Hydrogen a transportation fuel

Aug. 25, 2003
Tom Standing's letter "Making Hydrogen" (OGJ, Apr. 14, 2003, p. 10) raised the issue that energy expenditure for hydrogen production needs to be taken into account in promoting hydrogen as a new transportation fuel.

Although our own research at Argonne National Laboratory showed that hydrogen from electrolysis is not an energy efficient way to power motor vehicles, we concluded that electrolyzer efficiency is generally above 70%, not 60% as claimed by Standing (though some maintain that the efficiency could reach above 80%). Using an energy efficiency of 70-75% and considering the energy content of 115,000 btu (lower heating value) per kg of hydrogen, we estimated that one kg of hydrogen requires about 47 kw-hr of electricity, not 64.5 kw-hr implied by Standing. On the other hand, with an energy efficiency of 70% as adopted by Standing, a fuel cell could produce 23.6 kw-hr of energy per kg of hydrogen, a little higher than the 22.9 kw-hr of energy stated by Standing.

Considering energy losses during electricity generation and hydrogen compression (in the case of compressed hydrogen) or hydrogen liquefaction (in the case of liquid hydrogen), our well-to-wheels analysis indicated that electrolysis hydrogen is not a preferable way to deliver energy for hydrogen fuel-cell vehicles. However, one needs to go beyond energy efficiency calculations. With electrolysis hydrogen, energy feedstocks other than petroleum (such as natural gas, coal, hydropower) could be used for hydrogen production. Thus, even with relatively poor overall energy efficiency, electrolysis hydrogen fuel-cell vehicles will still be able to help reduce petroleum use in the US transportation sector—a key cause of US energy vulnerability. Furthermore, if renewable electricity such as hydropower is used for hydrogen generation, fuel-cell vehicles will achieve reductions in fossil fuel use and greenhouse gas emissions. Nonetheless, electrolysis hydrogen, which can be produced in refueling stations, serves at best as a transition pathway to other hydrogen production pathways as hydrogen distribution infrastructure is built.

Hydrogen production from natural gas via the steam methane reforming technology is a commercially mature technology. In fact, worldwide, almost all hydrogen is currently produced with this technology. Energy efficiencies and emission results with this production pathway are available from commercial size hydrogen plants in operation, not from pilot plants stated by Standing. Actual plant operation data show an energy efficiency of about 70% for this technology (based on the lower heating value of hydrogen, that is, excluding the energy in vapor from hydrogen combustion). If hydrogen plants are designed to coproduce hydrogen and steam, plant efficiencies can reach above 80%. By the way, these efficiencies already include energy penalties from purification processes such as pressure swing adsorption units in hydrogen plants to produce hydrogen at the purity level required by fuel-cell vehicles.

Standing correctly pointed out that compression of hydrogen requires energy. Based on robust thermodynamic calculations, we at Argonne National Laboratory expect that compression of hydrogen to 6,000 psi (not Standing's 4,000 psi) has an energy efficiency loss of 15% (not his 20%). On the other hand, if liquid hydrogen is to be used in fuel-cell vehicles, the energy loss could be as high as 30% from hydrogen liquefaction.

Unfortunately, Standing failed to point out key benefits of fuel-cell vehicles relative to internal combustion engine vehicles. Hydrogen fuel-cell vehicles could achieve more than 2.5 times fuel economy as much as gasoline-fueled internal combustion engine vehicles (on the energy basis). This large gain in efficiency helps fuel-cell vehicle offset more than the efficiency losses during hydrogen production (except for the electrolysis hydrogen pathway). In addition, hydrogen-fueled vehicles will produce far less noxious gases such as nitrogen oxides and particulate matter than internal combustion engine vehicles do.

