The BTE turbine is a lift type devices derived from the Darrieus wind turbine. The axis of rotation can be horizontal but is usually vertical, the more common name, is a Vertical Axis Turbine (VAT) or Crossflow Turbine.
Fixed pitch, straight blade Darrieus turbines have some advantages and some disadvantages compared to horizontal axial flow turbines. One disadvantage is that they shake and produce torque ripple. This problem is overcome by helical turbine or variable pitch blades but helical turbines are still fairly inefficient and lack starting torque.
BTE is using a variable pitch mechanism invented by the company to improve the performance of the turbine. The variable pitch improvement has been examined theoretically and proven by several world class scientist including Dr Brian Kirke and more recently (2015) Dr Pierre-Luc Delafin of Cranfield University which was published at the 4th Oxford University Workshop. Efficiencies around 40-45% are realistic with this turbine.
However, efficiency is not the only factor in making a commercially acceptable turbine. The most beneficial aspect of the crossflow turbine is its shape. The square or rectangular frontal shape lends itself to a far simpler and therfore lower cost device to construct and maintain than the horizontal axis turbine.
The sea is an alien place, electronics and electrical devices are not suited to this environment. The BTE turbine has its generator at the top of a stack of turbines positioned out of the splash zone, just below the water surface. With the BTE mounting structure the generator can be serviced or swapped out by raising the stack to above the water in a simple manoeuvre. There are no other electrics below the generator; no hydraulics; no actuators and no control requirements.
The mounting system is a very important part of tidal energy project. It is a much simpler objective to develop a super efficient turbine and produce a few kilo watts of power. This will add very little renewable energy to power our country. The main thrust of the turbine must be to enable large scale energy production and the only way to do that is to exploit the tidal stream resource efficiently. This would mean extracting the maximum MWh per metre of tidal stream.
The BTE concept for the mounting system is based on a modular system. The latest thinking regarding size of turbines is that it is better to have lots of small turbines than a few large ones. Even the horizontal axis turbine manufacturers have accepted this principal.
Using multiple turbines which are simple to manufacture reduces the cost per unit dramatically.
Using small turbines with the BTE mounting structure, also allows the turbines to be installed, maintained and replaced by small local vessels.
The mounting structure is based on the tried and testing pile system that has been used in the oil and gas industry for decades. BTE has designed a sliding rail system that will support a stack of turbines to suite the depth of water. The drive shaft of each turbine module is connected together with a flexible coupling which then connects to a single generator at the top of the stack. A unique ‘swing up’ mechanism is being developed to enable the whole stack to be floated to the surface for regular inspection or maintenance.
There is an adage in the oil and gas industry that says a $1 spent in the workshop will be $10 out at sea and $100 below the sea.
BTE does not recommend the depth of the stack to reach the bottom. However, in order to exploit the full depth of the tidal stream available, the number of turbine modules would ideally be the maximum possible to a level just above where the flow is slower, which is about ¼ of the total depth above the sea bed. The modular system allows this complete exploitation of the tidal resource, unlike a sea bed mounted horizontal turbine that is designed and built at a fixed diameter. The BTE system will allow a project to exploit any depth of water down to 10m using just one turbine module, for example.
Scalability is crucial in any good design and the BTE modular system accommodates this requirement. A basic configuration is two piles with a stack of turbines between the piles. To double the capacity just one more pile is required, and so on until the entire tidal stream area has been used.
As an added bonus with the BTE array as suggested, the design benefits from the 'barrage' effect. Oxford University has provided research data that an array as described will provide an extra 35% improvement in efficiency from the turbines.
The company is currently working on two patents - the variable pitch tracking mechanism and the mounting system.
Tracking Mechanism:
UK patent publication number GB2521166
International patent publication number WO2015087056
Mounting Structure:
UK patent publication number GB2527311
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Tank Testing at Cranfield video: