"As we increase the rotor diameter in our portfolio, the tower dimensions increase," explains Michael Baranowski, product manager for Repower's 3.XM turbine series, which includes turbine towers with hub heights up to 143 metres. "As the rotor gets bigger and bigger, it means that the turbine will be more specialised for low-wind sites, and at low-wind sites a project will only make sense if you can capture wind at higher altitudes."
It is therefore the manufacturers with turbines for low-wind sites in their portfolio - including Repower, Suzlon, Acciona and Nordex - that are offering taller towers. Although taller towers are more expensive, manufacturers can see that the greater energy yields available at higher altitudes make this extra investment worthwhile.
Christof Stork, Italy country manager for GL Garrad Hassan, says that taller towers at low-wind sites may also be less robust, and thus less expensive, than would be necessary for equally tall turbines located at prime wind sites.
In a cost-benefit analysis, taller towers typically make more sense at sites where the wind is blowing less strongly.
Gunter Steininger, product manager at Nordex, notes that the company has been turning out turbines with a hub height in excess of 100 metres since 2005. On the European market, the company currently offers its N117 turbine model at a hub height of up to 141 metres. "There is actually a lot of experience with taller turbines," says Steininger.
"What is new over the last few years is that there is a real market for them."
Steininger says the market has developed because the best wind sites, particularly in Europe, have already been taken up and the efficiency of turbines for low-wind sites has improved. "In the past there was a disadvantage at the economic level for these taller turbines, but that is no longer the case."
Steininger notes that in the wake of the Fukushima nuclear accident in Japan last year there has been a further push to boost turbine efficiency, and thus the business case for taller turbines. Countries now foregoing nuclear energy or reducing its weight in their future energy mix will need more power from renewables and less efficient sites than once planned.
"I think towers are going to be one of the next big areas of innovation in the industry," says Acciona Wind product-line director Scott Baron, "and that this innovation will involve how you offer these taller towers more cheaply."
Acciona Windpower's solution for both cutting costs and overcoming the structural limits associated with the use of steel has been to use an all-concrete structure for its 120-metre towers. Modular pre-cast tower sections are moulded on site and assembled at the plant location. Other manufacturers of taller turbines have opted for concrete and steel hybrid towers.
"If you think of the economics of choosing a tall concrete tower versus a slightly shorter steel tower, there are three main factors to keep in mind," explains Baron. One is the wind shear, or the rate of increase in wind speeds seen at higher altitudes at a specific site.
In this vein, Baron believes fine-tuning wind measurement at higher altitudes will be an important challenge for the industry.
Other factors include the relative cost of steel-tower transportation and project scale. "If your project is located right next to a steel factory then you have low transport costs, but if you're a decent distance away it can be quite expensive and you can avoid these costs with a concrete tower," explains Baron. "Larger projects also benefit from concrete towers more because you can spread the fixed cost over more turbines."
Clear of the turbulence
A secondary factor driving the trend towards taller towers is the growing number of wind farms that are being built in wooded areas, particularly in northern Europe. While blade tips must obviously be high enough to clear the tree tops by an ample margin, height is also important to ensure that the turbines can operate efficiently. "With longer blades, especially in woody and hilly areas, there is more turbulence and by going higher we can reduce that," says Baranowski.
The modular nature of the concrete or hybrid concrete and steel structures typically used for taller towers has the beneficial side effect of limiting the number of trees that must be cut down to transport and install turbines at a site.
While the occasional not-in-my-backyard (NIMBY) protests may be par for the course for wind farms in general, there does not seem to be any particular opposition from the public to taller turbines specifically.
Indeed, their growing spread on the market comes as the population of many countries has grown accustomed to wind farms.
But as the turbines get taller, permitting approval is likely to be an issue. Generally speaking, projects involving taller turbines may take a little bit longer to be authorised and may undergo a slightly more in-depth assessment process.
"In the US there can be permitting issues from an aviation perspective," says Baron.
"There is typically a lengthened process when the ground-to-tip height is above 500 feet (152 metres) and with a taller tower and a bigger rotor you can easily cross that threshold."
