Imminent changes in the world energy portfolio will amplify pressure on development of zero-emission vehicles, of which battery-powered vehicles are the nearest-term realisation.
The world energy portfolio will change rapidly in the coming decades due to pressures of supply, pollution, climate change, population growth and industrialisation. One of the obvious effects of the latter two pressures is the rapid growth in automobile usage on the planet, and its effect on combined emissions. This will amplify pressure on development of zero-emission vehicles (ZEV), of which battery-powered vehicles are the nearest-term realisation.
Currently, the US accounts for approximately one-third of the automotive carbon emissions worldwide. Rapid industrialisation will increase the contributions of other countries substantially. Importantly, the anticipated adoption of personal cars by large numbers of consumers in China, promises to increase the Chinese fleet by three to seven fold between 2002 and 2010. If these vehicles are powered by the existing internal combustion engine technology, increased greenhouse gas and pollutant emissions—carbon monoxide, hydrocarbons, nitrogen oxides, ozone, particles and toxic hydrocarbons, including benzene—are expected.
Thus, the needs driving the technology stem not only from global interest in averting climate change, but also the needs of the emerging economies of Asia. Emission effects will be localised in megacities, where air pollution already compromises human health. Change in drivetrain power technologies are thus motivated by global economics and climate change, known health effects of emissions, and demographic pressures. And, cleaner technologies will likely have North America as a practice market for the upcoming global needs.
The new drivetrains will require not only novel technologies, but also large numbers of creative technologists, who must be conversant in multiple power technologies, electronics and sectors. The connection between the automobile and the grid is growing rapidly, and the educated workforce required to complete the connection is substantial.
The weaning of the automotive fleet from fossil fuels is now less a pipe dream, than a set of impressive, but achievable technology goals to be met in the coming decades. Over 350,000 new hybrid vehicles were sold in 2006, representing rather modest savings when fuel economy is considered in tandem with higher vehicle costs. The importance of mitigation of greenhouse gas (GHG) emissions to consumer automotive purchases can be inferred from these purchases. The resulting new, greener element of the fleet, most of which is based in the US, presently represents less than one percent of the total market for new cars, but the rate of insertion of the technology has nonetheless been impressive, and has had multiple salutary effects for OEMs. These include greener brand images, improved materials, controls and drivetrain component technologies and intensified scrutiny of energy sourcing from grid power, with plug-ins now on the horizon. Moreover, when viewed in the context of not only environmental, but also strategic US needs for energy security, it is clear that greener drivetrains have a bright future.
The underpinning technologies for these new drivetrains span several engineering disciplines, including controls, dynamics, mechanics and thermodynamics. The anticipated electrification of the drivetrain and its progenitors in hybrid systems has particularly intensified research in a new area for the automotive sector, batteries. These are essential components in both fuel cell and gas/hybrid vehicles as the only practical energy storage device at present. Though the basic electrochemistries have been known since Edison, it is their implementation in novel, often nano-structures, which has revolutionised their performance.
The importance of batteries in clean vehicles is undisputed; whether fuel cells or reduced-greenhouse gas-emitting IC engines become dominant, batteries will be a near and mid-term requirement. Fuel cells require use of a storage device, and cost parity of recovery of energy in braking for cleaner IC engines versus a simpler drivetrain, has nearly been achieved. Activity in battery research and development is unprecedented, and the relatively small group of workers in electrochemistry, who have been working nearly in obscurity in the context of info/nano/bio/health funding increases by the Federal government, now find themselves working with new vigor and attention on new electrochemistries, and engineering of batteries. Also importantly, older electrochemistries are being reinvestigated with new materials technologies.
With improved design of materials, the horserace of electrochemistries i.e. combinations of anodes, cathodes and electrolytes, has sometimes yielded surprising results over the last two decades. Familiar electrochemistries include lead-acid, nickel metal hydride (NiMH), and nickel-cadmium (NiCad) cells, used for decades in consumer electronics. In the last decade, lithium batteries, because of their higher power and energy density, and longer lifetime, have displaced the former three electrochemistries in higher-end consumer electronics, including cell phones, personal digital assistants (PDAs) and laptop computers. Indeed, with the present, rough cost parity between commodity lithium ion (Li-ion) and nickel-metal hydride battery NiMH cells, it appears inevitable that Li-ion cells will shortly be ubiquitous in consumer electronics.
Similarly, Li-ion cells are rapidly emerging as the best hope for sufficient energy and power density and lifetime, for hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV) and plug-in electric vehicle (PEV) applications. The supplantation of NiMH in these applications has been accelerated by both basic science and engineering advances in Li battery design. In the coming years, batteries will be increasingly designed as other automotive components using a computer, using models that incorporate different elements of analyses. The additional challenge in vehicle applications is in the amount of material that will be required to manufacture sufficient numbers of cells, at low cost.
The consumption of energy in the US by the approximately 2 billion electronic devices using battery power is estimated to be 16TWh/year. Worldwide, around 10 billion electronic devices can be approximated as consuming 80TWh/year. With a US automotive fleet of 220 million vehicles, the total power consumption can be approximated to be 4,700 TWh/year (4,700 billion KWh/year) against the worldwide automotive fleet of 790 million vehicles (2005), which is expected to draw around 16,000TWh/year. Thus, the conversion of the fleet or even a small portion of it represents a daunting challenge to scaleups in battery production and reductions in cost. Presently, automobiles comfortably consume more than two orders of magnitude more energy than engineered devices powered by batteries.
