I would guess that any heat source could be used for a sterling. It's more a case of what is appropriate in a particular case in terms of economic or physical effort.
If you've only got a garden, then the muck heap rotting down organic matter would be a possible source of waste heat, bearing in mind that you need to keep the temperature up to keep the process going.
However, if you've got a silo, you've probably also got a large piece of land feeding it, with lots of sun to ripen your crops. In this case you might be better considering direct solar heating with the sun(admittedly only 8 - 12 hours a day at most) "wasting" more heat on any spare piece of land than your silo is producing in 24 hours.