Facebook has hired Google’s former “green energy czar” Bill Weihl, in a move designed to demonstrate the company's commitment to low-carbon computing and renewable energy. Weihl will begin the new post in January 2012, Facebook confirmed.

Weihl reportedly told online magazine Fresh Dialogues that he plans to “advance sustainability” at Facebook. While his job title and responsibilities at have not been decided, the focus will be on sustainability, clean energy and energy efficiency, he said.

The news comes just as Greenpeace winds up a long-running campaign calling on Facebook to “unfriend coal” as a source of energy for its data centres. The environmental campaign group singled out Facebook because of the company’s decision to site its first wholly-owned data centre in Oregon, using electricity from PacificCorp – an energy company which makes two thirds of its power using coal.

In its April report on cloud computing services, Greenpeace estimated Facebook's reliance on coal at 53.2 percent, second only to Apple's (at 54.5 percent) and far higher than Google's (34.7 percent) or Amazon's (28.5 percent). Greenpeace based its estimates on published figures for data centre power consumption and electricity utilities' reports of their energy sources.

Last week, however, Facebook promised to become less reliant on coal and start favouring data centre sites with access to clean and renewable energy. The company has already 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.

Facebook also plans 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.

The company has promised to “engage in a dialogue with our utility providers about increasing the supply of clean energy that power Facebook data centres”. Despite being powered by coal, however, Facebook's Oregon datacentre is actually very efficient, and has a PUE (power usage effectiveness) of just 1.15, which is better than the current best-practice figure of 1.2.

The datacentre uses free-air and evaporative cooling, meaning that it needs no mechanical chillers, which are normally the most significant consumers of electric power in a data centre, apart from the IT equipment. The company also says it is using the computers themselves more efficiently than other companies, addressing the factors beyond PUE that can improve power usage.

Google, meanwhile, is cooking up its own green energy strategy. Under the stewardship of Bill Weihl, Google invested over $850 million in renewable energy projects, entered into 20-year power purchase agreements with wind farm producers, and set up a company called Google Energy to trade energy on the wholesale market.

Weihl's departure from Google last month was a blow for the company, and was followed by the abandonment of an ambitious project to make renewable energy cheaper than coal. However, the company is still investing in clean energy, announcing just today a $94 million investment in four different solar photovoltaic projects in California.

“We believe investing in the renewable energy sector makes business sense and hope clean energy projects continue to attract new sources of capital to help the world move towards a more sustainable energy future,” said Axel Martinez, assistant treasurer at Google Treasury, in a blog post.