Electric cars are a pipe dream

[Syzygys]

Tesla Motors Inc., a U.S. maker of electric cars, said next year’s production of its new Model S has sold out and that the company should earn a profit in 2013...

http://www.bloomberg.com/news/2011-...el-s-sold-out-should-turn-profit-in-2013.html

 

[adoucette]

But even if they sell 6,500 units that really means absolutely nothing in a car market of over 12 million cars a year (in a bad year).

 

[Billy T]

... Indeed your math is filled with flaws, like: Note the tension in the walls of the square tube and the round tube is the same so the wall thickness is t for both with the same safety factor and same internal pressure...Arthur

No that is correct for the walls which have zero tendency to "bow outward" because both sides have the same pressure acting on them - I.e. for all the internal division or "tension web" walls of the "flat tank." They are in "pure tension" no flexing outward force acts on them, as in the single square tank. But thanks for finally trying to find an error in my math proof.

Two vertical flat internal division or "tension web" walls 1 cm apart, even if 4 cm tall, in the "flat tank" will have EXACTLY the same stress in them as is found everywhere in a one cm diameter circular cylinder tank, but of course it will be 60 times less thick than in a single 60 cm diameter tank storage tank such as now used in NG cars, so their thickness can be 60 times less.

My math model does ignore several minor (from a weight POV) details (and may have more important errors, so hope you will check it.) so I will tell a little more details about these internal tension web walls. They would be quite like those used internally in airplane wing, which keep the lower pressure on top of the wing from pulling the top out of shape by tying it with "tension wall to the flat bottom of the wing. I.e. along their midline the could have circular holes, but this would not reduce their weigh significantly as where the holes are the total cross section of web wall must be the same as where they tension walls join with the outer wall. There is also small "fillet" at this junction. One must, or at least should facilitate the smooth transition of the vertical tension in the webs to the horizontal tension in the outer walls, but this is getting into a level where computer modeling of stress distribution patterns is needed, so I have ignored these minor details.

 

[billvon]

That is true, but the Honda GX is just a very slightly modified Honda Civic, NOT a car designed for NG. I.e. the NG tank was just stuck in the trunk of a Civic, reducing the useful trunk space by 50%, etc.

Well, if your criteria for intentional design is "no reduction in trunk space" then the Nissan Leaf is not designed to be an electric car; it has a significant reduction in trunk space due to the battery pack.

A car designed for NG would have a full size trunk as the NG tank would be integrated into the body, probably a "flat panel" tank of more than 100 long, small cross section, rectangular tubes, adjoining side-by-side, so they share a common wall. I.e. each rectangle (except for one - one extreme edge of the "flat tank") has only three, not four, walls. This economy of walls, makes the weight of the tank very significantly LESS than the conventional single large round tank holding the same volume. This flat tank serves as the floor board and lowers the center of gravity, compared to a big round NG tank 100% above the floor board, improving the car's stability.

All natural gas tanks are spherical or cylindrical for strength, safety and economy reasons. The reason no one uses prismatic tanks is not that no one but you has thought of it - it is because they don't work well.

For example, they might be 1cm wide and 4 cm tall. If on average (they are shorter where the wheels are) in a front wheel drive car, they might be be 1.6 meters long. Thus, the average volume of one tube would be: 1x4x160 = 640cc or 0.64 liters. If the width of the car is 1.4 meters, then there could be about 125 parallel tubes.

If you have a (relatively) flat surface that is 1.4 by 1.6 meters, and you pressurize it to a standard 3600 PSI, the flat surface will have to withstand a lateral force of 12 million pounds. This would require insanely thick walls, and would likely be so heavy that cars could not carry it.

Then the stress in the two edge tanks would be EXACTLY the same as in 4cm radius round tube.

Not even close. Cylindrical and spherical tanks work by operating in tension rather than opposing lateral forces. Any force placed on the wall of a sphere merely increases tension on the wall; it does not need to resist lateral deformation since the forces working to deform it are trying to deform it into a sphere.

Once you remove sections of that sphere or cylinder you lose that advantage. That's why cylindrical tanks have their outlets at the top rather than along the sides. (That's also why the tops and bottoms are much, much thicker than the sides; they have to resist the forces trying to change them to a sphere.)

