Heat pumps work by altering a refrigerant between its liquid and vapor phases in a closed loop. This phase change process releases and absorbs heat, enabling the heat pump to move heat from one place to another—even if the heat source is colder than the heat sink. This refrigerant cycle or vapor-compression cycle is the principle behind nearly all air conditioners, heat pumps, and your kitchen refrigerator.
There are four main components to a heat pump—compressor, condenser, expansion valve, and evaporator, plus heat exchangers to deliver heated or chilled air to the living space. Gaseous refrigerant is mechanically squeezed in the compressor—a process that also raises its temperature. This hot vapor then enters the condenser, where the hot vapor cools and condenses into a liquid state. A heat exchanger in this part of the heat pump transfers heat to the surrounding air (because that air is cooler than the refrigerant), and that heated air is delivered to the house.
Next, the condensed liquid refrigerant flows through an expansion valve, where it experiences a sudden pressure drop. This further cools the refrigerant, which at this point is mostly liquid. From there, this liquid refrigerant flows into the evaporator, where it evaporates into a vapor, a phase change that requires absorption of a large amount of heat. The heat exchanger uses air to warm that vapor and, in the process, chills the air.
“The key idea,” says Temple, “is that we have a loop and there are two phase changes every time the refrigerant goes around that loop.” Heat is absorbed on one side of the loop (evaporation) and dissipated on the other (condensation).
Every heat pump has a heat source and a heat sink , and these can be swapped. In the winter, the heat source is the outside air (air-source heat pump, ASHP) or the ground (ground-source heat pump, GSHP), and the heat sink is the house that is being heated. In the summer, that is reversed, with the house air being the heat source and either the outdoor air or ground being the heat sink. The same coils are used, but in one season they serve as evaporator coils and in the other season they serve as condenser coils. A heat pump’s reversibility is one of its key benefits.
Imagine removing the door on your refrigerator. In the winter, you position the refrigerator in a doorway, with its opening facing the outside. In this configuration, the refrigerator works like a heat pump to heat your house—extracting heat from the outside air and transferring that heat to the room through the coils on its back. In the summer, you turn the refrigerator around and fit it back into the doorway so that its cool interior now faces the room. Now it works like a heat pump to cool your house.
A real heat pump doesn’t have to be physically moved, but uses valves to accomplish the same effects—the condenser becomes the evaporator and the evaporator becomes the condenser.
Confused? Don’t worry. The refrigerant cycle is complex and not very intuitive. But it works—as it has since Willis Carrier first perfected its use in 1900. In a heat pump, the components are elegantly housed in a box located inside the home or in separate indoor and outdoor units. The user doesn’t see those thermodynamic processes happening, but can just enjoy the results.
Ground-source heat pumps have been a darling of the green building movement—and for pretty good reason. Temperatures underground remain much more constant year-round than air temperatures. This makes the ground a better heat source in winter and a better heat sink in summer, boosting the efficiency of a GSHP compared to an ASHP.
Most GSHPs are closed-loop systems, in which long coils of plastic tubing (usually polyethylene) are buried in horizontal trenches or vertical bore holes. This tubing serves as a primary heat exchanger, transferring heat between the ground and an antifreeze (glycol) solution, which is brought to the heat pump where another exchange of heat with the refrigerant happens.
A specialized type of closed-loop heat pump uses the coil of tubing sunk in a body of water. The heat transfer to and from the water is very rapid, so the coil doesn’t need to be as long. However, this type of system is uncommon, since most homes don’t sit next to usable water sources.