Bigger turbines are not necessarily greener

WORLDWIDE: Years ago a former colleague was telling me about a new lightweight turbine that had impressed him. "Considering its dimensions, this turbine's head mass is so low that it beats any competition and scaling rules do not apply anymore here," he said.

His reference to scaling came from his knowledge of a mathematical principle known as the square-cube law, which describes the relationship between the volume and the area of a given object as its size increases or decreases. Generally, as an object grows in size, its volume - and hence its mass - increases faster than its surface area, so smaller objects tend to have larger surface-area-to-volume ratios than bigger objects.

The rule applies to living things and can have multiple effects, including heat retention or loss.

For example, a baby's large surface-area-to-volume ratio leads them to quickly lose body heat while elephants, with a smaller ratio, have a harder time cooling themselves.

In engineering too, the pistons of high-performance engines often require thermal control cooling support by oil sprayed against the inner bottom part. Here cylinder volume increases much faster with size than the piston mantle circumference area available for heat dissipation.

Contrary to my colleague's belief, turbines cannot escape scaling law effects, but they can be countered. Without clever innovations, large-scale installations would become extremely heavy in relation to their size - and uneconomical. Successful mass-curbing strategies have involved advanced modelling tools, lightweight design solutions, smart turbine control and the use of high-strength materials.

A combination of these with ever increasing experience through series production and technological progress over the last 30 years have produced impressive results. Prominent proof is the latest multi-megawatt onshore and offshore turbines, typically offering lower head mass as a valuable contribution towards pushing down life-cycle-based cost of energy.

A recent study, Wind Power Electricity: The bigger the turbine, the greener the electricity? by the Swiss ETH Zurich institute, not surprisingly generated substantial media coverage about scaling and environmental effects.

The study claims that larger turbines generate greener power - a consequence of pure turbine size-related effects as well as learning and experience with the technology over time. The study also claims that for every doubling of cumulative turbine production, the global warming potential per kilowatt-hour is reduced by 14%. It further indicates that hub height and rotor diameter can be used to estimate the environmental impacts for a generic turbine model.

Uncertain

The 14% figure immediately caught my attention, because such a precise conclusion would need a sound knowledge base of quantitative facts, the use of known verifiable variables and only a few uncertainties. Instead, a considerable number of assumptions were introduced in key areas, which implies the calculations can, at best, produce a rough estimate while the risk of introducing a major statistical error is likely.

One assumption in the study is that a generator's efficiency improves with size, but the reality is much more complex. Important efficiency differences between kW-size and MW-size generators remain unaddressed, as do variations in topology, speed and more. The study assumed an overall efficiency of 48% for turbines produced before 2000 and a constant 53% being a "best-case scenario" for more "modern" machines.

The environmental impact of turbines is assumed, according to the study, to be driven mainly by mass as that directly affects material production, processing and transport. However, manufacturing energy, hence environmental impact, also depends heavily on materials used, steel grade, size and function of components, machining requirement and intensity.

The environmental impact of a turbine's operation is defined in the study as primarily lubrication oil consumption and the diesel used to transport it to the site of the wind turbine. But this ignores the difference between geared and direct drive (no gearbox) and repair or replacement of components.

A clear omission is that small turbines developed decades ago were compared to bigger modern equivalents on a one-to-one basis. But, redesigning a smaller kW-class turbine with current knowledge today would certainly outperform the original in terms of yield and environmental impact.

So, while larger turbines may well produce greener energy, this report simply seems to lacks both necessary insight and careful consideration to convince me that this is indeed the case.

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

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