A thermal reservoir, also thermal energy reservoir or thermal bath, is a thermodynamic system with a heat capacity so large that the temperature of the reservoir changes relatively little when a significant amount of heat is added or extracted.
[1] As a conceptual simplification, it effectively functions as an infinite pool of thermal energy at a given, constant temperature.
Lakes, oceans and rivers often serve as thermal reservoirs in geophysical processes, such as the weather.
Since the temperature of a thermal reservoir T does not change during the heat transfer, the change of entropy in the reservoir is
of a heat bath of temperature T has the property
It thus changes by the same factor when a given amount of energy is added.
For an engineering application, see geothermal heat pump.