Just outside London, a test is underway to see if it’s possible to power commuter trains by solar power.

At a certain level, the railways are already using renewable supplies such as solar power, only they do so indirectly through supplies bought from the National Grid. Plugging solar panels into the railway may sound like an obvious and simple thing to do — which is exactly why it’s none of those at all.

There are some substantial advantages though, but only if they can overcome some really irritating technical issues.

One of the biggest benefits would be efficiency, as a conventional solar supply pumps into the National Grid, which converts the solar array’s DC electricity into its AC requirements. When the National Grid supplies it to the railways to power their trains, it’s turned back into DC supply — and that conversion process loses around 4-5% of the electricity originally generated.

Cut the losses by pumping DC solar power directly into DC absorbing railways and you lower the cost of providing the same amount of energy – making solar power more affordable.

The really big problem is supply and demand rarely align with each other.

Not just that solar power is variable thanks to the vagaries of the weather, as it’s often far more stable than you might expect from the British climate, but that the demand from the railways varies wildly depending on how often trains pass through the area being supplied by the solar panels.

Lots of trains on a busy railway and there’s always a train wanting power, but on quieter routes, you can have periods of no demand at all, followed by a big spike.

The whole project actually started down on the London-Brighton railway, in the town of Balcombe, where an opportunity to install a large solar array hit an expensive problem. It could have generated more power than the local town would need, but the National Grid in the area was already “full” and couldn’t take any more supply without an expensive upgrade.

The climate charity 10:10 which was developing the solar array wondered why they couldn’t sell the surplus to the mainline railway that just happened to run right through the centre of the town. In came a bunch of researchers, led by Professor Tim Green, Director of the Energy Futures Lab, Imperial College London – and they got to work.

Not only is solar power conveniently the same sort of DC supply that the railways want, most commercial solar panels output power at between 600 and 800 volts, which happens to be within the range that railway requires of 750 volts – thus reducing costs and complexity.

Typically, electricity substations on the railway are spaced 2km-8km apart, and they provide the power to their local section of track. As a train passes through, the power demand rises, then falls away. If there are lots of trains, it’s possible that the power consumption level will be relatively stable, but away from the centre of cities, it’s likely that power levels will rise and fall quite a bit, so the demand for electricity is — at a local level — very intermittent.

What’s been looked at by a company set up for this project, Riding Sunbeams is how to ensure that solar power pumped into the railway doesn’t overload during the quiet period and can be ramped up instantly a train demands more juice. Issues such as regenerative braking, where a slowing train converts braking energy back into electricity for the other trains in the circuit to use adds to the complexityy of managing a stable supply from the solar arrays.

High-speed discharge storage, such as supercapacitors and flywheels have been looked at to quickly dump a lot of stored energy into the railway when needed — amongst more conventional batteries to act as storage when power is generated, but not needed.

And of course, the whole lot cant interfere with radio and signalling systems. The research has reached a point though where live trials are about to begin.

Just outside Aldershot station on the line into Waterloo, the team at Riding Sunbeams have been building a solar array. Initially, their relatively modest-sized 30 kilowatts ‘First Light’ demonstrator array will connect to an ancillary transformer on the traction system to supply power to lights and signalling equipment.

Riding Sunbeams will then use real-world performance data from the test unit to build and validate modelling for much larger volumes of solar power to be injected directly into the DC traction network.

If the test is a success then it could open up an entirely new way of powering the railways. It’s not just that the power is clean, but that it’s also distributed in small pockets all along the railways, making the power supply more resilient to power cuts (which of course never happen any more — do they?).

It’s estimated that solar power could supply up to a tenth of the total power needed by the railways – or around 0.1% of the UK’s total electricity demand. Estimates by Riding Sunbeams suggest that widespread deployment in the Southern rail region would shave around 4% off their traction electricity bill – and 13% off the associated carbon emissions.

All going well with the tests, they hope to start pumping direct solar powered electricity into the live railway network from next year.

The trial project has been funded under the Department for Transport and Innovate UK’s First of a Kind competition.