Dynohub with a drum brake - a hub of choice for all-weather no-maintenance utility bikes. It runs on cartridge bearings and is advertised as perfectly sealed against weather, but I can't confirm it because I haven't seen the internals myself (yet). The hub is totally silent. It does have some little mechanical drag (freely spun wheel stops sooner than the same wheel with a similar hub without the generator), but I didn't measure it.
The brake drum diameter is 90 mm (smaller 70 mm X-FDD version is also available), it brakes very well in my 20" wheels and I wouldn't be afraid to use it for larger wheels too. Overheating at long descents is not as big a problem as with the 70 mm variant because there is an additional centimetre of aluminium between the brake drum and generator magnets (which shouldn't exceed 80 °C), but even a larger thermal capacity can get filled up. Wheel removal is no problem, the cable unhooks without any tools and the reaction arm pulls out of its bracket easily. You can see my special modification for a cargo trike on the photo: the arm is anchored by an M6 screw which goes through a hole drilled in it. Brake maintenance is trivial, just adjust the cable stop and you're done. I expect the brake pads to last almost forever, spare ones can be bought if needed.
Nominal power is 3 W (there is also a 2.4 watt version). I have the hub laced in 47-406 mm (20×1,75") rear left wheel of a trike and a speedometer in the front 50-406 (20×2") wheel with a circumference of 1543 mm, the difference between the two wheels is negligible. For wheels of different size, divide the speeds below by circumference of my wheel and multiply by circumference of yours. Output current was rectified by a bridge of four diodes 1N4004 and smoothed by a capacitor.
Open circuit voltage rises to about 45 V at 35 km/h. Premature end of some of the curves was caused by tired engine.
Absolute maximum current is 570 mA.
The nominal 3 watts would be generated with a 28" wheel at 20 km/h.
Pn is nominal power of 3 W. The zigzag shapes of some of the curves are caused by measurement errors.
Optimum load is something around 30 or 40 Ω. If you draw more current, you get more power at lower speeds, but less at high speeds. Compared to Shimano and SON generators, Sturmey gives a bit less power at higher speeds and with more scatter than SON.
Source data to download (XLS, Excel 97).