Small wind turbine WESpe generates power with a synchronous generator from Johannes Huebner Giessen
Customized generator delivers lasting improvement to efficiency balance
- Objective: Implement a type-tested small wind turbine with optimum operating characteristics
- Implementation: Individual modifications to a permanently excited 5 kW generator to suit the characteristics of the wind turbine and the systems management concept
- Result: Long service life and maximized feed-in tariff returns by making the best-possible use of the available wind energy source
Small wind turbines deliver huge energy yield– the WESpe 4-blade downwind system is able to take advantage of a wide range of wind speeds to achieve a high level of efficiency. This is not only made possible by the technical features of the system such as passive pitch control of the blade angle, the aerodynamic shape of the nacelle and passive yaw control but also through individual adaptation of the permanent magnet synchronous generator supplied by Johannes Huebner Giessen. Huebner adapted its generator so no heavy IEC adapter is required, the inverter operates in an optimum voltage range and the system runs virtually free of cogging torque.
The market for decentralized power supply is becoming ever-more interesting. Private households, communities, companies and agricultural farms seek to be independent of energy companies and generate their own power autonomously. And wind-generated electricity is increasingly gaining in popularity. Wind-generated electricity accounted for 8.9 per cent of gross electricity consumption in 2013, thereby consolidating its position as the most important source of power generated from renewable sources. However, this figure applies mainly to large wind turbines. Small wind turbines used predominantly to generate electricity on private property with an output below 100 kW have yet to become widely established in Germany. The reasons for this include regulations and legislation, which in comparison with those that apply to other regenerative sources of energy have still to be fully developed: For example, the feed-in tariff is significantly below that for solar generated electricity – and there are still no uniform certification and quality standards for the systems.
"The market for small wind turbines appears confused as far as the final customer is concerned. There is a great deal of potential for small wind turbines," explains Ludolf Ibs, managing director at WES energy GmbH. Specializing in concepts and systems in the field of renewable energies, the company based in St. Michaelisdonn, Schleswig-Holstein, Germany, developed the WESpe 5.0. Following a test phase beginning 2005 the WESpe 5.0 became ready for series production three years ago after successful type testing by TÜV Nord SysTec GmbH & Co. KG in compliance with DIN 61400-2. "The WESpe is one of the few systems to have gained TÜV approval. One of the benefits of this type test is that it can simplify the building approval process for the final customer", according to Ludolf Ibs.
The 4-blade downwind system with passive pitch control adjusts the angle of the rotor blades according to the rotor speed and centrifugal force beginning at approx. 150 rpm. The mechanical energy created by the rotating rotor is transferred to the gearbox/generator unit by means of a rotor shaft mounted in two bearings and a torsionally flexible coupling. Power is generated in a permanent magnet synchronous generator supplied by Johannes Huebner Giessen, which is flange-mounted directly to the gearbox. The Hessian manufacturer of motors and generators has made its name in many fields of industry for more than 20 years by providing customized designs and prototype production created to customer specifications. In this case the generator was adapted specifically for use with the gearbox in the WESpe and reduce the weight in the nacelle. Equally, the voltage range of the generator was designed to comply with the inverter rating according to the given speed range and characteristics of the turbine, and the maximum efficiency of the generator was coordinated with the specified turbine speeds and outputs. "Our generators are designed to operate almost free of cogging torque to achieve optimum starting performances at low speeds, which improves the energy yield", adds Prof. Ewald Ohl, head of the Energy Systems department at Johannes Huebner Giessen.
Generator helps to dispense with IEC adapter
Design changes making it possible to forego the so-called IEC adapter or IEC shaft adapter at the gearbox achieve considerable reductions in weight. The weight of the nacelle needs to be kept low for two reasons: on the one hand it is easier to erect wind turbine generators with a light nacelle, and on the other hand a low weight prevents natural frequencies arising. These would in turn generate oscillations that damage the system. The IEC shaft adapter weighing approximately four to five kilograms is usually located at the gearbox to allow the generator to receive a standard shaft end. Johannes Huebner Giessen built the generator so as to ensure it could be received directly in the gearbox. It was necessary to design the shaft specifically to encompass the gearbox including a custom shaft and custom end shield to make it possible to do without an IEC shaft adapter. "That is not a typical approach. Not every manufacturer is prepared to do that. We pay particular attention to customer requirements and do our utmost to fulfil their wishes", explains Prof. Ing. Ewald Ohl.
