If there is a theme to these posts, it is learning about sustainable energy. Staring at a wind farm or an array of solar panels can be instructive, but some personal experience is always useful, even with the limitations imposed by small models. These two models were made with materials which were lying around where were a length of drain pipe, some plywood offcuts and a random selection of Meccano. If I ever repeat this exercise, I would opt for a lighter construction and a small alternator as a load. An attempt at making a dynamometer was not wholly successful.
Wind in urban and rural areas is often turbulent and gusty with frequent changes of direction. There are a few small horizontal wind turbines in our area, the only ones which give the impression of running continuously when the wind is blowing are those mounted on tall masts. One of the attraction of vertical axis turbines is that the can adapt to frequent changes in wind direction without having to “hunt” for the wind, another is they are quite simple machines. Many Savonius turbines used in the Caribbean to drive irrigation pumps are just a 40 gallon oil drum which has cut in two along and welded back together. I am not fully convinced that rotating machinery has a place in the urban environment, however, the Savonius design has some inherent speed limitation. For these reasons I opted to mess with vertical axis designs.
The advantage of models is that they can be moved around on a bicycle and tried in various locations. Despite mounting the Savonius model on a 5 metre pole, it never turned continuously in my back yard whatever the wind was doing elsewhere, that was a useful lesson. About 7km to the west of where I live is a small airfield where the remains of some of the Second World War defences are accessible. The roof of a pillbox which had been build on an embankment is about 10m higher then the surrounding flat terrain, here the Savonius model spun freely and smoothly. The wind there is both smooth and steady. In contrast, performance on the upper level of multi-story car parks in the city centre was disappointing, the turbine would spin during gusts, then stay sullenly stationary for at least 30 seconds before starting up again. Performance on jetties along the sea front was good if the wind was blowing of the sea and poor if it was coming from the land. I did not investigate too closely, but the wind at the top of cliffs appeared to be complex, I may have formed a different opinion if the turbine had been on mast rather than at head height. It is important to be careful in public places.
I had no desire to draw attention to myself, but with the exception of a large Alsatian called Trooper I was allowed to engage in this bizarre activity without interference.
I had been intrigued by the Darrius design of turbine, however, the model refused to do anything useful, which with hindsight is probably a good thing. The most probable cause was poor construction which did not allow for setting an appropriate angle of incidence on the vertical blades. An afternoon spent figuring out the aerodynamics of this type of machine was useful.
Whilst this design looks simple, the aerodynamics and resulting stresses are complex, had the thing worked, it would have probably shaken itself to bits. I’m not certain of my facts, but I think that this design is capable of over-speeding under no-load conditions.
I became somewhat more absorbed in this project than I intended and when time permits I want to build another Savonius model. If I proceed with a second attempt, the design will incorporate a small alternator to act as a load and provide some indication of output. The first attempt used a fluid clutch attached to a torsion spring, this was a good illustration of the rule which says “if it looks complicated, it’s probably wrong”. Secondly, the rotor would consist of three segments each 120 degrees apart, this would smooth the output torque and prevent the rotor getting “stuck”. Most of the time the Savonius was self-starting, but occasionally act like a weather vane until given a sharp prod with a finger. I have seen some beautiful picture of Savonius turbines where rotor is a spiral which suggests the possibility of making a turbine into a garden feature.
Both models are being dismantled for firewood and the recovery of the Meccano bits.