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.