Electricity from the Wind – Part 2

In this article, we will look at some of the wind turbine design parameters that are specific to wind farm sites.

Wind turbine design parameters

Modern wind turbines are designed for specific wind conditions, which varies from site to site. Therefore, when planning a wind power plant careful considerations must be given to the wind turbine classification. The International Electrotechnical Commission (IEC) defines various wind turbine classes in its IEC 61400-1 standard to which manufacturers design their turbines. The standard wind turbine classes are defined in terms of wind speed and turbulence parameters, as shown in the following table.

The roman numbers I, II and III represent turbines designed for high, medium and low wind speed sites, respectively. S is to be used by manufacturers for turbines that fall outside of the I, II and III classifications. The wind speed classification is primarily dependent on the reference wind speed (V_ref), which is the extreme (maximum) 10-minute average (mean) wind speed expected, at the turbine hub height, with a recurrence period of 50 years. The average wind speed is derived from the reference speed as V_ave = 0.2* V_ref. The letters, on the other hand, define the reference turbulence intensity (I_ref) at 15 m/s, where class A, B and C represents turbines designed for sites with high, medium and low turbulence characteristics, respectively. Turbulence is dependent on surface roughness, terrain, surface heat flux and the wake from nearby turbines.

Therefore, the optimal turbine for a site must have design ratings that match or exceed the local site wind conditions. Thus, a turbine with an IEC IA rating is designed for sites with high wind speeds and high turbulence intensity, as further illustrated below in a brief study of the Wigton III wind farm in Jamaica.

Wigton III case study

As an example case study, let’s look at the turbine selection for Wigton III, which was commissioned in 2016 as the third phase of the Wigton wind farm. It consists of twelve (12) Gamesa G80 wind turbines and it is located at Rose Hill, Manchester, Jamaica. The G80 turbine formed part of Gamesa’s 2.0 MW platform, as shown in the table below and it has an IEC IA class rating.

In order to double-check the turbine selection for the Wigton III wind farm let’s take a looked at the mean wind speed and the turbulence intensity recorded at the Rose Hill site during the preliminary wind resource assessment stage of the wind farm’s development.

The figure below shows the mean diurnal wind speed profile at heights of 13, 40 and 60 meters (m) above ground. If we narrow in on the 60m profile, we will see that the diurnal wind speed varies between 6.90 to 10.88 meters per second (m/s) in an average day with a mean of 9.04 m/s and a standard deviation of 0.772 m/s. Given that the mean wind speed of the Rose Hill site is above the design values for the low (7.5 m/s) and medium (8.5 m/s) turbine wind speed classes, the optimum wind turbine for this site must be rated for high wind speed (i.e with a mean wind speed of up to10 m/s). This justifies Wigton’s selection of the G80 turbine, with its Class I rating.

The next figure shows the turbulence intensity versus wind speed of up to 25 m/s. We can see that the mean and representative turbulence intensities are approximately 0.08 and 0.12 at 15 m/s, respectively. This implies that a turbine rated for low turbulence intensity (i.e with mean turbulence intensity of 0.12) is adequate for this site. However, this turbulence intensity represents the case of an isolated wind turbine, but consideration must also be given to the turbulence resulting from the wake of other turbines installed on the wind site. This in addition to the proven options available from Gamesa, at the time, meant that only the G80 turbine would have met the wind speed class and hence Wigton’s selection.

Annual energy estimation

Once the optimal turbine has been selected for a particular wind site, the annual energy production (and capacity factor) can be estimated from the wind speed distribution of the site and the selected wind turbine power curve. For example, the power curve for the G80 is shown below. It has cut-in, rated and cut-out wind speeds of 3.5, 15, 25 m/s, respectively.

The wind speed distribution of the Rose Hill site is illustrated in the figure below using a Weibull distribution with c and k factors of 10.18 m/s and 2.43, respectively. The figure also shows the estimated energy production of the G80 turbine over its operating speed range in an average year, with peak energy production at 11 m/s wind speed. The aggregate energy is therefore estimated to be approximately 835,000 kWh per annum, which corresponds to a capacity factor of 48% or 4205 full-load hours per annum. This estimated annual energy production and capacity factor are, however, considered to be gross or theoretical as it ignores all the mechanical and electrical losses.

In summary

A wind turbine design must be evaluated against specific site conditions to ensure that the load assumptions of the wind turbine’s design are not exceeded. The following criteria must, therefore, be met for optimal turbine selection:

• The 50-year recurrence wind speed at the site is less or equal V_ref of the design class conditions.
  
• The annual average wind speed at the site is less or equal V_ave of the design class conditions.
• The probability density function of the wind speed at hub height is equal to or smaller than the probability density function of the design class conditions.
• The characteristic turbulence intensity occurring at the site, is equal to or lower than the one considered in the design class conditions. The effective turbulence intensity shall be considered where relevant.
• The normal and the extreme wind shear is equal or lower for all wind directions than those considered in the design class conditions.
• The average air density at the site should be lower than the design air density.
• The site specific extreme turbulence wind speed standard deviation does not exceed the extreme turbulence model (ETM) used in the design.

Other parameters that are also important when choosing the wind turbine for a site are the electrical considerations (such as the generator type, proximity to grid and compliance with grid codes), the mechanical and aerodynamic noise of a turbine, and transportation of equipment.

In the next article, we will look at some of the electrical considerations when integrating a wind farm into transmission and distribution systems.