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## Current Designs

It was considered that a 3 bladed design is optimum from a rigidity perspective, but that is all we have in terms real design guidance. By looking at small scale designs which have yielded experimental results, the ratio of chord to radius was set at 3/8 which gives a solidity of 0.179.

Because vertical axis turbines have two defining geometries, radius and height, in order to keep the number of permutations within reasonable bounds, it was specified that height would equal radius. This design would hopefully keep the centroid of the turbine at a reasonable height.

Again, the model was run for a series of radii, velocities and
RPMs, and again the maximum RPM was dictated by cavitation considerations.
The radius was increased in increments of 2.5m from 5m, the velocity
and RPM in increments of .5 ms^{-1} and 1 RPM from 0.5
ms^{-1} and 1 RPM. The results are as follows:

A detailed description & analysis of our three generic models produce some relatively intuitive results and some not so intuitive. As to be expected on an individual basis the power coefficient and resultant power of the Horizontal Axis MCT (HA) is the largest over the size range.

The Vertical Axis MCT (VA) produces comparable power to that of the horizontal axis turbine in the range of smaller sizes, with a decreasing efficiency as its inflow area increases. It is likely that where costs of small scale HA MCTs may limit deployment, smaller and cheaper VA MCT’s could be deployed in shallow fast flowing tidal streams with greater efficiency in terms of energy capture and cost.

Fig. 1 represents the relationship between each of the devices modelled. The Oscillating Hydrofoil (OH) having the lowest efficiency hence suffers from lower power outputs at smaller inflow areas and device sizes. This however will be seen in our case study not to be the only limiting factor of resultant farm power output. The device’s minimal impact hydrodynamic design and the support structure present relatively little resistance to tidal flow, enabling higher tidal velocities to be maintained.

Further comparative analysis of the devices in terms of farm
deployment, with numerous devices causes blockage effects and
significant impact factors to be taken into account. The relationship
between device power coefficient and resultant farm power output
is not a linear relationship and hence large scale deployment
of HAMCTs will not commonly be the best option. This comparison
is highlighted with further results in the conclusions section.

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