 

[adoucette]

Billy, just because you essentially corregated the outside of your rectangle shaped tank with little humps didn't make it suffer from the same weaknesses as any rectangular shape, and as pointed out, for the same thickness that shape can only hold bewteenb 1/20th and 1/120th of the pressure of a cylinder with the same thickness wall.

They don't make rectangular pressure vessels, even with your webs, because they aren't practical, not because no one considered it.

EPIC FAIL Billy.

 

[Billy T]

Well, if your criteria for intentional design is "no reduction in trunk space" then the Nissan Leaf is not designed to be an electric car; it has a significant reduction in trunk space due to the battery pack.

we have slight terminology problem I think. I agree that all cars sold in any volume, even just a dozen (and including all the NG cars on the market) are "intentionally designed" to be as they are. What I was trying to suggest with "designed for NG" is that never AFAIK has an NG car been designed from "scratch." All have started with an existing car and modified it.

... If you have a (relatively) flat surface that is 1.4 by 1.6 meters, and you pressurize it to a standard 3600 PSI, the flat surface will have to withstand a lateral force of 12 million pounds. This would require insanely thick walls, and would likely be so heavy that cars could not carry it.

Well let’s calculate a little, instead of guess:

I have a 1.6 meter long "tension web" spaced every cm apart. That in your 1.4 by 1.6 meters is 140 x 160 = 22,400 linear cm trying to hold the two outer plates together. Or each cm of length must support a tension of 12E6 / 0.224E5 pounds = 536 pounds per linear cm. I will not trouble to look up tensile strength of many materials but here is some data on one fiber/binder matrix:

“… Under the trade name of GraphliteTM this material is cost effective and consistent in properties. You can build wing spars {wing spars are much like my “tension webs” but hold the top and bottom surfaces of the wing together mainly against flexing stresses} faster with much greater reliability. Fabricated in one operation in a female mold, it cuts assembly time in half. {Note it is molded, not layered in place like old fiber glass cloth. binder construction} … It's more than six times stronger than 2024-T3 aluminum, twice as stiff and nearly half its weight. … Maximum performance is obtained in every fiber resulting in tensile strengths exceeding 350,000 psi {but that is with their extrusion process they call “pultrusion” as it get the fibers mainly aligned in the direction of the expected tension.}
From: http://www.marskeaircraft.com/carbonrod.html

I will assume only 10% of their max strength or 35,000 psi. Now a square inch is 6.45sq cm. Thus the tensile strength of the webs is 35E3/6.45 = 5426 pounds per sq cm. Thus with no safety factor less than 1mm thick web is all that is required, (to resist 536lbs/cm) but let’s assume the webs are 1.2 mm thick (and 160cm long and 4 cm tall) but weigh only about half what aluminum does with a 20+% safety factor.

I would not call 1.2mm thick webs, with a 20% safety factor and half the weight of Aluminum, “insanely thick” would you? (Now that we have quantative rather than intuitive results for their thickness)? Probably minor thin spots in the extrusion would make the design thickness 1.5mm, but perhaps we can do better than only my assumed 10% of max tensile strength? This flat tank floor board, when empty, would be significantly lighter than the current steel ones and probably stiffer, so I think the car could carry it OK.

SUMMARY: Point is the webs are NOT “insanely thick” or heavy as you suggest. Have more faith in math analysis and less in intuition.

 

[billvon]

I would not call 1.2mm thick webs, with a 20% safety factor and half the weight of Aluminum, “insanely thick” would you?

Nope. And if you made a cylindrical tank out of such material it would work well. Indeed, many tanks are so reinforced with composite material outside a thin metal tank.

However, you're talking about putting such material inside a tank, somehow bonded to a surface to resist a pull of 12 million pounds, as well as all the normal stresses (i.e. unequal expansion and contraction due to thermal cycling, expansion and contraction due to pressurization cycles, bending stresses due to road vibration and impacts etc.) If any small part of the bond fails, the load will be transferred to the bond nearest to the failure, and it will quickly propagate and cause a massive explosion.

It may be possible to develop such a technology, but it is far from straightforward, and currently cannot be done. That's why all CNG tanks are cylinders or spheres.

SUMMARY: Point is the webs are NOT “insanely thick” or heavy as you suggest. Have more faith in math analysis and less in intuition.

