Experimental Characterization of High Solidity Cross-Flow and Axial Flow Tidal Turbines

This paper outlines the experimental testing of high solidity cross-flow type hydrokinetic turbines in the towing tank at the University of Maine and axial flow turbines tested at MIT. These turbines are being developed for commercial scale tidal energy production at megawatt scale tidal energy sites. Details of the testing apparatus, experimental methods, instrumentation and data are presented. Hydrokinetic turbines extract the kinetic energy of a flowing stream and therefore differ from the more conventional head based hydroelectric turbine systems. Hydrokinetic energy farms have more resemblance to a wind farm placed underwater. The testing of such devices in scale model in Tow tanks presents special problems but is similar in many respects to the testing and characterization of propeller performance. Tow tanks provide excellent simulation of a flowing stream, providing a valuable experimental tool in the development of high performance, efficient hydrokinetic turbines. Important to the characterization of hydrokinetic turbines is the power extraction efficiency (in the form of power coefficient) versus the turbine tip speed ratio. In order to fully characterize the cross-flow turbine, a highly quality sensitive dynamometer and load control system has been implemented which addresses the issue of starting and maintaining a constant load on the turbine during the tow tank run. This system allows for a full range of performance data to be collected at a range of loading conditions. In addition the force data on the rotor is encoded so the phase-averaged force data versus rotation position can be collected. The details of the design of the instrumentation and control system are presented along with samples of the data collected. In addition the development and construction of a similar dynamometer for the testing of axial flow turbines is presented. Data is presented for a cross-flow turbine of relatively high blade solidity ratio (blade area greater than 20% of cylinder area). Though data for lower solidity ratio cross flow turbines have been frequently published, this work presents data for turbines with high blade area that are being developed for hydrokinetic applications. The data is being used to validate and improve numerical design models based on the vortex lattice method as a design tool for high solidity ratio tidal turbines. Prior numerical models were accurate in predicting performance of low solidity designs but suffered when solidity exceeded 20%. Results of the numerical models versus experimental data are presented for both overall performance measures as well as detailed phase averaged forces. Test data is also presented for an axial flow turbine designed by the numerical code OpenProp. The turbine was tested on the Axial flow dynamometer at the MIT water tunnel. The testing methodologies presented in this paper are also being utilized to draft engineering guidelines and procedures for the testing of scale model hydrokinetic turbines. Some discussion of the standards development activities underway and the relevance of this work to those efforts is discussed.