Wind output is higher in winter, solar output in summer, allowing the two types of generation to share infrastructure with little need for curtailment. The synergies can help reduce overall costs.
But support payments can make or break projects, as Vattenfall's experience shows. The Swedish utility completed its pilot co-location solar project at an existing wind farm in Wales, UK, in March 2016.
The 4.99MW-solar panel array, limited to comply with the 5MW cap on UK support for solar, was installed at Vattenfall's 8.4MW Parc Cynog wind farm and uses the same electricity and access infrastructure.
Proving the complementary nature of wind and solar output, the solar array has to be curtailed only around 3% of the time, according to Vattenfall.
However, after the UK government scrapped support for solar power in March 2016, margins for solar projects were slashed, and the company switched its attention to combining wind projects with batteries.
In 2018, it expects to install 22MW of batteries at its 228MW Pen Y Cymoedd wind farm, which came online in October 2016.
The battery project was selected by transmission system operator National Grid in August 2016 as one of eight projects to provide enhanced frequency response service to the grid network.
Vattenfall now hopes to trial co-locating wind and solar in the Netherlands, where solar support is available for solar plants with higher voltage grid connections — such as those used by wind farms.
Vattenfall's Dutch subsidiary Nuon is planning solar arrays at the Wieringermeer and Haringvliet wind farms.
If the permits are received in time, applications for subsidies under the SDE+ programme to encourage sustainable energy production will be made this year, according to Margit Deimel, Nuon's director of wind development.
The 99-turbine, 300-400MW Wieringermeer wind farm is due online in late 2019. It has been granted subsidy and has irrevocable permits in place.
The 18-24MW Haringvlieti wind farm has applied for subsidy: a permit has been granted, but this is not yet irrevocable, according to Nuon. It is due for commissioning in 2018.
Nuon is also examining installing a solar array at the existing 4.8MW Oudendijk wind farm.
"We have learned from the Welsh pilot project that wind and solar represent a perfect fit. Lessons have also been learned in respect of planning and safety at the location and during construction at an operating wind site," said Deimel.
But Nuon is also considering wind and battery co-location in the Netherlands, with a pilot project planned for the operating 122MW Princess Alexia wind farm.
Restrictions in Germany
In Germany, co-location is rarely possible because legally, ground-mounted PV plants are largely restricted to the 110-metre strips of land next to motorways and railway lines, and brownfield sites, such as former military property, which rarely have wind farms nearby, said Werner Tenschert, head of solar at renewables developer Energiequelle.
"Co-location makes sense. But it is difficult to get permits for the same location. And there are advantages in each project having its own electricity infrastructure, because this can avoid legal problems if, for example, the wind park is sold," said a spokesman for developer Energiekontor. The firm included a 1.46MW solar array as part of repowering the Debstedt wind farm in 2016.
Change is afoot, however. The government of the southern state of Bavaria passed a regulation in March allowing ground-mounted solar arrays in "agriculturally disadvantaged" areas, after complaining that east-German projects on brownfields sites are more successful at the auctions for ground-mounted solar support allocation.
On the other hand, co-use of electricity transmission infrastructure is already common in Germany, but goes largely unnoticed because the cables lie underground, Energiequelle said.
The firm has 900 kilometres of wholly or part-owned underground cables, with 450 medium-voltage substations where wind or solar projects feed into its cables, with a total capacity of 1.8GW connecting to the transmission system operators' grids.
Co-use of its cables spreads the cost of the infrastructure across more wind and solar projects. This is an attractive option when the Energiequelle cable is closer to a planned project than the local transmission system operator's next suitable grid connection point, said Tenschert.
At around €75,000-100,000/km for installing a 5MW transmission cable, savings here alone can be considerable, he added.