Thermal Reservoir - Thermodynamics:

Thermal Reservoir. A thermal reservoir is the part of environment that can exchange heat energy with system. It has adequately large capacity and its temperature is not affected by quantity of heat transferred to or from it.

The temperature of a heat reservoir remains constant. The changes that take place in thermal reservoir as heat enters or leaves are so slow and so small that processes within it are quasi static. The reservoir at high temperature that supplies heat to the system is called as HEAT SOURCE. For instance: Combustion chamber, Boiler Furnace, Nuclear Reactor. The reservoir at low temperature that receives heat from the system is called as HEAT SINK. For example: Ocean, Atmospheric Air, and river.