The words “mast” and “tower” are often used interchangeably. Both structures can be used to support instrumentation, such as anemometers at multiple heights or temperature sensors. Instruments to measure wind speed and direction must be carefully mounted on long booms that are often 4-6 times the tower cross-sectional width from the center of the tower to ensure minimum interference between the tower and sensors, even when the booms are upwind of the tower structure. This tower interference effect is documented in IEC 61400-12-1 (2017). Care must also be taken to avoid using data that are gathered when the sensors are in the shadow of the tower. This waked sector can be 30-60° wide, depending on tower size and porosity. Wake effects from towers have been documented in several studies, most recently McCaffrey et al. (2017). It is also important to design booms according to the types of instrumentation that they will carry. Because temperature sensors are relatively insensitive to vertical or horizontal vibration, booms used to mount those sensors can be relatively flexible, as long as the motion of the sensor is slow. By comparison, large vertical vibrations can impact the performance of cup anemometers, and sonic anemometers will measure any boom motion as a velocity superimposed on the true wind speed. For this reason, booms designed for sonic anemometers must be very stiff compared to those for cups, which increases their weight and can make the booms too heavy for tubular masts, and instead a lattice tower may be required (Figure 18). Lattice towers are also beneficial when “research” equipment is installed that may need frequent or unplanned maintenance, as the tower can support elevators for the booms or for personnel to access the instrumentation.
A complication of masts is the need to install guy lines. These lines can extend approximately one-half of the mast height out from the mast base in three or four directions, and may be a hazard to other site users and birds. Also, masts or towers require Federal Aviation Administration approval when installed near runways or airfields. They require power for lighting systems when they exceed 200 feet in height.
Tubular and lattice masts and towers up to 100 m in height are “off-the-shelf” items, and complete wind resource measurement systems can be bought online. Although a tower or mast up to 100 m in height may only cost $50,000$100,000 to purchase and equip, annual costs for equipment maintenance and calibration may add 20% or more per year to overall costs, depending on the number and type of sensors. Furthermore, the type of terrain and weather that the mast is deployed in may impact costs. For example, a remote mast that can only be accessed by helicopter is considerably more expensive to operate than a mast on a wind power plant. Also, masts and towers are at risk of lightning strikes and should be designed with devices that protect electronics and power supplies. Additionally, ice buildup can cause masts or towers to collapse, or make the area around the mast dangerous because of ice shedding after a freezing event.
Examples of towers designed for wind energy research include the 60-m inflow masts at the SWiFT facility, and the NWTC’s 135-m inflow towers. The NWTC towers are documented in Clifton et al. (2013b). Taller towers have been built for atmospheric research, such as the 300-m Boulder Atmospheric Observatory (Kaimal and Gaynor 1983), which was used in the XPIA study (Lundquist et al. 2017), although these tall towers are very difficult to keep operational and require a high level of funding commitment (Wolfe and Lataitis 2018).
Masts and towers for wind energy resource assessment and meteorological studies have also been deployed offshore on fixed and floating platforms. Although most of these have been self-supporting towers, there have been some masts deployed that use guy lines attached to the bottom and top of the masts and have intermediate bracing back to the mast. Offshore towers have been found to cost several million dollars to install and several hundred thousand dollars per year to operate.
Although numerical weather prediction models show promise for use in predicting offshore wind resources, a 2014 study by the Crown Estate in the United Kingdom showed that numerical weather prediction models were not as accurate as direct observations on site by instruments on towers (Crown Estate 2014). Since that report was published, floating lidar systems have been found to perform well if correctly deployed and operated, but at a considerably lower cost than towers; this cost advantage has contributed to the rapid adoption of floating lidar technology.