How to determine whether hybrid cars are really more sustainable than traditional cars?

Question

I purchased a Prius many years ago, but many of my friends have put forth claims that the batteries are very toxic and thus I may in fact be creating more harm than good in using a hybrid and that hybrid's are a less sustainable solution than non-hybrids?

How can one objectively measure the total footprint of a hybrid vehicle to determine whether the use of a hybrid over a traditional non-hybrid gasoline powered vehicle is lesser or greater? Has much research gone into the entire picture? For example, when the car reaches the end of its useful life, what happens to the batteries, can they be recycled or are they thrown away? What about the manufacture of the batteries, what negative effects come into play during manufacture? Are there other externalities that I've missed?

Answer 1

So the short answer is to look at 'life-cycle assessment' studies (LCA).

The longer answer is to ask what you mean by 'better', and then look at a bunch of LCAs and figure out what impact categories you care about most.

In either case, the goal of LCA is to collect all the different inputs and outputs for a product for all stages - not just use, but manufacturing and mining and disposal, etc. These impacts are collected into a smaller number of interpretable impact categories. Some impact categories to think about: carbon footprint/global warming potential, energy consumption, ecotoxicity, human health, water consumption. All of these are pretty commonly used. There is a lot of research on LCA methods, both on how to collect the basic data and how to transform it into these impact categories. Two common methods include the EPA's TRACI or the european ReCiPe. Ideally, LCA's provide a quantitative and clear answer, but that's rarely true - they are quantitative though, which is really important.

For some of your later questions, particularly around batteries, the answers will depend on the study. The technical terms here are allocation and boundary conditions. Do we model battery recycling? who gets the benefits of it? what technologies are we looking at, and what other products do those produce? What impacts or processes do we include or exclude? There are generally a lot of assumptions, which need to be made and defended to move forward - though often they need to be revised for new technology.

To answer your initial question, there was a poorly done report from years ago that purported to show that older Priuses were 'worse' than a Hummer in terms of energy usage. The report was debunked all over the place, but it is true that we've gotten better at making batteries over time. New hybrids have much lower impacts than the originals, and keeping the car longer will definitely decrease the relative impact (one of the assumptions is about how long cars last). Check out a newer study here. In terms of toxicity, here's an older and newer study on the topic - it can take some study to get a reasonable answer, and it's rarely clear-cut. There are almost always tradeoffs.

Answer 2

I find the site CarbonCounter.com, developed by MIT, useful in approaching this question. You can play around with some of the assumptions on e.g electricity supply, battery manufacture emissions and vehicle lifetime. Under most reasonable assumptions, for vehicles of comparable size, hybrids are better than pure ICE, and EVs better than hybrids (unless the electricity is mainly coal). So while yes 'it depends', after playing with the options for a bit, a rough rule of thumb for US average electricity is that hybrids are around 2/3 the lifecycle co2 of an ICE, and full electric are around 50% lifecycle co2 of an ICE. (Very roughly)

Answer 3

There are several environmental concerns when driving a hybrid.

One is the rare earth metals used in permanent magnet motors such as neodymium. However, it has been already established that electric cars end up being more sustainable than gasoline cars, and also the electric motors in hybrid vehicles aren't bigger than those in electric cars (actually in some cases they may even be smaller), so this is not an issue.

Another is the battery. However, a non-plug-in hybrid car has typically 1-2 kWh battery, whereas a plug-in hybrid car has 10-20 kWh battery and an electric car has 50-90 kWh battery. So you can see from this that the amount of metals in a non-plug-in hybrid is insignificant, because the battery is so small.

However, a non-plug-in hybrid very often uses a different battery chemistry. Prius uses nickel metal-hydride whereas electric cars practically all use lithium-ion today. In Prius, the main concern is lanthanum (10-15 kg in battery) whereas in electric vehicles the main concern is cobalt (10 kg in battery). World cobalt reserves are 7.1 megatonnes, and world rare earth reserves are 120 megatonnes. If we assume 25% of rare earth metals obtained from a mine is lanthanum, that would mean we have 30 megatonnes of reserves. So it can be seen that cobalt is limiting electric vehicles more than lanthanum is limiting non-plug-in hybrids. By the way, the lanthanum reserves would allow 2-3 billion hybrid vehicles using nickel methal-hydride batteries. That's more than the number of cars on the roads today.

Also it's possible to consider that battery recycling is very efficient and nearly all of the metals in batteries can be recycled. The battery can be treated as a deposit of valuable metals far richer than the deposits found in mines.

Overall, because gasoline cars use oil at a far greater rate than Prius uses oil, and because lanthanum isn't consumed by hybrid vehicles, it's probably better to reduce oil use by putting the lanthanum inside hybrid vehicle batteries and then at end of life recycle the lanthanum, than to leave the lanthanum in the ground and continue using oil at a great rate.

In the end, the main concern of non-plug-in hybrid vehicles is that they aren't good enough. We shouldn't reduce oil use and carbon dioxide emissions by 30% (which is what Prius does), but by at least 90% if not more (which is what electric vehicles in a clean electricity grid do).

Disclosure: I drive a Toyota non-plug-in hybrid, but my next car will be a fully electric car. The only reason I drive a non-plug-in hybrid was that back when I bought the car, the price of reasonable comparable electric cars was at least twice of what it is today, and plug-in-hybrids had concerns of battery wear and weren't available from Toyota back then (I only buy Toyotas because I like reliable cars). Also back then I didn't have a realistic possibility to charge a plug-in-hybrid, because the only electrical outlet available was behind a 2-hour clock switch.

Answer 4

In this case Life Cycle Assessment is indeed the way to go. A Life Cycle Assessment is the systematic analysis of the environmental impact of a product during its entire life cycle. So to determine whether hybrid cars are more sustainable than traditional cars, one would have to collect and assess all inputs and outputs during the entire life cycle of a hybrid car.

Answer 5

Here is a report from Union of Concerned Scientists on the total benefits of hybrid vehicles:

https://www.ucsusa.org/sites/default/files/attach/2017/11/cv-report-ev-savings.pdf

It was quoted in this general interest article:

https://www.nytimes.com/interactive/2021/01/15/climate/electric-car-cost.html

It discusses a full range of aspects of this question, including state-by-state electricity costs and fuel savings; and vehicle costs for a range of vehicles.

But the other question in my mind is how much the entire petroleum economy costs - to society as a whole. Which is perhaps outside the scope of this question.