A further measure undertaken to individually adapt the generator was to adapt the generator voltage for the inverter to meet specific application requirements. This was necessary because the inverter is able to withstand only a certain input voltage or rather work with a given input voltage. If the input voltage is too high, the electronics of the inverter will be damaged. "A special level of protection must be guaranteed to ensure that does not happen. Consequently, it was necessary to adapt the generator voltage to within the operating range of the inverter", explains Prof. Ewald Ohl. The result is the generator remains in step even when operating at overspeed on the one hand and on the other it also supplies power to the inverter when operating at low speed. The reason is there is a direct correlation between the operating range of the inverter and the wind speeds: the generator turns slowly at low wind speeds and should generate power, if possible; higher wind speeds threaten overspeed and, as a consequence, overvoltage that damages the inverter.
Safety concept and starting characteristics
Johannes Huebner Giessen also customized the generator to comply with the WESpe safety concept. According to TÜV specifications small wind turbines must be equipped with two mutually independent brake systems. A decisive factor was a short-circuit braking function, in other words stop the wind turbine by short-circuiting the three phases at the generator. It is a question of imposing a maximum torque as counter torque to the wind to decelerate and halt the turbine. Dynamic braking is employed when the generator is subjected to extreme loads and the counter torque is large enough so that the wind is not able to turn the wind turbine. "If the electronics in the nacelle fail, then the controls no longer function. In that event, there must be an option available to halt the generator manually by shorting the terminals of the generator", according to Prof. Ewald Ohl. The second component of the safety system is known as a passive pitch control system. This turns the blades out of the wind in the event overspeeds occur.
A further aspect of WESpe-specific configuration applies to the start-up characteristics. Johannes Huebner Giessen designed the generator rotor so that the starting torque is as low as possible. This design aspect ensures the wind turbine starts up easily and without any undesired jerking movements. There is a direct correlation between the cogging torque and system oscillations, which can have a greater negative impact if the generator cogging torque is too large. Undesired oscillations can lead to noise developing on the one hand and damage generator components on the other. "Start-up characteristics are a combination of the settings of the turbine blades and the comparative configuration of the individual components interacting to form a drive train", adds Ludolf Ibs. The cogging and start-up torque are mostly determined by the design of the generator. In addition, there is the resistance of the gearbox as a result of friction and high viscosity of the oil filling.
Long service life thanks to robust construction
Wind turbines operate in all weather conditions, so reliable operations must be guaranteed when wind conditions change as well as during high wind speeds. Housed in grey cast iron generators from Johannes Huebner Giessen are extremely robust by design. An example based on short-circuit braking makes the benefits plain: the grey cast iron housing of the generator is more robust than the aluminium housings generally used. A huge amount of excess energy needs to dissipate when short-circuiting the generator. "The grey cast iron housing is able to absorb the energy. That allows the generator to decelerate and brake reliably", explains Prof. Ewald Ohl.
What is more, the grey cast iron housing is more mechanically robust. It is finished with a salt-water proof coating to prevent corrosion damage resulting from exposure to salt air. Protection class IP55 confirms the generator is protected against ingress of foreign particles and hosing water jets from all directions. All Huebner generators are fundamentally mechanically stable by design. Prof. Ewald Ohl explains: "Our generators are dimensioned to suit application requirements." The robust construction is made clearly visible by the strengthened shafts and the strengthened end shields. That proves advantages as wind turbines are subjected to widely differing load forces. For example, powerful gusts of wind can result in shear forces in the wind turbine, which the strengthened shafts and end shields are able to absorb.
Effect of individual adaptation
A decisive factor in achieving a large energy yield is that it is possible to make use of energy at as many wind speeds as possible. This aim is achieved through the technical characteristics of the wind turbine such as aerodynamic housing, the optimized rotor concept with four blades as well as individual adaptations made to the generator. In contrast to an asynchronous generator the permanent magnet synchronous generator can make use of a wide range of wind speeds and guarantees a high level of efficiency over a long service life. In combination with variable speed operation this makes it possible to achieve a high power density in conjunction with all wind speeds. To ensure the turbine operates flawlessly and efficiently all components need interact 100 per cent seamlessly with one another.
The cooperation between Johannes Huebner Giessen and WES energy GmbH amounted to more than adapting the generator. Once it was ready for series production Johannes Huebner Giessen integrated the small wind turbine as one of the components of the EnergyContainer® – a hybrid energy concept for a mobile power supply consisting of the wind turbine described, a solar collector system and a diesel generator. The EnergyContainer® is a fully autonomous system that is able to generate power far away from the power grid, as is the case during aid operations following catastrophic events as well as to supply power to remote building sites and pipeline stations. Consequently, both the EnergyContainer® and WESpe make an important contribution towards autonomous power supplies – the EnergyContainer® predominantly for power supplies in remote locations and areas with difficult access, while the WESpe supports private households, communities, companies and agricultural farms to autonomously generate electricity with high yields and become more independent of energy companies in the future.