CFD proves key to turbine efficiency
Tom Shelley reports on the crucial role that computer modelling is playing in the design of a new, possible source of 'green' energy.
Software simulation is key to the development of a new kind of turbine that extracts maximum energy from tidal and river flows without some of the expensive infrastructure associated with some other solutions. The base technology, according to Michael Evans, CEO of Green-Tide, based in Cambridge, is to use vanes to induce a twist in the water flow and turn it through an angle, and scoop like- blades to extract maximum energy.
He said he came up with the idea trough thinking about ways of getting more energy from ocean currents. Pelton wheels, he says, are, "A lot more efficient than open bladed turbines", leading to the basic idea, which has since been extended through, "Adding the effects of suction".
This is, of course, the way that an aircraft wing achieves two thirds of its lift – only one third comes from air pressing against the underside of the wing, while two thirds comes from air flow speeding over the top, which has to reduce its pressure in order to do so, according to Bernoulli's theorem, providing lift.
Evans explained that the same effect is made use of in his turbines, with efforts made to accelerate flows in some areas. Energy output increases with flow velocity cubed, so the faster the flow, the more energy. "Full capacity", is to be achieved at an average flow rate through the turbine of 4m/s, although he claimed, "Useful" amounts of power can be extracted down to 1m/s, and up to 6m/s.
The exact shapes of the blades is crucial, and the team makes extensive use of SolidWorks Flow Simulation, which it has been using since June 2010. Evans describes it as, "Very intuitive", although he added that, "It was very useful to have somebody who knows what is going on" when inputting data and setting up boundary conditions.
Evans said that they use it to make a first pass, which on their Dell 8 core PC takes a couple of hours, which indicates where there are complex flows where the mesh needs to be refined. Subsequent full analyses take anything up to a week – one was run over the Christmas/New Year period.
However, the modelling is proving invaluable in the design of the blades with a view to optimising their performance, especially important in the next stage of development, which is to go on from the current 0.5m diameter laboratory scale installation to be found in the University of Cambridge Department of Engineering's old laboratory in Trumpington Street, or at an undisclosed location on the River Cam.
The next stage is to be a 1m diameter 'Run of the river' prototype which is to be tested and evaluated in QinetiQ's ocean replicating test tanks at Havant in Hampshire, normally used to evaluate ship hull shapes and submarine propulsion systems. The intention is to come up with a design that will be, 'Install and forget', with no variable pitch blades or other complex actuator and control systems, and thus be a cost effective source of electricity. It thus has to have a vane and turbine configuration that will function with maximum efficiency over the whole range of flow rates, hence the need for repeated modelling and design optimisation iterations.
In addition to shapes and configurations, it will also be necessary in the next stage of design work, to model and optimise surface finishes and their effects, and the mechanical construction of the turbines and their supports in order to maintain function during all kinds of flow events and impacts by debris.
• Turbines will be required to operate with maximum efficiency without pitch or other geometry changes over the a flow rate range from 1m/s to 6m/s
• Because of the very large number of possible design variants of turbine blade and guide vane combinations, CFD modelling is essential if the design is ever to be optimised
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