A wind turbine generator in a timber tower concept
At HASSLACHER Green Tower GmbH, sustainability and regional value creation are vital. Our innovative timber wind power towers are not only leading in terms of technology, but also blaze new paths with their ecological and economical balance sheet. We source our raw material wood from regional sources and additionally offer the possibility to source the wood directly from the wind power system’s location and feeding it into the HASSLACHER group’s delivery chain. If the wood meets the requirements, we can use it to built your wind turbine tower. This minimises truck journeys and reduces the associated carbon emissions. Every log is photographed and and integrated into our (quality assurance) system.
This way, we ensure maximum transparency in the (quality assurance) system and in raw material processing. Once the wind tower has reachedthe end of its useful life cycle, we offer options to buy back the wood, if the customer wishes. The wood can then be used on the next structural timber solution. This way, we complete the material cycle and promote the sustainable use of resources. At the moment, our production capacities allow for an output of 100 towers annually, based on a hub height of approx. 160 m. Thus, HASSLACHER Green Tower GmbH sets new standards in wind power technology by combining sustainability with efficiency.
The timber hybrid tower
HASSLACHER Green Tower GmbH’s timber hybrid tower for wind power systems is a resolved spatial timber frame structure using glued laminated timber beams made from spruce using a hybrid construction method (wood-metal composite). In the framework of the timber hybrid construction method the respective materials are used in such a way that their specific strength is optimally applied, resulting in great value for money for our customers. Our tower concept is based on four timber supports with diagonals made from laminated beams and tip consisting of a conventional steel tube structure. Our hybrid towers are structures that are freely exposed to the elements, showing off the wood they are made.
Contrary to most conventional wind power systems, our timber hybrid towers are not founded on circular flat foundations, but on four relatively small individual foundations that only protrude out of the ground at the supports. Compared to circular flat foundations, the individual foundations use 50% less cement, sand, gravel and reinforcing steel.
Additionally, their significantly lower volume markedly reduces construction time and has the benefit that, once they have reached the end of their useful life, the foundations can be removed without a trace easily and cheaply.
The entire construction is maximally pre-fabricated and produced with a high degree of automation, where the pre-assembled elements can be installed easily and quickly, regardless of the weather. Maintenance lift, ladder, power and control cables are run centrally through the frame, permanently protected from all weather by a “tower within a tower” and run all the way to their transition into the steel tube tower. HASSLACHER Green Tower’s timber hybrid towers fulfil all structural, dynamic and requirements resulting from operation. TÜV Nord’s certification is a generic type certification that allows for the construction of the timber hybrid structure in all climate zones of Europe. For the future, we are planning to acquire component certification for our tower, to make sure that new certifications can be processed even more quickly and efficiently.
A wind turbine with a total height of 200 m
At a hub height of 137 m, the wood volume of the support structure comes in at a total of approx. 320 m³. Since the steel structure for the upper part of the tower makes up approx. 14 m³, the material wood accounts for more than 95% of the entire structure’s volume. At a hub height of 137 m and a rotor blade diameter of 126 m, this results in a total system height of 200 m.
Our strengths
In the philosophy of HASSLACHER Green Tower GmbH, sustainability and regional value creation are vital. Our innovative timber wind power towers are not only leading in terms of technology, but also blaze new paths with their ecological and economical balance sheet
Low-cost logistics
Minimal material use
Resource savings
Regrowing raw material wood
Attractive price
Low maintenance costs
Low removal costs
High degree of prefabrication
Quality assurance in the production process
The supports and diagonals of HASSLACHER Green Tower’s timber hybrid tower are made from glued laminated timber (glulam). This is timber glued together from several board layers (so-called lamellas) in the same direction of growth.
Fire protection for timber structures
One of the most frequently asked questions concerns the structural load behaviour of our timber hybrid towers in the event of a fire. Since wood is known as a fuel, it is assumed that fires in a timber structure such as this would flare up easily and spread uncontrollably. However, does this assumption have any basis in reality?
