Transport and the Hydrogen Economy
(Updated April 2014)
- Hydrogen is widely seen as a future transport fuel, but that future is probably further off than popularly perceived.
- In the short term, hybrid and full electric vehicles will substantially increase the demand for base-load power from grid systems.
- The use of hydrogen in the production of transport fuels from crude oil is increasing rapidly and is vital where tar sands are the oil source.
- Nuclear energy can be used to make hydrogen electrolytically, and in the future high-temperature reactors are likely to be used to make it thermochemically.
- The energy demand for hydrogen production could exceed that for electricity production today.
Nuclear power is relevant to road transport and motor vehicles in three respects:
- Plug-in hybrid and pure electric vehicles use off-peak power from the grid for recharging.
- Nuclear heat can be used for production of liquid hydrocarbon fuels from coal.
- Hydrogen for oil refining and for future fuel cell vehicles may be made electrolytically, and in the future, thermochemically using high-temperature reactors.
Hybrid electric vehicles are powered by batteries and an internal combustion engine, the batteries largely being recharged by the internal combustion engine. Higher capital cost is offset by slightly lower running costs and lower emissions. Better batteries will allow greater use of electricity in driving, and also mean that charging them can be done from mains power, as well as from the motor and regenerative braking. These plug-in electric hybrid vehicles (PHEV) are on the verge of being practical and economic today, joining the better-established full electric vehicles (EV), which have a niche market. See also paper on Electricity and Cars.
Widespread use of PHEVs and EVs which get much of their energy from the electricity grid overnight at off-peak rates would increase electrcity demand and mean that a greater proportion of a country's electricity could be generated by base-load plant and hence at lower average cost. Where the plant is nuclear, it will also be emission-free.
While thermochemical production of hydrogen is a long-term objective, it will require dedicated nuclear plants running at high temperatures. Electrolytic hydrogen production however can use off-peak capacity of conventional nuclear reactors.
Coal to liquid hydrocarbon fuels
The Fischer-Tropsch process was originally developed in Germany in the 1920s, and provided much of the fuel for Germany during the Second World War. It then became the basis for much oil production in South Africa by Sasol, which now supplies about 30% of that country's gasoline and diesel fuel. However, it is a significant user of hydrogen, catalyzing a reaction with carbon monoxide. The hydrogen is now produced with the CO by coal gasification, part of the gas stream undergoing the water shift reaction. A nuclear source of hydrogen coupled with nuclear process heat would more than double the amount of liquid hydrocarbons from the coal and eliminate most CO2 emissions from the process.
Using simply black coal, 14,600 tonnes produces 25,000 barrels of synfuel "oil" (with 25,000 tonnes of CO2).
The hybrid system uses nuclear electricity to electrolyse water for the hydrogen. Some 4400 tonnes of coal is gasified using oxygen from the electrolysis to produce carbon monoxide which is fed to the Fischer-Tropsch plant with the hydrogen to produce 25,000 barrels of synfuel "oil". Very little CO2 results, and this is recycled to the gasifier.
Hydrogen is already a significant chemical product, about half of annual production being used in making nitrogen fertilisers via the Haber process and about half to convert low-grade crude oils (especially those from tar sands) into transport fuels*. Both uses are increasing significantly. Some is used for other chemical processes. World consumption in 2009 was about 70 million tonnes** per year, growing at about 7% pa. There is a lot of experience handling hydrogen on a large scale, though it is not entirely straightforward.
* Eg (CH)n tar sands or (CH1.5)n heavy crude to (CH2)n transport fuel. Upgrading heavy crude oil and tar sands requires 3 to 4 kilograms of hydrogen per barrel (159 litres) of product.
** in thermal terms @ 121 MJ/kg: 8470 PJ, equivalent to all of US nuclear electricity.
About 96% of hydrogen is made from fossil fuels: half from natural gas, 30% from liquid hydrocarbons and 18% from coal. This gives rise to quantities of carbon dioxide emissions – each tonne produced gives rise to 11 tonnes of CO2. Electrolysis accounts for only 4%.
Like electricity, hydrogen is an energy carrier (but not a primary energy source). As oil becomes more expensive, hydrogen may eventually replace it as a transport fuel and in other applications. This development becomes more likely as fuel cells are developed, with hydrogen as the preferred fuel, though storage at vehicle scale is a major challenge. Meanwhile hydrogen can be used in internal combustion engines, and it can also be reacted with carbon dioxide at 300-400°C to give methane, using the exothermic Sabatier process.*
* CO2 + 4H2 ⇒ CH4 + 2 H2O. This seems of more relevance to space travel than transport on Earth, though on a small scale it will scrub CO2 and CO from hydrogen streams for fuel cells.
