Solar balloon

In reality, the air contained in the envelope is not all the same temperature, as the accompanying thermal image shows, and so these calculations are based on averages.

For typical atmospheric conditions (20 °C, 68 °F), a hot air balloon heated to (99 °C, 210 °F) requires about 3.91 m3 of envelope volume to lift 1 kilogram (62.5 cu ft/lb).

Also, in the lower atmosphere, the lift provided by a hot air balloon decreases about 3% for each 1,000 meters (1% per 1,000 ft) of altitude gained.

Direct insolation is equal to the solar constant minus the atmospheric losses due to absorption and scattering.

For example, the energy received by a spherical, 5 metre radius, solar balloon with an envelope of black plastic on a clear day with direct insolation of 1000 W/m2, can be estimated by first calculating the area of its great circle: Then multiplying this with the emissivity of the plastic and the direct insolation of the Sun: 78.54 * 0.95 * 1000 = 74,613 Watts At sea level at 15 °C at ISA (International Standard Atmosphere), air has a density of approximately 1.22521 kg/m3.

On a clear day with a black body surface of 1 m2 perpendicular to the Sun and no heat loss, this would take a little over 6 seconds.

Records compiled for the FAI show that on 6 February 1978 Iranian Frederick Eshoo also made a solar flight in a balloon called Sunstat.

This used a standard balloon design, but used clear plastic on one side, allowing the Sun's radiation to reflect off the inner surface, heating the inside air.

[6] The first 100% solar weather probe, named Ballon ORA, was launched from the French Antarctic Dumont d'Urville Station in January 2011 by a joint team of students, scientists and engineers.

The idea was to assess the feasibility of using solar balloons as probes in remote area, where saving the use of lifting gas, helium or hydrogen, would be precious.

Unexpected clouds pose a serious risk, akin to regular hot air ballooning without reserve fuel.