Say that a scientific discovery was made that could effortlessly convert H20 into hydrogen and oxygen gas. For this discussion, imagine that the conversion is practially free. Something like a simple catalyst and sunlight is all it needs. From the resulting hydrogen and oxygen gas, how many MPG would a car likely get?
WAG without calcs?
2 to 3 times normal.
Based on this table, energy content of gasoline is 44.4 MJ/kg, and hydrogen contains 121 MJ/kg. But water is only 2/18 hydrogen (by mass), so if you separated 1kg of water into hydrogen and oxygen, you’ll only get back 13.4 MJ of energy. So the “MPG” would be roughly 30% that of a comparable gasoline engine.
But I hope you realize that separating H2O into hydrogen/oxygen takes more energy than you’ll get back by burning that hydrogen? So if you have an energy source that you can carry on your car and generate hydrogen from water, you’d be better of using that energy source to run the car directly.
Incidentally, a catalyst can change the rate of chemical reactions but it can’t change the amount of energy needed to convert H2O into H2 and O2.
If we have a car that does 45 MPG at 45 MPH, that’s an average power consumption of 33 kilowatt. If you convert it to water/solar power, you still need a 33 kilowatt power source to generate the hydrogen. Even if you invent a 100% efficient solar-powered H2 generator, sunlight only contains about 120 watt per square meter, so you need a 280 square meter collecting area. That’s about 3000 square feet. I think that attachment will make your car somewhat less efficient, so you need an even larger area to make up for that.
Man, I am good. I was right on the money with a factor of 3 (121/44)
Till I forgot that most of the wieght was oxygen, which gasoline doesnt have to carry around :smack:
You can probably make an internal combustion engine that burns pure hydrogen and oxygen significantly more efficient, another WAG would be 50 percent more efficient give or take.
So, that 30 percent MPG would probably be more like 45 percent give or take.
I read once if you could harness the molecular energy in a gallon of water, you could provide the electricity for a medium sized city for a year. I have no way of verifying this though.
(Bolding mine)
Ummm…
Try 1,000 Watts/square Meter.
Yeah, I realize it takes a lot of energy and it not really practical. I’m just wondering if a magical, super-advanced method was discovered such that it didn’t take any energy to split water into gas, how well would it work to have water as a fuel.
But… if you posit a magical, super-advanced method that effortlessly splits water, then the amount of energy in a gallon of water is infinite. You separate it into H2 and O2 for free, burn it for the energy, then recapture the water and you’re back where you started. There would be no end to that cycle. You’d build engines with one gallon of water each in a sealed container and never refill them.
According to my calculations, a gallon of water has 193 kJ of molecular bond energy. Even if you could get all of that, you couldn’t run a single house for a year, maybe not even a week. Maybe you read that if the matter in a gallon of water was directly converted to energy, you could run a city for a year. That would be around 3.40x10^14 kJ.
Perhaps. Perhaps not.
I can imagine some chemical compound you carry around that when mixed with water uses the Oxygen and liberates the Hydrogen, which would then be burned in an IC engine.
Since you dont have the O2 to burn, your back to the 30 percent give or take MPG. You would also have the chemical to carrry around. The chemical would be finite amount and would probably weigh in the same ball park as the water.
Not the the greatest system in the world, but still better than batteries that cost a fortune or take long periods of time to recharge.
Are there any chemical compounds that fit the bill?
There’s a method to get hydrogen from water using aluminum and gallium. Ignoring the energy and cost of making this, which is quite a bit, you can have a water powered car. Which is misleading as you have an aluminum powered car because once you run out of aluminum you will need to buy more.
Alternately, you could crack the water at some central location (using coal or nuclear or solar or whatever power), and just fill up the tank with the resulting hydrogen. The problem there is that even though hydrogen has more energy per mass than gasoline, it has less energy per volume, even when compressed down to a liquid. And keeping the hydrogen liquid would require either extremely sturdy tanks, heavy-duty cryogenics, or both, neither of which would be safe or practical in a consumer vehicle.
Uh, er, right, I knew that. :o I was in a hurry and glanced at this page and used that number, as I thought that was supposed to be a typical flux at ground on a fair weather day, but now I’m not sure if that was the right interpretation or not.
But even at 1000 W/m[sup]2[/sup] it’d take a 360 square ft solar collector.
But the fact is, hydrogen + oxygen (as separate gases) contains more energy than the water. So if you have a magical way to do it without using energy, what you have is a magical way of getting energy out of nothing. If we assume that’s possible, Laws of Thermodynamics are out of the door, and anything is possible. Perpetual motion and all that.
