Updated: Jul 7, 2022
A thermal energy reservoir is a hypothetical concept used for defining bodies with very high heat capacity, because of which their temperature does not vary significantly (or doesn't vary at all) when a finite amount of heat is added or removed from them.
Heat capacity is nothing but the product of mass and specific heat of the material
We know that,
Q = m c dT
from the above definition of heat capacity C,
Q = C dT
When a finite amount of heat is added to the thermal energy reservoir (or a finite amount of heat is taken out from it), its temperature does not vary because it has a very high thermal heat capacity
We can see from the above equation, that the transfer of heat does not change the temperature of the thermal energy reservoir.
What are the examples of thermal energy reservoirs (TER)?
Examples of thermal energy reservoirs (models) include rivers, large ponds, the atmosphere, the Sun, etc.
Sometimes phase change systems that work on heat transfer at constant temperature can also be considered thermal energy reservoirs. They just utilize the amount of heat provided to them in the form of latent heat and only undergo a phase change. And as we all know, during the phase change the temperature remains constant.
The thermal energy reservoirs usually have a very large mass/size, but it does not need to be always true. The thermal energy reservoir can also be of relatively smaller mass/size. For example, a CPU losing heat to the air in the room in which it is placed can also be considered a thermal energy reservoir.
Why the temperature of the thermal energy reservoir does not vary?
It is an ideal concept and in practice, the temperature does vary because of heat transfer.
Then you might say the efficiency should also vary along the process as the temperature is varying. For practical purposes we do not use absolute temperature values to calculate efficiency rather we take the ratio of work output and heat input to calculate efficiency.
We use absolute temperature values only in the case of ideal cycles like the Carnot cycle which is a hypothetical/ideal cycle.
What is the source and what is the sink?
The thermal energy reservoir that supplies energy in form of heat is called a source and the thermal energy reservoir that absorbs energy in the form of heat is called a sink.
What are the applications of source and sink in the thermodynamic machines/systems?
They are everywhere. No thermodynamic system is considered complete without the presence of both source and sink. There must be a source that is a high-temperature reservoir and a sink that is at a low-temperature reservoir to transfer heat and extract work. If the temperature of the source and sink is equal, that means there is work being extracted with a single reservoir which violates the second law of thermodynamics.
What can you do with all this information and knowledge?
Remember that the source and sink both must be present to make a thermodynamic system that continuously produces work or continuously transfers heat from lower temperature to higher temperature.
If you know this you can understand that the larger the temperature gap between source and sink the more work can be extracted continuously.