In 2019 Riding Sunbeams, an innovator focussed on decarbonizing rail traction networks through the development and connection of solar, wind and energy storage assets, bypassed the electrical grid and supplied traction power direct from solar panels to the third rail outside Aldershot, Hampshire in the UK. According to UK infrastructure operator Network Rail, this world-first has potential to provide 10% of traction power across England’s DC networks.
Riding Sunbeams is poised for its first full-scale commercial implementation and pursuing a solution to supply solar power direct to AC electrified lines. The enterprise germinated from the desire of climate charity Possible to connect a 5MW community solar farm at Balcombe, West Sussex in 2014.
“We had a sympathetic landowner and south-facing slopes, but no grid connection,” recalls Riding Sunbeams’ director, Leo Murray. “Meanwhile, it was rail-adjacent and connecting to the third rail just seemed like it ought to work.”
Energy Futures Lab director, Professor Tim Green saw no fundamental reason it shouldn’t and introduced Murray to postdoctoral researcher, Dr Nathaniel Bottrell. In 2016, the pair secured Innovate UK funding and Network Rail data support for a feasibility study.
The Renewable Traction Power project concluded that solar arrays and integrated energy-storage could supply 10% of energy needed to power trains on Britain’s electrified DC routes. The project proposed custom power electronics to bypass the grid entirely.
On the southeast commuter network, 200 rail-side solar farms could meet 15% of total traction demand. “The upshot of the feasibility study was that it’s feasible,” says Murray. “We came away confident we could supply traction energy at a lower unit price than Network Rail pays for grid power.”
Plugging in to the DC third rail
Initially, Riding Sunbeams assumed it would use a DC-to-DC converter to connect DC photovoltaic output to the DC third rail and minimize conversion losses. “But actually, we would need a bespoke converter,” adds Murray. “Nothing on the market was suitable and because rail is so heavily-regulated, developing new power electronics is prohibitively onerous and expensive.” But the existing supply architecture offered another way to plug in solar with off-the-shelf equipment.
Southern England’s DC network is dotted with traction substations at 3km to 5km intervals, connected via grid supply points (GSPs). Each GSP supplies about eight or ten substations with 33kV feeder cable; in each substation, a 12-pulse diode rectifier converts AC to DC and pushes 750V DC to the track.
Meanwhile, when light hits the semiconductor material in solar panels, they produce a DC current. In a solar farm, strings of panels are connected to an inverter, which converts their combined DC current to an AC output for distribution. Still following? Good.
“The 33kV AC feeders from GSPs to traction substations have similar electrical characteristics to distribution networks,” explains Murray. “We can use off-the-shelf inverters, designed to connect solar to distribution networks, to connect to ancillary transformers in substations. We’re not pushing solar power directly to the tracks, but supplying those feeder cables. It’s mass-manufactured technology at a low price-point and relatively trivial to install.”
“Commercial inverters use control algorithms to pull DC current from strings of panels and high-frequency switching to convert it to low-voltage AC output,” explains Bottrell, who now advises Riding Sunbeams as a principal consultant at Ricardo. “Using a transformer, we match the voltage required to export to Network Rail’s private AC feeder network.”
Cost makes batteries unviable, and without integrated storage, surplus generation may necessitate active curtailment. Land is another constraint and meeting winter demand with reduced photovoltaic output could require five-times larger sites.
“One challenge is matching solar generation to demand without integrated storage,” says Murray. “Traction demand is very peaky: a train passes, then demand drops back to nothing.” The DC-to-DC direct connection first envisaged would only supply one 5km track-section. “Supplying the feeders spreads our generation across all substations supplied by that 33kV feeder,” he explains. “It gets used anywhere a train accelerates on those electrified sections.
“We proved it works outside Aldershot Station in 2019,” Murray continues. The First Light project connected a 100-panel photovoltaic array with around 39kW total output to a DC substation ancillary transformer. Network Rail believes it was the first such effort to supply solar direct to traction systems anywhere in the world.
“It’s still there,” says Murray. “Network Rail owns the rig, and it will continue generating for 20 years.” Riding Sunbeams meanwhile is now moving from successful pilot to first full-scale deployment at Berwick, East Sussex.
“Cuckmere Community Solar [in Berwick] had planning permission for a solar farm but no grid connection: exactly the problem we set out to solve,” Murray explains. “It has an attractive end-customer next door: the electrified railway. Grids are at capacity, but railways are effectively a shadow-grid with their own demand and proximal native suppliers with no route-to-market.”
Pending exact confirmation, Riding Sunbeams envisages a 4MW peak solar farm on the Lewes-Eastbourne line. “It’s never been done before,” says Bottrell. “We’re learning how to connect renewable technology to railways together with Network Rail.”
In Britain, the 20% DC traction load-share Riding Sunbeams could theoretically supply diminishes to 10% due to land-use constraints. “It’s one in ten trains,” says Murray. “There are fixed costs and hard thermal and voltage limits to what we can connect at one location.”
But Riding Sunbeams has studied its potential impact in India. “It’s more like one in four trains there,” adds Murray. India’s climate offers a better generation profile and circumventing grid promises greater carbon-savings. “India’s grid power has four times more carbon-intensity than the partly-decarbonized UK grid.”
Nevertheless, Network Rail is Britain’s largest electricity consumer. “For any administrative bureaucracy, initial change adds costs,” says Murray. “But we’re confident we can provide direct, behind-the-meter supply at a lower unit price.”
It involves none of the policy and grid access costs bundled into Network Rail’s ten-year contracts, which will not insulate it indefinitely from global price-rises. “The energy crisis increases our business-case,” adds Murray. “Solar is already cheaper than gas-supplied power and continues its downward trajectory.”
Supplying renewable energy to overhead lines via bespoke converters
Railway electrification in Britain dates from the nineteenth century and 38% of routes are electrified. Of these, 64% use 25kV AC overhead lines and 36% the 660/750V DC third-rail system.
“Britain’s southern region was electrified first at 750V DC third-rail in the 1950s,” explains Ricardo principal consultant, Nathaniel Bottrell. “But as high-speed rail took off, it became clear DC couldn’t provide the required voltage-level and newer electrified mainlines use 25kV AC.”
“25kV AC is now the de facto technology but requires problematic clearance for overhead lines in built-up areas,” adds Riding Sunbeams director, Leo Murray. “DC is often used for city metros or subways with railways in tunnels.”
Just as Network Rail is Britain’s largest electricity consumer, London’s largest consumer is Transport for London (TfL). The London Underground operates at 630V DC, but subterranean urban geography limits its scope for direct solar supply.
Elsewhere, Riding Sunbeams believes it could supply 10% of DC lines with commercially available inverters, but proposes a parallel solution for the 64% of UK electrified lines using overhead 25kV AC.
“Connecting AC railway requires a different power electronics solution,” says Murray. “We’re developing a bespoke converter, because no available product does the necessary power-conversion at appropriate cost.” The market offers either 10kW single-phase inverters for residential roof-tops or high-power three-phase inverters for utility-scale installations.
“The 25kV traction networks are all single-phase,” Bottrell adds. “We can’t use utility-scale three-phase inverters, because it would create an imbalance on their single-phase system. But connecting many 10kW roof-top single-phase inverters to a MW-scale solar farm is costly and impractical.”
*This article first appeared in the March 2022 issue of Electric & Hybrid Rail Technology magazine.