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I'm working on a story set in a generation ship traveling through the stars. Its propulsion is a solar sail, which will accelerate it over many decades to a substantial fraction of the speed of light. However, its inhabitants need power for electricity, and don't want to use nuclear power (or radioisotope thermoelectric generators, RTGs), or fossil fuels.

Many spacecrafts today use solar panels or RTGs for power, but the latter is not renewable, and the former may not be useful when passing through regions of space where the light from stars is dim.

So, how can my ship be equipped to take advantage of renewable energy in space?

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  • $\begingroup$ Related but not a dupe; many of those power sources are not always available (e.g. my suggestion of a Bussard ramjet). $\endgroup$ – HDE 226868 Feb 18 '16 at 16:07
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    $\begingroup$ Lots and lots of hamster wheels? $\endgroup$ – Xandar The Zenon Feb 18 '16 at 16:16
  • $\begingroup$ Does nuclear power include fusion? $\endgroup$ – Jaywalker Feb 18 '16 at 16:29
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    $\begingroup$ @XandarTheZenon I think that violates the first law of thermodynamics. Where are the hamsters getting the energy from? $\endgroup$ – DaaaahWhoosh Feb 18 '16 at 16:30
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    $\begingroup$ I don't think solar sails can accelerate a ship to near light-speed; the force falls of quadratically. $\endgroup$ – k-l Feb 18 '16 at 17:07

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I'm going to summarize the comments and other answers.

If you wish to remain true to science:

There's no free lunch

Despite everything you've been told there is no free lunch.

Solar power is a bust

Even as close as the Earth is to the Sun, PV panels are marginal at best and they are a very dilute form of energy generation compared to other types. As you move away from the sun, the intensity of sunlight drops as $\frac{1}{r^2}$.

NASA does not consider solar as a viable energy source outside the orbit of Mars. You shouldn't either.

Renewable Energy Source

There is exactly one renewable energy resource available between the stars, this is interstellar hydrogen.

There are many possible ways for you to use this hydrogen, but they all break down into one of two ways:

  1. Fusion
  2. Antimatter annihilation

Having your cake and eating it too

Your question makes it very clear that you do NOT want to consider nuclear power.

Since you are presumably writing fiction, this means you can basically write into your story anything you want. You can claim humans found magnetic monopoles, flubber, or can use clean Zero Point Energy. None of these are actually real or viable as a means to power a space ship but if you insist on no-nuclear, you'll have to use some form of handwavium (aka McGuffinite) to do it.

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  • $\begingroup$ I get free lunches every week, at the Alumni center. $\endgroup$ – Serban Tanasa Feb 18 '16 at 22:47
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    $\begingroup$ They aren't free, you just didn't realize that you're paying for it. Just because you don't pay for something at the time of service does not mean its free. $\endgroup$ – Jim2B Feb 18 '16 at 23:02
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    $\begingroup$ Actually, NASA is using solar panels on Juno all the way out at Jupiter, due to the shortage of plutonium for RTGs--not to mention Rosetta and Dawn, both solar-powered, operating in the asteroid belt. Although to use PV in interstellar space you'd only need about ten million times as much collection area =) $\endgroup$ – 2012rcampion Feb 18 '16 at 23:33
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    $\begingroup$ I'm not sure if interstellar hydrogen counts as renewable... $\endgroup$ – user3757614 Feb 19 '16 at 0:48
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    $\begingroup$ @Molot, When the calculations involved using regular light hydrogen fusion, you are definitely right. I was thinking of an engine that might use $Li_7 + p -> 2He_4$ reaction in which the $p$ came from interstellar space. The problem is, there's not much advantage to using it since it only saves about 12% on the mass of your fuel/propellant. Of course you'll get better thrust if you just "burn" some of that and use the rest as propellant. $\endgroup$ – Jim2B Feb 23 '16 at 19:54
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You have a few options as far as I can see.

2 of them you need to bring with you. You need them to be very dense in power generation because you don't have lots of space.

Bring with you

  1. nuclear power you have already nixed.

