http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf
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.World demand for energy is projected to more than double by 2050 and to more than triple by the end of the century. Incremental improvements in existing energy networks will not be adequate to supply this demand in a sustainable way. Finding sufficient supplies of clean energy for the future is one of society’s most daunting challenges.
Sunlight provides by far the largest of all carbon-neutral energy sources. More energy from sunlight strikes the Earth in one hour (4.3 × 1020 J) than all the energy consumed on the planet in a year (4.1 × 1020 J). We currently exploit this solar resource through solar electricity — a $7.5 billion industry growing at a rate of 35–40% per annum — and solar-derived fuel from biomass, which provides the primary energy source for over a billion people.