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That won’t stop these investors from trying.

Sometimes it’s called the “missing middle,” sometimes, more ominously, the “valley of death.” Whatever the terminology, it’s undeniable that a chasm lies between a climate company’s early funding rounds and its eventual commercial scale-up, one that’s getting harder and harder to bridge. From the first half of last year to the first half of this one, total Series B funding declined by nearly a quarter; beyond Series C rounds — what the market intelligence platform CTVC calls “growth funding” — it declined by a third.
“The capital needs of these businesses have just outgrown their early stage backers,” Frank O’Sullivan, a managing director for S2G Ventures’ energy investments, told me. “But the infrastructure investors have absolutely no appetite whatsoever for taking on an unproven technology and scaling.” S2G makes both early stage and growth stage investments, and O’Sullivan co-authored a white paper last year on the problem of the “missing middle.” The paper found that of the $270 billion in private capital for clean energy raised between 2017 and 2022, just 20% was allocated to late-stage and growth-focused investments, while 43% went to earlier rounds and 37% toward deploying established tech.
Of course, some of climate tech’s funding gap can be attributed to broader trends in the venture market and economic landscape. Covid-related disruptions and low interest rates led investors to throw money at promising startups, only to see their valuations drop as inflation (with rising interest rates to match) and geopolitical uncertainty cooled down the overheated market. Other companies went directly onto the public market via special purpose acquisition companies, only to underperform expectations. “There is capital to be deployed,” O’Sullivan told me. “But a lot of the companies that need that capital are struggling, really, to swallow hard and take significant restructuring of their previous valuations.”
With clean tech in particular, there’s also frequently a mismatch between the abilities of venture firms, which often make their biggest returns on software startups, and the demands of climate tech. The latter tends to require huge investments in physical infrastructure and support for first-of-a-kind projects, and generally has a longer timeline to profitability than, say, an app. “Venture funding, in some sense, was built for scaling software companies,” Lara Pierpoint, managing director of the new catalytic capital program Trellis Climate, told me. “You’re talking about a capital light business that generally is creating something that enters a white space, and for which there’s huge amounts of market potential.”
It’s much more difficult to build expensive infrastructure that aims to displace fossil fuel facilities and the entire economy that relies on the cheap, reliable power they provide. So while VCs may be enthusiastic about taking a relatively small financial bet on a high-potential early-stage company, that may be all they’re able to do.
Trellis, on the other hand, is a part of the climate nonprofit Prime Coalition and funds first-of-a-kind climate projects with philanthropic capital. The nonprofit structure and philanthropy-focused funding model mean that Trellis can take a different tack on missing middle financing than traditional venture or equity investors. For example, Pierpoint told me it can choose whether to invest in a company or just a specific project. Trellis can also help de-risk projects by providing an “insurance backstop” — basically backup capital in case primary project funding falls short. “We’re looking at expanding the kinds of resources and dollars we can bring to the table in general for the ecosystem, because we think that venture can’t do this alone,” Pierpoint told me.
As with all nonprofits, generating big returns isn’t the focus for Trellis. But for traditional investors, that’s the primary goal. And while growth investments in more technically mature solutions are likely to generate consistent returns, O’Sullivan told me they don’t often provide the rarer but more alluring 10x returns that make early-stage venture capital particularly enticing. “So it’s a more balanced portfolio, typically, in that growth equity category. It’s just that you don’t see the high highs,” he said, explaining that a two to 3x return on investments is more realistic.
Brook Porter, a partner and co-founder at the growth-stage firm G2 Venture Partners, told me that focusing on the missing middle can be extremely profitable, though, and that the key to making real money is correctly identifying a company’s “inflection point” — that is, when it’s poised for significant growth and impact. That is, of course, every investor’s dream. But G2’s whole strategy revolves around identifying exactly when this critical juncture will be, tracking more than 2,000 companies per year to identify the ones best poised for breakout scale-up.
