When considering the design of air conditioners and how they work, it can be argued that understanding the basic principles of thermal energy are essential because the air conditioner has the function of working against the natural process of thermal energy transfer. Although the air conditioner has the function of pumping heat from cold to hot—opposite of thermal energy transfer—the process is identical to that of thermal energy transfer.
Thermal energy or more commonly, the energy of temperature is based on the movement of molecules. No matter what state a substance is in—solid, liquid or gas—the molecules that comprise matter are constantly in a random state of motion. This means that in any given object, at any given time the molecules that comprise an object can be found moving in many different directions and at a number of different speeds. While it is true that the motion of molecules in any state of matter is completely disorganized, scientists have developed two parameters for describing the motion of molecules within matter: molecules within any matter are confined within a definite space, and the vector sum of the velocities of all the individual molecules is zero.
In addition to the random motion that exists in all form of matter, physicists and physical chemists have identified the existence of internal vibrational or rotational motion within each individual molecule. This motion is equally as important as the random motion of the molecules because the summation of all the energies is what constitutes the internal energy, or the kinetic energy of the substance comprised by the particles. Thus, the thermal energy of any given material is the total kinetic and potential energy of all of its constituent particles and is dependent upon the amount of material present. Finally, heat is defined as the thermal energy that is transferred between two objects.
The working fluid in the air conditioner is perhaps one of the most essential parts of the appliance. Understanding the principles by which the working fluid transfers thermal energy is simply a matter of understanding how heat is transferred between two objects. For a specific substance, the amount of thermal energy will ultimately determine the temperature and vice versa. Therefore, if the temperature of the substance rises, it is due to an increase in thermal energy; likewise any decrease in temperature is a result of a decrease in thermal energy. Heat can exist as thermal energy both before and after a transfer; typically flowing form a warmer body to a cooler one.