When president Bush speaks of a hydrogen economy, he is talking about using hydrogen as a fuel. In this context, hydrogen is only an energy storage medium not an energy source. Energy had to be expended separating the hydrogen from water and because efficiencies are never 100%, less will be recovered than expended.
Hydrogen can allow a fixed energy source such as a nuclear power plant to provide energy for mobile applications such as vehicles and do so in a less environmentally harmful manner than fossil fuels and providing the primary source of energy is sustainable, it can do so in a sustainable manner.
Hydrogen can be a source of energy if we can utilize it in a fusion reactor but presently political and economic forces prevent implementation. Deuterium and tritium are the isotopes of hydrogen that are likely to find application in fusion reactors. Tritium doesn’t exist naturally except in trace amounts but could be readily bred by placing a lithium blanket around the reactor. Liquid lithium also has some properties that improve the performance of a fusion reactor.
In terms of hydrogen’s use as an energy storage medium for transportation and other portable applications it has potential with some caveats. Water vapor is a far more effective greenhouse gas than is carbon dioxide, so increasing water vapor content of the atmosphere will increase greenhouse effects however additional clouds will reflect more light, so how that will balance out remains a subject of speculation.
A group of scientists working on alloys for advanced semiconductors discovered that if you drop pellets of an aluminum-gallium alloy into water, it reacts with the water. The oxygen in the water reacts with the aluminum alloy to form aluminum oxide, the hydrogen is released and can be used as a fuel. The gallium is released and can be re-used. Gallium prevents aluminum from quickly developing an oxide layer that prevents significant reaction.
Scientists envisioned this as a way to retrofit gasoline cars to use hydrogen but this would be a poor use of this technology. Spark ignited internal combustion engines have a mechanical efficiency of around 20%. They can achieve up to about 37% at their optimum operating point but only a hybrid vehicle can approach this. Hydrogen is a low octane fuel and requires a reduction in engine compression with a corresponding reduction in thermal efficiency.
Fuel cells have achieved 50% efficiency. Between controller and electric motors, the drive train efficiency is around 85%, so 42% system efficiency can be achieved this way, far better than with an internal combustion engine and the generation of nitrous oxides is avoided.
The recombination of hydrogen and oxygen in a fuel cell is an exothermic reaction and can result in temperatures in the ceramic membranes of 900°C. If this heat is captured and utilized to generate power, significantly higher vehicular system efficiencies are possible. This doesn’t speak to the efficiency of hydrogen generation, around 85% is typically achievable in a commercial electrolyzer.
This means that your electricity to wheel efficiency with a fuel-cell vehicle is going to be around 35%, if hydrogen is burned in an internal combustion engine, only about 17%. In this scheme of aluminum – gallium pellets the actual efficiency in which electricity is converted to hydrogen is not documented but I suspect it’s going to be significantly less.
Consider an all-electric vehicle. A lithium ion battery can return about 80% of the energy used to charge it, that in connection with a 85% efficient drive train can yield an electricity to wheel efficiency of 68%, far better than hydrogen fuel cell vehicle.
For NiMH cells, the charge efficiency can be between about 65 and 98%. A lot of the efficiency with NiMH cells depends on the intelligence of the charger, they are very efficient right up to where they are almost fully charged, then efficiency drops rapidly at high charge rates so a good charger will have a thermal sensor, charge at a high rate until it detects the battery getting warm, then top it off with a trickle charge.
On the discharge curve, a high discharge rate will reduce efficiency, the degree to which this is true varies widely from one manufacturer to the next. For maximum efficiency, choose a cell with good high current discharge characteristics. Consider using an ultracapacitor to spread load spikes. In addition, an all-electric vehicle can use regenerative breaking to recover energy improving the overall efficiency even more.
An all electric vehicle wins hands down in terms of electricity to wheel efficiency over hydrogen fueled vehicles, even those using high end fuel cells. However, if electricity is generated from fossil fuels, even an efficient all electric vehicle will result in an overall fuel to wheel efficiency of only around 31%, and in that case a diesel hybrid will do better. Of coarse what is desirable is to utilize renewable or essentially inexhaustible energy sources to generate the electricity.
By essentially inexhaustible I mean something like controlled hydrogen fusion or properly managed fission generation. There is enough deuterium in the worlds oceans to provide our energy needs for 15 billion years.
Properly managed fission would utilize integral fast reactor facility instead of the one-pass system we have now. Such a system could extract approximately 96% of the energy content of uranium (or thorium) while producing no actinide waste thus eliminating the 50,000 year storage problem, contrasted by our current one-pass system that extracts less than 1% of fuels energy potential and creates waste that will need to be isolated for 50,000-100,000 years.
The high efficiency of such as system would make extraction of uranium from seawater a practical fuel source and would make nuclear power viable for millions of years. It would still produce short-term fission product waste but that represents a much smaller problem than plutonium and other actinides, approximately 300-500 years verses 50,000-100,000 years.
Right now though it happens that wind power is the least expensive means of generating electricity. It also happens that the variability of wind is not such an issue with fuel production as we only have to worry about average production rates and have enough hydrogen storage facilities to bridge the periods where production is inadequate due to slow winds.
Overall though I am skeptical of the hydrogen economy, I think all-electric vehicles will provide the main alternatives to fossil fuel powered vehicles.