Indirect Fuel Efficiency and CO2 Emissions – Power Station to Wheels

In the case of a Petrol, Diesel (or Hybrid) fuelled Internal Combustion Engined (ICE) vehicle the liquid fuel is provided directly to the vehicle and it is easy to determine the efficiency and emissions directly related to that vehicle type. The electricity supplied to the EV (Battery Electric Vehicle (BEV) or Plugin Hybrid Electric Vehicle (PHEV)) must of course be created somewhere. Fig 4 presents a diagram illustrating the energy supply process for a Diesel Vehicle and a comparable EV.

Comparison of Direct and Indirect Fuel Efficiencies for EVs with respect to Diesel ICE

The electricity delivered to the Consumer is composed of electricity created from a mix of renewable and fossil fuelled power stations. In the case of renewable wind or hydro sources, the electricity is supplied directly to the network. In the case of fossil fuel power stations, the fuel (gas, oil, coal or peat) must first be burned in an engine connected to a generator during which process as much as 60% of the energy is lost as heat or energy used on-site.  Considering this mix of renewable and fossil electricity generators together we can estimate the average energy efficiency for this group of machines. In the example in Fig 4, the average efficiency of the generators is currently approximately 49%. 

Next this energy must be conveyed along the power transmission and distribution lines where approximately 8% of energy transmitted will be lost.

The EV itself once it has received its electrical charge for a socket, has a direct energy efficiency of approximately 75%. The EV’s efficiency is high because the individual efficiencies of the charge, battery, electric motor are very high (typically 85-95%) added to which the regenerative breaking and vehicle idle performance provide further gains.

For a Diesel vehicle:

Direct Fuel Efficiency = 17% to 25%

For an EV vehicle (operating on Ireland’s electrical system with 2008 power mix):

Direct Fuel Efficiency = 75%

Indirect Fuel Efficiency = 34% (= 49% x 92% x 75%)

Importance of Direct vs Indirect Fuel Efficiency

When a Consumer fuel a vehicle, the Consumer is only paying for the electricity, petrol or diesel which is Directly consumed by the vehicle. For example, by using the Direct Fuel Efficiency figures for comparison, the Consumer must purchase 80% (Diesel efficiency = 17%) to 67% (Diesel efficiency = 25%) less energy units depending on the Direct efficiency of the Diesel and EV vehicle.

When comparing the Indirect figures, overall, the Electricity Energy Generators must use between 50% (Diesel = 17% efficient) to 27% (Diesel = 25% efficient) less fossil energy to generate the electricity required by the EV Consumer compared with a Diesel Pump Station supplying a Consumer travelling the same distance in a Diesel vehicle. The good news for Ireland is that as the number of wind – and eventually ocean – energy electricity generators continue to grow, the Indirect efficiency of the EV continues to improve each year. 

In 2013, 21% of Ireland’s electricity was generated from renewable energy resources. The largest growth in renewable electricity will come from wind power. Ireland’s target is to supply 40% of its electricity from renewable electricity generation by 2020.  Assuming that this renewable energy is mainly supplied from wind power, an approximate estimate of an EVs Indirect Efficiency on Ireland’s Electricity System in 2020 (see Fig 4) can be made as follows:

Direct Fuel Efficiency = 75%

Indirect Fuel Efficiency = 44% (= 63% x 92% x 75%)

Therefore the overall energy which Ireland must import for its transport requirements will continue to reduce for each EV as the % of wind, ocean or hydro continues to increase. For example, an industrial site using wind power generated on-site could have Indirect Fuel Efficiencies for its EV fleet of up to 75% if its EVs were charged each night using surplus wind power.

Find out more about the Cost Benefit of EVs for Ireland and achieving its 10% Target, < link to SEAI paper >

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