BESS owner-operator Return has signed a long-term toll with power firm Vattenfall for its 200MW/800MWh Sirius BESS project in the Netherlands, concurrent with financial close.
The long-term fixed-price toll with Vattenfall helped the project secure project financing under a €400 million (US$463 million) package from banks NG, NatWest, Deutsche Bank, ABN AMRO and Rabobank.
The 4-hour Sirius battery energy storage system (BESS) project in Winschoten, Groningen, will require €180 million in investment, some of which will come from the financing and some from Return.
Sirius will connect to transmission system operator TenneT’s grid in Meeden under a time-dependent transport rights (TDTR) agreement, similar to a flexible connection agreement (FCA) in Germany. It imposes some constraints on the BESS’ operation, but enables it to connect within existing grid constraints while supporting the wider system, Return said.
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Under the agreement, the BESS adjusts when it charges and discharges based on how much capacity is available on the grid at that moment.
Congestion is a significant challenge on the Dutch grid. So much so that some battery storage projects are being brought online with agreements with TenneT specifically to mitigate congestion.
The Sirius project should reach commercial operations in Q4 2027. Return said it is one of the largest fixed-priced tolls in the Netherlands to-date.
It is the second toll signed between Return and Vattenfall. In April 2025, the pair partnered on the 100MW/200MWh Antares project in Waddinxveen.
Sweden-headquartered Vattenfall was in the news last week when a 55MW offtake deal it entered into via the aggregating BESS tolling platform of Terralayr in Germany became fully operational. Vattenfall is also developing its own projects, but has mainly announced offtake roles.
Return is based in the Netherlands and was an early mover there with its platform SemperPower, which commissioned what were at the time the two largest BESS projects in the country in 2022/23. It also owns Lion Storage, which last year reached financial close on a 350MW/1,400MWh project in the Netherlands. Both platforms have now been folded into SemperPower. Since then Return has also expanded into Germany.
