There are actually three different application areas to consider for hydrogen:

1. As fuel for transportation vehicles and mobile equipment, replacing gasoline and diesel;
2. As a replacement for natural gas for a range of industrial and home uses;
3. As an energy storage medium for buffering electrical power.

In this article, we’ll cover some general issues and then focus on hydrogen as a transportation fuel. A later article will address hydrogen’s potential for replacing natural gas in heating and for use in power buffering.

Hydrogen Basics
Proponents of the hydrogen economy like to lead off with two observations about hydrogen:
a) It is the most abundant element in the universe, comprising some 99% of all atoms in the stars and galaxies; and
b) Free hydrogen gas, H₂, carries the highest chemical potential energy, relative to mass, of any chemically stable compound that exists or is ever likely to exist. One kilogram of hydrogen contains as much energy as about 2.4 kg of natural gas or 2.85 kg of gasoline.

The first point is perfectly true, but academic. We are hardly going to be mining hydrogen from the atmosphere of Jupiter to obtain energy. The second point is also true, but of little relevance unless one is choosing the fuel for an orbital rocket.

Cosmic abundance notwithstanding, the fact is that here on earth, there is no free hydrogen waiting to be tapped for producing energy. It must always be split away from compounds with which it is chemically bound, using some other energy source. Hydrogen for us is not an energy source, but an energy carrier. In that respect, it is like electricity.

The simplest way to get hydrogen is to split it from water, by electrolysis. Current industrial electrolyzers need about 54 kWh of electricity per kg of hydrogen [1]. In the future, better catalysts and electrolytes might enable as little as 40 kWh / kg, but that’s getting fairly close to thermodynamic limits.

Producing hydrogen by electrolysis is of interest because it’s clean, and because the leading renewable energy sources—wind, solar, and hydro—all produce electricity. But unless the electricity is priced below about $0.03 / kWh, the cost of producing hydrogen by electrolysis is greater than the cost of producing it from natural gas or other fossil fuels [2].

Producing Hydrogen
The most economical method for producing hydrogen for industrial use has long been steam reforming of methane. Steam reacts with methane at high temperatures, to produce carbon monoxide and hydrogen:

CH₄ + H₂O ? CO + 3H₂

By separating out the hydrogen and adding more steam, the CO can be shifted to CO₂ and more hydrogen:

CO + H₂O ? CO₂ + H₂

The method used to separate hydrogen from CO and CO₂ strongly influences the cost and efficiency of the process. In the past, the size and complexity of the equipment required made small-scale steam reforming plants impractical. Recently, ceramic membranes have been developed that are highly permeable to hydrogen but not to other gases [3]. Tubes formed from these membranes can operate at the temperatures and pressures at which the reforming and shift reactions are carried out. This avoids thermal cycling and improves net efficiency.

More importantly, it makes small-scale reformers feasible. Potentially, these membranes might enable on-board reformers to produce purified hydrogen for fuel cell vehicles [4].

Following the steep rise in natural gas prices in North America over the last few years, steam reforming of methane is no longer quite so cheap. In principle, it would now be more economical to produce hydrogen from gasification of coal or refinery waste—carbon in the form of “pet coke”.

The basic reactions involved are very similar to those involved in steam reforming of methane, but they start with plain carbon:

C + H₂O ? CO + H₂

This method isn’t widely used as yet. The capital cost for a gasification plant is high. Because of impurities present in coal and pet coke, the processes are messier and not as mature as they are for steam reforming of methane. Since no one is entirely certain that the cost of natural gas will remain at current high levels relative to coal, there is a reluctance to invest in these plants.

Hydrogen can also be produced from virtually any type of biomass. If biomass is heated to several hundred °C in the absence of oxygen, it breaks down through pyrolysis into a range of volatile hydrocarbons and a solid char that is mostly carbon. If either or both of the pyrolysis fractions is reacted with high temperature steam, the result is the same sort of CO + H₂ mix created in steam reforming of methane or in gasification of coal.

Gasification of biomass is economically attractive in certain situations. It’s actually easier than gasification of coal, because the biomass feedstock is largely free of sulfur, mercury, and other contaminants that complicate the latter. But growing biomass for energy requires a lot of land and water. It isn’t a practical replacement for fossil fuels at the rate we currently consume them. However, when a stream of biomass is a byproduct of another process, it can be cost-effective to tap it for local power and heat. Examples are described in [5].

