The ‘Hydrogen Economy’ is a concept discussed in the media and political circles as an option for reducing society’s reliance on finite fossil fuels and as a sustainable alternative to other renewable energy technologies.
There are potential benefits in using hydrogen in energy supply, but there are also significant challenges and costs. This briefing paper considers the issues of using hydrogen as an energy source.
Hydrogen is a highly reactive element and in the natural environment there is no free hydrogen available. This is in contrast to natural gas (methane) which is naturally occurring and relatively stable in air. Hydrogen is only available through in more stable compounds, particularly water (H2O). Therefore hydrogen is not an energy source, but an energy carrier, or vector, like electricity.
Hydrogen could be manufactured using a low carbon energy source and then stored to be used as a fuel, particularly in transport to reduce CO2 emissions. However, the amount of energy required to produce hydrogen is a significant proportion of the final energy delivered and this is a major factor in the concept of a ‘Hydrogen economy’ as discussed below.
Uses of hydrogen
Hydrogen has been used in a large number of applications, most spectacularly, in many of the space rockets used by NASA, e.g. the Space Shuttle, as a super cooled liquid fuel and combined with liquid oxygen.
It is currently produced on an industrial level from methane, primarily to create Ammonia (CH3) as fertilizer for agriculture and in the petrochemical industry to break down heavy crude oils into lighter fractions through ‘hydrocracking’.
As an energy source, it can be burnt directly for heat, used in gas turbines (static or in aviation), used in conventional internal combustion engines, or more promisingly, fed into modern fuel cells to produce (or re-produce) electricity, e.g., to drive electric motors in transport.
Production and storage
Production of hydrogen
Currently most industrial hydrogen is commercially produced by reforming methane under high temperature steam to create H2 and CO2. Longer term aspirations are to use renewable electricity in large quantities to break down water into hydrogen and oxygen through electrolysis. In addition, the gasification of biomass, as a partly renewable source, can produce large amounts of hydrogen. Finally, with thermal dissociation, water can be broken down into its constituent gases when subjected to very high temperatures (2000oC, or theoretically less) in concentrating solar collectors (in appropriate areas, e.g. equatorial deserts)
The energy cost of producing hydrogen as an energy carrier is a major factor. David Mackay, the government’s chief scientist, describes in his book “Sustainable energy without hot air”, that the calorific energy value of hydrogen is 39 kWh/kg whilst the best practice energy cost of making hydrogen (from gas) is 63 kWh/kg. Therefore, from this example, the manufacture of hydrogen is only around 60% efficient. The efficiency of modern electrolysis systems is also around 60%.
These figures are approximate as there are a number of different options in process, temperature, pressure, etc that may be applied. However, the efficiency figures also need to be factored by the additional costs (energy and financial) of production, compression, storage, transportation and, re-conversion to motive force.
The low density of hydrogen is a major constraint in its use as an energy carrier.
Hydrogen for industrial use in static applications is successfully stored, most commonly in underground salt caverns or depleted oil/gas reservoirs at pressures up to 100 bar, often in volumes of several thousand cubic metres. However, the physical characteristics of hydrogen make its storage in a transportable form a significant challenge. Current options include:
- Physical storage of compressed gas in tanks; at up to 700 bar (approx 10,000 psi)
- Storage at cryogenic temperatures (liquid at -235oC) and/or high pressure.
- Chemical molecular storage with different (and currently experimental) compounds by adsorption, absorption or in chemical combination (as hydrides).
In addition, hydrogen atoms are small and can percolate through many materials, making leakage a significant issue. Containers of steel and some common alloys are also subject to hydrogen embrittlement through chemical reaction, thus requiring inert internal barriers within storage tanks.
Hydrogen can be dispatched under pressure through relatively conventional pipelines, subject to its aggressive properties as noted above. Whilst it has been used extensively as ‘town gas’ before the advent of ‘natural gas’ (methane), its energy density at atmospheric pressure is around a quarter that of methane. One transitional option if created from renewable sources, is to use hydrogen in the existing gas transmission system at less than 20% by volume to reduce CO2 emissions whilst avoiding the drawbacks of a pure hydrogen fuel.
Hydrogen in Transport
The EU estimates that road transport accounts for 20% of current Green House Gas (GHG) emissions (15% by cars and vans). Whilst other GHG sources have seen reductions due to increased efficiency, the road transport sector saw its GHG emissions rise by 26% between 1990 and 2004, illustrating the need for significant improvements.
The EU and its member states are committed to an overall reduction in GHG of 20% from 1990 levels by 2020. Therefore new directives have been adopted to enforce better fuel efficiency on vehicles.
Hydrogen attracts considerable interest as an option to reduce transport GHG emissions because the hydrogen cycle is conceptually simple.
A current demonstration vehicle is the BMW Hydrogen 7 of which 100 have been issued on test loan to potential customers. Whilst delivering comparable performance to conventional vehicles, the car also shows the shortcomings of hydrogen power.
The car is powered by a conventional 6.0 litre V12 engine capable of running on both petrol and hydrogen fuel. The hydrogen fuel is stored in a large 45-gallon (170 liters) bi-layered and highly insulated tank, weighing 120 kg, that stores the fuel as liquid rather than as compressed gas, which BMW says offers 75% more energy per volume as a liquid than compressed gas (at 700 bar). The hydrogen tank’s insulation is under high vacuum in order to keep heat transfer to the hydrogen to a bare minimum. To stay a liquid, hydrogen must be super-cooled and maintained at cryogenic temperatures of, at warmest, −253 °C (−423.4 °F).
