Inside Wealth: Billionaire family offices bet on geothermal energy, brain implants in September
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Fall is off to a slow start for investment firms of ultra-wealthy families. In September, family offices made 42 direct investments in companies, down from 52 the month prior, according to data provided exclusively to CNBC by Fintrx, a private wealth intelligence platform.
However, while family offices are writing fewer checks, they are still making bold bets with startup mega-rounds. In September, Hillspire, the personal investment firm of former Google CEO Eric Schmidt, participated in a three billion euros (about $3.4 billion) Series D for Mistral. The three-year-old French startup builds a wide range of artificial intelligence models such as chatbots and coding assistants, including freely available open-weight models.
That same month, hedge fund titan Stanley Druckenmiller's Duquesne Family Office participated in a $500 million Series A for Fab2 and a $250 million Series D for Precision Neuroscience. Fab2 manufactures small semiconductor factories, and Precision Neuroscience develops brain implants that record neural signals to let users control digital devices.
Family offices' interest in clean energy startups continues, representing five of the month's transactions. John Arnold's Centaurus Capital and John Doerr's namesake family office led a $135 million Series B for Mazama Energy, a geothermal energy startup. Bill Gates' private venture capital firm also participated in the round.
Both Arnold and Doerr, who made their billionaire fortunes in hedge funds and venture capital, respectively, have backed multiple clean energy startups this year.
Mazama Energy drills into extremely hot rock then circulates water through engineered cracks to bring heat back to the surface to generate carbon-free electricity. This technology enables Mazama to generate more energy from fewer wells, reducing the cost of power, according to the three-year-old company.
In 2025, Mazama drilled into rock at its Oregon pilot site that reached 629°F — what the company says is the hottest engineered geothermal system on record.
Mazama CEO Sriram Vasantharajan said this proof of concept showed a clear path to commercialization and was key to attracting top investors.
"When you talk about family offices or even any other investor, what they look for is a demonstrated track record. It's one thing to create the tech platform, but what Mazama has done is actually applied it," Vasantharajan said.
Investing in cutting-edge technology often comes with long investment horizons. Vasantharajan said this isn't the case with geothermal energy. The Oregon site is already capable of producing 10 gigawatts of energy.
"The heat beneath our feet exists. Its temperature is proven," he said. "It is not a scientific problem that we're tackling. It is an engineering challenge."
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