The mainstream way to create electricity is to use thermal to mechanical. This seems to be one of the norms.
One of the ways to create thermal energy is from a compost heap... the centre of the pile is usually very hot (and some have known to burst in to flames)
why not harness the energy from the centre of a compost pile of steaming crud and then use the heat energy to convert to electricity.
its all a reaction with bacteria and such creatures.