Diesel or Hydrogen?

by Jack Peckham

San Diego-Futurists talk dreamily of a “hydrogen future” where the internal combustion engine and fossil fuels are replaced by “clean” hydrogen and fuel cells.

But practical reality tells a far different story today: Diesel/ distillate fuels (including jet fuel) do all the heavy lifting on this planet-for good reason.

That’s because distillates are cheap, abundant, available everywhere, and have by far the best energy density of any fuel.

Still, hydrogen proponents see a day coming when diesel and gasoline feedstocks (mostly from crude oil) dry up, and then something else has to replace them.

But according to most oil & gas experts, the replacement won’t be natural gas, since this is just another fossil fuel being depleted at virtually the same rate as crude oil.

But “global warming” concerns could sharply limit such cheap fuel feedstocks, since it’s so difficult and expensive to “sequester” C[O.sub.2] emissions from coal or oil/tar-sands gasification with today’s technology.

Coal could replace some liquid hydrocarbons, as Sasol does today with its gasified-coal gas-toliquids (GTL) plants, although at a cost higher than crude-based fuels.

Only two “non-carbon” fuels–hydrogen and electricity–potentially could replace fossil fuels, as U.S. Department of Energy office of heavy vehicle technologies director James Eberhardt told the Diesel Engine Emissions Reduction (DEER) workshop here.

However, both hydrogen and electricity today are produced (mostly) by fossil fuels, with only limited potential for hydropower, solar or wind to produce hydrogen or electricity to meet enormous energy demands at competitive prices.

Nuclear could do it, but “greens” hate flukes, and people are terrified at the consequences of nuclear waste transport and storage.

Even if some other hydrogen or electric energy “non-carbon” feedstock source could be found, it’s not clear that “clean” power could be produced at anything close to what most consumers would pay, especially in “third world” emerging economies.

Even if a hydrogen production breakthrough occurs, more breakthroughs would be required for practical, low-cost distribution and on-board energy storage, Eberhardt showed.

Here’s how heavy-duty highway diesel trucks compare to hydrogen-fuel cell trucks, for example:

“That’s 316 cubic feet of cargo space lost, with a potentially dangerous [hydrogen] pressure vessel,” Eberhardt pointed out here.

Even liquid hydrogen only has one-fourth the energy density of diesel fuel, and it would require very costly production and storage, as well as raise new safety issues.

If the same truck were to run on electricity, then 85% of the entire weight capacity of the truck would be taken up by batteries, just to go 500 miles (half the range of diesel), Eberhardt showed.

As an alternative to batteries or on-board pure hydrogen storage, one automaker has proposed sodium borohydride as a “compact and portable” way to carry hydrogen.

However, this is hugely inefficient from an energy standpoint, since it takes more energy to make sodium borohydride than the energy released or recovered in the fuel cell, Eberhardt showed. The required high-temperature (900[degrees]C) sodium borate to sodium borohydride reaction will result in large amounts of C[O.sub.2] emission.

What’s more, it would take 26 84-gallon tanks of sodium borohydride (13 tanks for the solution, 13 tanks for spent fuel) weighing over 15,000 pounds just to equal the energy from two 84-gallon diesel tanks on a heavy truck. This would be a huge, unacceptable weight and space penalty for trucking.

Plenty of other obstacles stand in the way of replacing diesel with hydrogen, including the big energy losses accompanying the electrolytic splitting of water to hydrogen, the need for efficient and low-cost on-board hydrogen storage (or an highly efficient on-board hydrocarbon fuel reformer), and the need for greatly reduced precious-metal catalyst loadings in the fuel cell stack/reformer system.

Meantime, diesels just keep getting cleaner and better, thanks to much-improved fuel quality (ultra-low sulfur diesel), greatly improved engine/combustion technology (direct-injection rate shaping, electronic controls) and emerging versions of homogenous-charge compression-ignition (HCCI at part load).

Exhaust catalyst technologies that will nearly eliminate nitrogen oxides (N[O.sub.x]) and particulate matter (PM), and electric-hybrid technologies, also are pushing diesels to the forefront of the world’s vehicle technologies, at far lower costs than alternative fuels/systems.

This means that clean, ultra-low sulfur diesel (and gasoline) will continue to dominate transportation fuels for many years to come.

“With no alternative yet identified, it appears that hydrocarbon-based fuels from a variety of feedstocks will be the future fuels for heavy-duty vehicles,” at least for the next 15-25 years, Eberhardt said.

Still, pursuing a hydrocarbon path won’t get the planet anywhere near the target United Nations greenhouse gas “stabilization” level, as James Dooley, staff scientist at Battelle/University of Maryland’s Joint Global Change Research Institute, showed here.

The only technologies that could make a big difference in “stabilization” would be carbon capture, geologic sequestration, hydrogen systems, better energy storage systems and commercial-scale biomass energy, he said. None of these are simple or cheap.

Change won’t happen because the earth is “running out of fossil fuels,” but rather, because of climate change issues, Dooley said.

Problem: “We cannot do this ‘at any cost,”‘ Dooley recognized. Slashing greenhouse gas (GHG) emission via technological innovation is likely to happen in the industrial and buildings sectors first, followed by transportation, he said. Each sector accounts for about one-third of human GHG emissions.

Carbon taxes likely will be much more effective in industry/buildings sector GHG reduction, but transport “decarbonization” looks a lot more complicated, he said. One critical issue is where to produce the “decarbonized” fuel–at a refinery, on-board a vehicle, or someplace else?

Meantime, while converting heavy-duty trucking to hydrogen looks exceptionally difficult, it might be possible to introduce hydrogen power into some limited-range, local delivery trucks, if not the long-distance Class 7/8 heavy trucks, according to Jay Keller of Sandia National Laboratories.

If cost-effective production and storage of hydrogen can be developed, then it may be possible to use hydrogen in truck fleets operating out of a centralized fueling site, for trucks that never go more than 100 miles/day, he showed.