The firm spun off in 2016 from Kleiner Perkins’ Green Growth Fund, where Porter and his three co-founders previously worked as senior partners. This is where they honed their theory of inflection point investing, funding companies such as Uber, drone-maker DJI, and Enphase Energy. Porter told me that helping startups move from proof-of-concept to building “that machine of a business” requires a lot of hand-holding, and that “there aren’t as many investors with that skill set,” so it could take a while for this approach to scale.
On the other end of the funding spectrum, large institutional investors like banks, hedge funds, and asset management firms certainly have the money to help bridge the missing middle, but O’Sullivan and Pierpoint told me they’re generally more interested in fulfilling their internal climate mandates by building out more wind and solar, which generates near-guaranteed returns. These investment giants then look at their remaining cash and think, “Well, we should do something more avant garde. Let’s put money into early-stage venture,” O’Sullivan explained. That’s how many seed and Series A-focused funds raise money.
As O’Sullivan sees it, what’s happening now is “a flaw of the structure of capital allocation at the very highest level.” He thinks we could start by reorienting incentives such that large investors such as banks, asset managers, and pension funds get paid in part for helping bring new climate solutions to market, as opposed to just funding the same old, same old. That would allow them to write “right-sized checks” on the order of $50 million to $100 million to ready-to-scale companies — larger than what a VC firm would write, but smaller than what the big infrastructure investors are used to.
How would those alternate funding models actually work? Well, that’s the real question. Pierpoint said she’s often asked whether a new kind of investor or asset class will be necessary to fill the gap, and while she doesn’t have an answer, what she does know is that the group of climate tech companies that’s ready to commercialize “can’t wait 15 years until we have the exact right form of capital.”
“There needs to be urgency on the part of philanthropists, on the part of infrastructure equity investors, on the part of venture capitalists, to really start showing that we can do this,” Pierpoint told me, “and that we can bring together the right capital stacks to make this happen.”
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The spinoff of Lawrence Livermore National Lab has a new 10-point plan to get onto the grid by the 2030s.
One of fusion energy’s newest startups, Inertia Enterprises, is betting that the fastest route to commercial fusion runs through one of the field’s oldest ideas. The company, which raised a $450 million Series A earlier this year, plans to build a power plant based on the laser-driven fusion system pioneered at Lawrence Livermore National Laboratory’s — the only tech yet to have produced more energy from a fusion reaction than it took to initiate it. Now, Inertia has shared its commercialization roadmap exclusively with Heatmap, detailing the 10 near-term capabilities it must demonstrate before this landmark experiment can become a grid-scale power plant by the mid-2030s.
The roadmap offers a route from the national lab’s impressive but commercially impractical fusion demonstrations to an economical power plant capable of producing electricity for the grid. At its core are a set of milestones — mostly aimed at developing cheap, mass-manufacturable components — that Inertia says it must clear before those individual systems can be integrated into a working plant. This road is not necessarily linear, however, as various teams will likely be working on many of these goals simultaneously.
At least the physics of Inertia’s approach are already proven, the startup’s CEO Jeff Lawson told me, pointing to the fusion experiments at Lawrence Livermore’s National Ignition Facility as a proof-of-concept. The lab’s demonstration of net energy gain caps more than six decades and $30 billion (in 2026 dollars) of U.S. fusion research. The remaining challenges, he argued, are all engineering-related, requiring “elbow grease, hard work, and smart people” rather than breakthroughs in fusion science.
"It seems to us like a startup or a commercial company of any variety should be focused on commercializing a proven scientific result, as opposed to actually trying to demonstrate the basic science to begin with," Lawson told me. Basic science, he argues, is better left to national labs and universities, where researchers can pursue "unbounded problems" that don’t align with the expectations and timelines of venture-backed startups.
Indeed, no fusion startup has yet achieved scientific breakeven, the milestone Lawrence Livermore first hit in 2022, and has since repeated numerous times. But leading players such as Commonwealth Fusion Systems and Helion Energy maintain that it’s only a matter of time before they validate the physics behind their own reactor designs, which they claim will be highly cost-competitive.
