October 6, 2003
The following is taken from a message I posted earlier the year to the Yahoo Groups Energy Resources (ER) list.
A Contrarian is Roused
The ER list has lately seen a lot of bashing of the hydrogen economy, and of research efforts by GM and others into fuel cell vehicles. Ordinarily, you would find me on the side of the bashers, pointing out that hydrogen has been overhyped by its partisans. It's only a bucket for carrying energy that you must have gotten from somewhere else. And it's a far from perfect bucket at that. I’ve sworn that I had to read one more popular article breathlessly noting that hydrogen is "the most abundant element in the universe", I would scream! As if that had anything to do with real issues!
But now the sheer volume of H2 bashing, albeit largely deserved, has roused my contrarian instincts. I feel compelled to take the other side, and offer a few words in defense of hydrogen and GM’s hydrogen vehicle program.
H2 vs. Other Manufactured Fuels
If one must use non-fossil energy to produce a transportation fuel--and before too long we will have to--then hydrogen really is about the most energy-efficient way to go. In terms of electric or solar energy in to motive power out, it’s hard to beat hydrogen feeding fuel cells to power an electric drive train. To be sure, it’s necessary to posit a viable solution to the hydrogen storage problem, which is uncertain. However, if one takes that leap of faith, then it makes sense to look to hydrogen as the likely fuel of choice for a post-petrol future.
GM anticipates a huge potential auto market in India and China, as those countries continue to advance economically. They are well aware that conventional oil will not be able to support that market. If it’s going to happen, it will have to be based on fuel derived from coal, or manufactured from electric, solar, or nuclear thermal energy. Should hydrogen be viable as a transportation fuel, then it would be the most efficient choice, regardless of the primary source of energy used to produce it. Other options do certainly exist, but they appear likely to deliver fewer ton-miles of transport per unit of energy invested than hydrogen.
The reason for hydrogen’s greater efficiency is not hard to understand: nearly all other candidates for a manufactured transportation fuel require production of hydrogen as a first step. For example, to produce methanol without resort to coal or biomass, the overall reaction is:
CO2 + 3H2 => CH3OH + H2O
A significant amount of energy is lost in reacting hydrogen with CO2 to produce methanol. It would be much more efficient if the hydrogen could be used directly.
The H2 Storage Problem
Some hydrogen advocates feel that the storage problem has been adequately solved already by the development of high performance tanks for storing compressed hydrogen gas. The tanks are filament-wound with Kevlar fiber for high strength and low weight. Tanks rated for 5000 PSIG are currently in use in a number of hydrogen vehicle demonstration programs. Tanks rated for 10,000 PSIG are expected to be used in production autos. At that pressure, the amount of gas that can be squeezed into tanks of acceptable size and weight is sufficient for a 5-passenger vehicle with performance and driving range comparable to today’s gasoline-powered cars.
The two big technical issues with this approach are the energy needed to compress hydrogen to such extreme pressures, and safety. Even with perfect isothermal compression, it takes almost half as much mechanical energy to compress hydrogen to 10,000 PSI as the decompressed hydrogen will deliver in chemical energy. In practice, this "energy tax" for compressing hydrogen is comparable to the energy that would be needed to produce methanol from hydrogen and CO2. Methanol is easy to store, and avoids the safety issues with highly compressed hydrogen.
Recovering Energy of Compressed H2
It’s possible that the "compression energy tax" might actually be turned to hydrogen’s advantage. Before it can be used in a fuel cell, the highly compressed hydrogen in the pressure tank must be decompressed. The cheapest and easiest way to do that is with a pressure regulator—a needle valve operated by a spring and diaphragm. The mechanical potential energy of the compressed gas is dissipated in forcing the gas through the needle valve. It’s simple, but extremely wasteful. If, instead, the pressurized gas is used to power a compressed gas motor, a good fraction of the energy put into compressing the gas can be recovered.
