Many industrial-scale wind energy projects have several 60-meter meteorological towers, with perhaps one hub-height tower used to gather wind data to determine if a project is financially viable during the pre-construction phase. Although the wind industry is maturing and turbine technology is advancing quickly, there have been relatively few advances in meteorological towers, however an emerging technology may bring about a major shift.
Capital City Renewables (CCR) successfully installed a 100-meter (330-foot) tilt-up meteorological tower last month. This is the second 100-meter meteorological tower of its kind in North America — one that can be both climbed and easily moved by tilting it down. The lightweight metal lattice tower was assembled on the ground, complete with anemometers and wind vanes, a safety wire for climbing, and Federal Aviation Administration safety lights before being erected and mounted on a steel plate base.
“Most hub-height met towers installed for the wind industry are actually steel communication towers that have been adapted slightly,” explains Kiril Lozanov, CEO of CCR. “Moving these heavy towers is costly and difficult, especially if the tower is mounted on cement. The met tower that we installed is special because it is designed to meet the unique needs of the wind industry and provides everything needed at a much lower cost.”
The communication industry for example is not concerned about how wind travels around a tower, as long as the equipment is stable. “We seek to limit the distortion of flow around the tower,” says Katy Briggs, senior engineer at DNV KEMA. “We want to measure the free stream, not the wind speed once it has slowed down or sped up due to tower influences. To minimize influences we typically focus on smaller diameters in tubular towers, lattice towers that let some of the wind flow through, and regardless of the tower type, long booms to get [the sensors] away from the tower.”
Unlike the tubular met towers that only reach 80 meters, there are hub-height steel lattice communication towers, but these towers are typically mounted on a cement base with cement anchors and are therefore not designed to be easily moved. In addition, the permitting process is often more involved and some landowners are concerned about the impact of the cement base and anchors on their property.
“The rest of the met towers in use by the wind industry are hollow tubes that cannot be climbed to make repairs; thus the tower needs to be taken down to replace instruments when needed,” says Lozanov. “This practice is costly and cumbersome.”
This inability can also impact the data quality. “If an instrument fails, depending on the criticality of the sensor, you might let a failed sensor go and continue measuring with the other sensors,” says Briggs in her experience with tilt-up tubular towers and steel lattice towers. “If a tower can be easily climbed at a lower cost than lowering and re-erecting a non-climbable tower, you’re more likely to keep it well maintained."
“When you’re climbing the tower, you’re only replacing one instrument at a time,” says Zachary Robinson, project manager for DNV KEMA in his experience with tilt-up tubular towers and steel lattice towers. “Unlike when you need to lower the entire tower for maintenance, with a climbable tower you’re not disturbing the other instruments or doing anything that would take the whole tower off-line. The work can be done in a wider range of weather conditions and on a shorter schedule. If an instrument is out, someone can get out there very quickly to replace an instrument and it is taken care of. To mobilize a whole team of people to lower a tower is going to be a lot more costly and take more time. On the other hand, tilt-up (non-climbable) towers tend to have a lower up-front cost.”
A recent trend in the wind industry is to boost wind data accuracy in the preliminary stages of a project to reduce uncertainties in wind speed measurements at hub height and associated financial risks while financing and constructing the wind farm. “The better you understand your future revenue [from a wind farm], the better financing you can get,” says Briggs. “If there is less uncertainty in the wind resource, it allows you to take on more debt, so you can spend your money elsewhere.”
To reduce risk, there is demand for wind data from higher elevations, closer to the hub height of the wind turbine. “The reason why you want to measure at hub height is because wind changes with height,” explains Briggs. “There are certain assumptions we can make to estimate wind speeds at heights above measurement height, but there can be errors in that. The more you can measure at hub height and across the turbine rotor, the better estimate you can make.”
Instrument reliability is paramount for gathering accurate data. Placing instruments at greater heights however, where they are exposed to stronger winds and a higher risk of lightning damage, increases the likelihood that instruments will need to be repaired or replaced. The ability to accurately gather wind data at higher elevations in a cost-effective manner creates a competitive advantage.
“It seems the industry is pushing to build turbines that are taller and taller,” Jim Fietzer, safety operations manager for CCR. “I think the use of this aluminum lattice tower mounted on a steel plate puts our client in an advantageous position in the future. If the developer decides to push for 120-meter turbines on their next project, they can simply add another 20 meters to their monitoring tower and use it again.”
CCR installed, documented, and tested the instruments on the ground for its new tower, allowing for greater accuracy and simpler, safer, and more effective troubleshooting. The tower, complete with a permanent safety climbing wire, can be climbed to repair instruments. The met tower meets and exceeds Federal Aviation Administration guidelines for aircraft visibility. It is properly painted and has FAA lights; the guy wires have marking balls, and the solar system with batteries contains a three-week backup supply capacity that powers safety lights and the data logger.
Credit: Kiril Lozanov of CCR
Unlike communication towers that often have a cement base and anchors, and remain in one location for a decade or more, it is ideal to be able to move meteorological towers every year or two during the pre-construction phase of a project. “When wind developers need to move communication towers, they often collapse them and buy new ones to avoid part of the high removal expense,” explains Lozanov. “This wasteful practice is eliminated with these new met towers.”
CCR’s recent installation includes a steel plate base, and the lightweight aluminum lattice tower dissembles into segments. The tower can be moved inexpensively and erected on a new site using a small and big gin pole or stored for future use. A steel plate base is appropriate for most soil types and for installations up to four years, according to Lozanov. He recommends a cement base and anchors for longer time periods.
“If you’re developing a project, there are risks that the project won’t go through due to market changes, environment permitting, low wind resource, or a variety of other reasons,” says Briggs. “So there is a balancing act of not investing too much before it's a sure thing, but investing enough to properly assess the value of the project. Part of this balancing act is the investment you make in towers and other measurement equipment. Having mobile towers or remote sensing devices can allow you to recoup some of your investment if a project falls through, by reusing your equipment at another project.”
Moveable towers give developers more options, particularly in the early phases of a project. “If you have a tower that is easy to move, you might use it in a different manner, such as early development where there is more doubt if the project will be built,” says Briggs. She also points out there are still costs associated with moving a tower, such as decommissioning, shipping, and reinstalling the tower and possibly replacing other components such as guy wires and sensors.
Although this tower is the second 100-meter tower of its kind, it may soon become an industry trend. “Climbable lattice towers will likely become more common due to the simple fact that it requires fewer resources to service the towers and accompanying sensors,” says Amy Sue Karshbaum, a crew leader for CCR. “As a result, it will be less expensive to achieve a higher standard of data quality for the client.”