Electric Vehicle Chargers

EV vs Gas Cars – Part 1 Efficiency

By on Jun 12, 2015 in Electric Vehicles, Environment |

Several articles and studies have been done recently regarding whether EV’s are really green. Are gas cars better for the environment? If we could switch to battery electric vehicles, should we?

In reviewing this question, there are three areas that I feel merit review:

  • Energy efficiency
  • Greenhouse gas (GHG) emissions
  • Cost comparison

Each is worth looking at independently. So I’ll look at each in a separate post, this being the first.

First Energy efficiency

Is the EV more energy efficient than a gas burning car? The short answer is yes.  In any given class, the EV is more energy efficient than a gas burning car.

How did I come to that conclusion and why is it so definitive?

First, electric motors are tremendously more energy efficient than gas engines used for vehicles.  About 80% efficient for the electric motor, and 20% efficient for the gas engine.  A quick look at some of the EVs on the road shows that the comparison holds with EVs getting about 3-4x the mileage of gas vehicles in the same class.  A Tesla gets about 98 MPGe vs 24 mpg for the average luxury sedan.  The best EVs get 100-120 mpge vs 30-40 mpg for the best small cars.  The BMW i3 is rated 137 mpge city, a full 100 mpg higher than just about any pure gas burning vehicle’s city mileage.  The above numbers are EPA ratings, many EV drivers get better mileage by driving in the EV’s economy mode most of the time. My personal average after 36,000 miles in a 2012 Leaf is 4.1 miles/kwh or ~135mpg combined city and highway.

But this phenomenal mileage may be a little misleading.  It is based on the power from the plug or pump to the wheels (PTW), the EV gets an advantage because the electricity has to be generated elsewhere.  What about the power required to generate the electricity and for the gas vehicle, what about the cost to drill, refine, transport and pump the gas?  So how bad can generation costs be?

Electricity can be generated a number of ways.  Solar generates electricity directly from light, wind and hydro electric use the motion of wind or water to generate electricity.  Nuclear power uses fission to generate heat to run steam turbines.  Natural gas, coal, and bio-combustion generators burn fuels to generate motion directly through an engine or to generate heat to run steam turbines.  All of these have different energy costs to build and energy costs to operate.  However, even fuel burning generators are more efficient when used to generate electricity rather than motive power.  This is mainly because the generator can always be run at the most efficient speed for energy generation while a car’s engine speed is based on the desired speed for the vehicle, which may not be most energy efficient.

Gasoline is pumped from ancient reserves of oil in the earth, refined from oil (or coal, for synthetic fuels) into gasoline, and transported to the gas station for use.  There is an energy cost for bringing gasoline to your local gas station that can also be measured and taken into account.

Fortunately, someone has already done most of the legwork on the energy cost for generation also known as the Well-to-Pump (WTP) cost. The Argonne National Laboratory has developed a program called GREET – Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation. This model is designed as a well-to-wheels model for many forms of transportation. The model allows users to add their own data for analysis while building in research that is already complete. I’ll be using data generated using this model for my analysis.

In addition to operational and well-to-wheel energy costs, there’s the energy cost to build.  The gas burning car uses about half the power to build that an EV does. The energy cost to build a gas car is equal to around 6-8 month’s worth of driving at the average of 12,000 miles per year.  For a gas car, this represents about 5% of the total energy used.  If the EV burned gas, the energy cost to build would represent 10% of the total energy used, or the EV uses five percent more than the gas car over the average life of an average vehicle driven an average number of miles.  For purposes of this analysis, I won’t be adding this energy cost to the comparison.  Just keep in mind that the EV starts with an energy deficit that is balanced out after 6-8 months of driving.

Operational Energy usage of Gas and Electric Vehicles of similar classes

EV vs Gas Post 1 Graph1

The chart above shows that from plug/pump-to-wheel, in each class, the BEV has a much higher energy efficiency than the gas burning car. However, the electricity has to be generated somehow and there are additional energy costs in drilling and transporting the gasoline. So the chart below adds those generation and drilling costs, to provide the well-to-wheel energy costs.

Total Energy usage of Gas and Electric Vehicles of similar classes, with multiple forms of generation

image004

The graphs show that the electric vehicles are more energy efficient, even if powered by coal.

A Leaf or i3 using a Coal generated source saves somewhere between 20-25% of the energy vs. a fuel efficient car. A Tesla using the same source saves about 30% vs an average car while the RAV4 EV saves about 17% vs its gas burning counterpart.

A Leaf or i3 using US Grid generated power saves somewhere between 45% – 50% of the energy vs. a fuel efficient car. A Tesla using the same source saves about 55% vs an average car while the RAV4 EV save about 33% vs its gas burning counterpart.

The real savings starts to show when solar power is used to generate electricity. All of the EVs get 75% or greater fuel efficiency than a gas burning car when renewable solar energy is used to generate electricity.

A note about hybrids

I deliberately am not including hybrids in this analysis.  There are may hybrids and electric assist vehicles on the market and some increase fuel efficiency while others use the electric motor to create more power.  To generalize, the hybrid will use more energy to build than a gas car and possibly more than even a battery electric vehicle, but possibly less depending on battery size.  The fuel efficiency will be between the gas car and the EV.  The hybrid trades lower fuel efficiency for higher range.

Conclusion

The EV is more energy efficient than the gas burning vehicle.  That applies even if coal is used to generate the electricity.  The transition from gas burning vehicles to electric vehicles will reduce energy consumption in the US. Combining that with solar electricity generation will free up tremendous amounts of power for other purposes.

Next, Part 2 – Green House Gas Emissions