In part 1 of this series, I introduced the two main types of electric vehicles: plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). Today, I want to talk about their energy efficiency.
Electric motors – such as those used in a PHEV or BEV – are around three times more efficient than internal combustion engines. However, this is only part of the picture. The production, transportation, conversion and consumption of “useful” energy tends to involve losses at each stage of the process. A vehicle may appear to be more, or less, efficient than another depending on how many stages are taken into account.
“Useful” energy is lost in several of the processes involved in charging and powering an electric vehicle. There are losses in the generation, transmission and distribution of electricity to the charging point. The generation losses can be substantial, (especially for geothermal plants), and the transmission and distribution involves a loss of around 7%.
You pay for these losses, indirectly, in your retail electricity prices. But let’s put them to one side, since they’re being costed anyway, and their emissions are negligible, which is ultimately where I’m heading with this.
For EVs, additional losses arise from the conversion of electricity into stored energy in the battery (charging the battery), and the conversion of that stored energy back into electricity to drive the motor (and move the car).
Similarly, for conventional vehicles, useful energy is lost in extracting, transporting and refining oil; large amounts of energy are lost within the engine as heat or friction, and so on. The overall efficiency of this process is referred to as “well-to-wheels” efficiency – an energy life cycle going all the way from the oil well through to the wheels of your car.
Although the process for EVs is substantially different and may not involve oil wells at all, the overall efficiency of generating electricity through to driving the car is also called “well-to-wheels” efficiency. Similarly, “tank-to-wheels” efficiency refers to just the latter stages of the process, from the fuel tank onwards for conventional vehicles, or the battery onwards for EVs. I’ll focus on this from now on.
On average, petrol cars in New Zealand consume around 10 litres of petrol for every 100 km they travel, a “tank-to-wheels” measure of their energy use. Given the energy content of petrol at around 35 megajoules per litre (MED, 2011a), these cars use 350 megajoules per 100 km.
On the other hand, a BEV might have a “tank-to-wheels” electrical efficiency of 20 kWh/ 100 km (or just 72 megajoules per 100 km). Certainly, you could be looking at an EV using 70% or 80% less energy than a typical car on this basis.
Anyway, this has been a fairly lengthy look at the energy efficiency of various types of cars – sorry about that! However, energy efficiency isn’t that important in itself. Where it becomes important is when you look at its implications for running costs, or energy security, or greenhouse gas emissions. I’ll look at those topics in future posts.