Beware the secondary effects of decarbonisation
No vehicle yet designed generates zero emissions. Despite much variation geographically, and much argument, battery electric vehicles probably, on average, halve lifecycle carbon dioxide (CO2) emissions when considering first-round effects such as manufacturing and operation. But are the advantages as clear if secondary effects – the side effects – of electrification are considered?
In a previous newsletter, we set out our Eight Principles of Decarbonisation required to meet a real net-zero target, as shown below. By "real net-zero" we mean actually net-zero in a similar way to the "absolute zero" set down in UK FIRES by Allwood et al in 2019.
Vehicle manufacturers are performing increasingly sophisticated lifecycle analyses of their products. However, most do not consider the secondary, or knock-on, consequences of these electrified vehicles. This is our Sixth Principle of Decarbonisation.
These side effects may include the knock-on consequences on energy infrastructure, vehicle design, driver behaviour and traffic patterns. Most immediately, electric vehicles are likely to increase demand on the electricity grid. To meet net-zero, this additional demand will need to be fulfilled with zero-carbon electricity. Cleaning existing electricity will not be sufficient. For any net-zero scenario it is a pre-requisite that the whole future grid is clean, but we will not look at this further here as it is outside the scope of this newsletter despite being foundational to any meaningful solution.
The most frequent concern with battery electric vehicles (BEVs) is that the additional weight compared to equivalent internal combustion engines (ICEs) leads to higher non-exhaust emissions, which may equal or exceed the eliminated exhaust emissions. These non-exhaust emissions from the vehicle come from its brakes and tyres. Road abrasion and resuspension are often included in non-exhaust emissions, but will be set aside here as they do not originate directly from the vehicle.
Emissions Analytics conducted a long-term study on the wear from Continental Contisport 6 tyres on a 2012 Mercedes C-Class driven on the public highway in consistent, normal conditions. For the first 1,200km, with no added payload, the average wear rate was 161mg/km, but over 31,000km the average wear rate fell to 76mg/km. Even this lower value is 15 times higher than the maximum permission exhaust particle mass emissions under current European regulations. Running the same 1,200km test but with 570kg of payload in addition to the driver, the wear rate increased to 194mg/km, an uplift of 21%. In other words, the average uplift was almost 6mg/km for every additional 100kg of payload. For example, on this basis the Jaguar I-Pace would emit 16% more tyre particle wear than the nearest equivalent Jaguar F-Pace, due to the 443kg additional overall weight.
As a reference, the maximum exhaust particle mass emissions permitted in the EU since 2009 is 5mg/km1. Often, real-world emissions on vehicles with a particle filter are well below 1mg/km. Therefore, for every 100kg of extra payload, the added tyre wear emissions may be as much as the maximum allowed out of the tailpipe in total, and more like five times more than the tailpipe emissions in practice.
However, this analysis may overstate the increase. The extensive regenerative braking of the BEV may well reduce brake emissions and the calibration of the electric motors may smooth driving dynamics to reduce tyre wear. On the other hand, ICEs are increasingly incorporating regenerative braking using 48V systems, and the higher torque of the BEVs (27% in the case of the Jaguars) may encourage more aggressive driving. While data is too limited to draw firm conclusions on these mitigating factors, the underlying upwards pressure on tyre wear remains.
Vehicle weight is clearly a crucial factor in vehicle performance and profitability. The Chinese-built Tesla 3 contains a lithium-iron phosphate (LFP) battery, whereas the US-built versions have the nickel-manganese-cobalt (NMC) version. For similar range, the LFP battery is 200kg heavier, but cheaper in construction. That additional 200kg leads to approximately 8% extra energy to propel the vehicle due to increased inertia and rolling resistance of a typical on-road driving cycle. This leads to greater CO2 emissions in the electricity generation, distribution and usage. At the same time, that extra weight may cause 12mg/km of tyre wear, other things being equal. These downsides could be offset by material light-weighting, power and torque limitations and advanced tyre materials – but all come at either added cost or reduced driver utility.
Switching to inside the vehicle, the preoccupation with maximising the range of electrified vehicles – to be competitive with ICEs – may lead to worse vehicle interior air quality. Incoming air to the vehicle’s ventilation system is usually filtered to remove first-and-foremost particles, but this process consumes energy due to the back pressure created by the filter. While this may be insignificant proportionately on an energy-consumptive ICE, it can be material on more efficient vehicles.
