By Paul Keegan, contributor

K.R. Sridhar looks nervous. The CEO of Bloom Energy, the much-hyped fuel cell start-up, sits in a conference room preparing to show off his magical “Bloom Box” for the first time in public. The 49-year-old scientist-turned entrepreneur has raised $400 million in venture capital for his Sunnyvale, California company, but until now Sridhar has revealed almost nothing about what his company has actually produced since it launched eight years ago.

“In our eight-year history, this is the first time I’m sitting down with anybody who’s not wearing a Bloom badge,” he says with a laugh. “So it’s a big deal.”

Thus begins the opening salvo of a full-bore media assault by this soft-spoken mechanical engineer that will soon be followed by a big press event on Wednesday in Silicon Valley. On the dais at Bloom’s coming-out party will be board member Colin Powell, California governor Arnold Schwarzenegger, and John Doerr of Kleiner, Perkins, Caulfield & Byers, the blue-chip venture capital firm that jump-started the Bloom bandwagon back in 2002 (New Enterprise Associates and Morgan Stanley were also early believers). The event will be held at eBay (EBAY) headquarters in San Jose, one of its first customers – along with Google (GOOG), Wal-Mart (WMT), FedEx (FDX), and Staples (SPLS) – and CEO John Donahoe is expected to rave about the potential of these little black boxes that Sridhar claims will “change the world.”

Those dark spots on photos of earth taken from outer space? Ablaze with light. That old, unreliable grid mostly powered by dirty coal? Obsolete, since Bloom boxes are basically tiny power plants installed right in your back yard, next to the dumpster at your corporate campus, or at your local electric-car charging station – though they can also be connected to an electrical grid just like your PC connects to the Internet. Hydrocarbons such as natural gas or biofuel (stored in an adjacent tank) are pumped into the Bloom Box – ceramic plates stacked atop each other to form modules that can be assembled into a unit of any size – and out comes abundant, reliable, cleaner electricity. The company says the unit does not vibrate, emits no sound, and has no smell.

Sridhar, an India-born PhD who once led a team of NASA scientists trying to develop the technology to sustain life on Mars, holds one of the modules in his hand. Stacking them into a bread loaf-sized unit, he says, can produce one kilowatt of electricity, enough to power an American home. Sridhar explains that it has taken so long to produce this contraption because he is building not just a company but an entire industry. “You are used to market sizes that start with a ‘B’,” he told venture capitalists when the company launched in 2002. “This is a market size that starts with a ‘T’.”

Alas, the fuel cell industry – for all the genuine promise and outrageous claims of the last 50 years – is still one with profit margins that start not with “T,” “B,” or even “M,” but with a minus sign. Bloom lost $85 million in 2008, according to venture capitalists that have seen its business plan, roughly on par with other fuel-cell companies building energy boxes much like Bloom’s. The reason is simple: fuel cells are still too expensive – at least until there is a market to produce them on a mass scale.

Google told Fortune that it has a 400 kilowatt installation from Bloom at its headquarters in Mountain View, California. But the real test, analysts say, is whether Google feels confident enough to use Bloom boxes to power its vast server farms upon which its business depends.

“I definitely think Bloom is over-hyped,” says Jacob Grose, senior analyst at Lux Research, which specializes in emerging technologies, though he stresses that he hasn’t seen the soon-to-be-unveiled Bloom box. “What Bloom offers does not seem to be unique – other fuel-cell companies are doing very similar things. The real question is whether Bloom has unlocked the secret of how to make these things cheap, and I’m very skeptical of that.”

One company that Grose points to as offering a similar product – sans the media circus – is Fuel Cell Energy, Inc., a small firm based in Danbury, Connecticut that went public in 1992 and has over 60 fuel-cell installations worldwide at companies ranging from Pepperidge Farm to Westin Hotels. Like Bloom, it also hasn’t figured out how to make money, losing $71 million last year on revenues of $88 million.

The stakes are high not only for Bloom, but also for Kleiner, Perkins, which staked its post-dotcom future on renewable energy and is looking for a monster hit. Bloom was actually the VC firm’s first foray into green technology, and Sridhar says he is the evangelist who opened the company’s eyes to its huge potential. “With this concept we can change the world,” Sridhar says he told the Kleiner partners. “So it was a very big leap of faith on their part.”

Sridhar stresses that he never promised immediate results – unlike Internet startups, a fuel-cell company is very capital intensive and requires a long gestation. “I told them in 2002, it will take roughly eight years to have a commercial product,” he says. “So we are on schedule. I said you are used to investing tens of millions of dollars before reaping the benefits. This is going to be hundreds of millions of dollars before you see the benefit.”

It took three years of development to produce the first in-house version of the Bloom box, and in 2006 the company shipped its first unit to be tested at the University of Tennessee under a contract with the U.S. Department of Energy. After two years of testing, the company shipped the first Bloom boxes to corporate customers in July of 2008 – twenty Fortune 100 companies in all.

So why is Sridhar going public with his product  now? Turns out it wasn’t his idea – his customers are forcing him to show his hand. “They are pushing,” he admits. “They are saying if you’re not going to say anything we’re going to go out and say we’re doing this.”

The reason these companies are so anxious to go public? It’s great PR – they want to let the world know how green they are. “The young people they are trying to recruit into these organizations are really asking questions in the interview process like, ‘How green are you?’ ” Sridhar says. “‘How sustainable are you?’”

Grose says even if a company like Google ends up spending more on energy by using the Bloom boxes, the positive publicity will more than make up for it. “Google likes to present itself as a green company for recruiting and burnishing its global brand,” he says. “Even if it loses money for awhile, it’s still a very good thing to do.”

So Sridhar is finally opening the door and sending his baby out into the world. Will it be able to stand up on its own? Stay tuned.