在热力学里，热库被视为是一固定温度的热来源。热库的温度不会因为热量增加或被抽离而有任何改变。

A thermal reservoir, a short-form of thermal energy reservoir, or thermal bath is a thermodynamic system with a heat capacity that is large enough that when it is in thermal contact with another system of interest or its environment, its temperature remains effectively constant. It is an effectively infinite pool of thermal energy at a given, constant temperature. The temperature of the reservoir does not change when heat is added or extracted because of the infinite heat capacity. As it can act as a source and sink of heat, it is often also referred to as a heat reservoir or heat bath.