Flexible from foundations to tip
The open timber frame construction of our timber hybrid tower is reflected in clear flows of forces, making the tower easy and quick to scale in terms of different hub heights, system outputs and rotor diameters. The concept allows for simple and flexible adaptation to respective customer and location requirements.
Competitive manufacturing costs are reached through minimal material use as the open timber frame structure boasts a converted wall thickness of less than 10 cm. Resource savings results from the use of small-volume individual foundations. Segmentable elements allow for low-cost logistics, as the entire structure can be moved to the construction site in a mere 25 truck loads. A high degree of prefabrication and exclusively mechanical joints allow for simple and quick assembly, almost independent of the weather.
Flexible from root to top!
Thanks to 10 years of development, HASSLACHER Green Tower GmbH’s construction delivers maximum efficiency and thus, an attractive purchase price. Intelligent wood protection measures result in low maintenance costs. At the end of its useful life, the wood has a material value exceeding the costs of its removal, leading to positive removal earnings. Low removal provisions for the counties lower capital commitments. Wood as a renewable raw material has stable purchase prices and has no CO2 pricing compared to steel and reinforced concrete. Simply through removing the connecting material in the timber structure and the time-saving cutting of the relatively small individual foundations, customers only accrue low removal costs. Parametrising all contributing factors can reduce the certification time for new systems.
Component certification can help transfer type certificates to other systems without excessive work and time. Hybrid towers are also flexible to assemble in terms of the weather conditions. Since the system doesn’t use glue joints, but only metal-to-metal joints at the construction site, the towers can be assembled regardless of the weather. The timber hybrid tower can even be used in complex site conditions. Its benefits lie in the scalability of the timber elements and their lower weight. This allows the timber tower elements to be brought to the wind power system’s location through lower corner radii, reducing the effect on the site. Small-format individual foundations allow for the entire wind power system to be removed easily and without leaving a trace behind.
Do you have any questions about the construction of the HASSLACHER Green Tower?
The HASSLACHER Green Tower timber hybrid tower is an open timber frame structure made from glued laminated timber (glulam), meaning it’s a rod-shaped structure. Glued laminated timber consists of wooden layers bonded in the same grain. Such lamellas made from solid wood are mainly used in mass timber engineering, meaning structures subject to structural and dynamic loads. Glued laminated timber beams are also known as glulam trusses. The hybrid tower structure’s lower part is made from four solid glulam supports connected by timber diagonals (also made from glulam beams). An adapter piece made from steel leads to the upper part of the structure, which consists of traditional steel tube sections with a maximum diameter of 4.3 m.
Depending on the configuration, the wood portion of the timber hybrid tower comes in at approx. 90 volume percent of all materials used in the tower. The reason for that is that the large-dimension timber elements in the lower part of the whole structure make up the majority in terms of volume.
Contrary to many other wind power systems, we use four individual foundations instead of a conventional base plate. These individual foundations are designed to be relatively small, meaning that they save approx. 50% of concrete compared to the usual 1,000 m3 of concrete used. The timber supports are connected to the four individual foundations via adapters that are pre-mounted in the factory. These four individual foundations consist of one base plate each and a plinth in the shape of a truncated pyramid. The anchor rods used to connect to the timber supports protrude out of those. The foundations are connected to an external earthing ring.
We are currently working on implementing deep foundations (anchoring technology using pilots). This will further reduce the volume of concrete used significantly. Thus, the foundations will be even easier and faster to remove.
Timber boasts excellent properties when it comes to loads in grain. Thus, a timber structure is very good at transferring tensile and compressive loads. Loads perpendicular to the grain as caused by torsional forces, however, present a significant challenge to timber as a construction material. Since these forces are often exceedingly large due to the wind power system’s rotor blades, a hybrid tower was chosen as the first development stage. The reason for this is that steel is very well suited to transferring high torsional forces in the tower’s head area thanks to its shear modulus. We are able to transfer the high torsional forces at the tower’s foot through spreading out the structure.