There are two likely design approaches for the decompression motor that occur to me. One is a modified Sterling cycle piston engine, whose displacer piston and thermal regenerator yield an approximation to the ideal isothermal expansion cycle. The other is a combined cycle system consisting of a simple adiabatic expansion piston engine or gas turbine, with the cold decompressed gas then serving as heat sink for a low temperature Rankin cycle turbine. (The heat source would be waste heat from the hydrogen fuel cell.)
I don’t know which approach would ultimately prove to be more cost-effective, or if an entirely different approach would be better. But if I were in charge of a hydrogen vehicle research program, I would put a high priority on finding out. The payoff from a reliable, manufacturable design would be substantial. If it were able to recover as much as 70% of the compression energy of the hydrogen gas, then it would reduce the size and weight of the hydrogen tanks and fuel cells by about 30%, for the same range between fill-ups.
The Killer Issue: Safety?
As to the safety issues with highly compressed hydrogen, I’d say that the jury is still out. I don’t think there’s any need for concern about spontaneous bursting of the pressure tanks themselves. That would be a worst-case safety scenario, but the tanks can be manufactured with a sufficient margin of strength to preclude it. More difficult to forecast is the safety of the tanks in a crash. In that respect, very high pressure is probably an advantage. Not only does the high pressure make the tanks smaller, but in order to withstand that degree of internal pressure, the tanks must be so strong that external impact forces become, by comparison, almost negligible. Hang a tank holding 10,000 PSI gas from a concrete abutment and crash a vehicle into it at high speed: the tank would likely survive intact. The metal of the crash vehicle would crumple around it.
The real safety exposure with high pressure hydrogen gas, to my mind, is not in the tank but in its plumbing connections. Those are much more vulnerable in the event of an accident. And even if the internal plumbing can be made safe, the connection to a high pressure supply tank that is necessary if the tank is to be refilled quickly is scary as hell. Safety in normal operation isn’t the issue. The issue is the potential for large scale disaster in cases where something manages to go badly wrong.
Say, a truck pulls in behind a car that’s refueling, but its brakes fail; it knocks the car forward while it’s still attached to the high pressure supply tank. The line breaks. Hydrogen gas is not explosive, in itself. But at 10,000 PSIG and with a speed of sound more than four times greater than the speed of sound in air, the result is the same as if it were. It will drive an intense shock wave through the air, with the hydrogen behind the shock wave diffusing into the air ahead of it and burning in milliseconds.
If the pressure lines are equipped with the fast acting safety valves, and the main vehicle tank and the supply tank remain intact, the blast from the released gas in the line may be equivalent to "only" a half stick of dynamite. If the small blast were to breech the main supply tank, however, the resulting explosion could level a city block.
OK, I'll admit that I can think of designs to avoid that problem also. Nonetheless, I suspect that the people planning to use high compression for storing and transporting hydrogen are in much the same position as the early developers of nuclear power. If you don’t think too hard about all the ways that something could go wrong, it sounds great. Probably any problem you do think about will have a solution. But the potential consequences of any mistake or oversight are enormous.
Other Storage Options
There are other options for storing hydrogen, of course, but none is ideal. Without turning this into a treatise on hydrogen storage, I'll say that the approaches that deliver good gravimetric and volumetric densities all involve binding the hydrogen chemically. By producing methanol, for example. That implies some loss of energy, compared to using hydrogen directly.
The least wasteful of these approaches is probably to produce ammonia—NH3. Reacting hydrogen with nitrogen is non-trivial; the gases have to be compressed and reacted at a few hundred degrees C in the presence of a catalyst. The result is an equilibrium mixture of hydrogen, nitrogen, and ammonia. The mixed gases are cooled in a heat exchanger and the ammonia is condensed out; then the hydrogen and nitrogen are returned through the heat exchanger to the reaction chamber. The process is not all that energy intensive (once the hydrogen is produced). In fact, the recycled gases don’t have to be reheated, since the reaction of hydrogen and nitrogen is mildly exothermic. If the heat exchangers are reasonably efficient, the reaction supplies enough heat to maintain the temperature in the reaction chamber.
