Measuring Instruments v4

8/13/2019 Measuring Instruments v4

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Measuring Instruments

Ameya Sathe


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Why know about measuringinstruments in wind energy?

Main fuel for energy is wind Main source of loading is also wind For offshore turbines it is also waves Very important for validation of models Very important for further research and sophistication

of models


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Basic terminology

Sensors Transducers Accuracy and precision Reliability Sampling rate Sensitivity Why learn basic terminology?

Uncertainty in measurements Make some decisions, for e.g. sampling rate

High accuracy,

but low precision

High precision,but low accuracy
http://en.wikipedia.org/wiki/Image:High_precision_Low_accuracy.svghttp://en.wikipedia.org/wiki/Image:High_accuracy_Low_precision.svg


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Types of instruments

Thermometer Strain gauge Wave buoys

SODARCup Anemometry LIDAR


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Cup Anemometer Rotation proportional to wind speed Mechanical counters for registering rotation Electrical or electronic voltage changes Lightweight, small and low friction sensors

* Requires met mast


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Sonic anemometer Wind speed derived from flight times sound pulses

Measures also turbulence, stability Flow distortion due to probe head

* Requires met mast


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Met mast anemometry

Usually 10 min averages recorded Absolutely essential to obtain quality anemometers 10% error in the measurement causes 33% error in

the energy content If this flawed wind speed is to be extrapolated at

higher heights then the error gets magnified Typical cost of a well calibrated anemometer with 1%

error is 700-900 $


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Met mast anemometry Best to fit the anemometer at the top of the mast at

hub height to reduce uncertainty Thin cylindrical towers to minimize tower shadow Data logging is done on electronic chips Artic conditions require a heated anemometer


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Met mast anemometry Corrections for tower shadow Corrections for mast movement Icing/Deterioration/Maintenance Calibration (wind tunnel, other anemometers)


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SODAR Based on Doppler effect

Emission of sound waves and backscatterring (from aerosol) Total distance travelled is measured (indicates position) A shift in the frequency is also measured, which gives the velocity

of the wind


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Doppler effect

Shift in the frequency related to the velocity of aerosol (and thus the wind)

speed aerosol


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SODAR

No tower is required Accuracy quite reasonable Calibration of the instrument is difficult A discrepancy in the temperature measurement can

give an error in the wind speed Problems in the use at offshore sites due to the noise

of waves There is a limit to which reliable wind speed

measurements could be obtained at greater heights


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LIDAR Also based on Doppler effect and measure wind

velocity Emission of laser and backscattering Doppler shift in the frequency Frequency shift is proportional to the wind speed and

the speed of light Vertical resolution of 5-20 m


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LIDAR

Does not require a measuring mast Low backscatter at extreme air clarity Difficulties in rainy conditions for measuring vertical

wind speed


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Comparison of the cup anemometer,SODAR and LIDAR measurements


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ADCP Acoustic Doppler Current Profiler

Measures water currents Mounted on seabed; 4 beams Doppler shift (sample rate 1-2 Hz) Currents (3D) + waves

Hs, Tz, direction, sea level,wave spectrum,


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ADCP

No long string current meters are required anymore Measures absolute speed of water Measures a water column upto 1000m long Runs out of batteries rapidly If water is clear then reliable data may not be obtained Bubbles in turbulent water can cause the instrument to

miscalculate the current

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Wave Buoys Measure 3D motion of water particles Contains an onboard accelerometer to measure

vertical acceleration Corrections are made to take into account the

horizontal motion of the buoys

May require manual data gathering or remote datagathering


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Strain gauge Used in load measurement Measurement of resistancechanges Resistance is proportional toarea Resistance is related to strain bya quantity known as gauge factor Installed on the blades or thesupport structure


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Strain gauge

Many types like foil, mechanical etc. A very small signal is produced and hence signal

conditioning is required It is important to keep the stretching within the elastic

limit of the material A Wheatstone or Kelvin bridge circuit is used

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Metbuoy OWEZ I

4 year, 2 kilometres south west ofmast site 10 m.a.s.l., stabilized 2 anemometers 1 wind vane, air temperature, seatemperature

ADCP


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Examples of measured variables

Wind speedTemperature


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Examples of empirical relations

subjected to various values based on measurements

Stable conditions

k = 0.35-0.4 von Karman constant


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RECAPThermal effects: similarity theory

Recall neutral case: dU/dz = u * /(k z)Non-neutral case: extra nondimensional parameter:=z/L m

Dimensional analysis: windshear should be similar forscaled flows (Monin-Obukhov)

k z/u * dU/dz = (z/Lm)for z/L m 0, 1 (neutral)

Integration leads to expressions for U(z)(similar expressions for temperature T)


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Example of measurement uncertainty


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Example of measurement uncertainty


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Experience gained from analyzing themeasurements

Obtain the uncertainties for all the variables under scrutiny Different measurement campaigns have different format of data

logging. First obtain the format with which you are mostcomfortable.

Carry out a sensitivity analysis on the variable which has asignificant effect on the analysis

Any analysis is based on theoretical assumptions. Hence filter thevalues which do not conform to the assumptions in the theory

Do not trust the previous research blindly and verify for yourselfthe applicability of the results from previous research


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Summary

Measurements are indisputable to any branch ofscience

For wind energy, wind speed, loads and wave

measurements are crucial Cup anemometers -> wind speed Wave buoys -> wave measurements Strain gauges -> Load measurements www.noordzeewind.nl gives the data at Egmond. You

can download and analyze the data

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http://www.noordzeewind.nl/http://www.noordzeewind.nl/


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Thank you

Measuring Instruments v4

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