DC-DC converter/regulator lead-acid battery charger design question
I'd start off with a buck regulator that is capable, when the input voltage is insufficient, of just acting as a milli ohm short. This gets you 15v for all voltages greater than about 16 volts going in. 

For slow speeds read on....

That's a decent beginning. Then you have to ask yourself how you are going to get 45 watts from an inefficient bicycle dynamo at slow speeds.

After a while, you might decide that theoretically you'll never be able to pedal hard enough from a standing start and hopefully soon realize that your spec is unrealistic and impossible to achieve.