Horizontal-axis turbines (axial-flow turbines) are similar in concept to the horizontal-axis wind turbine (HAWT) and are also a derivative of the Kaplan turbine described earlier. Prototype tidal turbines have been built and tested using this concept, and, in some cases, ducts may be used around the blades to increase the flow and power output from the turbine (Figure 3.24 (left)).
It appears that the British development of horizontal-axis tidal stream power devices can be traced back to a river current turbine project that ran from 19761984. One of the instigators was Peter Fraenkel, initially from within ITDG but then in the newly formed IT Power. The turbine used a vertical-axis Darrieus-type rotor, and was moored off the bank of the river Nile in Juba, Sudan, where it was used for irrigation. Following this ‘run of river’ project, IT Power began work on a turbine designed specifically to produce electricity from tidal currents. The 3.5 m diameter horizontal-axis turbine was suspended beneath a floating raft moored in
Figure 3.24 (Left) Ducted horizontal-axis tidal turbine from OpenHydro under test at the European Marine Energy Centre in the Orkney Islands (OpenHydro, 2008). (Right) Artist’s impression of Marine Current Turbines’ SeaGen twin-impellor device (Marine Current Turbines, 2008).
the Corran Narrows, at the entrance to Loch Linnhe in Scotland, and produced a shaft power of 15 kW. In the following years, Fraenkel established Marine Current Turbines and went on to design and construct the Seaflow and SeaGen devices (Figures 3.13 and 3.24).