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How do heat pumps work?
To explain how heat pumps work, we need to understand a process that is basic in all of nature, but isn't fully understood by most people. That process is called "Change of State".Change of state refers to the event of a substance changing from gas to liquid, or from liquid to solid. Water is an example that everyone is familiar with. Water can exist in different 'states' as a vapor, or as a liquid, or as a solid. When water is in each of these states, it is still water, it is still H2O.When water is in the vapor state, it behaves in every way as a gas would behave. It is highly compressible, it can expand to fill the container it may find itself in. But it is still water, still H2O. "Steam" is another term for water vapor, but the word 'steam' also carries the connotation of heat... which gives us a clue as to what is going on here.When water is in the liquid state, it behaves in every way as a liquid would behave. It is almost completely incompressible, but it can take the shape of the container it may find itself in. It is still water, still H2O. This is the most familiar form of water we know. When we hear the word "water", we think of liquid water. This is because in the temperature and pressure levels that humans are comfortable in, water is most frequently seen as a liquid.When water is in the solid state, it behaves in every way as a solid would behave. It is almost completely incompressible, but it will not take the shape of the container it may find itself in. It is still water, still H2O.What causes the differences between these different states of water? Why is water a vapor in some instances, and a liquid in other instances, and a solid in still other instances?Our ordinary experience give us a clue that heat has something to do with it. So you could take a pan, put some ice in it, put the pan on a stove and turn on the heat.After a while, you would see the ice start to melt, turn into liquid water, and then a while after that the heat from a stove would cause the water to boil and then turn to steam, or water vapor, until the pot was dry.OK, great... Is it reversible?So, where does the heat come from that is used to heat the house?Well, if you have ever pumped up a bicycle tyre you will probably have noticed that the pump body gets warm or hot. How come?When you compress a gas it heats up - pumping the pump with mechanical energy (your arm or leg) compresses the gas to a higher pressure (as the exit from the pump is a small hole so the pressure in the pump body increases) and the gas heats up.This is what the compressor does - compresses the refrigerant which is in the form of a cold gas with mechanical energy (a motor) and in doing so as the gas compresses and it heats up.The hot gas is then pumped to the place where the heat is removed (some form of heat exchanger) so you then have a cold liquid. This cold liquid is vaporised into an even colder gas through a metering device. The very cold gas then easily absorbs heat from the atmosphere or ground (in another heat exchanger) to become a cold gas and this then goes back into the compressor to be compressed back into a hot gas.Most residential heat pump (and air conditioning) systems use what is called a "vapor compression cycle" or "phase change cycle". They use a volatile chemical (like freon, puron, propane, etc) for a heat transfer fluid. The chemical "refrigerant" is in a sealed plumbing loop, and a compressor is used to move the chemical by pressure differential. The process is very stable and very energy efficient.This process takes advantage of the energies of evaporation and condensation or "boiling and distilling". As with water on the stove, it takes a lot less heat (sensible heat) to bring the fluid to boiling temperature than it does to actually boil all the water(latent heat). The heat pump takes advantage of this large latent heat transfer, illustrated below in the phase changes that occur when water is heated from ice to steam:As you can see, it takes many more joules of heat energy to melt the ice and boil the water at constant temperatures than it does to raise it from -50 to 0, 0 to 100, and 100 to 150 degrees celsius.In the sealed refrigerant loop, internal pressures determine the boiling point of the chemical rather than just raw temperature. So by changing the pressure inside the plumbing, the chemical can be forced to change from a liquid to a gas and vice versa at whatever temperatures we want or need them to. When the chemical changes from a liquid to a gas (evaporation), it must absorb heat to do so. When it changes from a gas to a liquid (condensation), it has to release the same amount of heat.The device that separates the pressures is called a metering device. It is basically a highly engineered blockage in the plumbing. When the compressor runs, it builds up pressure on the discharge side due to this blockage. As the discharge pressure builds up, the chemical gets hot inside and releases heat through the piping wall. This release of heat at high pressure causes the chemical to condense into liquid form. The higher the pressure, the more the liquid builds up in front of the blockage. The longer the wait, the more the liquid is cooled in the plumbing. If the liquid refrigerant is cooled below its boiling temperature, we say it is "subcooled".At some point, the built up liquid forces its way through the blockage. Making its way through, it encounters a massive pressure drop, which forces the liquid to boil violently and absorb heat in the process. The liquid has to seek a lower temperature in order to absorb heat, since at this lower pressure, the boiling temperature is many degrees lower than it was on the other side of the metering device. The longer the plumbing is between the blockage and the compressor intake, the more heat is absorbed along the way. When (and if) the liquid all boils off and begins to absorb sensible heat from the plumbing, we say the chemical has become "superheated".In well-designed heat pumps, the plumbing in between the compressor and metering device is made so the heat flows easily as the chemical changes states of matter (phase change).This area of plumbing surface is known as a heat exchanger. Heat exchangers are designed so that the refrigerant can absorb or release all the latent heat it needs to make the change and then some. The heat extracted or released can be upwards of 5 times as much as the energy used by the compressor to actually move the refrigerant.Below is a basic diagram of an air-to-air heat pumping system:In this picture, red represents high pressure gas, orange represents high pressure liquid, blue represents low pressure liquid, and green represents low pressure gas.
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By now, you've learned that air-source heat pumps use an outdoor fan to bring air over refrigerant-filled coils. Two sets of these coils transfer this heat indoors where it's then blown away from the coils by a second fan, and distributed through your home as cool goodness. Some air-source heat pump systems consist of a single packaged unit containing both sets of coils in one box. This box is then installed on the roof of a building with the ductwork extending through the wall. A lot of larger systems for commercial buildings are installed in this way.
Home heat pumps are usually split systems with an outdoor and an indoor component installed through the wall. Depending on the type of system, it may have one or more indoor components to distribute heat.
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Ground-source heat pumps (also known as geothermal heat pumps) are a little different. They absorb heat from the ground or an underground body of water and transfer it indoors — or vice versa. The most common type of ground-source heat pump transfers heat directly from the ground by absorbing it through buried pipes filled with water or a refrigerant.
These liquid-pumping pipes can be either closed-loop or open-loop systems, and they operate pretty much exactly how they sound. In a closed-loop system, the same refrigerant or water circulates through the pipes repeatedly. In an open-loop system, water is pumped out of the underground water source, like a well or a man-made lake. From there, the heat is extracted from the water and that water returns to the well or surface lake. More water is then pumped from the well to extract more heat in a continuous open loop.
If that's not enough to blow your mind, consider the absorption heat pump — air-source pumps that are powered by natural gas, solar power, propane or geothermal-heated water rather than by electricity. Absorption pumps can be used for large-scale applications but are now available for homes.
The main difference between a standard air-source heat pump and an absorption pump is that instead of compressing a refrigerant, an absorption pump absorbs ammonia into water, and then a low-power pump pressurizes it. The heat source then boils the ammonia out of the water — and the process starts all over again.
When you go to check out an absorption heat pump, it helps to know how they're rated. Manufacturers rate them using a measurement called a coefficient of performance (COP), which sounds pretty complicated. But all you need to know is to look for a COP above 1.2 for heating and above 0.7 for cooling. And don't worry, we'll discuss ratings for standard heat pumps a little later.
Air-source, ground-source and absorption heat pumps are the most common kinds of heat pumps, but they won't work in every situation. Read on to learn about other types of heat pumps.
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