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Various fluid turbine systems and methods are described. The turbine can be a vertical axis wind turbine configured to generate power from wind energy. The turbine system can have a blade assembly. The blade assembly can have a plurality of blades rotatable about an axis. The turbine system can have

Various fluid turbine systems and methods are described. The turbine can be a vertical axis wind turbine configured to generate power from wind energy. The turbine system can have a blade assembly. The blade assembly can have a plurality of blades rotatable about an axis. The turbine system can have a concentrator positionable upwind and in front of a return side of the blade assembly. The concentrator can define a convex surface facing the wind. The turbine system can also have a variable concentrator positionable upwind of a push side of the blade assembly. The variable concentrator can be adjustable between a first position and a second position, the variable concentrator being capable of deflecting more wind toward the turbine in the first position than in the second position.

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What is claimed is: 1. A fluid turbine system, comprising: a turbine comprising blades rotatable about an axis, each of the blades comprising a generally straight front surface portion spaced from the axis, a hook portion extending outwardly from the straight front surface portion, a curved rear su

What is claimed is: 1. A fluid turbine system, comprising: a turbine comprising blades rotatable about an axis, each of the blades comprising a generally straight front surface portion spaced from the axis, a hook portion extending outwardly from the straight front surface portion, a curved rear surface portion that is rearward of and curved relative to the straight front surface portion, and a window that occupies a substantial portion of a height of the blades and is positioned closer to the axis than the straight front surface portion, wherein a first plane parallel with and intersecting the axis divides the space surrounding the turbine into a return side and a push side opposite the return side, the turbine configured to rotate generally in an upstream direction on the return side and generally in a downstream direction on the push side relative to a fluid flowing nominally parallel to the plane; and a concentrator positionable in a concentrator position upstream of at least a portion of the turbine and at least partially or completely on the return side, the concentrator having the shape of a leading end of an airfoil or a parabola and comprising a first curved surface portion configured to extend from a first position upstream of the turbine to a second position further upstream of the turbine and further into the return side, wherein the first curved surface portion is configured to be convex facing an upstream direction of the fluid flow; wherein the first curved surface portion deflects at least some fluid toward the push side and creates a relative vacuum to draw at least some fluid away from the window of the blades in the concentrator position, the concentrator movable toward a blocking position further toward the push half relative to the concentrator position such that the concentrator blocks some fluid from reaching the push half of the turbine. 2. The fluid turbine system of claim 1, wherein the concentrator comprises a second surface portion positionable further into the return side relative to the first curved surface portion, the second surface portion configured to extend from a third position to a fourth position that is further into the return side and further downstream than the third position. 3. The fluid turbine system of claim 2, wherein the concentrator comprises a back surface portion configured to be concave facing downstream, the back surface portion comprising an upstream flow surface portion, an intermediate surface portion, and a downstream flow surface portion, the upstream flow surface portion shaped and positionable to direct at least some fluid flowing upstream from the turbine toward the intermediate surface portion, the intermediate surface portion shaped and positionable to redirect the at least some fluid flowing upstream to flow generally downstream toward the downstream flow surface portion, and the downstream flow surface portion shaped and positionable to receive the at least some fluid from the intermediate surface portion and direct the at least some fluid generally downstream into the fluid flowing nominally parallel to the plane. 4. The fluid turbine system of claim 2, wherein the concentrator is configured to extend in a direction further into the return side at least to an outer edge of the turbine such that the concentrator at least intersects a second plane, the second plane being tangent to an outermost edge of the turbine and parallel to the first plane. 5. The fluid turbine system of claim 4, wherein the second plane is separated from the first plane by a blade tip radius, and wherein the concentrator is configured to extend past the second plane at least twenty-five percent of the blade tip radius. 6. A fluid turbine system, comprising: a turbine comprising a blade assembly, the blade assembly comprising a plurality of blades rotatable about an axis, each one of the blades comprising a generally straight front surface portion spaced from the axis, a hook portion extending outwardly from the straight front surface portion, a curved rear surface portion that is rearward of and curved relative to the straight front surface portion, and an open section closer to the axis than the straight front portion of the blade, the turbine comprising a push half and a return half for a given direction of an overall flow of a fluid that defines an upstream direction and a downstream direction; a concentrator positionable in a concentrator position directly upstream of at least a portion of the return half of the turbine, wherein the concentrator has a shape of a leading end of an airfoil or a parabola and defines a convex surface facing upstream and a concave surface facing downstream when in the concentrator position, wherein when the concentrator is in the concentrator position, the convex surface deflects at least some fluid toward the push half of the turbine, the concave surface redirects at least some fluid flowing generally upstream from the return half of the turbine to flow generally downstream, the concentrator movable toward a blocking