Wind turbines have been known to generate electricity since the late 19th century. In the late 20th century, the global wind turbine industry has experience rapid growth due to escalating costs and environmental concerns related to conventional sources of electrical energy. A common trend in wind farms is to increase the energy extracted from each turbine installation by using taller towers and large swept areas. This trend presents challenges when considering wind shear effects on large rotors and transportation restrictions on very large turbine components. An alternative method of increasing the wind energy extraction is to use a Contra Rotor Wind Turbine (CRWT). A CRWT is a dual rotor wind generator in which the two rotors spin in opposing directions to extract more power from the wind compared to a single rotor wind turbine of the same swept area. In 2000, Dr. Kari Appa was awarded a grant from the California Energy Commission to demonstrate proof-of-concept of a CRWT using a pair of two-bladed 4 meter rotors and a pair of 3 kW permanent magnet alternators. The outcome of the study show that the leeward rotor of a CRWT could extract an additional 30-40% more power from a wind stream (Figures).

CRWT Benefits:  

CRWT technology has potential benefits for both small and large scale turbines. Small scale CRWTs produce higher frequency voltages which, when coupled with submersible pumps, can allow for more efficient water pumping from deeper wells. Utility scale CRWTs which use small components than their equivalent single rotor counterparts may be particularly useful at wind farm sites in which transportation restrictions present a hard stop to component delivery. Additionally, limiting the rotor diameter allows for closer lateral spacing among turbine installations in a wind farm yielding more power from the site, especially when sited in a linear fashion such as along ridges. Benefits in manufacturing can also be seen. Since the CRWT can be designed such that minimal changes are required to convert a SRWT to a CRWT, a manufacturer can produce two separate product lines of turbines using many common processes. CRWT Cost Comparison Direct manufacturing cost differences between CRWT and a SRWT designs are expected to primarily involve the rotor and the generator. In a CRWT, the cost of a second rotor certainly exceeds the cost of enlarging the rotor of an SRWT, but this can be offset by cost savings in the generator. It is important to note that blade cost increases exponentially with respect to length with a growth exponent estimated between 2.66 and 2.78 (Sandia National Laboratories). Therefore, in order to increase the swept area of a turbine by 30%, the direct manufacturing cost for the rotor may increase by at least 40% or more, which means the actual increase 
in cost for a CRWT rotor is less than 45%. On the other hand, a direct drive contra-rotating generator, due to essentially doubling the magnetic flux speed, can result in reduction of the generator weight and cost by about 50%. If it is assumed that the rotor and generator components comprise roughly a similar percentage of the total cost of the turbine, then in the model where a CRWT produces about 30% more power, the rotor and generator costs roughly offset each other. If performance exceeds the 30% assumption, than the CRWT design would demonstrate and favorable cost comparison.