Rail solar project seen as stepping stone for renewables

Riding Sunbeams is the company behind a project of the same name that is already delivering power to trains on Network Rail’s Wessex route from a site near Aldershot. That project is supported by the Department for Transport’s 'First Of A Kind' (FOAK) scheme to boost innovation in rail. The firm is now talking to the Welsh government about a possible role in electrifying the South Wales Metro.

Alex Byford is head of delivery at Riding Sunbeams with long experience of power projects. He explains that the Aldershot installation is quite small, at 37kW, but enables the company “to prove to Network Rail’s product assurance and safety teams that renewables could be integrated without causing any issues with the rail network.” A data logger at the site is gathering information about power quality, in particular, to ensure that the equipment will neither cause problems nor itself be damaged as a result of being connected to “quite a dirty supply” that has issues with harmonics, reactive power and voltage fluctuations.

Trains on the Wessex line are powered by a third-rail supply at 750V DC, so it’s surprising to hear that the solar installation uses inverters to convert its DC output to AC. Byford says this is a better way of doing things because Network Rail supplies the third rail from its own private distribution network at 33kV AC. Plugging directly into that enables the use of the same off-the-shelf inverters that are used by other solar sites to feed into the grid. It’s those inverters that are now going through the railway approval process.

Meanwhile, in Wales the devolved government and Transport for Wales are pushing ahead with plans to electrify 172km of what are known as the Valley Lines as South Wales Metro. The avowed aim is to do this with 100 per cent renewable energy, of which 50 per cent will be Welsh-generated. Work is currently in the technical design stages, addressing the physical requirements.

Riding Sunbeams comes in because it received funding from the Rail Safety and Standards Board (RSSB) to review the feasibility of integrating renewable generation into the newly electrified lines. An outcome of that will be direct recommendations to Transport for Wales on how much generation they can put in and where it should be.

Byford acknowledges that directly connected renewables can’t provide all the lines’ power requirements, but estimates that 20-30 per cent is achievable. “It’s still quite a large amount on a new line,” he says, and the alternative is to connect to the National Grid, which “still has a lot of old fossil fuels in it”.

At this stage, Riding Sunbeams is only making recommendations, but a solar connection to the South Wales Metro would be an important step for the company because it requires different technology from that used at Aldershot. This would demonstrate that the concept could be applied much more widely both in Britain and internationally.

The Valley Lines will be electrified with overhead power lines running on 25kV AC, but - significantly - the supply will be single-phase, unlike the national transmission and distribution networks, which are three-phase.

Byford explains why this matters: “Single-phase electricity is normally used in domestic installations at 240V, whereas this is at 25kV, so the issue is that no equipment has really been designed to change the output of the renewables to that single-phase electricity.”

One option is to use standard domestic inverters, “but they only go up to about 10kW. That’s important, because on an installation that’s likely to be around 5MW you need a huge number of them, which would be quite unwieldy and quite expensive. The other option is to use a very large piece of kit called a static frequency converter, but these are massive and they’re normally used at national grid level. They cost around £1m per megawatt.”

However, Riding Sunbeams thinks it has a solution. The company is proposing to develop a new piece of equipment called a converter, using repurposed railway equipment, that could meet the size and price requirements for a commercial product. That will depend on bringing in investment and establishing suitable partnerships, so progress is still at the early stage.

Byford says the first step would be to create another demonstrator site, similar to Aldershot, using readily available domestic inverters, while developing the converter in parallel. That would let the team connect solar panels into an overhead line network “and just prove it can be done”, before replacing the inverters with the new converter and using the demonstrator as an R&D site, to confirm that the device can be used on the operational railway and can be certified.

In January of this year, transport minister George Freeman visited the Aldershot installation to launch the 2020 round of FOAK funding and he’s met the company directors to discuss progress. Freeman is interested because if Riding Sunbeams does succeed in producing a commercially viable railway-certified converter, it would be eminently exportable.

Electric railways in many parts of the world use a 25kV single-phase supply: Byford cites China, much of Europe, India and Malaysia as examples. “Other countries are going to have the same issue in terms of equipment,” he notes. “It’s not technically difficult - there just hasn’t been the market to design it, so no one has.”