Well, I can’t answer the last question; but the first couple paragraphs are still missing the point. Burning the hydrogen is combining it with oxygen to create water. If the process of breaking up the water uses up the oxygen atoms, that’s OK–there’s lots of them floating around. So we suck in the air, burn the hydrogen & end up with oxygen-poor air with a high water content; then, precipitate the water out of our exhaust & feed it back into our magic electrolysis box to pull the hydrogen back out, ready to re-use. Not perfect, as we’re limited by the efficiency of our method for sucking the humidity out of the exhaust–but that process can be designed to be very, very efficient.
If this were somehow possible, energy issues would be resolved forever by some fairly simple engineering.
Not neccessarily.
Basically, you MIGHT have a chemical compound that has stored energy and releasing that energy is done by mixing it with water. That energy is then expressed as the production of Hydrogen and Oxygen. You can extract THAT energy for useful purposes by burning it in an IC engine or a fuel cell.
Of course its obviously going to take MORE energy to create that compound than you would get out your magically produced hydrogen and oxygen. But such a system may still be better under some circumstances than trying to store energy in batteries.
And as someone pointed out, its really not an water powered car anymore, its a magic compound powered car that uses water too. Its just we actually know how to usefully use hydrogen and oxygen to power a car once we have them in a separated form.
Wouldnt sodium, lithium, and pottasium used with water at least fit this bill?
And in the really weird category, I seem to remember one internet proposal for burning Boron as fuel. IIRC it had a good energy storage and the oxidized boron could be collected as a solid and reprocessed.
Well, I guess … Let’s see, ignoring the heat of reaction of the lithium with the water (which is pretty negligible, relatively speaking), we need about 3 gallons (12 pounds) of water to produce the same amount of energy as one gallon of gasoline. (We’ll need some remaining water to dissolve the resulting lithium hydroxide, but the water’s cheap, right?) We’ll need about 4 pounds of metallic lithium to liberate that much hydrogen; I find conflicting information about current prices, but it looks like $60/pound is a conservative estimate for wholesale.
So, to drive our car as far as it would get on one gallon of gas (costing about $2.50 and weighing about six pounds), we need water and lithium in amounts weight 150% more and costing almost exactly 100 times what the gasoline would cost.
Of course, lithium wouldn’t stay that cheap if we started burning it in our cars … It’s a much more limited resource than hydrocarbons. On the plus side, we’ll really put a dent in global warming, since water vapor is a less efficient greenhouse gas than CO2, is more efficiently cycled out of the atmosphere, and most especially because none of us will drive very much on the equivalent of $250/gallon gasoline.
What does it take to mine and refine lithium? I’d be absolutely stunned if it’s just sitting around in a highly reactive form, just waiting for someone to come pick it up.
Wiki claims that it’s produced by electrolysis of lithium salts. Meaning that on top of your calculation of the impracticality, it probably took the energy equivalent of a lot more than a gallon of gas to produce that 4 lbs of lithium. TAANSTAFL, and people have been trying to come up with clever ways to circumvent the reality of thermodynamics for centuries without success.
Is the lithuim spewed out the tail pipe in this process or can it be recaputured?
If it can be recaptured, then its “just” a matter of turning in the “used” lithium (or whatever) to be reproccessed back into pure lithium (or whatever compound you started with) when you go get more of the new stuff.
It doesnt matter if your magic compound cost thousands of dollars if its just a method of storing energy that can be recaptured and/or is never spewed out the tail pipe in the first place. At that point its just a different “type” of battery.
The whole point of doing something like this is if your “normal” battery technology isnt up to the task.
And, again, your car really isnt running on water, its running on the compound and if anything, the water should probably be considered the “catalyst”. But thats just semantics.
Of course, on a practical level, if batteries never really get good enough for the long distance driving many must do (or pretty much any useful flying) and you want to quit using fossil fuels (or run out) you “just” make synthetic alchohol or propane or methane or something like that and burn that in a normal fashion. If you pull the carbon you need to make it from the air, then its a carbon neutral process.
Oh, and of course all this crap could be anywhere from mildly ineffiecient to horribly ineffiecient when looked at total energy used point of view. However, some thing will probably never be practical to power by normal rechargeable batteries and will require higher energy storage levels.
Straight down noontime sun at the equator is about a kilowatt (1000 watts) per square metre.
But if you calculate your need for solar panels based on that number, you’ll be sorely disappointed. scr4, the number you used is much closer to a reasonable number than the one used by beowulff.