A couple of months ago, I came across an interesting article "Prius Outdoes Hummer in Environmental Damage". It seems quite out of place at first at a time when everyone is chanting for greenness. Unfortunately, I would have to agree with the author on this topic. Here, I would like to look at the issue from another perspective.
Before I begin to explain, I wanted to clarify that the title of this post is “Do Hybrid Cars Really Save Energy” not “Do Hybrid Cars Really Save Money”. Why would I make this differentiation? Well, given the upward trend of gasoline price for the entire life span (assuming more than ten years) of cars, hybrids may indeed save money at the end even though the actual amount might be quite small. But here I would argue that hybrid cars, from production till the useful life spans end, actually use more energy then the gasoline powered vehicles. The reasoning is as follows:
- Rechargeable lithium-ion batteries take significant amount of resources to make (which translate to the energy needed to build such batteries). While I do not have the numbers, it is conceivable that a lot of energy is required to extract Lithium and form the chemical compound that is used in side the battery.
- The energy density of batteries is inherently much less compared to gasoline. For lithium-ion batteries, the energy density is 0.54–0.72 MJ/kg. While for gasoline, the energy density is 46.9 MJ/kg. So from a MJ/kg perspective, gasoline is roughly 80 times more efficient than lithium-ion batteries. Even though a gasoline engine’s efficiency is typically around 30% and a electric motor’s efficiency can easily achieve 90%, factoring this difference, gasoline is still 20 times more efficient than lithium-ion batteries as far as power is concerned. Thus, when the vehicle moves, the power required to offset ground resistance is more for hybrids then for conventional vehicles given the same output powers of both powertrains.
- Hybrid powertrain also cannot be as reliable as gasoline only powertrain given the fact that it has many more moving parts (see my post simple is beautiful) . This is by pure law of physics. So hybrid cars need more maintenance then its gasoline only counterparts and maintenance itself requires energy.
- Worse yet, the failure modes of gasoline engines have been well understand for more than a century and the engine manufacturing processes have matured to a point that engines manufactured these days generally last for twenty or more years. And the output power of a given engine does not significantly degrade as the engine ages. Batteries, on the other hand, ages much faster. Given the way how hybrids’ batteries are constructed (hundreds of serially connected cells), any single cell’s malfunction translates into the failure of the whole battery. So the life span of battery packs is significantly less than engines. Even if all the cells remain functional the cell capacity degrades significantly over the time. So at the end of the life span, a battery pack might only have 60% of the capacity than when new and yet carries the same weight. All these translate into less efficiency over time and thus consume more and more energy to maintain the same performance.
The above points explain why at current stage, hybrid vehicles use more energy during their useful years than their gasoline counter parts. So if you really care about the environment, you might want to think twice before choosing your next vehicle. A conventional vehicle with a reasonably sized engine might be a wiser choice.