Not sure where to put this so I’ll reply to myself…
Tensairity Torus
I’ve been struggling to get this idea off the ground (pun intended) and (another pun) feel like I’m reinventing the wheel a bit here (given Roddy’s original daisy designs) but, for what they’re worth, here a few images and clips of where I’ve gotten to (and probably about as far as I’ll ever get!)
Creating or procuring inflatable tori of suitable dimensions isn’t easy…the solution I came up with was 2 litre sparkling water bottles (saved up over a year or two!) - dried and stuck in the freezer and then capped, which, assuming that the bottles were not significantly dented initially and a 30 - 35 deg temperature difference between the freezer and ambient air temperature when flying, might give a pressurisation of 1.1 bar. The bottles are held in position in sleeves made of sliced bottles. The compression elements are 20mm fairly stiff UPVC ducting pipe. In a true tensairity structure the pipe and bottle rings would have been have been uniformly bonded to one another, here they are attached intermittently, and the bottle ring is tensioned with strings looping along the ring. Originally I tried a 24 bottle ring but this wasn’t stiff enough: reducing to 18 bottles produced a reasonably strong ring of about 2 m diameter.
Wanting to stick with a inflatable transmission but running low on water bottles, I tried a milk container ladder, but they weren’t really up to the job, and I just ended up with a mess of tangled lines – see photos of triple torus on ground! Scaling back to a single turbine and using a 25 metre MDPE water pipe we at least managed to get airborne and spin round a bit (in an approx 20-25 km/hr breeze).
Obviously things are far from ideal in terms of aerofoil shape and the bicycle wheel attachment point was not spinning as freely as I’d hoped, and I’m not sure about any friction between the pipe and the kite line along it’s length – though I guess that if there was any major problem the kite would have gone fly about.
So…is there any point to this?? (Here commences a rant…so please disregard if you prefer.) The point does relate to some of the other concerns on the forum…the software vs hardware discussion and, more fundamentally, is there any future to AWE outside, perhaps, of some niche applications. I wonder what the debate would have looked like a century or so ago in relation to hydroelectricity had people had the same access to IT as we have today. I expect that the software enthusiasts would have been proposing a variety of ingenious fishlike devices packed with semiconductors weaving through fast flowing streams, reeling out yoyo lines or sending back electricity (in rather limited quantities) along their tethers. For better or worse, our predecessors, not blessed with microchips (or any electronics to speak of) resorted to the hardware approach, and a pretty extreme hardware approach at that, involving tens of thousands of tons of earth and concrete and some pretty unintelligent, slowly spinning turbines. (I’m not getting into a debate about the virtues/vices of hydroelectricity…but one does need to bear in mind that the lost habitats and displaced people have to be set against either the lack of electricity or the increased fossil fuel combustion which would have resulted had the rivers not been dammed.)
If one were to sum up the philosophy behind hydropower in a couple of words: massive and passive come to mind. Their applicability to the Hoover Dam or the Three Gorges is pretty self-evident, but the situation isn’t that dissimilar to any of the other proven forms of power generation, be it solar arrays, tidal barrages or conventional wind turbines (as well as nuclear and fossil fuel). Of course, all these forms of power generation are now monitored, controlled and integrated electronically, but none involve IT as an intrinsic part of the energy conversion process – they were all conceived of and in most cases implemented (at least initially) in a non-computerised world.
So is AWE a completely different beast? I’ve no pretensions to prophecy, nor to expertise, but it seems to me unlikely that, if it is to become a significant part of electricity production, it can avoid going down the same route. “Massive” may not seem the right word to apply to airborne structures – “bulky” might be better, but the idea is the same: in order to harness significant quantities of power above the reach of conventional wind turbines one will need to have large objects aloft (even allowing for as low a turbine solidity and as high a concentration at the blade tips as conceivable.) I’m also pretty skeptical about relying on active controls for safety critical functions – that may be OK for high performance aircraft (which, of course, are what Makani and others have been designing) but not for large scale AWE. In a very uncertain world, power regulation and overwind protection need to be as passive as possible – turbines and other airborne components need to deform or reorientate themselves to survive weather extremes and to be sited far away enough from population centres to mitigate the fallout from catastrophic failure.
Arrays: to array or not to array…is that the question? Again my own (limited) experience of tangled kitelines makes me a bit queasy when it comes to having lots of AWE components flying about in close proximity (and especially if one is relying on software to keep them out of each others way.) More fundamentally, one needs to consider where one is siting the installations. While the prairies of North America, the Siberian tundra, the Sahara or the Australian outback could host large arrays well removed from population centres, in the case of Europe and other densely populated regions one really needs to be focussing on what works offshore. Offshore, the case for single large structures over multiple smaller ones is compelling, and becomes ever more so the further out and deeper one goes.
…So how does this relate to a 2 metre water bottle torus? The principal question is obviously one of scalability. The key advantage of a torus as the backbone of the architecture is that it frees the blades from a structural role – ie. vis-a-vis a conventional HAWT where the blades are mainly there to support the blade ends – so that the blades can be optimised aerodynamically (and for longevity or any other relevant considerations). As far as the mass/swept area ratio and whatever unfavourable cube-square law it follows, this could be overcome (at least in part) by using a bit of the electricity generated to heat the torus or, heaven forbid and the Hindenburg not withstanding, filling it with hydrogen!..but perhaps not so outlandlish for tori tens of kilometres off the coast and flying at over 1000m, widely separated and protected as far as possible from lightning strikes and terrorist attacks.
Anyway…enough of my fantasy/rant…I agree in principal with the advice to start small and scale up gradually, but I think I’ve reached the limit of what I can do with improvised materials in my loft. If anyone thinks it worthwhile to obtain/manufacture a better torus – and to improve the transmission system (either Daisy fashion with scaled-down tori or some other way) I’d be interested – and would wish them luck!