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There is US Army convoy travelling through Czech Republic right now, and obviously, they had to make several stops to refuel. And, stupid idea arises in my head:

What if army wanted to switch to solar power?

I call you, fellow worldbuilders to help me investigate on that idea.

What would make solar powered army vehicle feasible?

  • Such vehicle should be "war ready." Not necessary tank, but you should be able to use it in direct combat. So no "back front support" vehicle
  • Such vehicle should use as much as possible power from easily obtainable natural resources: Wind and solar. Priority on solar. Goal is having vehicle powered at least 80% from these resources. Gasoline is allowed but should play secondary role
  • You should be able to operate it at any given condition (cloudy, night)
  • You can assume, that first prototypes would be used in places, where you get loads of sun naturally. Middle East is first destination of such vehicle
  • Other parameters should be comparable, or even better than its counter-party powered by nowadays resources (solar powered tank should be better choice than diesel one, the same for armed vehicles)
  • Money is not a problem. You may assume that you are given necessary budget for this.
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    $\begingroup$ A significant departure in the emissions from the Sun, perhaps if Sol were (ridiculous assumption) to go from Class G to Class F, or better yet to Class O $\endgroup$ – Everyone Mar 31 '15 at 16:36
  • $\begingroup$ Question: why "no 'back front support' vehicle?" After all, the Army doesn't send its tanker trucks into "direct combat," why should a solar 'battery tanker' have to? $\endgroup$ – 2012rcampion Mar 31 '15 at 16:41
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    $\begingroup$ Rain on, war off. Is suggesting that t-shrts saying "Make rain not war" would become popular a valid answer? $\endgroup$ – Twelfth Mar 31 '15 at 16:53
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    $\begingroup$ It's simply a matter of energy density. The energy density of the sun reaching the Earth is very low compared to fossil fuels (which are essentially concentrated chemical storage of solar energy). Furthermore, battery energy density is still very low compared to fossil fuels...the number usually bandied about is something like a x20 ratio, battery-to-petroleum. There's reason to believe that this will slowly improve over time, however. At that point, it's a matter of how you charge your batteries...it will still make a lot more sense to burn fossil fuels, or even better, charge them nuclearly. $\endgroup$ – Chris B. Behrens Mar 31 '15 at 17:24
  • $\begingroup$ @2012rcampion I am curious about "war ready" vehicle simply because I can imagine suppoorting vehicles running on solar. So to answer you... Pure curiosity if and how it could be possible for battle vehicle $\endgroup$ – Pavel Janicek Mar 31 '15 at 20:03
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2012rcampion's answer covers all of the numbers for why this generally doesn't work. So I want to mention the simple way it could happen:

If the world ran out of non-solar fuels, armies would use solar powered vehicles.

In war, you don't waste thought on silly things like ecofriendliness. We do try hard to be ecofriendly with our armies, but when rubber hits the road, there's someone shooting at you. If they have gasoline and you don't, you're at a major disadvantage.

However, war is also not idealistic. If there's no gas, they wont pine for gas. They'll move to the next best thing that works. If that thing happens to be solar, they will find ways to be more efficient with their energy usage so that solar becomes feasible. They won't have to face as powerful of opponents, because they'll be stuck on solar too, so it could work out!

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According to Wikipedia, a "large army division" can use around $6000~\text{Gal}/\text{day}$ of fuel, while the DoD as a whole uses $12.6~\text{MGal}/\text{day}$. The average power usage (in fuel alone) is $220~\text{MWh}/\text{day}$ for a division or $450~\text{GWh}/\text{day}$ for the DoD overall.

Based on this map, it looks like the Czech Republic gets an insolation of at most $3~\text{kWh}/\text{m}^2/\text{day}$. An army division stationed there would need around $160\,000~\text{m}^2$, or $40~\text{acres}$ of solar panels to provide their (fuel only!) energy needs (even with cutting-edge 45% efficcient panels).

If the DoD used solar fields in the American Southwest ($\approx 6~\text{kWh}/\text{m}^2/\text{day}$) to gather energy, and then shipped it out in battery form, they would need a $170~\text{km}^2$ ($65~\text{mi}^2$) field (assuming 100% availability; for practical reasons it would have to be larger). Assuming they stored the energy in primary (non-rechargeable) lithium cells—the highest-density common battery chemistry at about $1.8~\text{MJ}/\text{kg}$—they would process about $900\,000~\text{t}$ of batteries per day (yes, that's just under one million tons per day).

Remember that every gallon of gasoline contains the energy that something like 40 acres of plant matter gathered from the Sun over the course of many years. There is essentially no way that on-demand solar power will ever replace chemical energy storage for military use.

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  • $\begingroup$ and they'd need more than that just to fuel the gigantic fleet of cargo vehicles needed to ship it all. Batteries have lower energy density than do fossil fuels, so you need more and/or larger (and that becomes problematic as you'd end up being incapable of crossing bridges, smaller roads can't accept your trucks, etc.) vehicles than the current fuel bowsers. $\endgroup$ – jwenting Apr 1 '15 at 3:39
  • $\begingroup$ @jwenting I was thinking about that =) It's sort of like the rocket equation, except even worse, since rockets lose mass as they expend energy, batteries don't. $\endgroup$ – 2012rcampion Apr 1 '15 at 12:43
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I think it is completely unfeasible, for one fairly simple but uncircumventable problem:

All forms of natural energy exist outside the vehicle. That means that any reactor that draws energy from these sources, must logically also be exposed to the world outside the vehicle, because otherwise it's not connected to the energy source and thus cannot draw power from it.

