Abstract: An differential expansion system and method for balloon buoyancy control which includes a zero-pressure envelope and a superpressure envelope. A gas transfer device is positioned between the envelopes to facilitate the transfer of lift gases that are lighter than air between the envelopes. The superpressure envelope and the gas transfer device can be located inside the zero-pressure envelope. Alternatively, the zero-pressure envelope, the superpressure envelope, and the gas transfer device can be located inside a zero-pressure envelope.

Abstract: A photovoltaic power source includes a shadowing timer comprising at least one shadowing member (i.e. vane element) secured proximal to an array of photovoltaic solar cells electrically connected in series. The shadowing members are arranged such that they shadow individual solar cells or rows of cells as the sun moves through diurnal and annual cycles. The shadows cast by two or more vanes can modulate the array output power on two independent time scales: time of day and time of year. Because the individual solar cells are connected in series, a small shadow significantly reduces the array output power, thus enabling fine-scale temporal control.