Emissions Analytics conducted a test programme across 97 recent model year cars in the US market and found that many hybrids had relatively poor filtration. Tests were conducted on a standardised urban route around Los Angeles. Real-time particle number concentrations, with a lower size cut-off of 15nm, were measured simultaneously inside and outside of the vehicle and the integrated values ratioed over the test. Condensing Particle Counters from National Air Quality Testing Services (NAQTS)2 were used. The testing followed the methodology set down in a Society of Automotive Engineers (SAE) paper authored by Emissions Analytics and the University of California Riverside3.
Not all electrified vehicles performed poorly, but the majority did. The average cabin air quality index from hybrids was 55% worse than the other vehicles in the group, and the particle ingress on the worst was 3.6 times higher than the average of standard vehicles. In contrast, the Jaguar I-Pace BEV was one of the best performers. Although not part of this testing, Tesla’s ‘biohazard’ high efficient particulate air (HEPA) filter, which is now standard on the Models S and X, has excellent reported particle ingress performance, although it will still come at the cost of increased energy consumption.
To quantify this energy consumption, we can look at the mechanics of the ventilation system. A typical vehicle heating, ventilation and air conditioning system consumes from around 140W to 1.4kW depending on the setting4. The lower value is an approximation of the power requirement of the fan and the energy required to overcome the back pressure from the filter. At an average speed of 40km/h, the energy consumption would be between 0.28kWh and 2.8kWh per 100km driven. A typical BEV would consume 25kWh per 100km, so the ventilation system may add between 1.1% and 11% to overall energy consumption. For this reason, there is an incentive to reduce the amount of air filtered, the filtration efficiency or air conditioning activity on electrified vehicles, which would lead to higher particle exposures – and the resulting adverse health effects – of the occupants.
Thinking more widely at the transportation system level, a problem that may start to emerge is added congestion, caused by extra vehicle miles from electric vehicles, which then may adversely affect total emissions from the fleet. This would apply during the transition, while BEV penetration remains relatively low.
A BEV costs approximately 5 pence (5.5 Euro cents) per kilometre in energy costs, compared to 12 pence (13.2 cents) for a reasonably frugal ICE5. Other things being equal, this is likely to lead to more and longer journeys, and a switch to cars from other forms of transport: the income and substitution effects. Setting aside the effects on the economics of public transport, the additional traffic volume will lead to greater congestion, other things being equal. As the fleet will remain predominantly powered by ICEs for decades – due to the legacy light-duty fleet and diesel remaining prevalent for heavy-duty vehicles – this added congestion caused by BEVs is likely to cause increased emissions from these legacy ICEs.
Analysing Emissions Analytics’ database of over 2,000 light-duty vehicles, we can quantify the effect of this added congestion. To travel the same distance at the same speed (65km/h), a driving profile with stopping and starting between 30km/h and 90km/h can create higher emissions than steady-state driving. On average, CO2 emissions are 24% higher, NOx emissions 89% higher and particle number emissions 75% higher. For a period, a relatively small number of BEVs may adversely affect the emissions of the majority ICEs, increasing emissions and worsening air quality. This does not mean the push to BEVs is wrong, but the secondary effect in the short- to medium-term must be considered. One mitigation would be to push faster for BEV penetration.
As congestion leads to longer journey times, the rational response would be for some distance-based or road-access pricing. At least, this would need to compensate for the naturally lower marginal costs of operation of BEVs. More widely, there is a strong argument that motoring generally is under-priced. The pollution produced by an ICE is a negative externality not internalised, which leads to over-consumption.
In summary, these are just three of the potential side effects of electrification. This does not mean that electrification is bad, but that these secondary effects must be understood and controlled. With the large amounts of taxpayers’ money being requested to build the electric infrastructure, there should at least be a responsibility that this is well spent and not just the catalyst for swapping one problem for another.
Footnotes:
- Or 4.5mg/km under the PMP protocol
- NAQTS
- Researchgate Publication 333500744 Development of a Standard Testing Method for Vehicle Cabin Air Quality Index
- Impact of the Air-Conditioning System on the Power Consumption of an Electric Vehicle Powered by Lithium-Ion Battery, Modelling and Simulation in Engineering, 2013
- Assuming 2.5 miles per kWh and 12.5 pence per kWh for the BEV; 45mpg (6.3l/100km) and £1.20 per litre (€1.33) for the ICE