Getting any sort of strict comparison between EV’s and ICE cars is difficult. The comparison presented here is based on the 45 mpg figure for a large diesel ICE, representative of real world performance for combined cycle driving in the UK, and the 380 Whr/mile figure for the Tesla. The comparison is only intended to be illustrative, the cars are after all very different in functionality and there is no way of knowing how the actual driving patterns compare.
CO2 Emissions : EV's
A clear distinction should be drawn between point of use CO2 emissions and what are termed ‘life cycle’ figures. The latter figures allow not only for the CO2 released from any fuel at its point of use, but also the associated CO2 generated by construction of the power plant, operation and maintenance of the plant, decommissioning, and production of the fuel. Life cycle emissions can be difficult to assess accurately.
Ref 10 gives the actual CO2 emissions for the UK electricity mix in 2012 as 0.505 kg/kWhr. The greenhouse effects of other pollutants created during power production (NOx and CH4) can be included as their equivalent CO2. Including these gives a figure of 0.509 kg/kWhr. This is at the point of generation (i.e. before transmission or other losses). These figures are for 2012 – there is significant variability from year to year, principally due to variations in the amount of coal used. The latest figure for 2015 from ref. 16 is 0.462 . Transmission losses through the UK grid are around 7.9%.
This gives the Tesla running emissions as 0.509 / ( 2.24 / 1.079 ) = 0.245 kg CO2 /mile
The life cycle figure will be higher, but it is hard to find a reliable figure for the UK electricity generation mix. Some sources suggest a figure of around 0.59 kg/kWhr, which would put the Tesla emissions up to 0.284 kg CO2 (eq)/mile. From this point onward, the use of (eq) is used to denote life cycle figures.
For solar or wind generated power the associated CO2 emissions are very low. Low but not totally CO2 free. The life cycle CO2 figure for wind power can be around 0.012 kg (eq) / kWhr, and for solar PV around 0.048. This gives CO2 emission figures of 0.005 to 0.02 kg (eq) /mile. If the power came from biomass, the figure is somewhat higher, rising to around 0.1 kg (eq) /mile.
Some may argue that if they purchase ‘green’ electricity to charge their car then their emissions are very low or zero. However, there is a logic fallacy here. The nett effect of charging a car from the grid in the UK is that it creates a demand for power generation from non-green and non-nuclear sources. The reason is straightforward. Except for very small quantities, electricity cannot be stored and must be used as generated. Once built, there is no point not utilizing all the solar and wind generated power output available since this would simply waste the power available and leave assets laying idle, so all available solar and wind power is always used. Similarly, the marginal fuel cost of nuclear is essentially zero, and nuclear plants are run to provide base load.
It is also the case that coal fired stations produce at fairly even rates, in part because they are relatively hard to turn up and down. At present coal and gas each generate around 30% of UK electricity, but all UK coal plants are expected to be shutdown within 10 years. Extending the life of coal fired plant by deploying Carbon Capture and Storage (CCS) is unlikely, as the technology is almost certainly too expensive in this application.
It is the gas fired capacity that is varied to balance the grid (albeit with a bit of hydro power and import from other countries) and smooth out the swings created by solar and wind inputs, and by variations in demand.
There is however a caveat. Having large amounts of capacity on standby is expensive and inefficient (so called Short Term Operating Reserve – STOR). Surprisingly there is some back-up power to the grid that uses diesel generators with CO2 emissions not far short of those from coal fired plant. In December the National Grid announced contracts worth £175M to build 650MW of new diesel-driven back-up capacity. That power will not be used much, but it will be very expensive and very high in CO2 emissions.
Whatever the existing mix of electrical power sources, when charging a car from the grid in the UK the additional demand created comes not from zero carbon sources but from gas, and this will continue to be the case until the zero or low carbon electricity production exceeds the non-transport electrical demand. After that transition point is reached, any additional low carbon generation would gradually drive down the associated CO2 emissions for car use.
To get a feel for what this means, the 60% of electricity production presently generated by coal and gas would have first to be replaced by non/low carbon electricity generation. For simplicity, assume this is by nuclear (that way we avoid getting into the huge complication of how a grid dependent almost entirely on intermittent sources could possibly be managed, and the backup capacity that might be required). 60% of 35GW is 21GW, which might cost something like £125 billion to build. Alternatively it might be 60 GW of offshore turbines, at something like £186 billion.
For gas fired electricity generation the CO2 life cycle emission figure is 0. 41 kg (eq) / kWhr (9, assuming the most efficient combined cycle technology). The gas only figure is lower than the figure for the average, pooled electricity as assumed above (0.59). For coal the figure is 0.82 (median figure). These figures are taken to apply to the power at the point of generation.
