ESA Document - Emits - ESA

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4.3.5.4.2 Power storage
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 303 of 422
4.3.5.4.2.1 Flywheels
Flywheels are mechanical batteries that convert energy to mechanical motion and when needed,
convert that motion back to energy. They have a really high charge/discharge efficiency (85 to
95%) that also applied for high power demand.
The lifetime is estimated to be over 20 years.
The operating temperature range is interesting for Martian surface applications compared to
chemical batteries.
Also, the expected specific energy is higher than for the secondary batteries.
In [RD77] is presented a NASA demonstrator turning at 60 000 rpm that can store up to 7.5 MJ.
The main disadvantages for Martian surface operation is the important self-discharge. The
NASA demonstrator is discharged after only 12 hours. With eclipses lasting 14 hours, the
flywheel cannot be used for this type of application.
4.3.5.4.2.2 Secondary batteries
The current secondary batteries that provide the best characteristics for the requirements of the
surface operations are the Li-Ion cells.
The performances are:
• Specific energy: 100Wh/kg
• Efficiency Wh: 94%
• Temperature range: 0 to 40 °C
• Low self-discharge
• More than 5000 cycles
Some developments are in progress for extending the operational temperature range to –40 °C.
For this study, by looking the improvements during the last decades, a specific energy of
150Wh/kg is taken into account, which is expected to be available in 2015.
4.3.5.4.3 Fuel cells
4.3.5.4.3.1 Primary fuel cells
A fuel cell is a device that produces electricity through an electrochemical process within the
fuel cell itself. This is very similar to the way a battery produces electricity. However, unlike a
battery, a fuel cell only produces electricity while fuel is supplied to it. The primary fuel source
for the fuel cell is hydrogen.