, the philanthropic arm of Google, is pumping $10.25m into research into renewable energy sources.

The organisation has announced a massive investment into a technology called Enhanced Geothermal Systems (EGS), which basically produces electricity by harvesting underground heat.

"It's 24-7, it's potentially developable all over the country, all over the world, and we really do think it could be the 'killer app' of the energy world", said Dan Reicher, Google head of climate and energy initiatives.

This latest announcement is part of a wider initiative called Google's "Renewable Energy Cheaper than Coal".

This has so far seen investment in solar thermal power, advanced wind, and now EGS technologies.

Google has set a goal to produce 1 gigawatt of renewable energy capacity, which is enough to power a city the size of San Francisco, within "years, not decades".

Google also announced that it is now working on energy-efficient technology to power and cool its data centres in the US and is also generating electricity for its Mountain View headquarters from a 1.6 megawatt solar panel installation, which is claimed to be one of the largest in the States.