Going Places

Solar Trains Could Help Us Rethink Energy

Harnessing the sun could be the perfect way to convert existing train networks to clean power

7 min read

Trains and the sun.
Going Places logo depicting a ship, a train, and an airplane

The small English village of Balcombe became a center of anti-fracking activism in 2013. Now, after organized resistance from local residents, it’s a net exporter of solar power. But the biggest legacy from the summer that hundreds of people stood up to a giant fracking corporation may be the invention of an entirely new way to power trains with solar energy.

Balcombe, halfway between London and England’s south coast, was once the target of Cuadrilla Holdings, which procured a license to drill in the village for gas. After a tense few months during which activists made headlines for a mounted defense of their “fracking village,” Cuadrilla pulled out. The company officially cited structural issues with the quality of the rock formations beneath the Sussex countryside, but campaigners claimed it as a victory. Post-fracking, Balcombe’s residents switched their collective efforts toward the other end of the energy spectrum, seeking to become completely self-sufficient with co-op solar power. (The story of Balcombe, and the bittersweet construction of that solar farm, was told in more detail by our former editor, Alice Bell, in 2015.)

The British climate change campaign charity 10:10, which was directly involved in the Balcombe campaign, is now working with Imperial College London on a new project that could have global consequences.

“We went and organized an initial meeting, three days after Cuadrilla’s exploratory drilling license ended and the anti-fracking camp had packed up and gone,” explains Leo Murray, 10:10’s director of strategy. “Everyone was switched on at that point, engaged about energy politics and wondering what to do next–and we helped craft their headline mission, which was to generate enough community-owned energy locally to match their annual power demand as a village. To do that, we had to find somewhere to put about five megawatts of solar energy.”

As volunteers combed the nearby countryside for potential sites, they found a new train-stabling shed on the London-Brighton mainline, which runs through the village. The roofs would have made a perfect site for a solar array, but there was a problem: The local power grid was already completely saturated, and any spare power generated by the panels couldn’t have been sold back into the national grid. So, what to do with that spare power?

“A chap came along [to a local meeting] called Tim Green. He’s a local resident, and he’s the director of the Energy Futures Lab at Imperial,” says Murray. “We said, “‘Tim, we’ve had this idea–is this crazy, could we connect straight to the trains? Straight to the tracks?’ And Tim was like, “‘Well, I can’t see any reason why not.'”

The benefits of getting people to switch from cars to trains for their travel depend on a number of important factors. Trains carry more people, obviously, but you can’t just swap a diesel car for a diesel train, for example–that’s not much better for air quality or climate change.

Electrification solves these problems to an extent, as long as you’re getting your electricity from renewable sources–but there’s an issue there, too. In countries with old, established electrical grids like the United Kingdom, everything is set up for large power plants to generate a predictable, controllable amount of power and feed it down transmission lines like branches of a tree stemming from a main trunk. With a few exceptions–large hydro dams, mainly–power sources like wind and solar are best scattered across a landscape like seeds to the wind. Their power is green, but inconsistent, and often small-scale.

Railways are the largest single consumer of electricity in the U.K. Reducing carbon dioxide emissions means switching as much of the network across to renewables as possible.

But how can small-scale renewables work with the constant, heavy demand of a modern rail transit network?

The answer, maybe surprisingly, is a coincidence: The output of a typical photovoltaic (PV) farm is a DC current at between 600V and 800V; the operating voltage on the third rail of the local rail line in Balcombe is between 630V and 730V. “It looked as if you could crocodile clip onto a DC third rail and feed power into it, basically, anywhere on the line–you don’t even need a transformer,” says Murray.

It’s not hard to see why this is compelling. As railways run through different landscapes, they often leave pockets of brownfield land along their length. While unsuitable for, say, housing or a commercial building, this might be perfect for a series of small solar arrays. The minimal transformation required to match a solar panel’s DC voltage to the third rail’s required input further minimizes the amount of equipment necessary to fit in those spaces near the lines. Peak train demand also tends to be during daylight hours.

This likewise helps with a policy headache. In 2015, the British government eliminated or severely cut most of the subsidies supporting the country’s renewables industry, including the ones that made it economical to build solar arrays and sell excess generation back into the grid. As well as being a critical blow to the renewables sector in the U.K.–investment in renewables is projected to fall by 95 percent by 2020–it also left only niche types of solar arrays as financially viable.

“The whole game now for commercially generated solar is to displace retail electricity [from the national grid],” says Murray. “You need to have an industrial client with a very good credit rating who can underwrite a solar farm, and to be confident you can make your money back you need to sign a contract with someone who will remain for the lifetime of the equipment, which is 20, 25 years. When you narrow that down, the places where this works so far are places like factories and water treatment works. Belfast International Airport has a £5 million solar farm with a private wire into the airport and it uses everything it generates–it should meet about 25 percent of its demand over 25 years.”

