A Rwandan prison project, which reduces cooking fuel bills by using methane gas from inmates' toilet waste, has won a global environment award.
The biogas plants resemble giant brick beehives
The Kigali Institute of Science and Technology has helped prisons cut their firewood spending by $44,000 (£25,000).
The residue sewage is then used as fertiliser on crops to feed each institution's 10,000 prisoners.
"Biogas kills two birds with one stone," Ainea Kimaro, the Ashden Award winning project head told the BBC.
Mr Kimaro received his $53,000 (£30,000) prize at a ceremony in London on Wednesday for underlining the vital role which small-scale sustainable energy can play in tackling both climate change and poverty in Africa, the award organisers said.
Most biogas plants are small, but Mr Kimaro's big tanks resemble giant brick beehives - constructed in a pit which is covered on completion.
Cyangugu's prison officers have created gardens from the compost
Rwanda has a huge prison population with some 120,000 suspects awaiting trail for their alleged part in the 1994 genocide in which some 800,000 Tutsis and moderate Hutus were killed.
Five of the country's largest jails now boast biogas plants, either in operation or under construction.
At Cyangugu prison, with biogas now fuelling five of the nine boilers in the prison kitchen, their firewood bill has been cut by more than half.
"The firewood savings are excellent - they really make a difference for us," a Cyangugu prison warden said, adding that the odour-free compost had done wonders for the prison gardens.
"Look at all these bananas! This fertiliser really is the best," he said.
Mr Kimaro said the fact that the methane gas was generated from sewage had not put prisoners off their food.
"I myself I have a biogas plant in my house! It's a tropical solution to a tropical problem," he said.