Other Production Methods

Other methods under development are aimed at producing hydrogen directly from sunlight, using the energy of solar photons to split water molecules. Although some of these approaches are regarded as “promising”, efficiencies to date have been low. The amount of hydrogen produced per square meter of sunlight has been far less than what could be produced by high efficiency solar cells driving conventional electrolysis.

One further method for producing hydrogen is thermo-chemical splitting of water. Although not presently used commercially, it’s potentially very important. It is considered a practical way to produce hydrogen from a new generation of very efficient nuclear reactors. The hot coolant from the reactor would drive a thermo-chemical cycle for splitting water. Waste heat from the thermo-chemical cycle would drive conventional power turbines. An overall thermal efficiency of perhaps 60% has been projected for the combined cycles. [6], [7]

The prospect of large supplies of cheap hydrogen from hundreds of such reactors lends credence to the hydrogen economy. However, it dismays some of hydrogen’s original proponents. They oppose large central power plants in general and nuclear plants in particular. The notion that there could be a synergy between advanced nuclear power and the hydrogen economy is unwelcome.

Thermo-chemical production of hydrogen, however, might also work for decentralized power. In Canada, SHEC Labs is developing a small-scale thermo-chemical hydrogen production process. Focused sunlight, rather than nuclear power, drives its thermal reactors [8]. In principle, these reactors should be able to achieve much better efficiency for solar energy to hydrogen conversion than is possible using PV cells and electrolysis. If they can be made cheaply, concentrator dishes around five meters in diameter might supply hydrogen and electricity to small clusters of energy-efficient houses. Unfortunately, the technology is still unproven in mass production.

Hydrogen as Transport Fuel
Despite skepticism from hydrogen’s detractors, the fact is that if one must use non-fossil energy to produce fuel for transport, then hydrogen really is about the most energy-efficient way to go. In terms of electric or solar energy input versus power output at the wheels, no manufactured fuel beats hydrogen feeding fuel cells to power an electric drive train. The reason for hydrogen’s greater efficiency is easy 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 reaction is:

CO₂ + 3H₂ => CH₃OH + H₂O

A significant amount of energy is lost in reacting hydrogen with CO₂ to produce methanol. It is more efficient if the hydrogen can be used directly.

Another consideration favoring hydrogen is that it can, in principle, deliver to the wheels a much higher fraction of the energy it contains. That’s because it can be burned in a fuel cell to produce electricity at a high efficiency. Electricity, in turn, can be efficiently converted to traction power at the wheels. A great deal of internal friction from pistons, crankshaft, driveshaft, and gears that lower IC engine efficiency is avoided. [9] To realize those advantages, it’s necessary to first solve two formidable problems. The cost and durability of fuel cells for mobile vehicles must be dramatically improved, and a practical means must be found to store enough hydrogen in an acceptably small and lightweight fuel tank.

Pressurized Hydrogen
Many hydrogen advocates feel that the storage problem has been adequately solved already by the development of high performance tanks for storing compressed hydrogen gas. Such tanks are filament-wound with carbon fiber for high strength and low weight. Tanks rated for 5000 PSIG, manufactured by Quantum Technologies are in use in hydrogen vehicle demonstration programs. Tanks rated for 10,000 PSIG are planned for use in production vehicles. At that pressure, enough hydrogen can be carried to give a 5-passenger fuel cell-powered coupe a 300-mile driving range—comparable to today’s gasoline-powered cars.

The two main 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 about one-eighth as much mechanical energy to compress hydrogen to 10,000 PSI as the hydrogen will deliver in fuel cell output. This "energy tax" for compressing hydrogen approaches what would be needed to produce methanol from hydrogen and CO₂. Since methanol is easy to store and avoids the safety issues with highly compressed hydrogen, critics suggest that it might be a better just to produce methanol.

It’s possible that the critics are right. But the issue is far from settled. It may actually be possible to turn the “compression energy tax” to hydrogen’s advantage.

Energy Recovery
Before it can be used in a fuel cell, the highly compressed hydrogen from a pressure tank must be decompressed. A conventional pressure regulator does that job cheaply and easily, but also wastefully. The mechanical potential energy of the compressed gas is dissipated in forcing the gas through the regulator’s needle valve. If, instead, the pressurized gas is used to power a compressed gas motor, a good fraction of the energy put into compressing it can be recovered.

