At the Reinvent the Toilet fair, hosted at its Seattle campus this week, designs included a lavatory that used microwave energy to turn poo into electricity.

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Certainly, microwaves would work: a test carried out in 2008 transmitted useful amounts of microwave energy between two Hawaiian islands 148km (92 miles) apart, so penetrating the 100km of the atmosphere would be a doddle.

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But the safety of beaming it back--proposals call for converting the radiated energy into a narrow microwave beam, which would then be collected and converted into electrical energy on earth--makes many people uncomfortable.

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