In a straight choice between a new petrol car and a new electric car, on climate grounds the electric car wins easily.

But what if your existing petrol car is still working fine – or is even only a year or two old? Contrary to popular belief, the overall climate impact is lower if a new petrol car is scrapped in favour of an electric vehicle (EV), new research reveals.

Studies show that although EVs produce more carbon emissions when they are manufactured, they are responsible for much lower emissions when they are being driven compared with a petrol or diesel car. Overall, the whole-life emissions for an electric car than for combustion engine vehicles.

at the University of California, Santa Cruz and at UC Santa Barbara wanted to find out whether drivers should wait until their current combustion engine car is ready for the scrapheap before moving to electric.

Using US data, they analysed the emissions impact of scrapping a fossil fuel car and replacing it with an EV at different points, from scrapping a car that is just a year or two old to one at the end of its life. Their modelling accounts for the emissions associated with producing and running a new electric car, and other factors including variability in the grid electricity mix, vehicle efficiency, manufacturing emissions and mileage.

“We found that in the vast majority of cases, and even for a brand new gas vehicle, you would be far better off scrapping the gas vehicle and switching to electric,” says Campbell. “It takes more energy to build an electric vehicle, but the savings from driving the vehicle swamp all the upfront costs.”

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Campbell stresses he doesn’t expect people to immediately scrap their brand new cars. Instead, the study aims to show that there is zero environmental benefit in keeping a fossil fuel car on the road until the end of its life.

The findings hold true for almost any scenario, except where an electricity grid is very reliant on coal power, or when a fossil fuel car is driven less than about 7000 km per year.

“The idea here is just to simply show what a huge advantage electric vehicles offer, so that when the opportunity to make the transition [is] something that people are ready and excited to do, by all means go for it,” says Campbell. “There’s no environmental concern to hold you back.”

at the Yale University says the paper takes a “clever angle”. “I think what they’re talking about makes perfect sense. I’ve thought about this before [but] I’ve never thought to write a science paper on it, and I’m impressed.”

Electric cars now last as long as petrol and diesel counterparts

Records from UK vehicle safety tests show that the average lifespan of an electric vehicle is 18 years, and the reliability is still improving considerably from year to year

But at the University of Michigan points out that the study is considering an “extreme case”. “It’s unlikely somebody’s going to purchase a combustion engine vehicle and retire it after a year,” he says.

Instead, those cars will likely hit the second-hand market, which could have knock-on economic and behavioural impacts, such as lowering the price of second-hand cars so dramatically that it convinces more people to switch from public transport to car use. More research is needed to model those potential impacts, says Campbell.

But the study’s authors say their findings lend support to the idea of subsidised scrappage schemes, to encourage people to scrap older fossil-fuel cars in favour of electric vehicles. Such policies could accelerate the EV transition in the US and help to cut transportation emissions, which are now the single largest source of carbon emissions in the country.

An iron-air battery that stores and releases energy through a reversible rusting process has become the first of its kind to connect with a public electricity grid. On 30 July, the Dutch start-up Ore Energy announced its batteries had connected to the grid at Delft University of Technology in the Netherlands.

Batteries can help deliver a consistent supply of electricity by storing renewable energy from solar or wind farms, releasing it when necessary to ensure that a sudden change in sunlight or wind doesn’t mean an immediate drop in available electricity.

“You need to be able to store that excess energy when the wind is blowing and the sun is shining, to be able to deploy it when you need to during critical demand periods during the day,” says at The Faraday Institution, a battery research body in the UK. “Essentially, batteries can help to smooth out that power output to make it usable on the grid.”

Many grid-connected batteries are now lithium iron phosphate ones manufactured in China. But they typically hold power for just 4 to 6 hours and are prohibitively expensive, says Marie. In contrast, the iron-air batteries developed by Ore Energy can store power for 100 hours or longer and are made from cheap and widely available materials.

“Iron is the most mined metal in the world, it’s incredibly cheap,” says Marie. “And when you combine that with air, which is literally all around us and basically free, those are almost the two cheapest components that you could find.”

The system charges and stores energy by using electricity to convert iron oxides – a form of rust – into metallic iron. The iron can then discharge or release its stored energy by chemically reacting with oxygen from the air to form rust again.

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“When the battery is discharging, we are actually taking the iron and turning it into a special type of rust,” says , CEO of Ore Energy. “And when we are charging the battery, we are taking the rust back into iron, and we do this over and over again [while] the battery is breathing in and out the oxygen from atmospheric air.”

The batteries are stored in standard 12-metre shipping containers and can store multiple megawatt-hours of energy – with 1 megawatt-hour being enough to supply more than a month of electricity to a typical US home.

Separately, the Massachusetts-based company Form Energy has several iron-air battery projects in the works in the US. They are slated for installation in New England and the Midwest.

In addition to iron and air, such batteries incorporate water-based electrolytes that are also cheap and abundant – not to mention greatly reducing the risk of battery fires. “I wouldn’t want to be the one to say never, but you can’t set fire to water,” says Marie.

But the main goal of the battery technology is to help renewable power sources replace fossil fuels in electricity grids.

“Energy companies still rely a lot on gas-fired [power] generation to provide the flexibility needed when wind and solar are not sufficient,” says , business development manager at Ore Energy. “But in the long term we will need a different type of flexibility, and that is where our battery really excels – to provide this multi-day flexibility.”