  2. Anti-matter needs much less space, but does need some power to keep it safely contained. Also we can't yet generate enough to be useful.

reduce energy needed

  1. Put (almost) everyone in cryo-stasis and have enough energy source stored to last the journey.

renewable/pick up along the way

  1. Fusion. You can have a big scoop in front of the ship collecting hydrogen molecules to fuse into larger atoms. Currently the fusion machines we currently have are huge and dangerous.

  2. Somehow learn to tap dark-energy and dark-matter.

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  • $\begingroup$ 3 is definitely renewable although it might not yield much power(but that never was the question) number 5 would also fit this (I don't know anything about dark-energy or dark-matter so I assume they are "underneath" everything?) the other points are more of an alternative but not renewable, maybe group them seperat? $\endgroup$ – vanillagod Feb 18 '16 at 17:00
  • $\begingroup$ true I'll do a little reorg $\endgroup$ – bowlturner Feb 18 '16 at 17:02
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    $\begingroup$ The only renewable options you have are fission and fusion (1 and 4) but both are a class of nuclear power (which is nixed), the others are either not renewable or do not actually generate power. $\endgroup$ – Samuel Feb 18 '16 at 18:16
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    $\begingroup$ Nit-picking, but anti-matter is also nuclear power. $\endgroup$ – jamesqf Feb 18 '16 at 18:18
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    $\begingroup$ @jamesqf: That's a matter of definition, really. Some might argue that anti-matter annihilation really deserves its own category, just like you wouldn't lump nuclear fission (or fusion) together with chemical reactions. But yes, anti-matter does share many of the features of nuclear power (including the tendency to produce ionizing radiation, and the potential for big scary explosions) just by virtue of its high power density. Alas, the OP hasn't specified why they want to avoid nuclear power, so we can't tell if those reasons would also apply to anti-matter. $\endgroup$ – Ilmari Karonen Feb 18 '16 at 20:12
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Space Wind

Yeah, it's a real thing. The main issue here, as with most renewables, is that you're not going to get much power from it. Not only that, but the real power is coming from sapping your forward momentum (like putting wind turbines on a car, you're stealing from yourself). This is technically renewable, but not advantageous over the nuclear option. There just aren't a lot of energy options out there.

The idea is you'll have interstellar medium blowing past you. It has mass and it's moving with respect to you, that means you can reach out and harvest that energy. Either the kinetic energy, or as bowlturner suggested, collect the interstellar hydrogen for use in fusion. Yes, fusion is a form of nuclear power on your ship, but it is renewable. It's also how you got going in the first place, except the fusion was from a star.

Alternatively, you need to leave a beaming station near a star behind you to beam energy into your sails and your on-board batteries. This will allow you more power and a longer period of acceleration.

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What do you mean by renewable? I think the common use is something that you won't run out of, or something that's not depleting a previously made stockpile (that is, not fossil fuel). I think what you mean here is something external to the ship rather than something you brought with you.

But really, why? And in some cases, it's hard to make the distinction.

First of all, what's your energy budget? Look at other questions concerning how much energy it takes to accelerate the ship. It's more than the entire energy consumption of Earth now, by orders of magnitude. That's with billions of people and all the heavy industry. How much energy will the occupants need? The amount is a tiny tiny drip in the budget needed to accelerate and then slow down the ship.

ride the brakes

So what's wrong with tapping off a little of that? Planning a separate power source that lasts indefinitely in case the primary source is lost and they can't slow down at their destination and are doomed to cruise forever at relativistic speeds, then at least they can just keep living like that?

I'd like to point out that one storage reservoir of energy is the moving ship itself. Like I said, enough for a Type I civilization back home! You need to slow down anyway, so wouldn't it be nice if you could somehow utilize some of that energy as a side effect, as well as not having to spend that much energy for a second time to slow down?

A Bussard ramjet might not actually give a gain over drag, by modern calculations. But if you're breaking that's not a problem and the drag only adds to your breaking! Running the ramjet at a minuscule level during the cruise phase will provide power for the internal use of the ship, and make about as much of a dent in the ship's progress as the surrounding dust and gas would have anyway.