But anything on the outside of the vehicle is super vulnerable to enemy fire. Tanks have the engine on the inside and the armor on the outside for a very good reason. You simply cannot have that design with a solar or wind powered vehicle. In order to catch solar or wind energy you'd need panels covering the outside of your vehicle, but you couldn't possibly build a solar panel that's able to withstand the same kind of damage that a plate of pure armor could.

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    $\begingroup$ That's not completely true though. You would store the electric power in batteries then use the panels to recharge. Damaging the solar cells might limit you in a few hours, but it doesn't help the enemy right now. $\endgroup$ – Tim B Mar 31 '15 at 13:29
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    $\begingroup$ @TimB Some of the most effective attacks are those that destroy, deplete, or cut off your enemy's resources. Destroying the solar panels might not win you this battle, but if they can't be replaced in time you will win all the subsequent battles. $\endgroup$ – 2012rcampion Mar 31 '15 at 15:49
  • $\begingroup$ I agree today it is unfeasible. Heat is one form of natural energy which is inside the vehicle. Air can be allowed through in vents (I assume tanks use vents?). But neither of those will create much energy and will probably take up quite a bit of space to harness. Solar might be possible if we could develop "see-through" armor that lets light through? It would also help if the panels would still work even with slight or major damage. $\endgroup$ – DoubleDouble Mar 31 '15 at 18:49
  • $\begingroup$ @TimB same as attacking fuel depots now, to hinder any further enemy advance and force him to use a part of his forces to protect those depots. Solar farms and their battery charging and storage facilities, loading docks, etc. would be far larger and more vulnerable than current fuel depots (which can relatively easily be concealed, even placed partially underground in bunkers, can't do that with solar cells). So you'd need to divert even more combat troops and equipment to protect them, weakening yourself even further. $\endgroup$ – jwenting Apr 1 '15 at 3:42
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Not likely to happen unless several things come about. First solar panels that are much more efficient yet, near 'black hole' capacity where almost no light is reflected from surface and turned into electricity. Army vehicles use a lot of power and they can run non-stop for days. More efficient batteries that can be charged up quickly without overheating and can move a large loaded vehicle several hundred miles on one charge, since often troop movements are done under the cover of darkness. The diesel engine could be just a power plant to keep the battery charged.

Then last but not least. Solar needs large arrays preferably with optimal angles to the sun to catch the best light. Military vehicles try to be inconspicuous, hide in places out of the sun and move all the time. Not to mention damage from enemy fire, sand general use etc. I'm not seeing it as likely any time soon.

On a more positive note, almost all US military ground vehicles have a little solar panel that gives a trickle charge to the battery to that it would go dead at an inopportune time.

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While with current tech it seems safe to assume that a solar powered combat ready vehicle is not to be expected any time soon, it might be conceivable to have a vehicle (or, more likely, a convoy of them), that will handle the energy storage for your combat vehicles.

Assuming you wanted to use solar power, you could either store the electricity in batteries, which has the advantage that the batteries are fairly easy to handle, but the disadvantage of weight, limited number of charging cycles and all, or you could use the electricity to power electrolysis and gain hydrogen, which can then either be used by fuel cells, or burned, in a turbine or combustion engine.

A big advantage of the fuel cell would be that it works both ways: your combat vehicles could have backup solar cells, probably stowed away during combat, for emergency autonomous charging / resupplying with hydrogen. Also, electric motors are a lot more robust than combustion engines.

The overall setup would benefit from a range of factors: it could work fully autonomous as long as tehre is any source of water (since you will never have a fully lossless conversion), plus it could benefit from local power grids, or any source of electricity that can be obtained en route.

There is a major disadvantage, though: Fuel cells don't live forever, the catalysts (despite their name) get depleted, too.
And the resulting hydogen tends to be a bit on the combustible side, which does not go too well with the combination of being kept under pressure and being shot at at the same time...

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Military vehicles need to be able to move and fight on demand. Solar energy is simply too diffuse or intermittent for any sort of combat vehicle, except in niche roles (a high flying UAV powered by solar cells across the body and using low power passive sensors, for example).

In fact, what would be ideal for military vehicles would be some sort of light weight nuclear reactor, which would provide huge power to weight ratios and the ability to operate 24/7 and energize multiple systems at once (motive power, active and passive sensors, energy weapons systems like rail guns and lasers, environmental control systems etc.) Of course the need for shielding makes this impractical for ground based vehicles (even nuclear fusion reactors would emit radiation in the form of neutrons, or x-rays emitted due to bremsstrahlung radiation). With a high enough power to weight ratio, even the fabled "flying tank" would be possible.

The way around all this would be to "beam" power to vehicles. When in convoy or doing other administrative moves, they could be supplied by a cooperative station in orbit or other beaming station, and switch to on board power when in contact with the enemy. This would minimize fuel usage when out of contact, and make logistical operations much easier by reducing the amount of fuel needed to be brought forward.

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The energy could be stored and concentrated. The vehicle needs to take on fuel, and how does the fuel get to that spot in the first place? If there was an oasis of sorts it would save the trouble of getting fuel to the stopover points, which is a significant logistics problem.

A depot could have a algae bed producing hydrogen or methane, in a compact self-running unit. The hitch is that it needs lots of area to gather sunlight, and lots of time to stockpile it.

It would also become something that needs defending.

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The surface of a tank is not large enough to provide the power needed to be efficient in combat, even if you have the best power convertor ever made.

You can to externalize the solar panel and transfer the energy to the vehicles with micro wave or a LASER for example.

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