When not using fuel, the Hydrogen 7’s hydrogen tank starts to warm and the hydrogen starts to vaporize. Once the tank’s internal pressure reaches 87 psi, at roughly 17 hours of non-use, the tank will vent the building pressure. Because of this, over 10–12 days, the contents of the tank will be completely lost. The BMW Hydrogen 7 uses more fuel than many trucks, consuming 13.9 Lt/100 km for petrol (average UK car consumes ~8.6 lt/100km) and 50 Lt/100 km for hydrogen.
Another transport option is the use of fuel cells, which generate electricity by direct electrochemical conversion of a fuel by an oxidizing agent, offering the potential for high conversion efficiencies and low unwanted emissions. If the fuel is hydrogen or a hydrocarbon, the exhaust products are water or water and CO2 respectively. There are several types of low temperature fuel cells more suitable for mobile applications. Alkaline fuel cells (AFC) operate at 80 °C but require very high purity fuel and oxygen. Solid polymer fuel cells (also known as proton exchange fuel cells (PEM) also require external fuel reforming and operate at about 80 C but are not as susceptible to poisoning as the AFC. PEM fuel cells are most commonly associated with small scale fuel cell applications, such as for vehicles.
There is a debate that hydrogen-fuel cell cars would be more sustainable than current alternatives. However, Mackay also shows that a current hydrogen fuel cell car, the Honda FCX Clarity, has an energy consumption of 69kWh per passenger-100km. This is similar to an average petrol-fuelled car achieving around 30 mpg. Looking at current electric battery cars, these deliver transport at an energy cost of just 15 kWh/passenger-100km (Mackay Fig 20.23). Whilst hydrogen-fuelled vehicles are, and will be, used in areas of high pollution to reduce emissions (, e.g. London buses and in-factory loaders), it is unlikely that there will be a substantial move to a seems clear that there will be no major ‘Hydrogen Economy’ in transport in the immediate future, due to the technical challenges outlined above. Unless there is a significant breakthrough in hydrogen technology, e and that electric vehicles (cars, trains, trams, etc) are likely to remain the most energy efficient option and the least polluting. These advantages are , subject to electric vehicle battery charging being supplied from a national generating system that continues to reduce GHG emmissions (currently ~ 500gm/kWh).
Whilst hydrogen-fuelled vehicles are, and will be, used in areas of high pollution to reduce emissions (e.g. London buses and in-factory loaders), it is unlikely that there will be a substantial move to a major ‘Hydrogen Economy’ in transport in the immediate future, due to the technical challenges outlined above. Unless there is a significant breakthrough in hydrogen technology, electric vehicles (cars, trains, trams, etc) are likely to remain the most energy efficient option and the least polluting. These advantages are subject to electric vehicle battery charging being supplied from a national generating system that continues to reduce GHG emmissions (currently ~ 500gm/kWh).
Static generation of electricity
If hydrogen is unsuitable as a significant substitute fuel for transport as discussed above, could it be used to produce electricity at power stations? Clearly it could not be used for primary generation due to conversion losses. However, it could be used to store energy, for which there are limited options available.
There are various suitable types of fuel cell and configurations that can be deployed using hydrogen. For industrial or large scale commercial use, pressurized high temperature fuel cells, combined with a gas turbine and heat recovery for the exhaust gases would offer overall efficiencies in the region of 70 – 80% or even higher.
High temperature types of fuel cell such as solid oxide (SOFC), molten carbonate (MCFC) offer the best promise of internal fuel reforming, resistance to contaminated fuel and high efficiency through combination with heat recovery. They operate at about 600 – 900 °C. MW size installations have been commissioned and operating.
Hydrogen could also be used for electricity demand balancing using existing, predominantly gas, CCGT, generation, subject to an analysis of costs. However, there are alternatives:
- pumped storage (hydro) already provides balancing power at greater efficiency (75%),
- ‘Supergrid’ transmission links across Europe are expanding and could balance demand on a continental scale.
- Increased demand management is seen as a key factor in energy efficiency.
It has been proposed that ‘spare’ renewable energy, from nuclear, wind, etc, at times of excess production could be converted into hydrogen and released to fuel cells to balance peak demands. Whilst this is entirely possible, and already operational in some remote communities, there are two challenges here.
Firstly, the present contribution of renewable electricity to national demand is only 7.4% (2010), so that the direct use of renewable electricity within the national grid system will be more efficient than using it for electrolysis.
The second challenge is that of cost. No energy source is ‘free’ as each requires a combination of capital and operating expenditure. The cost of the generating plant must be added to any system of electrolysis and distribution of hydrogen. When all the costs are taken into account, it is unlikely that hydrogen will be economic compared to direct transmission of electricity, apart from very exceptional circumstances.
Research continues worldwide into the use of hydrogen as an energy carrier. However, in the immediate future, hydrogen is most likely to be applied to specialist “niche” applications for energy supply. A series of technological innovations and cost reductions will be needed before hydrogen could compete with other energy supply options on a mass deployment scale.