Railways, it turns out, fall into this kind of category, with power grids that are controlled independently of–and downstream from–the national power distribution network. “Railways represent an incredible opportunity,” Murray says.

So, what’s the catch? Well, for one, this is all still speculative, points out Nathaniel Bottrell. He’s a researcher in micro-generation at Imperial College London and part of the team there working on investigating the idea’s engineering challenges over the next six months.

“The first challenge,” he says, “is that a traction rail network will have trains going by a certain section only every 15 minutes, or half an hour, or maybe more frequently in London, so the load isn’t necessarily constant. The load will be intermittent, [and] PV is intermittent. Clouds, low light levels, it can only operate during the day when the sun is shining–so, can we match those two together? Do we need some form of storage [during gaps in load], or stop generation?”

Other challenges include designing the power electronics for loading current from a solar farm into a railway grid–”to my knowledge nobody has built a converter for PV to traction voltage,” he says–and figuring out how dense these insertion points will need to be alongside a track. Each farm needs to be big enough to generate a useful amount of energy, but not so big that it overloads the third rail at its point of connection. As for whether it’s best to have a handful of larger farms versus twice as many smaller farms (for example) over the same distance, Bottrell says these are all issues that need to be investigated.

“Our research will lead to an algorithm,” he says. “So if you have a network with trains of this size, and trains of this frequency, then you can connect this much generation. An algorithm that can size a PV farm, that can size how much you can connect at one particular point, because once we understand that, then we can start to apply it to other train networks.”

If the numbers work out, then 10:10, Imperial, and their other collaborator, the renewable energy cooperative Community Energy South, will seek funding for a pilot trial to run on the Brighton main line. The success of that trial will hopefully inspire its adoption elsewhere in the world.

Whether it will work outside of a small, 50-mile [81 kilometer] railway in the south of England is also as yet unknown. The biggest limiting factor is that this is all about DC power, not AC–and when railways are electrified, they tend to be converted to AC over DC. This is because AC is more efficient for larger trains carrying heavier loads at higher speeds, whereas DC is more efficient at lower speeds and with smaller trains, and the equipment is cheaper to build and maintain. Those advantages tend to make more sense in metro and subway systems that have to operate in small tunnels where overhead wires can’t fit, and the trains haven’t got the room to carry AC transformers.

Right now, India is embarking on an ambitious electrification efforts–and that includes a stated target of installing 100 gigawatts of solar power by 2022. (That’s more than half the installed solar capacity of the entire world back in 2014.) Like many developing countries, it’s leapfrogging some of the technological solutions that nations elsewhere adopted when the economic and environmental situation was very different, and the technologies involved weren’t as mature.

The U.K.’s electricity distribution network is heavily centralized–and deliberately structured to make it difficult to install small-scale renewable sources and sell that power into the grid like a major company can do with a large gas or nuclear plant. India, however, is pushing solar power not just because of its suitability as an energy source in a part of the world with high levels of sunlight, but because it fits in nicely with a decentralized energy network.

As a report on the topic by the Indian Ministry of New and Renewable Energy argued last year, solar farms boost local employment rates, reduce reliance on fossil fuels, and solve the critical issue of bridging the “last mile” between people and their nearest power source. With 16,777 miles [27,000 kilometers] of track already electrified, but almost as much again still yet to switch over, there’s clearly huge potential for trackside solar.

But as much as it might seem like the perfect solution, there is a catch. India’s newly electrified railways almost all run on 25,000V of AC (not DC) power. Figuring out how to economically step up from a few hundred volts to tens of thousand is the second goal of the research project, Murray explains.

“The bigger prize here in terms of market potential is connecting straight to the AC route, but the engineering challenge is different and it might not make sense to do that,” he says. “There are city metros all over the world with DC at the right voltage, and if we could actually crack the AC connection, you’re talking about something that could be genuinely very significant at decarbonizing the transport and the power sectors.

“India has a vast, vast network–it’s got a very aggressive electrification target of 2,000 kilometers [1,242 miles] electrified every year, and it’s also got an extremely aggressive target in terms of solar. But analysts at Bloomberg are skeptical about them reaching that target, and a key reason is they don’t have the infrastructure to accommodate it, [because of] poor generation and transmission networks. So if we can make it work to connect solar directly to DC rail, you can see that there’s potential for gigawatts of that 100-gigawatt target not to even connect to the grid–just to power trains. It’s huge.”

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How We Get To Next was a magazine that explored the future of science, technology, and culture from 2014 to 2019. This article is part of our Going Places section, which looks at the impact of transportation technology on the modern world. Click the logo to read more.