Ammonia can be used directly in high temperature fuel cells. The energy required to break the ammonia back down to hydrogen and nitrogen is supplied by waste heat from the fuel cell itself, and doesn’t subtract from the electrical output of the fuel cell. Apollo Energy Systems of Ft. Lauderdale, FL, is planning to use alkaline fuel cells powered by ammonia in a line of fuel cell vehicles. Unfortunately, ammonia doesn’t get off Scott free on the safety front. It’s widely used as a fertilizer, and farmers are accustomed to handling it. But it’s similar to propane, in that it requires moderate pressurization to keep it liquid at normal temperatures. A tank rupture won’t cause an explosion or fire, but in high concentrations, ammonia is toxic. There have been fatalities due to accidental releases of ammonia at chemical plants or from crashes of tanker vehicles carrying ammonia.
Two other options worth investigating are the "hydrogen on demand" system from Millennium Cell, and the lithium hydride slurry approach from Safe Hydrogen LLC. The former uses sodium boro-hydride (NaBH4) in an aqueous solution, while the latter uses a stabilized slurry of lithium hydride particles (LiH) in a light mineral-oil carrier. Both are similar in that they produce spent solutions that must be stored in a separate spent fuel tank for recycling. In both cases, the liquid fuels are resistant to combustion, and present no fire hazard in the event of a crash. They also suffer from the same downside: the energy required to regenerate the fuel is substantially greater than the energy that would be needed to produce the hydrogen directly by electrolysis of water. I don’t have figures, but I suspect that recycling the spent fuel is even less energy-efficient than producing methanol from CO2 and hydrogen.
A Note on Hydrogen Pipelines
One final note before I wrap this up. A lot of ER regulars are familiar with the article by Eliasson and Bossel titled The Future of the Hydrogen Economy: Bright or Bleak? While it’s an excellent and informative article that I highly recommend, its analysis of hydrogen pipelines is flawed.
The authors show, with rigorous fluid flow calculations, that for a given pipeline diameter and energy flow, the pumping energy required to transport hydrogen is about 4.6 times higher than that required for methane. In natural gas pipelines (with natural gas being predominantly methane) a typical figure for pumping energy is 0.3% per 150 km of pipeline. (I.e., 0.3% of the gas flowing through the pipeline is used at each pumping station to power the pumps, with 150 km being a typical distance between stations.) Multiplying this by 4.6, they conclude that a hydrogen pipeline would need to consume 1.4% of its flow every 150 km. They use this result to show that a proposed hydrogen pipeline from solar PV plants in North Africa to Europe would be able to deliver to users only 60 – 70% of the hydrogen initially produced.
What they neglect to mention is that the same fluid dynamic equations they use to get their results show that the pumping energy is inversely proportional to the pipeline diameter raised to the power 3.75. So, while it takes 4.6 times as much energy to push an MBTU of hydrogen vs. an MBTU of methane through a given pipeline, increasing the diameter of the hydrogen pipeline by 50% gets you back to the same 0.3% per 150 km as methane. The pipeline sections would be roughly twice as expensive per meter as the smaller natural gas pipeline. However since much of the cost of any pipeline is in right-of-way acquisition, road building, and excavation, the capital investment for the hydrogen pipeline, overall, is no more than about 50% greater than it is for natural gas. In most cases, the difference would probably be smaller.
The viability of long distance hydrogen pipelines is an important issue, because the most economical way to produce hydrogen once oil and natural gas become expensive will be from wind power in places like North and South Dakota. Wind power and a hydrogen pipeline are natural complements. Part of the wind generated electricity is fed directly to the grid, and part is used to generate hydrogen. The hydrogen is pumped into the pipeline. When the wind is not blowing, hydrogen is withdrawn from the pipeline and burned in fuel cells for electricity. The pipeline supplies storage for hydrogen as well as transport.
The bottom line is that, once oil and natural gas become expensive, hydrogen will be produced. Coal, nuclear, and renewable sources will all be utilized. The only real issue is whether it will be possible to use the hydrogen directly as an automotive fuel, or whether it will be necessary to bind it chemically to carbon, nitrogen, or other elements.