Biofuels
The great attraction of liquid biofuels is that they are sustainable ‘drop-in’ replacements for fossil-based petrol and diesel that power most land vehicles. Liquid biofuels offer similar performance characteristics to petrol or diesel. Petrol can be replaced by bioethanol or biobutanol, and diesel fuel by biodiesel. Biomethane can also be substituted for fossil fuels, although engines needs to be converted to run on gas. The storage tanks on board the vehicles are identical to those currently in use.
There are, however, some warranty-type issues regarding the direct use of these fuels in existing internal combustion engines, which still need to be resolved by many manufacturers. The current position is that liquid biofuels are most commonly blended with mineral-derived fuels.
Given the EU-wide commitments, under the renewable energy directive, for 10% of energy used in the transport sector to be derived from renewable resources by 2020 (the UK’s figure for 2009 was 2.4%), it seems highly probable that liquid biofuels will have to provide the vast majority of this. With first-generation biofuels, there were understandable sustainability concerns centred around the ‘food versus fuel’ debate and whether or not arable land should be prioritised for feeding the world’s growing population.
The UK government commissioned the Gallagher review in 2008 to address this issue. The industry responded with second-generation biofuels, based on feedstocks that do not compete with food crops such as using oils from trees such as jatropha, ligno-cellulosic crops (woody materials not suitable for eating) and waste products from agriculture.
More recently, third-generation biofuels have been developed that are derived from microalgae and, at least theoretically, have substantially greater yields than other biomass sources. These microalgae require sunlight and CO2 to grow, so are potentially suitable for biofuel production in the developing world. However, algae oil is more expensive than colza, jatropha or sunflower oil; it also requires very high-tech processes and is far from ideal for developing world applications.
Electrical systems
Although electrically powered road vehicles have been available for the past century, they have never really achieved their potential, with the disadvantages usually outweighing the advantages. This is in marked contrast to electric railway vehicles, which have continued to develop and have, even in the UK, become the most desirable form of propulsion system. However, railway vehicles are predominantly supplied with electricity from overhead or third-rail systems.
The current push for electric vehicles in the UK largely stems from the King review of 2007-08, which concluded that electric vehicles (EVs) were the preferred way forward for road transport. This move has been further encouraged by proposals to meet CO2 emissions targets by decarbonisation of the transport sector.
True EVs use an entirely different transmission system to conventional vehicles with traction motors driving the wheels directly. This has the benefit of allowing the use of regenerative braking to help reduce electrical consumption and preserve battery life. Hybrids and plug-in hybrids tend to use more conventional drivetrains with electric motor assistance. In either case, the energy to power the drive motors is stored in batteries of one kind or another.
The flexibility of lithium-ion technology (from small high-power batteries for power buffering in hybrids, through medium-power batteries providing both electric-only range and power buffering in plug-in hybrids, to high-energy batteries in electric-only vehicles) makes it ideal for use in EVs. However, the grounding of the entire fleet of Boeing 787 Dreamliners in early 2013 following fires in lithium-ion batteries has raised serious doubts about the flammability of this battery type.
One of the attractions of EVs is that there are no tailpipe emissions. This is indeed beneficial in urban environments as the air quality is not impaired by the vehicles. There will, of course, be greenhouse gas emissions from the tailpipes of hybrid and plug-in hybrid vehicles but these are significantly reduced by the predominant use of the electric motors in urban areas. Nevertheless, the ‘zero emissions’ claim for many EVs is misleading, as in the UK the large proportion of electricity is generated from fossil fuels and this has to be taken into consideration. Although greenhouse gas emissions from power stations are anticipated to reduce over time, as non-fossil-fuel electricity becomes more widespread, this is unlikely to be a significant proportion of the mix for the foreseeable future.
Range remains a concern for electric vehicles that have to travel even quite moderate distances. Battery exchange systems have been proposed and could well allay some concerns, although public acceptance of, for instance, putting an old used battery into a new car is, as yet, unknown. As with all electrical systems, the ideal conductor material is copper, which is in short supply. As global population and mobility increase, there are likely to be major sustainability issues with the supply of this material.
Hydrogen
Hydrogen is a useful high-calorific value fuel that can be used in a variety of ways in vehicular transport. However, owing to its extremely low boiling point (-253°C), it is difficult and expensive to produce in liquid form. To allow practical quantities of gaseous hydrogen to be transported in vehicles, the gas must be compressed to the range 350-700bar, which is technically difficult and expensive.
There are two main types of hydrogen vehicle: those using hydrogen as a fuel for an internal combustion engine, and those using hydrogen fuel cells to provide electric power. The former uses a conventional transmission and wheel-drive system, whereas the latter powers the wheels through traction motors as in a normal EV.
Whichever system is used, the issue of portability arises. Because hydrogen is the lightest known element, it has to be compressed to very high pressures so that the volume of the gas is reduced sufficiently to be portable. Although special composite materials have been developed for storage tanks, these are expensive and are unlikely to decrease sufficiently in price with mass production to be in any way competitive with conventional fuel storage tanks.
Furthermore, filling stations will require hydrogen compression equipment to deliver the gas at up to 700bar into the vehicle for storage. There are significant health and safety, as well as cost, concerns around this practice. Nevertheless, a number of such filling stations are being built, particularly in the US. Also, the distance that the vehicle could travel on a full tank of hydrogen is currently similar to that of an EV.
