Assessing Wind Potentials

Investigation of available wind velocity data for the proposed site

As a first step in assessment of wind potentials available data from conducted measurements or other sources has to be gathered. In case an open access atlas for regional or national wind velocity has been prepared, it probably is available at the following websites:

• winddata.com: [[4]]
• UNEP Solar and Wind Energy Resource Assessment (SWERA): [[5]]

Other possibilities for wind data enquiry:

• airports
• Meteorological institutes
• Universities dedicated to wind energy research

Paul Gipe explains in his Guide to Small and Micro Wind Systems, that in case of micro wind systems, it is often better to install a small wind turbine and to monitor the output during the first year, than to conduct an own measurement: If the wind turbine output does not suitable values, the turbine could be resold easily as there is a large market for used small and micro wind turbines. Nevertheless as project size, related investments and – according to this – the responsibility of the project developer, increases, wind measurement becomes a central component of wind project development.

Site selection and wind measurement in the height of 10 m

The following characteristics have to be considered for inital site selection and placement of the measurement masts:

• Indications for suitable wind conditions (As a guiding value an average wind velocity of 6 m/s in the height of 10 m is generally considered suitable). Besides the available information about wind velocities, statements of the local population or landscape characteristics (inclined trees caused by the prevailing wind direction) could be used as indicators.
• Proximity to the electricity grid, especially to suitable connections points (transformer stations for larger projects)
  
• A sufficient transport-infrastructure for transport of the large and heavy parts of wind turbines to the site (Curve radii of roads, inclination of roads, carrying capacity of bridges, width and height of bridges to be passed through etc.)
• Proximity to cities and their infrastructure to locate a center for maintainance and service of the wind park
• Conflicting interests of site utilization: Hill sides providing good wind conditions are usually prefered placements for telecommunication masts as well. Telecommunication processes can be disturbed by the installation of wind turbines. Airports, radar-stations and areas of environmental protection could be constraining factors, too. 

In case only scarse information about wind conditions at the site is available, wind speed measurements should be conducted at 10 m height as a pre-study. In this way costs for measurements are kept low, since the suitability of the site still has a speculative character. Wind speed is measured by anemometers, while information about wind directions is gathered by a vane. It is of essential importance to calibrate the anemometers before their application. For large wind projects calibration is often done by accredited institutes. Calibration has to be conducted according to international guidelines like the ISO 3966, IEA-Guidelines for calibration or the standardized methods of the MEASNET-Group. 

For the storage of the measured wind data a data-logger is necessary. This device is installed in a case at the lower part of the measurement mast. The data-logger is supplied with electricity by a battery or a PV-module combined with a small battery storage. Supply of the data-logger has to be very reliable, because loss of data will result, if the data-logger is out of electricity. To prevent thievery of the PV-Module it should be fixed at a higher point of the measurement mast.

Collected wind data should be read out regularly: The most comfortable read-out-method is realized by installing a mobile communication device to read out data from afar. Remote connection to the data-logger also allows observation of the measurement devices. However it is necessary to visit the site in periodically to inspect the mast and the measurement devices on-site.

Costs of all described tools for wind measurement are listet in the following. The given prices are examples from two manufacturers of anemometers, vanes and data loggers, Ammonit and Thies. Good experiences have been gathered with measurement devices of these providers. 

• Anemometer 550 €
• Calibration of one Anemometer 300 €
• Vane 450 €
• Data-logger 1.200 €
• Additional equipment (cabel, etc.) 500 €

The measurement masts have to be equipped with appropriate consoles to fix measurement tools. A lightning conductor as well as anchored guy cables are standard parts for the erection of a measurement mast. For site evaluation in developing countries the measurement devices often have to be imported, while the mast for measurement could be acquired or constructed in the country. For example project developers used telephone masts of 10 m height to conduct peliminary wind measurement in Ethiopia.

A minimal measurement period of one year is needed to generate a complete survey of the local wind conditions. Seasonal variations can be captured only by such relatively long-term measurements.

Based on the analysis of wind conditions in the height of 10 m, a first reliable estimation of the economical feasibility of the wind   park at the proposed site can be worked out.

An average wind velocity of 6 m/s at a height of 10 m is generally considered as a guiding value for a suitable wind project. The necessary wind conditions for a specific wind project can deviate from this: If the costs of alternative electricity sources is comparatively high, a wind project could be economically viable at average wind speeds below 6 m/s. A wind regime with an average wind speed higher than 6 m/s can not be considered as a guaranty for economical feasibility as well:

The returns of a wind project as well as financing conditions are heavily influenced by national, regional and even local political framework conditions. Investigation and monitoring of the political framework conditions is a central task to prepare reliable statements about economical feasibility of the project.

Including these informations and taking into account the analysis of the collected wind data at the height of 10 m the developers may decide, whether the project should be continued or not. If the measurement has revealed a suitable outcome the next measurement campaign has to be planned. In any other case the project will be ceased at this point. Speaking generally the outcome of measurement at 10 m height is a conceptional 'rated break point' applied in every large wind project.

Specification of measurement devices and calibrated sensors

The necessary devices include

• Wind vane (accuracy ≤ 5°, definition ≤ 5°)
• Anemometer (Impuls-output with a minimal definition of 1 Puls per 0.1m wind-track, measurement characteristics in inclined flow and accuracy have to be chosen accordingly the IEA-guidelines)
• Data-logger for capturing time series (Storage of 10-Minute values of: the average value of wind velocity, the standard deviation of wind velocity as well as minimum and maximum velocities during the interval; sampling rate should be for minimum 0,5Hz) Data transfer via storage card or chip
• Battery or combined PV-Panel-storage system
• additional equipment like cables
• Notebook Computer
  

Installation of measurement masts and equipment

• Transport of masts for 10m-measurement to the site; planning of the steps for installation of the equipment.
• Positioning of measurement masts; adjustment of anemometer to avoid inclined flow
  
• Inspection of the installation of the measurement masts, including anemometer and vane adjustment as well as functioning of the data logger and reliability of its electricity supply. Safety risks and potential sources for failures have to be identified and removed.
  
• Compiance with national and international safety regulations for all persons involved in the installation process.
• Testing of operability and test-measurements for commissioning
  
• Documentation of installation of measurement system by a protocol for approval
• Determination of geographic positions of measurement masts via GPS.
• Description of the direct surrounding of the measurement masts concerning potential obstacles for wind flow and estimation of roughness parameters.
  

Measurement and periodical data analysis to assess wind energy potential

Wind measurement contains the following tasks: 

• Read-out of wind data after periods of 1, 3, 6, 9 und 12 Months
  
• Checking data for completeness or failures
• Data analysis includes consistency, plausibility and identification of missing values. In case the measurement reveals significant shortcomings, possible methods to enhance data quality have to be considered.
• Wind measurement should result in a time series, which can be easily processed with the planning project WA^SP. According to this an appropriate data format has to be chosen.

Rating of the wind data concerning the quality of wind potential at the site

The developer works out a report about wind energy potential, based on the first wind measurement at the height of 10 m. This report contains information concerning the following aspects:

• Analysis and rating of data quality and its impacts of the outcome of the measurement;
• Annual average value, Histogram, seasonal variations in the monthly averages of wind speed, typical variations of wind speed during a day
  
• Distribution of wind potentials sorted by wind directions (wind rose);
• Weibull-parameters sorted by wind-directions;
• roughness-parameter and obstacles in the surrounding sorted by wind direction or visualized in a map;
• Degree of turbulence wind speed maximum at the site

The report recommends whether the project development should continued. Additionally recommendations for additional measurements at a height of 40 m is given.

