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Athermal reservoir, alsothermal energy reservoir orthermal bath, is athermodynamic system with aheat capacity so large that thetemperature of the reservoir changes relatively little when a significant amount ofheat is added or extracted.^[1] As a conceptual simplification, it effectively functions as an infinite pool ofthermal energy at a given, constant temperature. Since it can act as aninertial source and sink of heat, it is often also referred to as aheat reservoir orheat bath.

Lakes, oceans and rivers often serve as thermal reservoirs in geophysical processes, such as the weather. Inatmospheric science, largeair masses in the atmosphere often function as thermal reservoirs.

Since the temperature of a thermal reservoirT does not change during theheat transfer, the change ofentropy in the reservoir is$${\displaystyle d{S}_{\text{Res}}=\frac{\delta Q}{T}.}$$

Themicrocanonical partition sum${\displaystyle Z(E)}$ of a heat bath of temperatureT has the property$${\displaystyle Z(E+\mathrm{\Delta }E)=Z(E)e^{\mathrm{\Delta }E/k_{\text{B}}T},}$$where${\displaystyle k_{\text{B}}}$ is theBoltzmann constant. It thus changes by the same factor when a given amount of energy is added. The exponential factor in this expression can be identified with the reciprocal of theBoltzmann factor.

For an engineering application, seegeothermal heat pump.

• Heat sink
• Thermal battery
• Thermal energy storage

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