Kepler Energy Limited, a development stage company, intends to develop and manufacture tidal turbines which will have the potential to harness tidal energy. The company was founded in 2010 and is based in the United Kingdom. Kepler Energy was established in 2010 to exploit, with an exclusive licence, an internationally patented 2nd generation tidal turbine developed from 2007 to 2010 by three senior academics at the University of Oxford Department of Engineering Science. Initial work focused upon establishing the performance characteristics of the rotor at 1:20 prototype scale, financed initially by the University, and then, for a second time, with more detailed instrumentation for stress and other measurements, financed by the Technology Strategy Board and by the company`s shareholders. Subsequent work has involved developing designs for full scale turbines, combined into tidal fences, undertaken with supply chain partners who are each leaders in the offshore and tidal energy arenas.
Kepler has no direct economic competitors in providing access to tidal stream resource where first generation turbines are not economic, and represents the first disruptive technology to positively impact on the economics of power production from this predictable source of sustainable energy. This is because the Kepler turbine is very different to all others on the market - it is larger, scaleable, intrinsically more powerful and patented. It has been optimised to open up new resources in low tidal stream velocity flows (below 2.5 m/s), the most commonly found current strength in coastal waters across the globe. The rotor is deployed in standardised units within a tidal fence arrangement, with the key advantage of generating revenue incrementally during the construction phase, to provide certainty in realising expected equity returns.
The University of Oxford Department of Engineering Science has developed an innovative, second generation tidal turbine (THAWT). Testing has successfully demonstrated potential effectiveness significantly higher than existing axial flow designs.
The Kepler Transverse Horizontal Axis Water Turbine (THAWT) is designed to be operated either in arrays or in stand-alone form. It is highly effective and efficient in terms of extracting the maximum power from given stretches of tidal or river current. Theoretical analysis and modelling, confirmed by testing, has shown outputs several times higher than those achievable by propeller type turbines placed in the same site. This advantage arises from (1) the greater rectangular swept area of a THAWT rotor compared with the depth limited circular swept areas of multiple propeller type rotors and (2) the fact that greater powers can be extracted from tidal flows by optimising the blockage ratio (swept area of turbine divided by flow area). Theoretical analysis has shown that, because of the free water surface in tidal flows, tidal turbines, unlike wind turbines, are not subject to the Betz power limit. The THAWT rotor design is better suited to this optimisation than conventional propeller type turbines.
The structure itself is novel, the blades themselves being configured so that the turbine rotor (called a triangulated stressed truss) needs no enveloping supporting structure (the truss structure feature is patented in key global markets). This leads to lower parasitic drag, and hence low power losses and simple, cheap construction with longer rotors across the flow. This is in contrast to all other transverse horizontal axis turbines on the market, which are limited in their power output by their lack of structural rigidity and consequent limited maximum size
The basic generating unit comprises two turbines with a central direct drive generator, with only four supporting bearings and three foundation supports required. As a consequence of this design, the weight of the unit will be significantly lower than that achievable by propeller type units. The design philosophy throughout is focused on extreme reliability combined with ease of installation and maintenance with minimal or no ecological impact. The illustration at the head of the page shows one possible configuration, a THAWT turbine assembly in fence format. The transverse horizontal axis configuration works equally well with flow from either direction and hence requires no adjustments as the tide changes direction.
The turbine is scalable to suit different marine sites. A typical turbine rotor would be 10 m diameter and 60 m long sited in a tidal flow with a mean depth of 20 m. Documented flume tests on a scale model have shown that the basic 10m diameter, 120 m long unit (two turbines with one generator) should generate more than 4.4 MW at a water velocity of 2m/sec, and more than 5.2 MW at a water velocity of 2.5m/sec. In addition, the unit will operate with reasonable efficiency at low water velocities.
The simplicity of the design – only one rotating unit, with no yaw mechanism, no complex pitch changing mechanisms and with electrical equipment in a dry column – means that the Kepler turbine arrays will have very significantly lower life-cycle costs than those of first generation machines.
The characteristics (in terms of swept area and ability to exploit blockage) of the turbine in tidal fence format (for example in 20m of water, with peak spring flows of 2m/sec) are such that it should have around two to three times the power output of an array of conventional axial turbines, with levelised costs proportionally even lower. As an example, a 14 km tidal fence would have a peak output in excess of 600 MW and is likely to have lower levelised costs than offshore wind.