Wind measurement in major heights

As wind velocity normally increases with height the wind 10 m - measurement allows only estimation of wind velocity at hub height. In case the report of the first measurement has recommended to continue project development, a second measurement at major heights is implemented. 

Measurement at hub height would reveal the most reliable outcome. It is technical feasible to install measurement masts with heights of 100 m, but the related costs of measurement are very high. It is generally considered sufficient to conduct additional measurements at the height of 40 m. The gathered data of this measurement may be used for extrapolation of wind speeds at hub height with an appropriate reliability. The measurements at the height of 10 and 40 m have to be conducted at the same placements, because the increase of wind speed with height depends on the specific placement of the mast: within flat ground conditions without major obstacles the wind speed increase with height is less than at a placement with high roughness values. The difference in wind velocity at the heights of 10 and 40 m can amount 2 m/s at several sites. On the other hand a lower wind speed is measured at 40 m than at 10 m. Thus applying additional measurements at 40 m height is fundamental for generating reliable wind data. 

Measurement masts of 40 m height cost approximately 10000 €. Lattice towers have a better stability than masts consisting of tubes, but their price is significantly higher.

Measurement is conducted for minimum for one year again. As it is known, that wind energy yield can vary significantly between years and measurement of one year does not provide enough information to determine the representative average value for the site, which is necessary for reliable further calculations. To approximate this long-time average value for wind velocity, outcome of measurements are compared with wind data available for other sites. If the correlation of the different data sets is sufficient, it will be possible to deduct the amount of deviation between the long-time average value and the outcome of the measurements. In case no data is available for comparison, a rough approximation can be calculated using so-called „reanalysis wind-data“ from US National Center for Atmospheric Research and National Center for Environmental Prediction (NCEP/NCAR). These data sets are generated by global climate-models, which include global measurement data to compute wind velocity data for different sites.

It is possible to skip measurement at 10 m to safe time in project development. As measurement in 40 m causes high costs, the 10 m measurement should only be skipped, if initial data collection has revealed the general suitability of the site. As the availability of wind data for developing countries is rather low, a measurement campaign containing the two phases is recommended. After the measurement at 40 m is finished and data is analysed, developers decide again, whether the project should be continued or ceased. 

Identification of measurement placements

Based on the outcome of the first measurement, proposals for continuing the measurements are worked out. Beside choosing the placements for the 40 m measurement, the following aspects have to be considered:

• All partners and project developers agree to the choice of placements
  
• 40m-masts and equipment are available;

permissions for the installation of the masts and the conduction of the measurement has been granted by local authorities

The installation of masts and equipment and the conduction of wind measurement and data analysis is conform to the implementation of the first measurement phase.

Data analysis and forecasting of wind energy yield

Measurement data and additional long-time data from comparable sites is used as input for model-calculations.

The resulting report contains the following information:

• Analysis and rating of data quality and its impacts of the outcome of the measurement;
  
• Annual average value, Histogram, seasonal variations in the monthly averages of wind speed, typical variations of wind speed during a day
• Distribution of wind potentials sorted by wind directions (wind rose);
• Weibull-parameters sorted by wind-directions;
• roughness-parameter and obstacles in the surrounding sorted by wind direction or visualized in a map;
• Degree of turbulence wind speed maximum at the siten;
• Correlation to long-time data;
• annual energy yield for different types of wind turbines depending on hub height and placement
• annual energy yield based on adjustments by correlation analysis with long-time data;
• Rating of uncertainty and potential deviations of energy yield from the forecasted values. Calculation of the probability by which the forecasted outcome will be exceeded.

Micro-siting

To evaluate the energy yield of the specific turbines within a wind park a project layout is generated: The potential decrease of annual energy yield for specific turbines is calculated considering the effects of shading between the turbines. This model calculations are used to generate proposals for the conduction of a feasibility study. The final report of the measurement campaign contains the outcome of the model calculations as well as a recommendation, whether the project should be continued or ceased.

References

Portal:Wind