Two generators thrumming in a cramped Silicon Valley parking lot represent a clean power technology so new that most potential customers don’t know it exists. For Adam Simpson and his startup, Mainspring Energy Inc., that’s both a curse and an opportunity.

Inside the generators, steel cylinders wrapped with magnets race back and forth through copper coils 12 times per second, generating electricity. There’s fuel involved, but no combustion: Nothing burns. The generators’ fast, muffled drumming—about as loud as traffic on the Bayfront Expressway a few yards away—makes them sound like engines, but they aren’t.

Mainspring calls them linear generators, and as the company’s chief product officer and co-founder, Simpson often finds himself explaining them to the world. Most of the technologies powering the clean energy transition—the solar cells, wind turbines, batteries and fuel cells—have been commercially available in one form or another for decades, even if they’re only now taking off. Businesses understand them and have grown comfortable slapping them on rooftops or planting them next to offices. Not so with the linear generator, which Mainspring started deploying in 2020. As far as the company and its backers can tell, no one else sells one.

“There’s a lot of education we need to do,” says Simpson, who also leads Mainspring’s government affairs team. “It’s a brand-new category of generation that customers and grid planners didn’t know about, a tool they didn’t know they had.” To help them grasp it, Simpson sometimes takes policymakers through the company’s small assembly plant in Menlo Park, where units awaiting shipment to customers are tested in the parking lot. “It’s real,” he says. “They can come touch it.”

Mainspring has managed to persuade some big names in the energy world to give its generators a chance. The world’s largest producer of renewable power, NextEra Energy Resources LLC, signed a $150 million agreement in 2021 to buy and deploy the generators as well as finance purchases for other customers. Even to NextEra, the technology was new. “I’d never heard of anything like this,” says Matt Ulman, NextEra’s vice president for distributed generation. In recognition of that newness, Mainspring was named a 2023 Pioneer by BloombergNEF, which every year awards a selection of early-stage companies working on potentially game-changing climate tech.

The switch to clean power, in fact, is spurring a host of new technologies that may take years to gain traction—if they survive at all. But even as the bulk of the world’s $1 trillion annual clean energy investment goes to renewable power and electric cars, startups are pursuing better nuclear reactors or new ways to generate electricity from the Earth’s own heat, says Jesse Jenkins, an assistant professor of mechanical and aerospace engineering at Princeton University.

“Some of them will fail, which is always the case with innovation,” says Jenkins, who models ways to decarbonize. “But there are enough shots on goal that some of them will succeed.”

He’s been briefed on Mainspring’s generator and sees several potential uses, including replacing heavily polluting “peaker” power plants, which run only when electricity demand on the grid soars, or charging electric semitrucks, which will require far more power than the typical EV.

“There are some unique characteristics to the linear generator that make it an interesting tool,” Jenkins says. “It’s a fundamentally new way to produce electricity.”

The basic idea of the linear generator stretches back 80 years. But for much of that time, it was considered a potential design for a combustion engine in cars. Mainspring took a very different approach.

Each generator core consists of two cylinders, called oscillators or translators, that move in opposite directions—like a two-pronged pogo stick lying on its side. As they swing inward, they compress a mix of fuel and air until the fuel molecules break down and push the oscillators outward. Air springs catch them and send the oscillators racing toward each other again. Although they’re housed in a casing, the oscillators glide on a thin cushion of air to minimize friction. Magnets on the cylinders pass back and forth through copper coils to generate electricity. Mainspring packages two of the 20-foot-long cores side by side within a modified shipping container to create each generator, which can produce 230 kilowatts of electricity. That’s enough for a typical retail store, according to the company.

For now, the generators’ fuel is natural gas, biogas (which landfills or dairy farms can produce) or a blend of gas with hydrogen. But the units that ship next year will be able to run on any of those fuels, as well as ammonia, and switching fuels won’t require a hardware upgrade. Running on natural gas, the generators produce carbon dioxide, but on hydrogen, the only byproduct will be water. Because there’s no combustion, the generators spew almost no nitrogen oxides—air pollutants that can trigger breathing problems and contribute to smog.

Simpson and co-founders Shannon Miller and Matt Svrcek developed the idea while all three were studying at Stanford University’s Advanced Energy Systems Lab, in the mid-2000s. The point wasn’t to revive an old auto industry design; it was to find the most efficient process they could to tap the energy within chemical bonds, as in a fuel. Miller, Simpson and Svrcek settled on compressing fuel and air, then started devising a machine around the process. As the concept began to gel, they had to design the individual pieces themselves and find machine shops to build them.

“We didn’t think you could just snap your fingers and build a whole new type of power generation system overnight,” says Miller, now Mainspring’s chief executive officer.

The team caught a key break when venture capital firm Khosla Ventures LLC hired Simpson to do some research. Khosla, perhaps best known for funding payment companies Square and Stripe and delivery service DoorDash, has a long-standing interest in cleantech and had Simpson assess an energy storage startup the company was considering funding. Managing Director Samir Kaul said he was impressed with Simpson’s diligence and attention to detail—more impressed than with the company Simpson was hired to research.

“I told him, ‘We’re going to pass on this company, but what are you working on?’ ” Kaul says. He met with Miller and Svrcek and saw potential in the trio and their idea. When Mainspring formed in 2010, Khosla Ventures became its first funder.

Mainspring started shipping its initial pilot generators to customers in 2020 and soon picked up support from NextEra. Ulman says NextEra decided to sign an agreement because the generator offers several big selling points for potential customers. Although he and Mainspring won’t discuss precise costs, Ulman says businesses in places with high electricity prices can save money relying on the generator rather than the grid. They also gain protection from blackouts, a growing problem in much of the US. The ability to switch to climate-friendly fuels such as hydrogen without an expensive equipment upgrade gives customers the chance to future-proof their operations and meet their climate goals. And because the generator instantly turns on and off, it can easily be paired with a rooftop solar array, powering up when sunlight fades.

NextEra’s endorsement goes a long way to persuading potential customers to take a look, Miller says. But Ulman says there’s a spectrum of willingness to try unusual tech. “You have some customers who are very progressive in what they do in energy, and they want to be in the forefront of trying new solutions,” he says. “And there are other customers who want you to prove it first.”

Other big-name backers have come on board. To date, the company has raised more than $500 million from investors that include Shell Ventures, utility American Electric Power Co. and Bill Gates.

Cold food storage specialists Lineage Logistics LLC decided to try a linear generator at a warehouse in the Southern California city of Colton, matching it with a rooftop solar array. The Michigan-based company operates more than 400 facilities and can’t risk losing power in its climate-controlled warehouses. Now the company has generators installed at three sites and is actively planning 15 more. They’re currently running on natural gas, but Lineage is looking at biogas and ultimately wants to move to hydrogen. The novel technology is working, says Jesse Tootell, Lineage’s senior manager for energy analytics.

“We haven’t had a single month when they’ve missed a single spec,” he says. “They’ve delivered on every technical dimension we care about.”