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. Since it can act as an inertial source and sink of heat, it is often also referred to as a heat reservoir or heat bath.

Lakes, oceans and rivers often serve as thermal reservoirs in geophysical processes, such as the weather. In atmospheric science, large air masses in the atmosphere often function as thermal reservoirs.

Since the temperature of a thermal reservoir T does not change during the heat transfer, the change of entropy in the reservoir is

$${\displaystyle dS_{\text{Res))={\frac {\delta Q}{T)).}$$

The microcanonical partition sum ${\displaystyle Z(E)}$ of a heat bath of temperature T has the property

$${\displaystyle Z(E+\Delta E)=Z(E)e^{\Delta E/k_{\text{B))T},}$$

${\displaystyle k_{\text{B))}$

For an engineering application, see geothermal heat pump.