According to a representative from sodium-ion (Na-ion) battery energy storage system (BESS) startup Peak Energy, the company will pilot a 3.1MWh system at energy firm RWE’s Eastern Wisconsin lab in the US.
Speaking to Energy-Storage.news, the representative explained that Peak is in ongoing discussions with RWE to establish the project’s success criteria and timeline, as well as to evaluate a pathway to full commercial development.
Peak’s BESS is designed without moving parts and features active cooling and ventilation. This design, as the company states, minimises the common failure points seen in typical BESS systems.
Lithium-ion (Li-ion) battery cells within BESS require active cooling and ventilation to extend their lifespan and reduce the risk of fires.
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Lower system degradation can decrease the need for upgrades, meaning operators might not need to add extra batteries or components during the project’s lifetime to maintain storage capacity.
The company claims its sodium-ion phosphate pyrophosphate (NFPP) cells reduce the lifetime cost of energy stored by an average of “US$70/kWh, which is equivalent to approximately half of the total price of a battery system today.”
ESN asked for a breakdown of the US$70/kWh cost. Peak responded, “All of the US$70 comes from long-term OpEx savings. Augmentation, auxiliary power, and operations and maintenance, as mentioned. There are also revenue advantages from the flexibility of our performance guarantee, eliminating requirements such as average state of charge (SoC) constraints and allowing for multiple discharges per day, as well as higher system availability.”
Na-ion generally has a lower round-trip efficiency than Li-ion. Peak’s representative said its system’s RTE is “96% at beginning of life, which is similar/better than what you see on the Li-ion side.”
The company also highlights that its BESS can safely operate in a wide range of temperatures, “without a performance decrease.”
Its representative noted to ESN, “Ambient operating range is -40 to 55 °C. We have passive heaters in the system to combat extreme low temperatures, just as Li-ion systems will use heating to keep their systems within their narrow operating temperature bands.”
The company also noted the system’s location in the MISO service area. In 2025, a report from American Clean Power Association (ACP) and Aurora Energy Research stated that MISO will need a 500% rise in battery storage by 2035.
According to the report, the RTO is experiencing increasing load demands that its ageing energy resources cannot support. Due to MISO’s extensive coverage area and limited transmission infrastructure, electricity prices often vary significantly.
It was noted that in summer 2025, MISO’s capacity prices surged by over 2,000%. Without implementing energy storage solutions, models predict that peak electricity prices for consumers will keep rising and become less dependable over time.
The companies will collaborate to develop a storage platform that integrates Peak Energy’s Na-ion batteries with Energy Vault’s proprietary digital stack, emphasising Energy Vault’s AI infrastructure portfolio. They state this strategy accelerates deployment, enhances safety, and reduces costs by eliminating “complex legacy systems.”
In November 2025, Peak announced a multi-year phased agreement with developer Jupiter Power to provide up to 4.75GWh of Na-ion BESS. Under this deal, Peak will supply the BESS to Jupiter Power for deployment from 2027 to 2030. The total contract value could exceed US$500 million.
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