ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 186 of 422
(see [RD19],[RD20] & [RD21]) data are considered for the design. The electrical mounting, the
cells, their coverglasses and the substrate are the only part taken into account in the solar panels
mass budgets for the power subsystem. The other components of the panels (deployment
mechanisms, motor, yoke,..) are integrated in the mechanical subsystem.
For the multi-junction cells, coverglasses of 150 µm are assumed (see Figure 3-43).
For the thin film cells, a value of 600 g/m 2 is assumed (see [RD22])
Option 1: 140 µm
(kg/m 2 Option 2: 100 µm
)
(kg/m 2 )
Triple junction cells 0.720 0.514
150 µm coverglass 0.397 0.397
Coverglass adhesive 0.067 0.067
Interconnects 0.013 0.013
Cell adhesive 0.213 0.213
Bus/wire/diodes 0.307 0.307
Substrate
50 µm kapton
1.144 1.144
Total 2.878 2.878
Figure 3-43: Mass budget for multi-junction cells options
15 panels of 1 metre make up one polar wing. Changing the cells and the substrate will require a
new qualification. The length of the panels will be computed for this study.
To cope with a mechanical failure during the folding/unfolding manoeuvres, the solar wings are
sized taking into account one panel loss.
The solar panels are mounted with a driving mechanism. Except during eclipses and
manoeuvres, the solar panels are Sun pointed.
The sizing mode for the solar panels is the “Orbiting on Mars” when the eclipse is the longest
(41 minutes) and the solar irradiance at its minimum. In that case, the solar arrays are designed
for being able to fully recharge the battery before the start of the next eclipse. A failure of a
complete solar panel is also taken into account for this computation.
3.3.4.3.2 Power storage
The power storage has to supply power during the eclipses on LEO and in Mars orbit. But the
sizing cases are the manoeuvre phases in which the solar panels may be stowed up to 6 hours.
Alternatively, there are other interesting storage technologies:
• Secondary batteries. The most efficient are the Li-Ion cells with round trip efficiency
around 94%. The specific mass nowadays is around 100 Wh/kg. Improvement to 150
Wh/kg for 2015 is expected (cf [RD3],[RD4] and [RD5]).
• Regenerative fuel cells. The most advance ones are the PEM providing electricity and
water by combining Oxygen and Hydrogen. On the other hand, the charge is performed
by electrolysed water. The efficiency is estimated to be about 50 to 60%.
The regenerative fuel cells may be interesting for high energy requirements such as in this
mission, but their poor efficiency involves a huge increase of the solar generator size. Since this
part is already a critical point, the use of secondary batteries is preferred.