy device or buoy.
It is good practice to ensure each wave is run for at least two periods, just like in other system identification tests described above. From the buoy or marine energy device motion and shape data, the numerical models can be enhanced. From the enhanced numerical models, the researcher can see how accurate the model predicts the tension in the mooring line(s) under more extreme wave conditions that may not be possible in a tank, flume, or basin setting. This method of gathering data in a test facility and enhancing numerical models ensures the reduction of overall testing costs (labor, materials, manufacturing) since the system’s open-water behavior can be modeled accurately in a virtual environment before actually having to deploy.
Instrumentation and Measurements: Motion tracking camera to track the dot on the mooring line, 6DOF load cell for documenting the buoy or marine energy device motion, and uni-directional load cell to measure the tension in the mooring line(s).
Pull Test
Overview: The first step in characterizing the seakeeping of a MEC is a pull test, which provides a quick estimate of the roll and pitch period. This is an important test because the righting periods (either roll or pitch) can be separately calculated without cross-coupling effects from motions in other degrees of freedom. This test can also provide another rough estimate of the GM.
The pull test should be performed during calm conditions with the MEC connected to its mooring and all moving parts locked down. The MEC should be ballasted to its working state. The basic procedure is as follows:
- Measure the draft of the MEC
- Apply load to the pull line, which should be attached as high as possible, until the MEC is at the desired tilt angle (between 5 and 15 degrees). For ocean testing, the pull line is typically loaded by a winch aboard a moored vessel.
- Rapidly release the load by parting the line; this can be done using a line release mechanism. Care should be taken at this stage to protect crew and equipment from the line whipping back.
- Repeat steps 2 and 3 so that 3 separate pull tests are completed for each axis of rotation (roll and pitch).
If it is possible to pull the MEC from a point that is off the centerline, to cause a rotation in yaw but not significant tilt, then the yaw period can also be measured. For laboratory tests, the MEC can be pushed down and released to measure the heave period.
Instrumentation and Measurements: The MEC should be equipped with a set of instruments to capture the MEC’s 6 DOF motion and their derivatives (e.g. velocity and acceleration) for both body-fixed and inertial reference frames. For laboratory tests, a motion tracking system and a tilt sensor may be sufficient but for open ocean testing, a full inertial navigation system (INS) level of measurement is recommended.
Power Performance
Purpose: The purpose of a power performance test is to determine the MEC’s electrical power production performance as a function of the environmental conditions at a specific test site.
Overview: Early in the development timeline, it is important to quantify the efficiency of a MEC at converting the energy of a resource to power. At small scales in laboratory testing, developing an accurate PTO may not be possible, therefore mechanical power captured by the absorbing element should be measured instead. For open-water testing, the power performance is one of the most important parameters to characterize but can be complicated due to the wide range of capture methods and technologies. However, as a MEC advances toward commercialization, the power performance characteristics (the device power capture curve/matrix) must be accurately estimated to provide credible data to assure investors of the technology’s energy generation potential.
References: The IEC has developed Technical Specifications for wave, tidal, and river energy converters that should be used to guide testing and analysis of the power performance measurements[1][2][3]. Several other power measurement references are provided for additional information on testing procedures[4][5][6][7][8][9][10][11].
WEC Power Performance
Overview: The power performance of a WECs should be determined by simultaneous measurements of the wave climate and the power output from the WEC at the test site, as well as external factors such as wind, current, water depth and mooring line tensions (as applicable).
Prior to testing, an analysis of the test site should be completed to measure the incident wave climate and infer the incident wave power at the location of a WEC from the location of the wave measurement buoy. For this evaluation, a Wave Measurement Instrument (WMI) should be deployed at the proposed WEC test site location and used for correlation with the long-term WMI at the site. The analysis should continue until seasonal variations are captured and a transfer function for all wave height, periods, and directions can be developed.
Based on IEC TS 62600-100[1], WEC power should be reported via a normalized power matrix that is constructed using the “method of bins”, as defined in Section 9. In summary, this method entails collecting WEC power output samples in 20 minute long samples. The data are then binned in a power matrix according to the measured Hm0 and T0 for that same 20 minute sample period. This requires a time synchronization of the power output samples with the WMI data. The bin specification for the power matrix are as follows:
- Significant wave height with a maximum bin width of 0.5 meters
- Energy period with a maximum width of 1.0 seconds
With power measurements divided into a 2-dimensional array of bins, the measurement period for each combination of bins must be long enough to establish a statistically significant database over the desired ranges of significant wave height and energy period. According to IEC 62600-100 TS Ed 1[1], if the WEC is connected to the grid, the grid needs to be monitored for export capacity that might limit the WEC output capacity. IEC compliant WEC power performance data processing codes are available in MHKiT for both Python and Matlab.
