Light Detection And Ranging Wind Measurement Method Engineering Essay

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This report intends to summarize the Light Detection and Ranging (LIDAR) wind measurement method as it relates to wind resource assessments for the installation of wind turbines. To do so, the basic construction features of the technology will be discussed, followed by some technical data from existing LIDAR devices. In addition, a description of the measurement procedure according to the Measuring Network of Wind Energy Institutes (MEASNET) will be provided. Lastly, this report attempts to analyze the advantages and disadvantage of the LIDAR method.

II. Applications

In many cases, LIDAR is used as a short-term data collection method in conjunction with anemometer measurements. In other cases, LIDAR can be used for initial site investigation, in the absence of any meteorological towers. More recently, the technology has been used for offshore wind resource assessments and wind turbine control systems, wherein information regarding rapid changes in wind speed and direction can be used to optimize turbine operation.

III. Construction and Function

LIDAR works on the principle of Doppler Shift. From the LIDAR apparatus, a laser pulse is sent into the atmosphere and the Doppler Shift of this radiation, which is scattered by natural aerosols carried by the wind (such as dust, pollen, water molecules, pollution, or salt crystals) is measured.

The following is a brief summary of the basic components of a LIDAR system:

Laser: The active component in the LIDAR system is the laser itself. Laser pulses are emitted into the atmosphere at the rate of 4,000 to 100,000 pulses per second. The resulting beam forms a conical measuring area.

Laser Detector: A telescopic device, which is mounted in conjunction with the laser, detects the light that is reflected back to the device. The telescope reflects the received light beams towards an optical fiber, which provides all the necessary input data for creating a 3D - profile of the wind (speed, direction, turbulence).

Scanning Mechanism: There are several variations of scanning mechanisms. The most common is the oscillating mirror.

A simplified diagram is depicted below:

Figure 1: The laser is scanned around the scene being digitized, in one or two dimensions (middle), gathering distance measurements at specified angle intervals.

Figure: Photo of a Lidar scanner

Timing Electronics: Each laser pulse emitted from the device can reflect up to five return pulses. Each of these return pulses needs to be precisely timed to make accurate readings.

Computer: Given the large amount of data collected by a LIDAR apparatus, significant computing power is required to record and process the data into three dimensional coordinates. The computer takes data collected through the optical sensor and creates plots and graphs which are then used to assess the wind profile.

Figure 6: Schematic representation of a Doppler Lidar system main components.

IV. Examples and Specifications of LIDAR Devices

In this section, the technical specifications of three LIDAR devices are summarized. These devices are manufactured by WINDCUBE and LEOSHERE are used in the wind power industry for wind resource assessments.

WINDCUBE®v2

Range

40 m to 200m

Data Sampling rate

1s

Number of programmable heights

10

Speed accuracy

0.1m/s

Speed range

0 to 60 m/s

Operating Temperature Range

-30°C to 45°C

Transportation

Size

System: 543x552x540 mm

Transparent Case: 685x745x685 mm

Weight

System:45 KG

Data Storage and Transfer

Data Format

ASCII

Data Storage

SSD and compact flash ( backup storage)

Data transfer

LAN/USB

Out data

1s/ 10min Horizontal &vertical wind speed

min &max. direction, SNR

Quality factor ( data availability)

GPS coordinate

Power

Power supply

18-32V DC / 100 to 230V AC 50-60 Hz

Power Consumption

45W

Data derived from: http://www.lidarwindtechnologies.com/pdf/windcube.pdf

Doppler Wind LIDAR System: WINDCUBE®v2

Cloud & Aerosol Micro LIDAR System: ALS 300 LIDAR LEOSPHERE

ALS300

Range

.15 km to 12km

Data Sampling rate

30s

Vertical resolution

1.5/15m

Laser Type

Nd-Yag solid state

Emitted wavelength

355nm

Operating Temperature Range

-15°C to 40°C

Transportation

Size

Optical Head: 650x356x190 mm

Electronics : 480x500x300 mm

Weight

System:36 kg

Data Storage and Transfer

Data Format

ASCII,binary,HDF

Data transfer

Ethernet

Power

Power supply

100 to 240V AC 50-60 Hz

Power Consumption

750W

Data derived from: http://www.leosphere.com/pdf/BrochureDef_BD.pdf

Cloud & Aerosol Micro LIDAR System: ALS 450 LIDAR LEOSPHERE

ALS450

Range

400 m to 20km

Data Sampling rate

30s

Vertical resolution

1.5/15m

Laser Type

Nd-Yag solid state

Emitted wavelength

355nm

Operating Temperature Range

-15°C to 40°C

Transportation

Size

Optical Head: 650x356x190 mm

Electronics : 480x500x300 mm

Weight

System:36 kg

Data Storage and Transfer

Data Format

ASCII,binary,HDF

Data transfer

Ethernet

Power

Power supply

100 to 240V AC 50-60 Hz

Power Consumption

750W

Data derived from: http://www.leosphere.com/pdf/BrochureDef_BD.pdf

V. Measurement Procedure

This section is intended to summarize the aspects of the MEASNET procedure "Evaluation of site-specific wind conditions" which pertains to the use of the LIDAR measurement techique.

MEASNET is a network created by different measurement institutes in order to harmonize measurement procedures for wind energy related measurements. Each member conforms to the agreed upon "MEASNET procedures", which specify the quality and procedure of measurements. Member institutes also evaluate one another. In particular, "Evaluation of site-specific wind conditions" is the most complete and internationally accepted MEASNET procedure in terms of quality, traceability and comparability.