For breaking, there is also magnetic and electric scoops. The ramjet might not work well enough no matter how well the engineer it, so turn that on its head and make the scoop as bad as possible instead, and you have a parachute. Without having to gather the hydrogen and feed it to a fusion engine, you can just use the energy of it passing through the scoop as-is, in the manner of a windmill.

A long conductive tether can provide for magnetic interactions with the galactic magnetic field, and serve as a break and a huge source of electricity.

Maybe you get a laser boost from Earth to get moving for a short time and then have to run the break the entire rest of the journey since the breaking thrust is not as large as the launching thrust. Or maybe you cruise for some years and slow down when you get there, able to break quite fiercely. In the latter case, you can still run the break by a minuscule amount to generate power. The amount of power you need is orders of magnitude less than the amount of kinetic energy in the ship, so you won't notice.

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  • $\begingroup$ I like the bussard parachute concept! Not sure how feasible it would be for any decent deceleration, but it's a nice idea! $\endgroup$ – Joe Bloggs Feb 23 '16 at 10:33
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Robert Forward is your friend.

To accelerate a light sail to near $c$, the sail is powered by banks of terawatt lasers in close solar orbit (the general version of this is to have the solar power stations and lasers orbiting Mercury, but you can arrange this however you like). The sail rides on the power beam, and as it moves farther and farther away from the Solar System, giant fresnel lenses suspended in space focus the beam on the starship. (see http://www.lunarsail.com/LightSail/rit-1.pdf)

enter image description here

Terawatt beams are a lot of energy, so it is quite feasible to tap a bit of the energy shining on the lightsail to reflect on a power conversion device on the spacecraft itself to provide "renewable" energy. Since it is essentially Solar energy, it fits the definition of "renewable".

The added bonus is the system as described can be used to decelerate. The two tricky questions are: will there be anyone manning the laser and lens system when it is time to decelerate, and; will the system function with enough efficiency to decelerate a lightsail moving at a large fraction of $c$? Forward himself suggested a much more modest performance for the system.

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Summary: a Bussard Scoop is your only hope, but it is a very unsafe bet.

As others have pointed out: no, it cannot be done in the traditional sense of "renewable energy" as we know it on Earth. Space is terrifyingly empty of everything, including energy to harvest.

But I want to ask you to challenge the premise. My question to you is this:

Did you mean "renewable" or "sustainable" energy?

Why do you require your energy source to be "renewable"? Is there a story element that requires it, or did you tack that label on in hopes that "renewable" means "good"? Where "good" in turn means "sustainable", "does not produce waste", "reliable", "available", "does not use consumables", "just works and works for centuries"?

If there is a plot element behind it, then as I said: you are bust. If your travelers' motivation for demanding "renewable energy" on their ship does not amount to anything other than "We just want it that way", then the answer from their ship builder will be a harsh but truthful "Then you are not going into space". Do keep in mind that travelling in space is not a leisure cruise. It is not for the squeamish and happy-go-luckies. There are some hard decisions to make, and you will not have several of the "luxury" options available to you in space that you have on Earth. Renewable energy is one of these luxuries.

But if you were simply looking for an energy source that will sustain your travelers for centuries, then you have used the wrong approach by assuming that renewables are the only thing that works.

The safest bet is nuclear power. Already fission power provides a quite ridiculous energy density in its fuel.

enter image description here

Source: xkcd 1162, "Log Scale"

Assuming fourth generation nuclear fission power, approximately 50 grams of nuclear fuel will sustain one person with electricity and other energy for a normal human lifetime. That is quite manageable. You are going to need much more in terms of other consumables and renewables for every person on that ship. If you were worried about having to haul fuel along, this is not a problem, because the premise alone means that you will be hauling stuff along to sustain the people anyway, and a load of fuel for nuclear power does not add any significant amount of weight.

If we then assume fusion power, the energy density becomes even more ludicrously high. Assuming for instance that the Polywell reactor works...

enter image description here EMC2's "Wiffle Ball 8" reactor in operation.

