Electric Cars Energy Savings
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Fuel Economy
according to the EPA U.S. vehicle fuel economy has risen to record 24.7 mpg. A US gallon contains 130 MJ of energy. Dividing by 24.7 mpg obtains 5.26 MJ/mile. The EPA official range for the 2017 Model S 100D, which is equipped with a 100 kWh (360 MJ) battery pack, is 335 miles (539 km). Dividing 335 miles into 360 MJ obtains 1.08 MJ/mile.
Note: The 2020 Model S 100 long range has an improved permanent magnet motor giving the 100 kWh battery a 373 mile (600 km) range! This obtains 0.965 MJ/mile!
It takes 1.08/5.26 = 0.2053 ~ 20.5% as much energy to move a vehicle using a battery than using an internal combustion engine.
Note: The 2020 model 18.4% as much energy!
A gallon of petrol in the USA costs $2.83 and that takes a vehicle 24.7 miles or 11.457 cents per mile.
A kWh of electricity costs $0.12 in the USA. A kWh contains 3.6 MJ of energy. A Tesla battery is 93% efficient. So it takes 1.075 MJ of electricity to get 1.000 MJ to the wheels so $0.12 * 1.075/3.6 *1.08 = 3.87 cents per mile.
Note: For the 2020 model 3.46 cents…
A gallon of petrol releases 8.9 kg of CO2 for each gallon burned (2.8 kg). This is 360 grams of CO2 per mile traveled.
Using a wind turbine, solar panel, hydroelectric plant, or nuclear plant produces zero CO2 electricity. So this source of electricity has zero carbon footprint.
Crude oil is 86% carbon by weight and releases 46.4 MJ of energy per kg of fuel. Natural gas is 75% carbon by weight and releases 55.6 MJ of energy per kg of fuel and steam coal used in power plants is 78% carbon by weight and releases 29.2 MJ of energy per kg of fuel.
Thermal generators are typically 42% efficient making electricity from heat.
The carbon in the fuel just described is combined with oxygen to form carbon dioxide, which adds the mass of oxygen to each carbon atom. So on a molar basis;
- C + O2 → CO2
- 12 grams of Carbon plus 32 grams of Oxygen yield 44 grams of Carbon Dioxide.
So now we can compute how much carbon dioxide is released for each MJ electrical of energy consumed.
- Oil Fired Plant
- 0.86 * 44/12 / 46.4 = 68 grams/MJ thermal → 161.8 g/MJ electrical → 174.8 g/mile
- Note: for 2020 model 156.2 g/mile
- Natural Gas Fired Plant
- 0.75 * 44/12 / 55.6 = 50 grams/MJ thermal → 117.8 g/MJ elec → 127.2 g/mile
- Note: for 2020 model 113.7 g/mile
- Coal Fired Plant
- 0.78* 44/12 / 29.2 = 98 grams/MJ thermal→ 233.2 g/MJ elec → 251.9 g/mile
- Note for 2020 model 225.1 g/mile
- Direct Petrol Burning → 360 g/mile (above)
So, NO MATTER WHAT THE SOURCE - NO MATTER WHAT THE METRIC- electric autos use less energy, cost less and pollute less than petrol autos.