Why Hydrogen Cannot Replace Fossil Fuels as an Energy Source

Introduction

The world's population is growing, and so are its energy needs. Fossil fuels continue to be consumed, their combustion byproducts are intensifying greenhouse gas emissions, and the day of their depletion draws closer. Furthermore, disasters such as the Deepwater Horizon/BP Gulf Coast oil spill have made the environmental and economic costs of fossil fuel dependence impossible to ignore. Scientists have long searched for an alternative energy source with fewer harmful global effects, and many have focused on hydrogen. However, hydrogen as an energy source is unlikely to replace fossil fuels. The overall hydrogen fuel process is inherently costly and inefficient, increases greenhouse gas emissions, and — due to its low energy output — would require extensive development of hydrogen processing infrastructure.

The Hydrogen Fuel Cell

There is some potential for hydrogen in small-scale power applications, but its viability for powering car engines or households is limited. A hydrogen fuel cell functions essentially as a storage battery for energy derived from other sources. In a fuel cell, hydrogen and oxygen are fed to the anode and cathode of each cell. Electrons stripped from the hydrogen produce direct current (DC) electricity that can power a DC electric motor, such as those found in kitchen appliances.

As an energy source for vehicles, however, hydrogen is inefficient partly because it is highly reactive. When hydrogen gas contacts metal, it decomposes into individual hydrogen atoms. These atoms are small enough to penetrate metal — and most other materials — causing hydrogen to leak even from well-insulated containers. For this reason, hydrogen stored in tanks will always evaporate, at a rate of at least 1.7% per day.

The required size of hydrogen fuel tanks presents an additional obstacle. In gaseous form, approximately 6,287 gallons of hydrogen gas are needed to match the energy capacity of just 20 gallons of gasoline. Compressed hydrogen has been the primary form used in hydrogen-powered vehicles to date. Yet because of its low density, compressed hydrogen does not give a vehicle a range comparable to gasoline. A compressed hydrogen fuel tank would also be at risk of developing pressure leaks that could lead to explosions. The energy costs of liquefying hydrogen and maintaining it in a liquid state further reduce the net energy return.

Producing Hydrogen for Use

Hydrogen does not occur freely in nature in useful quantities. It must therefore be split from molecules — either from methane derived from fossil fuels or from water. Currently, most hydrogen is produced by treating methane with steam, following the reaction: CH₄(g) + H₂O + energy → 3H₂(g) + CO(g).

Carbon monoxide gas (CO) is a byproduct of this reaction, which undermines the very purpose of seeking an alternative fuel: eliminating the production of greenhouse gases. Moreover, the first and second laws of thermodynamics dictate that this process results in severe energy loss. The first law states that the energy output of any process cannot exceed its energy input; the second law holds that each process degrades energy. The production of methanol from natural gas results in an initial net energy loss of 32% to 44%, and the subsequent steam treatment to extract hydrogen results in a further 35% energy loss.

Several processes are being explored to derive hydrogen from water, which is theoretically an inexhaustible source. However, the reaction — 2H₂O + energy → 2H₂(g) + O₂(g) — requires a substantial energy investment per unit of water (286 kJ per mole). This investment, again demanded by both laws of thermodynamics, renders the electrolysis of water unprofitable in terms of energy return relative to energy invested.

Any functional hydrogen economy would require an infrastructure capable of using zero-carbon power to electrolyze water into hydrogen, transporting this highly diffuse gas over long distances, and pumping it at high pressure into vehicles or large stationary fuel cells in homes. The hydrogen would then need to be converted back into electricity to drive an electric motor or power utilities. The cumulative process — electrolysis, transportation, pressurized pumping, and fuel-cell conversion — would yield only 20 to 25% of the original electricity as usable energy.

Conclusion

Because of the second law of thermodynamics, hydrogen fuel cells will always deliver a limited energy return, and — depending on the production method — will continue to contribute to greenhouse gas emissions. Furthermore, a hydrogen-based economy would require large-scale infrastructure development and land use that would likely prove cost-prohibitive. For these reasons, hydrogen is not a feasible replacement for fossil fuels as a primary energy source. Alternative approaches, particularly plug-in hybrid electric vehicles, offer far greater energy efficiency and present a more practical path toward reducing dependence on fossil fuels.

Bibliography

Romm, Joseph J. The Hype About Hydrogen: Fact and Fiction in the Race to Save the Climate. Island Press, Washington, DC, 2004.

Romm, Joseph J., and Andrew A. Frank. "Hybrid Vehicles Gain Traction." Scientific American, April 2006.

Business Wire. "Hydrogen Technology Applications and UTEK Corporation Announce Strategic Alliance Agreement." Business Wire, April 2002.