Assuming that marginal power is generated only by gas and allowing for transmission losses, the CO2 figure for the Tesla becomes = 0.41 / ( 2.24 / 1.079 ) = 0.197 kg (eq) /mile.
CO2 Emissions : Diesel Cars
Diesel has a density of 0.83 kg/litre, and releases around 2.61 kg CO2 per litre. For the diesel ICE at 45 mpg the CO2 emissions are 2.61 / 9.91 = 0.263 kg/mile (164 g/km). This is higher than the 154 g/km for the average UK diesel car (10), a figure which is also based on estimates of ‘real world’ fuel usage. The figures from ref. 10 for diesel cars in the UK range from 89 to 211 g/km; this covers cars 1998 to 2014 - new/newer diesels will give lower emissions.
For standard diesel the life cycle figure (9) is 3.128 kg CO2 (eq) /litre (20% higher than the tail pipe figure of 2.61), pushing the overall figure for the Volvo up from 0.263 to 0.315 kg (eq) /mile.
However, a small diesel car can achieve around 0.17 kg (eq) /mile, easily competitive with the Tesla.
What this shows is that EV’s are not necessarily that much better than IC engines, and in some cases may show little or no benefit.
Liquid fuels do not have to come from non-renewable oil extraction; the simplest example is biodiesel. The life cycle figure for biodiesel produced from rapeseed oil is 1.334 kg CO2 (eq) /litre (9). Allowing for the lower energy density of biodiesel compared to petroleum diesel, the Volvo figure for running on biodiesel drops markedly, to 0.146 kg/mile, 26% lower than the 0.197 figure for the Tesla.
Production of biodiesel from rapeseed (or other plant oils) requires land, and consideration of what proportion of transport energy requirements could be satisfied from such sources is for another discussion, but the fact remains that biodiesel can offer substantially better environmental performance than EV’s.
For a small diesel car running on biodiesel the life cycle CO2 production is 60% lower than the Tesla. To match this performance an EV would have to achieve something around 6.5 miles / kWhr from a charged battery. For the Nissan Leaf, the manufacturer’s claimed usage (14) under the most favourable conditions (low speed, a/c off, 20C ambient) equates to only 5.2 miles/kWhr, and will be as low as 3.2 miles/kWhr at low speeds (52 mph) on the motorway with the a/c on.
It is possible to generate drop-in liquid fuels starting with renewable electricity – the overall efficiency will be much lower than using the electricity directly, but storage of liquid fuels is easy, the existing distribution infrastructure can be used, and this route provides a possible means of utilizing excess renewable power and addressing supply/demand imbalances. Production via electrolysis of water has a low overall efficiency if the co-product oxygen cannot be utilized, but efficiencies can be improved by developments in water electrolysis technologies. There are other approaches to generating liquid fuels using electrical power.
Conclusions On CO2 Impact of Moving to EV’s
In the UK, with the current methods of electrical power generation, and in the context of reducing CO2 emissions from UK car usage, EV’s have a broadly similar environmental performance to existing diesel cars using petroleum derived fuel, and are worse than diesel cars using bio-derived liquid fuels such as bio-diesel. EV’s are not a panacea for reducing UK CO2 emissions, and could well have limited impact.
A recently published report (7) comparing ‘gas’ cars (i.e. petrol) to electric vehicles in the US came to a similar conclusion, consistent with the numbers presented here. Simply put, the study concluded that the possible benefit from adopting EV’s depends on how the electricity is generated and on the assumed efficiency of the ‘gas’ car. With present power generation systems in the US, the study found that there are no grounds for the subsidy of electric vehicles in the eastern US as they make pollution worse. On the east coast there is a case. The study compared ICE and EV cars on (as far was possible) a like-to-like basis. The study makes the point that gains in environmental performance arising from the use of EV’s are obtained by exporting emissions from the point of use to the point of power generation.
A second US report by EPRI (15, 2013) looked at the relative economics of owning comparable ICE, hybrids and fully electric vehicles (EV’s). One EV considered was the Nissan Leaf. The study concluded that the overall lifetime costs were within about 10% of each other, but only where the cars were used in short journey, urban driving. To quote from the report ‘some drivers have driving patterns that are poorly matched to the characteristics of a given PEV and would experience a negative impact for a PEV purchase’ (in other words, it would be more expensive!). The study assumed 35 mpg (combined cycle, UK gallons) for the conventional car, and assumed gasoline (i.e. petrol). The study does not seem to have even considered diesel. One of the conventional cars the study considered was the Ford Focus. Real world data from ref. 13 suggests assuming diesel fuel at 55 mpg would have a fairer comparison.
EV05
留言