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International Space Station


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NASA’s Marshall Space Flight Center in Huntsville, Ala., is responsible for the design, construction and testing of regenerative life support hardware for the International Space Station, as well as providing
technical support for other systems that will provide the crew with a comfortable environment and minimize the resupply burden.

The Environmental Control and Life Support System (ECLSS) for the Space Station performs several functions:
• Provides oxygen for metabolic consumption;
• Provides potable water for consumption, food preparation and hygiene uses;
• Removes carbon dioxide from the cabin air;
• Filters particulates and microorganisms from the cabin air;
• Removes volatile organic trace gases from the cabin air;
• Monitors and controls cabin air partial pressures of nitrogen, oxygen, carbon dioxide, methane, hydrogen and water vapor;
• Maintains total cabin pressure;
• Maintains cabin temperature and humidity levels;
• Distributes cabin air between connected modules.


Background
Earth’s natural life support system provides the air we breathe, the water we drink and other conditions that support life. For people to live in space, however, these functions must be done by artificial means.
The life support systems on the Mercury, Gemini and Apollo spacecraft in the 1960s were designed to be used once and discarded. Oxygen for breathing was provided from high pressure or cryogenic storage tanks. Carbon dioxide was removed from the air by lithium hydroxide in replaceable canisters. Contaminants in the air were removed by replaceable filters and activated charcoal integrated with the lithium hydroxide canisters.
Water for the Mercury and Gemini missions was stored in tanks, while fuel cells on the Apollo spacecraft produced electricity and provided water as a byproduct. Urine and waste-water were collected and stored or vented overboard.
The Space Shuttle is a reusable vehicle, unlike those earlier spacecraft, and its life support system incorporates some advances. But it still relies heavily on the use of consumables, limiting the time it can stay in space.
The International Space Station includes further advances in life support technology and relies on a combination of expendable and limited regenerative life support technologies located in the U.S. Destiny lab module and the Russian Zvezda service module. Advances include the development
of regenerable methods for supplying oxygen (by electrolysis of water) and water (by recovering potable water from wastewater).
These advances will help to reduce the cost of operating the Space Station because it is expensive to continue launching fresh supplies of air, water and expendable life support equipment
to the Station and returning used equipment to Earth.

Providing Clean Water and Air
The Space Station Environmental Control and Life Support System includes two key components – the Water Recovery System (WRS) and the Oxygen Generation System (OGS). The systems have been jointly designed and tested by the Marshall Center and Hamilton Sundstrand Space Systems International in Windsor Locks, Conn. They are packaged into three refrigerator-sized racks that will be located in the U.S. Lab of the Station.
The Water Recovery System provides clean water by reclaiming wastewater, including water from crewmember urine; cabin humidity condensate; and Extra Vehicular Activity (EVA) wastes. The recovered water must meet stringent purity standards before it can be used to support crew, EVA, and payload activities.
The Water Recovery System is designed to recycle crewmember
urine and wastewater for reuse as clean water.
By doing so, the system reduces the net mass of water and consumables that would need to be launched from Earth to support six crewmembers by 15,000 pounds (6800 kg) per year. The Water Recovery System consists of a Urine Processor Assembly (UPA) and a Water Processor Assembly (WPA). A low pressure vacuum distillation process is used to recover water from urine. The entire process occurs within a rotating distillation assembly that compensates for the absence of gravity and therefore aids in the separation of liquids and gases in space. Product water from the Urine Processor is combined with all other wastewaters and delivered to the Water Processor for treatment. The Water Processor removes free gas and solid materials such as hair and lint, before the water goes through a series of multifiltration beds for further purification. Any remaining organic contaminants and microorganisms are removed by a high-temperature catalytic reactor assembly. The purity of product water is checked by electrical conductivity sensors (the conductivity of water is increased by the presence of typical contaminants).
Unacceptable water is reprocessed, and clean water is sent to a storage tank, ready for use by the crew.
The Oxygen Generation System produces oxygen for breathing air for the crew and laboratory animals, as well as for replacement of oxygen lost due to experiment use, airlock depressurization, module leakage, and carbon dioxide venting. The system consists mainly of the Oxygen Generation Assembly (OGA) and a Power Supply Module.
The heart of the Oxygen Generation Assembly is the cell stack, which electrolyzes, or breaks apart, water provided by the Water Recovery System, yielding oxygen and hydrogen as byproducts. The oxygen is delivered to the cabin atmosphere while the hydrogen is vented overboard. The Power Supply Module provides the power needed by the Oxygen Generation Assembly to electrolyze the water.
The Oxygen Generation System is designed to generate oxygen at a selectable rate and is capable of operating both continuously and cyclically. It provides from 5 to 20 pounds (2.3 to 9 kg) of oxygen per day during continuous operation and a normal rate of 12 pounds (5.4 kg) of oxygen per day during cyclic operation.
The Oxygen Generation System will accommodate the testing of an experimental Carbon Dioxide Reduction Assembly (CReA). Once deployed, the reduction assembly will cause hydrogen produced by the Oxygen Generation Assembly to react with carbon dioxide removed from the cabin atmosphere to produce water and methane. This water will be available for processing and reuse, thereby further reducing the amount of water to be resupplied to the Space Station from the ground.

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Bron: www.nasa.gov