Mainspring Energy Lands $150M Deal To Deploy Its Linear Generators With NextEra

Over the past decade, Mainspring Energy has been at work on a novel "linear generator" that it says can provide on-site electricity with lower emissions than fossil-fueled engines and microturbines, and greater flexibility than fuel cells. 

On Tuesday, Mainspring's vision garnered a vote of confidence from U.S. utility and renewables giant NextEra in the form of a $150 million agreement with its business services arm NextEra Energy Resources to purchase, finance and deploy Mainspring's devices across the country. 

Mainspring started testing its linear generators last summer with an unnamed national supermarket chain, which has agreed to expand its use to up to 30 grocery stores, according to Tuesday’s announcement. The Menlo Park, Calif.-based startup,  formerly called EtaGen , has also shipped products to big-box retailers and utility customers, and it is in discussions with other Fortune 500 companies. 

The core concept behind Mainspring’s generator — capturing the back-and-forth motion of pistons, or oscillators in Mainspring’s terminology, to generate energy — is shared by a wide class of devices, ranging from Stirling engines to linear alternators and motors. But Mainspring’s system differs in several key ways from the variety of similar technologies developed over the decades, CEO Shannon Miller said in an interview.

“It’s a full systems integration: making sure the emissions are low, making sure the efficiency is high, making sure we have the controls in place to use different fuels,” said Miller, a former mechanical engineer at Tesla who developed the idea behind Mainspring’s technology at Stanford University with company co-founders Matt Svrcek and Adam Simpson. 

Low-emissions, fuel-flexible, dispatchable on-site power

One of the key innovations is Mainspring’s use of “air springs” and an “air-bearing system” as cushions of air   that hold the magnet-equipped oscillators in place within the structure that captures their back-and-forth motion to generate electricity. Using air instead of mechanical bearings or oil reduces friction and does away with the key mechanical points of failure that have challenged other linear generator designs, said Miller. 

In simple terms, “it’s like an air hockey table that you’ve wrapped around into a tube,” Miller said. In terms of its sophistication, getting this air-based system to work smoothly and reliably has required “a lot of good engineering,” helped along by research and development funding. Mainspring has raised  $133 million  to date from investors including Microsoft co-founder Bill Gates, Khosla Ventures, utility AEP and the venture arms of oil giant Statoil and energy company Centrica. 

Mainspring has also tapped advanced power electronics developed for electric vehicles, solar inverters and other digital power conversion and control technologies to “adjust the position of the floating tubes very...