Product Code Database
A solar balloon is a balloon that gains buoyancy when the air inside is heated by solar radiation, usually with the help of black or dark balloon material. The heated air inside the solar balloon expands and has lower density than the surrounding air. As such, a solar balloon is similar to a hot air balloon. Usage of solar balloons is predominantly in the toy market, although it has been proposed that they be used in the investigation of planet Mars, and some solar balloons are large enough for human flight. A vent at the top can be opened to release hot air for descent and deflation.
The lift generated by 100,000 ft3 (2831.7 m3) of dry air heated to various temperatures may be calculated as follows:
| 68 °F, 20 °C | 1.2041 kg/m3 | 7517 lb, 3409.7 kg | 0 lbf, 0 kgf |
| 210 °F, 99 °C | 0.9486 kg/m3 | 5922 lb, 2686.2 kg | 1595 lbf, 723.5 kgf |
| 250 °F, 120 °C | 0.8978 kg/m3 | 5606 lb, 2542.4 kg | 1912 lbf, 867.3 kgf |
The density of air at 20 °C, 68 °F is about 1.2 kg/m3. The total lift for a balloon of 100,000 cu ft heated to (99 °C, 210 °F) would be 1595 lbf, 723.5 kgf. 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). The precise amount of lift provided depends not only upon the internal temperature mentioned above, but the external temperature, altitude above sea level, and humidity of the surrounding air. On a warm day, a balloon cannot lift as much as on a cool day, because the temperature required for launch will exceed the maximum sustainable for the envelope fabric. 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.
Over the course of a year the average solar radiation arriving at the top of the Earth's atmosphere is roughly 1,366 per square metre Satellite observations of total solar irradiance (see solar constant). The radiant power is distributed across the entire electromagnetic spectrum, although most of the power is in the visible light portion of the spectrum. The Sun's rays are attenuation as they pass through the atmosphere, thus reducing the insolation at the Earth's surface to approximately 1,000 watts per square meter for a surface perpendicular to the Sun's rays at sea level on a clear day.
A black body absorbs all the radiation that hits it. Real world objects are gray objects, with their absorption being equal to their emissivity. Black plastic might have an emissivity of around 0.95, meaning 95 percent of all radiation that hits it will be absorbed, and the remaining 5 percent reflected.
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:
\mathrm{Area} = \pi \times (5m)^2 \approx 78{.}54m^2
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. The density of air decreases with higher temperatures, at the rate of around 20 grams per m3 per 5 K. Around 1 kilojoules of energy is needed to heat 1 kilogram of dry air by one kelvin (see heat capacity). So, to increase the temperature of 1 m3 of air (at sea level and at 15 °C) 5 °C requires around 5 °C * 1 kilojoules/(kilogram*kelvin) * 1.225 kilograms = 6.125 kilojoules. By doing so, you've reduced the mass of 1 m3 of air by around 24 grams. 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.
Ėout= tσπr2(TS4-TF4) + hπr2(TS-TF)
Δs = ∫(cv/T)dT + Rgasln(V2/V1)
Δs = cvln(T2/T1)
Manned flights carry special risks. Unexpected clouds pose a serious risk, akin to regular hot air ballooning without reserve fuel. Solar balloons can descend rapidly when cooling occurs, making ballast very important.
==Gallery==
|
|
