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Supercaps and solar for outer planet missions
orbitaldescent
post Mar 10 2013, 03:37 AM
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Hello UMSF! I've been a reader for some time but am a first time poster this evening.

Last month, I read a few articles concerning a new capacitor technology that was invented using graphene. (Graphene being a 1-atom thickness sheet of carbon atoms arranged in hexagonal formation.) At UCLA they appear to have made capacitors with this stuff that will fully charge in seconds. There are many articles available but most of the research papers are behind paywalls and I haven't been able to read them. On sciencedaily, I read that these capacitors could be integrated into solar panels to greatly increase their efficiency. The lattice component is demonstrably cheap and potentially anybody can make them using the LightScribe function on their consumer DVD-RW drives.

Here is a link to the article: New Technique Scales Up Production of Graphene Micro-Supercapacitors

I was very excited to read this. I instantly thought of how it might be useful on probes, and in particular, probes bound for the ice giants or Kuiper belt. My understanding was that we have sent few probes to Uranus or Neptune because the RTG requirement makes them very costly. I frequently think on the successes of the Cassini mission and all of the interesting things we've witnessed at Saturn just by having an orbiter there. What wonders have we been missing at Uranus and Neptune? Might this capacitor technology make the ice giants accessible for a Discovery-sized budget?

I'm by no means an expert on electrical engineering or solar energy, so I'm unsure if even with greatly improved panel efficiency there would be enough energy to power the instrumentation and temperature control systems. I wanted to reach out to this community and maybe get your opinions.
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djellison
post Mar 10 2013, 05:43 AM
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QUOTE (orbitaldescent @ Mar 9 2013, 07:37 PM) *
My understanding was that we have sent few probes to Uranus or Neptune because the RTG requirement makes them very costly.


If you read a document like this (which explains the reasoning behind the Cassini RTG's for example) - http://saturn.jpl.nasa.gov/spacecraft/safety/eisss2.pdf - you will see it's not just cost.

Massive massive solar arrays are heavier than an RTG, more complex to deploy, require accurate pointing and articulation, and constrain slew rates etc.

And - RTG's present a less expensive option. In FY93$ a solar array option for Cassini would cost around $190 - $225M.

The RTG option only cost $117M

It's not the cost of the RTG's that are the problem. It's the lack of a healthy planetary science budget.
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Keatah
post Mar 10 2013, 07:48 AM
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Capacitors, in any form, Super, Micro, Electrolytic, Ceramic Disc, any kind.. are devices designed to store energy, like a battery, but through different physical processes.

A capacitor's advantage over a battery is the ability to charge quickly, and release all its energy even faster. Much like an old-school photo flash unit.

A capacitor will not allow a solar panel to make more energy than it already does. And that *is* the problem with outer planet missions. You're going to need bus and airplane sized panels.

A capacitor bank (embedded into the cell as implied in the SD article) will help smooth out the flow and current variations as the spacecraft goes about its business of operating various instruments. Much the same as a battery. Perhaps more efficiently for smaller transients. A capacitor is also more durable than than a battery.

The technology and fabrication techniques used in making thin-film SuperCapacitors may be applicable to solar cell mfg to gain a few percent more efficiency. But just connecting the capacitors to the cells alone will do nothing to boost the amount of power generated.

Any ice giant mission will need Juno-sized (or larger) panels. The other option is RTG.

I hope that makes some sense for you.
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stevesliva
post Mar 10 2013, 04:51 PM
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QUOTE (djellison @ Mar 10 2013, 01:43 AM) *
Massive massive solar arrays are heavier than an RTG, more complex to deploy, require accurate pointing and articulation, and constrain slew rates etc.


I still wonder about JUICE's operational constraints.

Also, subsequent L class request for proposals:
http://sci.esa.int/Call-WP-L2L3

... will be interesting to see whether any call for RTGs.
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orbitaldescent
post Mar 10 2013, 11:53 PM
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If solar cell efficiency were improved, would that not translate into higher wattage per square meter?
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nprev
post Mar 11 2013, 03:00 AM
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Obviously. And there are a tremendous number of other 'ifs' that would make things great.

Bottom line here is that virtually any planetary mission requires something between five years and a decade of planning, fabrication & integration before launch in most cases, and once you select a power schema there's very little flexibility to change that, or even to augment it.

It ain't like upgrading an iPhone...not at all.

Let's please just keep it real.


--------------------
A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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orbitaldescent
post Mar 11 2013, 04:56 AM
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Understood. So what I can glean from all the input is that any efficiency improvements from this new technology are unlikely to change solar's current viability in the ice giant environments. Hopefully something else will come along before the Plutonium runs out!
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stevesliva
post Mar 11 2013, 04:20 PM
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QUOTE (orbitaldescent @ Mar 11 2013, 12:56 AM) *
Hopefully something else will come along before the Plutonium runs out!


Ideally, more plutonium will be made. The new RTG types look great.
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