Wind turbine tower heights have experienced a steady increase in the last 20 years. The average tower height in Belgium, France and the Netherlands has increased from approximately 60m in 2000 to 100-120m in the last years, as can be seen in the following graph:
In 2013, 75% of the installed capacity in Belgium and 70% in France has a tower height equal or taller than 100m. The main reason behind this tendency is of course the potential increase of the expected production yield due to higher wind speeds at greater heights. The trend of using bigger and bigger rotors for low wind sites (and therefore the necessity for more clearance from the ground) and the fact that wind turbines are often constructed in obstacle-rich environments, further contributes to this trend. As for every change, this trend comes with a few challenges which have already been or will be addressed in the nearby future.
From a design point of view, taller towers require a change in technology to be cost effective. Unlike for offshore wind, onshore wind turbine components are mostly transported by road, and such transport is regulated by a number of maximum allowed weight and dimension requirements, especially to go through tunnels and under bridges. Such restrictions usually limit the tower section diameter to 4.5-5m. Therefore, tubular steel tubular walls have to be thickened considerably to withstand the loads when tower height exceeds 100-120m, resulting a significant increase in tower price. As a consequence, hybrid towers -a combination of concrete and steel towers- are gaining importance as they allow towers to reach higher heights without an exponential cost increase. Furthermore, new technology solutions are currently being investigated, resulting in innovative designs such as lattice steel towers wrapped in plastic fabric (currently prototyped by GE) or wooden towers combined with laminated panels which could offer a cheap alternative (currently being investigated by Vensys). Further fine-tuning of these technical alternatives will offer competitive tower designs, allowing tower heights to further increase without compromising the CAPEX of the project.
Wind condition assessment at greater heights also represent a potential challenge. Due to cost efficiency and practical restrictions, most commonly met masts are measuring at a maximum height of 80-100m, as well as one or two lower heights in order to measure the wind speed increase with height (wind shear). The assessment of the wind conditions beyond this maximum mast height is carried out by a vertical extrapolation based on the measured wind shear (adjusted to take into account terrain features). However,this post-measurement process will however increase the uncertainty over the annual energy yield estimate. Additional LIDAR measurements could provide a suitable solution as this will allow to perform the measurements at different locations and heights simultaneously in order to better assess the wind speed increase with height. Experience shows than 3 months of measurement reduces the uncertainty over the vertical extrapolation by half. Consequently, the WAsP wind flow model can be further fine-tuned and the uncertainty on the vertical extrapolation can be further reduced, resulting in a smaller gap between P50 and P90, than when no Lidar measurement is performed.
Without the presence of a met mast on site, the challenge will even be higher as generic wind data (e.g. from meteorological stations) are often available only at lower heights. Furthermore, In Belgium, France, etc, no operational data is yet available for wind farms with tall towers; hence, the currently used wind data, models and assumptions cannot be validated for higher turbine heights yet. Consequently, without the availability of wind data or extra operational data, the uncertainty on the vertical extrapolation will stay above 1.5-2% for the wind speed estimation at 120-140m (compared to 0.4-0.6% when using a met mast), resulting in a wider gap between P50 and P90, when compared to the same gap for turbines with a 100m hub height.
At last, tall towers also represent a potential risk in terms of certification. Over the last year, some turbine types have been offered with a non-common (tall) tower height for which no full Type Certificate has yet been received. As a consequence, TSA/EPC contracts are being signed for turbine types that are not holding a valid certificate. Type Certificate availability is often a requirement for Financial Close and/or start of construction (in some country specific regulations). In addition, full insurance cover during operation, will only be applicable upon the availability of a suitable certificate.
Mitigation measures will be project-specific and shall be customized to the project characteristics, however the availability of a design life statement from the manufacturers will be a requirement in all cases. In addition, extra mitigating solutions might be either the delivery of the Type Certificate as a condition precedent to the TSA/EPC contract, or the mandatory availability of the design approval statement (which is the main step in the Type Certification process) at contract signature and the delivery of the full type certificate as condition precedent to project take over. If no certificate nor design approval are available at contract signature and if the developer/investor does not want to incur a delay in construction, a suitable mitigation strategy could be a customized payment schedule, including delay liquidated damages (that cover the revenue loss due to the lack of operation) in case the certificate is delivered later than a specific date.
Further manifestation of the tall tower trend is expected in the coming years with a potential for tower heights to go up to 180-200m. This will result in the obvious benefits of higher energy production and decreasing fatigue loads. However, it might also result in some difficulties and risks for which suitable mitigation measures shall be applied.