Directors of hydrogen vehicle research program might wish to give some priority to developing compressed gas motors for use with hydrogen. The payoff from a reliable, cost-effective 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 10%, for the same range between fill-ups.

Safety
As to the safety issues with highly compressed hydrogen, I think the jury is still out. There’s probably no need for concern about spontaneous bursting of the fuel tanks. A fuel-tank rupture would be a worst-case scenario, and would create a truly horrendous explosion. However, the tanks can be made with a sufficient margin of safety to preclude spontaneous bursting [10].

Very high pressure is possibly a safety advantage in crashes. High pressure allows the tanks to be smaller. Moreover, to withstand that degree of internal pressure, the tanks must be so strong that, by comparison, external impact forces become less significant.

The plumbing between the tank and the fuel cells is more vulnerable, and would likely be severed in a crash. But Quantum has addressed that problem by developing pressure regulators that mount inside the tank. External lines carry only low-pressure gas. Safety valves at the regulator immediately cut off gas flow if pressure in the external lines is lost. So it seems that the system would be adequately safe for driving. Refilling the tanks is more problematic. The only way to get fast refilling is to have a direct connection between the fuel tank and a high-pressure supply tank. Although it is almost certainly possible to design hoses and connectors that are capable of handling hydrogen at 10,000 PSI, it’s not easy. Equipment of that sort is not something to be handled by untrained citizens at neighborhood filling stations. The potential for disastrous accidents is too great.

Possibly, the refilling issue can be addressed by making the hydrogen tanks swappable. To fill up, a driver would pull up over a sunken bay similar to those now used at fast oil-change stations. A robotic tool would remove the depleted tank and replace it with a full one. The depleted tank would then be conveyed to an underground room where refilling could be done slowly and safely.

Unfortunately, there is in these days one final safety issue with high-pressure hydrogen tanks. A heavy-caliber armor piercing round fired into a car’s high-pressure hydrogen tank would cause an explosion equivalent to at least a case of dynamite. The hydrogen gas itself is not explosive, but pressurized to 10,000 PSI and with a speed of sound four times faster than that of air, it might as well be. It will escape so rapidly and with such force from a pierced tank as to drive an expanding spherical shock wave through the air ahead of it. Hydrogen behind the shock wave will diffuse into the superheated shock zone and burn in a matter of milliseconds. The result would be hard to distinguish from a “true” explosion.

I don’t know it that particular problem has any technical solution.

Other Storage Options
There are other less scary options for storing hydrogen, though none is ideal. The approaches that deliver good densities all involve binding the hydrogen chemically. That invariably means some loss of energy, compared to using hydrogen directly.

The least wasteful of these approaches is probably to bind the hydrogen with nitrogen, producing ammonia (NH₃). Ammonia can be used directly in certain types of high temperature fuel cells. The energy required for breaking the ammonia down to hydrogen and nitrogen is supplied by waste heat from the fuel cell, and doesn’t subtract from the electrical output of the 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 is not free from safety issues of its own. In high concentrations it is toxic to breathe. The risk is moderate; ammonia is used for fertilizer, and farmers routinely handle it safely. But pressure is needed to keep ammonia liquid at normal temperatures. It will escape rapidly and disastrously from a broken valve. There have been fatalities due to accidental release of ammonia at chemical plants or in crashes of tanker trucks carrying it.

Two other interesting options 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 mineral-oil carrier. Both are similar in that they produce spent solutions that must be held on board for recycling. In both cases, the liquid fuels are resistant to combustion, and present no fire hazard in the event of a crash. Though much safer than gasoline, the spent solutions are caustic, and can be hazardous if their tanks are breached.

The cost of regenerating fuel is also a significant downside for both approaches. The energy needed is very much greater than that needed to produce the same amount of hydrogen directly by electrolysis of water. Neither company quotes figures, but I suspect that recycling the spent fuel is even more costly than producing methanol from CO₂ and hydrogen.

Cost and Durability Issues
Whether hydrogen finds widespread use as a transportation fuel will ultimately depend on how well certain cost and durability issues can be addressed. Production of hydrogen at this point is not a killer problem. But both the cost and durability of hydrogen fuel cells for automotive use are big problems, as is the cost of lightweight high-pressure tanks. The cost of hydrogen fuel cells must be cut by a factor of ten before they are cheap enough to be attractive for automotive use. That won’t be easy, but it may be possible. Over the last five years, costs have already dropped by one factor of ten; with more research and tooling for mass production, another factor of ten is conceivable. Some major players, like GM, are betting on it.

