> > There used to be a reasonable sized experimental ducted wind turbine at > Waikaretu > (~80km south of you, 40km north west of me). Don't know what became of it= .. > > Ducted wind turbines pop up ongoingly, and then vanish. I'm thinking of trying one for somewhat different reasons to the usual. At low wind speeds power in air is abysmally low. At 10 m/s it's 600 Watt/m^2 At 4 m/s it's ~=3D 40 Watts/m^2 At 2 m/s ~=3D 5 W/m^2 You can recover 20-30% of that in a good small WT so at say 25% that's 150, 10, 1.25W. 1.25W average over 24 hours is 30 Wh. If you can save and deliver half of that over 6 hours it's 2.5W continuous for 6 hours or about 300+ lumens of LED light. That's extremely useful in many developing country applications. But a 1 m^2 (1.1 m diameter) WT has its issues. 2 m/s is much much much more available at low heights above ground and in a range of environments. At such low power levels the required strength and rigidity is minimal, but a long spindly rotor subjected to gusts of say 5 m/s has to deal with 15 x the power and at 10 m/s 100+ times the power. All sorts of overspeed regulation systems are possible but when the blades get long and gangly it gets harder to protect them - and robust 1m+ diameter blades are financially unattractive (at least) at such power levels. So too does a ducted unit, but if you can concentrate the flow to say 0.1 m^2 (350 mm dia) to 0.2 m^2 (500mm dia) or even smaller then air velocity increase inversely and 2 m/s becomes 10 m/s to 20 m/s (getting silly) and is much easier to handle with small solid rotors. A very significant advantage of small rotor areas with proportionally high velocities is that rotor velocity increases both with decreasing diameter in a given wind speed for the same tip-to-wind-speed (TSR) ratios, AND increases linearly with absolute velocity. So a 1m+ rotor may spin at say 180 RPM with a TSR of 2 in a 2 m's wind. At the same TSR with an area concentration ratio of 4 the rpm doubles due to diameter reduction and increases by 4x due to velocity increase so rotor speed is 8x or ~~~~ 1400 RPM in this case. For very low RPM (in the 100's of RPM range) alternators need to either have many poles to get reasonable voltage or a high number of turns on each coil. This has 'costs' not just in copper used and resistance but in eg making coils larger so harder to maximise flux with given magnets. Higher RPM (within limits) increase the range of small alternators that can be utilised* and/or make it easier and cheaper to fabricate low cost alternators. (* or more usually, small motors suited to use as alternators). Rotors such as are found in small pedestal and table fans or "muffin fans" or ones of similar ruggedness (bits of vent Al or plastic or carved from wood) become useful. Instead the concentrating shroud needs to become robust or at least high-velocity survivable, or be unimportant to sacrifice occasionally. Paper or cloth or plastic or .... may be adequate. The tradeoffs may well not be worthwhile. But, they may. TBD. That's impressive about hydro turbines. It's nice to see machines where > efficiency > works out great, and haven't need any great technological advancements. > Did some > investigations on large AC motors some years ago and found them to be > something > like 96-98% efficient =3D good enough, focus on improving other things. > Top grid-tie inverters that convert energy from PV panels at voltages of typically 100-1000 VDC into AC mains voltages achieve efficiencies usefully above 95%. AFAIR I've seen claims of up to around 98%. 1 to 2 years ago (from memory) Google was promotong competitive entries for units that increased then available efficiencies. Russell --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .