Author: Graham Brennan, transport programme manager SEAI
This two-part article examines why after 100 years, the EV is today emerging as the leading contender to displace the use of combustion engines on our roads. Part 1 will examine the technical issues around the vehicle and Part 2 will deal with the manufacturing costs and infrastructure requirements going forward.
Motor transport is a difficult area to address in terms of efficiency and emissions because of the dominance of the Internal Combustion Engine (ICE) and the requirement for high energy density fuel to supply it. The combustion engine has been with us since the 19th century and offers little scope for major gains in thermodynamic efficiency.
Even before the combustion engine, electric motors and batteries were well understood and, in the 1890s, the electric automobile began to emerge as the first passenger car. With a range of 30km, it was regarded as a safe, reliable and clean method of city transport for the wealthy.
The discovery of oil in the USA and the development by Henry Ford of the ICE at prices that many Americans could afford meant that by 1912 the Electric Vehicle (EV) was soon surpassed in sales by the combustion engine vehicle and was eventually consigned to the scrap heap.
The advent of power-hungry laptop computers in the 1980s and portable electronic devices created a need for high-energy density batteries. Lithium was known to offer one of the highest standard electrode potentials. When combined with a corresponding chemical on the opposite end of the scale such as cobalt, iron ferrate or permanganate, peak voltages of 4V could be achieved in a single battery cell.
This contrasted dramatically with cell voltages of 2V for a lead acid battery. Therefore a lithium battery could transmit up to twice the energy per electron charge delivered. With lead being such a heavy element, the first generation of lithium ion batteries in practice offered three times the energy density of lead batteries.
The automotive industry realised that they could now consider electric vehicles which could hold enough energy to propel a vehicle of sufficient size and performance to meet the expectations of a motorist. This was in stark contrast to the types of vehicles powered up to that point by lead batteries.
With manufacturers under steady pressure from environmental regulations and consumers expecting ever more energy-efficient, powerful and environmentally friendly vehicles, the electric vehicle began to be viewed by manufacturers as an option worthy of serious investment.
Lithium ion battery principles
Fig. 1 shows a cross-section of a typical lithium ion battery in use today. Graphite carbon is used to store the lithium between sheets of carbon on the anode side during discharge. The electrons cannot pass through the electrolyte and must follow the external electrical circuit. The electrolytes allow the positive ions to travel through the separator to the cathode.
Electrode and electrolyte chemistries are selected for the anode side to maximise supply of electrons while on the cathode side chemistries are selected to maximise absorption of electrons. Once the circuit is made, the reaction can proceed. The same number of positive charges flow through the battery as negative electrons flow through the external electrical circuit. The rate of flow is determined by the rate of current required by the electric motor and the internal and external resistances of the battery and circuit.
As the store of lithium ions begins to deplete, there are fewer ion charge carriers available and the internal resistance (R) of the battery begins to rise causing the battery terminal voltage to steadily drop. Increasing amounts of the battery’s power are thus lost through this process of internal heat generation. The current flow (I) of ions within the battery causes this heating effect which is determined by the relationship I2
During the charging process, an external DC source is applied to reverse the polarity and pull electrons out of the cathode thus replenishing the anode ready for use again.
Overheating during discharge or charge is therefore a key design concern for the engineer. In the worst case the temperature could be high enough to start exothermic reactions in the electrolyte leading to a fire in the cell. Lithium batteries have a minimum voltage below which the chemistry can deteriorate and surface depositions can develop on the electrodes. In addition, cell charging must be uniform to ensure each cell is working to its optimum output. Carefully designed battery management systems are therefore required to prevent these scenarios from occurring.
Efficiency and emissions
A key argument made by critics is that EVs do not actually reduce emissions when the CO2 contained in the electricity is accounted for. The electricity system efficiency and CO2 intensity for Ireland’s electricity network are published by SEAI annually and in 2013 this figure was 0.469 kg of CO2 per kWh supplied. The electrical system efficiency, measured as electricity supplied divided by energy inputted, in 2013 was 48.3 per cent with the remaining energy lost in thermal energy in the generators, power consumed to run the power plant and electricity transmission system losses.
The quoted energy performance of the Nissan Leaf on the New European Drive Cycle (NEDC) is 150Wh/km, which means the customer must purchase 150Wh of electricity for each km driven. The CO2 contained in the electricity supplied to the car is 70g/km. Applying the system efficiency to this number we find that the energy needed to create and deliver this electricity is 313Wh/km. The fuel efficiency of a new Toyota Auris 1.4 litre diesel car tested on the NEDC is 380Wh/km (or 3.8 L/100km) with a corresponding CO2 rating of 99g/km.
Comparing the primary energy supplied to make the electricity and associated CO2 with the performance numbers for the diesel car, we can see that the primary energy saving is 18 per cent and that the CO2 saving is 29 per cent. EVs operating on Ireland’s electricity system therefore offer higher efficiency and lower emissions than an equivalent class diesel car on the market at present.
