Inmates in a small prison in the Brazilian state of Minas Gerais are being offered the chance to reduce their jail terms in return for generating electricity by pedalling on exercise bicycles.

The bikes are hooked up to portable batteries, which light up the humble boardwalk along this small country town's river each night. In its first months, the program has proved so popular that guards have reported a jump in good behavior, which moves candidates to the top of the waiting list.

It all began when a local judge, Jose Henrique Mallman, ran an absent-minded Google search for "renewable energy."

"The system has totally failed us. Jails have become a human depository and nothing else. Men waste away sitting in there, just discussing criminal strategies, and come out exactly the same or worse," he says in his office, decorated with art created by prisoners.

Every night just before sunset, a guard drives the charged battery from the prison, on the outskirts of town, to the downtown promenade. He hooks it up to the converter and a few minutes later the 10 street lamps begin to glow a soft white, like full moons suspended over the rushing waters of the river.

Long abandoned after dark, the newly illuminated promenade now attracts dog walkers, joggers, kids on bikes and couples walking arm-in-arm.

Another guard comes in the morning to pick up the battery and ferry it back to the prison, where 133 inmates are serving sentences ranging from a few months for burglary and drug charges to up to 34 years for murder.

The 36-year-old said that the time on the bike also gives him time to reflect on his situation.

While he's pedaling, "I think about my imprisonment, about my freedom, my wife, my kids," he said.