Any such tank made with current technology would indeed be insanely thick. This is not an issue unique to cars; LNG tankers could save millions of dollars a year by using prismatic pressurizable tanks, rather than low pressure prismatic tanks (much faster boiloff) or high pressure spherical tanks (lower capacity; will not fit through many canals.) Yet they do not - because of the problems involved with using prismatic tanks. That is a good indication that it cannot be economically done.

But if you feel it could be, I encourage you to give it a try. Needless to say, high pressures can be deadly, so be careful if you try this.

 

[Billy T]

... However, you're talking about putting such material inside a tank, somehow bonded to a surface to resist a pull of 12 million pounds, as well as all the normal stresses (i.e. unequal expansion and contraction due to thermal cycling, expansion and contraction due to pressurization cycles, bending stresses due to road vibration and impacts etc.) If any small part of the bond fails, the load will be transferred to the bond nearest to the failure, and it will quickly propagate and cause a massive explosion. ...

No both in the post you just read and in several others I have made it clear there is NO Bonding - The flat tank is completly extruded*, even the corner "fillets" which smoothly transfer the vertical tension in the webs to the horizontal external surfaces. This change in the direction of tension is part of the reason there is no effort to aligned the fibers and why only 10% of the max strength with quasi-lined fibers was assumed (and then an additional 20+% safety factor was added to the design. - Sort of like assuming only 8% of the max strength was available!)
-------
* Except for the tiny 1 by 4cm slightly rounded, end caps of each tube. They could be hollow "cups" that extend into the tube a cm or so to keep them from being "blown out" by the pressure - or if 1 cm penetration (10 sq cm of bonding surface) into the tube is not more than enough, whatever is necessary.

...{part of post 2188} Fabricated in one operation in a female mold, it cuts assembly time in half. {Note it is molded, not layered in place like old fiber glass cloth. binder construction} … Maximum performance is obtained in every fiber resulting in tensile strengths exceeding 350,000 psi {but that is with their extrusion process they call “pultrusion” as it get the fibers mainly aligned in the direction of the expected tension.}
From: http://www.marskeaircraft.com/carbonrod.html ...

AND:

[part of post 2175}...
Below I give a crude cross sectional picture of the extruded floor board, which is also called the "flat NG tank", but image each of the "interior Cs" has only three straight sides:

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCO This final O tube is the one that has four sides, but as I will soon discuss and illustrate below, it is really a "flattened D". ...As the flat tank is extruded*, the two extreme sides of it could have thicker walls. Thus these two outer most tubes could only "bow out" 2 cm wide instead of be only 1 cm wide. Effectively the tank would have only 123, not 125 tubes or hold perhaps only 21 gallons,...

* The extrusion would used a binder and short carbon fibers for strength - probably the same mix as used in Boeing's new 787 "dreamliner."

PLUS OTHERS

BTW, the "L" in LNG is for Liquid. The tankers hauling very cold LNG, in rapidly increasing volumes, all over the world use spherical tanks, I think, mainly for thermal reasons - least possible surface for heat transfer into the LNG so least possible loses in transit. I don't know but strongly suspect the density of LNG is much greater than the CNG used in cars. ("C" being for Compressed natural GAS) These spherical tanks are as large as will fit on the ship as the thermal loss area only increases with the square of the diameter but the transported volume with its cube. For transport of LNG, my "flat tank" would be an abomination sent from the worst part of Hell.

 

[adoucette]

Billy claims he has designed a flat CNG tank that he claims is 35% more efficient than any cylinderical tank in existence when scientific journals on the subject proclaim that cylindrical pressure vessels are able to carry 20 to 120 times higher pressure for equivalent wall size then rectangular ones and our Billy doesn't even work in the industry.

Too bad he's too old to make a mint off of his brilliant reasoning, and so, the philanthropist that he is, he shares freely and openly with the world this amazing insight into structural engineering, a design which could probably have made him a billionaire.

All HAIL Billy, If he can't design it, Nobody Can.

Arthur

 

[adoucette]

If the width of the car is 1.4 meters, then there could be about 125 parallel tubes. So the flat tank's total volume could be: 125x0.64= 80 liters, which is 21.134 gallons, bigger than most gasoline tanks, yet making the floor less than two inches thick.

And if only CNG had the energy density of gasoline.

It takes 728 cubic inches of CNG to equal 1 gallon of Gasoline.

Your supposed revolutionary engineered flat tank doesn't have the equiv of 21 gallons of gasoline Billy, only 6.7 gallons.

