9
$\begingroup$

The sub would be about the size of:

  • the Sōryū class, which displaces 4200 tonnes (implying about 4200m³ volume) and has a 6000kW propulsion system
  • the Oyashio class, which displaces 4000 tonnes (implying about 4000m³ volume) and has a 5780kW propulsion system
  • On the smaller side: Collins Class, 3407 tonnes displaced, 4200kW

Nuclear tech is not available in this world.

They could fuel up on hydrogen from bases, and I might go that route if the answer to this is a hard no, but I would prefer to keep them autonomous. Even this anti-hydrogen-economy article concedes that submarines might be a good niche application for hydrogen: https://phys.org/news/2006-12-hydrogen-economy-doesnt.html

The question is: can such a sub harvest enough energy from the currents, chemistry, and thermal gradients of the sea around it to run??

Reduce power demand

Of the above, Sōryū demands 1.428kW/tonne, Oyashio 1.445kW/tonne, Collins 1.23kW/tonne.

Note that the bigger they are, the more energy they use proportionally. Is this an absolute law? Something to do with drag, or some square-cube law? Are smaller subs always gonna be more efficient? (If so I may make all the subs in my world smaller.) Or is it just because bigger subs are fancier and have more gizmos?

This trend continues in a more pronounced way if we consider really small subs: the DSRV-1 Mystic displaces 37 tonnes with 11.2 kW for a score of 0.3027kW/tonne, and this one 61 tonnes with 50 kW for a score of 1.02kW/tonne

Can the efficiencies of large subs be brought down to something like 0.3027kW/tonne? Because that would be amazing. Or is it impossible? What's the physics here?

It does seem that with aircraft size is bad for efficiency: https://web.archive.org/web/20180302044700/https://theicct.org/blog/staff/size-matters-for-aircraft-fuel-efficiency

PS: I see now that the DSRV-1 Mystic, as well as using about a-fifth the energy, achieves about a-fifth the top speed, so maybe speed rather than size is the issue.

PPS: There's some debate over whether pump-jets are more efficient than propellers (all the subs detailed here use propellers). Here's an article on the side of that debate against pump-jets, but it does seem to accept that they'd be better at high speeds. (It also says "the square law for drag means that the energy required for propulsion becomes extremely small at very low speeds", which is good news for this thread.) Certainly the drag is proportional to the square of the speed, so halving the speed means only one-quarter as much drag, and drag is a large part of energy consumption though not all.

PPPS: Some comments on increasing efficiency by those guys who got defeated by a flock of emus – https://apps.dtic.mil/sti/pdfs/ADA428039.pdf – including the interesting proposal of coating the surface with some sort of polymer to make the flow laminar rather than turbulent on the first few meters of the nose.

Generating from ocean currents

Let's say we don't get the efficiency way down, and our heroes need 6000kW to go.

Energy can be harvested from ocean currents. This can be done with turbines, but this tech trumps all - https://minesto.com/our-technology - because it claims "Small in size and lightweight. Up to 15 times less per MW than competing technologies." Its compact size makes it perfect for our purposes of a generator the sub can carry. This is credible tech, with government partners, not some scammy website, and it is something like an underwater kite carrying turbines.

Could this tech be scaled up to 6000kW? I think it would weigh 70 tonnes then (3MW version weighs 35 tonnes, the 250kW version, one-twelfth the capacity, weighs 3 tonnes, so I guess the weight scales up proportionately with the capacity?) What sort of wingspan would be needed? I see no reason not to make the wings foldable/collapsible.

Other power sources

Anærobic digestion of the crew's waste? Assume 23 submariners; how many watts could be generated?

Electrically-active microbes in the sea - https://tos.org/oceanography/assets/docs/25-1_girguis.pdf - how much could be generated here?

OTEC - https://www.nrel.gov/docs/legosti/old/254.pdf envisions a unit generating 32kW or 50% more with advanced materials. The dimensions aren't given, but the figure on page 19 shows it about six feet tall, but I don't think the weight is mentioned. How would this scale up? What size OTEC unit would the sub need to deploy to approach 6MW?

