Correction: An earlier version of this article included an outdated title for General Electric’s Danielle Merfeld. She now serves as leader of the solar product line for GE’s renewable-energy business; she is no longer director of the solar technology platform at GE’s Global Research Center. This version has been corrected.

As federal stimulus dollars for investment in renewable energy begin to dry up, will the private sector rush in to fill the void?

Maybe not.

Venture capital investments in what the industry calls “clean tech” companies fell to $1.1 billion in the second quarter this year, a 44 percent decline from the second quarter of 2010, according to an analysis by the firm Ernst & Young. The number of deals involving clean-tech firms dropped 12 percent during the same period, according to a new analysis by the center-left think tank Third Way.

“The decline in funding for clean tech firms means there are fewer opportunities to create new technologies that are born in the United States,” said Joshua Freed, who directs the clean energy program at Third Way. Freed said “we risk losing out on” a global “clean energy market” that he thinks will eventually total $2.3 trillion.

Part of this drop stems from broader financial trends. Overall U.S. venture capital investment rebounded last year from 2009’s slump, but still totaled $22 billion, 26 percent below the 2007 level. That has translated into less funding for the nation’s start-up companies in general.

Some venture capital experts warn that trends in clean-tech investment are difficult to discern. “Investment flows are pretty choppy,” said Tim Carey, practice leader for U.S. clean tech at PricewaterhouseCoopers. Because biofuels and solar projects are capital intensive, he said, “quarterly trends in investment could vary pretty widely, just based on timing.”

Indeed, Third Way’s figure for clean-tech venture capital in the second quarter this year was still one of the four or five highest figures ever for venture capital investments in clean tech energy projects.

“Five, six years ago, there was a wave of VC investments coming into clean tech,” said Tim Newell, senior adviser to US Renewables Group, which has made 23 investments in renewable power and biofuel projects. “It was a wave of not only new capital but new investors. . . . Sectors get hot. That shakes out over time.”

But Freed worries that the level of venture capital investment in clean tech will worsen as the federal spigot for renewable-energy funding dries up, and with the recent flap over the now-bankrupt Solyndra — a solar manufacturer that had $535 million in federal loan guarantees — the spigot isn’t likely to be turned back on.

The loan guarantee program that provided backing to renewable energy projects expired Sept. 30. Part of the 2009 American Recovery and Reinvestment Act, it provided a little more than $16 billion of loan guarantees to 28 projects. The terms of those loan guarantees included demands for equity from private investors, which Freed said created a temporary spike in investment in the first two quarters of 2010.

The stimulus bill also provided $7.7 billion in cash under a tax credit provision to 15,125 renewable projects, which created 11.6 gigawatts of new capacity.

Even before the stimulus bill poured money into renewable energy grants and loan guarantees, there was a relatively modest amount of money devoted to what is broadly known as “clean technology” investments, industry experts say. Now, an abundance of cheap natural gas extracted from shale, the death of climate legislation and fierce competition between existing renewable energy companies have combined to make venture capital investors hesitate even more.

Instead, some of the biggest commitments to renewable energy projects are coming from the nation’s traditional industrial giants. Siemens has a venture capital unit; it recently invested in a firm developing software to improve the electricity grid.

General Electric announced this month that it would build a plant in Colorado to develop thin-film photovoltaic modules using cadmium telluride, an alternative to the polysilicon used in conventional PV panels.

Danielle Merfeld, leader of the solar product line for GE’s renewable energy business, estimated that her company has devoted $600 million so far on its solar program, about half for the factory itself. The rest went to purchase PrimeStar Solar, the firm that originally developed the technology, and to pay for additional research.

Freed worries that big companies won’t invest in firms at the earliest stages. He said that venture capital funds have turned their dollars toward more mature companies, rather than riskier, early-stage ventures. According to Third Way, almost 80 percent of the total venture dollars invested in the second quarter this year, $865.2 million, went to companies that were already generating revenue. Of these companies, two-thirds qualified as late-stage investments.

Newell said, however, that investments in later stages of corporate development “is what you’d expect to see if those companies are succeeding.” Otherwise, he added, “that would mean the technologies are not getting past the early stage.”

Lynn Jurich, president of the solar home financing firm SunRun, said ventures that are less mature than her company are having trouble luring significant private investment, especially given the lack of clarity from energy policymakers in Washington.

“Clean tech has become more challenging to invest in,” Jurich said. “I think what you’re seeing is the political and regulatory uncertainty.”

Tim Woodward, who specializes in clean energy investment as managing director of the San Francisco-based venture capital firm Nth Power, said some of the venture capitalists who migrated from the Internet sector to renewable energy are pulling back at this point.

“I think that’s a healthy retrenchment on some level,” Woodward said, noting that for some Silicon Valley investors who switched gears, “ultimately they realized it’s a different technology, it’s a different business.”

But, he added, in some sectors, such as solar energy, some deserving start-ups are encountering cash challenges. Woodward noted that his group has identified a promising solar manufacturing venture, but finding a second investor has been challenging.

“It’s hard to find another party that wants to join us around the table,” he said.

This shift has opened the door to newer investors such as Google, which has put millions into building a backbone for wind power transmission off the East Coast and concentrated solar thermal power generation out West.

Urs Hoelzle, Google’s senior vice president for infrastructure, said his company has stepped in as banks and other clean tech investors have shied away from new projects.

“There were 15 investors, and it went down to four,” Hoelzle said in an interview, adding entrepreneurs have approached Google with promising renewable energy projects, “and nine months later, they’re still not funded.”