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The 5MW Multibrid -- Sometimes truly innovative ideas come good

WORLDWIDE: A friend of mine once said to me that he did not care whether a story was factually true as long as it was entertaining. For similar reasons, I enjoy new technological innovations. I take for granted that some of them might have unrealistic claims attached or may even be based upon obvious ignorance.

When you want to promote a new idea as special and groundbreaking, it is not unusual to present it as the outcome of "out-of-the-box" thinking. The looming prospect of energy shortages is one area that continues to attract many innovative thinkers who unleash upon us the results of their creative efforts.

One of the many potential solutions that resourceful inventors have come up with of late — and which received significant publicity in my native Netherlands — is an energy-generating dance floor. The concept is that moving bodies generate a substantial amount of kinetic energy, the sum of which can be put to use for producing electricity.

While the idea is quirky and even elegant, the engineering challenges of transferring minimal dance-floor vertical movements to generators integrated into the floor structure are huge. For a start, you would have to include a sufficient degree of movement absorption to reduce dancer discomfort, at the same time as guaranteeing operational stability and meeting stringent safety standards.

But apart from the many practical challenges, the claimed sustainability objectives proved to be its Achilles’ heel. An engineer who was very familiar with the proposal revealed to me that the power generated via human movement in this project would have a pay-back time of around 1,000 years.

However, while not every proposed innovation will result in a realistically useful application, the wind industry has proven time and time again that groundbreaking out-of-the-box innovations have been important stepping stones for it to gain its current status as a cost-effective primary renewable-energy source.

A genuine example of unconventional conceptual thinking is the 5MW Multibrid, an innovative wind-turbine concept developed and patented by German engineering consultancy Aerodyn Energiesysteme in 1996-97. Advertised as a quantum leap into the multi-megawatt class, it was presented at Germany’s Hanover industrial fair in April 1998 and developed into an M5000 prototype six years later. Today, it is a series product of French firm Areva Wind for the offshore market.

Multibrid is based on several comparable drive systems and other studies. Favourable top-head mass and the highest-possible systems reliability were assessed as the two most important criteria during its development.

Direct-drive (gearless) technology at the time did not perform well against its competitors, with excessive weight, high investment costs for the labour-intensive generator and poor overall economics claimed as its main disadvantages. Geared-drive models underperformed in terms of reliability expressed in annual repair and maintenance costs. Multibrid was a clever mix between fast-speed geared and direct-drive technologies.

Novel ideas

The initial concept incorporated a number of interesting but unusual ideas such as a sea-water cooled generator. Other unusual innovations were variable-speed operation and stall power-output control, dictated by reasons of simplicity and ease of maintenance. And the initial award-winning nacelle-cover design proved in practice too narrow for offshore service activities and emergencies like staff evacuation.

The researchers adopted a novel lifetime-based upkeep strategy with a focus on component reliability that incorporated redundancy when necessary, consequent monitoring and preventative maintenance. In case of a major system failure, the complete nacelle would be exchanged. The key driver for this decision was the consideration that full nacelle exchange is expensive but at the same time economic because it allows restoring the design life of all the main components back to 100%.

It is not often that completely new wind-turbine concepts are developed. And success is never guaranteed. But the Multibrid continues to fascinate me to this day. It evolved over time in major leaps, but also through several small refinements, confirming that technological development and commercialisation always require much more time and effort than anticipated.

Today there are still few Multibrid or other hybrid turbines in operation and they have as many critics as supporters. But the fact that Gamesa, Vestas, WinWind and drive specialist Winergy all have decided to pursue new — and different — hybrid solutions makes me optimistic that this technology is here to stay.

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