Electric Vehicle – Thermodynamic Efficiency

Aug 12, 2020

Updated 2020

Electric Vehicle – thermodynamic efficiency & emissions

Regardless of how fuelled, all road vehicles emit pollution (and hence health issues). Emissions are in two main forms. One includes haze and particulate matter. The other are ‘greenhouse gases’ such as carbon dioxide and methane. This article, by Collyn Rivers, discusses electric vehicle thermodynamic efficiency and emissions.

Vehicle pollution – 2019. Pic: Original source unknown

Particulate matter from tyres

Particulate matter is constantly shed from tyres. It is mainly soot and styrene-butadiene. Moreover, the smaller particulates are airborne. Furthermore, they are a minor cancer risk. https://ncbi.nlm.nih.gov/pmc/articles/PMC1567725/.

The larger particles are washed into lakes, streams and rivers etc. Related data is scarce. Sweden, however, calculates particulates from tyres as about 150 tonnes a year. Because battery-electric vehicles are heavier than those fossil-fuelled, tyre emissions may slightly increase.

Particulate matter from brake linings

Brake linings also cause particulate emissions. These were initially a mix of asbestos cadmium, copper, lead, and zinc. All were banned in the 1980s. They are now a mix of fibres of glass, steel, and plastic. There are  antimony compounds. There are also brass chips and iron filings, plus steel wool to conduct heat. These brake-related particulates disperse directly into the air. Furthermore, the antimony (Sb) content may increase cancer risk.

Electric vehicles primarily reduce speed via regenerative braking. This substantially reduces brake lining emissions. Regenerative braking is explained later in this article.

Tailpipe emissions

Electric-only vehicles produce virtually no direct emissions (excepting via brake and tyre wear). Hybrids produce no tailpipe emissions when in all-electric mode. They do have evaporative emissions, mainly during refueling. Overall however, their direct emissions are lower than those of fossil-fuelled vehicles.

Emissions from energy drawn from fossil-fuel power stations 

An Australian electricity power station. Pic: SMH.com.au.

With electric vehicles, those charged from grid power must include emissions from power stations. Most of Australia’s are fossil-fuelled. Most are far from efficient. Only four are above typical global efficiency.

Their average emissions are about 920 kg CO2-per MWh. None rivals the 670–800 kg of CO2 per megawatt/hour achieved by China. India has many inefficient fossil-fuelled power stations. It does, however, lead the world in large-scale solar power.

Currently, coal fired generation (both brown and black coal) makes up 78 per cent of electricity generation across Australia’s NationalElectricity Market This is followed by gas. It accounts for just under 10%.

Regardless of whatever they burn, no fossil-fuelled power station converts more than 40% of heat into electricity. Australia’s power station emissions are listed at the end of this article.

Currently, due to Australia’s power stations emissions, there is little point in using an electric car powered via the grid network. In the longer term, however, (when battery capacity permits) it is better by far to go all electric, and powered by solar. Or possibly via hydrogen fuel cells.

Future power stations

Australia is unlikely to build more-efficient fossil-fuelled power stations. Even marginally reducing their existing pollution is enormously costly. If done, their output would inevitably be undercut by ever-increasing renewable energy. Wind plus solar and hydro, is cheaper and simpler. Furthermore, (apart from manufacturing and erecting costs) wind, solar and hydro is virtually pollution free.

Oil-well to vehicle emissions must include extracting, refining and distributing to fuel stations. When burning fossil fuels, vehicle engines are only 25% or so efficient: 75% of the energy is lost.

Quantifying petrol vehicle emissions

Overall, every litre of burned petrol results in 3.15 kg of CO2 emissions. About 81% is caused in burning the petrol, 13% by extraction and transportation, and around 6% from refining.2. Burning petrol also releases nitrous oxide. This has 300 times the global warming potential of CO2.

A typical fossil-fuelled Australian passenger car uses about 9.0 km/litre. Driving just one kilometre generates close to 350 grams of CO2 equivalent being emitted into the atmosphere. This is about 4.8 tonnes of CO2 equivalent emissions per car per year.

Reducing diesel engine emissions was initially hindered by outright deception of some major European vehicle makers. Huge technical efforts have since made to limit fossil-fuel powered vehicle emissions. It is now, however, recognised it is not feasible to reduce them further. This is particularly so of diesel. The alternatives are reduced vehicle weight and performance. Or, as is already (2020) happening, by going all-electric or hybrid, e.g. fossil fuel plus electric.

Currently, battery technology restricts range between charging. All-electric cars are fine for typical commuting to and from work. Furthermore, for general use right now however, hybrids make more sense.

Most cars are driven about 14,000 km/year. They emit about 4.8 tonne/year. The Toyota Prius hybrid averages just under 30 km/litre. It emits 31% CO2 (about 1.5 tonnes a year). That is 3.3 tonnes less than a comparable petrol-powered car.

Regenerative braking

Regenerative braking (in hybrid and electric cars) is primarily of benefit in ongoing stop/start driving in traffic and/or very hilly areas. When braking the electric drive motor acts as a generator. Moreover, it charges the vehicle’s  batteries. Furthermore, by doing so the vehicle’s kinetic energy is recovered for use.

Regenerative braking is of thermodynamic efficiency in all electric vehicles, not just hybrids. It also reduces particulate emissions from brake linings.

Regenerative braking: whilst braking the drive motor acts as a generator, thereby charging the vehicle’s batteries. By doing so the vehicle’s kinetic energy is saved and stored for propulsive use. Pic: reworked from a concept of the Porter & Chester Institue, Connecticut, USA.

Toyota Prius Hybrid. Pic: Toyota

Australia’s main power stations – ages and emissions

Those known in terms of year built, and kilograms of CO2 per megawatt/hour (MWh) actually produced.

Stanwell (1996): 969 kg per MWh.

Bluewaters (2009): 982 kg per MWh.

Muja CD (1985): 982 kg per MWh.

Mt Piper (1996): 997 kg per MWh.

Collie (1999): 1004 kg per MWh.

Eraring (1982): 1011 kg per MWh.

Vales Point (1979): 1018 kg per MWh.

Callide B (1989): 1019 kg per MWh.

Bayswater (1986): 1031 kg per MWh.

Gladstone (1976): 1052 kg per MWh.

Lidell (1973): 1066 kg per MWh.

Muja AB (1969): 1285 kg per MWh.

Worsley (1982): 1324 kg per MWh.

A few of the above have now been (or soon will be) closed down.

Caravan & Motorhome Electrics

The essential handbook for anyone with an RV of any kind.  Everything you need to know to make the electrics in your rig work for you.

eBook versions

Paperback version

Any bookshop, whether online or bricks and mortar, can order copies of Caravan & Motorhome Electrics
Just ask.
ISBN: 978-0-6483190-8-5.

Caravan & Motorhome Electrics

The essential handbook for anyone with an RV of any kind.  Everything you need to know to make the electrics in your rig work for you.

eBook versions

Paperback version

Any bookshop, whether online or bricks and mortar, can order copies of Caravan & Motorhome Electrics
Just ask.
ISBN: 978-0-6483190-8-5.