The main attraction of hydrogen as a transport fuel is that there are no greenhouse gas emissions from the tailpipe. The hydrogen reacts with oxygen in the atmosphere to form water, which can be considered environmentally benign. However, although hydrogen is in abundant supply globally, it does not exist in free form and has to be manufactured. Currently only 4% of the world’s hydrogen comes from renewable sources and the production processes for this ‘green’ supply are very inefficient.
An alternative use of hydrogen for vehicles is as an additive to other liquid fuels. This is already commonplace as hydrogen is added to conventional fossil fuels to give a leaner burn and reduce greenhouse gas emissions. However, the most promising way of using ‘green’ hydrogen for transport is to create a CO2-neutral liquid fuel by chemically combining it with CO2 that has been captured in a process such as carbon capture and storage. This produces a synthetic form of methanol and may be thought of as chemically liquefying hydrogen. The product methanol can then be used as a liquid fuel in an internal combustion engine in a similar manner to conventional fossil fuels.
AIR/NITROGEN
The idea of nitrogen as a transport fuel is not new. Scientists first liquefied nitrogen in 1883, and within 20 years the Liquid Air Car Company had produced a vehicle that would run on it. But it never took off.
While various prototypes were produced over the years, the engine was always very inefficient and was soon eclipsed by the internal combustion engine. In principle, any piston-type engine can be made to run on compressed air or nitrogen but, until recently, there were few that were dedicated to run on them. This changed in the early 2000s with the development of the Dearman engine, which uses a patented, novel and far more efficient approach.
The engineering breakthrough behind the Dearman engine was that, instead of using bulky external heat exchangers to gasify the nitrogen, the liquid could be made to boil after entering the engine cylinder, simply by injecting a small amount of water and antifreeze to provide the necessary heat. The ‘thermal fluid’ has more thermal mass than the nitrogen, and so provides plenty of heat to boil it and drive the piston, yet cools by only a few degrees itself. After passing through the cylinder, the fluid is circulated through a radiator to warm back up to ambient temperature. Without this ‘thermal fluid’, the engine would have to be multi-stage, which is cumbersome, inefficient and expensive. Other than the novel engine design, the vehicle uses a conventional transmission layout.
Although not customarily used as a fuel, liquid nitrogen is a product of a conventional air separation unit (ASU) and is widely available throughout the world. Many ASUs, particularly those related to synthetic fertiliser production, already produce liquid nitrogen and have spare production capacity immediately available, without the need for any additional investment. Others, dedicated to providing liquid oxygen, such as for steelworks, could be retrofitted to produce liquid nitrogen at relatively low cost.
The production and storage of liquid air and/or nitrogen is a well-known, well-proven and inexpensive process. The operating fluid is ambient air, one of the most plentiful resources, and no special or unsustainable materials are used in any part of the process. The exhaust from the vehicle is predominantly nitrogen and there are no greenhouse gas emissions at the tailpipe. Another advantage is easy refuelling, which would take minutes rather than the hours needed to recharge an EV. The energy transfer rate is almost as good as with liquid fossil fuels and, because the energy density of liquid nitrogen is low, the range would be similar to that of an EV. The lower cost and greater convenience of liquid nitrogen are likely to be significant advantages.
There are, however, some disadvantages. Widespread adoption of hydrogen systems would require new nationwide filling station infrastructure to handle/dispense liquid air/nitrogen. It would also require new engine configurations, although it could work with existing low-cost transmission systems.
Flywheels
Flywheels can be viewed as kinetic or mechanical batteries; they use electric motors to accelerate a rotor (flywheel) to a very high speed, which stores the energy in mechanical/rotational form.
In railway and tramway applications, the flywheel is turned by a motor/generator and spun up to speed from a third rail or small on-board engine. The flywheel then turns the motor as a generator, to produce electricity for the traction motors that turn the wheels. The flywheel allows direct capture of brake energy (when slowing down or descending gradients) and its reuse for acceleration (‘regenerative braking’). Since the short-term power demand for acceleration is provided by the energy stored in the flywheel, there is no need for a large engine. Flywheel-driven railcars developed by PPM have been successfully used on a short branch line at Stourbridge in the West Midlands since 2009.
Flywheel systems are also being trialled in buses and high-performance cars in hybrid applications. The Flybus consortium has developed a system using a Ricardo Kinergy flywheel as the energy storage medium and a Torotrak continuously variable transmission as the means of transferring energy between the wheels and the flywheel. Expectations are that the Flybus system will be available at significantly lower cost than an electric hybrid, with fuel savings in excess of 10%.
Jaguar’s Flybrid project uses a very similar flywheel hybrid system in a high-performance car. The flywheel system provides a 60kW power boost, for up to seven seconds at a time, and tests indicate fuel economy improvements of up to 20%. While flywheel systems offer high power capacity, low maintenance and negligible environmental impact, they are disadvantaged by low energy density, and high-precision machining requirements result in relatively high unit costs
The content of this article came from a report – Energy Storage: The Missing Link in the UK’s Energy Commitments – published by the IMechE, available for download here.