Our industrially bonded weathering layer made from pine heartwood is at least 4 cm thick and lasts 20 years, but can also be made thicker depending on the wind power system’s life cycle. The fact that this weathering layer protects the load-bearing timer structure well, was proven in series of tests.
When it comes to material fatigue, we are able to reference many tests done on the timber elements by material testing institutes. Comprehensive material fatigue tests have shown that timber initially shows fatigue tendencies similar to those of steel or concrete. Interestingly, however, timber virtually stops fatiguing after a certain period of time. This proven effect is shown in fatigue curves or so-called Wöhler curves.
This effect can be explained through the natural adhesives in the wood as well as the dynamic type of load transfer of the timber materials. This is why we assume that the timber structure will last significantly longer than 20 years. Furthermore, our timber structures are always designed in such a way that all “predetermined breaking points” are located within the steel elements, meaning the timber structure’s life cycle always exceeds that of the mechanical parts (both in terms of connecting means and the wind power system itself).
Bonding processes for load-bearing timber elements must be performed under controlled conditions (meaning in a factory hall) and may only be done by certified companies. Critical weather conditions such as exceedingly low temperatures or high humidity, but also dust particles, can compromise the quality of the bonded joints to the point where their structural integrity can no longer be guaranteed. Thanks to the high rental costs for tall cranes, wind power towers must be erected in a minimum number of days. This does not leave time for the two-component adhesive to cure in a closely timed assembly process. Furthermore, possible bonding defects can no longer be corrected once the elements are installed at the construction site. This is why we only use mechanical connections at the construction site. This has the further benefit that assembly crews that specialise in timber construction are not required.
Do you have questions regarding the technology used on the HASSLACHER Green Tower timber hybrid tower?
The open timber frame structure leaves much more room to install electrical components in the tower’s foot and inside the tower’s structure than with any other tower construction method. That’s why an efficient technical equipment station that can be adapted to the available space can be offered. Due to the larger spaces available, the technical equipment station can be further optimised in size to allow for accessibility and maintenance friendliness of the electrical components. The technical equipment station can be equipped with a fire-proof liner and can be made explosion-proof on request. Visually, a technical equipment station made from wood fits a timber tower better than possible containers made from steel or concrete.
Do you have questions regarding the logistics/assembly of the HASSLACHER Green Tower timber hybrid tower?
The timber hybrid tower’s structural elements can be segmented. That means that we are able to manufacture and deliver to the construction site specific elements adapted to the access roads and available assembly areas. Thus, we are able to reach complex locations (e.g. alpine sites) for which no logistics concepts were previously available. However, we have to make you aware in this context that this will require additional support stacks and thus fastening means as well as additional assembly processes.
Do you have questions regarding the risks involved with the construction of a HASSLACHER Green Tower timber hybrid tower?
Obtaining type certification for a tower structure is an indispensable necessity for the erection of commercial wind farms and for a viable business model. Such certification processes are very challenging. This is likely the reason that were are currently and so far (2023) the only company in the world that has received type certification for a timber tower structure. The timber hybrid tower (as Green Tower timber hybrid tower – T-7003/22-1 Rev. 0) and the associated foundations (surface foundations without lift – T-7003/22-2 Rev. 0) received type certification by the inspection office for structural design and wind power systems at TÜV NORD CERT GmbH according to DIBt regulation. In this context the timber hybrid tower was certified with a VENSYS 126 (incl. Vers. 2), a hub height of 136.9 m and a nominal output of 3.8 MW. We furthermore possess type certification for a Vensys 120 (137 m, nominal output: 3.0 MW). Other certifications with other hub heights, tower versions and system manufacturers are pending.
For this purpose, a round wire leading to an earthing ring that runs around the foundations is installed. Additionally, there are various concepts on how to better distribute lighting. To protect the timber structure from direct hits, lighting spikes are installed on it and connected to the conductor wire in a lightning-proof way. The spikes of the outer lightning protection system in this area are positioned in such a way that here, no lightning hit can be expected when observing the specific lightning current parameters.
