According to the abstract, the feasibility study looked at whether wind, water and solar (WWS) power could provide "worldwide energy for all purposes" - and the answer was yes. That's 100% of our global energy requirements, without fossil fuels and without nuclear power.
The study further analyzed the technological requirements to meet this goal, and here's the shopping list: 
3,800,000 5 MW wind turbines 49,000 300 MW concentrated solar plants 40,000 300 MW solar PV power plants 1.7 billion 3 kW rooftop PV systems5350 100 MW geothermal power plants270 new 1300 MW hydroelectric power plants720,000 0.75 MW wave devices490,000 1 MW tidal turbines
The numbers are daunting but the technology is there. According to the Huffington Post, Germany and China have already built offshore turbines, and "utility-scale" solar plants exist. The materials to build the requisite facilities are already available or attainable through further mining or recycling.
The greatest hurdle, it would seem, is human will. According to Discovery.com, 80% of the world's energy currently comes from fossil fuels - and that's big business for some. "With so many new Republicans on Capitol Hill who don't believe in the science of climate change (which will continue until the year 3000), not to mention many members of the public," warns Discovery contributor David Teeghman, "it may be a challenge to achieve such a task and get off fossil fuels."
A total energy overhaul would not only suspend our reliance on fossil fuels, but would also reduce the amount of energy required. The Energy Policy study suggests that a "WWS infrastructure reduces world power demand by 30% and requires only 0.41% and 0.59% more of the world's land for footprint and spacing, respectively."
The study's final recommendation? "We suggest producing all new energy with WWS by 2030 and replacing the pre-existing energy by 2050."