Bioenergy

Several people have brought up that fish, whales, dolphins do what I'm talking about: swim around on ocean energy by using bio-energy. Whales and dolphins do this while also supporting a warm-blooded metabolism. Technology can often do things much more efficiently than biology, so could we ingest biochemicals from the water, digest them more efficiently than an animal, and use them for an efficient propeller?

$\endgroup$
1
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Sep 15, 2021 at 14:06

9 Answers 9

17
$\begingroup$

Almost Certainly Not

6 MW is a lot of power, and all of the proposed mechanisms can only sip at available power. (Or wouldn't work at all.)

Efficiency

As with anything that moves through a medium, most efficiency gains are realized by aero or aqua-dynamics. So a knife-nosed submarine would see more efficient movement forwards, but would have enormous power demands when turning. The reason that small subs have high efficiency is because their cross-section (and the amount of water they have to shove out of their way) is small regardless of other considerations. Realizing small-sub efficiencies with a large sub is physically impossible.

Current Generation

As with all turbine generation, tidal or current generation depends on resistance. The currents want to sweep through an area, the turbines block the currents from doing so until some of their power pushes the turbines around. In the case of Minesto's "kite" generator, the "kite" is anchored to the sea floor and whips around in a figure-eight in response to the currents, generating more power than a normally-oriented tidal or flow turbine.

But the anchoring is key. Without the anchor holding the turbine in place, it would simply move with the currents - as would your submarine. Current and tidal generation cannot work with a non-anchored turbine. Unless your submarine anchors in place and charges batteries, it's not going to get anything from that.

Waste Processing

Quite aside from the fact that the facilities required to process waste into energy tend to be large and non-portable, you'd need an entirely separate generative system that burned the resultant fuel. It's hard to imagine the poop of 23 submariners making more sense than just burning alcohol.

Biological Fuel Cells

Ocean microbes still need food. While a galvanic current can be produced by certain microbes around hydrothermal vents, you'd need to provide the chemical and environmental needs of the microbes - again, it's almost impossible to imagine this as being any more efficient than just burning a fuel directly.

OTEC

As with any thermal gradient generation, the gradient is very important. The paper you linked assumed a 20C gradient, which is achieved by having your warm water intake pipe at the surface of the ocean, and your cold water intake pipe about 3 km down. Not portable. Not feasible.

Edit: (Additionally, the linked paper also makes clear what would be required to scale up their proposed model. About two thousand tonnes of copper-nickel alloy, 500 tonnes of pure copper, and 600 tonnes of thermoelectric material - though that last could be improved by modern thermoelectrics, this is just the generator, not the pumps or pipes.)

So no. None of these proposed methods could realistically power a submarine, let alone a nuclear-scale submarine.

$\endgroup$
14
  • 3
    $\begingroup$ This is true, but if all you're doing is generating power and storing it while remaining motionless, we're back to batteries or hydrogen being a wildly more practical solution than hauling thousands of tonnes of additional equipment along. $\endgroup$
    – jdunlop
    Sep 8, 2021 at 20:36
  • 9
    $\begingroup$ (I should note, though, that several km of pipe of the gauge required to move enough water to be useful is not something I would expect to find on any large vessel.) $\endgroup$
    – jdunlop
    Sep 8, 2021 at 20:37
  • 1
    $\begingroup$ (Also, note that you typically need 5–7 times the amount of chain as the depth of the water you're anchoring in. Anchoring with the bottom 100 feet below you requires 500–700 feet of chain.) $\endgroup$ Sep 9, 2021 at 13:12
  • 2
    $\begingroup$ All submarines weigh about 1 tonne per cubic meter – less with the ballast tanks empty, more with the ballast tanks full. What you say is true of other vehicles. $\endgroup$
    – Humphrey
    Sep 9, 2021 at 13:43
  • 2
    $\begingroup$ @PabloH - as for the efficient vs. feasible thing - carrying kilometers of oil-derrick pipe for a generative system that cannot be used when operating and that uses up 60+% of your available mass goes beyond "costly" and well into "this vehicle cannot be used for its expressed purpose". Waste processing and biological fuel cells either aren't within a reasonable order of magnitude of the required power or require fuel that defeats the stated goal of self-sufficiency. I'd call that a feasibility fail, not just efficiency. $\endgroup$
    – jdunlop
    Sep 9, 2021 at 16:17
5
$\begingroup$

What is a sub's role?

