VENTURI DUCT - COST EFFECTIVE
At the heart of the LTT is a venturi shaped duct which acts as an accelerator on tidal flows which pass through the turbine and therefore increase the energy that can be captured by blades of a given diameter. Because the size of turbine for a given power take off can be reduced cost benefits in manufacture, operation and maintenance are available. At the heart of Lunar Energy’s business philosophy are the words ‘simple’ and ‘economic’. The LTT venturi duct fulfils these criteria.
Diagram 1 to the left illustrates the manner in which the duct accelerates fluid through the turbine blades that in turn drive the hydraulic pump and generator that are all housed within the sub-sea removable centre module.
The use of a duct ensures that the energy contained within the tidal stream is always straightened to flow perpendicularly through the turbine itself unlike an un-ducted turbine where the flow will be at whatever angle that the flow meets the turbine. (See diagram 2 to the left.) By straightening the flow the duct ensures that the turbine operates at optimum efficiency at all times.
The contrasting efficiency of ducted/non-ducted turbines are clearly illustrated in the graph below which shows the result of Glasgow Tank Tests (May 2004) on both types of duct.
The graph shows the relative power outputs from a ducted and un-ducted turbine as the flow moves away from the turbine axis. There is a sharp drop off in power with the un-ducted turbine (magenta) whereas the ducted turbine enjoys an INCREASE in power of up to around 25 degrees off axis flow (black). This graph is a reflection of empirical data gathered during 1/20th scale model tank testing. In summary a ducted turbine receives greater augmentation of power/energy during off turbine axis flows and peaks at around 25 degrees off angle. At the 25 degree yaw angle the estimated difference in power output is 23%. This means that the output of the ducted turbine is almost exactly twice that of the un-ducted turbine at the same yaw angle. It is markedly more efficient as a source for collecting the renewable energy contained within the tidal stream. It is a better way of harnessing tidal power.