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 = V_a, Kinetic energy = K_a) is directly fed to the turbine; the turbine extracts (K_b), a fraction of the kinetic energy in the wind; the remaining kinetic energy is expelled (K_c) with wind speed = V_c. In this scenario, the turbine is able to extract K_b, which will be less than (K_a – K_c); 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 K_a.

   At this point let us ask the question, what if the exhaust system reduced the amount of energy used from K₀ to K₀ – K_b? This would lower the exhaust kinetic energy from K_a to K_c, 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-K_b, rather than place a turbine to extract K_b energy. Both lead to expulsion of K_c 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.

Article written by Dr. Pramod Jain

Email Pramod at pramod@frombeginningtowind.com

Visit Wind Energy Consulting and Contracting Inc.