The €1.3 billion Borssele project, scheduled to start construction later this year and operating in early 2021, is being developed by a consortium that also includes oil major Royal Dutch Shell PLC, a Mitsubishi Corp. subsidiary, maritime contractor Van Oord NV and private equity firm Partners Group Holding AG.

Shell and Eneco are each buying half of the output under 15-year agreements with the consortium. Eneco has signed PPAs to market its share of the output to stack revenues on top of the guaranteed price of €54.49/MWh the project will receive under the Dutch government's renewables subsidy program.

"We have nearly sold out our half," Spruit said, adding that the volatility of power prices was the main driver to find additional off-takers. "You need the additional income to make sure the project is profitable."

Beyond subsidies

Eneco is up for sale, and Shell, along with a partner, expressed its interest in bidding for the utility in January. The two companies are involved in a joint bid to develop another offshore project in the Netherlands, where auctions are now taking place without direct subsidies for the power they will produce thanks to steep declines in technology costs over recent years.

In countries from Spain to Finland, PPAs with corporate consumers have been emerging as a preferred option for locking in long-term revenues and thus securing financing to develop unsubsidized renewables projects, so far mainly onshore.

Although corporate PPAs are projected to keep rising for both wind and solar projects in the coming years, analysts have warned that there is a limited pool of viable off-takers on the continent, restricting the potential of such deals to keep financing developments as state subsidies are phased out.

"The market for those [long-term] PPAs is very thin," Marc Groves-Raines, the head of renewables at asset manager Allianz Capital Partners GmbH, which has invested in offshore wind farms, said at an industry event in April.

For now, developers remain bullish on their opportunities. Ørsted, the biggest offshore developer, is hoping to follow-up its first offshore PPA with Northumbrian Water Ltd. in the U.K. by selling parts of its current and future production in other countries such as the Netherlands and Germany, CEO Henrik Poulsen told analysts May 1.

"We will, of course, continue to look for corporate PPAs that will help offset any merchant risk we have across our portfolio in Europe," Poulsen said.