(October 2009)
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
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 is about 70 million tonnes** per year, growing at about 7% pa. There is a lot of experience handling hydrogen on a large scale.
* 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.
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 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 imminent, 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 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:
The first three are essentially cogeneration. The last three are described in detail in the paper in this series: Nuclear Process heat for Industry .
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.
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 are used in test programs in the USA. The cars have already covered more than 2 million kilometres in test programs around the globe. BMW is currently the only car manufacturer using hydrogen stored in its liquid state.
However, eventually its main use is likely to 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. 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%.
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 most 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. This disadvantage is coupled with a weight penalty due to the storage system, which is about 50 times heavier than the hydrogen it stores - the target is to get it down to 20 times as heavy by 2010, and perhaps ten times as heavy one day.
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 trialled 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 and have less downtime. The first fuel cell electric cars running on hydrogen are expected to be on the fleet market soon after 2010. Japan has a goal of 5 million fuel cell vehicles on the road by 2020. (Current electric car technology relies on heavy storage batteries, and the vehicles have limited endurance before slow recharge - see Appendix.)
Current fuel cell design consists of bipolar plates in a frame, and developer of the 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 automotive division of Ballard is being sold to a Daimler-Ford joint venture (50.1% Daimler, 30% Ford, 19.9% Ballarda), allowing Ballard to concentrate on stationary applications. (Re fuel cells: see Ballard web site.) Other 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 will need to drop from US$ 50,000 to $6000 and 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, natural gas or hydrogen). Figures of over $1000 per kilowatt are quoted, compared with $100/kW for conventional internal combustion engine.
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 are already 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.
Main Sources:Forsberg, C. 2005, What is the initial market for hydrogen from nuclear energy? Nuclear News Jan 2005.
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