To utilize effectively tidal stream, the authors have developed a counter-rotating type tidal stream power unit composed of tandem propellers and a peculiar generator with double rotational armatures. This paper prepares a prototype power unit with a synchronous type generator with net output of 1.5 kW, where the front propeller with a diameter of 1 m has three blades and the rear propeller with a diameter of 0.95 m has five blades. The propellers start rotating and generating power at a moderate stream velocity and the unit reaches a maximum output (power generation) at a relative tip speed ratio specified in the unit design. The hydrodynamic forces acting on a pile attached to the unit are affected by the velocity in the stream direction, but the force vertical to the stream is obviously tiny as the rotational moment is counter balanced in the power unit.
It has been requesting, for building a sustainable global civilization, to exploit clean and renewable energy resources. Tidal stream, whose power can be predicted sequentially, may be one of favorable and abundant resources (Bahaj and Myers, 2003; Bahaj, 2011), and many types of power units have been proposed to utilize effectively the stream. Beneficial data for a hydrodynamic design of horizontal axis type tidal stream turbines has been proposed by numerical simulations and experiments in a cavitation tunnel and a towing tank (Batten, Bahaj, Molland and Chaplin, 2007; Bahaj, Molland, Chaplin and Batten, 2007; Bahaj, Batten and McCannb, 2007). The cavitation, undersea noise, and wake flow of the propeller have also been investigated experimentally (Wang, Atlar and Sampson, 2007). Marine Current Turbines Ltd. had installed a two-bladed turbine at Devon coast, which is the world's first tidal current turbine in open sea conditions without a connection to the grid system (IT Power, Seacore, Gesamthochschule Kassel, and Jahnel-Kesterman, 2005). The European Marine Energy Centre (EMEC) Ltd. established at 2003 in Orkney Island has provided test sits for developers of both wave and tidal energy converters. They are, for instance, 250 kW open centered turbine of OpenHydro in 2006, 500 kW turbine of Tidal Generation Ltd's (TGL, now Alstom) in 2010, 1 MW horizontal axis turbine AR1000 of Atlantis Resource Corporation in 2011, 1 MW pre-commercial tidal turbine of ANDRITZ HYDRO Hammerfest in 2012, 1 MW horizontal axis turbine HyTide 1000 of Voith in 2013, and so on (http://www.emec.org.uk/).