Standing compared hydrogen's well-to-wheels efficiency with the efficiency of 30% for natural gas-fired turbines. In fact, natural gas-fired, combined-cycle gas turbines for electricity generation have an energy efficiency of above 50%. However, such comparison is not meaningful. The current interest in hydrogen is primarily motivated by motor vehicle applications. Motor vehicles cannot readily use electricity, except for battery-powered electric vehicles, which are not viable replacements for today's fully function vehicles. Comparison of energy efficiencies of hydrogen and electricity, each of which has different applications, can be misleading.

Energy efficiency calculations are a starting point to evaluate energy and environmental effects of a new technology such as fuel-cell vehicles, but not the ending point. Energy efficiencies based on total energy inputs from various energy sources fail to address the fact that different types of energy sources—such as petroleum, fossil fuels, and other resources—could be used to produce energy products. To adequately address energy issues, one ought to take into account the types of energy sources as well as the amount of energy used for producing energy products. Furthermore, energy efficiencies are not a good indicator of environmental effects—both criteria pollutants and greenhouse gases—as long as different energy source types are involved.

Needless to say, the availability of a large-scale hydrogen production and distribution infrastructure and the associated costs are major challenges that a hydrogen economy faces. These challenges, along with those on the utilization side, are being tackled worldwide with significant hydrogen R&D efforts. As an energy carrier, hydrogen could help the transportation sector diversify its energy supply sources to continue serving mobility needs. One cannot simply conclude hydrogen and fuel cells are a "mere illusion" on the basis of some simplistic energy efficiency calculations.
Michael Wang
Center for Transportation Research
Argonne National Laboratory
Argonne, Ill.

Hydrogen and fuel cells:

A response to Michael Wang's letter

Michael Wang provides important references that apparently will quantify energy inputs for all stages of producing, and then utilizing hydrogen in fuel cells. I thank him for the opportunity to study the "well-to-wheels" analysis by Argonne National Laboratory.

Throughout the numerous reports from the Department of Energy and some national laboratories that I have studied previously, I have seen occasional quantitative analyses, but only as isolated fragments of the entire hydrogen energy path.

Reports have largely consisted of futuristic descriptions of a hydrogen-based energy system and how it will lead to energy security, abate pollution, and mitigate global warming. Two books on the hydrogen economy, by Peter Hoffmann and Jeremy Rifkin, published in 2001 and 2002, respectively, are full of rhetoric aimed to inspire readers about hydrogen energy, but are devoid of analysis. Such a tiresome search forced me to devise "simplistic calculations" of heat and material balances, using estimated energy efficiencies for hydrogen production.

I am not surprised at Argonne's conclusion that electrolysis of water is a poor choice for producing hydrogen.

On the face of it, electrolysis has a convoluted energy path. Fossil or nuclear fuel generates heat to produce steam that spins turbines to generate electricity, with the usual losses during the heat and/or steam stage. The electricity, then, is used to break the hydrogen-oxygen bonds in liquid water to produce hydrogen, with a 30% loss during electrolysis.

The hydrogen is stored until needed, at which time fuel cells convert the hydrogen back into electricity in the work-producing stage, with a conversion loss. In addition, the enthalpy difference between liquid water and water vapor means that 15.4% of the electricity used to decompose liquid water is not recovered in the fuel cell reaction that exhausts the water as vapor. It means that a 100% efficient fuel cell delivers 15.4% less energy than the energy needed to decompose water.

In view of Argonne's low rating of electrolysis hydrogen, I am mystified as to why electrolysis remains a strategy to reduce oil imports. Hydropower capacity in the US has not grown in 20 years. With environmental opposition to dams and impoundments, and even movements to demolish some dams, hydropower will be fortunate to hold onto today's capacity over the next 20 years. The kilowatt-hours thus generated will be consumed immediately to satisfy our growing demand for existing end-uses.

Replacing vehicular propulsion with electrolysis hydrogen would necessitate a gigantic leap in electrical generation. From "simplistic calculations," I estimate that 17 Grand Coulee dams would generate the electricity to replace the consumption of US gasoline and diesel fuel. US hydropower would have to be seven times as great as today to do the job. Can wind and solar generate that much electricity?

