Savonius Rotor for Offshore Wind Energy Conversion
Authors: Gaul R.D. , Brancart Claude P. , Gaul Vincent 
Published: May 2011

Abstract
Analysis of performance is presented for wind energy conversion by a Savonius type vertical axis rotor configured for generation of electrical power. The technical approach capitalizes on the high torque property of a large Savonius rotor at rotation rates dictated by the ratio of the wind speed to the peripheral speed of the rotor vanes. This property enables generation of AC power by means of a multiplicity of generators positioned around the periphery of the rotor that are sized and have gear ratios matched to selectable ranges of wind speed. The system can produce electrical power over a range of wind speeds and capacities that are significantly beyond the practical and economical operating envelope achievable with horizontal axis wind turbines (HAWT). A performance comparison is made with conventional HAWT rated at 1.5 megawatts. Potential performance is examined for systems with rated capacities in excess of 6 megawatts with the feature of increased capacity at higher wind speeds. Adaptation of this technology to offshore sites and the implications for cost-effective performance in wind farms are discussed.

Introduction
This paper presents an entirely new concept, referred to as SavRot, for implementation of vertical axis Savonius rotor technology that will have significant advantages over highly developed horizontal axis wind turbines (HAWT). This new adaptation of mature technology can provide systems that are quieter (noise from both vanes and generators); pose no danger to birds in flight; are easily maintainable by ground crews; have a greater tolerance for storm conditions; capture gusty energy independent of wind direction; can have higher power generation capacity; and have lower capital and operating costs. The inherent design flexibility of SavRot technology enables a broader range of applications than are suitable for HAWT. The technology is especially attractive for offshore wind farms that need higher power generation capacity at single sites, endurance and power production in storms, lower cost of ownership, and fewer hazards to navigation.
Changing conditions in the marketplace are considered insofar as they bear on the demand for and specification of more capable and less expensive equipment than now is available for wind energy conversion. An overview is provided of technological impediments to the capability of HAWT systems to respond to emerging market demands. A performance comparison is drawn between HAWT rated at 1.5 megawatts (MW) and an equivalent SavRot system. Performance potential of larger SavRot systems is illustrated for a scaled set of rotors with standard capacity ratings up to 6 MW and power generation capability greater than 10 MW at higher wind speeds. Consideration is given to the application of large SavRot systems in the offshore wind farm market.

Wind Energy Market
The U.S. wind energy conversion market is underpinned by the Federal mandate to have renewable sources provide 20% of national electricity consumption by 2030. Wind is the most cost effective source to meet the demand. The wind energy conversion goal for 2030 has been set at a total capacity 300,000 MW by the U.S. Department of Energy (2008). Total U.S. capacity at the end of 2006 was 11,500 MW. At the end of 2009 it was 35,000 MW (American Wind Energy Association, 2010), or about 11.7% of the 2030 goal. To meet the goal, the required annual increase of installed capacity has been estimated by the Department of Energy (2008) to level off at 16,000 MW per year beginning in 2018, at which time the plan calls for 110,000 MW to be in place.
Virtually all of the growth in the U.S. wind energy market has been in the form of terrestrial wind farms. The wind farm typically is an aggregation of HAWT systems tied to a large scale electrical network that has primary power supplied by multiple conventional power plants. Wind farms have been subsidized on many levels from the inception of the wind energy conversion industry. The subsidies are an essential ingredient for counteracting low return on investment.