...then we are talking about needing less than 1 gram of fuel per person and lifetime. Also with Polywell you can use aneutronic fusion, which means no radioactive waste products. In its aneutronic mode, Polywell uses 1 atom of Boron and 1 atom of Hydrogen to make 3 Helium per reaction... Helium that is not radioactive and that can be safely vented to space.

So I ask you again: why did you put "renewable" in the question? Was that really what you wanted? Or were you simply looking for a sustainable source of energy?

You will never get one that works forever without making "landfall", near or in star systems. But you can get one that works for a very long time, in nuclear fission or fusion power.

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    $\begingroup$ Thanks for challenging the premise - I needed that. And yes, I suppose I going for something more along the lines of sustainable. $\endgroup$ – HDE 226868 May 11 '16 at 22:13
  • $\begingroup$ @HDE226868 Glad I could help. :) Well like I said: if it is sustainable you want, then Polywell or any other fusion concept you would like to think of is the way to go. Polywell is my personal favourite because 1) it is compact 2) it is credible and 3) it looks incredibly sci-fi. :D $\endgroup$ – MichaelK May 12 '16 at 5:25
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I'll take the easy way out and more or less invent a technology based of of a branch of physics that's not mature yet:

Your folks will use Unruh-Energy. Unruh demonstrated theoretically that the notion of vacuum depends on the path of the observer through spacetime. From the viewpoint of the accelerating observer, the vacuum of the inertial observer will look like a state containing many particles in thermal equilibrium—a warm gas. If this perspective change can somehow be induced without having to reach near-light speeds, the false vaccuum could be harnessed. This will allow space travel via reactionless drives, and reduce the fuel weight to that of the Unruh effect modulators (whatever they may be).

Since by definition there is no point in the Universe without vacuum energy, you literally can't run out. It's just a matter of deploying subtle enough physics to take full advantage of the effect. It's unclear, for instance, just how large the energy density that is recoverable is. I've heard physicists talk of a single cup's worth of volume containing enough vacuum energy to boil all of the earth's oceans, but it's not clear by any means what fraction of that is recoverable, especially at an early stage of Unruh-drive technology deployment.

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    $\begingroup$ Despite the widespread use of this trope in fiction, there's no way to turn ZPE into usable energy. Your average reader may not understand why but your scientifically literate ones will. $\endgroup$ – Jim2B Feb 18 '16 at 22:16
  • $\begingroup$ When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong... I think it's more accurate to say that as of Feb 2016, most physicists believed that there is no known way to harness ZPE. $\endgroup$ – Serban Tanasa Feb 18 '16 at 22:20
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    $\begingroup$ If you ever meet someone that can violate the 3 laws the thermodynamics, I will seriously consider worshiping them. $\endgroup$ – Jim2B Feb 18 '16 at 22:31
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    $\begingroup$ Don't get me wrong, when writing fiction, the story comes first. But don't include ZPE for "scientific accuracy" (because it isn't), only include it if it advances your story. $\endgroup$ – Jim2B Feb 18 '16 at 22:40
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    $\begingroup$ Interestingly, if a lower energy state ever did appear in our Universe, it might catalyze a chain reaction converting our Universe to a new (lower energy) type. This would annihilate everything - down to subatomic particles. So finding such a lower ZPE state would be the last thing anyone ever does. The phase change propagation wave would spread at the speed of light. It would give no warning. One second we would be here not aware of any problems and the very next one we (and every physical object - stars & black holes included) would be a fine mist of spreading subatomic particles. $\endgroup$ – Jim2B May 10 '16 at 17:14
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Is your ship following the rule of cool aka "to boldly go where no man has gone before."?

Renewable energy sources in our time usually expect to have some storage/distribution infrastructure built around them - you're not taking your wind turbine/solar panel with you when riding e-car.

In hundreds of years before launch, automatic refuel stations were deployed along the expected path of generation ship to process interstellar gas into more convenient form of energy to be collected; the mission was to establish safe route and avoid having too much generations to die on ship; thus, higher acceleration and no dangerous technology onboard

Or, the stations may have alien origin and be scattered across galaxy - so the crew gets bonus points for being pioneers (without previous multi-stage launches to meet..)