This is not the real prize for Ireland, however. Each time a unit of wind energy (or hydro or ocean) is supplied to the electricity system instead of any thermal generation, it can displace approximately two units of fuel energy supplied to the thermal plant (assuming, for example, a plant thermodynamic efficiency of 50 per cent).
Nearly 13 per cent of the primary energy inputs (i.e. all fossil fuel and energy inputs needed to create Ireland’s electricity) to Ireland’s electricity was supplied from renewable resources in 2013, which was a mixture of wind, hydro and thermal biomass. For simplicity let us assume that all of the remaining energy was imported. In this case we could say that 272Wh/km of the energy for our EV was imported resulting in a 28 per cent reduction in transport energy imports compared with the diesel car.
Now let us roll forward to 2020 when Ireland’s wind generation has doubled and 28 per cent of our primary input to the electricity system is from renewable energy. In this case we could say that 225Wh/km of the energy for the EV would be imported, which would represent a 40 per cent reduction in transport fossil fuel imports with respect to the selected diesel vehicle. The levels of import would continue to fall as Ireland realises its full offshore wind and ocean energy potential.
Fig. 2 shows the indicative energy density for a range of batteries currently available in the market. For instance, the Nissan Leaf batteries have an estimated energy density of 100Wh/kg. Nissan manufactures its own bespoke battery cells. Tesla uses tried and tested laptop batteries produced by Panasonic with an energy density of 225Wh/kg. Several manufacturers including A123 and Kokam are also offering lithium batteries in the 200Wh/kg range.
If we consider that the Nissan Leaf has a range of 200km, by switching chemistries, the Leaf could be expected to have a range of 400km for the same mass of batteries as it has today. Nissan and others are known to be testing the next generation of EVs at present, so we can expect EVs which will exceed the 300km mark in the next two years. With that driving range, EVs could offer the Irish motorist the chance to travel the country with one stop only.
Lithium sulphur batteries use a metallic plate of lithium instead of carbon for the anode and cheaply available sulphur for the cathode. This provides a higher concentration of lithium and avoids the use of more expensive metals such as cobalt in the cathode. Sion Power and Oxis are examples of two companies offering this technology commercially at present. Currently available Li-Sulphur batteries offer energy densities above 300Wh/kg, which could see affordable vehicles with a 600km range become a reality before the end of the decade.
Looking at the longer-term future, advanced Lithium sulphur, silicon fibres and other nano structures offer the prospect of very high energy densities with excellent mechanical resilience. Nano structures offer very high surface areas enabling high absorption and release rates of ions leading to greatly improved fast charge and discharge capabilities.
Fig. 3 illustrates the types of failure mechanisms that commonly affect the storage capacity and therefore useful life of lithium batteries. Three of the common causes of loss of capacity and cell failure include:
- Mechanical fatigue due to thermal expansion and contraction causing electrode cracking;
- Formation of tree-like-salt dendrites on the electrode surfaces, eventually piercing the separator and causing a short circuit;
- Repeated over-discharging, leading to unwanted deposits forming on the surface of the electrodes thereby eliminating that section of the battery from use.
Developing solutions to eliminate all of these effects provides the main focus for researchers around the world.
When considering battery life, the first question to ask is how many charge cycles does the vehicle need during its lifetime? This depends a lot on the range of the car. For example a Leaf (200km range) would require 1,500 cycles and a Tesla S (500km) would require 600 cycles over its expected service life. Interestingly, this means that as the vehicle electric driving range increases, the required cycle life for the battery’s cells reduce. Fig. 4 presents a graph of battery cell cycle life collected mainly from researchers’ and battery manufacturers’ data.
While there are ad-hoc and unofficial tests that seem to confirm Nissan and Tesla’s claims with respect to life, a formal test standard is now required to provide assurance and transparency to the consumer. Depth of discharge, rate of charge/discharge and environmental conditions all influence the life of the battery and these need to be considered in any standard. In the meantime, car manufacturers keep this type of information confidential. Battery manufacturers on the other hand do release some data and two in particular, A123 and Kokam, would both claim to have commercially available lithium ion cells capable of more than 7,000 charge cycles at 80 per cent depth of discharge. This sounds promising but more real world evidence is required.
The main conclusion here is that as batteries get cheaper and packs become larger with higher energy density, the number of charge cycles required will begin to fall. Couple this with the encouraging efforts by manufacturers to make battery cells with longer cycle lives and it can be seen from Fig. 4 that it is likely that EVs will evolve to offer more than sufficient battery life to meet an average consumer’s expectations. This ‘surplus life’ could then be put to other uses such as the purchase of cheap rate electricity and the provision of power services to the electricity network which will be discussed in Part 2.