Based in the north of Germany, Elegant Energy GmbH & Co KG is a small, but enormously innovative and capable company motivated by the belief that every human being has the right to clean and limitless energy. In support of this belief the company has developed Marlene, a new vertical axis wind turbine that is versatile, efficient, compact and cost effective. Central to Marlene’s design and operation is a high performance NX-series grid converter from Danfoss Drives.
When embarking on the development of Marlene, the engineers at Elegant Energy quickly rejected horizontal axis designs as these need more space to install than is available to most users. However, the engineers also knew that vertical axis turbines, despite their quiet operation and unobtrusive appearance, had a reputation for poor performance and efficiency. They addressed these issues with a novel and elegant high-efficiency rotor design, and by using an induction generator rather than adopting the more common option of a permanent magnet generator.
As is the case with all wind turbines, the output voltage and frequency from Marlene’s generator varies with wind speed. To convert this output to the stable voltage and frequency needed to supply most types of load and to feed into the power grid, an inverter system, often known as a grid converter, is required. To provide a grid converter suitable for Marlene, Elegant Energy approached Danfoss Drives.
“We chose to go to Danfoss Drives because the company has an excellent reputation for product quality and performance, and its strong commitment to supporting renewable energy projects means that it has standard products that accurately meet our needs,” said Kim van Wagtendonk, co-owner of Elegant Energy. “We also knew that Danfoss offers sales and support worldwide, which is very important to us, and the price of the Danfoss products was also very attractive!”
By opting for a standard product rather than a specially developed custom grid converter, Elegant Energy eliminated development costs. Even more importantly, it gained the benefit of having a converter with proven performance and reliability which had already been type tested and shown to be fully compliant with relevant standards and regulatory requirements.
For use in Marlene, which has an 8kW generator, Elegant Energy chose grid converters from the Danfoss Drives NX family. These are available in a wide range of sizes with power ratings up to several megawatts, so scalability of the Marlene design is assured, should there be a demand for uprated versions to be used in areas with higher than average prevailing wind speeds. A further important benefit is that, as a result of Danfoss Drives’ in-depth understanding of and forward-thinking approach to renewable energy applications, the NX converters have standard pre-programmed functions that Elegant Energy found extremely useful.
The first of these functions is “flying-start.” To make best use of wind energy when wind speeds are low, Marlene is designed so that the rotor can start to rotate even when there is insufficient wind to generate electricity. Then, as soon as the wind reaches the critical level, the generator is connected to the converter so that it can immediately start to produce electricity. This means that the generator must be able to handle sudden connection to an aerodynamic rotor that is already rotating, and this is exactly the functionality provided by the flying-start option.
The second function useful to Elegant Energy is the “prohibited frequency” function. An issue that complicates the design of all wind turbines is resonance; over a small range of rotor speeds, potentially damaging resonant vibrations build up in the turbine structure. One solution is to add more structural bracing, but this adds cost and increases the weight of the turbine. The prohibited frequency function provided by NX converters offers a better solution.
When this function is used, the converter senses when the rotor speed approaches the lower end of the resonant range. It then loads the generator to prevent further speed increases until the wind speed rises sufficiently to drive the rotor at speeds above the top end of the resonant range. At this point it restores normal operation. The function works in a similar way to block operation in the resonant range when the wind speed is falling.
“The prohibited frequency function has greatly simplified the design of Marlene,” said Kim van Wagtendonk. “It meant we could eliminate resonance problems without incurring any additional costs, and it was exceptionally easy to use. All we had to do was to specify the upper and lower limits of the rotor speed band where resonance could occur – just a few minutes work to solve a potentially difficult and expensive problem!”
A further key benefit of using an NX grid converter in Marlene is that it has integral facilities and support for the connection of energy storage in the form of almost any type of secondary battery. This will allow Marlene to be delivered as a hybrid system, with batteries that are charged when wind is available so they can continue to supply the loads when the wind speed is below that required for adequate power generation. Elegant Energy has even designed the Marlene mast base to house all electronics, including room for basic batteries, enabling a compact design.
In Marlene, the Danfoss Drives NX grid converter is used in conjunction with a Beckhoff Industrial Programmable Controller that provides overall control and emergency functions, such as the automatic application of an electromagnetic brake in the event of an equipment failure. Both the converter and the Industrial PC proved easy to program and integration was straightforward, thanks to the technical benefits of the joint development agreement between Danfoss and Beckhoff.
At the time of writing, the first Marlene vertical-axis wind turbine has been operating for five months and, throughout this time, the Danfoss NX grid converters have performed faultlessly.
Currently Elegant Energy is using its experience from the first Marlene to help prepare series production, which is expected to commence in August 2020. First orders have been placed and international interest is even greater than expected.
“Danfoss Drives has without doubt played a key role in the success of the Marlene project,” said Kim van Wagtendonk, “and the company has been a delightful partner to work with. Although we are a comparatively small operation, Danfoss Drives has always supported us unstintingly and enthusiastically, and whenever we have needed help or advice, the Danfoss team has provided a fast, accurate and helpful response. I would recommend Danfoss Drives to anyone working in the renewable energy field – it really is a ‘can-do’ company with great products and great people.”