Investors in the much-talked-about clean-energy company Bloom Energy are doubling the size of the company’s current financing round.

The company’s series-G financing round closed last year at US$150m, but the investors have decided to expand it by exactly that much more, Dow Jones VentureWire reports, citing an email from the deal’s marketer to potential investors. If successful, the expansion will bring Bloom’s total value to $3bn.

Bloom makes fuel cells that convert natural gas or biogas into electricity. The company has a business unit focused solely on sales to data center operators and has already attracted a number of high-profile ones: Apple, AT&T and NTT America, among others.

Data center operators buy fuel cells for onsite electricity generation. Bloom’s product gives them some comparatively clean backup or primary power capacity.

In March, Bloom appointed Peter Gross, who was before at the helm of HP’s Technology Consulting business, to lead its mission-critical unit. Gross was one of the founders of EYP Mission Critical Services, which HP bought in 2007 and made its data center consulting business.

Apple is installing a 5MW Bloom system at its new data center in Maiden, North Carolina. In combination with two massive solar farms and clean-energy purchases, Apple says the data center will run on 100% renewable energy.

NTT America deployed a 500kW fuel-cell system at its data center in San Jose, California, and AT&T bought a total of 7.5MW of generation capacity to install across 11 of its California sites, including some data centers.

Fuel cells have been around for more than a century and have been used “in every NASA mission since the 1960s,” according to Bloom’s website. They have not been widely used because of high cost, but Bloom claims that it has found a way to make them cost-effectively.

Instead of burning fuel, fuel cells use a chemical process to convert it into energy.