Natural gas is an unlikely contributor to producing the volumes of hydrogen that Wang envisions. Constrained supply, firm demand, and low storage have pushed gas prices to unprecedented summertime levels. Energy data by sector show that natural gas consumption would double if the energy equivalent of gasoline and diesel used in the US were shifted to natural gas. Even a fraction of such an increase would necessitate fast-tracking the natural gas pipelines from Alaska's North Slope and Canada's Mackenzie River delta.

In addition, federal areas are under drilling moratoriums or otherwise off limits to drilling: California's offshore, Gulf of Mexico off Florida, Atlantic OCS, and the Rocky Mountains, will have to be opened to area-wide leasing. Can anyone foresee controversy here? Without these developments, we would have to import natural gas as LNG, perhaps from Russia, Iran, or Qatar, but does that make sense?

If natural gas does not supply the feedstock to convert a major portion of gasoline consumption to hydrogen, then many thousands of megawatts of coal-fired generating capacity will have to be constructed. But coal-fired power plants are only slightly more popular among politicians and the general public than is nuclear power.

Regarding steam reforming of natural gas to produce hydrogen, Wang should be careful not to overstate the scope and commercial status of this process. While processing natural gas to produce purified hydrogen has been commercial for nearly a hundred years, and liquefied hydrogen has been used to propel rockets into orbit since the 1960s, these quantities of hydrogen are miniscule compared to the quantity needed to replace gasoline and diesel fuel for vehicles.

The plant efficiencies of 70-80% for steam-methane reforming that Wang mentions, deserve to be defined precisely. I trust that the Argonne study spells it out fully. Plant efficiency should mean that the lower heating value of the product hydrogen equals 70-80% of the entire heat load for making all process steam from liquid water at ambient conditions to the temperature and pressure required to initiate and sustain the endothermic reforming reaction, plus the heat value of the fuel being reformed, plus the energy required to purify the hydrogen in the product stream to fuel cell specifications.

A complete energy balance should also include the energy needed to manufacture the considerable mass of catalyst for the reactors, plus the energy needed for regenerating or replacing deactivated catalyst.

Another of Wang's points worthy of clarification, is that fuel cell vehicles are 2.5 times more fuel efficient than conventional gasoline vehicles "on the energy basis." Does he mean that fuel cells and gasoline engines are also compared on the basis of equal power output and load?

In a hypothetical test, equal btu values for gasoline and the lower heating value of hydrogen would propel the gasoline vehicle and the fuel cell vehicle, respectively. The power output, weight, and aerodynamics of both vehicles—and driving conditions—would have to be identical.

Under those conditions, the fuel cell vehicle would travel 2.5 times as far as the gasoline vehicle. I would expect that the Argonne study precisely defines such comparisons and tests.

A major selling point of hydrogen-powered fuel cells is that they are pollution-free. But nitrogen oxides and carbon dioxide will result from raising great quantities of heat and steam to reform natural gas.

Carbon dioxide is also a byproduct of the reforming stage. Greenhouse gases and pollutants must be removed following hydrogen production if the entire energy cycle is to be pollution-free.

Since natural gas and hydropower will be unlikely contributors to hydrogen production, the burden then falls on coal, where particulates and many additional pollutants must be removed. How much more effective will be the pollution abatement equipment at new coal-fired plants, compared to catalytic converters on gasoline engines? Will such improvements justify the complete revamping of our energy system to hydrogen?

I thank Michael Wang for sharing some of the findings from Argonne's studies. It will be interesting to see how our industries and national laboratories progress toward a hydrogen energy system. However, because of the great requirements for energy inherent to hydrogen production, and the escalating cost of natural gas, we may always appear to be on the threshold of the hydrogen revolution.
Tom Standing
San Francisco