They are everywhere. No thermodynamic system is considered complete without the presence of both source and sink. There must be a source that is a high-temperature reservoir and a sink that is at a low-temperature reservoir to transfer heat and extract work. If the temperature of the source and sink is equal, that means there is work being extracted with a single reservoir which violates the second law of thermodynamics.
They are everywhere. No thermodynamic system is considered complete without the presence of both source and sink. There must be a source that is a high-temperature reservoir and a sink that is at a low-temperature reservoir to transfer heat and extract work. If the temperature of the source and sink is equal, that means there is work being extracted with a single reservoir which violates the second law of thermodynamics.