Height restrictions in some countries mean the tallest towers in a manufacturer's portfolio may not be suitable for all markets. For example, Baranowski of Repower says that in Germany - a key market for the sale of taller turbines - blade-tip restrictions kick in only at 200 metres and do not currently represent an obstacle even for its 143-metre towers. "In countries like Italy, France and Poland, there are tip-height restrictions at 150 metres," he adds. However, these restrictions are expected to ease shortly in Poland opening a new market for the sale of taller towers.
A CLOSER LOOK TURBINE TOWER DESIGN AND THE EVOLUTION OF TALLER towers by EIZE DE VRIES
With high-wind onshore sites increasingly difficult to find, developers have been seeking to boost yield levels through turbine models featuring large rotors per megawatt power rating. Higher towers are expected to be the next big area for technological development in the wind sector.
Wind speeds are typically highest in coastal regions and decrease further inland. They are lowest near ground level and increase with height. Power in the wind is directly related to wind speed cubed, so a relatively small increase in wind speed can produce proportionally much more power. Typical yield increment rates at inland locations are quoted as 0.75-1% per extra metre hub height, but tend to diminish above a certain point depending on the terrain.
Higher towers are especially beneficial at lowand medium-speed inland sites, enabling substantially higher yield levels per megawatt as well as achieving a reduction in cost of energy. And as wind flow at greater heights becomes less turbulent and more stable, reinforced by today's trend for larger rotors, the use of higher towers could increase a turbine's operational lifetime. A tower similar to or bigger than the rotor diameter is also considered more aesthetically pleasing.
Tubular steel towers have traditionally offered the most common wind-industry solution, and usually consist of several sections depending on hub height and transportation logistics. To allow safe passage through road obstacles like tunnels, a maximum diameter is usually restricted to 4.2-4.3 metres. That puts a limitation on maximum hub height, often around 100 metres.
An exception to this rule is Vestas. For its 3MW V112-3.0MW flagship turbine, the Danish manufacturer currently offers a 119-metre tubular steel tower with a 4.2-metre diameter at its base. These soft towers have relatively thick steel walls, making them an expensive solution. Other suppliers are considering alternative high-tower solutions.
Perhaps the world's highest turbine tower at present is a 160-metre four-legged lattice steel structure with a 2.5MW Fuhrlander turbine operating in Germany. Advantages of bolted lattice towers include low mass, rather small inexpensive foundations per leg, and easy component transportation.
However, there are downsides to this design. Most notably the time required for assembly, a need for yearly bolt-connection checks, and potential visual restrictions. Some alternative lattice-tower solutions focus on speeding up assembly and reducing maintenance demands.
Enercon has for many years dominated the concrete tower sector. An initial design for the company's 500kW turbines comprised two or three bolted cylindrical segments made in rotating moulds. The next step was in-situ chimney-type towers comprising a short steel transition piece on top, used with 1.5MW direct-drive E-66 turbines.
These towers are built using a sliding shuttering method characterised by a long construction period and uncontrolled weather and manufacturing conditions. Other suppliers have built in-situ concrete towers, but only in limited numbers and designed as a concrete-steel hybrid structure.
Enercon's later generation concrete towers were prefab designs, built from multiple coning rings with a short steel transition top section. After the towers are assembled onsite, steel cables are run through slots inside the concrete mantle, providing a stiff and strong single assembly.
The industry's biggest concrete tower is currently for the 7.5MW E-126 turbine, which features a 14.5-metre base and 135-metre hub height. In general, a wider base and the excellent stiffness characteristics of both concrete and concrete-steel hybrid towers allow for bigger hub heights.
Many believe concrete-steel hybrids are the way forward. In these, the concrete bottom sections match a standard tubular steel upper section with a 4.2-4.3 metre base diameter. Enercon has just announced a new hybrid tower with 149-metre hub height for its 3MW E-101 model, most likely a new record.
Apart from height restrictions, a possible limit for any future tall wind turbine tower design is, according to experts, a trade-off between additional investment costs set against the expected lifecycle-based cost of energy.