The axial concept has a rotational axis of rotor which is parallel to the incoming water stream. This is illustrated in figure 3. The inclined axis turbines (a) have mostly been studied for small river energy converters. The horizontal axis (b, c and d) turbines are common in tidal energy converters and are very similar to modern day wind turbines from design and structural point of view. The cross-flow concept (figure 4) on the other hand, has a rotational axis of rotor which is parallel to the water surface, but orthogonal to the incoming water stream. The advantage of cross-flow turbines is that they can rotate unidirectional even with bi-directional fluid flow.