One size does not always fit all

WORLDWIDE: There was a time when I used to spend hours every week in the waiting area of our local music school during children's choir rehearsals and instrument practice. Staff and visitors entering or leaving the building would swing open the inner door leading from the entrance hall to the waiting area and often leave it that way.

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Sitting there, especially on cold winter nights, meant being exposed to an uncomfortable chill when both the external and internal doors were open simultaneously. One night I decided to get up and close the door every time someone left it open, but it did not make much difference.

Another night I asked the caretaker for a solution that would keep the door shut most of the time. I also pointed out this would have some added benefits in terms of the building's energy efficiency.

I was surprised to see how widely our opinions differed. He argued that regular cold air entering the building would result in a higher load factor and consequently optimise operating efficiency for the heating system. Expanding on his line of thought, I suggested that removing the entire building roof would ensure the highest possible energy efficiency all year round. He didn't agree so I decided to leave it at that.

Most of us would agree it is a myth that wasting energy in one area can achieve better overall energy efficiency, although the caretaker had a point that a high load factor is important for optimised system efficiency.

It puzzles me how many rankings in the wind-power sector ignore efficiency and productivity aspects and instead focus only on megawatts of installed capacity. Two typical examples are annual listings of the world's top wind-power markets and of leading wind-turbine suppliers.

Let us assume that a supplier offers two models, each with a 100-metre rotor diameter, but with 2.5MW and 1.6MW power ratings respectively. This company would achieve the highest ranking by selling more 2.5MW turbines. However, the 1.6MW model could prove to be more efficient for low and medium wind-speed sites. Apart from saving on capital investment, indirect benefits such as reduced power transport cable length and transformer capacity requirements might be derived.

Behind current MW-based ranking methods lies an assumption that wind turbines are the same as base-load conventional or nuclear power stations. Coal, lignite and nuclear power plants typically operate all year round at near-full load and are generally only switched off for annual maintenance and occasional downtime. Annual full-load operating hours result in capacity factors sometimes in excess of the 75-85% range. Wind turbines by contrast face continuously changing wind speeds and other operational conditions that result in substantially lower capacity factors.

The right fit for each site

Not long ago vendors typically offered only one model in a given power-rating. More recently, they have begun to recognise the importance of products tailored to specific wind conditions, such as large-rotor models for inland wind farms. A German statistic from the mid-1990s shows these efforts to customise and optimise equipment to site conditions had led, over a mere decade, to inland turbines matching the yield levels of coastal turbines ten years earlier.

Raising performance per megawatt has become a primary strategy for driving down the cost of energy, both for onshore and offshore application. A major focus at present is the development of long slender rotor-blade designs that curb turbine loads while boosting yields.

In December, LM Windpower invited me to see the world's largest 73.5-metre slender GloBlade - which was designed for a 6MW Alstom offshore turbine - being manufactured. Remarkably, this huge blade is made of glass-fibre reinforced polyester composite, without carbon, and yet boasts a mass of only 26 tonnes.

LM held the previous 61.5-metre rotor-blade size record for some five years. These days, records tend to be broken much quicker than that. At the EWEA 2011 offshore event in Amsterdam in November, Siemens announced a 6MW offshore turbine with 75-metre rotor blades and 154-metre rotor diameter, and Mitsubishi unveiled a 7MW turbine equipped with a novel hydraulic drive system with a diameter of more than 165 metres - beating the 164-metre record held by Vestas for a few months.

Watching turbine size and other wind-technology records being repeatedly broken is truly exciting. But the key consideration remains that each new product development further advances the best possible performance per megawatt at the lowest achievable lifecycle cost, both offshore and onshore. This is essential if ambitious renewable-energy objectives are to be met.

Eize de Vries is Windpower Monthly's technology and market trends consultant.

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