It’s hard not to get caught up in the optimism on the exhibition hall at the ARPA-E Innovation Summit, which is taking place this week in the D.C. area. Nearly every booth features a promising—indeed, potentially revolutionary—energy technology concept or prototype.

But it’s also hard to ignore the fact that very few of the projects the Department of Energy’s six-year-old agency has funded have made a commercial impact. And with venture funding for energy startups in short supply, it’s worth asking: should the government do more to help commercialize transformative energy technologies?

President Obama clearly thinks the answer is yes. His latest budget request for the Department of Energy includes a 21 percent increase for 2017 in clean-energy R&D funding. That includes a 20 percent increase in funding for ARPA-E, which is charged with identifying and investing in promising early-stage technologies. Obama also proposed a so-called “ARPA-E Trust” for developing “larger-scale investment-ready outcomes,” which would begin with $150 million in funding in fiscal 2017 and provide $1.85 billion over five years to ARPA-E.

ARPA-E director Ellen Willams gives her opening remarks Monday at the ARPA-E Innovation Summit.

It’s not clear yet how exactly the additional money would be used if the president’s budget requests are approved. But the new proposals reflect Obama’s commitment to the recently launched Mission Innovation, a 20-nation effort to “reinvigorate and accelerate global clean energy innovation,” which the group says is not occurring quickly enough.

Some venture capitalists at the ARPA-E Summit agree that the agency, which generally doles out three-year grants worth a few million dollars at a time, plays a critical role in advancing laboratory concepts that could have a significant impact but wouldn’t normally get funding. It’s also successfully handed over some of those projects to the private sector. Ellen Williams, ARPA-E’s director, boasted during her opening remarks that 45 projects have secured $1.25 billion in follow-on private investing. But consider that ARPA-E has invested a total of $1.3 billion in 475 projects.

Commercializing energy technologies is very time-consuming and expensive. An ARPA-E grant isn’t enough, and private-sector funding often comes up short. Here in the U.S., it’s been proved that the conventional venture capital model just doesn’t work for energy the way it does for software or even biotech. Dozens of solar, biofuels, and battery companies have failed in the past six years because they couldn’t transition from promising advance to mass-produced product.

The problem is not that VCs don’t have an appetite for low-carbon energy technologies, says Vijit Sabnis, a partner at Khosla Ventures. But for energy technologies, “the incubation period tends to be longer and more arduous” than it is for other technologies VCs routinely fund. The time horizon for a typical venture fund is often too short for that. What’s more, the big energy players tend to be risk averse and have only rarely acquired startups. As a result, VC-backed energy startups often achieve pilot-scale production capabilities but then struggle to secure the funding needed to reach full-scale production. Sabnis says they need a new funding mechanism that “comes after venture capital.”

When ARPA-E started in 2009, it was thought that government loan guarantees could play that role. Then a string of high-profile failures, including solar startup Solyndra, left the loan guarantee program vulnerable to political contentiousness, and the government pulled back.

Today, one popular option is for startups to partner with a large energy company looking to make a strategic investment. Other companies, such as the solar company and ARPA-E awardee 1366, have gone outside United States to secure funding in countries such as China, where there is a greater need and desire to deploy new energy technologies.