Power Systems
RTG Power
Due to the very long mission duration, the best choice for
the Deep Space 9 probe will be a Radioisotope Thermoelectric Generator (RTG)
based power system.
RTGs have been used since 1961, and thanks to NASA efforts,
they keep improving their performances in terms of longer lifetime, higher
output power and reliability.
The most important element in an RTG is the radioisotope
(fuel) which releases heat through its radioactive decay; heat is then
converted into electric power by a thermocouple.
We can compute the output power using the following model:
where  is the thermal to
electric conversion efficiency, E is the decay energy,  depends
on the half-life  ,  is
Avogadro’s number, m is the total fuel mass and M is the atomic weight in amus.
It is clear that power is inversely proportional to the
isotope half-life; hence we need an isotope with half-life long enough to
provide an almost constant power for the whole mission duration, but not too
long in order to obtain an acceptable power level.
The most common radioisotope used for space RTGs, e.g.
Cassini and Galileo spacecrafts, is Plutonium 238. Nevertheless it will not be
suitable for our mission because of its half-life duration. In fact, the time
needed to get to the target planet is on the order of a thousand years, while Pu-238's half-life
is just 87.74 years. This means that after 87.74 years the RTG
will output only half of the initial power.
More suitable isotopes are the ones used in nuclear power:
Plutonium 239 (=24200 yrs, E=5.245 MeV),
Uranium 233 (=160000 yrs), and Uranium 235 (=700 million yrs, E=4.679 MeV).
All of these have sufficient half-lives, hence to maximize
the output power we will choose the one with shortest half-life and higher
decay energy. Therefore, we have selected Pu-239 for Deep Space 9.
Another important parameter is the conversion efficiency
from thermal to electric power. We will use thermionic converters instead of classic thermocouple, which nearly doubles the conversion efficiency.
An average thermionic converter has 20% efficiency;
accounting for losses due to time degradation we assume that our converter has
a mean 15% efficiency over the whole mission duration.
With such a thermionic converter and m = 3000 kg of Pu-239,
the RTG will initially output a total power of 860 W.
The following graph shows the power degradation over 10000
years:
Sources:
- M. Ragheb, ‘Radioisotopes power production’, 2/10/2010
- Wikipedia
- Gabor Miskolczy and David P. Lieb , ‘Radioisotope Thermionic Converters for Space Applications’ Thermo Electron Technologies Corporation
- www.nasa.gov
Solar Panels
Solar
panels, also known as photovoltaic panels, are used to convert light from the sun,
which is composed of particles of energy called "photons", into
electricity that can be used to power electrical loads. Light from the sun is a
renewable energy resource which provides clean energy, produced by Solar panels.
How Do Solar Panels Work?
 Solar panels
collect clean renewable energy in the form of sunlight and convert that light
into electricity which can then be used to provide power for electrical loads.
Solar panels are comprised of several individual solar cells which are
themselves composed of layers of silicon, phosphorous (which provides the
negative charge), and boron (which provides the positive charge). Solar panels
absorb the photons and in doing so initiate an electric current. The resulting
energy generated from photons striking the surface of the solar panel allows
electrons to be knocked out of their atomic orbits and released into the
electric field generated by the solar cells which then pull these free
electrons into a directional current. This entire process is known as the
Photovoltaic Effect.
There are seven typical perfect electric conductor materials. The following
table shows their characteristics.
|
materials
|
Voc
|
Jsc(mA/cm2)
|
FF
|
PCE.
|
|
Single-crystal Si
|
0.5-0.69
|
42
|
0.7-0.8
|
16-24
|
|
Poly-Si
|
0.5
|
38
|
0.7-0.8
|
12-19
|
|
Single-crystal GaAs
|
1.03
|
27.6
|
0.85
|
24-25
|
|
AlGaAs/GaAs tandem
|
1.03
|
27.9
|
0.864
|
25
|
|
GaInP-GaAs tandem cell
|
2.5
|
14
|
0.86
|
34
|
|
CdTe thin film (poly)
|
0.84
|
26
|
0.73
|
15-16
|
|
Single-crystal InP
|
0.88
|
29
|
0.85
|
21-22
|
PCE: power conversion efficiency FF:fill
factor
Power
conversion efficiency = 
Fill
factor
= 
With the development of the space satellite industry,
multi-junction compound semiconductor cells from the III-V groups are starting to be used, so we have chosen a GaInP-GaAs tandem cell as the material for our solar panel, which will improve the efficiency. The figure below is the structrue of the GaInP-GaAs tandem cell.
In conclusion, a solar panel in the range of 10-13 square meters, which weighs 17-20 kg, is necessary to generate….needs 5-6 kW of power to achieve the desired launch velocity. Because
the standard extraterrestrial level for mean solar irradiance at one
astronomical unit from the Sun is approximatively 1367 W/m2,
and the efficiency of our material is 34%, so the total power we can get from
sun is 465W/ m2. Therefore the size of our solar panel needs to be
10-13 m2. As for weight, the solar panels produce approximately 300 W/kg, so the weight of our solar panel will be approximately 17-20 kg
Sources:
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