Site assessment of wind conditions consists of measurements (performed on site and documented), processing, and interpretation of meteorological data. All phases should comply with reference documents (e.g. IEC 61400-12-1 Wind turbines - Part 12-1: Power performance measurements of electricity producing wind turbines, 1st Ed., 2005 and others).

Deviation from the guidelines can arise when not all requirements on input data or measurement methods are met: such deviations must be identified and described in the assessment report.

The MEASNET "Evaluation of site-specific wind conditions" in Annex C addresses remote sensing techniques, including LIDAR, as an Alternative Wind Measurement Procedure to be used in addition to classical procedures and defines cup anemometers and wind vanes mounted on met masts as state of the art techniques to measure wind speed and direction (also according to IEC 61400-12-1).

The LIDAR procedure requires the site to be visited in order to choose the best location for positioning the equipment. Any obstacle or moving object that can influence the measurements must be documented and avoided if necessary.

The following is a list of considerations for the optimal operation of LIDAR measurement equipment, as specified by MEASNET:

The LIDAR's north direction should be the same as the actual North and it must be erected in a upright position, which must be checked using a bubble gauge or similar device.

Means for keeping the wash and wipe system working properly should be provided and freezing should be avoided (anti-freeze).

All operating parameters of the LIDAR shall be reported (e.g. measurements heights, speed of rotation of laser beam on cone, cone angle etc.)

If the LIDAR is operated in different modes (e.g. different heights, cloud correction On/Off) start and ending points of the modes must be documented.

The accuracy of LIDAR should be evaluated by a comparison to cup anemometer and vane measurements before each application. Specifications for the calibration and comparison of measurements are given in Annex C.

For assurance of quality standards data shall be filtered and all data reductions or filtering have to be documented and reported.

Measurement failure shall be reported and should be divided at least in three componenets: System failures, Arrangement and Installation failures, Site depending influences.

The measurement period should not be smaller than three months and season and time period must be considered and documented.

Regular service activities and on-site system checks are required to ensure proper functioning of equipment.

The accuracy of LIDAR measurements may be affected during times of heavy precipitation, heavy mist, or if there are moving parts in the laser cone's view angle.

VI. Advantages & Disadvantages

Advantages

Measurement height: Lidar can measure wind speed and direction up to 200 metres from the ground. Within this area, multiple heights can be programmed for measurement so that the wind profile along the entire swept area can be measured. In areas of complex terrain, multiple potential hub heights can be evaluated to determine the most favourable (however, great care must be taken in the interpretation of Lidar data in complex terrain).

Simple installation/portability: The technological components are small and can quickly and easily be deployed and transported to different areas. Additionally, since there is no meteorological mast involved, Lidar instrumentation can be set up without a permit.

No moving parts: Since Lidar has no moving parts, it is less susceptible to mechanical failure. In addition, it is not limited by the mechanical components and can therefore result in greater accuracy of measurements compared to mechanical anemometers, which can be influenced by the inertia of the physical components (i.e. the cups will speed up faster than they will slow down and thus may not be able to reflect turbulence and gusts with as much accuracy).

Measurements unaffected by meteorological tower: Lidar devices can be placed directly under a meteorological tower and measurements will not be affected by its presence. This contrasts with Sodar devices, which must be placed at least one tower height away. As a result, the accuracy of data correlation with that being collected by anemometers on the tower can be achieved with greater confidence. However, it should be noted that there are limitations to the amount of stationary objects that can be in the laser cone. If there are too many objects, the accuracy of measurements may be affected.

Simple propagation of light waves: Since the speed of light is much faster than the speed of sound, sound ray propagation in the atmosphere is much more complicated than that of light. As a result, there are inherent advantages to using Lidar compared to Sodar in terms of achieving accurate measurements.

Disadvantages

Expensive to purchase

High energy requirement: Lidar uses large amounts of power compared to other measurement methods. This means that equipment needs to be checked on a regular basis to ensure a constant power supply and also implies that it is more costly to run.

Not considered acceptable by banks as sole measurement source: In order to obtain project financing, wind energy developers are not able to use Lidar as the sole data source. Instead, Lidar measurements will need to be used in conjunction with other sources, such as anemometers on meteorological masts.

Requires regular on-site check-ups and maintenance: The following items require constant attention:

Power supply

Wash and wipe fluid and condition of wiper

Unchanged vertical position and North facing orientation

Theft

Underdeveloped technology: Lidar is still a relatively young technology that is underdeveloped compared to Sodar and anemometry. As a result, the merits and limitations of the technology are not fully documented or known and there is still some uncertainty with data reliability.

Interference of moving parts: Any moving parts in the range of measurement need to be avoided, including rotor blades and tree branches. The presence of such moving objects in the laser cone will disrupt measurements.

Adversely affected by precipitation: During times of heavy rainfall, snow, or fog (or low clouds) the instrumentation may become inaccurate.

Limitations in complex terrain: Lidar measurements are highly accurate in simple terrain, generally resulting in errors in the mean wind speed of only a few percent. However, when considering more complex terrain, Lidar measurement have been shown to have errors of 10%. This is due to the fact that the measurements assume the flow over a conical area to be homogenous in order to deduce the horizontal wind speed. In areas with complex terrain, the flow may not be homogenous and thus lead to greater error.

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