Jupiter Icy Moon Orbiter. The case for solar power.
The JIMO has a planned power requirement of 100kw, and NASA has decreed that this will best be provided by a nuclear reactor. Suggested weight for the reactor is 100kg to 150kg.
What would be the mass of a solar powered solution?
Spectolab is the main supplier of Solar cells for space.
http://www.spectrolab.com
For naked Solar cells the weight 840g per Square meter, with a power of 378 Watts. This gives a weight of 222kg for a 100kw bank. Given the planned payload of 1500Kg, this is not unreasonable.
However This is for naked Solar cells in earth orbit. The weight for solar panels is at least 624kg. It gets worse. The light intensity is only 4% of Earth's around jupiter. If efficiency was similar we would need about 7500 square meters of solar panels, weighing about 250 Tons.
Clearly simple solar panels will be too heavy. However all is not lost. We don't need 7500 Square meters of solar cells, we can use solar concentrators.
http://www.sunpowercorp.com/html/Tec...ers/ProgPV.pdf
we just need mirrors or fresnel lenses, with a combined surface area of that figure. There IS a suitable material for a mirror, with a weight of 5 grams per square meter.
http://www.space.com/businesstechnol...il_000302.html
This would give a weight of 37.5kg! Given that it has the stiffness of cardboard, and would be operating in zero gravity, and very near zero acceleration, it would need little or no further support structure to be acceptably ridged. This weight doesn't seem to include the weight of the mirror coating, or mounting, but it looks promising.
So what about the weight of the solar cells? Solar cells work happily at light concentration of 200X earth normal sunlight, and plans are afoot for 1000X concentrations. Assuming the lower figure, we have about 1.5 square meters of solar cells, of negligible weight, and a serious cooling problem. Not nearly as serious a problem as in the nuclear power option however, for the following reasons:
1) Space grade solar cells currently operate at 28% efficiency, and likely to rise. This is far higher than most planned nuclear solutions.
2) It's a lot easier to build lots of small heat sinks that one large one, for the simple reason that the heat doesn't have to travel so far before it's radiated.
We have one final problem. such a large solar array must be assembled in space. It's about time we came to terms with this. Nuclear powered solutions will also need space assembly. Take a look at the heat sinks on the initial nuclear designs, and imagine launching them pre-assembled. We need to develop a robot based assembly facility at the ISS.
It's not only JIMO that would benefit from assembly in space. Think of the advantages of shipping satellites to the ISS, fitting them with high power, low weight solar concentrators, and bolting on the antenna. The satellites are then boosted into their proper orbit ether with onboard solar powered ion drives, or an unmanned, solar powered, reusable "space tug". ISS could host a satellite assembly, repair and maintenance facility that could revolutionise the communications industry.
