Several major obstacles remain in the path of progress, so is the hydrogen infrastructure just an impossible dream? Tony Wheatley investigates.
In 1874, science fiction author Jules Verne fantasised that water would someday provide an abundant, low-cost energy source.
We’re still chasing that seemingly elusive dream today, with hydrogen often cited as the fuel of the future and the answer to our ever-evolving energy needs.
The quest for hydrogen adoption and a possible hydrogen infrastructure is much talked about and the subject of ongoing research efforts.
In 1970, John Bockris a professor of Chemistry at Texas A&M University coined the phrase ‘Hydrogen Economy’ while describing a method he claimed to have discovered, to free hydrogen from water molecules using sunlight.
Fuel or energy carrier
Hydrogen is described as an energy carrier rather than a fuel, because although it is the most plentiful gas in the known universe, no source of pure hydrogen has been found on earth.
Instead it has to be extracted from stable compounds including water (hydrogen dioxide) and methane (carbon 4 hydride).
Separating hydrogen molecules requires the application of sufficient energy to break the chemical bonds and this is the energy that hydrogen molecules carry, to be stored and released when required.
Hydrogen reacts with oxygen to provide physical energy, either as heat through combustion with oxygen, or as electricity through fuel cells – pure water is the only by-product.
In this way hydrogen can perform a similar function to electricity that transmits energy from the generation plant to the many points of use, where it is required by consumers and industries.
The advantage of hydrogen over electricity is that it can be accumulated and stored at lower cost and in greater quantity. In this role as energy carrier, hydrogen competes with battery technologies.
The proposal of a Hydrogen Economy is based on adopting hydrogen gas as a universal energy carrier, to replace all of the fuels and energy delivery systems in use today.
The proposal assumed that techniques to facilitate the storage and transport of hydrogen would be developed so that hydrogen-based energy would be abundant, inexpensive, convenient, safe, efficient and of course, economically viable.
The future growth of the Hydrogen Economy was proposed to be driven by the availability of low cost, efficient hydrogen generators that would allow vehicle refuelling stations, industries, business and even households to produce hydrogen for their own consumption, using electrical energy delivered via the distribution grid and water as the feedstock.
Initially however, hydrogen production plants will necessarily depend on centralised electricity generation plants – many of which use fossil fuels and this should facilitate the capture and sequestration of greenhouse gases produced.
The future goal is that renewable energy sources like solar, wind, hydroelectric, geothermal, wave, tidal or nuclear reactors will power generation plants and thus eliminate greenhouse gas emissions.
The most obvious benefits of an economy based on hydrogen are of an environmental nature, because no greenhouse gases or any other pollution result from the use of hydrogen energy.
This promises to end the threat of climate change, global warming and increasing carbon dioxide levels in the atmosphere and the oceans. To deliver on this promise, hydrogen production must be independent of fossil energy.
A very large part of the motivation for establishing a Hydrogen Economy is the desire of most countries to be energy independent, because of the fluctuating price and availability of fossil fuels.
Transportation remains the sector that depends most heavily on oil as the primary source of liquid fuel and in the US consumes around 60% of the total oil consumption.
Obviously a successful energy carrier must supply the needs of this energy demand that is inextricably linked to population growth and socio-economic development.
In the Hydrogen Economy, it is envisioned that hydrogen highways, waterways and skyways will revolutionise transport systems as trucks, minivans, sport utility vehicles, motorcycles, buses, trains, watercraft and aircraft will all be adapted to operate using clean hydrogen energy.
Hydrogen promises far greater efficiency because it carries over three times the amount of energy per kilogram than petrol or diesel.
The first practical hydrogen-air fuel cell was made in 1959 by Francis T. Bacon of England’s Cambridge University, where he demonstrated it powering a welding machine.
NASA later used his design to provide electrical power in space vehicles.
Fuel cells are electrochemical devices and theoretically more efficient than heat engines like those in conventional cars.
The development of cost effective fuel cells is seen as fundamentally important to success when implementing a Hydrogen Economy.
Environmental groups and health experts have long argued that the US should work harder to develop vehicles that produce less air pollution and greenhouse gases.
The world’s most ambitious hydrogen infrastructure development initiative began in 2003, when ex-president Bush announced $1.2bn to fund research into the use of hydrogen fuel cells in automobiles, as well as studies of how to create, store and transport hydrogen fuel.