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Piracy: Travel via busy transport routes and keep nicking people's energy cells. Downside: Only works in a which includes populated space.

Trade: As above, but more boring.

Big batteries: Have extremely good power recapture, lots of huge batteries and travel via star-systems rather than in a straight line. Downside: Probably not possible using actual science, but handwave the battery technology enough and it could work in a story.

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Since the 80's almost all space missions have used special solar panels to cover the energy demand on board, but if we need to travel much further in space, we would need a different form of energy, which could be produced on board, not by the Sun, which might be pretty far away from the ship to count as an energy source.

Even in our solar system, if we decide to travel to one of Jupiter's moons (no further), the Sun being so far away from us, will not count as much as a viable energy source.

On Pluto for example, the Sun looks almost like any other star, so we need to consider all the energy that can be produced on board as a viable energy source that can power all the activities that are taking place on the ship.

Lately, it proved that human waste could be a good source of clean energy. Human pee is a good source of clean energy because about 5 liters of urine can produce 1 liter of biogas. Giving the fact that an average person produces more than 1 liter of urine per day, the mission, which may consist of four to six people can produce enough biogas to power their daily activities.

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  • $\begingroup$ "Since the 80's almost all space missions have used special solar panels to cover the energy demand on board". As soon as we need more energy than just a little satellite, or if we are going deep into space (i.e. to Jupiter and beyond), then solar panels have not been the primary energy source. The Inverse-Square Law is a harsh mistress. Also: urine is only re-cycled energy. It is not "original" energy. The original energy comes from the food the human is eating. $\endgroup$ – MichaelK Nov 24 '16 at 10:03
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The spacecraft is built like a giant "joule thief" and uses the planets' and stars' magnetic field to create electricity when it passes through. Incidentally that protects the people inside the toroid from radiation damage.

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  • $\begingroup$ In most of space, magnetic fields will not be strong enough to provide a source of any energy. $\endgroup$ – HDE 226868 Feb 18 '16 at 22:04
  • $\begingroup$ thats why its built like a giant "joule thief" $\endgroup$ – Erik vanDoren Feb 18 '16 at 22:05
  • $\begingroup$ I don't understand how that would work. $\endgroup$ – HDE 226868 Feb 18 '16 at 22:06
  • $\begingroup$ its a booster, some people think they can get energy out of thin air using similar stuff. Which isnt possible here on earth but you dont need something we can build now right? its better than using exploding tachyons ;) $\endgroup$ – Erik vanDoren Feb 18 '16 at 22:11
  • $\begingroup$ It would ideally be built with current or near-future tech, yes. But I really don't understand what the device would do or how it would work. $\endgroup$ – HDE 226868 Feb 18 '16 at 22:13
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Although this is kind of close to fossil fuels: Could your space ship not just rely on bio mass as a fuel source? All organisms create waste which can be used as a fuel source. Unfortunately this does mean that you lose the option of travelling indefinitely by recycling bio waste into food and other useful materials.

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  • $\begingroup$ No. That concerves energy but you are just getting more out of the energy used to feed the bio mass. $\endgroup$ – JDługosz Feb 18 '16 at 17:19
  • $\begingroup$ I know it conserves energy but combusted biowasze is harder to recycle and therefore effectively lost. $\endgroup$ – Jaywalker Feb 18 '16 at 17:24
  • $\begingroup$ "this does mean that you lose the option of traveling indefinitely by recycling bio waste into food and other useful materials." That was never an option. Any long distance starship will employ recycling in their life support systems. While this conserves matter, there is always an energy loss in recycling. You are effectively proposing a perpetual motion machine. $\endgroup$ – Schwern May 10 '16 at 18:49
  • $\begingroup$ Biomass does not create energy out of nothing. Photosynthesis harvests energy from light in order to build up biomass. No light = no photosynthesis = no biomass = no energy from biomass = no light... and so on. TANSTAAFL still applies... $\endgroup$ – MichaelK May 11 '16 at 7:10

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