A number of government initiatives are promoting research into and development of a ‘hydrogen economy’. But is this really the way to a sustainable future?
Ulf Bossel, European Fuel Cell Forum, Oberrohrdorf, Switzerland
Hydrogen is the topic of the day. The ‘Hydrogen Initiative’ of US President George W. Bush has led politicians from many countries including within the European Union to follow suit without questioning the promises of the hydrogen enthusiasts. Hydrogen is presented as the ultimate solution to all energy problems and claims to solve Third World problems, to clean the environment and to provide everlasting energy. Representatives from industry or public life are seen on television sipping the drip-off from tail pipes of hydrogen fuel cell vehicles. Indeed, the conversion of hydrogen back to its natural state (water) is clean, but how benign is the fabrication and distribution of this synthetic energy carrier?
Figure 1. Energy cascade for energy transport by electrons or hydrogen
Certainly, hydrogen is the most abundant element in the biosphere. Unfortunately, it appears only in chemical compounds like water. More energy is needed to split water than can ever be retrieved from the generated hydrogen. How much energy is really consumed to make, package, distribute and transfer hydrogen? Where does the energy come from? How efficient is the distribution of the lightest, thus most impractical of all energy gases? How much energy is needed to run a hydrogen economy?
These questions need to be answered before investments are made in a hydrogen future. It will cost trillions of dollars to convert the entire energy system to hydrogen. Therefore, it would be diligent to question the optimistic claims of hydrogen promoters before tax money is spent for research, development and hardware. Any new energy technology must be based on a sound platform of physics, engineering and economics. There is no room for vision.
Hydrogen is not a new source of energy, but merely another energy carrier. Like electricity, it provides a link between an energy source and energy consumers. The energy source may be natural gas, coal and oil, or electricity. With few exceptions, conversion of fossil fuels into hydrogen cannot improve the overall efficiency of the energy system or reduce emissions of greenhouse gases. Carbon dioxide is released into the atmosphere when natural gas is reformed to hydrogen. Hydrogen is clean only if made from renewable electricity. However, electricity of any source, conventional or renewable, can be transmitted to the consumer by power lines, pollution-free and with hight efficiency. Why use electricity to split water by electrolysis, spend more electricity to package hydrogen by compression or liquefaction to make it marketable, use energy to distribute it to the consumer and convert it back with considerable losses to electricity in fuel cells? In a sustainable future the cheaper power will come from the grid!
In future renewable electricity will be the main source of energy, generated near consumer sites to minimize transmission losses. Electrolysis and fuel cells may be used for temporary energy storage with hydrogen, but for the sake of overall efficiency renewable electricity will be transmitted directly by electrons and not by synthetic chemical energy carriers. Today, only ten per cent of the electrical energy is lost by optimized power transmission between power plant and consumer. However, if renewable electricity is converted to hydrogen, and hydrogen is later reconverted to electricity, more energy is needed to drive the process. In fact, only about 25 per cent of the original electrical energy may be recovered by the consumer.
At first glance this may sound unbelievable, but the high losses are related to the two electrochemical conversion processes and the difficulty of distributing the light energy carrier. The energy consumption associated with all significant stages of a hydrogen economy has been analysed and the results have surprised the hydrogen community world-wide, but the underlying physics can neither be debated nor improved by additional research and development.
In Table 1 representative numbers are presented for all significant stages of a hydrogen economy. In most cases electricity is consumed. All energy losses are scaled by the true energy content of hydrogen, i.e. its higher heating value HHV (142 MJ/kg).
A hydrogen economy will be based on optimized mixes of these analysed stages. Hydrogen may be compressed to 100 bar for distribution to filling stations in pipelines, and then compressed to 850 bar for rapid transfer into pressure tanks of automobiles. Liquefaction of hydrogen may be preferred to compression to save transportation energy, or onsite production of hydrogen with less efficient electrolysers may offer economic advantages over hydrogen production in large centralized plants and distribution by pipelines.
Whatever scheme is selected, a hydrogen economy will be wasteful compared to today’s energy system and also compared to a sustainable energy future based on the efficient use of renewable energy, i.e. the direct use of electricity and liquid fuels from biomass.
