What blade profile have you decided upon?
What is the cl/cd?
Solidity?
Have you drawn out the blade profile at the various stations reynolds numbers?
What method will you emply for protecting the machine against high winds?..forget a mechanical brake, they are not reliable enough. They are for emergency shutdown only and used for brief periods. Forget electrical braking, it will fail eventually.
My suggestion for maximum energy caputure and production is a large swept area, greater than what the math spells out for your choosen gen size. The adjustments can be made later on to take this mod into account. Build a dual rotor gen and be happy with the cutin far lower than ANY other gen, and the higher efficiency. It is also best to run with a higher voltage than you'd expect. 48v is the optimum figure when other sources of RE come in such as PV and Hydro. I build 3 phase dual rotor and switch between start and delta wiring depending on the gen RPM. The switching must be done sensibly and not immediately, to counter killing relays and casuing undue stress in the gen. The nature of wind is not predictable and we cannot say for certain what its speed will be in x seconds from now. So a more relaxed approach to switching wiring schemes works best.
Run the 3 phases down the tower and into your rectifier block in the battery shed, then into charge controller and dump load. Now your cooking!
Matching blades to a given and known gen, is a science and not something to just jump in and try. Paying a little attention now to your design and the carving of a set of blades will pay you back many times more than a wind turbine that is just hacked togther. It will also make the difference on low wind days of having power, and not having any. Nobody likes to sit in the dark.
I have built many home made machines and they run along side my Lister CS. Everything is home made and to a high standard, I cant stand cheap engineering. Sliprings are made here too, no shortcuts and no commercial junk.
With a little care you will out perform any of the small scale machines, none of them have the build quality or longevity of a properly engineered machine. I guess most buyers will not spend the money. Its a bit like a Lsiteroid and a Lister, the same but different!
Above all, have fun with it. There is no greater thing to get you grinning than really free power.
Emerald
Emerald,
Thank you for the input and questions. I haven't given a lot of thought to the blade yet. I won't need blades for a while yet. What I
have envisioned for the first set of blades will be formed from steel and will use a section of the 4-1/2" O.D. X0.125" wall steel irrigation tubing I have in stock as the root. The airfoil will be sheet metal welded on. I will likely fill them with expanding foam to stabilize them and keep out water that could turn to ice and cause unbalance. I expect to bury an aircraft cable within the spar of each blade to keep it from launching from the hub if the root or spar fractures catastrophically anywhere along it's length, although the most likely place is at the connection area to the hub. If you have seen the historically important Smith Putman machine you see what I am roughly thinking of for two blades. As I said earlier above, diameter 36-40 feet. My solidity will be in the 10-15% range I think. Probably won't have a twist but will be tapered. I'm proposing 4 feet wide at the hub and 2 feet at the tip.
I agree that a mechanical brake is not to be relied on for speed regulation. I have it in the system as a way to stow the blade and to haul it to a stop quickly in case of an emergency. It will have an automatic mechanical activation which will be triggered by excessive vibration or unusual/uncontrolled yawing. It will also be under electrical control. My speed will be controlled by adjusting the blade pitch with an electric servo system. The blade can be feathered to not spin even if pointed straight at the wind. In addition, the azimuth is motor driven and the blade can be actively set to track a desired offset angle to the true wind direction if desired. I have no plan to try to slow the turbine by overloading the generator head.
Because I can actively adjust the blade pitch I am planning that my blade will be bigger as you suggest so that I can take better advantage of low-medium wind days. No sense having a machine that makes it's rated power only just before a storm. I expect to see a steady 3-5 kW happening most of the time with this machine and full power of 20 kW plus on the cold windy days in the winter when I need the power for resistance heaters.
I want to see several hundred volts AC, most likely 3-phase coming down thhe tower and 750 feet across the field to the shop where the controller and any active ratio transformer system, rectifiers and batteries will be located, in a heated building out of the weather.
No plans for a battery bank that can process the full capability of the turbine, nor inverters of that capacity. Batteries make 'free wind based electricity' more expensive than grid electricity. I may have a forklift battery and several 1 kW inverters for loads in the house that require precision AC. This battery can also be topped up by my diesel plants. No need for a dump load. The excess wind power when available either won't be generated if not needed (throttle back the turbine) or used to run a dedicated high wattage incandescent lighting buss in the shop in the summer for light, and also resistance heaters for space heating in the house and shop in winter. Look at that fool he leaves all the light on all the time!
I'm not actually planning to go off-grid entirely just now. That's a
big step. My main need for the big turbine is as an electric furnace to offest my crippling winter propane heating fuel expenses. Resistance heaters don't care about voltage and frequency.