If natural gas also becomes expensive, or constraints are put on carbon dioxide emissions, non-fossil sources of hydrogen will become necessary. Sun, wind, hydro and nuclear are all possible.
Like electricity, hydrogen for transport use will tend to be produced near where it is to be used. This will have major geo-political implications as industrialised countries become less dependent on oil and gas from distant parts of the world.
Electricity and hydrogen are convertible one to the other as energy carriers.
In the short term, hydrogen can be produced economically by electrolysis of water in off-peak periods, enabling much greater utilisation of base-load generating plants, including nuclear ones. In future, a major possibility is decomposition of water by direct use of heat from nuclear energy, using a chemical process enabled by high-temperature reactors.
World oil refineries and chemical plants today have a demand for hydrogen* which is drawing close to the US nuclear output in thermal terms. The rapidly-growing demand for hydrogen favours technologies with low fuel costs, and the scale of hydrogen demand is appropriate to its production by nuclear reactors. Limited hydrogen pipeline networks already exist, allowing production facilities to be some way from users.
* 50 million tonnes in thermal terms @ 121 MJ/kg: 6050 PJ, equivalent to 70% of US nuclear electricity.
In the USA, 11 Mt/yr of hydrogen production has thermal power of 48 GWt, and consumes 5% of US natural gas usage, releasing 77 Mt CO2 annually. The use of hydrogen for all US transport would require some 200 Mt/yr of hydrogen*, though this scenario is a long way off.
* 89,88g hydrogen occupies 1 m3 at STP; 1 tonne hydrogen occupies 11,126 m3 at STP.
Each tonne of hydrogen from natural gas gives rise to 11 tonnes of carbon dioxide.
All this points to the fact that while a growing hydrogen economy already exists, linked to the worldwide chemical and refining industry, a much greater one is in sight if storage and handling problems at automotive scale can be overcome. The development of affordable fuel cells will also be vital. With new uses for hydrogen as a fuel, the primary energy demand for its production may approach that for electricity production.
Meanwhile, a significant increase in electricity demand is likely due to wider adoption of plug-in hybrid and full electric cars. Charging the batteries of these from mains power will be cheaper than using any internal combustion engines. This demand is becoming significant, and well within the planning horizons for new generating plants.
see US National Renewable Energy Laboratory information and http://www.h2data.de/ for hydrogen data.
Nuclear energy for hydrogen production
Nuclear power already produces electricity as a major energy carrier. It is well placed, though beyond the capability of most current plants, to produce hydrogen if this becomes a major energy carrier also.
The evolution of nuclear energy's role in hydrogen production over perhaps three decades is seen to be:
- electrolysis of water, using off-peak capacity,
- use of nuclear heat to assist steam reforming of natural gas,
- high-temperature electrolysis of steam, using heat and electricity from nuclear reactors, then
- high-temperature thermochemical production using nuclear heat.
The first three are essentially cogeneration. The last three are described in detail in the paper in this series: Nuclear Process heat for Industry . See also 2013 IAEA technical report: Hydrogen Production Using Nuclear Energy.
Use of hydrogen as fuel for transport
Burning hydrogen produces only water vapour, with no carbon dioxide or carbon monoxide. However, it is far from being an energy-dense fuel, and this limits its potential use for motor vehicles. By March 2014, there were 186 hydrogen refuelling stations worldwide, 26 of thee in Germany, and mainly used in trials of fuel cell vehicles.
Hydrogen can be burned in a normal internal combustion engine, and some test cars are thus equipped. Trials in aircraft have also been carried out. In the immediate future the internal combustion engine is the only affordable technology available for using hydrogen. announced hydrogen-powered passenger vehicles. For instance, one hundred BMW Hydrogen 7s have been built, and 25 were used in test programs in the USA. The cars covered more than 2 million kilometres in test programs around the globe. BMW is the only car manufacturer to have used hydrogen stored in its liquid state. The low energy density of 10.1 MJ/L is the limiting factor (cf 3.5 times this for petrol), coupled with the need to cool the 170-litre tank to minus 253°C. BMW has abandoned this development and is collaborating with Toyota on fuel cell vehicles, with a view to marketing a joint mid-sized platform and fuel cell stack by 2020.
For transport, hydrogen's main use will be in fuel cells. A fuel cell is conceptually a refuelable battery, making electricity as a direct product of a chemical reaction. But where the normal battery has all the active ingredients built in at the factory, fuel cells are supplied with fuel from an external source and oxygen from the air. They catalyse the oxidation of hydrogen directly to electricity at relatively low temperatures and the claimed theoretical efficiency of converting chemical to electrical energy to drive the wheels is about 60% (or more). However, in practice about half that has been achieved, except for the higher-temperature solid oxide fuel cells – 46%.
Apart from carbon-free production and then distribution to users, on-board storage is the principal problem for hydrogen as an automotive fuel – it is impossible to store it as simply and compactly as gasoline or LNG fuel. The options are to store it at very low temperature (cryogenically), at high pressure, or chemically as hydrides. The last is seen to have considerable potential, though refuelling a vehicle is less straightforward. Pressurised storage is the main technology available now and this means that at 345 times atmospheric pressure (34.5 MPa, 5000 psi), ten times the volume is required than for an equivalent amount of petrol/gasoline. By 2010 however 680 atmospheres (70 MPa) was practical, and the weight penalty of a steel tank was reduced by use of carbon fibre. Earlier, the tank had been about 50 times heavier than the hydrogen it stored, now it is about 20 times as heavy, and the new target is ten times as heavy.
One promising hydride storage system utilises sodium borohydride as the energy carrier, with high energy density. The NaBH4 is catalysed to yield its hydrogen, leaving a borate (NaBO2) to be reprocessed.
Fuel cells are currently being used in electric forklift trucks and this use is expected to increase steadily. They apparently cost about three times as much as batteries but last twice as long (10,000 hours) and have less downtime. The first fuel cell electric cars running on hydrogen were expected to be on the fleet market soon after 2010, but 2015 is now the target. Fuel cell buses have clocked up over 2 million kilometres and a fleet of 20 has been used in Vancouver. Another project has three Mercedes Citaro buses in each of 11 cities. Japan had a goal of 5 million fuel cell vehicles on the road by 2020.
Current fuel cell design consists of bipolar electrode plates in a frame with electrolyte between, and developer of the most common proton exchange membrane type, Dr Ballard, suggests that a new geometry is required to bring the cost down and make the technology more widely available to a mass market. The normal PEM type operates at about 80°C, using a catalyst which requires a pure hydrogen stream as fuel. The automotive division of Ballard was sold to a Daimler-Ford joint venture (50.1% Daimler, 30% Ford, 19.9% Ballard), allowing Ballard to concentrate on stationary applications. (Re fuel cells: see Ballard website.) Many reviews point out that fuel cells are intrinsically not simple and there are no obvious reasons to expect them to become cheap.
Fuel cells using hydrogen can also be used for stand-alone small-scale stationary generating plants – where higher temperature operation (eg of solid oxide fuel cells) and hydrogen storage may be less of a problem or where it is reticulated like natural gas. Cogeneration fuel cell units for domestic power and heat are being deployed in Japan under a subsidy scheme which terminates in 2012, by which time unit costs were expected to drop from US$ 50,000 to $6000 and also they will need to last for a decade.
But at present fuel cells are much more expensive to make than internal combustion engines (burning petrol/gasoline or natural gas). In the early 2000s, PEM units cost over $1000 per kilowatt, compared with $100/kW for conventional internal combustion engine. The target cost for a PEM fuel cell stack is below EUR100/kW, which will require reducing the amount of palladium catalyst.
The direct methanol fuel cell (DMFC) technology is practical for portable electronic uses but not favoured for automotive use. Other fuel cell technologies include the high-temperature PEMFC operating up to 200°C which is less vulnerable to catalyst poisoning by CO, phosphoric acid fuel cell (PAFC) – well developed but high-temperature and used only for stationary power generation, the solid oxide fuel cell (SOFC) – operating over 800°C, hence mainly used for stationary power generation, and the molten carbonate fuel cell (MCFC) – high-temperature (about 650°C) and accepting a variety of fuels, for stationary power generation.
The webpage H2Mobility provides information on some 400 current and past developed prototype cars, buses, trucks, bikes, ships and aircraft as well as specialty vehicles.
The initial use of hydrogen for transport is likely to be municipal bus and truck fleets, and prototypes have already been on the road in many parts of the world. These are centrally-fuelled, so avoid the need for a retail network, and onboard storage of hydrogen is less of a problem than in cars.
For fuller information on fuel cells in motor vehicles see companion paper Electricity and Cars.
Other, large-scale hydrogen uses
A peak electricity nuclear system would produce hydrogen at a steady rate and store it underground so that it was used in large banks of fuel cells (eg 1000 MWe) at peak demand periods each day. Efficiency would be enhanced if by-product oxygen instead of air were used in the fuel cells.
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