A new 6 megawatt (MW) battery energy storage system (BESS) with 48 megawatt-hours (MWh) of capacity, along with a new gas-fueled combustion generator unit with a maximum capacity of 16 MW, are likely to end for now the need for a third subsea transmission line to serve electric customers on Nantucket Island.

Research from the Energy Department’s Pacific Northwest National Laboratory (PNNL) suggests that the newly commissioned energy resources could deliver a range of ancillary services that likely will benefit National Grid, which serves the island off the Massachusetts coast.

The PNNL report said that total 20-year lifecycle benefits of the new resources are roughly $145.9 million, yielding a 1.55 return on investment (ROI). It said that roughly three quarters of the benefits are tied to the deferred investment in a third subsea transmission cable for 13 years. Another $18.8 million would result from regulation services. Outage mitigation is estimated to yield $12.3 million in benefits, forward capacity market operations $4.1 million and spinning reserves another $1.2 million. The report said that volt-VAR/CVR operations would yield negligible benefits.

“We knew the grid energy storage facility and new backup generator would defer the need for a third undersea transmission line,” said Rudy Wynter, president and COO of National Grid’s Wholesale Networks in a statement. “But once we made the decision to invest in the energy storage system, we asked PNNL to help us figure out what else we can do with this investment to create additional value.”

Two subsea cables currently serve the island. Source: PNNLNetwork model

As part of their work, PNNL researchers created a distribution system network model to assess a range of energy storage-enabled use cases for Nantucket Island. The cases included outage mitigation, volt-VAR optimization and conservation voltage reduction programs, frequency regulation, spinning reserves and forward capacity market participation. PNNL also found that the BESS and generator could reduce the duration of outages experienced by Nantucket residents by nearly half.

National Grid supplies electricity to the island via two undersea transmission lines and two small, aging generators located on the island. PNNL said that previous studies showed that if one of those lines failed, electricity demand on Nantucket Island would likely exceed the grid’s capacity.

To meet the island’s growing energy needs, National Grid developed a plan, called “IslandReady,” to upgrade the island’s electricity infrastructure. In it, the utility identified three project components:

Substation upgrades: Relocating three existing switchgear boxes housed at the utility Candle Street substation (used to control and protect substation equipment), along with a new switchgear box, placing them on a newly constructed platform for reliability and safety. Transformer controls also will be relocated from the sides of the existing transformers to inside a brick control house, providing protection from potential flood waters.

Distribution system upgrades: Installation of a new feeder to create increased capacity for the east side of Nantucket, improve switching ability on the island’s distribution network and reduce the potential for outages on other feeders, which serve critical facilities, including the island’s hospital, airport and wastewater treatment facility. Work is expected to be completed by spring 2020.

An existing underground cable located downtown will be upgraded — including the installation of new underground conduit and larger manholes — to improve reliability and decrease outages, and to meet increased capacity needs.

Back-up generation and battery storage: The new diesel-powered generator replaces two aging generators at National Grid’s Bunker Road facility and is sited next to the Tesla-supplied BESS.

PNNL said that the Nantucket energy storage project could be used to convince regulators elsewhere to allow a rate-based asset to participate in energy markets. “The key,” said one project leader, “is understanding how grid energy storage investments will perform under realistic grid operating conditions using real-world use cases and factoring in market dynamics.”