IEC Classification of Turbines: Selecting the right turbine for the site based on wind data

The International Electrotechnical Commission (IEC) creates and publishes standards for wind turbines among other electrical and electronics equipments. The IEC 61400 deals with wind turbine generators (WTG). This blog entry will explain turbine classes. Turbine classes are determined by three parameters the average wind speed, extreme 50-year gust, and turbulence. The following table explains the classifications.

WTG Class	I	II	III	IV
Vave average wind speed at hub-height (m/s)	10.0	8.5	7.5	6.0
V50 extreme 50-year gust (m/s)	70	59.5	52.5	42.0
I15 characteristic turbulence Class A	18%
I15 characteristic turbulence Class B	16%
α wind shear exponent	0.20

For standards purposes, wind speeds are measured every 3 seconds, and every 10 minutes wind speed and standard deviation are recorded. For design load calculations purposes the wind speed over 10 minutes is assumed to be a Rayleigh distribution.

All wind speeds in the above table are at hub height. The extreme wind speed are based on the 3 second average wind speed. I₁₅ Turbulence is the standard deviation of wind speed measured at 15 m/s wind speed.

As an illustration consider GE 1.5sle, a Class IIA WTG and GE 1.5xle a Class IIIB WTG. The Class IIA WTG has a rotor diameter of 77m and hub heights of 65m and 80m. It is designed for average wind speed at hub height of 8.5 m/s with turbulence of 18%.

The Class IIIB WTG has a rotor diameter of 82.5m and hub height of 80m. Because the Class IIIB WTG is designed for lower wind speed (7.5 m/s at hub height) and lower turbulence (16%), the design loads are going to be smaller, therefore its blades are larger and hub height is taller. Bigger rotors of Class IIIB WTGs therefore capture more wind energy and yield higher capacity factors compared to Class I or II WTG.

In conclusion, a wind resource assessment that is based on onsite wind measurements can provide not only the annual average wind speed, but also provide turbulence and extreme wind conditions. This data is necessary to select the class of a turbine. Wind data that is typically used for prospecting like reanalysis data and 10m airport wind data do not provide information about turbulence.

Article written by Dr. Pramod Jain

Email Pramod at pramod@frombeginningtowind.com

Visit Wind Energy Consulting and Contracting Inc.

Wind Energy from Rooftop Turbines—Does it make sense?

There is immense interest in capturing wind energy with turbines installed on rooftops. This blog entry and the associated whitepaper will answer the questions: Does it make sense to place a wind turbine generator on a roof?

Examples of prominent rooftop installs include: Twenty 1KW Aerovironment turbines at Boston's Logan Airport, the Brooklyn Naval Shipyard, and on top of comedian Jay Leno's garage.

The results of rooftop installs are not encouraging. The Massachusetts Technology Collaborative (MTC) sampled 19 small wind turbines installed using MTC grants. The data revealed that the actual average power output is only 27 percent of that estimated, with the high being 59 percent and the low an abysmal 2 percent. As a result of poor performance, in the fall of 2008 MTC cancelled the small wind initiative.

What is not to like about rooftop turbines? These are some of the positive considerations: Wind speeds increase with height; the wind tends to accelerate as it rises over the eaves of the building; there is nothing on the roof anyway; and, energy is produced very close to where it will be used.

Some of the negatives are: Due to the eaves and building contour, there tends to be a sharp increase in turbulence that causes excessive and unbalanced loads on the turbine that lead to premature component failure; residential and most commercial roofs are not suitable as they were not designed to carry the additional weight, dynamic load and vibration of the wind turbine generator; commercial metal roofs are not suitable because of vibration induced noise; turbulence causes energy output to reduce significantly; turbulence causes the life of turbine to be significantly shorter; the orientation of the building significantly impacts the airflow; rooftops produce the rated amount of energy only when the wind direction is in a small 30 degree sector, and in all other wind directions there is a sharp drop in energy production.

At the recent 2009 American Wind Energy Association annual convention, Brad Cochran of CPP presented a paper on "Optimizing the Placement of Building Integrated Wind Turbines." The authors contend that:

• Proper placement of turbines on the roof is essential. The wind speeds can range from 0.1 to 1.5 times that wind speed at eave height. A location closest to the eave that is perpendicular to the predominant direction of wind is the best.
  
• Building orientation with respect to predominant direction of wind is important. The widest part of the building should be perpendicular to the predominant direction of wind. Rooftop installs makes sense only in situations where the most favorable wind conditions are in a 30 degree sector.
  
• Height of building and height of turbine above roof are important. A 400 ft building will experience significantly higher wind speeds at roof level compared to a 40 ft building. 30 to 50 feet above the rooftop will experience normal turbulence levels; any turbines below this height will encounter high turbulence intensities.
  

In conclusion, a rooftop turbine install makes sense in the following situations:

• Building is in a high wind area and the building is tall. The average wind speed at hub height should be at least 6 m/s, preferably higher.
  
• The predominant energy from wind is in a 30 degree sector. The orientation of the building must be such that the broad side of the building is perpendicular to the predominant wind direction.
  
• Turbine should be at least 30 ft (preferably 40 to 50 ft) above the rooftop and any other taller structure in the vicinity. For shorter buildings (20 ft or lower), consider other alternatives like installing turbine on a 70 to 100 ft pole. Any hub height less than this will not see sufficient wind resource.
  
• Rooftop must be able to withstand the moments due to forces on a 30 ft cantilever. Roof must also be able to withstand the weight of the turbine. Roof must be of thick concrete so it does not vibrate.
  
• The turbine should be located as close to the eave as possible.
  
• The selected turbine must be tested in high shear and high turbulence environment because a roof will experience such conditions.
  

Failure to follow these guidelines will lead to significant reduction in wind turbine output.

Article written by Dr. Pramod Jain

Email Pramod at pramod@frombeginningtowind.com

Visit Wind Energy Consulting and Contracting Inc.