Author: Alicia Webb
In this article we look at the technical aspects of monitoring mast design. There are strict standards describing how instruments can be mounted on the mast, and what instruments can be used. These standards ensure that good quality wind data is captured.
The international standard for mast design is IEC 61400-12-1: Power performance measurements of electricity producing wind turbines. It is the responsibility of your mast designer to ensure that your mast complies with all the relevant standards. However, reading the below might assist you in making sure no corners are cut.
Mounting instruments
Boom arm length
A wind monitoring mast obstructs wind flow, which means wind close to the mast is not representative of free-flowing wind. To stop this happening, and to get an accurate measurement of the wind, anemometers and vanes are mounted away from the mast on horizontal poles called ‘boom arms’. It’s important to get the length of these right. Too short, and the instruments will be affected by mast disturbance. Too long, and they might vibrate or move around too much.
The ideal distance of instruments from the tower depends on the tower's diameter. According to international standards, it’s best to mount instruments at least six times the mast diameter away from the centre of the mast, and eight is even better. This means for a mast with a 30cm diameter, instruments should be at least 180cm away from the centre of the mast, and 240cm is even better.
Upstand length
Another small complication is that the boom arm also disturbs the wind. To avoid this happening, it's important to mount instruments on a stand, above the boom arm. This is typically known as an 'upstand' or a 'riser' and should be at least 15 times as long as the diameter of the boom arm.
Boom direction
You also need to think about the direction the boom arms face. They should never point in such a direction that they are sheltered by the mast with respect to the predominant wind direciton.
The IEC standards say that the best mounting direction is as follows:
- tubular mast — 45 degrees to the predominant directon
- lattice mast — 90 degrees to the predominant direction
A good way to ensure a mast satisfies IEC requirements is to ask the mast supplier to put its compliance in writing.
Anemometers
Anemometers measure wind speed. They look like three cups on a rotating stand and measure wind in the horizontal plane. Good wind sensors are calibrated by a third party, usually a member of the Measuring Network of Wind Energy Institutes (MEASNET). MEASNET has 11 laboratories in its network — mainly in Europe and the United States. A MEASNET-calibrated anemometer costs more than an uncalibrated one, but it greatly increases the quality of measurements, so most wind energy consultants recommend it.
Anemometers generate a pulsing or switching signal directly proportional to wind speed. Some versions don’t need a supply voltage, as magnets inside the instrument supply a low amplitude sinusoidal pulse signal. The ‘switch-closure’ type, however, does need a voltage supplied across it. As it spins, it opens and closes the switch, sending a square shaped pulse signal to the logger.
The linear relationship which converts the frequency of these pulses into wind speed is supplied in the calibration documentation for each instrument.
Generally speaking, a higher quality instrument has better bearings so reacts quicker to changes in wind speed, and will operate at lower wind speeds.
Wind vanes
Vanes sense direction using electrical resistance, so they need voltage supplied across them. They usually have three wires — two for supply voltage and earth, and a third for the output signal, or wiper. Output varies linearly between supply voltage (360 degrees) and zero (zero degrees).
It’s important to note that the zero reading is a physical attribute of the instrument, usually marked on the casing. Although it won’t always be possible to point it true north, it's absolutely vital to know the direction this zero marking is pointing in. This direction then becomes an offset and is added or subtracted from the reading to get an accurate direction.
As with the anemometers, a higher quality instrument usually has better bearings so will react faster to changes, and can operate at lower wind speeds.
In addition, wind vanes have a 'dead band' which is the gap in the resistor around the zero mark. Cheap instruments have a wider dead band, sometimes around 8 degrees. This means that they measure direction data between 4 and 356 degrees only. More expensive instruments have smaller dead bands so might be able to measure between 2 and 358 degrees.
Temperature, humidity, pressure
It’s standard for wind monitoring masts to measure temperature. Some also measure humidity and/or pressure, usually at the base of the tower near the logger. These environmental parameters interact to define the density of the air, which in turn defines the power available in wind. There are plenty of different varieties available and they vary in quality and price. The measurements from these instruments need to be reasonably accurate, but they are less vital than the wind speed sensors.
Loggers
A logger is a computer for recording and storing data on site. Each of the instruments on the mast is physically wired into the logger. It's usually powered by a solar panel and rechargeable batteries, and mounted in a weatherproof box strapped to the base of the tower. Sometimes logger boxes are mounted a few metres in the air if the area has cattle, or protesters who like to sabotage equipment.
Telemetry for communications
Commercial wind monitoring towers have telemetry gear to transmit data from the mast to a computer — usually over the GSM or NextG mobile phone network. Telemetry systems include:
- a NextG modem
- a SIM card
- an aerial
- a power source
- connection to the logger
Some telemetry systems, such as the NRG ipack, will send the data automatically at defined time intervals, such as once a week. Other types, such as the more expensive Campbell Scientific logger, will wait for a computer to place a phone call and collect the data.
If the mast is easy to get to, it can be cheaper to go there regularly (usually once a month), and physically collect data and change the data storage card, or download the data straight onto a laptop.
Power system
Monitoring masts don't need much power to run. Masts without telemetry (for remote communications) use very little power and can run for months on normal 12V batteries. If the mast's visited on a regular basis for data download purposes, it's a good idea to check batteries at the same time and replace them if necessary.
Systems that use telemetry need much more power, usually from a rechargeable battery connected to a small regulator and a solar panel. It’s important to mount the solar panel facing north, and preferably on an angle of approximately 45 degrees, though this varies according to the site.