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Use of Wind Turbines to Capture Energy from Air Exhaust

There are several air exhaust applications where large volume of air is pumped externally continuously. Examples are: Coal mines; covered parking lots; and, industrial air handling systems. The first two applications extract contaminated air necessary to maintain satisfactory air quality, and the third application is to use air to transport materials, remove moisture, etc.

Using the above examples, a natural assumption is that the exhaust emissions can be captured and converted into useable energy. Let us consider a few scenarios:

  1. Attach a wind turbine directly to the exhaust. In this scenario the exhaust air (speed = Va, Kinetic energy = Ka) is directly fed to the turbine; the turbine extracts (Kb), a fraction of the kinetic energy in the wind; the remaining kinetic energy is expelled (Kc) with wind speed = Vc. In this scenario, the turbine is able to extract Kb, which will be less than (Ka – Kc); losses will reduce the amount of electrical energy generated.

    Assume that the original exhaust system used K0 amount of energy, which produced an exhaust with kinetic energy of Ka.

    At this point let us ask the question, what if the exhaust system reduced the amount of energy used from K0 to K0 – Kb? This would lower the exhaust kinetic energy from Ka to Kc, assuming no losses. So placing an exhaust turbine is equivalent to reducing the input energy of the system. 

    Therefore, when losses are taken into account, it is more energy efficient to reduce the input energy rather than install an exhaust turbine. Of course reducing the input energy does not cost anything, whereas installing a turbine incurs a cost.

    To recap, it is much more efficient to reduce the overall energy consumption of the system from K0 to K0-Kb, rather than place a turbine to extract Kb energy. Both lead to expulsion of Kc amount of energy.

    So in this scenario an exhaust turbine does not make sense.

  2. Place a wind turbine at some distance from and in relation to the exhaust duct. In this situation significant energy will be lost to the outside and the other energy that hits the turbine can be conceptually viewed as a tube of air that is subject to exactly the same analysis as was conducted in item 1. So for this tube of air it is better to reduce the amount of input energy as opposed to recovering it using a wind turbine.

In conclusion, although the exhaust wind turbine idea seems promising conceptually, it is not.

Now is the time to communicate with our elected officials!

The American Recovery and Reinvestment Act (ARRA) of 2009, also known as the “Economic Stimulus” package provides several alternatives for improving the economics of potential wind projects. Now is the time to let your elected officials know of your interest, and what you are doing about it.
The provisions of the law are intended to help projects become more economically viable by lessening the impact of high initial costs. Thus making wind projects even more attractive to you, the ones contemplating the project and to your elected officials who have a great interest in promoting renewable energy projects in their areas.
Many of the programs being established involve government grants and incentives. Your elected officials, at every level, may be in a position to influence the decisions that will assist you with your projects…but they have to know about them first!
Below is a sample letter that WECC has given to our clients to do just that. It is intended as a starting point. Copy and paste the letter, modify it as required, put it on your letterhead and explain to your officials what it is you, their constituent, are trying to accomplish.
Once you establish contact, WECC recommends periodic updates to ensure they have the up to date information to assist you as you move forward.
If you thought wind was not a cost-effective solution for your situation, now is the time to take a closer look. WECC can help, call us to learn how.


Dear (Elected official)
As our nation moves forward with programs aimed at reducing our dependence on foreign oil and allocates resources towards the development of alternative energy sources, I wanted to take this opportunity to describe to you our efforts in the pursuit of renewable, wind energy.
( Company/ City etc. Name) has commissioned/ will commission a (What you have done i.e. preliminary wind resource study) for our facility/ location in (city, state) Working with a professional wind consulting company, we have determined that the wind resource will support our expectations for a wind project to offset and stabilize our energy costs and assist in reducing the amount of non renewable energy that we use. We are aggressively pursuing our next step in the process, a (met tower, Turbine erection etc).and hope that your schedule will allow a personal update in the near future.
As we move through the complicated maze of project financing and execution, we would ask your assistance with
(federal money, net metering legislation, whatever) as that would greatly enhance our ability to produce the results we seek in the emerging and all important field.
Thank you for the opportunity to inform you of our efforts in this project, we will keep you apprised of our progress.


Monday, May 18, 2009

Energy, Economic Impact of the Stimulus, and Enthusiasm

An Overview of the American Recovery and Reinvestment Act (ARRA) of 2009 – The Stimulus Legislation… in recap form is now posted on our website “from beginning to wind.com.” It relates to both the features of the stimulus legislation that apply to the tax-paying segment and features that apply to the tax-exempt segment is now posted on our website It provides an update on the extension of the Production Tax Credit and the expanded Investment Tax Credit are described here. In addition the update describes some additional aspects of the incentives and features of the stimulus package that are favorable for those involved in wind energy projects. The white paper can be downloaded at: http://www.wind-consulting.com/white_papers.htm

Wednesday, May 13, 2009

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





Vave average wind speed at hub-height





V50 extreme 50-year gust (m/s)





I15 characteristic turbulence Class A


I15 characteristic turbulence Class B


α wind shear exponent


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.

Tuesday, May 12, 2009

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.

Tuesday, April 28, 2009

Renewable Energy and Leadership in Energy and Environmental Design (LEED) Certification

In the recent years there has been a big push toward attaining LEED certification of buildings. In this entry I will describe the connection between renewable energy and LEED points, and describe ways to finance a renewable energy project that does not impact the cost.

When a "Design and Build" company proposes a LEED design to a developer, it chooses the most cost effective components of the design such that the points add up to the desired LEED level. For instance, if a designer wants to achieve a Gold level of LEED certification, then it needs 39 to 51 points. Inserting renewable energy generation into a project is a very expensive way to achieve this Gold target. There are several other significantly less expensive design options to accomplish the same goal.

My whitepaper argues that such a simplistic way of evaluating a renewable energy project is seriously flawed. A more sophisticated look reveals that on a variety of financial measures, a LEED design with renewable energy can be a very attractive investment in the long run. A wind project in a Class 3 wind area can yield substantial positive cash flow.

The whitepaper also describes financing mechanisms like performance contracting or other similar methods to pay for wind projects with no upfront costs.

SODAR based Wind Measurements for Prospecting

Sonic Detection and Ranging (SODAR) is a ground based remote sensing technique for measuring wind speed in the three directions. It is based on Doppler shift in the frequency of the sound waves that are backscattered by temperature fluctuations in the atmosphere.

As the hub heights and blade lengths of turbines have increased, met-tower based measurements at 40, 50 and 60 meters, or sometimes 80 meters height are inadequate to provide an accurate estimate for wind speed at the hub height, let alone over the entire turbine rotor. With both hub heights and rotor diameter above 85m, met-towers of height 150m or more would be required to measure the wind speed over the entire turbine rotor. This would be cost prohibitive. SODAR provides an economical method to measure wind speed in this range of heights.

I have written a whitepaper that describes how SODAR may be cost effectively used for prospecting. With SODAR based measurements a developer is able to evaluate multiple sites in a short amount of time. For instance, over a period of 6 months, a developer may be able to evaluate 6 to 7 potential sites with real measurements at heights of 50 to 200 meters in increments of 10 meters. In most cases, short-term (4 weeks) SODAR measurements are sufficient for this task; it requires that the correlations with longer-term reference wind data be within acceptable range.

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