There has been a good deal of myth-making of late with regard to micro and small wind turbines, particularly with regard to the pros and cons of building-attached wind turbines. Derek Taylor lays out the facts ...
This article is based on a document that was prepared in response to a request to clarify the likely plausible outputs of wind turbines with rotor diameters between one and two metres.
To assess the power output performance of a wind turbine one requires a power versus wind speed curve (sometimes called simply a power curve) ideally obtained under standard test procedures and preferably produced by an independent source. The power output of a wind turbine varies with the wind speed (V) experienced at any point in time and this will generally be below the rated wind speed (Vr) for most of the time. Therefore to be able to estimate its likely variation of power output over time and its likely annual electricity production in kilowatt-hours (kWh) one needs to have this information.
It is also worth pointing out that the rotor swept area of a turbine (and thus its diameter) with a given rated power output at a higher rated wind speed Vr will be smaller than one with a rated power (P) for a lower rated wind speed Vr. Historically, wind turbines designed for lower wind speed sites will usually have a lower rated wind speed, so the rotor diameter will be usually larger for low wind speed sites compared to high wind speed sites.
However in addition to power versus wind speed curve of the turbine, one has also to take account of the differences in wind speed characteristics at different sites. Every site has a different wind speed frequency distribution - in other words the number of hours that the wind blows at each wind speed; i.e. the number of hours over the year that the wind blows at 1m/s, 2m/s, 3m/s, 4m/s and so on. Sites with the same annual mean wind speed (Vm) can have different wind speed frequency distributions. This is important because the power contained in a wind stream is proportional to the cube of the instantaneous wind speed (V), so failing to take account of this can give an overestimate or an underestimate of the likely annual electricity production (E) in terms of kilowatt-hours per year (kWh/y) ...
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