position further toward the push half relative to the concentrator position such that the concentrator blocks some fluid from reaching the push half of the turbine; and a variable concentrator having a forward end and a rearward end, the variable concentrator having an airfoil shape and positioned such that the rearward end is upstream of the turbine, the variable concentrator located on the push half of the turbine, the variable concentrator comprising a deflection surface extending between the forward end and the rearward end and operable to deflect fluid, the deflection surface positionable to extend generally parallel to the axis along a substantial portion of a height of the turbine, wherein the variable concentrator is moveable, in response to a speed of the overall flow of fluid, between a first position at lower fluid speeds and a second position at higher fluid speeds, and wherein the variable concentrator is configured to deflect more fluid toward the blade assembly in the first position than in the second position. 7. The fluid turbine system of claim 6, wherein the plurality of blades define a sweep path, and wherein an edge of the sweep path furthest from the push half defines an outer return half edge, and wherein at least a portion of the concentrator is configured to extend past the outer return half edge of the sweep path measured in a direction perpendicular to the overall flow and away from the push half, and wherein the concentrator is shaped and configured to draw a drag portion of fluid upstream away from the sweep path and redirect the drag portion of fluid to flow downstream. 8. The fluid turbine system of claim 6, wherein the plurality of blades comprises four blades, the four blades extendable from the axis, each of the four blades comprising a tip and each of the four blades being positionable in a lift position, a push position, a catch position, and a return position; wherein the lift position is defined by a blade positioned such that the tip of the blade in the lift position is located generally upstream of the axis and the curved rear surface portion of the blade in the lift position is convex facing away from the return half of the turbine; wherein the push position is defined by a blade positioned such that the tip of the blade in the push position is located on the push half of the turbine and the straight front surface portion of the blade in the push position faces generally upstream; wherein the catch position is defined by a blade positioned such that the tip of the blade in the catch position is located generally downstream of the axis and a front catch surface portion of the hook portion of the blade in the catch position faces generally upstream; wherein the return position is defined by a blade positioned such that the tip of the blade in the return position, is located on the return half of the turbine and the curved rear surface portion of the blade in the return surface position faces generally upstream. 9. The fluid turbine system of claim 8, wherein the turbine is positionable in a torque position in which a first of the four blades is in the lift position, a second of the four blades is in the push position, a third of the four blades is in the catch position, and a fourth of the four blades is in the return position, the concentrator and the four blades adapted to provide favorable torque characteristics in response to the overall flow of the fluid; wherein the first blade is configured to accelerate at least some fluid flowing across the curved rear surface portion, thereby providing torque; wherein the second blade is configured to provide torque by at least some fluid impinging upon the straight front surface portion; wherein the third blade is configured to provide torque by at least some fluid impinging upon the front catch surface portion of the hook portion; and wherein the concentrator is configured to reduce drag on the fourth blade by blocking at least some fluid from impinging upon the return curved rear surface portion. 10. The fluid turbine system of claim 9, wherein the plurality of blades define a sweep path, and wherein an edge of the sweep path furthest from the push half defines an outer return half edge, and wherein at least a portion of the concentrator is configured to extend past the outer return half edge of the sweep path measured in a direction perpendicular to the overall flow and away from the push half. 11. The fluid turbine system of claim 9, wherein the first blade, the second blade, and the third blade are configured to provide torque simultaneously when the turbine is stationary in the torque position, the open section of the second blade located such that at least some fluid passes through the open section of the second blade and impinges upon the catch surface portion of the hook portion of the third blade. 12. The fluid turbine system of claim 6, wherein the open section defined by each of the blades is the same size and shape as the open section of the other blades. 13. The fluid turbine system of claim 6, wherein the variable concentrator is configured to be biased toward the first position. 14. The fluid turbine system of claim 13, wherein the variable concentrator is configured such that the flow of fluid is operable to adjust the variable concentrator from the first position toward the second position. 15. The fluid turbine system of claim 6, wherein the variable concentrator is configured to deflect at least some fluid toward the blade assembly when in the second position. 16. The fluid turbine system of claim 6, wherein the concentrator and variable concentrator are moveable around an outer perimeter of the turbine, the fluid turbine system further comprising a tail fin adapted to be coupled to the concentrator and variable concentrator, the tail fin shaped and configured to position the concentrator and variable concentrator generally upstream of the turbine. 17. The fluid turbine system of claim 16, further comprising a guide motor adapted to be coupled to the concentrator and variable concentrator, the guide motor configured to selectively control a position of the concentrator and variable concentrator. 18. The fluid turbine system of claim 6, wherein the concentrator and variable concentrator are moveable around an outer perimeter of the turbine, the fluid turbine system further comprising a guide motor adapted to be coupled to the concentrator and variable concentrator, the guide motor operable to position the concentrator and variable concentrator upstream of the turbine.