GUANTANAMO BAY NAVY BASE, Cuba -- Solar-powered lights serve as sentries where U.S. Marines once faced-off along the Cuban frontier. A team of Navy cops now rides bikes rather than gas-guzzling patrol cars in the searing Caribbean sunshine.

In this remote corner of Cuba that is better known as a lab for Pentagon justice and interrogation, the U.S. Navy has been quietly engaging in more low-profile offshore experimentation — seeking environmentally friendly alternatives to reduce its whopping $100,000-a-day fossil fuel dependence.

It’s a Navy-wide goal to halve dependence on fossil fuels by 2020. But the greening of Gitmo, as this base is known, comes with a particular challenge.

The base that today houses 6,000 people makes all its own electricity and desalinates its own water. It has done so ever since the 1960s when Rear Adm. John Bulkeley, then base commander, faced down Fidel Castro and cut off the naval station from Cuba’s water and power supply.

Everything from diesel fuel to spare parts arrives by ship or aircraft, more than tripling the price of power, according to base estimates.

“From my perspective certainly the greening of Gitmo is important,” says U.S. Navy Capt. Kirk Hibbert, the base commander. National security is paramount, he said, but the Navy mandate to curb consumption “has an effect on almost everything we do here.”

Hibbert’s the man who put a pair of Navy cops on bikes to patrol the base rather than sit inside air-conditioned sport utility vehicles, an $800-a-year savings that sends a symbolic message. And it’s been on his watch that a contractor has started building a huge solar array behind the base’s high school.

Guantánamo can strike visitors as a small slice of Americana, with its trailer parks and tract housing, a hilltop church, McDonald’s, cinemas and schools. But it’s a base behind a Cuban minefield with the Navy controlling who may come and who may go and who gets water and electricity.

Commanders like to compare it to a ship at sea — except this one is towing the most expensive prison on earth.

By base estimates, it costs $32,000 a day, or $11.7 million a year, to keep the lights on and water flowing to the 171 captives at the Pentagon’s prison camps and 1,850 U.S. forces and contractors who work there.

The Defense Department set up the detention center a decade ago, temporarily, at a time when the Navy was already tinkering with energy efficiencies.

In 2005, a Massachusetts firm installed four 270-foot-tall windmills on Guantánamo’s highest hill with visions of capturing up to 25 percent of the base’s power consumption from the Caribbean trade winds. But that analysis did not consider the never-ending nature of detention operations here, a venture that tripled the base population and sent construction costs soaring, from the coastal prison camps to the crude war court compound built atop an abandoned airfield.

“We get a lot of attention here because we are such an expensive base in the Navy,” says Arthur Torley, a senior civilian worker at Guantánamo’s version of a small-town Department of Public Works. “Gitmo, to me, is even more of a priority because of the expense. They would much rather spend money fixing planes and ships than dumping fuel into Gitmo.”

So he’s got his workers using a fleet of 24 solar-powered minis, squat little electrical vans with panels on their roofs. They arrived this summer, and can go about 35 miles before needing a charge, just about right for a week’s worth of work on the 45-square-mile outpost.