Heat pump mechanics

Although the ground loop that exchanges heat with the earth is the distinctive feature that gives this technology its name, it is the interior heat pump units that transfer heat between the ground loop and the conditioned spaces of a building. This page explains the basic principles underlying the mechanics of heat pump operation.

natural-heat flow

Heat energy is the collective random motion of molecules, and temperature is a measure of how fast the molecules are moving. Heat energy spontaneously flows “down the temperature scale” from a warmer region to a cooler one, as shown in adjacent figure. This is one way of stating the Second Law of Thermodynamics, which also can be be stated as: "The natural tendency of the universe is to become more disorganized." Observe a school classroom over any period of time, and you will see that this is true. The random activities of students will cause loose papers and books to spread into previously organized areas. Likewise, when a group of fast-moving (warmer) molecules is placed next to a group of slow-moving (cooler) molecules, random collisions will cause the slower molecules to speed up and the faster ones to slow down, thus transferring energy from the warmer group to the cooler group.

Latent heat is the energy needed to overcome the molecular interactions that tend to organize matter, such that it can undergo a phase change into a more disorganized state. Solids are more organized than liquids, which in turn are more organized than gases. Consider a block of ice being warmed by an external heat source ( bottom most figure). Although the ice molecules contain some heat energy, their vibrations are not sufficiently energetic to overcome their interactions that hold them together in a solid structure. As heat energy is absorbed by the ice, its molecules vibrate faster within the solid structure, and its temperature rises. When its temperature reaches 0°C, however, the ice begins to melt, and its temperature stops rising. This is because the molecules now have enough vibrational energy that any additional heat will break down the solid structure into a more loosely connected liquid state. Instead of causing increased molecular vibration within the solid state, all heat energy absorbed by the ice at its 0°C melting point goes into changing its state from solid to liquid until all molecules are in the liquid phase.


Any heat energy absorbed after all the ice has melted can again create faster molecular motions, but now within a liquid structure, and the water temperature begins to rise. Note that the temperature at any given time represents the average speed of random molecular motion; some individual molecules move faster and some move slower. When the faster-moving molecules at the water's surface have enough energy to break free of the cohesive forces that hold them in the liquid structure, they escape into the gas phase. This is the process of evaporation.

As escaping molecules accumulate in the gas phase above the liquid water surface, their chances of randomly striking the surface increase, and when they do collide with the surface, they spontaneously lose heat to the slower-moving molecules remaining in the liquid phase. If they lose enough energy to be "captured" by inter-molecular cohesive forces, they return to the liquid phase. This is the process of condensation.

When the rate of evaporation equals the rate of condensation, the air above the liquid is said to be saturated with vapor, and the pressure exerted by the molecules in the gas phase is called the saturated vapor pressure. If the liquid is contained in a closed volume, and that volume is suddenly compressed (as in a piston pump for example), then the concentration of water vapor molecules will increase. This is only a temporary condition, however, because the increased concentration of vapor molecules causes more of them strike the liquid surface and condense. There is a net flow of molecules back into the liquid until the vapor pressure returns to its original saturated value and evaporation exactly balances condensation. Therefore, the saturated vapor pressure of a liquid depends only on its temperature and not on the volume in which it is contained.

Returning to the graph in above figure, we now understand that as water is heated above its melting point, its saturated vapor pressure increases along with its temperature. When the water temperature reaches 100°C, its saturation vapor pressure equals normal atmospheric pressure, and at this point, any additional heat energy causes the internal pressure of water vapor bubbles within the liquid to exceed atmospheric pressure, enabling them to rise to the surface and release their vapor molecules to the gas phase. This is the process of boiling. All heat energy absorbed by the water at its 100°C boiling point goes into expanding the water vapor bubbles, which continue to break the surface until all molecules are in the gas phase.


From the above description, it is evident that a liquid can be boiled at any temperature simply by reducing the applied pressure below its saturated vapor pressure at that temperature. This is exactly what occurs when you press down the spray nozzle of a pressurized "canned air" duster (such as used for cleaning electronic components).

When the nozzle is depressed, the drop in pressure causes the propellant liquid to boil, absorbing latent heat and cooling the surrounding can.

When the spray nozzle is up, the can's internal pressure equals the saturated vapor pressure of the propellant liquid. When the spray nozzle is depressed, the can's interior becomes open to the atmosphere, and its internal pressure drops as propellant vapor is expelled. This reduces the pressure over the liquid propellant, causing it to boil. The propellant vapor absorbs its latent heat of vaporization from the liquid phase, which in turn absorbs heat from the surrounding can metal, and the can begins to feel quite cold. In fact, if the nozzle is held down too long, the hand holding the can will be in danger of frostbite!

A final basic principle underlying heat pump mechanics is that when a gas is compressed, the number of molecular collisions increases due to the increased concentration of molecules, and its temperature rises. This is sometimes referred to as Gay-Lussac's Law, which is a special case of the Ideal Gas Law, and it can be easily demonstrated with a deflated bicycle tire and hand pump. As the tire is inflated and its internal pressure rises, even greater external pressure is needed to force air into the tire. Increased compression is required with each additional stroke, and the temperature of the pumped air volume increases according to Gay-Lussac's Law. After several strokes this creates a significant temperature gradient, and heat spontaneously flows from the compressed air to the surrounding pump body, which begins to feel quite warm.