More to the point, Three 10.1" diameter conventional CNG tanks, 33.7" long, weighing just 31 lbs each would hold the equiv of 2.6 gallons each, which would be 16% more than Billy's Unobtanium Flat Tank and could easily be placed in the vehicle, and unlike Billy's taken out for inspection when necessary (after any crash), and unlike Billy's unobtanium tank is available off the shelf.

http://www.luxfercylinders.com/prod...e-fuel-cylinder-specifications?tags=undefined

Finally Billy, just to show how ABSURD your measurements are for your Unobtanium Tank (the one you claimed would make "the floor less than two inches thick" (LOL)).

Consider these specifications for Cylinderical Tanks.

A1134D holds 112 liters but has a radius of 321 mm and a length of 1829 mm

Which would make it's walls about 20 mm thick.

You specified a tank only 4 cm tall, so like this tank, your walls would also be at least 20 mm thick, in other words, all wall and no CNG.

Or compare the sq meters of tank sides to the volume.
The cylinderical tank specified above has FAR more volume and uses FAR less surface area, making your tank FAR less efficient (not even counting the internal walls)

My calculations show that even if you could make your thin and wide Unobtanium flat tank (I've made it 8 cm tall so that it has an internal height of 4 cm that you specified) and use the same thickness walls (it would in fact be much thicker) it would still have THREE times the surface wall area of the A1134D tank, and so while your tank holds only 6.7 GGE, or 40% less CNG than the A1134D, it would weigh about 3 times as much (about 320 lbs empty, so NO, you can't put it on the roof).

Epic Fail Billy.

Arthur

 

[billvon]

No both in the post you just read and in several others I have made it clear there is NO Bonding - The flat tank is completly extruded*, even the corner "fillets" which smoothly transfer the vertical tension in the webs to the horizontal external surfaces. This change in the direction of tension is part of the reason there is no effort to aligned the fibers

Ahh. Well, then you can't use graphlite. It depends on aligned and equally tensioned fibers.

And no, you can't just assume "well, if I don't align the fibers I will get 10% of the strength." Nor can you extrude carbon fiber composite and keep control of the alignment of fibers in complex shapes.

However, if you could, and could create tanks that had fibers oriented so that their long axes took the loads both in your webs and in your surfaces, that might work OK. Your problem then is that at the join between the top surface and the web there would be a lot of pressure (again, in the millions of pounds) trying to force those square joins into cylindrical shapes. Since the fibers would be good at resisting tension, but lousy at resisting attempts to separate the fibers, the joint would rapidly fail and allow the web+surface to conform to a more cylindrical shape.

Try this experiment. Make your own tank out of Soarcoat or other zero porosity fabric (available from Gelvenor.) Sew the seams and seal them with tent sealant. Alternatively, use an engineered fabric, of the sort Atair makes for sails and parachutes. This has to be bonded, so choose a good glue (similar to the matrix in the carbon composite you talk about above.)

Then pressurize it to low pressures, just in the 10's to 100's of PSI. I think you'll learn quite a bit about how such structures handle pressure,.

BTW, the "L" in LNG is for Liquid. The tankers hauling very cold LNG, in rapidly increasing volumes, all over the world use spherical tanks, I think, mainly for thermal reasons - least possible surface for heat transfer into the LNG so least possible loses in transit.

The vessels that use spheres keep the LNG under pressure to reduce boiloff rates, thus requiring less insulation and losing less cargo. The drawback there is that spheres are not efficient at using the space within the hull, so some cargo capacity is lost.

The vessels that do not use spheres, that use basically hull-shaped tanks, must keep pressures much lower to avoid deforming the tanks (and thus the hull) - these rely on thick insulation to keep boiloff rates low, thus reducing the cargo they can carry.

Shipwrights would love to have economical, oddly shaped pressure vessels so they could reduce the amount of insulation needed and increase cargo capacity. Unfortunately such a design is not available.

 

[Billy T]

And if only CNG had the energy density of gasoline.

It takes 728 cubic inches of CNG to equal 1 gallon of Gasoline.

Your supposed revolutionary engineered flat tank doesn't have the equiv of 21 gallons of gasoline Billy, only 6.7 gallons.... Arthur

You are again putting words in my mouth. I never said the flat tank had the "equiv of 21 gallons of gasoline." Working in metric units, I calculated the tank volume as 80 liters (and then as many have more feeling / understanding of/ for gallons, I converted 80L to 21+ gallons.)

