Making sense of small wind turbine comparisons

Comparing small wind turbines can be tricky without an understanding of exactly what affects the amount of energy they produce.

This short guide explains a few important differences between the recent comparisons made between a Gaia 133-11kW turbine and its nearest competitor.

The small wind industry is relatively new, and some manufacturers of small turbines have had significant problems with reliability. The Gaia 133-11kw is a well established and tested design with a 10 year record of reliable operation in many different environments.

Bigger is Better!

The best way of comparing two small wind turbines is to first look at the most obvious comparison – size. More specifically look at the size of the rotors.  The technical term applied to the rotors is called the ‘swept area’, which literally means the size of the area ‘swept’ by the blades as they turn.

comparison of collector area or 'swept area'

The bigger the rotor, the more wind it captures and therefore the more energy it generates. The relationship between swept area and output is linear, so (all other things being equal) if you double the swept area of a turbine you get double the output. The Gaia on the one hand has a swept area of 133m2 and its main competitor’s is 63.6m2 and in fact the same applies to all of Gaia’s competitors in the 5-20kW range.

Quite simply, the Gaia has a much bigger swept area and produces more power as a result.

The Rated Power Myth

Unfortunately, it has become common for wind turbines to be compared by looking at their maximum power output or ‘rated power’ in kilowatts (kW). The problem with this is that many manufacturers rate their products at very high wind speeds (usually 12m/s or more). This means that the turbine will only produce its rated power when the wind is blowing very hard – which is not that often.

It makes a lot more sense to see how much power a turbine will generate in the kind of wind speeds that are frequent in most parts of the UK – in the range of 4 – 9m/s. Because these lower speeds are much more common, a turbine that performs well in them will generate much more energy over the course of a year than one that performs best in the 10 – 15m/s range.

Tower Height

Also critical to performance is the height above the ground. Gaia’s main competitor is supplied on a 15m tower, whereas as Gaia 133-11kW is typically supplied on an 18m tower, where the energy in the wind is 10% higher.

Annual Energy Capture

So how much electricity could you expect a Gaia turbine to generate per annum against the competition?  Lots of different factors influence energy capture, but it is possible to estimate and make a rough comparison. The table below shows that on a site with an average wind speed of 4.3m/s at 10m height, a Gaia 133-11kW should outperform its nearest competitor by about 20%. Assuming an electricity price of 10p per unit, this translates to approximately an extra £1,800 per year including the new feed-in tariff proposed by the Government.