The main steps to follow for a wind assessment campaign are:
- Identify potential sites
- Wind resource evaluation of the site
- Micrositing of the selected site to help determin the futur wind turbine location on that site
The minimum wind monitoring duration should be one year, because it is important to evaluate the wind resource during every season to be able to determine seasonal variability and availability of the wind. But two years or more of wind monitoring will provide more reliable results that will allow better estimation of energy production and in the mean time reduce investment risks. Golden rule, the longer the wind campaign the less uncertainty of long term prevision.
The most important characteristics when choosing a site are:
- Wind resource: Wind resource estimates can be obtain from sources like Wind Atlas, National Weather Service stations, Universities and air quality monitoring. Although those data were not collected for wind energy assessment purposes they can provide a general description of the wind resource within the analysis area.
- Topographic indicators: The analysis of topographic maps is an effective way of identifying suitable terrain features. It allows to identify features that are likely to present greater wind speed than the general surrounding area.
- Availability of the land
- Position of existing roads and buildings
- Land cover
- Political boundaries
- Proximity to transmission lines
When choosing the exact location of the tower are important:
- Tower should be placed as far away as possible from any obstructions. In fact, obstructions can significantly affect the analysis of the site wind characteristics.
- General rule is to locate sensors at an horizontal distance of at least 10 times the height of the obstruction in the dominant wind direction.
- Select a location that is representative of the majority of the site.
- Tower should be located at a distance of at least 1.5 times the height of the tower from electrical installations, streets, roads, building, park or any other structure or location where the collapse of the tower could cause people injury of material damage.
Before the installation of a tower it is the client's responsability to :
- Verify with local authorities if any permits are required. Normally, a standard building permit is required, as well as authorization from air navigation and transport.
- You might need a formal lease agreement with the landowner to protect parties involved. This should include : tower location, total area required for monitoring period, insurance, access to premises and payment schedule.
- Verify if lighting and color marking are requested by local air traffic regulation
The core of monitoring program is the collection of wind speed, wind direction and air temperature data. Those parameters are needed to evaluate resource related wind energy feasibility issue.
- Wind speed is the most important indicators of a site's wind energy resource. In fact, wind power is a cubic function of the wind speed. Calibrated instruments are recommanded but all depends of the quality control specification of your particular wind assessment campaign.
- Wind direction is important for identifying preferred terrain shapes and orientations and for optimizing the layout of wind turbines of the wind farm.
- Finally, the air temperature is an important descriptor of the wind farm's operating environment. It is also used to calculate air density and a wind turbine's power output.
When selecting the measurement sensors for wind speed, wind direction and air temperatureIt is important to determine:
- Accuracy of data
- Reliability of the sensor
- Calibrated sensor vs non-calibrated sensors
- Climatic environment (heated or none heated sensor)
- Power requirement (solar panel, small wind turbine, etc.)
- Sensor height
- Sensor shape (to minimize turbulence)
- Sensor boom lenght to reduce mast influence
When selecting a data logger it is important to determine:
- Desired data transfer method
- Compatibility with the sensors type, numbers of sensors, measurement parameters and desired sampling and recording intervals
- Power requirement (solar panel, wind turbine, etc.)
Data loggers offer different methods of data transfer which are grouped in two main category :
- Manual Data transfer:
- Remove and replace current storage device or data card
- Transfer data directly to a laptop computer
- Remote Data transfer:
- By direct wire cabling
- Phone modems
- Cellular modems (numerical or analogic)
- RF modems
- Satellite communication
- Internet connection
Both methods presents advantages and disadvantages.
Manual data transfer is leading to a visual on-site inspection of the equipment which allows to detect any problems with the instruments or tower that could cause data loss or tower failure. On the other hand, this method is requiring additional data handling steps which increases potential data loss with no backup available. Also, it requires frequent site visits and if for some reason there is a problem, you will only notice when the data card transfer is done and you might loose good data for a certain period.
With the remote data transfer, you can retrieve and inspect data more frequently than you conduct site visits. This allows you to promptly identify and resolve site problems. Plus, you have a backup of your files in the memory card. Disadvantages include the cost and time required to purchase and install the equipment but it might be worthwhile in the long term if data monitoring problems can be spotted early and quickly remedied or if site is hard to access during certain time periods.
If available, 120V AC power can be used as a direct source of system power.
In most cases, AC power is unavailable and solar panels are used along with batteries built for cold climate to provide power.
If there is important power requirement (heated sensors, etc.), a small size wind turbine can be use to provide power.
Tubular tilt-up towers are recommended for their ease of installation. In fact, the tower can be assembled and sensors mounted and serviced at ground level. Also, minimal ground preparations is required and you don't need an easy access road to be able to install the tower at site since all material needed can be transport by ATVs or any small off-road vehicle. There is no need for expensive concrete base and no crane is required to erect tower. Finally, a tubular tower system cost less than lattice tower.
The tower should
- Have erected height sufficient to attain highest measurement level in reference to the hub height.
- Resist wind and ice loading expected for location.
- Be structurally stable to minimize wind-induced vibration.
- Be guy-secured with proper anchor type determined by the soil condition and pull resistance.
- Be equipped with lightning protection measures.
- Be secured against vandalism and tower climbing.
- Have ground level components clearly marked to avoid collision.
- Be protected against corrosion from environmental effects.
- Be protected from cattle or other grazing animals.
The tubular tilt-up tower can be installed almost anywhere.
Of course the task is much easier if the terrain is relatively flat, but it can be installed on a sloped site. In that case, it is important to first determine the best direction to raise the tower. It should be done in a way that the tower is assembled in the uphill direction in priority and in dominant wind direction if possible. Also, depending on the field, the side guy wires might need to be adjust more often as the tower is raised.
It is interesting to note that with the use of AAT unique gin pole assembly (patent pending), which allows lifters guy to be adjust throughout the raise, the angle of the gin pole can be modified at the end of the erection to fit site slope.
See drawing below :
It is also important to note that a maximum grade of 5% is allowed to erect a tower, considering the length of guy wire cable provide. Higher
grade can be accomodated but requier longer guy wires and more wire adjustment during raise.
Erection of the tower near a summit should be avoided because the assembling of the tower would be impossible. It is then recommended to lower the base downhill to allow correct tower assembly.
See drawing below :
Sensors and equipments installation is a very important aspect of a wind assessment campaign. In fact, the accuracy of the data obtained from instrumentation mounted on tall towers is directly related to the mounting technique. The support hardware, other instrumentation and the tower itself can all affect the airflow in the vicinity of the sensors. Therefore, careful placement of sensors will limit the interference and help ensure high data quality.
It is important that wind sensors must be mounted onto the tower with independent booms. Also, mounting hardware must be horizontally leveled and long enough to minimize reading errors.
Here are the general rules to follow for tubular towers, schematically resumed: