Wind Resource Assessment

A wind farm or wind park is a group of wind turbines in the same location used to produce electricity. A large wind farm may consist of several hundred individual wind turbines and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm can also be located offshore.

Many of the largest operational onshore wind farms are located in Germany, China and the United States. For example, the largest wind farm in the world, Gansu Wind Farm in China has a capacity of over 6,000 MW of power in 2012 with a goal of 20,000 MW by 2020. The Alta Wind Energy Center in California, United States is the largest onshore wind farm outside of China, with a capacity of 1,020 MW. As of April 2013, the 630 MW London Array in the UK is the largest offshore wind farm in the world, followed by the 504 MW Greater Gabbard wind farm in the UK.

A wind mill converts the kinetic energy of moving air into mechanical energy that can be either used directly to run the machine or to run the generator to produce electricity.

A wind resource assessment program is similar to other technical projects. It requires planning and coordination and is constrained by budget and schedule limitations. It demands a clear set of objectives so the best assessment approach is selected. Its ultimate success rests on the quality of the program’s assembled assets—sound siting and measurement techniques, trained staff, quality equipment, and thorough data analysis techniques.

Approaches and Objectives

Several approaches are available when investigating the wind resource within a given land area. The preferred approach will depend on your wind energy program objectives and on previous experience with wind resource assessment. These approaches can be categorized as three basic scales or stages of wind resource assessment: preliminary area identification, area wind resource evaluation, and micrositing.

Preliminary Area Identification – This process screens a relatively large region (e.g., state or utility service territory) for suitable wind resource areas based on information such as airport wind data, topography, flagged trees, and other indicators. At this stage new wind measurement sites can be selected.

Area Wind Resource Evaluation – This stage applies to wind measurement programs to characterize the wind resource in a defined area or set of areas where wind power development is being considered. The most common objectives of this scale of wind measurement are to

• Determine or verify whether sufficient wind resources exist within the area to justify further site-specific investigations
• Compare areas to distinguish relative development potential
• Obtain representative data for estimating the performance and/or the economic viability of selected wind turbines
• Screen for potential wind turbine installation sites.

Micrositing – The smallest scale, or third stage, of wind resource assessment is micrositing. Its main objective is to quantify the small-scale variability of the wind resource over the terrain of interest. Ultimately, micrositing is used to position one or more wind turbines on a parcel of land to maximize the overall energy output of the wind plant.

Measurement Plan

Common to all monitoring programs is the need for a measurement plan. Its purpose is to ensure that all facets of the wind monitoring program combine to provide the data you need to meet your wind energy program objectives. Therefore, the program’s objectives should dictate the design of the measurement plan, which should be documented in writing, and reviewed and approved by the project participants before it is implemented. The plan should specify the following features

• Measurement parameters
• Equipment type, quality, and cost
• Number and location of monitoring stations
• Sensor measurement heights
• Minimum measurement accuracy, duration, and data recovery
• Data sampling and recording intervals
• Data storage format
• Data handling and processing procedures
• Quality control measures
• Format of data reports.

Monitoring Strategy

How the measurement plan is carried out is the basis for the monitoring strategy. Its core is good management, qualified staff, and adequate resources. Everyone involved should understand the roles and responsibilities of each participant, and the lines of authority and accountability. Everyone should be familiar with the program’s overall objectives, measurement plan, and schedule. Communications among the players should be often and open.

Because of the complexities of siting and monitoring, the project team should include at least one person with field measurement experience. Data analysis, interpretation, and computer skills are also necessary assets. Available human and material resources must be commensurate with the measurement program’s objectives. High standards of data accuracy and completeness therefore require appropriate levels of staffing, an investment in quality equipment and tools, prompt responsiveness to unscheduled events (e.g., equipment outages), access to spare parts, routine site visits, and timely review of the data.

Quality Assurance Plan

An essential part of every measurement program is the quality assurance plan, an organized and detailed action agenda for guaranteeing the successful collection of high-quality data. The plan should be prepared in writing once the measurement plan is completed.

• Quality Assurance Policy: The program manager must first establish and endorse the quality assurance plan. This will lend credence to the party assigned the responsibility of enforcing the plan.
• Quality Assurance Coordinator: The link between the plan and the program management should be the quality assurance coordinator. This person should be knowledgeable of the routine operation requirements for collecting valid data. If the quality assurance plan is to be taken seriously, this person must be authorized to ensure that all personnel are properly trained, correct procedures are followed, and corrective measures are taken. In addition, the coordinator should maintain the proper documentation in an organized format.

Data quality is usually measured in terms of representativeness, accuracy, and completeness. The quality assurance plan relies heavily on the documentation of the procedures involved to support claims of data quality. The components of the plan should include the following

• Equipment procurement tied to the program’s specifications
• Equipment calibration method, frequency, and reporting
• Monitoring station installation and operation and maintenance checklists
• Data collection and retrieval forms
• Data analysis guidelines (calculations, etc.)
• Data validation methods, flagging criteria, and reporting format
• Internal audits to document the performance of those responsible for site

Monitoring Duration and Data Recovery

The minimum monitoring duration should be one year, but two or more years will produce more reliable results. One year is usually sufficient to determine the diurnal and seasonal variability of the wind. With the aid of a well-correlated, long-term reference station such as an airport, the interannual variability of the wind can also be estimated. The data recovery for all measured parameters should be at least 90% over the program’s duration, with any data gaps kept to a minimum (less than a week).