Some key Isentropic AWE work

To EnerygKiteSystems,                                                                                             July 20, 2009

The following documents [see below*] are a pretty good summary of where we got to in 2004. The main inventions were all made by our CTO, Jon Howes, who was previously Head of Loads and Dynamics at the UK CAA - consequently he knows quite a bit about aero structures, control, and fatigue. The document that compares VAWT [Variable Area Wind Turbine] vs a conventional turbine was the result of our most detailed theoretical model and shows the key points. The main conclusion was that the optimum cable length was about 9 times the glider width and you could generate about 3 times the power of a fixed turbine blade - beyond this cable length the cable drag overcame the benefits of more swept area. We were assuming solid gliders (no flaps etc..) and steel cables as we had concerns about the life of the more exotic materials like DyneemaŽ.

The key features we liked about this proposal was that the control of the gliders flight path is totally automatic as any deviation away from the main circular path is self correcting (see patent description for more detail). This means no clever electronics or control systems to go wrong. The power is generate by the models flying ahead of the arms on the generator and effectively leading them by a certain angle - so fairly simple all round. You control the length of the cable with torque, this automatically meant that if the wind drops the gliders wind in and this keeps generating lift so they fly in, likewise they automatically launch when the wind speed is high enough to support them. The The gliders are flying at about 1/3 speed of sound to avoid compressibility issues and keep the torque right down. The loads are still pretty ferocious and we were looking at 2inch steel cables for bigger versions.

The bits we did not like--and still do not like--is that the glider and cable will see a pretty horrible load cycle (much worse than a conventional aircraft) as each revolution you will go from full load to part load and back again. This is equivalent to continual take-off/landing events but every couple of seconds. We think the fatigue issues are substantial. We had concerns about complexity compared to a conventional wind turbine. You ideally want something that can be left in place and not worried about - it seemed that with cables you will always have a higher complexity and hence maintenance requirement. Lastly we were worried about cable wear at the base and around the rollers. None of this are insurmountable, but we could not convince ourselves that it gave a sufficient advantage over conventional turbines to pursue - especially when no one else believed us or saw the need for wind turbine structures that could be based offshore.

As usual with anything new we were rejected by the UK DTI - I have attached their rejection letter as an advert for what happens when you think up something really different. The cable-wear issue is a fair point, but all of the aerodynamic stuff is wrong. The irony is that the UK government (via the Energy Technologies Institute) is now spending a small fortune trying to develop offshore floating turbines.

On the physical side we only did a small amount of small scale model work, which showed that you need some torque control on the line for controlled launching and recovery and the launch gear needs to be reasonably slippery (low profile). We did have some designs for a multi-stacking one, where each wing launches from the top of the one below (too complex) and for one with more than two wings (too much risk of interference between them).

We probably should have done some more testing, but for the reasons mentioned above and our small company size, we let this drop.

Anyway I thought you might like to pass it on to anyone you think might be interested..

Best regards,
James Macnaghten
Isentropic Ltd
+44 845 2062070
+44 7968 110946