Not only do costs have to come down, however; durability has to go up. The best automotive fuel cells are currently good for only a couple of months of driving before their PEM membranes fail. Researchers at Du Pont are working on various incremental improvements for their Nafion® PEM membranes [11]. These have so far been the standard for automotive fuel cells. However, it’s not clear whether incremental improvements will suffice. Current PEM fuel cells prefer stable operating environments. Their membranes are physically and chemically delicate, and don’t stand up well to the repeated swings in temperature and humidity that they encounter in automotive use.

Recently, the British firm PolyFuel has announced the development of a promising new fuel cell membrane. If their claims hold up, they may be able to deliver the improvements in cost and durability that are needed. It’s too early, however, to know.

The most I can say for sure at this point, after surveying fuel cell development work reported on the web, is that fuel cell technology is an active field in which rapid progress is being made. Statements by hydrogen detractors that “researchers have been trying unsuccessfully for 80 years to make fuel cells practical” are unfair and misleading.

The Alternatives
If hydrogen fuel cell vehicles were the only option for dealing with the coming shortfalls in oil supplies, then their future would be assured. However, that’s not the case. The following developments could all serve to reduce oil consumption enough to keep crude oil prices from rising quickly above the $60 a barrel level toward which they now seem headed:

• Reduction in miles driven as drivers adjust to higher gasoline prices;
• A swing back to smaller, higher mileage vehicles;
• Increasing numbers of high-mileage hybrid vehicles;
• Emergence of “plug-in hybrids” with external battery charging provisions;
• Expansion of fuel production from oil sands, heavy and sour crudes, and coal.

The first three points are obvious, but their potential for reducing oil consumption is limited. The last point is potentially more significant. I’ll have more to say about it in Part 2, in connection with hydrogen’s use in oil refining and synthetic fuel production. It’s the fourth point, however—the emergence of plug-in hybrids—that poses the most direct challenge for hydrogen vehicles.

Gal Luft wrote about plug-in hybrids in an earlier EnergyPulse article this summer [12]. They are hybrid vehicles whose batteries can be recharged from an external source, like those of a battery electric vehicle. For trips under ten miles or so—which comprise the large majority of trips actually driven—they can run entirely on batteries. Fuel consumption may thus be cut by 85%. That’s substantial enough to allow us to track dwindling oil supplies for quite some time before we’re actually forced to abandon the stuff. That means that, absent a heavy carbon tax on gasoline and diesel fuel, hydrogen as a transportation fuel will have to compete economically with oil at prices not much higher than we see today.

Whether it will be able to do so depends heavily on the hydrogen production and distribution infrastructure we are able to put in place. That, in turn, depends at least partly on issues that will be explored in the Part 2, Hydrogen and Utilities.

Endnotes and References
[1] Summary of Electrolytic Hydrogen Production, http://www.nrel.gov/docs/fy04osti/36734.pdf
[2] Assumes current prices for fossil fuels. Naturally, as competition for diminishing supplies drive fuel prices higher, the price point at which electrolytic hydrogen becomes competitive rises correspondingly.
[3] http://www.physorg.com/news906.html
[4] Early efforts to develop on-board reformers as a way to supply hydrogen for fuel-cell vehicles have fallen by the way. The simple reformers that were feasible could not produce pure hydrogen, and efforts to develop automotive fuel cells that would tolerate CO and CO₂ in the fuel stream were unsuccessful.
[5]http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/merit03/30_gti_david_bowen.pdf
[6] http://www.uic.com.au/nip73.htm
[7] http://gcep.stanford.edu/pdfs/hydrogen_workshop/Schultz.pdf
[8] http://shec-labs.com/process.htm
[9] Of course, internal combustion engines and mechanical drive systems are not the only alternative to hydrogen fuel cell vehicles. See section on alternatives.
[10] U.S. codes for pressure vessels call for a 2.5:1 safety margin. A 10,000 PSI tank must survive testing to 25,000 PSI. Ultrasonic microphones can monitor the tank on refilling, and computer analysis of the sound can detect aging tanks that might be starting to weaken/
[11] http://www.qtww.com/home.shtml
[12] http://www.energypulse.net/centers/article/article_print.cfm?a_id=790

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