Is there a significant loss in a voltage doubler circuit?
If your generating apparatus is producing AC, it's easy to build a charge pump using diodes and capacitors; a voltage doubler is the most common form, but it's possible to build higher multiples as well.  The steady-state current drawn from the generator will be doubled but the voltage will be somewhat less than double.  Some of the loss will be in the diodes, but some will also occur because capacitors will be called upon to supply current continuously but are filled intermittently.  Consequently, by the time one capacitor reaches maximum voltage, the other capacitor will have sagged somewhat from its maximum.