His charitable organisation, the Bill & Melinda Gates Foundation, is looking for future loos that can improve sanitation around the world.

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.

Another turned excrement into charcoal, while a third used urine for flushing.

In total 28 designs were shown off at the fair and the winner was a team from the California Institute of Technology.

Led by Prof Michael Hoffman, the toilet they designed was solar-powered and generated hydrogen gas and electricity. They won a $100,000 prize.

"We couldn't be happier with the response that we've gotten," Bill Gates said at the event.

The project challenged inventors to come up with a toilet that operated without running water, electricity or a septic system. It needed to operate at a cost of no more than five cents (3p) a day and would ideally capture energy or other resources.

Walter Gibson, from a team of scientists at the London School of Hygiene and Tropical Medicine, showed off a toilet that used black soldier fly larvae to process waste and create environmentally-friendly animal feed.

The toilet is already being field tested in South Africa.

Traditional flush toilets waste tons of drinking water and are often impractical in many areas of the developing world.

The UN estimates that disease caused by unsafe sanitation is responsible for half of the hospitalisations in the developing world. About 1.5 million children die each year from diarrhoeal disease.

The Gates Foundation has committed $370m (£235m) to its future toilet initiative and hopes to field test the prototypes within three years.

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