A solar balloon is a
balloon
A balloon is a flexible bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, and air. For special tasks, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), or light so ...
that gains
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
when the air inside is heated by
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre ( ...
, 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
A hot air balloon is a lighter-than-air aircraft consisting of a bag, called an envelope, which contains heated air. Suspended beneath is a gondola or wicker basket (in some long-distance or high-altitude balloons, a capsule), which carries ...
. 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
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
, 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.
Theory of operation
Generating lift
![Infarot 9](https://upload.wikimedia.org/wikipedia/commons/d/d4/Infarot_9.jpg)
Raising the air temperature inside the
envelope
An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.
Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a sh ...
makes it less dense than the surrounding (ambient) air. The balloon floats because of the buoyant force exerted on it. This force is the same force that acts on objects when they are in water and is described by
Archimedes' principle
Archimedes' principle (also spelled Archimedes's principle) states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces. Archimedes' ...
. The amount of lift (or
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
) provided by a hot air balloon depends primarily upon the difference between the temperature of the air inside the envelope and the temperature of the air outside the envelope.
The lift generated by 100,000 ft
3 (2831.7 m
3) of dry air heated to various temperatures may be calculated as follows:
:
The
density of air
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...
at 20 °C, 68 °F is about 1.2 kg/m
3. 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 m
3 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.
Solar radiation
Insolation is a measure of
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre ( ...
energy received on a given surface area in a given time. It is commonly expressed as average
irradiance in watts per square meter (W/m2).
Direct insolation
Direct insolation is the insolation measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the ...
is the solar irradiance measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the sky). Direct insolation is equal to the
solar constant minus the atmospheric losses due to
absorption and
scattering. While the solar constant varies with the
Earth-Sun distance and
solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
s, the losses depend on the time of day (length of light's path through the atmosphere depending on the
Solar elevation angle
The solar zenith angle is the zenith angle of the sun, i.e., the angle between the sun’s rays and the vertical direction. It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between th ...
),
cloud cover,
moisture
Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...
content, and other
impurities.
Over the course of a year the average solar radiation arriving at the top of the Earth's atmosphere is roughly 1,366
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s per
square meter
The square metre ( international spelling as used by the International Bureau of Weights and Measures) or square meter ( American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square ...
[
] (see
solar constant). The radiant power is distributed across the entire
electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
, although most of the power is in the
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
portion of the spectrum. The Sun's rays are
attenuated 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
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
absorbs all the radiation that hits it. Real world objects are gray objects, with their absorption being equal to their
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
. 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.
Estimating energy received
![Great circle hemispheres](https://upload.wikimedia.org/wikipedia/commons/0/04/Great_circle_hemispheres.png)
If the balloon is imagined as a
sphere
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is th ...
, the sunlight received by this sphere can be imagined as the cross-section of a cylinder with the same radius as this sphere, see diagram. The
area of this circle can be calculated via:
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/m
2, 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
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
at 15 °C at ISA (
International Standard Atmosphere
The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a ...
), air has a density of approximately 1.22521 kg/m
3. The
density of air
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...
decreases with higher temperatures, at the rate of around 20 grams per m
3 per 5 K. Around 1 kilojoules of energy is needed to heat 1 kilogram of dry air by one kelvin (see
heat capacity
Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Heat capacity ...
). So, to increase the temperature of 1 m
3 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 m
3 of air by around 24 grams. On a clear day with a black body surface of 1 m
2 perpendicular to the Sun and no heat loss, this would take a little over 6 seconds.
Estimating rate of energy lost
Below is the energy balance equation of the rate of energy lost of a solar balloon when drawing the boundary line around the balloon. The Solar Balloon experiences heat transfer due to convection and heat transfer due to radiation.
File:Estimated Rate of Energy Loss of a Solar Balloon.jpg, Energy Balance Equation for Solar Balloon
Ėout= tσπr
2(TS4-TF4) + hπr
2(TS-TF)
Estimated Change in Entropy
Tds=du+PdV
Δs = ∫(cv/T)dT + Rgasln(V2/V1)
Δs = cvln(T2/T1)
Equilibrium
The system is in equilibrium when the energy lost from the balloon through convection, radiation and conduction, equals the energy received through radiation from the Sun.
History
In 1972,
Dominic Michaelis, a British architect and the inventor of many solar utilities and projects, invented and built the first solar balloon, with a clear external surface and dark, heat-catching internal walls.
Manned flight
The first human carrying pure solar balloon flight was made on 1 May 1973 by
Tracy Barnes
Charles Tracy Barnes (August 2, 1911 – February 18, 1972) was a senior staff member at the United States' Central Intelligence Agency (CIA), serving as principal manager of CIA operations in the 1954 Guatemalan coup d'état and the 1961 Bay of P ...
in his balloon 'Barnes Solar Firefly Tetrahedron'. This balloon was made from a spiral tube of fabric that was formed into a
tetrahedron
In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all th ...
.
Dominic Michaelis is recorded as having owned the first pure solar balloon in Europe. This balloon was flown by
Julian Nott
Julian Franklin Keith Nott (born 23 August 1960) is a British composer and conductor, mostly of animated films. He is known for his work on ''Wallace and Gromit'' and ''Peppa Pig''.
Biography
Nott was born in Marylebone, London, and was edu ...
across the English Channel. 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.
First antarctic solar weather balloon flight
The first 100% solar weather probe, named Ballon ORA, was launched from the French
Antarctic Dumont d'Urville Station
The Dumont d'Urville Station (french: Base antarctique Dumont-d'Urville) is a French scientific station in Antarctica on Île des Pétrels, archipelago of Pointe-Géologie in Adélie Land. It is named after explorer Jules Dumont d'Urville, wh ...
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
A lifting gas or lighter-than-air gas is a gas that has a density lower than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in lighter-than-air aircraft, which include ballo ...
,
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
or
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
, would be precious. The flight was a success, approaching . The savings do not only concern the lifting gas in itself. The ORA Balloon alleviates the need for the transportation, in and out, of the heavy gas canisters.
Use in planetary exploration
California Institute of Technology's Jet Propulsion Laboratory has conducted a study on the use of solar balloons on several planets and moons in the solar system, concluding they are a viable option for Mars, Jupiter and Saturn.
Safety
Planning and airspace permission may be required by local or national airspace authorities.
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
Image:cm529_sb_tube.jpg, A tube-shaped solar balloon made from garbage bags
Image:cm29_sb_standard1.jpg
References
External links
History and current development on solar ballooningEOSS-43: "Mars Micro Balloon Probe" Study of using a solar balloon on Mars*{{cite web , url=http://www.solar-balloons.com/ , title=Make and fly your own , archive-url=https://web.archive.org/web/20110810141507/http://www.solar-balloons.com/index.html , archive-date=2011-08-10
Register solar balloons and print tracking tagsApril 1982 Popular Mechanics article about flight over the English Channel
Balloons (aeronautics)