 

[Billy T]

... And no, you can't just assume "well, if I don't align the fibers I will get 10% of the strength." ...

The company I took my tensile strength data from has an extrusion process they call "pultrusion" which for rods does get the fibers mostly aligned along the rod. I don't know what it is but guess they use fibers with length several times the rod diameter and squeeze the randomly aligned mix of fibers out slowly (in a long slowly reducing cone so the "transverse fibers" at least get their ends bent into the rod axis direction.) They also, as I quoted, make airplane spars BY MOLDING that are stronger than those made of the best aluminium. Based on this information I though I could assume (and then later add a 20+% safety factor) 10% of their max PSI tension rating. Do you have any reason to believe randomly aligned fibers are worse than 10% of max strength? Or is that just your opinion?

... The vessels that use spheres keep the LNG under pressure to reduce boiloff rates, thus requiring less insulation and losing less cargo. ...

I don't know anything about why spheres are used. I assumed it was to have the lowest surface to volume ratio for least heat transfer in per unit volume of LNG. Do you really know that is not the reason OR are you just assuming too. If not give a link telling the typical operating pressure.

In another thread, about a year ago, I asked if anyone knew how they were insulated - expressed my believe that the were to big to use the best insulation. - I.e. could not be just a huge "vacuum bottle" due to their weight and more importantly, the stresses that an "angry sea" tossing them about would make. No one offered an answer to my question. Can you, if you know facts about LNG tanks on ships?

As I have worked on space craft design, I know that many (>20) layers of "krinkled metal foil" is very good insulation there especially out side the pressurized part. As the tiny space between foil is then in gas free, the heat transfer is largely radiative and easy to predict /calculate. A series of T^4 surfaces, but the tiny metal contact conduction must be included also. Thus, in ignorance, I would guess they wrap the LNG with "krinkled metal foil" and also have ~1% of its surface covered with high strenth to therml conductivity "pads" = perhaps glass?

Making them pressurized would make this structural problem even worse, so I doubt it is done but I could be wrong. Again do you have a link telling how much above atmospheric the LNG is under, or were you just guessing they are pressurized. Unlike CNG, you don't get more natural gas into the tank by pressurizing it more - LNG, like water, is essentially an incompressible LIQUID.

 

[adoucette]

You are again putting words in my mouth. I never the flat tank had the "equiv of 21 gallons of gasoline." Working in metric units, I calculated the tank volume as 80 liters (and then as many have more feeling / understanding of/ for gallons, I converted 80L to 21+ gallons.)

Well then you shouldn't have compared it to the volume of a gasoline tank since the implication was quite misleading:

which is 21.134 gallons, bigger than most gasoline tanks, yet making the floor less than two inches thick.

Because the Gasoline Equivalent Volume is less than 7 Gallons which is much less than the quite small 12 gallon tank in the Prius.

Don't matter Billy, you can't build the tank, and even if you could it would take far more wall area (= cost + weight) to hold a tiny amount of CNG.

Arthur

 

[Billy T]

Well then you shouldn't have compared it to the volume of a gasoline tank since the implication was quite misleading... Arthur

Perhaps for you who frequently extend what I actually say to be something false you can then attack, but reason I continued the 80L is 21+ gallon to comparison to a volume of a small car's tank was many don't even have much feeling for the size of a 21+ gallon tank. I.e. I wanted to show it was about twice the size of a small car's fuel tank, yet only adding less than two inches to the thickness of the floor board.

BTW, I don't expect the first use of the "flat tank" to be as LNG car's floor boards, but on top of LNG trucks (for no loss of cargo space and very little increase in wind resistance) or even as the roof of city buses,* but this thread is about cars so I am suggesting that someday an LNG car may be DESIGNED, instead of just be slightly modified gasoline car with tank stuck in the trunk (or under it as is done by the taxis in Sao Paulo, which need to keep full trunk capacity for people's bags, and can on city roads give up some road clearance.)

* In neither of these cases would it be necessary to cut out part of the extrusion to make clearance for the wheels.

 

[Billy T]

... Because the Gasoline Equivalent Volume is less than 7 Gallons which is much less than the quite small 12 gallon tank in the Prius.