Lawson, on the other hand, readily acknowledged that Lawrence Livermore’s tech is uneconomical in its current form. His bet is simply that the more predictable path to a commercial reactor is to drive down the cost of the lab’s validated fusion approach, known as inertial confinement. This system relies on high-powered lasers firing at a millimeter-scale pellet of fusion fuel, compressing it to extreme temperatures and pressures until the atoms fuse. Today, the National Ignition Facility makes each individual fusion target by hand, a workable solution given that it only uses about a dozen per year.
That production model, however, isn’t remotely plausible for a grid-scale power plant. Because each fusion reaction lasts just a fraction of a billionth of a second, a commercial facility must fire its lasers at a fresh target about 10 times per second to generate continuous electricity — requiring the production of hundreds of millions of targets each year.
Scaling production to roughly a million pellets per day and making them inexpensive enough for commercial operation without compromising the strength or precision required for fusion ignition is central to Inertia’s roadmap. That includes goals five, seven, eight and nine — industrializing the manufacturing of the carbon shells that hold the fusion fuel, making the thin films that hold those carbon shells both durable and cheap, scaling up and automating fusion target assembly, and speeding up how fast targets are filled with the requisite deuterium-tritium fuel.
The other central focus of the roadmap is the laser system, which will ultimately consist of 1,000 individual units operating in concert to compress and heat the fusion fuel. Key priorities include reducing the system’s cost (goal two), dramatically increasing its firing cadence (goal three), and bolstering its durability to withstand high-intensity operations (goal four). Goal six also complements these efforts, calling for the development of a control system capable of tracking moving fusion targets to precisely align each laser shot.
Goals one and 10 bookend the journey with some broader milestones. The first focuses on increasing the fusion target’s energy gain — the ratio of fusion energy produced to laser energy delivered — to more than 25 times ignition. Today, the National Ignition Facility’s best-performing laser shot has yielded a gain of just over four times what it took to start the reaction. Goal 10 then zooms out to the ultimate objective: integrating all these technologies into a commercially viable power plant that can deliver either electricity or industrial heat to end customers.
To reach that point, Inertia has embarked on an industrial engineering hiring spree, recruiting folks with experience taking complex hardware systems from prototype to mass production, “not unlike the processes that are used in the semiconductor or consumer electronics world,” Lawson explained. The company has been making progress on its component development goals since the beginning of the year, he told me, and expects to announce the successful demonstration of a few of these milestones in the coming months. Lawson ultimately expects Inertia to complete the core components of its laser and target manufacturing systems by the middle of next year.
The team will spend the next two to three years integrating these individual pieces into two fully operational subsystems, a prototype laser system and a target manufacturing line. Around 2030, the company will begin combining those subsystems into a first-of-a-kind fusion power plant, which will also serve as the proving ground for the target chamber, tritium fuel breeding system, and power conversion system that turns fusion heat into electricity. By the middle of the next decade, Inertia aims to be generating power from this first plant, setting the stage for the company to build and connect additional grid-scale commercial power plants.
There are plenty of engineering trade-offs that the company will have to solve for. Take the decision around how to size the target chamber, for example. “If you make it bigger, your walls have an easier time and survive longer, but it’s more expensive. If you make it smaller, your walls have a tougher time because they’re closer to all the heat and energy that the fusion reaction is creating, but now your power plant costs less to build.”
But to Lawson, this represents exactly the type of problem Inertia was built to solve: complex engineering issues that come to the fore once scientists have demonstrated the fundamental physics are sound. He thinks other fusion companies may someday reach this stage, as well — though he’s unwilling to hazard a guess on exactly what approach or startup is best positioned to do so.
“There have been generations of scientists who’ve made their predictions about fusion energy and gotten it wrong,” he told me. “I’m not going to pretend to be smarter than them. All I’m here to say is, just knowing that one did work, we can commercialize it.”