Original Article - New Scientist
For those who dislike the sight of wind turbines on the horizon, would a spectacular hot-air balloon farm be more acceptable?
Ian Edmonds, an environmental consultant with Solartran in Brisbane, Australia, has designed a giant engine with a balloon as its "piston". A greenhouse traps solar energy, providing hot air to fill the balloon. As the balloon rises, it pulls a tether, which turns a generator on the ground. Once the balloon has reached 3 kilometres, air is released through its vent and it loses buoyancy. This means less energy is needed to pull the balloon back down again, resulting in a net power gain (Renewable Energy, DOI: 10.1016/j.renene.2008.06.022). "It is like a huge two-stroke engine, with a capacity of 45 million litres, a stroke of 3 kilometres, and a frequency of one revolution per hour," says Edmonds.
For roughly the same cost as wind power, Edmonds has calculated that a large 44-metre-diameter recreational balloon could generate 50 kilowatts, enough to supply energy to about 10 homes. Doubling the diameter of the balloon would increase power production tenfold, substantially reducing costs, he says.
Using air heated by the sun to generate power has been attempted before: solar towers use the rising air to turn turbines. But a prototype solar tower in Manzanares in Spain proved too short even at 200 metres, limiting the amount of energy that could be captured from the rising air. Building towers of 500 metres or more has so far proved too expensive.