Hydrogen as an additive to diesel fuel?

[CRGreathouse]

In this situation i want to charge a battery from my power grid and use it to power a electrolyzer to get better gas millage from a gas powered el camino. While i am still putting in the same amount of energy. The cost of charging the battery at home can offset the current cost of gasoline prices. But i only want to do this if it make my engine considerably more efficient.

Alright. Electricity might cost $0.10 per kilowatt-hour, and gas $4 a gallon. Given an 80% recharge efficiency, every 32 kilowatt-hours would need to save a gallon of gas to break even. Can any of the physics gurus give advice here on how much energy it would take to save a gallon of gas?

Of course this neglects the weight of the battery and additional equipment...

 

[mgb_phys]

every 32 kilowatt-hours would need to save a gallon of gas to break even. Can any of the physics gurus give advice here on how much energy it would take to save a gallon of gas?

A litre of gas contains 35MJ of energy that's about 10KWH. Electricity is cheaper than gas/petrol, otherwise power companies would use gas/petrol powered powerstations!
The problem with electricity as many threads here point out is storing the stuff.
1Litre of gas/petrol fits in a large cup, 10KWH of batteries don't.

 

[CRGreathouse]

I meant using the battery to power an electrolyzer as kl2345 suggests, not to run the whol car. If this would exhibit a "very small (under 10%) improvement in combustion and thermodynamic efficiency", at what watt cost and for what %age savings?

 

[mgb_phys]

It doesn't matter how you use it - the laws of thermodynamics still apply.
The problem is that a car battery (12V 80AH) only contains about 1KWH of energy, the same as half a wine glass of gasoline.
The reason for the whole hydrogen addition industry/scam is the idea that because you are supposedly unlocking extra hidden energy in the fossil fuel somehow the law of conservation of energy doesn't apply.

 

[CRGreathouse]

I'm a fan of the laws of thermodynamics. I'm not looking for a 'way around'; I'm trying to disprove as large a fraction of these systems as possible.

Any system claiming, for example, to burn hydrogen obtained from onboard electrolysis is trivially impossible by the second law of thermodynamics. But a system claiming to increase the efficiency of an engine is harder to disprove, since car engines are typically inefficient -- I typically see figures around 20%, I don't know what the experts around here think of as a typical efficiency.

So since I couldn't rule it out a priori, I thought I'd go with the next step: find reasonable figures for the efficiency of a system, find the additional load on the system of carrying the extra weight, and show that it fails. This seemed the best path to me, since batteries are (as you point out!) much larger/heavier than gasoline for the same energy.

But as I'm not an engineer, I hoped to get some information from those around here who are (or know reasonable figures, regardless). I have no idea what an electrolyzer would even do in the system, let alone how much efficiency it might cause...

 

[russ_watters]

The numbers are there, just not in a good form. It actually works out nicely, though:

Gasoline has an energy content of about 36 kwh and costs about $3.60, or $.1 per kWh.
Electricity costs about $.1 per kWh (that can vary though - it can be as much as $.15.

That should set off the warning bells - unless the hydrogen can improve the efficiency of the car by more than the loss from creating the hydrogen, you'll lose money on the deal.

According to this paper, using a 7% mixture of hydrogen in gas (by mass) yields a 7% improvement in efficiency (from 30% to 32%). http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770016170_1977016170.pdf

Now since hydrogen actually has 3x the energy density of gas, that's actually replacing 21% of the gas's energy with hydrogen energy. In other words:

1kW gas = .30 kW of output mechanical energy
.79 kW gas + .21 kW hydrogen = .32 kW output mechanical energy

That's a 7% efficiency improvement for zero extra energy (or money) input.

But since your electrolyzer efficiency will be around 50%, you actually need .42 kW of electricity to realize that 7% efficiency improvment and .21 kW hydrogen input. Now your input energy is greater than your improvement in performance: .21 kW of extra energy input for .02 kW of extra energy output.

 

[russ_watters]

But a system claiming to increase the efficiency of an engine is harder to disprove, since car engines are typically inefficient -- I typically see figures around 20%, I don't know what the experts around here think of as a typical efficiency.

Car engines are inefficient, which makes such a belief easy to swallow. But the problem is that, car engines are inefficient for a reason: the laws of thermodynamics dictate how efficient they can be. In reality, if a car's thermodynamic efficiency is, say, 30%, it's theoretical maximum efficiency is likely only to be perhaps 35%. If there was more efficiency to be sqeezed-out, that would simply mean that car engines were poorly designed.

 

[russ_watters]

Electricity is cheaper than gas/petrol, otherwise power companies would use gas/petrol powered powerstations!

I don't understand this statement. Electricity is not cheaper than petrol (on a per btu basis) precisely because the power companies do use petrol (actually oil and natural gas, mostly) powered power stations.

Even after you take out mark-ups for the oil company profit (the power company pays less for oil than you do), a power company still has to deal with the efficiency of the power plant. They top out at about 60%, so on a per kWh basis, 1 kWh of electricity requires 1.67 kWh of oil to produce. So that means that the power company would have to be paying only 60% of what you pay for oil for this scenario to break even.

Now when it comes to your car, though, what you want out is mechanical energy. That's why it's better to use the power plant's far superior thermodynamic efficiency to drive your car with electricity instead of gas. For heating your home, on the other hand, gas is much cheaper than electricity (resistance heating anyway) because the efficiency is 80-95% instead of the 30% in your car.

 

[russ_watters]

Heh - I just realized this thread was locked. Sorry, but after spending an hour on those posts, I'm leaving them. In any case, hope they were helpful.