Ever since I’ve been experimenting with Joule Thiefs I’ve done so with the intention of using the circuit for boosting a low voltage source to power a higher voltage circuit. So I think this Analog Devices ADP5090 is very useful for doing just that: powering circuits up to 200 mW from very low voltage power source. This chip requires 0.38 volts to start, but can then draw power from a source of less than a tenth of a volt. It has MPPT, maximum power point tracking, which matches its load to the power source. More information about this is right here.