Engineering anything involves maximizing strengths whilst minimizing weaknesses within a specific framework of constraints. Unfortunately, it seems the baby might have been lost with the bathwater when focusing specifically on size and power whilst ignoring (or at least understating) questions of mission-lifetime, surge-demand, and reliability.

A submarine's role isn't simply to be "The sneakiest boat in a Navy!" (for varying criteria for "sneaky")... some stupid-simple inflatable rubber dingy full of relevant personnel can "sneak" a hell of a lot closer to any port-of-interest than any modern nuclear sub can do.

Determining the size/power needs of your sub won't simply depend on the size/power needs of current subs... it will depend on the landscape of possibilities that your no-nuclear-powered-subs-world represents.

What role do nuclear-powered submarines (currently) fill?

Currently, most nuclear subs seem to be focused on a mission of providing minimally-traceable launch platforms for fast-attack nuclear missiles. Therefore, our current timeline relies on both the existence of nuclear-energy and nuclear-bombs.

However, (despite nuclear-power being a reality in this timeline) not all modern subs are nuclear-powered. Why? Because not all nations want/need first-strike nuclear capability. This means that, for many mid-sized militaries having subs is less about long-term force-projection and more about short-logistics strategic defense... and under such a design constraint going small and diesel-electric is perfectly capable of (if not ideal for) getting the job done.

What role do your subs fill?

If nuclear-power is off-the-table, is M.A.D. force-projection with nuclear-bombs also off-the-table? Do your subs exist as launch-platforms, or more as battleship-killers and recon?

If your subs mostly operate in-territory... then off-base recharge may simply be irrelevant; refuelling with diesel* or hydrogen may simply be best based on the assumed-logistical-capability and mission-selection for subs in your world's near-term future. * (Consider that, although for aircraft MJ/kg is often king, in seacraft MJ/m3 may be just as likely - if not more-so - to be a driving economical factor.)

If your subs mostly operate ex-territory... then what role do your subs serve where parking-to-recharge is absolutely worth the risk of the wait? Any "ocean-energy" technology is basically relying on the economics of remote-generation offsetting the costs of either long down-times or long-term deployments. Either way, you're relying on a never-be-seen-in-the-first-place kind of sneakiness at the cost of losing get-outta-dodge energy-reserves-for-running capacity. So, you're basically pidgeon-holing these subs into mostly recon or infiltration/exfiltration kinds of slow long-term missions.

$\endgroup$
3
  • 2
    $\begingroup$ This is a wonderfully comprehensive answer to a different question. Much of it might have made very fine comments to the OP that they might think about what they need it for etc.. Please in future use answers for answering and comment the rest. You might be interested to read our meta post on frame-challenges - a different type of answer - which is something you might have considered in this case. $\endgroup$ Sep 9, 2021 at 11:45
  • $\begingroup$ Missiles? Enemy territory? There's nothing like that in my question, did you not read it? $\endgroup$
    – Humphrey
    Sep 11, 2021 at 13:55
  • $\begingroup$ I think this gets right to the heart of the matter here - they need to define what the sub is needing to do before we can tell them how to do it. I posted a comment that they just needed to use their sub as a sailing ship - almost no energy requirements whatsoever! But then, why would you need a sub? $\endgroup$
    – DWKraus
    Sep 11, 2021 at 15:34
3
$\begingroup$

On the energy content of human waste

The amount of energy remain in the gently used food varies widely based on the person and type of food. But, the highest figures I found during my searches was that about 25% of the original calories passed through undigested in the case of some kinds of nuts - the overall average being considerably lower in people with healthy digestion.

So, lets consider a Collins class sub with a compliment of 42 crew. Lets assume 2500 kCal per day / per person intake, with a unrealistically high yield of 25% calories in the waste stream.

