What's 100 feet long by 50 feet wide, cost $17.5 million to build and runs on 38,000 photovoltaic cells? It's the Turanor PlanetSolar, a massive catamaran powered solely by the sun.

The what? Turanor is a word taken from J.R.R. Tolkien's "Lord of the Rings" saga and translates into "the power of the sun". PlanetSolar is the team of 100 engineers and others working to be the first to circumnavigate the globe with a solar-powered boat.

"This is a milestone in the progress of solar mobility," Immo Stroher, a German investor who partnered with Swiss adventurer Raphael Domjan to bankroll the showcase for solar power, said in a statement. "It is my vision to see solar power take its rightful place — not only on rooftops, but also on the roads, seas and in the skies of the future."

After arriving from Europe last Saturday, the catamaran and its six-member crew spent a few days in Miami, Fla., and then left Tuesday for a leg to Cancun, Mexico, where delegates to the U.N. climate change summit are meeting.

"This voyage around the world is meant to test the long-term performance," Domjan stated. "When it comes to solar shipping, brilliant innovation is required: less weight, less friction, efficient propulsion, a reliable solar energy store and finally, the production of electric energy."

With a "wave-piercing" design, the 24-foot-tall catamaran "slices" through the water instead of riding waves, which takes more energy.

Moreover, the catamaran has a surface area of more than 5,700 square feet, allowing for 825 modules that house the 38,000 photovoltaic cells. The crew can adjust how many cells are used by extending the length and/or width of the catamaran by several feet.

Electricity from the sun powers four electric motors that propel the catamaran's two drive shafts.

Finally, a 13-ton lithium battery in the hull store enough electricity to run the catamaran for up to three days without direct sunlight.

But the ride is slow, with only just enough power to keep the catamaran at around 7 knots (about 8 mph) during favorable conditions — slow by sailing standards.

And then there's the criticism of why bother with solar when there's a much cheaper renewable energy for shipping: the wind.

"Some people say, 'You have built the most unnecessary vessel in the world because it is running without an engine, but that's what people have done with sails for 3,000 years,''' Stroher told the Miami Herald. "They are right, of course. It's not my main purpose to have boats or ships run on solar but to show what renewable resources are able to achieve.''

Now a month since leaving Monaco, the Turanor PlanetSolar is expected to take another nine months to complete the around-the-world journey.

For its backers, each stop along the way is a chance to excite others about the possibilities.

"We want to motivate engineers and scientists to develop innovative technologies," said Domjan. "This is to show that visions are about to become reality."