Grantham Scholar Fiona McBride researches the potential to power electric vehicles using the energy generated by trams and metro trains. In this blog, Fiona explains how exactly electricity gets wasted by trams and metro trains and how we can save the energy by using it to charge fleet electric vehicles.
When you think about electric cars and vans, where does your brain go first? Is it to a shiny Tesla, or to comparing petrol and electricity prices? Or do you wonder where we’ll be able to plug all these things in? For many people, where and how often they could charge an electric vehicle is a big concern.
And it’s a reasonable one: as more electric vehicles make their way into cities, they put more pressure on our electricity supply systems. The grid systems that deliver electricity across our cities can only supply so much power in one go. So we need to get creative to make sure we can charge all of the vehicles we’ll need.
Well, they don’t spit out any carbon dioxide or other pollutants directly where you’re driving them. There are some emissions released from producing the electricity to power them, but improving our renewable energy supplies will go a long way towards sorting that out. And if you’re travelling across a Low Emission Zone, electric vehicles won’t rack up charges.
Although switching to electric can give our cities cleaner air and a lower carbon footprint, it’s worth looking at other ways we can make getting from place to place more planet-friendly. Zero-emission public transport, such as electric buses and trams, can move lots of people around urban areas. A public transport season pass comes in at a much lower price tag than a personal car. And you don’t have to worry about where to park or charge a tram or a bus (unless you’re the driver, of course).
Trams and metro systems don’t need to be plugged into a charger, but they do face their own electricity-related problem. When a tram or metro train brakes, the motor inside runs backwards to turn speed into electricity. This causes the tram or train to slow down. Some of this energy gets diverted to power the lights and heating or aircon onboard. If there’s another train or tram nearby, it can use up some of the extra energy too. However, if there’s nowhere else for that energy to go, it ends up being wasted. It’s estimated that if this energy could be saved, the total electricity bill for tram companies could be cut by as much as 30%. Solving this problem would save money and electricity.
The tram problem:
Electricity gets wasted by a tram or metro system when there’s nowhere particular for it to go. So we need to give it somewhere to go – like batteries next to the tracks. This kind of system means that electricity can be collected up and saved for later, either by the tram system or by someone else.
The electric vehicle problem:
Power supply grids are going to run out of charging capacity. Tram systems have their own electrical supply grids. If we could tap into these to charge electric vehicles, it would take some pressure off the main city supply grid.
Since electric vehicles have batteries in, they could absorb the extra energy wasted by trams. They could either use it themselves, or return it to the tram network later using vehicle-to-grid charging. Even if there’s not enough wasted energy for all of the electric vehicles, they could draw extra power through the tram supply grid. This helps to reduce the pressure on the main city energy system.
We already have all of the technology we need to make a solution like this work. Now, the challenge is to figure out when, where, and how much electricity gets wasted by the tram system, and how many electric vehicles could be charged from it. And that’s what we do in the Electric Vehicle Logistics research group here at the University of Sheffield.
Firstly we need to work out when and where trains slow down, as they only produce energy when they’re braking. We do this by tracking the speed of a tram along a journey with a GPS (similar to using Strava to record a run or bike ride).
Next we combine information about how fast the tram is moving with details we get from its location – like whether it’s going up or down a hill, or around a sharp corner. All of these details help us to figure out how much force it takes to keep a tram moving or slow it down. And this tells us how much energy the tram is using or releasing. Since this approach is all based on GPS data, we already know how far along the track it is. So we can also see how much energy it could send to another tram or to an electric vehicle nearby.
Modelling energy use never gives us exact answers, but it can give us an estimate that allows us to work on using that energy.
There’s still a lot to do to link up tram networks with electric vehicle charging. But the estimates we have mapped out so far suggest there’s lots of potential for this system to work.
We’ve focused on ways we can charge fleet electric vehicles, like buses and delivery vans. This is partly because these vehicles do pre-planned routes, so it’s easier to figure out what charge they need than for private cars. It’s also because big vehicles tend to have higher carbon outputs and higher emission zone charges than smaller ones. Therefore focusing on improving charging access here has a bigger impact. Buses in particular also help us to provide better, cleaner public transport. This means that fewer cars are needed in the first place.
For an initial study on Edinburgh, we’ve estimated that one day of tram journeys wastes about 4.3 MWh of energy. That’s enough to charge five big refrigerated electric supermarket trucks, five electric buses and thirty electric cars.
Linking up electric vehicle charging with tram systems would make it easier to charge electric vehicles, and save energy from tram systems that’s currently being wasted. It could make it easier for big vehicles in city centres to switch to electric. This would save them congestion charges and make the air cleaner for everyone. All of this is technologically possible, and we’re working on ways to link things up so that it actually works.
The future of our cities is electric. Linking up our systems to save energy and make charging easier is part of building that future. But it’s only one piece in a much bigger puzzle. Developments in battery technology, in renewable energy generation, and even in how we move around our world, are all part of the process of creating a more sustainable transport system. With technology and teamwork, we are building a cleaner, more sustainable future.