2500 kCal * 42 * 0.25 = 52,500 kCal = 219.66MJ

So over a period of 86,400 secs per day - your total poo will yield continuous power of 2.19E8 / 86,400 = 25.35 watts (not considering conversion losses). Clearly not enough to be useful for powering the sub.

How about harvesting plankton / krill?

We have the example of whales that survive and propel themselves (frequently underwater) based on the energy they derive from feeding in the ocean. Whales also share size and speed characteristics that are similar to some submarine usages.

Developing systems to harvest the available food, convert it into useful energy, and dispose of waste will be complicated, but certainly theoretically possible knowing that whales do it.

The engineering challenges in creating a useful harvesting submarine would be severe. You would also need to harvest - filtering large amount of seawater, etc. essentially at the surface of the water - deep water food concentrations are much lower.

Even the largest whale is smaller than common submarines - even the most common Nazi u-boat was over twice as long. But the Electro-boat type XXIII coastal sub was about the same length. Note that this sub was so small it was limited 2 torpedo tubes that could not even be loaded internally.

jDunlop already provided some detail on the limitations of the other proposed methods.

$\endgroup$
4
  • $\begingroup$ Thanks for doing the maths on this. I was planning to, but you saved me the effort. This is useful in lots of worldbuilding scenarios (space habitats etc.) $\endgroup$
    – Humphrey
    Sep 8, 2021 at 20:14
  • 1
    $\begingroup$ It's worth noting that the largest whale is dwarfed by the smallest nuclear submarine ever, so if we're using nukes as the measuring stick (notwithstanding the much larger models specified by the OP), the whale model is unlikely to scale. (Harvesting biomass for energy might still work, but with a lot of compromises.) $\endgroup$
    – jdunlop
    Sep 8, 2021 at 20:51
  • $\begingroup$ See: Mixing Seawater with a Little Wastewater to Produce Bioenergy from Limnetic Algae doi.org/10.1016/j.tibtech.2017.12.002 $\endgroup$
    – Humphrey
    Sep 8, 2021 at 21:22
  • 2
    $\begingroup$ @Humphrey - I've long been a fan of algal biofuel, but an important component of algal growth is photosynthesis. You couldn't do this on board a submarine. It would be great if making fuel at a base and then loading the submarine, but then all you have is a diesel submarine, no self-sufficiency. The reason why there are lots of startup projects/research projects on it is because natural algal growth is insufficiently oil-rich and energy-rich to be efficiently processed into fuel. $\endgroup$
    – jdunlop
    Sep 8, 2021 at 21:25
1
$\begingroup$

No, no non nuclear method can generate useful levels of power.

Ocean currents require you to be affixed in a location and have extensive support technology requirements that weigh a lot. Unless you want your submarine affixed to the ground floating in a hoop, the submarine won't generate useful powers.

Submarines are required to move around, often in locations that are not ideal for power generation.

enter image description here

They can't just float around in a figure 8 pattern near the coast tethered to support equipment.

Hunting is also a slow power generation method

Blue whales just eat 6 giga joules of food per day. That's enough for about a 1000 seconds of power, or 20 minutes. Unless you are notably better at hunting than blue whales, you're not gonna get enough food. They hunt for the optimal food source in the ocean, on the surface with lots of sunlight. Your methods aren't gonna do better.

Thermoelectric power generation isn't a reliable power source for moving ships.

You need a few hundred tons of pipes going down deep into the ocean. Those pipes could hit things, will weigh a huge amount, could break, and could have lots of issues. The mass would be massively more than you can carry.

What about bursts of activity?

You could have a sub that could hunt for food like a whale. They're not gonna sustain 6 megawatts, or anything close to that, but just by floating around and using passive sonar to feed, they can probably slowly eat quite a lot. They can build up large reserves of fat, or hydrogen fuel cells, for bursts of activity in emergency, or to top up existing power.

They wouldn't be as mobile as a normal submarine, or as powerful, but they could extend their on board supplies for a while by hunting. When a threat called they could burn their fat or hydrogen reserves for bursts of activity.