... but 7 gallons of gas at 35 or 40 mpg fuel efficiency will take you up to 280 miles between fill ups. I.e. at least twice what most even pure electric cars can do, and at less than $1/gallon equvillent cost per mile.

Again, this thread is about Electric cars and their competitive alternatives, not about the efficiency of gas cars like the Prius, but what MPG dose it give? (is my 35 to 40, mpg a reasonable guess?) If the cost of CNG keeps falling and the cost of gasoline keeps rising, then someday a DESIGNED FOR CNG car will be made - I won't live to see it -I'm too old.

 

[adoucette]

Perhaps for you who frequently extend what I actually say to be something false you can then attack, but reason I continued the 80L is 21+ gallon to comparison to a volume of a small car's tank was many don't even have much feeling for the size of a 21+ gallon tank. I.e. I wanted to show it was about twice the size of a small car's fuel tank, yet only adding less than two inches to the thickness of the floor board.

BTW, I don't expect the first use of the "flat tank" to be as LNG car's floor boards, but on top of LNG trucks (for no loss of cargo space and very little increase in wind resistance) or even as the roof of city buses,* but this thread is about cars so I am suggesting that someday an LNG car may be DESIGNED, instead of just be slightly modified gasoline car with tank stuck in the trunk (or under it as is done by the taxis in Sao Paulo, which need to keep full trunk capacity for people's bags, and can on city roads give up some road clearance.)

* In neither of these cases would it be necessary to cut out part of the extrusion to make clearance for the wheels.

The Unobtainium Flat Tank will never be built Billy because it has been shown to be impossible to build to your specifications.

As Billvon already explained to you, they don't build flat tanks because no one has thought of them, they don't because the physics behind the flat tank makes it unworkable.

 

[Billy T]

... As Billvon already explained to you, they don't build flat tanks because no one has thought of them, they don't because the physics behind the flat tank makes it unworkable.

Here I think you are putting words in his mouth - he raised a physical objection and I answered it. He, like you, believes they must be unattractive or they would be used. He suggested that ocean transport would be using them. I replied to that too - noting that from thermal considerations in that use, the flat tank would be: "An abomination sent from the worst part of hell."

He has not attacked me personally, but has been helpful with comments based on physics, which thus far I have been able to refute.

 

[adoucette]

... but 7 gallons of gas at 35 or 40 mpg fuel efficiency will take you up to 280 miles between fill ups. I.e. at least twice what most even pure electric cars can do, and at less than $1/gallon equvillent cost per mile.

But that's a problem Billy, because of the lack of filling stations for CNG you need greater range then gasoline powered cars to get adoption (outside of fleet use). Which is one of the issues with the Civic GX and it has an 8 gallon gasoline equiv tank and an average mileage of 25 mpg, giving it but an ~200 mile range. With your much heavier and smaller tank you would get only 167 miles on a car as small as the Civic.

http://alternativefuels.about.com/od/naturalgasvehiclereviews/fr/2009HondaGX.htm

Again, this thread is about Electric cars and their competitive alternatives, not about the efficiency of gas cars like the Prius, but what MPG dose it give? (is my 35 to 40, mpg a reasonable guess?)

The EPA estimate for the Prius is 50 mpg.

 

[adoucette]

Perhaps for you who frequently extend what I actually say to be something false you can then attack,

No Billy, your statement was totally misleading to anyone who didn't know that a 21 gallon CNG tank has only the equiv capacity as a 6.7 gallon gas tank does, and you did not even hint at that, but that conversion is not common knowledge.

but reason I continued the 80L is 21+ gallon to comparison to a volume of a small car's tank was many don't even have much feeling for the size of a 21+ gallon tank. I.e. I wanted to show it was about twice the size of a small car's fuel tank, yet only adding less than two inches to the thickness of the floor board.

No Billy, even in a Cylinder, which has much thinner walls for the same pressure, the wall thickness of a tank holding about the same as you are specifying is 20 mm, so your 4 cm tank height is the internal height, otherwise it would be all tank wall. My rough guess is your flat tank would be about 12 cm tall and weigh about 600 lbs and hold about 7 GGE.

Which is an EPIC FAIL Billy.

Worse, it would be nearly impossible (expensive) to inspect, and these tanks have to be inspected after any possibility of damage, and so a tank that was part of the vehicle would doom the entire vehicle just because of a small fender bender.