Some 10 years ago, there was great interest in kinetic free flow turbines able to generate electric power from rivers, ocean currents and tidal currents as well as from coastal ocean wave conversion. While still being considered in some regions, interest in the technology has declined.

Introduction

The practice of converting power of rivers to mechanical work dates back over some 4,000 years to Ancient Greece, when innovative people built a mechanism to use river energy to convert grain such as wheat into flour to make bread. Over the millennia and centuries that followed, inventors improved on the river-powered waterwheel and used the energy to do a variety of task such as driving saws to convert logs into planks, grind whole grains into flour and drive lathes, drills, looms and a variety of human and animal powered tools of production.

The waterwheel was usually supported by a massive driveshaft that protruded from a building that had been constructed at the river’s edge and where river water flowed fast. During operation, only a small section of the waterwheel was submerged as it rotated under the force of river power. A variation built at waterfalls involved flowing water from higher elevation over the top of the wheel. The year 1812 saw the development of the wholly submerged Francis Turbine where a fast flow of water from higher elevation exerted force on blades built around the entire wheel and delivered much greater power.

Kinetic Turbines

The ongoing search for low-cost renewable energy led to the development of the submerged 3-bladed windmill suspended from overhead from a boat hull. Given that water has a density some 850-times that of air, a slow flow of river water of just over two meters per second (six feet/second) generated electric power from a comparatively small diameter three-bladed turbine. As the technology began to show future promise and governments made easy research and product development funding available, a variety of competing, entirely submerged kinetic turbine configurations appeared that involved parallel-axis, transverse-axis and vertical-axis variants.

A few governments were actually willing to invest money into installations in fast river streams. One installation along the St Lawrence River near Montreal involved a ducted 100mW turbine assembly weighing 120 tonnes that had to be lowered to the river floor in water flowing at seven knots. It was during real world service that some kinetic turbines revealed their shortcomings that included breakage of turbine blades as water speed increased to 11 knots. Undertaking repair and maintenance on submerged turbines required their removal from the river, at high expense and to the point of making large and heavy turbines economically unattractive.