$\endgroup$
2
  • $\begingroup$ To be clear: I wasn't picturing the SUB floating around in a figure-8 pattern; I was picturing it anchored in the position of the grey rectangle below the kite in the image you posted. Thanks for the good answer; I think it basically all comes down to rhythm: periods of high, medium, and low activity, which can provide interesting plot opportunities. $\endgroup$
    – Humphrey
    Sep 9, 2021 at 14:43
  • $\begingroup$ Yeah. That's more plausible for them. It means they don't need to have absurdly magical super materials or energy generation. $\endgroup$
    – Nepene Nep
    Sep 9, 2021 at 14:47
0
$\begingroup$

For Endurance, Solar Might Work

This is very dependent on water conditions, but light can penetrate dozens of feet of water. If you're at periscope depth, the hull is only a few feet below the waterline. If required, you could potentially extend some kind of semi-submerged or floating array - it would be easy to see in satellite imagery, but hard to pick up on radar since its so low to the water.

Submarines are most useful at PD

Periscope Depth is the place where a submarine can employ the most tools - the 'scope, cruise missiles, and comms masts are all denied to a submarine that is deep. So spending most of your time at PD makes good sense anyway.

You can't be Fast

Nuke boats are the only real option for going fast at depth - nothing else has the energy density to keep up. Even very modern diesel boats move slowly to conserve battery charge.

The Solar Sub is basically created a manned mine, which limps out to a given spot, floats there, and releases a barrage of torpedoes against any interlopers. This is more or less what diesel boats are today.

The solar sub is less steathy, but has the ability to crawl further from its supply lines.

$\endgroup$
0
$\begingroup$

It Could Easily Power Itself as Long as it is Staying Still

The niche of Nuclear powered ships is long term deployment. But being deployed for a long time does not have to mean moving around a lot. LetEpsilonBeLessThanZero's answer sort of touches on this idea, but missing the mark on recognizing the usefulness of a stationary sub. The subs you are referencing only need those massive power outputs when moving, but when just sitting on a sea shelf, you significantly reduce your power needs to little more than passive surveillance equipment and life support which can all be met by even a relatively tiny tidal generator.

So, in this respect you could use a normal diesel engine to get where you are going, then deploy some kind of stealth netting to make your sub look like its part of the sea floor to enemy sensors, then use tidal energy for months on end as you silently await orders.

While nuclear cruise missiles are the big one we use behind enemy line subs for today, it's not the only option your civilization could have for needing something like this. Your sub could be armed with biological weapons, dirty bombs, EM warfare systems, sea-mines, surveillance equipment, drone swarms, or something totally made up. It does not matter what is is as long as you can imagine something worth keeping deployed near an enemy shore for long periods of time.

$\endgroup$
0
$\begingroup$

The New Age of Sails:

Ships circumnavigated the globe using the power of the sea centuries before modern vessels, and they didn't even need hydrogen fuel. They used one of the oldest power sources on your list. They used sails.

Today, modern ships are returning to their roots and developing sail technology to off-set the very power concerns you have. Deliver the ship to where it needs to go without external inputs at the lowest cost possible. By trying to use all these power sources to generate electricity, you bind yourself to engines, to mass, to sitting still for long periods. If you're going to slow down the trip anyway, why not open up a deployable sail and just drift along?

Many innovative designs are coming out to allow easily deployable sails that don't take up large amounts of deck space. these same designs can be stowed when the ship needs to submerge. similar set-ups can harvest some of this wind energy for electricity that can then be used to charge batteries or possibly even generate hydrogen to refill the fuel tanks you'll still need to submerge.

I could even see designs where you have lighter-than-air sails filled with that same hydrogen fuel you're making. pump out and pressurize the hydrogen to submerge, folding up the sails. So you'd use wind to generate hydrogen AND keep the sails inflated, only pressurizing it when you had excess. Thus the ship sails, makes it's own fuel for later, and your submarine may even just look like a low-tech vessel sailing across the ocean anonymously. If your world still uses wind power to transport goods cheaply due to fuel constraints, it's the other end of stealth - hide in plain sight.

For much of the history of submarines, they traveled on the surface because it took less fuel and solved the problem of air. Hulls were optimized for surface travel, only diving when needed (in your case, whenever the plot calls for it). Nuclear power pushed the subs off the surface because they no longer had to be there.