“Hydrogen fuel cells represent one of the most encouraging, innovative technologies of our era,” said President George W. Bush in 2003 in his State of the Union address.
“The first car driven by a child born today could be powered by hydrogen and pollution free.”
The transition from petroleum-based fuels to hydrogen in the transportation sector is complicated by the need for refuelling stations to be in place before a market for hydrogen vehicles can be developed.
It has been calculated that at least 12,000 hydrogen fuel stations will be required in the US alone, for hydrogen to be conveniently available to 70% of the population and serve a potential 10 to 20 million cars. This compares to 180,000 existing fuel stations.
As things stand, while significant progress has been reported in the fields of fuel cell manufacture and the design and testing of prototype vehicles, several major obstacles remain in the path of progress.
Obstacle 1 – Production of hydrogen
The existing market for hydrogen gas has grown at a rate of about 10% p.a. and now exceeds 70 million metric tons annually.
The demand would be 10 times greater to support a global Hydrogen Economy.
Most of this volume is split between ammonia production for agricultural fertilizer and the refining of liquid fuels from low quality, crude feedstock.
The emerging demand for hydrogen to supply refuelling stations, together with various other applications, forms part of the ‘merchant market’ and is supplied by the industrial gas industry.
Only 4% of this volume is currently produced by electrolysis from water, while 18% is sourced from coal, 30% from oil and 48% is extracted from natural gas.
It is expected that over the next decade or two, fossil fuels will continue to be the primary source for the Hydrogen Economy, although prices will increase as resources are diminished.
The most energy efficient process yet developed for converting the chemical potential energy stored in fossil fuels into useful physical energy, is the Combined Cycle Power Plant.
Unsurprisingly, most of the newer plants in Europe and America are of this type and are fuelled by natural gas, because they have been proven reliable to achieve at least 60% efficiency in the output of electric power alone and over 85% when the waste heat energy is also utilised.
If the production of hydrogen depends in any way on the use of fossil fuel, then obviously the primary objective of the Hydrogen Economy has not been achieved.
The original dream of a process that inexpensively extracts pure hydrogen from water has not yet come to fruition, although pilot plants powered by renewable energy are in operation.
Obstacle 2 – Storage
The most inconvenient characteristic of hydrogen is that at ambient pressure and temperature it is not liquid like the petrol and diesel fuels in use today.
Instead, it is a gas of very low density and very difficult to contain, because its minute molecule escapes by diffusing through many materials.
The hydrogen molecule is small enough to be readily soluble in steel, creating the risk of brittle fractures and requiring that pipelines and containers receive a protective coating for hydrogen service.
Another challenge created by its low density is that hydrogen’s energy is difficult to concentrate and even when compressed to 700 bar, a tank of six times the capacity of a conventional petrol tank is needed to hold the energy equivalent quantity of hydrogen.
In the case of fuel cell cars, their higher conversion efficiency offsets this to some extent – but their range is still limited to about 400km because of the weight and size of pressure tank required.
Hydrogen can be liquefied to decrease the required storage volume, but the process is energy intensive and produces a cryogenic liquid that can only be stored in expensive, vacuum insulated containers.
Liquid hydrogen still only carries only 30% of the energy that petrol carries per unit of volume.
Obstacle 3 – Distribution
The same characteristics that make hydrogen difficult to store also complicate its distribution.
Pipelines are the most cost effective mode of transport, but are extremely expensive because of the need for protection against embrittlement and high pressure.
Compressed hydrogen is transported either in steel cylinders or in trailers fitted with high pressure storage tubes.
The low energy density of hydrogen reduces the energy carrying capacity of a tanker truck to 10% of the equivalent volume of petrol and over a distance of 250km, the fuel it consumes exceeds the energy equivalent of 10% of the load.
Obstacle 4 – Energy efficiency of vehicles
The massive contribution of road transport vehicles to the demand for energy from the existing supply network makes superior efficiency a critical success factor for the Hydrogen Economy.
When the unavoidable energy losses involved in generating hydrogen by electrolysis, storing it and distributing it through refuelling stations for use in fuel cell cars is calculated, it is disappointing to find that the maximum theoretical efficiency to be expected is 22% for compressed hydrogen and only 17% in liquid form.
This does not compare favourably with advanced diesel-fuelled vehicles that are capable of exceeding 25% efficiency.
Similar calculations for hybrid electric cars suggest a potential efficiency of 33% and electric cars with battery storage 39%, subject to limited distance capability without recharging.