This could store a lot of heat: as much as 4, 000 terawatt-hours, according to Dr Henkel.

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In 2009, Japan generated 956.5 terawatt hours (TWh) of energy, 30% of it from natural gas and 29% of it from nuclear sources.

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About 10 percent of the power is lost with each conversion step, and that can add up to terawatt-hours of power that just goes to waste.

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The proposed cable would be capable of exporting around five terawatt-hours (or five billion kilowatt-hours) each year, which is enough to power around 1.25 million European households.

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It's a tantalizing prospect: Based on an estimate of the number of rivers around the world with suitable conditions, it's a potential source of 1, 700 terawatt hours a year of energy globally, according to estimates by Statkraft's scientists.

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The Department of Energy and Climate Change's (DECC) Renewables Roadmap estimates that biomass could potentially provide between 68 teraWatt hours (TWh) and 100TWh of renewable heat and electricity by 2020, which is 26-42% of the total needed to meet the EU renewables target.

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