This is simply a return to the world before nuclear power. Goods were still being shipped via wind power till around the end of WWII. For a really autonomous submarine, flirt with being a sailing ship.

Sails for modern ships

$\endgroup$
1
  • $\begingroup$ I agree sailing ships are very cool and promising, regardless of the tech level. I've been wondering if a submarine could use a sail underwater, treating ocean currents as winds. As long as you can catch the Bernoulli principle making the current billow one side of the sail and create lift, it should work the same in water as air. $\endgroup$
    – Humphrey
    Sep 11, 2021 at 17:46
-1
$\begingroup$

Ok worldbuilders, can we have a chat about METHANOL

There's certainly methanol in seawater and this paper – Mincer, T. J., & Aicher, A. C. (2016). Methanol Production by a Broad Phylogenetic Array of Marine Phytoplankton. PLOS ONE, 11(3), e0150820. doi:10.1371/journal.pone.0150820 – says the following:

Stan-dards were prepared using analytical grade methanol (99% purity, Sigma-Aldrich, St. LouisMO, USA) in sparged natural seawater (bubbled through a sparging stone with nitrogen at 50mL/minute,5 hours at room temperature). Methanol standards were included in every run,in triplicate technical replicates, with typical concentrations in the range of 0.240–24 micromolar (μM), a typical calibration curve is represented in S1 Fig.

Now is that saying there are 0.240–24 micromolar of methanol in seawater or not? To cut to the chase: what IS the concentration of methanol in seawater?

You wouldn't need to combust it (using oxygen you don't have) to generate energy; you could use one of these: https://en.wikipedia.org/wiki/Direct_methanol_fuel_cell

https://www.sciencedirect.com/topics/engineering/air-independent-propulsion mentions methanol fuel cells as a possible form of air-independent propulsion for submarines.

$\endgroup$
2
  • $\begingroup$ Once again, the articles you've linked directly contradict your proposed usage. The final equation for the direct methanol fuel cell is 3 O2 molecules for every 2 methanol molecules, because oxidation is crucial to fuel cell operation. The Air-independent-propulsion methods in the paper all involve oxygen, just stored independently, rather than snorkeled, as stated explicitly in the paper. $\endgroup$
    – jdunlop
    Sep 17, 2021 at 23:08
  • 1
    $\begingroup$ Though there is some dispute over the exact biogenesis of methanol in seawater, the maximum reasonable quoted concentration is 400 nanomolar. The heat of combustion (inherent energy, naively assuming no losses) of methanol is 726 kJ/mol. If we assume 100kW constant generation (extremely conservative), that's 8.6 GJ. You would need to process twenty-nine and a half million cubic metres of water a day to gather the required methanol - again, assuming no energy cost of harvesting the methanol, nor any losses in burning it for energy. $\endgroup$
    – jdunlop
    Sep 17, 2021 at 23:13
-1
$\begingroup$

So your criteria are [1] autonomous, and [2] non-nuclear. I think there's a way to fulfill [1] if you compromise on [2]

There are 3.3 micrograms of Uranium in a liter of seawater (http://large.stanford.edu/courses/2017/ph241/jones-j2/docs/epjn150059.pdf)

Therefore there are 3.3 milligrams per cubic meter

That's 0.0000033 kilograms per cubic meter

Uranium contains 22,394,000 kWh/kg

0.0000033 × 22,394,000 = 739.002, the number of kilowatt-hours of Uranium energy in a cubic meter of seawater.

You'd need to process 81.2m³ per hour to get 6000kW worth of Uranium (or twice that at 50% extraction efficacy, ten times that at 10%, etc.)

Metal-organic frameworks (MOFs) could be used in a futuristic setting to extract the uranium from the seawater.

Not sure how uranium refinement/enrichment fits in to this picture.

$\endgroup$
1
  • $\begingroup$ This is explicitly a nuclear submarine, which is emphatically against what the OP requested. $\endgroup$
    – jdunlop
    Sep 22, 2021 at 22:25

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .