Impact of wind farms on low-frequency radio astronomy observations with LOFAR.

Mark Bentum, Wim van Cappellen, Michiel Brentjens ASTRON

Impact of wind farms on

low-frequency radio astronomy

observations with LOFAR

Content

! Context

! Radio astronomy

! The LOFAR radio telescope ! Wind farms and LOFAR

! Expected problems ! Measurements ! Direct measurements ! LOFAR ! Mitigation ? ! Conclusions

The 5 biggest questions about the Universe

! What is dark matter?

! What is dark energy?

! What came before the big bang?

! What’s inside a black hole?

! Are we alone?

3

The answer: observe the universe

!"##"$%"& '$%"& ()*%"+,-).* /,0,1).

234%"%.5 6,7%-8"+. 6",39:"5,- ;-89<%.=>.37&

!

Celestial sky: has been mapped for nearly every

New revival at this moment - AAs

5

The Low Frequency Array

! Low Frequency Array

! 10 - 90 MHz (LBA) ! 110 - 250 MHz (HBA)

! Arranged in stations

! Station connected with

fibres

! Station correlation

! Central correlator

LOFAR locations

LOFAR core and environment

! Large receiving surface ! Short baselines ⇒ Susceptible to interference ⇒ Radio quiet zone coordination zone of ~ 6 km Wind turbines!

Wind farm near LOFAR

9

Expected problems

! Presence of a windturbine can

change the RFI environment ! RFI increase

! Time variance of RFI ! Additional RFI

! Degradation mechanisms

! Direct RFI ! Scattering ! Diffraction

Time variability

11

! The RCS of an object is described as the ratio between the

incident power density at the object and the scattered power

density from the object at a receiving antenna. Expressed in m2

or dBm2 _{(~ 50 dBm}2 _{at 100 MHz)}

! RCS depends on size, shape, material, smootness

! The shape of a wind turbine is time-varying

LOFAR observations

13

Levels

15

! Astronomical sources measured in Jansky’s

! 1 Jy = 10-26 W/m2/Hz

! Brightest sources : 10.000 Jy

! Most sources: mJy and !Jy (so, dynamic range 100 dB …)

Harmful interference levels for RA

! Interference levels are considered to be harmful to the Radio Astronomy Service when the rms fluctuations of the system noise at the receiver output increase by 10% or more due to the presence of interference (after integration).

! ITU-R RA.769-2:

! Contiuum observations: ! Spectral observations

with f the frequency in MHz and S the maximum acceptable power flux density in dBWm-2_Hz-1

4850 | 150 | 18 259₊ − − = f S 4500 | 327 | 14 244₊ − − = f S

Measurements

! Hondtocht windpark,

! First measurements on windparks ! Emission

! Reflection

! Measurements at the WSRT

! Measurements at LOFAR station RS509

17

Hondtocht wind park

! Annual production: 24,671,000 kWh (7,260 households)

! Local authority: Dronten

! Wind turbine: Vestas - V66 - 1,75 MW

! Number of wind turbines: 8

! Installed capacity: 14,75 MW

! Rotor axis height: 67 metres

Mobile RFI monitoring station

! Antennas

! R&S HE010 :

! Active antenna ! 9 kHz – 80 MHz

! Single vertical polarization

! Schwarzbeck Vulp9118G:

! Single polarization log-periodic antenna ! 35-1500 MHz

! Receiver

! R&S ESMB monitor receiver

! Storage on PC

! Calibrated offline

19

Emission inside the windturbine (2)

21

Scattering

! FM-signal (89.8 MHz)

! Bandwidth is 1 MHz

! Period is 4 seconds, which is exactly the rotation speed of

the windmill.

! The signal strength changes about 4 dB.

23

Omroep Flevoland transmitter

! 22.5 km

! Relative

Measurements at the WSRT

! Purpose: Check propagation models

! ITU – recommendation P1546-4 ! Okumura-Hata

! Check RCS model by using

the WSRT telescope

! Check scattering and diffraction using

a WSRT telescope

25

Results

! Transmitter at Smilde with known parameters

! Calculated loss:

! Okumura and ITU ! 110 dB ! Measured ! 114 dB

! Results:

! Propagation models fit very well ! RCS changes are seen

Measurements at RS-509

! Measured at LOFAR station RS-509 and at Windmolenpark

Eemshaven.

27

Specifications

! LOFAR station RS509

! Oosteinde station

! Distance to Windmolenpark = 4.0 kilometer

! Measured the energy in “Radio Noord” @ 97.5 MHz ; location Hoogezand.

! Distance to Hoogezand 29.2 kilometer

! Windmolenpark

! Close to windturbine #49

! Distance to windturbine = 440 meter

! Distance to Radio Noord = 32,0 kilometer

29

Results

! LOFAR RS509

! 100 kHz bandwidth

! Antenna towards Hoogezand (south) : -46.5 dBm with fading of about 0.5 dB in power

! Antenna to the East : -51.5 dBm with fading of 0.5 dB in power ! Antenna to the West: -55.0 dBm with fading of 0.5 dB in power ! Antenna towards windmill park : -63.5 dBm with fading of about

Results

! Windmolenpark Eemshaven

! Towards Hoogezand: -52 dBm

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The numbers @ RS-509

“0

dB

”

The numbers @ windfarm

32 km 32 km

“0

dB

”

Impact on LOFAR observations

35

LOFAR imaging of wind turbines

! 3 large turbines at ˜3.2 km

Spectral maps

37

LOFAR impact

! Reduced sensitivity on the short baselines. So, no

observations of large structures ..like the EoR. Observations in danger.

! Slow pulsar will be impacted due to the scattering on the

rotor blades.

Compared to standard industrial norm

39

EN55015 norm

Covenant

! Agentschap Telecom establishes a method to measure the

improvement in cooperation with ASTRON and the wind farm developers

Flux density as function of distance

41

Mitigation possibilities

! Nulling the wind turbines in the aperture array

! Possible in the case of > 35 dB attenuation wrt EN55011 ! Becomes more difficult in case of wind farms

Conclusions

! Wind turbines influence the RFI pattern

! Direct RFI due to electronics is potential source of emission. Scattering and diffraction are measured. AM modulated signals are measured.

! There is substantial impact on the LOFAR observation.

Attenuation of the direct emission is needed.

! Small baselines (! large structures in space) will be a

problem

! Slow varying pulsars are difficult to distinguish from impact

wind farms

! Internal Project: Windfarm Mitigation Measures