Greenpeace has ended a long-running campaign calling on Facebook to "unfriend coal" as a source of energy for its data centres after Facebook agreed to promote clean and renewable energy, the companies announced yesterday.

In late October, Facebook announced plans to build a new data centre in Lulea, Sweden, using hydroelectric power for the servers and relying on the local climate to cool the data centre for free. At the time, though, Facebook made no commitment to use clean and renewable energy in its other data centres.

Greenpeace rated Facebook's existing data centres as among the dirtiest on the planet in its April report on cloud computing services, "How dirty is your data?"

Back then, the campaign group estimated Facebook's reliance on coal at 53.2%, second only to Apple's (at 54.5%) and far higher than Google's (34.7%) or Amazon's (28.5%). Greenpeace based its estimates on published figures for data centre power consumption and electricity utilities' reports of their energy sources.

Because the local utility, Pacific Power, obtains 63% of its energy from coal, even Facebook's newest data centre in Prineville, Oregon, was rated badly - and this despite its industry-leading PUE (power usage effectiveness) score of just 1.07, indicating that almost all the power it consumes is used for computing rather than ancillary functions such as cooling or lighting.

The company should become less reliant on coal, though, as in the future Facebook will favour data centre sites with access to clean and renewable energy, it said yesterday.

For Facebook's existing data centres, though, expect slow evolution, not a revolution, in energy supply: "We'll engage in a dialogue with our utility providers about increasing the supply of clean energy that power Facebook data centres," a company statement said.

The company has also promised to distribute the results of its research into energy efficiency through the Open Compute Project, an organisation it set up to promote low-cost, low-energy computing infrastructure. Greenpeace will help promote the project's findings, it said.