Wood only burns when the flames are supplied with oxygen from all sides. The massive diameter of the supports and diagonals prevents this oxygen delivery from three sides, meaning that only the side exposed to the fire would slightly char. Thanks to the density of the solid wood construction (glued laminated timber) and the associated protective carbon layer, the comparatively low heat conductivity of wood (cool core) and its limited oxygen supply, only a thin layer (several mm) of the surface would char before the fire would be go out automatically. The timber structure’s structural integrity is therefore not substantially threatened by a fire.
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The timber hybrid tower’s development is not only based on 125 years of group experience, but also on many years of development. Over the years, many material tests, fatigue verifications, climate studies and various scientific studies were conducted by renowned institutes. In the course of this, the Technical University of Graz, the University of Stuttgart, the Technical University of Munich, the Technical University of Braunschweig and the RheinMain University were involved. Additionally, tests were undertaken at the material testing facility in Leipzig. Moreover, a number of specialised design offices were consulted for development work. TÜV Nord was responsible for the system’s type certification. From this perspective, we have combined the current scientific insights with more than 125 years of experience of a production company to develop a safe and economical tower concept for our customers.
The HASSLACHER group is a family-run company with more than 120 years of history in timber processing and engineered timber construction. The group operates a dozen production facilities all over Europe and employs 2,000 staff. The timber hybrid tower structure consists of glued-laminated beams. The HASSLACHER group is now the world leader for glued laminated timber. Its experience with production and quality assurance is accordingly vast.
Cutting, planing and glued laminated timber production now exhibit an extremely high degree of precision when it comes to construction element production. Apart from a number of quality-assurance measures (see FAQs quality assurance) cutting, planing and joinery processes take place with a precision of +/- a tenth of a mm (1/10 mm). Moreover, all construction elements are once again 3D-measured by a laser unit and compared to the blueprints. Laser measurement is used across the entire assembly process, too, by the way. In the framework of the certification process, load cases must be calculated assuming a possible tilt of the wind power system. Such possible tilts exceed the internal tolerances in production by many factors.
Unlike synthetic materials like steel and concrete, wood, as a natural product, has inherent variations in density and strength. To ensure structural integrity, a high degree of material homogeneity must be achieved through careful selection and processing. In addition to the quality assurance measures in the cutting area, a range of quality-assuring measures follow in the glued laminated timber production facilities. For example, every single lamella that is used on a glued laminated beam to be used in a glued laminated timber structure is checked several times. Each lamella is exposed to defined tensile loads, x-rayed in the framework of an imaging process (e.g. to expose deep defects), electronically measured for their moisture content, and subsequently acoustically struck to determine the lamella’s strength from the returning sound. For all analysed individual lamellas, a stiffness spread is created and only those lamellas that lie within a defined range are approved for production. Furthermore, the position of every single lamella inside the beam is defined on the basis of its strength and documented. Thanks to these comprehensive quality-assurance measures, an extremely high degree of homogeneity and thus, material safety, can be reached for this natural material.
The success of structural mass timber construction in the last 20 to 30 years is based in large parts on the safe adherence to the above mentioned quality processes. The HASSLACHER group is a pioneer in this area and regularly invests in state-of-the-art production technology to further advance this commitment to quality. The HASSLACHER group’s many references involving demanding construction schemes that were and are realised all over the world are the best proof.
Wind turbines are highly stressed structures and experience shows that such systems experience frequent issues. This was clear to us from the very beginning of development. This is why we only use technology that has been tested and proven for decades for our tower concept. We possess decades of experience with tower and bridge structures from glued laminated timber that are exposed to the weather and dynamic loads.
Even in the use of rod dowel connections for dynamically stressed structures, we can draw on decades of experience. Additionally, we avoid all bonding processes at the construction site, that – according to our experience – can hardly be performed in a quality-assured way under critical weather conditions and associated time and cost pressure. 125 years of experience on how to develop and build timber structures, combined with state-of-the-art quality assurance methods and concepts flow into our pilot system projects. This will confirm the tower’s structural stability that was proven in the test report.
to wonders.
