The Darwind5 Trekker series of portable wind turbines are billed as next-generation Darrieus "eggbeater" powered generators. The design is said to deliver 35% better annual power production than any current wind turbine at the same price point and sweep area.
The Darrieus design was patented by a French aeronautical engineer of the same name in 1931. Its configuration is theoretically as efficient as the propeller type under constant wind speed, but real designs have typically had problems realizing this efficiency.
When the Darrieus rotor spins, the airfoils rotate. The aerodynamic principles which rotate the rotor are the same as those for autogiros, and normal helicopters in autorotation. As the airfoil moves around the back of the apparatus, the angle of attack changes to the opposite sign, but the generated force is still obliquely in the direction of rotation, as when the airfoils are in the front. The rotor spins at a rate unrelated to the windspeed, and usually many times faster.
One problem with the classic design is that the angle of attack changes as the turbine spins, so each blade generates its maximum torque only at the front and back of the turbine. This leads to a pulsing power cycle that complicates design. If this pulsing happens at a natural frequency of the blades, the resulting resonance can destroy the turbine.
The eggbeater blade shape is a means of reducing the centrifugal stresses that would otherwise arise if blades were straight such that their mass concentrated at the periphery of the turbine. In the eggbeater configuration, most of the stress is in the part of the blades which torque against the generator sitting at the bottom of the turbine. The only forces to be balanced out vertically are the compression loads caused by blades flexing outward (thus attempting to "squeeze" the tower), and the wind force trying to blow the turbine over.
The Darwind5, developed by the Canadian company Harvistor, uses a five-blade design. Harvistor says it used CFD to design the airfoils with a special angle of attack and to make the turbine more solid. The airfoils are designed to work four times per rotation instead of just two times. These factors, says Harvistor, help boost Darwind5 performance by 25% compared to current vertical turbines.
Harvistor also says it devised what it calls a passive Smart Pitch Angle Regulation Strut System that changes the local angle of attack of the rotor given the current wind speed and direction. This feature is billed as a first for Darrieus eggbeater turbines.
One or more generator sets sit at the base of the turbine to produce from 500 W to 1.5 kW in a 1.2-m working diameter. The Model 12xx series of the turbines scales on the same platform with blade and shaft mid-section kits along with additional generator sets of 500 W.
Harvistor: http://www.harvistor.ca/news.php
Wikipedia has a page on the Darrieus design: http://en.wikipedia.org/wiki/Darrieus_wind_turbine