Instrumentation and Measurements: The net electric power should be measured at the output terminals of the MEC using a power measurement device (e.g. power transducer), based on measurements of current and voltage on each phase. The power transducers should be placed as close to the generator as possible to limit line losses. If measurements are recorded distant from the generator, the line losses must be accounted for. Sample rates should be a minimum of 2 Hz and the full-scale range of the power measurement device should be set to -50 % to +200 % of MEC rated power. For certification purposes, transducers must meet the requirements of the following standards and should be class 0.5 or better – specific details are given in 62600-100 TS Ed 1[1], IEC 62600-200 TS Ed 1[2], and IEC 62600-300 TS Ed 1[3]
- Power transducers IEC 60688[9]
- Current transformers IEC 61869-2:2012[12]
- Voltage transformers IEC 61869-3:2011[13]
Additionally, MEC control system status should be monitored to determine the state of the WEC and data rejected for anyy of the following reasons:
- WEC is not operating because of a fault or because it is shut down
- Wave climate is outside of WEC operating range
- Wave direction is outside the range of site calibration
Finally, the WEC load (if not grid connected) should be recorded in the case of a variable load bank.
Power Quality
Purpose: Power quality tests are used to characterize the power output from a component of a MEC to ensure that it is compatible with a downstream component or system.
Overview:Characterization of power quality is important for MEC developers aiming to certify their technology for both grid and remote power applications. The most common power quality test is to verify that the power being output from a MEC is compatible with the grid that it will be connected to. Other potential objectives for power quality testing of MECs include the following:
- Assess voltage issues at the shore point of common coupling from the use of power electronic conversion equipment and subsea cables that may induce a phase change between the current and voltage.
- Study output fluctations from devices containing internal energy storage in the power conversion system that may provide power smoothing characteristics.
- A wide range of electrical generation machines from classical induction generators to more novel outer-rotor rim-type direct-drive generators of custom new designs are currently being considered for marine power devices.
- Evaluate potential power quality issues from multiple power electronics converters within single-machine/single-device or multimachine/single-device interactions.
References:The IEC has developed a Technical Specification for the power quality assessment of MECs that should be used to guide testing and analysis[14]. In addition, several other power measurement references are provided that give further information on testing procedures [12][13][15][16][17][18].
Instrumentation and Measurements:The measurement procedures described in IEC TS 626000-30[14] are valid for a single or farm of converters, as well as for any size of MEC. However,the procedure only requires MECs intended for point of common coupling (PCC) at Medium Voltage (MV: 1kV to 35kV) or High Voltage (HV ≥ 35kV) to be tested and characterized as specified. In addition, a simplified measurement and reporting procedure is outlined for MECs connected at Low Voltage (LV ≤ 1kV) networks. MV–connected and LV-connected devices are defined as:
- Medium voltage connected (MV - connected) units - typically multiple three-phase converters operating as a marine power farm and delivering bulk power through anHV or MV network.
- Low voltage connected (LV - connected) units - typically single-phase or three-phase units deployed in isolated, hybrid or micro-grid type systems supplying small-scale loads.
The measurement procedures are designed to be as non-site specific as possible so that power quality characteristics measured at a test site can be considered valid at other sites provided that the same MEC unit configuration and operation modes and controls are the same. Considering the nascent status of the marine energy sector, the following limitations of this technical specification should be recognized:
- Voltage fluctuations under switching operation – the current revision only considers voltage fluctuations under continuous operation.
- Resource classifications – to categorize the measured flicker quantities, various resource classes are suggested only as guidelines. The user is advised to use these resource classes judiciously.
- The measured characteristics are only valid for the specific configuration and operational mode of the assessed MEC. Other configurations and operational modes, including altered control parameters that cause the marine energy converter to behave differently with respect to power quality, require separate assessments.
All measurements for a power quality assessment should be made as outlined in the IEC TS 62600-30 Technical Specification [14]. Sections of IEC TS 62600-30 for specific power quality assessments include:
- Section 6.3.2: Voltage Fluctuations for continuous operation for medium voltage systems
- Section 6.3.3: Voltage Fluctuations for continuous operation for low voltage systems
- Section 6.4: Current harmonics and inter-harmonic and higher frequency components under continuous operation
- Section 6.5: Testing the response of a MEC to voltage drops
- Section 6.6: Covers active power, which requires measuring the following quantities:
- Average power over 600s, 60s and .2s
- A ramp rate power limitation test, if available
- A power output set-point control test from 20 to 100% power set points in 20% increments in a prescribed sequence.
- Section 6.7: A reactive power capability test
The IEC TS 62600-30 also lists specific test equipment requirements for each of the above power quality tests, as well as measurement locations. IEC compliant MEC power quality data processing codes are available in MHKiT for both Python and Matlab.
