ASHRAE Journal - July 2020 - 13

TECHNICAL FEATURE

limited amounts of water and gases that create human
breathing air may be present. With space agencies
interested in sending humans to the moon and Mars,1,3
understanding their atmospheric conditions would be
ideal to modify current HVAC technologies for facilities
there.
The moon's atmosphere is far less dense than Earth's
and is comparable to the conditions on the outer edges of
the Earth's atmosphere encountered by the International
Space Station (ISS) in low-Earth orbit. Gases mainly found
on the moon's atmosphere include helium-4, deuterium
(hydrogen-2), methane, ammonia, carbon dioxide, and
differing isotopes of neon and argon. Gravity is present
on the moon, although it's 16.5% of the Earth's gravity.7
Heat transfer and rejection equipment developed for the
ISS may be a starting point for developing environmental
control equipment for lunar facilities.
In contrast, the atmosphere on Mars is mostly composed of carbon dioxide, nitrogen and argon,8 and
the gravity on the planet is about 38% that of Earth's.9
Atmospheric pressure on Mars is much lower than
Earth's, and the planet's increased distance from the
sun, compared to the Earth, results in lower surface
temperature.10 Assessing each planet's surface conditions for human inhabitation is necessary to develop and
manufacture HVAC equipment for the respective planet.
Harsh environmental conditions necessitate accurate
thermal management systems for spacecraft.11 Several
studies have been devoted to direct and inverse heat
transfer techniques for measurement and monitoring of the spacecraft's surface temperature and critical
parameters under extreme conditions such as atmospheric entry12 and other similar applications.13 - 15
While keeping spacecraft safe during space missions is
critical and challenging, maintaining a livable condition for the astronauts is even more complex. On Earth,
HVAC systems are considered important because they
provide thermal comfort for the occupants of built
environments. For outer space applications, however,
proper environmental control is not just about comfort,
but survival. The ISS is an excellent example for environmental control needs in space, with three to six crew
members onboard for long durations. Providing a livable and comfortable condition for the crew members is
critical at ISS, and various challenges in this regard must
be addressed including temperature control, ventilation, oxygen production and water.

A Living Example: The International Space Station
The ISS is at an altitude of about 260 miles (418 kilometers) and orbits the Earth in about 92 minutes,6 with
temperature extremes for deep space as noted earlier.
A signiﬁcant amount of heat is also generated inside ISS
by the hundreds of electronics and equipment onboard,
which lead to a massive cooling load.6 The large temperature gradients as the result of the harsh environment and signiﬁcant internal loads necessitate effective
temperature control systems for ISS.
Temperature control at the ISS is performed using a
combination of passive techniques and an active thermal control system.6 Passive techniques include the use
of multilayer insulation (MLI), surface coatings with
selected radiative properties and heaters and heat pipes.
The active thermal control system consists of cold plates,
heat exchangers, ammonia and water as coolant ﬂuids
and radiators to maintain appropriate internal and
external temperatures.
The surface of the ISS, apart from windows, is covered
by MLI that slows down the heat transfer rate through
an effective use of multilayer radiation shields. MLI used
for ISS consists of an outer cover (aluminized biaxially
oriented polyethylene terephthalate), followed by several
reﬂector layers (double aluminized poly [4,4'-oxydiphenylene-pyromellitimide]), separated by polyester netting.16
The multilayer structure of the insulation allows nearly
100% reﬂection of incident heat ﬂux. However, it is not
adequate to maintain livable conditions inside the ISS due
to the large internal loads. An active thermal control system addresses this challenge (Figure 1).
The ISS active thermal control system consists of two
loops with water and ammonia as their working ﬂuids.
Water circulates through the interiors and use cold plates
and heat exchangers to absorb heat. The heated water
then transfers its thermal energy content to another loop,
which uses ammonia as the working ﬂuid. Ammonia, as a
widely used refrigerant, is selected due to its desired thermal characteristics and mainly because of its low freezing
temperature of −107.9°F (−77.7°C). The ammonia then
circulates in tubes throughout six large 6 ft by 10 ft (1.8 m
by 3 m) radiator panels and cools down before circulating
back to the interior. The radiator panels allow effective
heat dissipation to deep space.17
Ammonia is also used to provide cooling for the
photovoltaic panels that are responsible for electricity
generation. This is because the efﬁciency of solar arrays
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ASHRAE Journal - July 2020

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