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Is the calculation of CO2 emissions from electric vehicles correct?
CO2 Emissions of Electric Vehicles and Marginal Power Source Theory
Daughter Eve is compared to renewable energy generation and her parents to fossil fuel generation. If additional expenditures are calculated assuming that the parents' share of the burden remains the same, they do not match the actual additional expenditures. Similarly, the concept of an average for all power sources, which assumes that the emission factor remains the same, results in a large discrepancy between the calculated value and reality. The marginal power source approach is to calculate the actual additional expenditure/increase.
IASTEC : Open Letter to the EU-Council and Representatives of EU-states, 2022, https://iastec.org/open-letter-2
Generally, the average emission factor for all power sources is used, and the CO2 emissions of a BEV (Nissan LEAF) are calculated to be 3.9 kg for a 100 km trip. In some specialized cases, the emission factor for marginal power sources is used, which is 9.3 kg, more than double the value. On the other hand, the HEV (Prius) is calculated to be 7.1 kg based on fuel consumption.
The bar graph on the left shows annual electricity generation and CO2 emissions in the absence of BEVs. From the top, it shows renewable energy, nuclear, natural gas, and coal-fired power, but CO2 emissions from coal account for more than half of the total emissions. considering the additional demand for BEVs recharging (right), the power supply mix will change as the amount of electricity generated increases. The concept of marginal power supply takes into account changes in the power supply mix.
The various power sources are listed in order of operating cost, with generation capacity and power demand on the horizontal axis and operating cost on the vertical axis. Based on the economic principle, when electricity demand increases, the power generation is increased from left to right, and when it decreases, the operation is controlled in the opposite direction. As a result, when demand is M, coal becomes the regulating power source; when demand is H, natural gas becomes the regulating power source; and when demand is L, renewable energy becomes the regulating power source because nuclear power cannot be regulated. The CO2 emission factors for each source and the average annual emission factors for all sources and marginal sources are shown as dashed lines.
Olivier Corradi : Estimating the marginal carbon intensity of electricity with machine learning, Published in Electricity Map, Jul 3, 2018
Power sources are divided into two categories: fossil fuel generation and renewable and nuclear power. The above panel shows the case without BEVs, and the below panel shows the case where charging demand is added. In the normal case (left), renewable and nuclear power with low operating costs are generating power at full capacity at the time, so fossil fuel power is adjusting supply and demand with a margin. When a surplus of electricity is generated (right), fossil fuel power generation is reduced to the last possible amount, and the surplus electricity is then used to adjust supply and demand by curtailing renewable energy generation.
Calculating the difference in emissions between 10,000 kWh and 10,100 kWh using the average emission factor for all power sources yields 25 kg. On the other hand, since thermal power generation actually increases the amount of electricity generated, the increase is 60 kg when calculated using the emission factor for thermal power generation. The "difference in emissions" calculated using the average coefficient for all power sources and the actual "increase" do not match.
Taking electricity demand on the horizontal axis and CO2 emission factors on the vertical axis, the gray area is CO2 emissions from the power system. Using the average approach for all power sources (left), the increase in CO2 emissions ΔX due to an increase in demand of 100 kWh would be 25 kg. In reality, an increase in electricity demand causes thermal power generation to generate more electricity, so the emission factor for the average of all power sources increases slightly (right). Although the increase in the emission factor is small, multiplying this by total demand yields a significant value of ΔY of 35 kg.
CO2 emissions are F, the emission factor is M, and the electricity use is D. The change in CO2 emissions ΔF for a ΔD change in electricity demand was calculated using the formula for the derivative of a product of functions. The first term is consistent with the formula using the average of all power sources. The second term is not zero because the change in emission factor ΔM is very small but the demand D is very large.
If N electrical devices are using grid power, we do not know which power plant each device is using, but hypothetically, we randomly assign the power plants used by each device to ① through N. When an electrical device is turned on or off (or considered) here, that device has selected N+1 marginal power plant when turned on and N marginal power plant when turned off, so CO2 emissions can be calculated.
