In 2014, 10:10 was working with a community energy project in the village of Balcombe, Sussex, to develop a community owned solar farm – but found that the local grid network did not have the capacity to accept the power that would be generated. Balcombe is right next to the Brighton Main Line, so they wondered – why can’t the solar farm be connected to the railway line to power the trains?
So in 2017 10:10 won funding from Innovate UK to answer the question by running a technical feasibility study with the Energy Futures Lab at Imperial College London, in consultation with Network Rail. Railfuture were at the recent launch of the report from the feasibility study, which showed that it is indeed viable for solar farms to provide some of the power for trains.
The reduction in subsidy for solar power and the fact that in many places the distribution network is at capacity means that community energy projects are now only viable if there is an on-site client that can take all the power generated. DC electrified railways fill that bill neatly because substations are closer together than for AC electrification so there are more places where power can be fed in; and at 750v DC the voltage is close to that produced by solar panels, so provided the solar farm is close to the substation the power conversion losses are low. Consultation with Network Rail quickly brought the realisation that placing solar panels on railway land alongside the track would be unrealistic because access would be subject to Network Rail safety procedures, so the solar farm must be on land outside the railway boundary.
Although nominally 750v DC, the actual third rail line voltage can vary between 500v and 950v depending on how many trains there are in the section supplied by the substation at the time, and whether they are accelerating. Therefore the solution proposed is for the solar farm to feed a power converter which continuously matches the voltage supplied to that of the third rail. In locations where trains are less frequent so demand is intermittent, battery storage can also be provided to store the power available when there is no demand.
There are major opportunities to deploy direct solar traction power on the Southern network (Kent, Sussex and Wessex routes), on Merseyrail, and on the above ground parts of the London Underground network. Other light rail and tram networks in the UK could also benefit, and there is enormous scope on railways around the world, particularly in nations with year-round sunshine.
It is estimated that in the UK it would be commercially viable for solar power to provide around 10% of the traction electricity for these third rail networks. This is limited by the availability of sites for solar farms and the variability of light, and therefore power generated, though the day and the year – so any more than 10% would be uneconomic.
For example, the Southern routes use around 1.4Twh annually at a cost of £114m (around 0.4% of total UK demand for all purposes). Solar power can be generated more cheaply (and the difference will increase over time) so if solar power were to supply 10%, then there would be an annual saving of 4% - about £4.5m. The team have already identified 50 potential sites on the Southern network.
Railfuture supports this initiative but have identified two potential areas of risk:
- The decision to feed DC power directly to the substation was made in part because the original idea was to site the solar panels alongside the track. However this means that the solar farm must be near the substation, reducing the number of possible locations, and that storage is required to prevent power being drawn to adjacent sections when there is no train in section, which would increase power losses. Converting the solar farm output to AC and feeding it to the 11kV line connecting substations would avoid these costs and losses, and would reduce the need to develop specific power conversion equipment for this project.
- Providing a proportion of the traction power required from solar farms will increase the volatility and variability of the demand on the grid supply, which Network Rail contracts from just one of the ‘big six’ energy suppliers. This increased variability and volatility may have an impact on the price which the energy supplier charges Network Rail, wiping out the savings expected from solar power.
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