European Air Revitalisation System:
A Cost Saving Option for International Space Station Operations
G.B.T. Tan
Thermal Control and Life Support Division, ESTEC
H. Preiss
DASA-Dornier
Contractors:
DASA-Dornier GmbH (D)
Funding:
Basic Technology Research Programme.
and General Support Technology Programme
harmonised with DLR and German national activities.
Introduction and background
Following the assembly of the International
Space Station (ISS) a serious attempt at a
long-term presence of human beings in space
is imminent. Supporting life in space is a
complex and costly undertaking. All essential
consumables like food, water and breathable
air have to be brought from Earth. However,
major cost savings can be achieved by recycling
waste water and revitalising air for re-use.
The baseline design adopted by the United
States for its portion of the ISS uses an
open-loop system for breathable air. The
exhaled carbon dioxide is filtered from the
cabin air and dumped into space. Gaseous oxygen
is re-supplied from Earth. Although the
first step in closing the air loop has been
planned by NASA for Node 3 of the ISS, built
in Italy by Alenia Spazio, Europe has additional
technology on offer in this field.
Since 1985 ESA has conducted research into an
air revitalisation system and this has led to
the development which allows the effective
closure of the oxygenloop. A technology
demonstrator (ARSD), suitable for a three-person
crew and consisting mainly of commercial
hardware, has been assembled within half
an ISS payload rack (ISPR) and has been
successfully tested. Figure 1 shows the rack
holding the air revitalisation system and
the individual assemblies.
Figure 1. The full prototype air revitalisation
system showing in the top three sections of the rack,
the CCA (upper) CPA (middle) and OGA (lower). Expanded
views of these latter elements are given in Figures
2,3 and 4.
System description
The system consists of a chain of three processes:
carbon dioxide concentration (CCA) (Figure 2),
carbon dioxide processing (CPA), (Figure 3)
oxygen generation (OGA), (Figure 4).
Figure 2. The carbon-dioxide concentration assembly.
Figure 3. The carbon-dioxide processing assembly.
Figure 4. The oxygen generation assembly.
The CCA is a two-bed cycling system,
alternately set to absorption and (steam)
desorption modes. The desorbed carbon dioxide
(99%) is fed via a buffer system into the CPA,
which is a two-reactor Sabatier system. Part
of the carbon dioxide reacts with hydrogen
to produce water and methane, reaching an
optimum carbon dioxide conversion efficiency
of 98%. The surplus carbon dioxide is waste gas.
The water produced by the air revitalisation
system, or water from an external source, is
fed into the OGA, a high efficient (fixed)
alkaline electrolyser which closes the oxygen
loop. The hydrogen by-product is recycled.
The methane is currently handled as waste gas.
To maintain a balance additional water has to
be supplied; hydrogen is lost through the
production of methane.
Operational costs savings
A seven-person crew produces 2500 kg of carbon
dioxide per annum and consumes 2150 kg of
oxygen. Taking into account the ISS transferable
residual oxygen from the Orbiter, an upload of
1900 kg oxygen has to be brought to ISS.
Assuming an upload cost of 22 000 Euro per
kilogram and a packaging factor of
1.25, this results in an annual upload cost of
60M Euro. Half of this may be saved annually
throughout the operational phase assuming that,
to preserve hydrogen balance, only half of the
carbon dioxide is converted.
Conclusions
The European air revitalisation system has been
developed and its technology demonstrated. A
crew of seven persons can be supported using an
ISPR volume. With reference to the current ISS
oxygen traffic model an upload saving equivalent
to 30M Euro (at 1999 price levels) annually may
be achieved.
Preparing for the Future Vol. 9 No. 2
Published September 1999.