Figure 2. A hydrogen economy requires a hydrogen infrastructure and an extension of the electric power grid
The need for renewable electricity: Let us symbolize renewable electricity by windmills. Assume the electric power output of one windmill is supplied to a certain number of consumers by electrons, i.e. by conventional electric power lines. If hydrogen is used as the energy carrier, four windmills must be installed to provide these consumers with the same amount of energy. Essentially, only one of these windmills produces consumer benefits, while the remaining three are needed to compensate the energy losses arising from the hydrogen luxury.
New infrastructures for hydrogen and electricity: Electric power can be transmitted by a modestly upgraded version of the existing electric power distribution system. For the energy transport by hydrogen, a new infrastructure must be established and, in addition, the electric grid must be extended to deliver power to all active elements of the hydrogen infrastructure like pumps and compressors, hydrogen liquefiers, onsite hydrogen generators, etc.
Energy mix in a sustainable energy future: A sustainable energy future will be based on renewable energy from various sources. With the exception of biomass, renewable energy is harvested from solar, wind, hydro or ocean power plants. One may assume that 80 per cent of the renewable energy becomes available as electricity while only 20 per cent is derived from biomass or used directly for heating. This picture is a complete reversal of today’s scenario which is characterized by 80 per cent of the energy is fossil-derived and only 20 per cent comes from physical sources.
Cost of energy: Simplified, one can say that a customer receives only 50 per cent of the original renewable electricity energy with hydrogen gas and that the losses rise to 75 per cent or higher when this hydrogen is converted back to electricity. The conversion is done by fuel cells.
Today, natural gas prices serve as reference for the cost of electricity. Based on its energy content grid power is about four times more expensive than natural gas. In a sustainable energy future electricity will be the price-setter. It will cost more than today, but it will be the cheapest energy in the market. Because of the energy losses associated with the hydrogen economy, the following energy price relations may be expected for electricity-derived hydrogen:
- At filling stations hydrogen will cost at least twice as much as electrical energy from the grid
- Electricity from hydrogen fuel cells will cost about four times as much as electricity from the grid.
Consequently, for stationary applications natural gas will hardly ever be replaced by hydrogen, but small electric heaters and heat pumps will be used to comfort well-insulated buildings.
Where does the electricity come from? A hydrogen economy is characterized by a massive increase of electric power needs. It is unlikely that this demand can be satisfied from renewable sources. Coal fired and nuclear power plants will continue to be in use with all known consequences on environment and safety. Therefore, before a hydrogen economy is established, the source of electrical energy has to be identified and developed. Together with rational use of energy the renewable sources may be sufficient to match the reduced energy demand worldwide. However, it is unlikely that renewable generation capacity can be stretched three-fold to cover the losses of the hydrogen luxury.
Hydrogen and cogeneration
Presently, hydrogen is made from fossil fuels, i.e. from energy carriers used in most cogen applications. There is no indication that the hydrogen detour offers benefits over the direct use of hydrocarbons with respect to overall efficiency and greenhouse gas emissions. Recent ‘well-to-wheel’ studies based on the true energy content of chemical fuels conclude that hydrogen is not a promising energy carrier at this time.
For years to come, hydrogen cannot beat natural gas with respect to overall efficiency, ecology and economy. Advanced hydrogen technologies like fuel cells cannot compensate for the losses and energy consumption associated with hydrogen production and distribution. Polymer fuel cell co-generators in the 200 kW-class have hardly ever provided line power at LHV efficiency above 32 per cent.
But molten carbonate or solid oxide fuel cells may soon become a viable cogeneration technology. With modest fuel conditioning, these cells convert fossil fuels directly with up to 50 per cent electrical LHV efficiency. They will compete with conventional cogeneration equipment in a natural gas economy. Although still too expensive, high temperature fuel cells are clean converters of hydrocarbons into electricity. Also, high temperature waste heat can be recovered easily for many uses. Time will show the potentials of cogeneration with high temperature fuel cells.
In a distant sustainable energy future with most renewable energy being harvested as electricity, the role of cogeneration must be redefined. For energy efficiency, cogeneration will remain important for biomass-derived chemical energy. It is unlikely that hydrogen will be produced from biomass or by electrolysis and to be converted to electricity in cogeneration facilities.