Heat pumps

What they are, how they work

When it comes to residential heating and cooling systems, few types of systems are as energy efficient as heat pumps. But what exactly are they and how do they work? In short, a heat pump is simply an electrical device that is capable of transferring heat from one place to another. Though it may seem counterintuitive at first, heat can actually be extracted from cold air and added to warmer air.

By Jason Ng Cheng Hin

SUBSCRIBE TO THE DIGITAL OR PRINT ISSUE OF ECOHOUSE CANADA FOR THE FULL VERSION OF THIS ARTICLE.

To explain this in a more practical manner, let us step back a little bit to understand the basics. First of all, for the sake of an analogy, let’s imagine heat as a big body of water. Logically, water would flow from a higher place to a lower place. Similarly, heat naturally flows from a hotter place to a colder place.

So how is it that we can extract heat from a cold place and send it to a warmer place? Well, it is a little bit like moving water from a low place to a high place. All you need is a pump! Heat is actually defined by the movement of the molecules that make up matter. So in essence, all of the air in the world that is warmer than absolute zero [-273 °C, the temperature at which molecules actually stop moving] contains some amount of heat. So technically, it is possible to extract heat from air of any temperature and send it somewhere else, all it takes is a little energy.

Heat pumps are actually quite commonly used in our everyday lives. In fact, this process is happening almost all the time in your home right now, as refrigerators are actually a typical form of heat pump.

Heat pumps are able to extract heat from air by using a liquid refrigerant [more on this later] to absorb and remove heat and what’s known as the vapour-compression refrigeration cycle, which is a fancy name for a thermodynamic process that is commonly used for heat transfer applications.

There are four basic steps:
1. The process starts where the refrigerant is in a state known as a saturated vapour. This saturated vapour enters a compressor where the pressure is increased and, consequently, the temperature as well.
2. Hot vapour is then passed through a condenser, where it is condensed back into liquid form. The result of this condensation is that the refrigerant will lose its heat. This is what is what is occurring right now in your household refrigerator, and why it is warm in the back.
3. The liquid refrigerant then passes through an expansion valve, where the pressure drops and the liquid becomes much colder. At this point, the fluid is typically colder than the space that needs to be cooled.
4. Lastly, the cold fluid, which is now partially evaporated due to the pressure drop, passes through an evaporator, which typically consists of a coil or long tubes. A fan then blows air over the coil or tubes, cooling the air. This causes the refrigerant to evaporate within the tubes, returning it to its original saturated vapour state.

Jason Ng Cheng Hin, Eng, M.A.Sc. Jason is a consulting engineer who specializes in energy efficiency in buildings and building energy simulations. Our thanks to Jeff Hoogveld of Thermo Matrix in Kelowna, and to Maggie Yeun, P.Eng of Mitsubishi Electric in Toronto for their help with this article.