The challenge intrinsic to the transition to electrified drivetrains, is the addition of electrochemistry to the mix. Battery design has largely been the domain of empiricists, with incremental improvements reflecting the relatively light demands on the technology for primary (i.e. single-use, non-rechargeable) cells. The needs of rapidly shrinking portable electronics spurred development of smaller, more reliable and longer life rechargeable cells, but without the very strict cost and mass requirements of vehicles. Indeed, the automotive requirements for batteries may be the toughest challenge yet for electrochemical storage. Simply put, automobiles cannot be designed to spend too much energy carrying around their own power supplies, and so systems must be small, lightweight, long-lived and reliable.
The Li-ion cell has been successively improved by adopting new cathode electrochemistries, from LiCoO2 to higher-capacity LiNi1-xCoxO2. The more recent entry of nanoarchitectured materials (FePO4, and NiPO4), and the development of other materials improvements including separator technologies, hold the promise of even greater rate of insertion of portable electronics technologies into automobiles. Indeed, the march towards higher-voltage systems is now being paved with greater scrutiny of LiMexMn(2-x)O4, the so-called “high-voltage insertion compound”, whose nominal voltage exceeds that of present Li electrochemistries.
However, currently, capacity fade due to thermal ageing and even calendar life and/or uncontrolled generation of flammable gasses during operation in commodity Li cells, remain persistent problems. And though several technologies have emerged which offer safe, stable operation (LiFePO4, titanate systems (LiTi5O12), several manganese oxide systems (LiMn2O4, LiCoxMn2-xO4, LiMnVO4, LiNi1/3Co1/3Mn1/3O2), their costs remain unacceptably high for most HEV, let alone EV, applications. Full insertion of Li technologies into automobiles awaits significant cost reduction in more stable, high cycle-life and power dense materials.
Transition in battery manufacture to high-tech methods, while maintaining focus on materials technologies, which are both less expensive and readily available, comprise the most critical two tasks in the race to realisation of electric drivetrains. Nanoarchitectured material production incurs unacceptably high cost at present. But given the demonstrated performance of more highly controlled and high surface area architectures, the pressure is not to find solutions, so much as to engineer them to be competitive with the IC engine.
Engineering of new batteries includes not only selection and manufacture of the materials comprising the basic elements of the cell, but also the external shape and package. Form factors for automotive application are changing; larger format cells are being considered as they become safer. Indeed, a key reason for selecting smaller, more common sizes of packages is the mitigation of risk should a single cell fail, smaller cells simply have less available energy than larger cells, and thus pose a lower risk if they explode, however unintentional. The 18650 battery, a cylindrical device with a diameter of 18mm and a length of 65mm, is a commodity cell available in several Li electrochemistries, with a present capacity of approximately 2.2Ah. The FreedomCAR and Vehicle Technologies (FCVT) Program under US Department of Energy Office of Efficiency and Renewable Energy has set specific targets for high-power storage devices for HEVs and for minimum and maximum levels of power-assist performance. Similarly, the United States Advanced Battery Consortium (USABC) has set a goal for high-energy batteries powering EVs. But critical efforts are needed in both research and development, in assuring improved rate performance, reduced thermal and calendar ageing, and safety.
Without additional regulatory pressure or significant scarcity in oil, to create cost parity with gas-burning vehicles, insertion of batteries into a much greater share of vehicles (HEV, PHEV or EV), may take over a decade. It has been argued, sometimes successfully, but only on a regional scale, that harmful emissions from gas vehicles should be mitigated through either taxation or by compulsory regulation at either the vehicle or fleet scales. But, the true societal costs of emissions are still being assayed, though nowhere is the evidence clearer than in the developing world.
To design and manufacture electric drivetrains that the world needs, dramatic changes are required in education of science and technology workers. Just as IC engines form the basis of traditional mechanical engineering, study of other drivetrain elements, including batteries, must take their rightful place in engineering curricula of the future. Policy elements, including instruction in both the nature and outcomes of regulations of carbon emission, fuel economy and trade, are needed so that engineers are not simply automatons carrying out prescribed technology tasks. Instead, the next generation of automotive technologists must be equipped with improved decision-making skills for selection of appropriate power sources for different vehicles, economies and objectives. A new program in Energy Systems Engineering in the University of Michigan aims to provide a creative and talented cohort of engineers critical to the ambitious goals of the automotive industry in reshaping the central power element of the drivetrain. This program comprises a 30-credit degree which provides advanced instruction in three key technologies: portable energy technologies, which have informed power devices at larger scales for decades; automotive technologies, whose conversion to electric drivetrains represents a seismic shift in design; and grid power technologies, which will reshape to absorb the demands of the automobile, supplanting petroleum dispensed from gas stations. Other universities will surely, and hopefully quickly, follow suit to connecting energy and the automobile, by creating curricula and programs that contribute to the next generation of workers.
* Acknowledgments
The author gratefully acknowledges the contributions of Mr. Fabio Albano and Dr. Chia-Wei Wang to the data reported here.
